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Cells in New Light: Ion Concentration, Voltage, and Pressure Gradients across a Hydrogel Membrane

Magdalena Kowacz*
Magdalena Kowacz
Department of Bioengineering, University of Washington, Box 355061, Seattle, Washington 98195, United States
*Email: [email protected]
More by Magdalena Kowacz
http://orcid.org/0000-0001-5729-6816
and
Gerald H. Pollack
Gerald H. Pollack
Department of Bioengineering, University of Washington, Box 355061, Seattle, Washington 98195, United States
More by Gerald H. Pollack

Cite this: ACS Omega 2020, 5, 33, 21024–21031

Publication Date (Web):August 11, 2020

https://doi.org/10.1021/acsomega.0c02595

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Abstract

The ionic compositions of the intra- and extracellular environments are distinct from one another, with K⁺ being the main cation in the cytosol and Na⁺ being the most abundant cation outside of the cell. Specific ions can permeate into and out of the cell at different rates, bringing about uneven distribution of charges and development of negative electric potential inside the cell. Each healthy cell must maintain a specific ion concentration gradient and voltage. To account for these functions, various ionic pumps and channels located within the cell membrane have been invoked. In this work, we use a porous alginate hydrogel as a model gelatinous network representing the plant cell wall or cytoskeleton of the animal cell. We show that the gel barrier is able to maintain a stable separation of ionic solutions of different ionic strengths and chemical compositions without any pumping activity. For the Na⁺/K⁺ concentration gradient sustained across the barrier, a negative electric potential develops within the K⁺-rich side. The situation is reminiscent of that in the cell. Furthermore, also the advective flow of water molecules across the gel barrier is restricted, despite the gel’s large pores and the osmotic or hydrostatic pressure gradients across it. This feature has important implications for osmoregulation. We propose a mechanism in which charge separation and electric fields developing across the permselective (gel) membrane prevent ion and bulk fluid flows ordinarily driven by chemical and pressure gradients.

Introduction

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Two general features characterize a healthy cell: its ability to maintain an ion concentration gradient across the cell membrane and its ability to generate an electric potential difference between the cytosol and the extracellular fluid. (1) Inside a cell, the concentration of potassium ions (K⁺) is much higher than that of sodium ions (Na⁺), while the situation is reversed outside of the cell. It is recognized that the difference in ion concentrations is maintained by adenosine 5′-triphosphate (ATP)-driven (i.e., energy-consuming) pumps located in the cell’s membrane. An associated theory is that of membrane channels, which allow ions to move through the membrane down their concentration gradients, but at different rates. Because K⁺ ions move out of the cell at a faster rate than Na⁺ ions that penetrate inward, a difference in voltage across the cell membrane is created. This results in a negative voltage inside of a cell with respect to the outside. Both membrane pumps and channels are highly specific biological entities permeable to only particular types of ions.

Some concern has been expressed that the cell’s own metabolic energy may not suffice for supporting the functioning of all ionic pumps. (2,3) Experiments in which ion concentration gradients could be sustained even without the intact cell membrane (4) have implied that other mechanisms might contribute to this phenomenon. In accordance with those earlier results, a recent study has in fact shown that the bare cytoskeleton of membrane-permeabilized neurons can generate voltage oscillations underlying action potentials. (5) This feature is commonly thought to result solely from the performance of membrane pumps and channels. Yet, this electrical activity was shown to be due to microtubule bundles acting as ion-selective barriers, thanks to the negative charges and porosity of their cytoskeleton-building network. (5)

The cytoskeleton, able to induce intracellular electric fields, (5,6) was also demonstrated to transmit information between cell’s organelles via electric signals. (6) Therefore, some electrical properties manifested in a cell could result from features of the cytoskeleton, even without the pumping and channeling activity of membranous components. Other works have shown that some synthetic hydrogels (porous, negatively charged networks) can be a good experimental approximation of the cell’s cytoskeleton in terms of its mechanical and electrical properties. (7−10) Furthermore, the cytoplasm of mammalian cells has been shown to resemble a hydrogel. (11)

In this work, we use an alginate gel to examine the role of gelatinous components of a cell in sustaining its main electrochemical properties. Alginate is a naturally occurring polysaccharide derived from brown algae, where it constitutes the main structural component of the algal cell wall. (12) Due to its many carboxyl functional groups, just as those present within a cytoskeleton, the alginate hydrogel forms a negatively charged porous network within a cell. We explore the ability of the alginate hydrogel to support a Na⁺/K⁺ ion concentration gradient across the gel membrane, generate an electric potential difference, and respond to osmotic or hydrostatic pressure gradients.

Results and Discussion

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The calcium–alginate hydrogel, as used in our study, consists of a highly porous three-dimensional network with reported pore sizes ranging from few nanometers to micrometers (depending on the technique used to determine pore diameter). (13) Such pores are large enough to allow for diffusion of ions and water molecules throughout the network, unless specific interactions are present. Indeed, it is recognized that ions and larger solutes diffuse through the gel at rates sufficient to support the metabolism of gel-encapsulated cells (alginate is commonly used for cell immobilization). Motions of those solutes are restricted to different degrees, mainly due to steric obstructions created by the gel framework. (14,15) Nonetheless, the supply of solutes, from the bathing solution, through the gel, to the embedded cell, is assured. Yet, in the particular arrangement of our experimental system, neither ions nor water molecules could be exchanged across the alginate gel between the two aqueous reservoirs, as implied by conductivity measurements and observations on the effect of pressure gradients (see the Results and Discussion section). Thus, in our case, alginate creates a virtually impermeable barrier, so that interconnected solutions do not mix. Below, we present details of the experimental results, as well as specific conditions that need to be fulfilled to prevent ion and water penetrations across the gel. The underlying phenomena of charge separation at the gel/water interface, manifested in the measured electric potential difference across the gel, are also discussed.

Conductivity changes are directly correlated to ion concentrations (for ionic solutions of low concentration). In the two ionic solutions used in our experiments, these changes allow us to monitor the extent of solution mixing through the interconnecting bridge. We found that the conductivity of the salt solutions of different ionic strengths, separated by the gel, did not change over a time scale of a few days (Figure 1). This implies that there is no net migration of ions across the barrier between the solutions.

Figure 1. Conductivity measured over time in the interconnected-by-alginate-bridge solutions of (a) the left panel, from top to bottom: seawater, 200 mM KCl, 200 mM NaCl, 100 mM KCl, 100 mM NaCl, and DI water (right panel); (b) KCl (left, from top to bottom: 100 and 50 mM) and NaCl (right, from top to bottom: 100 and 50 mM). Note the 3-orders-of-magnitude difference in scale values for salt and DI water containers. The average values of the electric potential difference (mV) registered for (a) ionic solutions with respect to DI water and (b) KCl with respect to NaCl solution are depicted and yield (a) −180, −150, and −120 mV for 100 mM (KCl, NaCl, and seawater, respectively) and (b) −50 mV for interconnected solutions of KCl and NaCl of different ionic strengths, and −100 mV for equimolar solutions.

Although ions are expected to move down their concentration gradients to reach a uniform concentration in the two communicating vessels, such mixing does not occur. On the contrary, a stable ion concentration gradient is maintained across the gel barrier, as confirmed by conductivity measurements. In fact, a very slight increase in the conductivity of the DI water reservoir can be observed with time. Yet, that conductivity remains 3 orders of magnitude lower than in the interconnected salt solution. Therefore, although a very limited number of ions may possibly penetrate the gel barrier, no substantial mixing occurs between the two reservoirs that would lead to concentration equilibration.

Moreover, the imposed difference between the liquid column heights of the salt solutions and DI water across the gel barrier is also stably maintained. This indicates no variation of the hydrostatic pressure gradient over time (see Figure 2). The gradient is sustained, despite the porosity of the bridge interconnecting the two reservoirs. The pores, as already stated, should be large enough to allow for diffusion of water molecules. Indeed, if the gel separates two columns of different heights containing only water, these columns do eventually equilibrate to attain the same level, according to the principle of communicating vessels. Yet, if the interconnected solutions have different ionic strengths (DI water as the salt solution), no net water flow tending to equilibrate the pressure difference could be registered. Even when the osmotic force and hydrostatic pressure act in the same direction, the liquid levels in the reservoirs remain unchanged. That situation is illustrated in the case when a KCl solution is adjoined to a column of DI water of greater height (right-hand side, Figure 2). Thus, in these cases, the presence of a gel barrier cancels the effect of hydrostatic and osmotic forces.

Figure 2. Changes in solution levels (liquid column heights) in the two vessels interconnected by the alginate gel bridge (G) and containing DI water and 100 mM KCl solution. The light blue columns represent the liquid level at the beginning of the experiment, while the dark blue dotted columns show a steady liquid level at the end of the experiment.

The reason for the latter effects originates from the intrinsic properties of the uniformly charged porous alginate hydrogel, further in the study termed also “gel membrane” due to its function to separate the solutions.

Figure 3. (a) FTIR-attenuated total reflection (ATR) spectra of a 0.05 M K₂SO₄ solution and (b) a difference spectrum of the K₂SO₄ solution before and after a piece of alginate gel was soaked and swelled in it. Note an increase in an absorption band of SO₄^2– (highlighted in yellow), expressed as the positive peak in difference spectra, after the solution contacts with a gel.

Alginate Hydrogel as a Permselective Membrane

The alginate hydrogel is a negatively charged porous matrix that allows only cations (but not their counterions) to penetrate through. (18) Mobile ions from the bathing solution may migrate into the gel interior, driven by an attraction exerted by the fixed gel matrix charges. (18) Many anionic polyelectrolyte gels are known to exhibit permselectivity similar to that of alginate, being largely impermeable to anionic species. (16) Such behavior of the alginate gel is supported also by our spectroscopic measurements. In these experiments, water imbibition by the gel (swelling) results in an increased anion absorption band (thus concentration) in the bathing solution (Figure 3). This means that only water and/or cations can infiltrate the gel, while the anion concentration in the solution effectively increases due to reduced solvent volume, now partially accommodated within the gel matrix.

Figure 4. Optical microscope images of the ion-depleted exclusion zone (EZ) water, expressed as a zone devoid of the crystals (black dots in the image) formed in the solution next to the alginate gel surface. The ion-depleted EZ could be observed under the microscope only for gels prone to swelling (after shorter dialysis time). The scale bar is 200 μm.

When only cations selectively penetrate into the alginate gel, their parent solution is left with excess negative charges. Separated charges—positive ions migrating into the gel and negative ions repelled from the gel—create an electric potential difference. In our experiments, a negative voltage is always registered at the ionic side for the following solution combinations: KCl/water, NaCl/water, or seawater/water (Figure 1a). This result is in agreement with the inferred mechanism, where only cations permeate within and/or across the gel barrier, making their parent solution negatively charged with respect to the DI water reservoir. The magnitude of the registered negative voltage can be understood by considering differences in ion affinity toward alginate. K⁺ ions, which form the weakest ion pairs with carboxylate groups of alginate, can penetrate the gel network to a greater extent than Na⁺ ions with their stronger affinity toward COO^–. (17−19) The ease of penetration translates into a greater separation of charges, provided by cations infiltrating the gel and leaving their counterions behind in solution. This consideration is in agreement with the measured voltage, being the highest (most negative) across the gel in contact with the KCl solution and the lowest (least negative) in the case of the gel exposed to seawater. In the latter scenario, minor components of seawater, like Ca²⁺ or Mg²⁺, form strong complexes with alginate. Therefore, these ions are trapped in the gel’s pores and effectively screen fixed negative charges of alginate that are responsible for the gel’s selective permeability and voltage-generating capacity. For the KCl/NaCl system, a negative potential develops always on the KCl side (Figure 1b). Such an outcome can be explained by the fact that K⁺ ions penetrate the gel more readily than do Na⁺ ions; (17−19) hence, the K⁺ ions migrate away from their solution at a faster rate than do the Na⁺ ions. Therefore, the KCl side becomes negatively charged with respect to the interconnected NaCl solution.

The question, then, is why the cross-migration of ions stops altogether, so that there is no net mixing between the solutions, but the concentration gradient, as well as the electric potential difference, is stably maintained across the barrier (Figure 1). This can be understood by considering how the gel’s permselectivity affects ion distribution in the gel vicinity.

Ion Exclusion Zone

After some of the cations penetrate the alginate gel, the excess anions left in the solution are now electrostatically repelled from the negatively charged gel surface and cannot approach it closely without their counterions. As a consequence, a zone depleted in ions (cations and anions) is formed next to the gel interface. This phenomenon has been already widely exploited in designing water desalination and separation/preconcentration devices. (20−28) Water that is free of ions and other charged species could be collected in the immediate vicinity of the permselective membrane. At the same time, the solutes tended to preconcentrate at the ion-free/bulk solution boundary. The existence of such solute-free water layers has been observed next to many hydrophilic surfaces and, thanks to its properties, was given a generic name of exclusion zone (EZ). (29−33) We were able to confirm the presence of an ion-depleted EZ water layer adjacent to the alginate hydrogel in experiments in which crystal precipitation was induced in situ in a solution containing the alginate gel. Crystals formed from dissolved ions do not appear close to the gel surface (Figure 4). This implies that the region in the immediate vicinity of the gel is free of any ions that could serve to build crystals.

Due to the charge separation across the EZ layer (positive ions within the gel and their negative counterions at the EZ/bulk solution boundary), an electric force is created that opposes the chemical force driven by the concentration gradient. Thus, cations migrating away from the solution and within the gel are attracted back to their negatively charged counterions. After the equilibrium between these forces has been established, a stable concentration gradient and voltage are maintained (Figure 5), as seen in our experiments.

Figure 5. Schematic illustration showing the mechanism of formation of the ion-depleted EZ layer next to the negatively charged gel. Salt ions migrate according to the concentration gradient from the KCl solution to water. Due to the negative charge of the gel matrix, only cations can pass, and the KCl solution is left with excess negative charges. These negative charges are now repelled from the negatively charged gel body, thus creating an ion-depleted exclusion zone of water. The initially ion-free water reservoir (right side) gains some extra positive charges. The resulting electric force (cations attracted back by their counterions) opposes the chemical force due to the concentration gradient of ions between the two solutions.

Very notably, this is the situation analogous to that existing in a cell, where different ionic compositions between inside and outside, with the electric potential difference, are expressed across the cell membrane. The intrinsic properties of a permselective hydrogel are thus sufficient prerequisites to express these features, without the energy expenditure required in the case of membrane pumps.

In fact, the ion-depleted EZ, sustaining charge separation (by balanced chemical and electric forces), should develop on both sides of a gel membrane, but more extensively on the side exposed to K⁺-containing solution—the larger EZ being due to K⁺ migrating toward/within the gel faster than does Na⁺. Therefore, the electric potential difference between solutions separated by the gel membrane is determined by the relative partitioning of mobile ions from solution within the gel matrix (Figure 6).

Figure 6. Schematic illustration of the mechanism of development of electric potential difference between two ionic solutions separated by a permselective gel. K⁺ migrates within the gel at higher rates than does Na⁺. As their Cl^– counterions are left behind (cannot penetrate negatively charged pores), those anions are repelled by a charged gel creating an ion-depleted EZ at both sides of the gel membrane. Due to the faster migration of K⁺, the KCl solution is left with more excess negative charges than the NaCl solution. This relative difference is expressed in the measured negative electric potential of the KCl solution.

Yet, it is interesting to note that if the potential difference was measured between the gel and its adjacent solution on either side, the gel matrix should exhibit a negative voltage relative to the solution due to gel excess fixed negative charges. This is a known property of anionic polyelectrolyte gels. In principle, the presented mechanism of voltage generation across the gelatinous barrier is consistent with the ideas put forward in Ling’s association-induction theory. (2−4,34,35) Ling suggested that the electric potential of a cell results from the selective association of particular ions with components of the cytoplasm. If, rather than cross-migration of ions, we assume preferential accommodation of K⁺ ions within the alginate gel, then our experimental results could be interpreted in the view of Ling’s theory. K⁺ ions infiltrating the gel membrane, even without crossing it, to a greater extent than Na⁺ ions, leave behind a larger number of unbalanced anions in their parent solution. Therefore, the solution in the K⁺-rich side of the barrier becomes more negative in relation to the Na⁺-rich side.

The reason behind the apparent impermeability of the gel barrier toward ions can now be understood by considering the development of an ion-depleted EZ supporting sustained charge separation.

Interfacial Water Ordering and Pressure Gradients

Apart from the migration of ions, there are still forces, both osmotic and hydrostatic, that should drive the flow of water molecules between the interconnected solutions. Nevertheless, as shown in our experiments with different solution levels (Figure 2), there is no net water flow across the gel.

Generally, the liquid in communicating vessels tends to equilibrate the pressure felt at any point within the liquid’s entire volume by adjusting the liquid level in each compartment to the same height. This is because the pressure at any point in the homogeneous liquid depends only on the depth of the point in the liquid. In our case, however, the solution levels remain unequal regardless of the hydrostatic and osmotic pressure gradients.

The law of communicating vessels applies only to liquids, where intermolecular forces cannot counteract any pressure-driven displacements. In our system, it seems that forces acting between water molecules within the gel are strong enough to counteract any such pressure-induced translational motions. It is important to remember that this resistance to hydrostatic (as well as osmotic) pressure could be observed only in the presence of ions. Any physical pore clogging by the ions should not be possible. In fact, the alginate gel swells in the presence of a low concentration of monovalent salts, so pore volume increases. (16,17) Then, both Na⁺ and K⁺ ions form fully soluble complexes with alginate. (17) The observed resistance to a pressure-gradient-driven advective flow of water, as illustrated in experiments with different liquid columns’ heights, can be explained by considering the invoked charge separation and electric potential development, as described in the next paragraph.

In our experiments, voltage emerges across the porous gel network separating the two solutions. It has been previously shown that an electric field applied to a nanochannel can enhance the alignment of water molecules inside the channel, forming a liquid-crystalline phase. (36) Weak electric fields, of the strength comparable to that acting across the cell membrane, have been demonstrated to restrict water flow within a nanochannel due to this increased water structuring. (36) Water structure, resembling the liquid-crystalline state, has been previously suggested to exist next to many hydrophilic surfaces, termed the EZ water layer. (29−31) The voltage that develops in our experimental system (comparable in magnitude to that of the cell resting potential) (37) can therefore be expected to make water molecules in the porous network of the gel more ordered. Such water may resist the transfer of pressure information between the two communicating vessels in our study. Osmotic flow is also driven by pressure gradients, just as is hydrostatic flow. (38) Therefore, water molecules confined within alginate gel pores and/or adjacent to the gel–water interface could create an effective barrier, attenuating the impact of both osmotic and hydrostatic forces and effectively restricting flow.

Cell-Like Features of Gels

The alginate hydrogel is generally considered mainly as the structural component of the algal cell wall. (12) Our study shows it to be able to sustain a Na⁺/K⁺ concentration gradient, support the development of an electric potential difference, and attenuate osmotic pressure. These features in fact closely resemble the characteristics of each healthy cell: the ability to maintain a specific chemical identity of the intracellular environment, generate voltage, and maintain constant volume.

We have observed that the size of the gel-induced EZ increases on gel swelling. This can be understood by the advective water influx carrying ions to the gel matrix and has been confirmed by our microscopy observations (Figure 4) and indirectly by Fourier transform infrared (FTIR) measurements (Figure 3). The increase of ion exclusion on gel swelling is also in agreement with previous findings. (22) The size of the ion-depleted EZ is indicative of the extent of charge separation, and the greater the separation, the higher should be the resulting voltage. We did not perform systematic experiments on the latter relationship, but our individual observations on the effect of gel swelling on the registered voltage support such a notion. Then, the voltage affects water structuring in small pores, manifested as resistance to (osmotic) pressure. For marine algae living in tidal areas and experiencing periodic desiccation, an increase of EZ on rehydration (Figure 7) may support these organisms in maintaining critical cellular homeostasis in salty environments. Especially, large EZ developing next to the swelling algae can help to prevent osmotically driven water outflux from the cells. This is because cells are not in contact with salty water anymore, but are separated from it by the surrounding ion-depleted EZ. The negative electric potential of a gel, mimicking the cytoskeletal components—a porous, negatively charged network—also becomes more negative after gel swelling. (7) This observation is in agreement with our considerations on the correlation between swelling, EZ size, and voltage, and because swelling can be induced by preceding mechanical contraction, it has been suggested that the intrinsic properties of the gel-like cytoskeleton could be mainly responsible for the mechanoelectric response of cells. (7)

Figure 7. Optical microscopy image showing the EZ water layer, expressed as a zone devoid of microspheres, developing next to a rehydrating algal blade. Note the solute enrichment (dark region) at the boundary between the EZ and bulk solutions, a feature consistent with the formation of an ion-depletion zone. (20−27) The scale bar is 4 mm.

Notably, it is recognized that in the immediate vicinity of almost every cell there is a layer of unstirred water, within which flow is highly restricted. (39,40) This layer is also known as a layer of stagnant solutes. The largest of the known unstirred layers develops next to the intestinal membranes within their highly mucous (gelatinous) environments. (41) Our results imply that electric fields acting next to permselective membranes may well contribute to the establishment of unstirred water layers (UWLs). This is because he advective flow of water and solutes (ions) is restricted across/next to the gel, just as it is within UWL.

Conclusions

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Our results show that a permselective, porous, and uniformly charged gel such as alginate can effectively constitute a membrane across which an ion concentration gradient and an electric potential difference can be stably sustained. The latter features are the fundamental characteristics of healthy cells and are now commonly ascribed exclusively to the activity of channels and energy-consuming pumps embedded within the cell membrane. Our results thus imply that the plant cell wall and/or the animal cytoskeleton, being charged gelatinous networks, can potentially contribute to maintaining those essential physicochemical properties of living cells.

We propose a mechanism allowing the gel barrier to restrict the net flux of ions or water molecules despite concentration or pressure gradients imposed across the gel. The underlying cause for this apparent impermeability may be the presence of an ion-depleted EZ water layer developing next to a permselective barrier. Charge separation across the EZ layer generates a potential difference. The electric force acting in the direction opposite to the chemical drive creates a steady state, in which no net ion flow is observed. Furthermore, the potential difference acts on water molecules, making them more ordered within the gel pores so that a pressure gradient cannot be easily transmitted across the gel. Therefore, any pressure-driven water advection is highly attenuated. It seems plausible that homeostasis of a cell in terms of its ionic composition, electric potential, and volume (resistance to osmotic stress) can be supported by the cell’s gelatinous components without the need for energy expenditure.

Experimental Section

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Stock solutions of NaCl, KCl, CaCl₂, BaCl₂, Na₂SO₄, K₂SO₄, and sodium alginate (all chemicals from Sigma-Aldrich) were prepared with deionized (DI) water from a Barnstead D3750 Nanopure Diamond purification system and diluted to the desired experimental concentration. Artificial seawater was prepared according to the recipe by Cold Spring Harbor Protocols. (42)

Alginate Gel Preparation

To make an alginate gel bridge, a glass capillary (internal diameter 1.12 mm, World Precision Instruments, Inc.), cut to a length of 2 cm, was filled with 2% solution of sodium alginate and immersed in a bath of 0.2 M CaCl₂ solution for a minimum of 24 h for complete gelation to occur. Ca²⁺ ions, as well as other divalent cations, act as cross-linking agents and transform soluble sodium alginate into a hydrogel. (43,44) Gel-filled capillaries were then dialyzed against deionized water for a minimum of 48 h to remove any nonspecifically bound ions. The capillaries were then stored in DI water until the time of the experiment.

The preparatory method for alginate gels used for microscopic observations and spectroscopic measurements varied slightly from that used for the gel bridge, above. Instead of a capillary, a small Petri dish (35 mm × 10 mm, Falcon) was filled with sodium alginate solution so as to just cover the dish bottom. The CaCl₂ solution was then gently poured over it. Before experiments, the formed gel was cut with a stainless steel blade to pieces of the desired size, usually about (5 mm × 5 mm).

For those experiments examining gel swelling, the gel substrate was dried in air and then fully rehydrated in water before use. During dehydration, some unspecific linkages are formed between those gel compartments that are not cross-linked. (45,46) On immersion in a solution of monovalent salts of low ionic strength, water-rehydrated gels swell additionally due to substitution of nonspecifically bound divalent cations by monovalent ones. (45) Shorter dialysis times, resulting in noncomplete removal of free Ca²⁺ entrapped within the gel network, were also used in our work to permit increased swelling when the gel was in contact with monovalent salts.

Experimental Setup

The alginate gel bridge was used to interconnect two reservoirs containing either NaCl or KCl at different concentrations relevant to the cell environment. Solutions included artificial seawater or DI water in different combinations: salt/salt, salt/DI water, and seawater/DI water (see Table 1). We used 2 mL Eppendorf microcentrifuge tubes as reservoirs. Holes were drilled at the 1 mL level mark of each tube, and the ends of the capillary were inserted through the holes. The reservoirs were filled to the top with the respective solutions. Solutions fully covered the capillary entries.

Table 1. Composition of Solutions Interconnected by the Gel Bridge

compound	conc. (mM)		conc. (mM)	compound (mM)
KCl	200	gel		DI water
NaCl	200
KCl	100
NaCl	100
seawater	600 (NaCl)^a
	100		100
KCl	100		50	NaCl
	50		100

^{^a}

Molarity of the main component of seawater.

Conductivity

To quantify ion permeability through the gel, we followed conductivity changes of the reservoir solutions. Conductivity results from the presence of mobile ions in solution, so changes of conductivity therefore directly relate to the permeability of the gel. Conductivity was measured with the use of a conductivity meter (OAKTON, CON 100 Series).

Electric Potential Difference

To check for permeability of the barrier to specific ions, we measured the development of a potential difference between the two reservoirs. For voltage measurement, a digital multimeter (TENMA 72-7750) was used with either steel or gold probes. The electric potential difference (voltage) between the two containers can develop in a situation when one solution becomes positively/negatively charged with respect to the other. Solutions of salts have no net charge, as each cation is counterbalanced by an anion. When only one type of ion, either positively or negatively charged, can selectively pass through the barrier, charges on both sides of the bridge may no longer remain balanced. Conductivity and voltage changes were followed for the course of at least 48 h and up to 120 h.

Pressure Gradient

To check for the effect of hydrostatic and/or osmotic pressure differences, the solution levels in the two reservoirs were made uneven (1 mL vs 2 mL) for salt/water and water/water interconnected systems: 1 mL 100 mM KCl/2 mL DI water; 2 mL 100 mM KCl/1 mL DI water; and 1 mL DI water/2 mL DI water. For these experiments, the alginate bridge was inserted closer to the bottom of the reservoirs to assure full coverage of capillary entries by the solutions. Changes in the solution levels were tracked over the course of the experiment (120 h). In communicating vessels, pressure is expected to equilibrate by adjusting the liquid level to the same height in each compartment. This is due to the fact that pressure (P) exerted at any point in the static fluid depends only upon the depth of the fluid (h), the density of the fluid (

), and the acceleration of gravity (g), P = h

g. Because g is constant and density (

) of the DI water and of 100 mM KCl is identical up to the third decimal place, the pressure felt at any point in the liquid in our communicating vessels should depend only on the liquid column height.

Crystallization Experiments

For microscopic evaluation of ion-depletion-zone development next to the alginate gel, crystallization experiments were employed. The gel specimen (approx. 5 mm× 5 mm × 2 mm) was immersed in 0.01 M Na₂SO₄ or 0.01 M K₂SO₄, covered with a glass slide, and left to equilibrate for 15 min. A cover slide was supported on gel pieces, so that a solution-filled clearance of about 2 mm persisted between the cover and the bottom of the setup. Then, with the use of an automatic pipette, a solution of 0.01 M BaCl₂ was added to the equimolar bathing solution of either Na₂SO₄ or K₂SO. The Ba²⁺ and SO₄^2– combine to form a very insoluble BaSO₄ precipitate that is observed immediately after mixing of the solutions.

We inspected our samples in situ for the presence of crystallization-free zones next to the alginate surface. The layer void of crystals means that there are no ions from which crystals could form. Both fully swollen and prone to swelling (rehydrated or with shorter dialysis time) gels were used in these experiments.

Behavior of Natural Algae

To visualize the solute-free zone forming next to the alginate-containing natural seaweed, we used a dried Wakame (Undaria pinnatifida) blade and immersed it in a suspension of 1 μm polystyrene microspheres with carboxyl functional groups (Polysciences Inc., 08226). The ratio of the aqueous microsphere suspension to DI water was 45 μL (1 drop) to 15 mL. The formation of a zone void of microsphere solutes was observed on blade rehydration and swelling.

Spectroscopic Experiments

We performed Fourier transform infrared (FTIR) spectroscopy measurements using an attenuated total reflectance (ATR) sampling technique to analyze the ion-selective permeability of the alginate gel by following the procedure described further in this section. A Thermo Scientific Nicolet Nexus 670 Spectrophotometer was employed. Spectra were recorded using 256 infrared-beam scans and a wavenumber resolution of 4 cm^–1. OMNIC software was used to perform the analysis. We used the K₂SO₄ solution as a gauge for gel permeability due to a characteristic infrared absorption feature of aqueous SO₄^2– at 1104 cm^–1.

After collecting the spectra of 0.05 M K₂SO₄, a piece of alginate hydrogel (of a volume comprising about 1/3 of a K₂SO₄ solution volume) was immersed in the solution. As the gel swelled, changes in the height of the SO₄^2– absorption band were monitored in the solution. To assure swelling (solution infiltration), previously dehydrated gels (as described in the Alginate Gel Preparation section) were used in the experiments. Due to the negative charges of gel pores provided by fixed carboxyl groups, the alginate gel is expected to be selectively permeable to cations but impermeable to anions. (47) Therefore, an increased concentration of SO₄^2– anions in solution (expressed as the height of the absorption band) can be expected when only water molecules (and cations), but not anions, penetrate into the gel.

Author Information

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Corresponding Author
- Magdalena Kowacz - Department of Bioengineering, University of Washington, Box 355061, Seattle, Washington 98195, United States; http://orcid.org/0000-0001-5729-6816; Email: [email protected]
Author
- Gerald H. Pollack - Department of Bioengineering, University of Washington, Box 355061, Seattle, Washington 98195, United States
Funding
The Software AG Foundation (SAGST) and Anonymous.
Notes
The authors declare no competing financial interest.

Acknowledgments

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The authors thank Arazi Pinhas and Zheng Li for editing suggestions on early versions of this manuscript.

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This article references 47 other publications.

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The physical state of water and ions in living cells and a new theory of the energization of biological work performance by ATP
Ling, Gilbert N.
Molecular and Cellular Biochemistry (1977), 15 (3), 159-72CODEN: MCBIB8; ISSN:0300-8177.
A review with 84 refs. Various models are considered. The effects of ATP interaction with cellular proteins on cell energization are considered.
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Thoke, H. S.; Bagatolli, L.; Olsen, L. Effect of Macromolecular Crowding on the Kinetics of Glycolytic Enzymes and the Behaviour of Glycolysis in Yeast. Integr. Biol. 2018, 10, 587– 597, DOI: 10.1039/C8IB00099A
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Effect of macromolecular crowding on the kinetics of glycolytic enzymes and the behaviour of glycolysis in yeast
Thoke, Henrik S.; Bagatolli, Luis A.; Olsen, Lars F.
Integrative Biology (2018), 10 (10), 587-597CODEN: IBNIFL; ISSN:1757-9694. (Royal Society of Chemistry)
Water is involved in all aspects of biol. activity, both as a solvent and as a reactant. It is hypothesized that intracellular water is in a highly structured state due to the high concns. of macromols. in the cell and that this may change the activity of intracellular enzymes due to altered binding affinities and allosteric regulations. Here we first investigate the kinetics of two glycolytic enzymes in artificially crowded aq. solns. and show that crowding does indeed change their kinetics. Based on our kinetic measurements we propose a new model of oscillating glycolysis that instead of Michaelis-Menten or Monod-Wyman-Changeux kinetics uses the Yang-Ling adsorption isotherm introduced by G. Ling in the frame of the Assocn.-Induction (AI) hypothesis. Using this model, we can reproduce previous exptl. observations of the coupling of glycolytic oscillations and intracellular water dynamics, e.g., (i) during the metabolic oscillations, the latter variable oscillates in phase with ATP activity, and (ii) the emergence of glycolytic oscillations largely depends on the extent of intracellular water dipolar relaxation in cells in the resting state. Our results support the view that the extent of intracellular water dipolar relaxation is regulated by the ability of cytoplasmic proteins to polarize intracellular water with the assistance of ATP, as suggested in the AI hypothesis. This hypothesis may be relevant to the interpretation of many other biol. oscillators, including cell signalling processes.
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Ling, G. N. Maintenance of Low Sodium and High Potassium Levels in Resting Muscle Cells. J. Physiol. 1978, 280, 105– 123, DOI: 10.1113/jphysiol.1978.sp012375
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Maintenance of low sodium and high potassium levels in resting muscle cells
Ling, Gilbert N.
Journal of Physiology (Cambridge, United Kingdom) (1978), 280 (), 105-23CODEN: JPHYA7; ISSN:0022-3751.
The healthy region of a frog sartorius muscle effectively membraneless open-ended cell prepn. accumulated 42K+ to a level higher than that in Ringer's soln. and excluded 22Na+ to a level below that in Ringer's soln., much as a normal uncut muscle does in its normal environment. Ouabain (10-4M) reduced the cellular K+ concn. The diffusion coeff. of Na+ in normal muscle cytoplasm at 25° was 2.07 × 10-6 cm2/s, roughly 1/6 that of the diffusion coeff. of Na+ in a 0.1N NaCl soln. The results are discussed in terms of the assocn.-induction hypothesis.
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Cantero, M.; Villa Etchegoyen, C.; Perez, P.; Scarinci, N.; Cantiello, H. Bundles of Brain Microtubules Generate Electrical Oscillations. Sci. Rep. 2018, 8, 11899 DOI: 10.1038/s41598-018-30453-2
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Bundles of Brain Microtubules Generate Electrical Oscillations
Cantero Maria Del Rocio; Perez Paula L; Scarinci Noelia; Cantiello Horacio F; Villa Etchegoyen Cecilia
Scientific reports (2018), 8 (1), 11899 ISSN:.
Microtubules (MTs) are long cylindrical structures of the cytoskeleton that control cell division, intracellular transport, and the shape of cells. MTs also form bundles, which are particularly prominent in neurons, where they help define axons and dendrites. MTs are bio-electrochemical transistors that form nonlinear electrical transmission lines. However, the electrical properties of most MT structures remain largely unknown. Here we show that bundles of brain MTs spontaneously generate electrical oscillations and bursts of electrical activity similar to action potentials. Under intracellular-like conditions, voltage-clamped MT bundles displayed electrical oscillations with a prominent fundamental frequency at 39 Hz that progressed through various periodic regimes. The electrical oscillations represented, in average, a 258% change in the ionic conductance of the MT structures. Interestingly, voltage-clamped membrane-permeabilized neurites of cultured mouse hippocampal neurons were also capable of both, generating electrical oscillations, and conducting the electrical signals along the length of the structure. Our findings indicate that electrical oscillations are an intrinsic property of brain MT bundles, which may have important implications in the control of various neuronal functions, including the gating and regulation of cytoskeleton-regulated excitable ion channels and electrical activity that may aid and extend to higher brain functions such as memory and consciousness.
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Frieden, B. R.; Gatenby, R. Signal Transmission through Elements of the Cytoskeleton Form an Optimized Information Network in Eukaryotic Cells. Sci. Rep. 2019, 9, 6110 DOI: 10.1038/s41598-019-42343-2
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Signal transmission through elements of the cytoskeleton form an optimized information network in eukaryotic cells
Frieden B R; Gatenby R A
Scientific reports (2019), 9 (1), 6110 ISSN:.
Multiple prior empirical and theoretical studies have demonstrated wire-like flow of electrons and ions along elements of the cytoskeleton but this has never been linked to a biological function. Here we propose that eukaryotes use this mode of signal transmission to convey spatial and temporal environmental information from the cell membrane to the nucleus. The cell membrane, as the interface between intra- and extra-cellular environments, is the site at which much external information is received. Prior studies have demonstrated that transmembrane ion gradients permit information acquisition when an environmental signal interacts with specialized protein gates in membrane ion channels and producing specific ions to flow into or out of the cell along concentration gradients. The resulting localized change in cytoplasmic ion concentrations and charge density can alter location and enzymatic function of peripheral membrane proteins. This allows the cell to process the information and rapidly deploy a local response. Here we investigate transmission of information received and processed in and around the cell membrane by elements of the cytoskeleton to the nucleus to alter gene expression. We demonstrate signal transmission by ion flow along the cytoskeleton is highly optimized. In particular, microtubules, with diameters of about 30 nm, carry coarse-grained Shannon information to the centrosome adjacent to the nucleus with minimum loss of input source information. And, microfilaments, with diameters of about 4 nm, transmit maximum Fisher (fine-grained) information to protein complexes in the nuclear membrane. These previously unrecognized information dynamics allow continuous integration of spatial and temporal environmental signals with inherited information in the genome.
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Shklyar, T. F.; Safronov, A.; Klyuzhin, I.; Pollack, G.; Blyakhman, F. A Correlation Between Mechanical and Electrical Properties of the Synthetic Hydrogel Chosen as an Experimental Model of Cytoskeleton. Biophysics 2008, 53, 544– 549, DOI: 10.1134/S0006350908060146
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Shklyar, T.; Safronov, A.; Toropova, O.; Pollack, G.; Blyakhman, F. Mechanoelectric Potentials in Synthetic Hydrogels: Possible Relation to Cytoskeleton. Biophysics 2010, 55, 931– 936, DOI: 10.1134/S0006350910060084
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Blyakhman, F.; Safronov, A.; Shklyar, T. Biomimetic Sensors of the Mechanoelectrical Transduction Based on the Polyelectrolyte Gels. Key Eng. Mater. 2015, 644, 4– 7, DOI: 10.4028/www.scientific.net/KEM.644.4
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Blyakhman, F.; Safronov, A.; Zubarev, A.; Shklyar, T.; Dinislamova, O.; Lopez-Lopez, M. Mechanoelectrical Transduction in the Hydrogel-Based Biomimetic Sensors. Sens. Actuators, A 2016, 248, 54– 61, DOI: 10.1016/j.sna.2016.06.020
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Mechanoelectrical transduction in the hydrogel-based biomimetic sensors
Blyakhman, F. A.; Safronov, A. P.; Zubarev, A. Yu.; Shklyar, T. F.; Dinislamova, O. A.; Lopez-Lopez, M. T.
Sensors and Actuators, A: Physical (2016), 248 (), 54-61CODEN: SAAPEB; ISSN:0924-4247. (Elsevier B.V.)
The study addresses the phenomenon of mechanoelec. transduction in polyelectrolyte hydrogels and, in particular, the search of the driving force for the change of the elec. potential of a gel under the applied mech. stretch. Polyelectrolyte gels of calcium and magnesium salts of polymethacrylic acid were synthesized by the radical polymn. in water soln. Their elec. potential measured by microcapillary electrodes was neg. and fall within 100-140 mV range depending on the nature of a counterion and the networking d. of a gel. The rectangular samples (∼10 mm in length and 2× 2 mm in cross-section) of gel-based sensors underwent the dynamic axial deformation, and the simultaneous monitoring of their geometrical dimensions and the elec. potential was performed. Sensor elongation resulted in the overall increase of gel vol., and it was always accompanied by the gel potential change toward the depolarization (diminishing of the neg. values). Theor. model based on the assumption of the total elec. charge conservation in the course of the dynamic deformation of a filament was proposed to describe the dependence of the elec. potential of a gel on its vol. Good agreement between the predictions of the model and the exptl. trend was shown. The proposed mechanism of mechanoelec. transduction based on the stretch-dependant vol. changes in polyelectrolyte hydrogels might be useful to understand the nature of mech. sensing in much more complex biol. gels like the cell cytoskeleton.
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Fels, J.; Orlov, S.; Grygorczyk, R. The Hydrogel Nature of Mammalian Cytoplasm Contributes to Osmosensing and Extracellular pH Sensing. Biophys. J. 2009, 96, 4276– 4285, DOI: 10.1016/j.bpj.2009.02.038
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The hydrogel nature of mammalian cytoplasm contributes to osmosensing and extracellular pH sensing
Fels, Johannes; Orlov, Sergei N.; Grygorczyk, Ryszard
Biophysical Journal (2009), 96 (10), 4276-4285CODEN: BIOJAU; ISSN:0006-3495. (Cell Press)
The cytoplasm is thought to have many hydrogel-like characteristics, including the ability to absorb large amts. of water and change vol. in response to alterations in external environment, as well as having limited leakage of ions and proteins. Some gel-like behaviors have not been rigorously confirmed in mammalian cells, and others should be examd. under conditions where gel vol. can be accurately monitored. Thus, possible contributions of cytoplasm hydrogel properties to cellular processes such as vol. sensing and regulation remain unclear. Here, the authors used 3-dimensional imaging to measure vol. of single substrate-attached cells after permeabilization of their plasma membrane. The permeabilized cells swelled or shrinked reversibly in response to variations of external osmolality. Vol. changes were 3.7-fold greater than obsd. with intact cells, consistent with cytoplasm's high water-absorbing capacity. Vol. was maximal at neutral pH and shrunk at acidic or alk. pH, consistent with pH-dependent changes of protein charge d. and repulsive forces within cellular matrix. The vol. shrunk with increased Mg2+ concn., as expected for increased charge screening and ionic crosslinking effects. The findings demonstrated that mammalian cytoplasm resembles a hydrogel and functions as a highly sensitive osmosensor and extracellular pH sensor. Its high water-absorbing capacity may allow rapid modulation of local fluidity, macromol. crowding, and activity of the intracellular environment.
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Michel, G.; Tonon, T.; Scornet, D.; Cock, J.; Kloareg, B. The Cell Wall Polysaccharide Metabolism of the Brown Alga Ectocarpus Siliculosus. Insights into the Evolution of Extracellular Matrix Polysaccharides in Eukaryotes. New Phytol. 2010, 188, 82– 97, DOI: 10.1111/j.1469-8137.2010.03374.x
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The cell wall polysaccharide metabolism of the brown alga Ectocarpus siliculosus. Insights into the evolution of extracellular matrix polysaccharides in Eukaryotes
Michel, Gurvan; Tonon, Thierry; Scornet, Delphine; Cock, J. Mark; Kloareg, Bernard
New Phytologist (2010), 188 (1), 82-97CODEN: NEPHAV; ISSN:0028-646X. (Wiley-Blackwell)
Brown algal cell walls share some components with plants (cellulose) and animals (sulfated fucans), but they also contain some unique polysaccharides (alginates). Anal. of the Ectocarpus genome provides a unique opportunity to decipher the mol. bases of these crucial metabs. An extensive bioinformatic census of the enzymes potentially involved in the biogenesis and remodeling of cellulose, alginate and fucans was performed, and completed by phylogenetic analyses of key enzymes. The routes for the biosynthesis of cellulose, alginates and sulfated fucans were reconstructed. Surprisingly, known families of cellulases, expansins and alginate lyases are absent in Ectocarpus, suggesting the existence of novel mechanisms and/or proteins for cell wall expansion in brown algae. Altogether, our data depict a complex evolutionary history for the main components of brown algal cell walls. Cellulose synthesis was inherited from the ancestral red algal endosymbiont, whereas the terminal steps for alginate biosynthesis were acquired by horizontal gene transfer from an Actinobacterium. This horizontal gene transfer event also contributed genes for hemicellulose biosynthesis. By contrast, the biosynthetic route for sulfated fucans is an ancestral pathway, conserved with animals. These findings shine a new light on the origin and evolution of cell wall polysaccharides in other Eukaryotes.
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Simpliciano, C.; Clark, L.; Asi, B.; Chu, N.; Mercado, M.; Diaz, S.; Goedert, M.; Mobed-Miremadi, M. Cross-Linked Alginate Film Pore Size Determination Using Atomic Force Microscopy and Validation Using Diffusivity Determinations. J. Surf. Eng. Mater. Adv. Technol. 2013, 3, 1– 12, DOI: 10.4236/jsemat.2013.34A1001
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Cross-linked alginate film pore size determination using atomic force microscopy and validation using diffusivity determinations
Simpliciano, Cheryl; Clark, Larissa; Asi, Behrokh; Chu, Nathan; Mercado, Maria; Diaz, Steven; Goedert, Michel; Mobed-Miremadi, Maryam
Journal of Surface Engineered Materials and Advanced Technology (2013), 3 (4A), 1-12CODEN: JSEMBC; ISSN:2161-489X. (Scientific Research Publishing, Inc.)
The deficit of organ donors has fueled the need for advances in tissue engineering and regenerative medicine. Microencapsulation in alginate immuno-isolation membranes has been used to treat many disabling metabolic disorders, namely, phenylketonuria, kidney failure and diabetes mellitus. Systematic nutrient flux detns. are hindered by the lack of exptl. data on alginate-based membrane topog. and the pore size thus preventing the full therapeutic potential of the bio-membranes to be reached. In this study, samples of cross-linked alginate membranes were subjected to the following anal. characterization: 1) pore size characterization using at. force microscopy operated in contact mode to detect and measure pore size; 2) differential scanning calorimetry to confirm biopolymer crosslinking; and 3) diffusivity measurements using spectrophotometry and fluorescence microscopy to confirm the presence of through pores and to calc. reflection coeffs. The pore sizes for the pre-clin. std. formulation of 1.5% (w/v) medium viscosity alginate cross-linked with 1.5% CaCl2 and 0.5% (w/v) alginate and chitosan cross-linked with 20% CaCl2 are 5.2 nm ± 0.9 nm and 7.0 nm ± 3.1 nm, resp. An increase in the glass transition temps. as a function of cross-linker concn. was obsd. Diffusivity values obtained from the inward diffusivity of creatinine into macrocapsules (d = 1000 μm ± 75 μm) and the outward diffusivity of FITC dextrans from macrocapsules (d = 1000 μm ± 75 μm) and microcapsules (d = 40 μm ± 5 μm) were shown to correlate strongly (R2 = 0.9835) with the ratio of solute to pore sizes, confirming the presence of through pores. Reflection coeffs. approaching and exceeding unity correlate with the lack of permeability of the membranes to MW markers that are 70 kDa and greater.
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Golmohamadi, M.; Wilkinson, K. Diffusion of Ions in a Calcium Alginate Hydrogel-Structure is the Primary Factor Controlling Diffusion. Carbohydr. Polym. 2013, 94, 82– 87, DOI: 10.1016/j.carbpol.2013.01.046
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Diffusion of ions in a calcium alginate hydrogel-structure is the primary factor controlling diffusion
Golmohamadi, Mahmood; Wilkinson, Kevin J.
Carbohydrate Polymers (2013), 94 (1), 82-87CODEN: CAPOD8; ISSN:0144-8617. (Elsevier Ltd.)
The diffusion of solutes was evaluated in an alginate hydrogel as a function of its structure. The role of solute and gel charge on the diffusion measurements were of particular interest. Diffusion coeffs. were measured using fluorescence correlation spectroscopy as a function of solute charge and size, bulk soln. ionic strength and pH, and gel d. Diffusion coeffs. of fluorescent dextrans with hydrodynamic radii up to 6 nm were reduced by 30% in a 1.8% (wt./wt.) hydrogel whereas they were reduced by only 2% in a 0.2% (wt./wt.) hydrogel. The role of ionic strength was examd. for various concns. (0.1-100 mM) and compns. of ions (Na+, Ca2+ or mixts. thereof). The diffusion coeff. of a small charged probe (rhodamine 6G, R6G+) did not change significantly with increasing ionic strength when sodium was used as the counter ion. The diffusion coeff. was only moderately influenced by the charge of solutes (from +1 to -2). Similarly, pH variations from 3 to 9 had little impact on the diffusion coeffs. of R6G+ in the gel. The addn. of Ca2+ had a significant impact on gel compactness, which led to a significant redn. in solute diffusion. For the calcium alginate hydrogels, structural modifications resulting from Ca binding were much more important than electrostatic effects due to modifications of the gel Donnan potential.
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Lundberg, P.; Kuchel, P. Diffusion of Solutes in Agarose and Alginate Gels:1H and 23Na PFGSE and 23Na TQF NMR Studies. Magn. Reson. Med. 1997, 37, 44– 52, DOI: 10.1002/mrm.1910370108
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Diffusion of solutes in agarose and alginate gels: 1H and 23Na PFGSE and 23Na TQF NMR studies
Lundberg, Peter; Kuchel, Philip W.
Magnetic Resonance in Medicine (1997), 37 (1), 44-52CODEN: MRMEEN; ISSN:0740-3194. (Williams & Wilkins)
Cells immobilized in gels experience potential metabolic restrictions in the form of reduced diffusion rates of metabolites and ions and their possible selective adsorption on the gel matrix. Diffusion and relaxation characteristics of common solutes in agarose and barium alginate gels were investigated at 37° by using 1H PFGSE and 23Na TQF NMR spectroscopy. Glucose, glycine, alanine, lactate, sodium ions, and HDO were studied. There were no selective interactions between any of the metabolites and the gel materials but the diffusion coeffs. were uniformly reduced. The effects of metabolite diffusion and utilization, in gel beads and threads contg. cells, were simulated by using a reaction diffusion model incorporating the measured diffusion coeffs. Metab. is expected to be very significantly limited by diffusion of solutes to and from the cells that are centrally located within gel threads or spheres of radius ∼2.0 mm, which is a commonly used size.
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Guo, H.; Kurokawa, T.; Takahata, M.; Hong, W.; Katsuyama, Y.; Luo, F.; Ahmed, J.; Nakajima, T.; Nonoyama, T.; Gong, J. Quantitative Observation of Electric Potential Distribution of Brittle Polyelectrolyte Hydrogels Using Microelectrode Technique. Macromolecules 2016, 49, 3100– 3108, DOI: 10.1021/acs.macromol.6b00037
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Quantitative Observation of Electric Potential Distribution of Brittle Polyelectrolyte Hydrogels Using Microelectrode Technique
Guo, Honglei; Kurokawa, Takayuki; Takahata, Masakazu; Hong, Wei; Katsuyama, Yoshinori; Luo, Feng; Ahmed, Jamil; Nakajima, Tasuku; Nonoyama, Takayuki; Gong, Jian Ping
Macromolecules (Washington, DC, United States) (2016), 49 (8), 3100-3108CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)
We report, for the first time, the quant. measurement of the local elec. potential of brittle polyelectrolyte hydrogels using the microelectrode technique (MET). Given the solid-like nature of the hydrogels, the difficulty of applying MET is how to make a good contact of the microelectrode to the hydrogel. Poor local contact substantial underestimates the potential. We obsd. that, the potential measured decays exponentially with the increase of capillary diam. of the microelectrode. This behavior is related to the capillary wall thickness that dets. the contact distance of the electrode probe to the hydrogel. The characteristic decay length in resp. to the wall thickness is very close to the local Debye length around the capillary. The latter is much larger than that of the bath soln. due to the reverse osmosis effect. By using microelectrodes with a tip wall thickness less than the local Debye length, the Donnan potential of polyelectrolyte gel could be accurately measured. Using a micromanipulator, the inserting process of the microelectrode is precisely controlled, and the depth profile of elec. potential in the hydrogels can be measured with a spatial resoln. down to ∼5 nm. From the spatial distribution of potential, the microstructure of hydrogels both in bulk and near the surface, the thickness of ultrathin hydrogels, and the heterogeneous layered structure of composite gels, can be detd. accurately. The MET established in this work provides a powerful tool for direct characterization of the spatial distribution of elec. potential of hydrogels.
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Podlas, T.; Ander, P. Interactions of Sodium and Potassium Ions with Sodium and Potassium Alginate in Aqueous Solution with and without Added Salt. Macromolecules 1970, 3, 154– 157, DOI: 10.1021/ma60014a007
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Interactions of sodium and potassium ions with sodium and potassium alginate in aqueous solution with and without added salt
Podlas, Thomas J.; Ander, Paul
Macromolecules (1970), 3 (2), 154-7CODEN: MAMOBX; ISSN:0024-9297.
The interaction of Na and K ions with Na and K alginate were investigated by measuring the counterion activity coeffs. in aq. solns. of the polyelectrolytes with and without added simple electrolyte at 25°. In salt-free solns., the counterion activity coeffs. increase with increasing diln. and the activity coeffs. of K ions were higher than those of Na ions throughout the concn. range. Counterion activity coeffs. decreased with increasing polyelectrolyte concn. at const. simple salt concn. and decreased with decreasing salt concn. at a const. polyelectrolyte concn. Deviations from the additivity rule were most pronounced when the salt and polymer concns. were approx. equal. Good agreement was obtained when Manning's line-charge polyelectrolyte model was compared with the exptl. results for the lowest simple salt concn.
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Aziz, E.; Ottosson, N.; Eisebitt, S.; Eberhardt, W.; Jagoda-Cwiklik, B.; Vácha, R.; Jungwirth, P.; Winter, B. Cation-Specific Interactions with Carboxylate in Amino Acid and Acetate Aqueous Solutions: X-Ray Absorption and ab initio calculations. J. Phys. Chem. B 2008, 112, 12567– 12570, DOI: 10.1021/jp805177v
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Cation-Specific Interactions with Carboxylate in Amino Acid and Acetate Aqueous Solutions: X-ray Absorption and ab initio Calculations
Aziz, Emad F.; Ottosson, Niklas; Eisebitt, Stefan; Eberhardt, Wolfgang; Jagoda-Cwiklik, Barbara; Vacha, Robert; Jungwirth, Pavel; Winter, Bernd
Journal of Physical Chemistry B (2008), 112 (40), 12567-12570CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
Relative interaction strengths between cations (X = Li+, Na+, K+, NH4+) and anionic carboxylate groups of acetate and glycine in aq. soln. are detd. These model systems mimic ion pairing of biol. relevant cations with neg. charged groups at protein surfaces. With oxygen 1s X-ray absorption spectroscopy, we can distinguish between spectral contributions from H2O and carboxylate, which allows us to probe the electronic structure changes of the at. site of the carboxylate group being closest to the countercation. From the intensity variations of the COO-aq O 1s X-ray absorption peak, which quant. correlate with the change in the local partial d. of states from the carboxylic site, interactions are found to decrease in the sequence Na+ > Li+ > K+ > NH4+. This ordering, as well as the obsd. bidental nature of the -COO-aq and X+aq interaction, is supported by combined ab initio and mol. dynamics calcns.
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Okur, H. I.; Hladílková, J.; Rembert, K.; Cho, Y.; Heyda, J.; Dzubiella, J.; Cremer, P.; Jungwirth, P. Beyond The Hofmeister Series: Ion-Specific Effects on Proteins and Their Biological Functions. J. Phys. Chem. B 2017, 121, 1997– 2014, DOI: 10.1021/acs.jpcb.6b10797
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Beyond the Hofmeister Series: Ion-Specific Effects on Proteins and Their Biological Functions
Okur, Halil I.; Hladilkova, Jana; Rembert, Kelvin B.; Cho, Younhee; Heyda, Jan; Dzubiella, Joachim; Cremer, Paul S.; Jungwirth, Pavel
Journal of Physical Chemistry B (2017), 121 (9), 1997-2014CODEN: JPCBFK; ISSN:1520-5207. (American Chemical Society)
A review. Ions differ in their ability to salt out proteins from soln. as expressed in the lyotropic or Hofmeister series of cations and anions. Since its first formulation in 1888, this series has been invoked in a plethora of effects, going beyond the original salting-out/salting-in idea to include enzyme activities and the crystn. of proteins, as well as to processes not involving proteins like ion exchange, the surface tension of electrolytes, or bubble coalescence. Although it has been clear that the Hofmeister series is intimately connected to ion hydration in homogeneous and heterogeneous environments and to ion pairing, its mol. origin has not been fully understood. This situation could have been summarized as follows: Many chemists used the Hofmeister series as a mantra to put a label on ion specific behavior in various environments, rather than to reach a mol. level understanding and, consequently, an ability to predict a particular effect of a given salt ion on proteins in solns. In this Feature Article the cationic and anionic Hofmeister series can now be rationalized primarily in terms of specific interactions of salt ions with the backbone and charged side chain groups at the protein surface in soln. At the same time, the authors demonstrate the limitations of sepg. Hofmeister effects into independent cationic and anionic contributions due to the electroneutrality condition, as well as specific ion pairing, leading to interactions of ions of opposite polarity. Finally, the authors outline the route beyond Hofmeister chem. in the direction of understanding specific roles of ions in various biol. functionalities, where generic Hofmeister-type interactions can be complemented or even overruled by particular steric arrangements in various ion binding sites.
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Lee, S. J.; Lee, J.; Kim, K. Pressure-Driven Spontaneous Ion Concentration Polarization Using an Ion-Selective Membrane. Anal. Biochem. 2018, 557, 13– 17, DOI: 10.1016/j.ab.2018.07.005
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Pressure-driven spontaneous ion concentration polarization using an ion-selective membrane
Lee, Sang Joon; Lee, Jaehyeon; Kim, Kiwoong
Analytical Biochemistry (2018), 557 (), 13-17CODEN: ANBCA2; ISSN:0003-2697. (Elsevier B.V.)
In this study, the spontaneous ion concn. polarization phenomenon induced by pressure via a cation-selective membrane was theor. and exptl. investigated. Unlike conventional electrokinetic ion concn. polarization, which uses elec. current as a driving flux of cations through the membrane, advection caused by pressure is used as a transmembrane driving flux of cations to spontaneously and stably form an ion depletion zone in the present ion concn. polarization technique. The ion depletion zone produced in a simple exptl. setup was used to filter electrolyte and preconc. ions and microparticles. Different from the general assumption of the negligible thickness of the elec. double layer in microchannels, the low concn. in the ion depletion zone considerably increased the length of the elec. double layer. This enhanced the formation of the ion depletion zone. The present results can improve the understanding on ion transport in the ion concn. polarization system and can be utilized to develop a portable water desalination device for rural/remote areas and for preconcg. biomols.
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Kim, S. J.; Ko, S. H.; Kang, K. H.; Han, J. Direct Seawater Desalination by Ion Concentration Polarization. Nat. Nanotechnol. 2010, 5, 297– 301, DOI: 10.1038/nnano.2010.34
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Direct seawater desalination by ion concentration polarization
Kim, Sung Jae; Ko, Sung Hee; Kang, Kwan Hyoung; Han, Jongyoon
Nature Nanotechnology (2010), 5 (4), 297-301CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
We report a process for converting seawater (salinity ∼500mM or ∼30,000 mg/L) to freshwater (salinity <10mM or <600 mg/L) in which a continuous stream of seawater is divided into desalted and concd. streams by ion concn. polarization, a phenomenon that occurs when an ion current is passed through ion-selective membranes. During operation, both salts and larger particles (cells, viruses and microorganisms) are pushed away from the membrane (a nanochannel or nanoporous membrane), which significantly reduces the possibility of membrane fouling and salt accumulation, thus avoiding 2 problems that plague other membrane filtration methods. To implement this approach, a simple microfluidic device was fabricated and shown to be capable of continuous desalination of seawater (∼99% salt rejection at 50% recovery rate) at a power consumption of <3.5 W-h/L, which is comparable to current state-of-the-art systems. Rather than competing with larger desalination plants, the method could be used to make small- or medium-scale systems, with the possibility of battery-powered operation.
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Park, S.; Jung, Y.; Son, S.; Cho, I.; Cho, Y.; Lee, H.; Kim, H.; Kim, S. Capillarity Ion Concentration Polarization as Spontaneous Desalting Mechanism. Nat. Commun. 2016, 7, 11223 DOI: 10.1038/ncomms11223
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Capillarity ion concentration polarization as spontaneous desalting mechanism
Park, Sungmin; Jung, Yeonsu; Son, Seok Young; Cho, Inhee; Cho, Youngrok; Lee, Hyomin; Kim, Ho-Young; Kim, Sung Jae
Nature Communications (2016), 7 (), 11223CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
To overcome a world-wide water shortage problem, numerous desalination methods have been developed with state-of-the-art power efficiency. Here we propose a spontaneous desalting mechanism referred to as the capillarity ion concn. polarization. An ion-depletion zone is spontaneously formed near a nanoporous material by the permselective ion transportation driven by the capillarity of the material, in contrast to electrokinetic ion concn. polarization which achieves the same ion-depletion zone by an external d.c. bias. This capillarity ion concn. polarization device is shown to be capable of desalting an ambient electrolyte more than 90% without any external elec. power sources. Theor. anal. for both static and transient conditions are conducted to characterize this phenomenon. These results indicate that the capillarity ion concn. polarization system can offer unique and economical approaches for a power-free water purifn. system.
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Mogi, K. A Visualization Technique of a Unique pH Distribution Around an Ion Depletion Zone in a Microchannel by Using a Dual-Excitation Ratiometric Method. Micromachines 2018, 9, 167 DOI: 10.3390/mi9040167
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Mogi, K.; Hayashida, K.; Yamamoto, T. Damage-Less Handling of Exosomes Using an Ion-Depletion Zone in a Microchannel. Anal. Sci. 2018, 34, 875– 880, DOI: 10.2116/analsci.17P462
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Damage-less handling of exosomes using an ion-depletion zone in a microchannel
Mogi, Katsuo; Hayashida, Kei; Yamamoto, Takatoki
Analytical Sciences (2018), 34 (8), 875-880CODEN: ANSCEN; ISSN:0910-6340. (Japan Society for Analytical Chemistry)
Exosomes are of increasing research interest because they are integral to cell-cell communication and are implicated in various disease states. Here we investigated the utility of using an ion-depletion zone in a microfluidic device to conc. exosomes from the culture media of four types of cell lines. Furthermore, we eveluated the extent of damage to the exosomes following concn. by an ion-depletion zone microchannel device compared with exosomes concd. by a conventional ultra-centrifugation technique. Our results conclusivedly demonstrate that significantly less damage is incurred by exosomes following passage through and concn. by the ion-depleted zone microchannel device compared to concn. by ultra-centrifugation. Our findings will help extend the utility of exosomes to various applications.
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Huicochea, A.; Siqueiros, J.; Romero, R. Portable Water Purification System Integrated to a Heat Transformer. Desalination 2004, 165, 385– 391, DOI: 10.1016/j.desal.2004.06.044
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Portable water purification system integrated to a heat transformer
Huicochea, A.; Siqueiros, J.; Romero, R. J.
Desalination (2004), 165 (1-3), 385-391CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)
H2O is a natural resource essential for life and for most economic activities developed on earth. Population growth and lack of H2O in some regions of the world led humans to design and implement new technologies to use H2O in an efficient way. H2O quality requirements in the industrial sphere are higher everyday. Development of desalination technol. through a water purifn. system integrated to heat pumps has taken more than two decades. Absorption heat pumps use low quality energy in a waste heat form and a small quantity of high quality energy. This characteristic has made possible using these systems in quite a few places. The following work presents the results of the exptl. tests applied to a portable water purifn. system integrated to a heat transformer (TTAPPA), where the low quality waste heat is simulated and LiBr-H2O was used as a working soln.
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Ma, B.; Chi, J.; Liu, H. Fabric-Based Ion Concentration Polarization for Pump-Free Water Desalination. ACS Sustainable Chem. Eng. 2017, 6, 99– 103, DOI: 10.1021/acssuschemeng.7b03679
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Lee, D.; Lee, J.; Lee, H.; Kim, S. Spontaneous Selective Preconcentration Leveraged by Ion Exchange and Imbibition Through Nanoporous Medium. Sci. Rep. 2019, 9, 2336 DOI: 10.1038/s41598-018-38162-6
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Spontaneous Selective Preconcentration Leveraged by Ion Exchange and Imbibition through Nanoporous Medium
Lee Dokeun; Lee Jung A; Kim Sung Jae; Lee Hyomin; Kim Sung Jae; Kim Sung Jae
Scientific reports (2019), 9 (1), 2336 ISSN:.
Manipulating mechanism of particle's motion has been extensively studied for the sample preparation in microfluidic applications including diagnostics, food industries, biological analyses and environmental monitoring. However, most of conventional methods need additional external forces such as electric field or pressure and complicated channel designs, which demand highly complex fabrication processes and operation strategies. In addition, these methods have inherent limitations of dilution or mixing during separation or preconcentration step, respectively, so that a number of studies have reported an efficient selective preconcentration process, i.e. conducting the separation and preconcentration simultaneously. In this work, a power-free spontaneous selective preconcentration method was suggested based on leveraging convective flow over diffusiophoresis near the water-absorbing nanoporous ion exchange medium, which was verified both by simulation and experiment. Especially, the velocity of the convective flow by an imbibition deviated from the original tendency of t(-1/2) due to non-uniformly patterned nanoporous medium that has multiple cross-sectional areas. As a result, the direction of particle's motion was controlled at one's discretion, which led to the spontaneous selective preconcentration of particles having different diffusiophoretic constant. Also, design rule for maximizing the efficiency was recommended. Thus, this selective preconcentration method would play as a key mechanism for power-free lab on a chip applications.
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Lee, J.; Lee, D.; Park, S.; Lee, H.; Kim, S. Non-Negligible Water-Permeance Through Nanoporous Ion Exchange Medium. Sci. Rep. 2018, 8, 12842 DOI: 10.1038/s41598-018-29695-x
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Non-negligible Water-permeance through Nanoporous Ion Exchange Medium
Lee Jung A; Lee Dokeun; Lee Hyomin; Kim Sung Jae; Park Sungmin; Lee Hyomin; Kim Sung Jae; Kim Sung Jae
Scientific reports (2018), 8 (1), 12842 ISSN:.
While the water impermeable constraint has been conventionally adopted for analyzing the transport phenomena at the interface of electrolyte/nanoporous medium, non-negligible water-permeance through the medium results in significant effect on ion and particle transportation. In this work, a rigorous theoretical and experimental analysis of the water-permeance effect were conducted based on a fully-coupled analytical/numerical method and micro/nanofluidic experiments. The regime diagram with three distinctive types of concentration boundary layers (ion depletion, ion accumulation, and intermediate) near the ion exchange nanoporous medium was proposed depending on the medium's permselectivity and the water-permeance represented by an absorbing parameter. Moreover, the critical absorbing parameters which divide the regimes were analytically obtained so that the bidirectional motion of particles were demonstrated only by altering the water-permeance without external stimuli. Conclusively, the presenting analysis of non-negligible water-permeance would be a substantial fundamental of transport phenomena at the interface of the ion exchange medium and electrolyte, especially useful for the tunable particle/ion manipulations in intermediate Peclet number environment.
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Zheng, J.-m.; Chin, W.; Khijniak, E.; Khijniak, E.; Pollack, G. Surfaces and Interfacial Water: Evidence That Hydrophilic Surfaces Have Long-Range Impact. Adv. Colloid Interface Sci. 2006, 127, 19– 27, DOI: 10.1016/j.cis.2006.07.002
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Surfaces and interfacial water: Evidence that hydrophilic surfaces have long-range impact
Zheng, Jian-ming; Chin, Wei-Chun; Khijniak, Eugene; Khijniak, Eugene; Pollack, Gerald H.
Advances in Colloid and Interface Science (2006), 127 (1), 19-27CODEN: ACISB9; ISSN:0001-8686. (Elsevier B.V.)
It is generally thought that the impact of surfaces on the contiguous aq. phase extends to a distance of no more than a few H2O-mol. layers. Older studies, however, suggest a more extensive impact. The authors report here that colloidal and mol. solutes suspended in aq. soln. are profoundly and extensively excluded from the vicinity of various hydrophilic surfaces. The width of the solute-free zone is typically several hundred microns. Such large exclusion zones were obsd. in the vicinity of many types of surface including artificial and natural hydrogels, biol. tissues, hydrophilic polymers, monolayers, and ion-exchange beads, as well as with a variety of solutes. Using microscopic observations, as well as measurements of elec. potential and UV-visible absorption-spectra, IR imaging, and NMR imaging, the solute-free zone is a phys. distinct and less mobile phase of H2O that can coexist indefinitely with the contiguous solute-contg. phase. The extensiveness of this modified zone is impressive, and carries broad implication for surface-mol. interactions in many realms, including cellular recognition, biomaterial-surface antifouling, biosepn. technologies, and other areas of biol., physics and chem.
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Zheng, J.-m.; Pollack, G. H. Long-Range Forces Extending from Polymer-Gel Surfaces. Phys. Rev. E 2003, 68 DOI: 10.1103/physreve.68.031408 .
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Chai, B.; Pollack, G. H. Solute-Free Interfacial Zones in Polar Liquids. J. Phys. Chem. B 2010, 114, 5371– 5375, DOI: 10.1021/jp100200y
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Solute-Free Interfacial Zones in Polar Liquids
Chai, Binghua; Pollack, Gerald H.
Journal of Physical Chemistry B (2010), 114 (16), 5371-5375CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
Large, solute-free interfacial zones have recently been described in aq. solns. Found next to hydrophilic surfaces, these "exclusion zones" are commonly several hundred micrometers wide and represent regions of water that appear to be more ordered than bulk water. We report here that other polar solvents including methanol, ethanol, isopropanol, acetic acid, D2O, and DMSO show similar near-surface exclusion zones, albeit of smaller magnitude. Microelectrode measurements show that these zones are neg. charged and grow in response to incident IR radiation, similar to exclusion zones in aq. solns. Hence, near-surface exclusion zones appear to be features characteristic not only of water but of other polar liqs. as well.
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Florea, D.; Musa, S.; Huyghe, J.; Wyss, H. Long-Range Repulsion of Colloids Driven by Ion Exchange and Diffusiophoresis. Proc. Natl. Acad. Sci. U.S.A. 2014, 111, 6554– 6559, DOI: 10.1073/pnas.1322857111
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Long-range repulsion of colloids driven by ion exchange and diffusiophoresis
Florea, Daniel; Musa, Sami; Huyghe, Jacques M. R.; Wyss, Hans M.
Proceedings of the National Academy of Sciences of the United States of America (2014), 111 (18), 6554-6559CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
Interactions between surfaces and particles in aq. suspension are usually limited to distances smaller than 1 μm. However, in a range of studies from different disciplines, repulsion of particles was obsd. over distances of up to hundreds of micrometers, in the absence of any addnl. external fields. Although a range of hypotheses were suggested to account for such behavior, the phys. mechanisms responsible for the phenomenon still remain unclear. To identify and isolate these mechanisms, we perform detailed expts. on a well-defined exptl. system, using a setup that minimizes the effects of gravity and convection. The obsd. long-range repulsion is driven by a combination of ion exchange, ion diffusion, and diffusiophoresis. A simple model was developed that accounts for this data; this description is expected to be directly applicable to a wide range of systems exhibiting similar long-range forces.
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Huszár, I.; Mártonfalvi, Z.; Laki, A.; Iván, K.; Kellermayer, M. Exclusion-Zone Dynamics Explored with Microfluidics and Optical Tweezers. Entropy 2014, 16, 4322– 4337, DOI: 10.3390/e16084322
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Exclusion-zone dynamics explored with microfluidics and optical tweezers
Huszar, Istvan N.; Martonfalvi, Zsolt; Laki, Andras Jozsef; Ivan, Kristof; Kellermayer, Miklos
Entropy (2014), 16 (8), 4322-4337CODEN: ENTRFG; ISSN:1099-4300. (MDPI AG)
The exclusion zone (EZ) is a boundary region devoid of macromols. and microscopic particles formed spontaneously in the vicinity of hydrophilic surfaces. The exact mechanisms behind this remarkable phenomenon are still not fully understood and are debated. We measured the short- and long-time-scale kinetics of EZ formation around a Nafion gel embedded in specially designed microfluidic devices. The time-dependent kinetics of EZ formation follow a power law with an exponent of 0.6 that is strikingly close to the value of 0.5 expected for a diffusion-driven process. By using optical tweezers we show that exclusion forces, which are estd. to fall in the sub-pN regime, persist within the fully-developed EZ, suggesting that EZ formation is not a quasi-static but rather an irreversible process. Accordingly, the EZ-forming capacity of the Nafion gel could be exhausted with time, on a scale of hours in the presence of 1 mM Na2HPO4. EZ formation may thus be a non-equil. thermodn. cross-effect coupled to a diffusion-driven transport process. Such phenomena might be particularly important in the living cell by providing mech. cues within the complex cytoplasmic environment.
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Tamagawa, H.; Ikeda, K. Another Interpretation of the Goldman–Hodgkin–Katz Equation Based on Ling’s Adsorption Theory. Eur. Biophys. J. 2018, 47, 869– 879, DOI: 10.1007/s00249-018-1332-0
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Another interpretation of the Goldman-Hodgkin-Katz equation based on Ling's adsorption theory
Tamagawa, Hirohisa; Ikeda, Kota
European Biophysics Journal (2018), 47 (8), 869-879CODEN: EBJOE8; ISSN:0175-7571. (Springer)
According to std. membrane theory, the generation of membrane potential is attributed to transmembrane ion transport. However, there have been a no. of reports of membrane behavior in conflict with the membrane theory of cellular potential. Putting aside the membrane theory, we scrutinized the generation mechanism of membrane potential from the view of the long-dismissed adsorption theory of Ling. Ling's adsorption theory attributes the membrane potential generation to mobile ion adsorption. Although Ling's adsorption theory conflicts with the broadly accepted membrane theory, we found that it well reproduces exptl. obsd. membrane potential behavior. Our theor. anal. finds that the potential formula based on the GHK eq., which is a fundamental concept of membrane theory, coincides with the potential formula based on Ling's adsorption theory. Reinterpreting the permeability coeff. in the GHK eq. as the assocn. const. between the mobile ion and adsorption site, the GHK eq. turns into the potential formula from Ling's adsorption theory. We conclude that the membrane potential is generated by ion adsorption as Ling's adsorption theory states and that the membrane theory of cellular potential should be amended even if not discarded.
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Tamagawa, H. Mathematical Expression of Membrane Potential Based on Ling’s Adsorption Theory is Approximately the Same as the Goldman–Hodgkin–Katz Equation. J. Biol. Phys. 2018, 45, 13– 30, DOI: 10.1007/s10867-018-9512-9
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Ritos, K.; Borg, M. K.; Mottram, N. J.; Reese, J. M. Electric Fields Can Control the Transport of Water in Carbon Nanotubes. Philos. Trans. R. Soc., A 2016, 374, 20150025 DOI: 10.1098/rsta.2015.0025
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Electric fields can control the transport of water in carbon nanotubes
Ritos, Konstantinos; Borg, Matthew K.; Mottram, Nigel J.; Reese, Jason M.
Philosophical Transactions of the Royal Society, A: Mathematical, Physical & Engineering Sciences (2016), 374 (2060), 20150025/1-20150025/19CODEN: PTRMAD; ISSN:1364-503X. (Royal Society)
The properties of water confined inside nanotubes are of considerable scientific and technol. interest. We use mol. dynamics to investigate the structure and av. orientation of water flowing within a carbon nanotube. We find that water exhibits biaxial paranematic liq. crystal ordering both within the nanotube and close to its ends. This preferred mol. ordering is enhanced when an axial elec. field is applied, affecting the water flow rate through the nanotube. A spatially patterned elec. field can minimize nanotube entrance effects and significantly increase the flow rate.
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Abdul Kadir, L.; Stacey, M.; Barrett-Jolley, R. Emerging Roles of the Membrane Potential: Action Beyond the Action Potential. Front. Physiol. 2018, 9 DOI: 10.3389/fphys.2018.01661 .
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Marbach, S.; Bocquet, L. Osmosis, From Molecular Insights to Large-Scale Applications. Chem. Soc. Rev. 2019, 48, 3102– 3144, DOI: 10.1039/C8CS00420J
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Osmosis, from molecular insights to large-scale applications
Marbach, Sophie; Bocquet, Lyderic
Chemical Society Reviews (2019), 48 (11), 3102-3144CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
Osmosis is a universal phenomenon occurring in a broad variety of processes and fields. It is the archetype of entropic forces, both trivial in its fundamental expression - the van 't Hoff perfect gas law - and highly subtle in its phys. roots. While osmosis is intimately linked with transport across membranes, it also manifests itself as an interfacial transport phenomenon: the so-called diffusio-osmosis and -phoresis, whose consequences are presently actively explored for example for the manipulation of colloidal suspensions or the development of active colloidal swimmers. Here we give a global and unifying view of the phenomenon of osmosis and its consequences with a multi-disciplinary perspective. Pushing the fundamental understanding of osmosis allows one to propose new perspectives for different fields and we highlight a no. of examples along these lines, for example introducing the concepts of osmotic diodes, active sepn. and far from equil. osmosis, raising in turn fundamental questions in the thermodn. of sepn. The applications of osmosis are also obviously considerable and span very diverse fields. Here we discuss a selection of phenomena and applications where osmosis shows great promises: osmotic phenomena in membrane science (with recent developments in sepn., desalination, reverse osmosis for water purifn. thanks in particular to the emergence of new nanomaterials); applications in biol. and health (in particular discussing the kidney filtration process); osmosis and energy harvesting (in particular, osmotic power and blue energy as well as capacitive mixing); applications in detergency and cleaning, as well as for oil recovery in porous media.
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Hibino, H.; Takai, M.; Noguchi, H.; Sawamura, S.; Takahashi, Y.; Sakai, H.; Shiku, H. An Approach to the Research on Ion and Water Properties in the Interphase Between the Plasma Membrane and Bulk Extracellular Solution. J. Physiol. Sci. 2017, 67, 439– 445, DOI: 10.1007/s12576-017-0530-3
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An approach to the research on ion and water properties in the interphase between the plasma membrane and bulk extracellular solution
Hibino, Hiroshi; Takai, Madoka; Noguchi, Hidenori; Sawamura, Seishiro; Takahashi, Yasufumi; Sakai, Hideki; Shiku, Hitoshi
Journal of Physiological Sciences (2017), 67 (4), 439-445CODEN: JPSOE2; ISSN:1880-6546. (Springer Japan)
A review. In vivo, cells are immersed in an extracellular soln. that contains a variety of bioactive substances including ions and water. Classical electrophysiol. analyses of epithelial cells in the stomach and small intestine have revealed that within a distance of several hundred micrometers above their apical plasma membrane, lies an extracellular layer that shows ion concn. gradients undetectable in the bulk phase. This "unstirred layer", which contains stagnant solutes, may also exist between the bulk extracellular soln. and membranes of other cells in an organism and may show different properties. On the other hand, an earlier study using a bacterial planar membrane indicated that H+ released from a transporter migrates in the horizontal direction along the membrane surface much faster than it diffuses vertically toward the extracellular space. This result implies that between the membrane surface and unstirred layer, there is a "nanointerface" that has unique ionic dynamics. Advanced technologies have revealed that the nanointerface on artificial membranes possibly harbors a highly ordered assembly of water mols. In general, hydrogen bonds are involved in formation of the ordered water structure and can mediate rapid transfer of H+ between neighboring mols. This description may match the phenomenon on the bacterial membrane. A recent study has suggested that water mols. in the nanointerface regulate the gating of K+ channels. Here, the region comprising the unstirred layer and nanointerface is defined as the interphase between the plasma membrane and bulk extracellular soln. (iMES). This article briefly describes the physicochem. properties of ions and water in the iMES and their physiol. significance. We also describe the methodologies that are currently used or will be applicable to the interphase research.
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Oh, Y.; Lee, H.; Son, S.; Kim, S.; Kim, P. Capillarity Ion Concentration Polarization for Spontaneous Biomolecular Preconcentration Mechanism. Biomicrofluidics 2016, 10, 014102 DOI: 10.1063/1.4939434
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Capillarity ion concentration polarization for spontaneous biomolecular preconcentration mechanism
Oh, Yoonjee; Lee, Hyomin; Son, Seok Young; Kim, Sung Jae; Kim, Pilnam
Biomicrofluidics (2016), 10 (1), 014102/1-014102/12CODEN: BIOMGB; ISSN:1932-1058. (American Institute of Physics)
Ionic hydrogel-based ion concn. polarization devices have been demonstrated as platforms to study nanoscale ion transport and to develop engineering applications, such as protein preconcn. and ionic diodes/transistors. Using a microfluidic system composed of a perm-selective hydrogel, we demonstrated a micro/nanofluidic device for the preconcn. of biol. samples using a new class of ion concn. polarization mechanism called "capillarity ion concn. polarization" (CICP). Instead of an external elec. voltage source, the capillary force of the perm-selective hydrogel spontaneously generated an ion depletion zone in a microfluidic channel by selectively absorbing counter-ions in a sample soln. We demonstrated a reasonable preconcn. factor (∼100-fold/min) using the CICP device. Although the efficiency was lower than that of conventional electrokinetic ICP operation due to the absence of a drift ion migration, this mechanism was free from the undesirable instability caused by a local amplified elec. field inside the ion depletion zone so that the mechanism should be suitable esp. for an application where the contents were elec. sensitive. Therefore, this simple system would provide a point-of-care diagnostic device for which the sample vol. is limited and a simplified sample handling is demanded. (c) 2016 American Institute of Physics.
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Figure 1
Figure 1. Conductivity measured over time in the interconnected-by-alginate-bridge solutions of (a) the left panel, from top to bottom: seawater, 200 mM KCl, 200 mM NaCl, 100 mM KCl, 100 mM NaCl, and DI water (right panel); (b) KCl (left, from top to bottom: 100 and 50 mM) and NaCl (right, from top to bottom: 100 and 50 mM). Note the 3-orders-of-magnitude difference in scale values for salt and DI water containers. The average values of the electric potential difference (mV) registered for (a) ionic solutions with respect to DI water and (b) KCl with respect to NaCl solution are depicted and yield (a) −180, −150, and −120 mV for 100 mM (KCl, NaCl, and seawater, respectively) and (b) −50 mV for interconnected solutions of KCl and NaCl of different ionic strengths, and −100 mV for equimolar solutions.
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Figure 2
Figure 2. Changes in solution levels (liquid column heights) in the two vessels interconnected by the alginate gel bridge (G) and containing DI water and 100 mM KCl solution. The light blue columns represent the liquid level at the beginning of the experiment, while the dark blue dotted columns show a steady liquid level at the end of the experiment.
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Figure 3
Figure 3. (a) FTIR-attenuated total reflection (ATR) spectra of a 0.05 M K₂SO₄ solution and (b) a difference spectrum of the K₂SO₄ solution before and after a piece of alginate gel was soaked and swelled in it. Note an increase in an absorption band of SO₄^2– (highlighted in yellow), expressed as the positive peak in difference spectra, after the solution contacts with a gel.
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Figure 4
Figure 4. Optical microscope images of the ion-depleted exclusion zone (EZ) water, expressed as a zone devoid of the crystals (black dots in the image) formed in the solution next to the alginate gel surface. The ion-depleted EZ could be observed under the microscope only for gels prone to swelling (after shorter dialysis time). The scale bar is 200 μm.
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Figure 5
Figure 5. Schematic illustration showing the mechanism of formation of the ion-depleted EZ layer next to the negatively charged gel. Salt ions migrate according to the concentration gradient from the KCl solution to water. Due to the negative charge of the gel matrix, only cations can pass, and the KCl solution is left with excess negative charges. These negative charges are now repelled from the negatively charged gel body, thus creating an ion-depleted exclusion zone of water. The initially ion-free water reservoir (right side) gains some extra positive charges. The resulting electric force (cations attracted back by their counterions) opposes the chemical force due to the concentration gradient of ions between the two solutions.
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Figure 6
Figure 6. Schematic illustration of the mechanism of development of electric potential difference between two ionic solutions separated by a permselective gel. K⁺ migrates within the gel at higher rates than does Na⁺. As their Cl^– counterions are left behind (cannot penetrate negatively charged pores), those anions are repelled by a charged gel creating an ion-depleted EZ at both sides of the gel membrane. Due to the faster migration of K⁺, the KCl solution is left with more excess negative charges than the NaCl solution. This relative difference is expressed in the measured negative electric potential of the KCl solution.
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Figure 7
Figure 7. Optical microscopy image showing the EZ water layer, expressed as a zone devoid of microspheres, developing next to a rehydrating algal blade. Note the solute enrichment (dark region) at the boundary between the EZ and bulk solutions, a feature consistent with the formation of an ion-depletion zone. (20−27) The scale bar is 4 mm.
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References
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This article references 47 other publications.
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  Sensors and Actuators, A: Physical (2016), 248 (), 54-61CODEN: SAAPEB; ISSN:0924-4247. (Elsevier B.V.)
  The study addresses the phenomenon of mechanoelec. transduction in polyelectrolyte hydrogels and, in particular, the search of the driving force for the change of the elec. potential of a gel under the applied mech. stretch. Polyelectrolyte gels of calcium and magnesium salts of polymethacrylic acid were synthesized by the radical polymn. in water soln. Their elec. potential measured by microcapillary electrodes was neg. and fall within 100-140 mV range depending on the nature of a counterion and the networking d. of a gel. The rectangular samples (∼10 mm in length and 2× 2 mm in cross-section) of gel-based sensors underwent the dynamic axial deformation, and the simultaneous monitoring of their geometrical dimensions and the elec. potential was performed. Sensor elongation resulted in the overall increase of gel vol., and it was always accompanied by the gel potential change toward the depolarization (diminishing of the neg. values). Theor. model based on the assumption of the total elec. charge conservation in the course of the dynamic deformation of a filament was proposed to describe the dependence of the elec. potential of a gel on its vol. Good agreement between the predictions of the model and the exptl. trend was shown. The proposed mechanism of mechanoelec. transduction based on the stretch-dependant vol. changes in polyelectrolyte hydrogels might be useful to understand the nature of mech. sensing in much more complex biol. gels like the cell cytoskeleton.
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11. 11
  Fels, J.; Orlov, S.; Grygorczyk, R. The Hydrogel Nature of Mammalian Cytoplasm Contributes to Osmosensing and Extracellular pH Sensing. Biophys. J. 2009, 96, 4276– 4285, DOI: 10.1016/j.bpj.2009.02.038
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  11
  The hydrogel nature of mammalian cytoplasm contributes to osmosensing and extracellular pH sensing
  Fels, Johannes; Orlov, Sergei N.; Grygorczyk, Ryszard
  Biophysical Journal (2009), 96 (10), 4276-4285CODEN: BIOJAU; ISSN:0006-3495. (Cell Press)
  The cytoplasm is thought to have many hydrogel-like characteristics, including the ability to absorb large amts. of water and change vol. in response to alterations in external environment, as well as having limited leakage of ions and proteins. Some gel-like behaviors have not been rigorously confirmed in mammalian cells, and others should be examd. under conditions where gel vol. can be accurately monitored. Thus, possible contributions of cytoplasm hydrogel properties to cellular processes such as vol. sensing and regulation remain unclear. Here, the authors used 3-dimensional imaging to measure vol. of single substrate-attached cells after permeabilization of their plasma membrane. The permeabilized cells swelled or shrinked reversibly in response to variations of external osmolality. Vol. changes were 3.7-fold greater than obsd. with intact cells, consistent with cytoplasm's high water-absorbing capacity. Vol. was maximal at neutral pH and shrunk at acidic or alk. pH, consistent with pH-dependent changes of protein charge d. and repulsive forces within cellular matrix. The vol. shrunk with increased Mg2+ concn., as expected for increased charge screening and ionic crosslinking effects. The findings demonstrated that mammalian cytoplasm resembles a hydrogel and functions as a highly sensitive osmosensor and extracellular pH sensor. Its high water-absorbing capacity may allow rapid modulation of local fluidity, macromol. crowding, and activity of the intracellular environment.
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12. 12
  Michel, G.; Tonon, T.; Scornet, D.; Cock, J.; Kloareg, B. The Cell Wall Polysaccharide Metabolism of the Brown Alga Ectocarpus Siliculosus. Insights into the Evolution of Extracellular Matrix Polysaccharides in Eukaryotes. New Phytol. 2010, 188, 82– 97, DOI: 10.1111/j.1469-8137.2010.03374.x
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  12
  The cell wall polysaccharide metabolism of the brown alga Ectocarpus siliculosus. Insights into the evolution of extracellular matrix polysaccharides in Eukaryotes
  Michel, Gurvan; Tonon, Thierry; Scornet, Delphine; Cock, J. Mark; Kloareg, Bernard
  New Phytologist (2010), 188 (1), 82-97CODEN: NEPHAV; ISSN:0028-646X. (Wiley-Blackwell)
  Brown algal cell walls share some components with plants (cellulose) and animals (sulfated fucans), but they also contain some unique polysaccharides (alginates). Anal. of the Ectocarpus genome provides a unique opportunity to decipher the mol. bases of these crucial metabs. An extensive bioinformatic census of the enzymes potentially involved in the biogenesis and remodeling of cellulose, alginate and fucans was performed, and completed by phylogenetic analyses of key enzymes. The routes for the biosynthesis of cellulose, alginates and sulfated fucans were reconstructed. Surprisingly, known families of cellulases, expansins and alginate lyases are absent in Ectocarpus, suggesting the existence of novel mechanisms and/or proteins for cell wall expansion in brown algae. Altogether, our data depict a complex evolutionary history for the main components of brown algal cell walls. Cellulose synthesis was inherited from the ancestral red algal endosymbiont, whereas the terminal steps for alginate biosynthesis were acquired by horizontal gene transfer from an Actinobacterium. This horizontal gene transfer event also contributed genes for hemicellulose biosynthesis. By contrast, the biosynthetic route for sulfated fucans is an ancestral pathway, conserved with animals. These findings shine a new light on the origin and evolution of cell wall polysaccharides in other Eukaryotes.
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13. 13
  Simpliciano, C.; Clark, L.; Asi, B.; Chu, N.; Mercado, M.; Diaz, S.; Goedert, M.; Mobed-Miremadi, M. Cross-Linked Alginate Film Pore Size Determination Using Atomic Force Microscopy and Validation Using Diffusivity Determinations. J. Surf. Eng. Mater. Adv. Technol. 2013, 3, 1– 12, DOI: 10.4236/jsemat.2013.34A1001
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  13
  Cross-linked alginate film pore size determination using atomic force microscopy and validation using diffusivity determinations
  Simpliciano, Cheryl; Clark, Larissa; Asi, Behrokh; Chu, Nathan; Mercado, Maria; Diaz, Steven; Goedert, Michel; Mobed-Miremadi, Maryam
  Journal of Surface Engineered Materials and Advanced Technology (2013), 3 (4A), 1-12CODEN: JSEMBC; ISSN:2161-489X. (Scientific Research Publishing, Inc.)
  The deficit of organ donors has fueled the need for advances in tissue engineering and regenerative medicine. Microencapsulation in alginate immuno-isolation membranes has been used to treat many disabling metabolic disorders, namely, phenylketonuria, kidney failure and diabetes mellitus. Systematic nutrient flux detns. are hindered by the lack of exptl. data on alginate-based membrane topog. and the pore size thus preventing the full therapeutic potential of the bio-membranes to be reached. In this study, samples of cross-linked alginate membranes were subjected to the following anal. characterization: 1) pore size characterization using at. force microscopy operated in contact mode to detect and measure pore size; 2) differential scanning calorimetry to confirm biopolymer crosslinking; and 3) diffusivity measurements using spectrophotometry and fluorescence microscopy to confirm the presence of through pores and to calc. reflection coeffs. The pore sizes for the pre-clin. std. formulation of 1.5% (w/v) medium viscosity alginate cross-linked with 1.5% CaCl2 and 0.5% (w/v) alginate and chitosan cross-linked with 20% CaCl2 are 5.2 nm ± 0.9 nm and 7.0 nm ± 3.1 nm, resp. An increase in the glass transition temps. as a function of cross-linker concn. was obsd. Diffusivity values obtained from the inward diffusivity of creatinine into macrocapsules (d = 1000 μm ± 75 μm) and the outward diffusivity of FITC dextrans from macrocapsules (d = 1000 μm ± 75 μm) and microcapsules (d = 40 μm ± 5 μm) were shown to correlate strongly (R2 = 0.9835) with the ratio of solute to pore sizes, confirming the presence of through pores. Reflection coeffs. approaching and exceeding unity correlate with the lack of permeability of the membranes to MW markers that are 70 kDa and greater.
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14. 14
  Golmohamadi, M.; Wilkinson, K. Diffusion of Ions in a Calcium Alginate Hydrogel-Structure is the Primary Factor Controlling Diffusion. Carbohydr. Polym. 2013, 94, 82– 87, DOI: 10.1016/j.carbpol.2013.01.046
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  14
  Diffusion of ions in a calcium alginate hydrogel-structure is the primary factor controlling diffusion
  Golmohamadi, Mahmood; Wilkinson, Kevin J.
  Carbohydrate Polymers (2013), 94 (1), 82-87CODEN: CAPOD8; ISSN:0144-8617. (Elsevier Ltd.)
  The diffusion of solutes was evaluated in an alginate hydrogel as a function of its structure. The role of solute and gel charge on the diffusion measurements were of particular interest. Diffusion coeffs. were measured using fluorescence correlation spectroscopy as a function of solute charge and size, bulk soln. ionic strength and pH, and gel d. Diffusion coeffs. of fluorescent dextrans with hydrodynamic radii up to 6 nm were reduced by 30% in a 1.8% (wt./wt.) hydrogel whereas they were reduced by only 2% in a 0.2% (wt./wt.) hydrogel. The role of ionic strength was examd. for various concns. (0.1-100 mM) and compns. of ions (Na+, Ca2+ or mixts. thereof). The diffusion coeff. of a small charged probe (rhodamine 6G, R6G+) did not change significantly with increasing ionic strength when sodium was used as the counter ion. The diffusion coeff. was only moderately influenced by the charge of solutes (from +1 to -2). Similarly, pH variations from 3 to 9 had little impact on the diffusion coeffs. of R6G+ in the gel. The addn. of Ca2+ had a significant impact on gel compactness, which led to a significant redn. in solute diffusion. For the calcium alginate hydrogels, structural modifications resulting from Ca binding were much more important than electrostatic effects due to modifications of the gel Donnan potential.
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15. 15
  Lundberg, P.; Kuchel, P. Diffusion of Solutes in Agarose and Alginate Gels:1H and 23Na PFGSE and 23Na TQF NMR Studies. Magn. Reson. Med. 1997, 37, 44– 52, DOI: 10.1002/mrm.1910370108
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  15
  Diffusion of solutes in agarose and alginate gels: 1H and 23Na PFGSE and 23Na TQF NMR studies
  Lundberg, Peter; Kuchel, Philip W.
  Magnetic Resonance in Medicine (1997), 37 (1), 44-52CODEN: MRMEEN; ISSN:0740-3194. (Williams & Wilkins)
  Cells immobilized in gels experience potential metabolic restrictions in the form of reduced diffusion rates of metabolites and ions and their possible selective adsorption on the gel matrix. Diffusion and relaxation characteristics of common solutes in agarose and barium alginate gels were investigated at 37° by using 1H PFGSE and 23Na TQF NMR spectroscopy. Glucose, glycine, alanine, lactate, sodium ions, and HDO were studied. There were no selective interactions between any of the metabolites and the gel materials but the diffusion coeffs. were uniformly reduced. The effects of metabolite diffusion and utilization, in gel beads and threads contg. cells, were simulated by using a reaction diffusion model incorporating the measured diffusion coeffs. Metab. is expected to be very significantly limited by diffusion of solutes to and from the cells that are centrally located within gel threads or spheres of radius ∼2.0 mm, which is a commonly used size.
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16. 16
  Guo, H.; Kurokawa, T.; Takahata, M.; Hong, W.; Katsuyama, Y.; Luo, F.; Ahmed, J.; Nakajima, T.; Nonoyama, T.; Gong, J. Quantitative Observation of Electric Potential Distribution of Brittle Polyelectrolyte Hydrogels Using Microelectrode Technique. Macromolecules 2016, 49, 3100– 3108, DOI: 10.1021/acs.macromol.6b00037
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  16
  Quantitative Observation of Electric Potential Distribution of Brittle Polyelectrolyte Hydrogels Using Microelectrode Technique
  Guo, Honglei; Kurokawa, Takayuki; Takahata, Masakazu; Hong, Wei; Katsuyama, Yoshinori; Luo, Feng; Ahmed, Jamil; Nakajima, Tasuku; Nonoyama, Takayuki; Gong, Jian Ping
  Macromolecules (Washington, DC, United States) (2016), 49 (8), 3100-3108CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)
  We report, for the first time, the quant. measurement of the local elec. potential of brittle polyelectrolyte hydrogels using the microelectrode technique (MET). Given the solid-like nature of the hydrogels, the difficulty of applying MET is how to make a good contact of the microelectrode to the hydrogel. Poor local contact substantial underestimates the potential. We obsd. that, the potential measured decays exponentially with the increase of capillary diam. of the microelectrode. This behavior is related to the capillary wall thickness that dets. the contact distance of the electrode probe to the hydrogel. The characteristic decay length in resp. to the wall thickness is very close to the local Debye length around the capillary. The latter is much larger than that of the bath soln. due to the reverse osmosis effect. By using microelectrodes with a tip wall thickness less than the local Debye length, the Donnan potential of polyelectrolyte gel could be accurately measured. Using a micromanipulator, the inserting process of the microelectrode is precisely controlled, and the depth profile of elec. potential in the hydrogels can be measured with a spatial resoln. down to ∼5 nm. From the spatial distribution of potential, the microstructure of hydrogels both in bulk and near the surface, the thickness of ultrathin hydrogels, and the heterogeneous layered structure of composite gels, can be detd. accurately. The MET established in this work provides a powerful tool for direct characterization of the spatial distribution of elec. potential of hydrogels.
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17. 17
  Podlas, T.; Ander, P. Interactions of Sodium and Potassium Ions with Sodium and Potassium Alginate in Aqueous Solution with and without Added Salt. Macromolecules 1970, 3, 154– 157, DOI: 10.1021/ma60014a007
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  17
  Interactions of sodium and potassium ions with sodium and potassium alginate in aqueous solution with and without added salt
  Podlas, Thomas J.; Ander, Paul
  Macromolecules (1970), 3 (2), 154-7CODEN: MAMOBX; ISSN:0024-9297.
  The interaction of Na and K ions with Na and K alginate were investigated by measuring the counterion activity coeffs. in aq. solns. of the polyelectrolytes with and without added simple electrolyte at 25°. In salt-free solns., the counterion activity coeffs. increase with increasing diln. and the activity coeffs. of K ions were higher than those of Na ions throughout the concn. range. Counterion activity coeffs. decreased with increasing polyelectrolyte concn. at const. simple salt concn. and decreased with decreasing salt concn. at a const. polyelectrolyte concn. Deviations from the additivity rule were most pronounced when the salt and polymer concns. were approx. equal. Good agreement was obtained when Manning's line-charge polyelectrolyte model was compared with the exptl. results for the lowest simple salt concn.
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18. 18
  Aziz, E.; Ottosson, N.; Eisebitt, S.; Eberhardt, W.; Jagoda-Cwiklik, B.; Vácha, R.; Jungwirth, P.; Winter, B. Cation-Specific Interactions with Carboxylate in Amino Acid and Acetate Aqueous Solutions: X-Ray Absorption and ab initio calculations. J. Phys. Chem. B 2008, 112, 12567– 12570, DOI: 10.1021/jp805177v
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  18
  Cation-Specific Interactions with Carboxylate in Amino Acid and Acetate Aqueous Solutions: X-ray Absorption and ab initio Calculations
  Aziz, Emad F.; Ottosson, Niklas; Eisebitt, Stefan; Eberhardt, Wolfgang; Jagoda-Cwiklik, Barbara; Vacha, Robert; Jungwirth, Pavel; Winter, Bernd
  Journal of Physical Chemistry B (2008), 112 (40), 12567-12570CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
  Relative interaction strengths between cations (X = Li+, Na+, K+, NH4+) and anionic carboxylate groups of acetate and glycine in aq. soln. are detd. These model systems mimic ion pairing of biol. relevant cations with neg. charged groups at protein surfaces. With oxygen 1s X-ray absorption spectroscopy, we can distinguish between spectral contributions from H2O and carboxylate, which allows us to probe the electronic structure changes of the at. site of the carboxylate group being closest to the countercation. From the intensity variations of the COO-aq O 1s X-ray absorption peak, which quant. correlate with the change in the local partial d. of states from the carboxylic site, interactions are found to decrease in the sequence Na+ > Li+ > K+ > NH4+. This ordering, as well as the obsd. bidental nature of the -COO-aq and X+aq interaction, is supported by combined ab initio and mol. dynamics calcns.
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19. 19
  Okur, H. I.; Hladílková, J.; Rembert, K.; Cho, Y.; Heyda, J.; Dzubiella, J.; Cremer, P.; Jungwirth, P. Beyond The Hofmeister Series: Ion-Specific Effects on Proteins and Their Biological Functions. J. Phys. Chem. B 2017, 121, 1997– 2014, DOI: 10.1021/acs.jpcb.6b10797
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  19
  Beyond the Hofmeister Series: Ion-Specific Effects on Proteins and Their Biological Functions
  Okur, Halil I.; Hladilkova, Jana; Rembert, Kelvin B.; Cho, Younhee; Heyda, Jan; Dzubiella, Joachim; Cremer, Paul S.; Jungwirth, Pavel
  Journal of Physical Chemistry B (2017), 121 (9), 1997-2014CODEN: JPCBFK; ISSN:1520-5207. (American Chemical Society)
  A review. Ions differ in their ability to salt out proteins from soln. as expressed in the lyotropic or Hofmeister series of cations and anions. Since its first formulation in 1888, this series has been invoked in a plethora of effects, going beyond the original salting-out/salting-in idea to include enzyme activities and the crystn. of proteins, as well as to processes not involving proteins like ion exchange, the surface tension of electrolytes, or bubble coalescence. Although it has been clear that the Hofmeister series is intimately connected to ion hydration in homogeneous and heterogeneous environments and to ion pairing, its mol. origin has not been fully understood. This situation could have been summarized as follows: Many chemists used the Hofmeister series as a mantra to put a label on ion specific behavior in various environments, rather than to reach a mol. level understanding and, consequently, an ability to predict a particular effect of a given salt ion on proteins in solns. In this Feature Article the cationic and anionic Hofmeister series can now be rationalized primarily in terms of specific interactions of salt ions with the backbone and charged side chain groups at the protein surface in soln. At the same time, the authors demonstrate the limitations of sepg. Hofmeister effects into independent cationic and anionic contributions due to the electroneutrality condition, as well as specific ion pairing, leading to interactions of ions of opposite polarity. Finally, the authors outline the route beyond Hofmeister chem. in the direction of understanding specific roles of ions in various biol. functionalities, where generic Hofmeister-type interactions can be complemented or even overruled by particular steric arrangements in various ion binding sites.
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20. 20
  Lee, S. J.; Lee, J.; Kim, K. Pressure-Driven Spontaneous Ion Concentration Polarization Using an Ion-Selective Membrane. Anal. Biochem. 2018, 557, 13– 17, DOI: 10.1016/j.ab.2018.07.005
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  20
  Pressure-driven spontaneous ion concentration polarization using an ion-selective membrane
  Lee, Sang Joon; Lee, Jaehyeon; Kim, Kiwoong
  Analytical Biochemistry (2018), 557 (), 13-17CODEN: ANBCA2; ISSN:0003-2697. (Elsevier B.V.)
  In this study, the spontaneous ion concn. polarization phenomenon induced by pressure via a cation-selective membrane was theor. and exptl. investigated. Unlike conventional electrokinetic ion concn. polarization, which uses elec. current as a driving flux of cations through the membrane, advection caused by pressure is used as a transmembrane driving flux of cations to spontaneously and stably form an ion depletion zone in the present ion concn. polarization technique. The ion depletion zone produced in a simple exptl. setup was used to filter electrolyte and preconc. ions and microparticles. Different from the general assumption of the negligible thickness of the elec. double layer in microchannels, the low concn. in the ion depletion zone considerably increased the length of the elec. double layer. This enhanced the formation of the ion depletion zone. The present results can improve the understanding on ion transport in the ion concn. polarization system and can be utilized to develop a portable water desalination device for rural/remote areas and for preconcg. biomols.
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21. 21
  Kim, S. J.; Ko, S. H.; Kang, K. H.; Han, J. Direct Seawater Desalination by Ion Concentration Polarization. Nat. Nanotechnol. 2010, 5, 297– 301, DOI: 10.1038/nnano.2010.34
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  21
  Direct seawater desalination by ion concentration polarization
  Kim, Sung Jae; Ko, Sung Hee; Kang, Kwan Hyoung; Han, Jongyoon
  Nature Nanotechnology (2010), 5 (4), 297-301CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
  We report a process for converting seawater (salinity ∼500mM or ∼30,000 mg/L) to freshwater (salinity <10mM or <600 mg/L) in which a continuous stream of seawater is divided into desalted and concd. streams by ion concn. polarization, a phenomenon that occurs when an ion current is passed through ion-selective membranes. During operation, both salts and larger particles (cells, viruses and microorganisms) are pushed away from the membrane (a nanochannel or nanoporous membrane), which significantly reduces the possibility of membrane fouling and salt accumulation, thus avoiding 2 problems that plague other membrane filtration methods. To implement this approach, a simple microfluidic device was fabricated and shown to be capable of continuous desalination of seawater (∼99% salt rejection at 50% recovery rate) at a power consumption of <3.5 W-h/L, which is comparable to current state-of-the-art systems. Rather than competing with larger desalination plants, the method could be used to make small- or medium-scale systems, with the possibility of battery-powered operation.
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22. 22
  Park, S.; Jung, Y.; Son, S.; Cho, I.; Cho, Y.; Lee, H.; Kim, H.; Kim, S. Capillarity Ion Concentration Polarization as Spontaneous Desalting Mechanism. Nat. Commun. 2016, 7, 11223 DOI: 10.1038/ncomms11223
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  22
  Capillarity ion concentration polarization as spontaneous desalting mechanism
  Park, Sungmin; Jung, Yeonsu; Son, Seok Young; Cho, Inhee; Cho, Youngrok; Lee, Hyomin; Kim, Ho-Young; Kim, Sung Jae
  Nature Communications (2016), 7 (), 11223CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
  To overcome a world-wide water shortage problem, numerous desalination methods have been developed with state-of-the-art power efficiency. Here we propose a spontaneous desalting mechanism referred to as the capillarity ion concn. polarization. An ion-depletion zone is spontaneously formed near a nanoporous material by the permselective ion transportation driven by the capillarity of the material, in contrast to electrokinetic ion concn. polarization which achieves the same ion-depletion zone by an external d.c. bias. This capillarity ion concn. polarization device is shown to be capable of desalting an ambient electrolyte more than 90% without any external elec. power sources. Theor. anal. for both static and transient conditions are conducted to characterize this phenomenon. These results indicate that the capillarity ion concn. polarization system can offer unique and economical approaches for a power-free water purifn. system.
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23. 23
  Mogi, K. A Visualization Technique of a Unique pH Distribution Around an Ion Depletion Zone in a Microchannel by Using a Dual-Excitation Ratiometric Method. Micromachines 2018, 9, 167 DOI: 10.3390/mi9040167
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24. 24
  Mogi, K.; Hayashida, K.; Yamamoto, T. Damage-Less Handling of Exosomes Using an Ion-Depletion Zone in a Microchannel. Anal. Sci. 2018, 34, 875– 880, DOI: 10.2116/analsci.17P462
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  24
  Damage-less handling of exosomes using an ion-depletion zone in a microchannel
  Mogi, Katsuo; Hayashida, Kei; Yamamoto, Takatoki
  Analytical Sciences (2018), 34 (8), 875-880CODEN: ANSCEN; ISSN:0910-6340. (Japan Society for Analytical Chemistry)
  Exosomes are of increasing research interest because they are integral to cell-cell communication and are implicated in various disease states. Here we investigated the utility of using an ion-depletion zone in a microfluidic device to conc. exosomes from the culture media of four types of cell lines. Furthermore, we eveluated the extent of damage to the exosomes following concn. by an ion-depletion zone microchannel device compared with exosomes concd. by a conventional ultra-centrifugation technique. Our results conclusivedly demonstrate that significantly less damage is incurred by exosomes following passage through and concn. by the ion-depleted zone microchannel device compared to concn. by ultra-centrifugation. Our findings will help extend the utility of exosomes to various applications.
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25. 25
  Huicochea, A.; Siqueiros, J.; Romero, R. Portable Water Purification System Integrated to a Heat Transformer. Desalination 2004, 165, 385– 391, DOI: 10.1016/j.desal.2004.06.044
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  Portable water purification system integrated to a heat transformer
  Huicochea, A.; Siqueiros, J.; Romero, R. J.
  Desalination (2004), 165 (1-3), 385-391CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)
  H2O is a natural resource essential for life and for most economic activities developed on earth. Population growth and lack of H2O in some regions of the world led humans to design and implement new technologies to use H2O in an efficient way. H2O quality requirements in the industrial sphere are higher everyday. Development of desalination technol. through a water purifn. system integrated to heat pumps has taken more than two decades. Absorption heat pumps use low quality energy in a waste heat form and a small quantity of high quality energy. This characteristic has made possible using these systems in quite a few places. The following work presents the results of the exptl. tests applied to a portable water purifn. system integrated to a heat transformer (TTAPPA), where the low quality waste heat is simulated and LiBr-H2O was used as a working soln.
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26. 26
  Ma, B.; Chi, J.; Liu, H. Fabric-Based Ion Concentration Polarization for Pump-Free Water Desalination. ACS Sustainable Chem. Eng. 2017, 6, 99– 103, DOI: 10.1021/acssuschemeng.7b03679
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  Lee, D.; Lee, J.; Lee, H.; Kim, S. Spontaneous Selective Preconcentration Leveraged by Ion Exchange and Imbibition Through Nanoporous Medium. Sci. Rep. 2019, 9, 2336 DOI: 10.1038/s41598-018-38162-6
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  Spontaneous Selective Preconcentration Leveraged by Ion Exchange and Imbibition through Nanoporous Medium
  Lee Dokeun; Lee Jung A; Kim Sung Jae; Lee Hyomin; Kim Sung Jae; Kim Sung Jae
  Scientific reports (2019), 9 (1), 2336 ISSN:.
  Manipulating mechanism of particle's motion has been extensively studied for the sample preparation in microfluidic applications including diagnostics, food industries, biological analyses and environmental monitoring. However, most of conventional methods need additional external forces such as electric field or pressure and complicated channel designs, which demand highly complex fabrication processes and operation strategies. In addition, these methods have inherent limitations of dilution or mixing during separation or preconcentration step, respectively, so that a number of studies have reported an efficient selective preconcentration process, i.e. conducting the separation and preconcentration simultaneously. In this work, a power-free spontaneous selective preconcentration method was suggested based on leveraging convective flow over diffusiophoresis near the water-absorbing nanoporous ion exchange medium, which was verified both by simulation and experiment. Especially, the velocity of the convective flow by an imbibition deviated from the original tendency of t(-1/2) due to non-uniformly patterned nanoporous medium that has multiple cross-sectional areas. As a result, the direction of particle's motion was controlled at one's discretion, which led to the spontaneous selective preconcentration of particles having different diffusiophoretic constant. Also, design rule for maximizing the efficiency was recommended. Thus, this selective preconcentration method would play as a key mechanism for power-free lab on a chip applications.
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28. 28
  Lee, J.; Lee, D.; Park, S.; Lee, H.; Kim, S. Non-Negligible Water-Permeance Through Nanoporous Ion Exchange Medium. Sci. Rep. 2018, 8, 12842 DOI: 10.1038/s41598-018-29695-x
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  Non-negligible Water-permeance through Nanoporous Ion Exchange Medium
  Lee Jung A; Lee Dokeun; Lee Hyomin; Kim Sung Jae; Park Sungmin; Lee Hyomin; Kim Sung Jae; Kim Sung Jae
  Scientific reports (2018), 8 (1), 12842 ISSN:.
  While the water impermeable constraint has been conventionally adopted for analyzing the transport phenomena at the interface of electrolyte/nanoporous medium, non-negligible water-permeance through the medium results in significant effect on ion and particle transportation. In this work, a rigorous theoretical and experimental analysis of the water-permeance effect were conducted based on a fully-coupled analytical/numerical method and micro/nanofluidic experiments. The regime diagram with three distinctive types of concentration boundary layers (ion depletion, ion accumulation, and intermediate) near the ion exchange nanoporous medium was proposed depending on the medium's permselectivity and the water-permeance represented by an absorbing parameter. Moreover, the critical absorbing parameters which divide the regimes were analytically obtained so that the bidirectional motion of particles were demonstrated only by altering the water-permeance without external stimuli. Conclusively, the presenting analysis of non-negligible water-permeance would be a substantial fundamental of transport phenomena at the interface of the ion exchange medium and electrolyte, especially useful for the tunable particle/ion manipulations in intermediate Peclet number environment.
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29. 29
  Zheng, J.-m.; Chin, W.; Khijniak, E.; Khijniak, E.; Pollack, G. Surfaces and Interfacial Water: Evidence That Hydrophilic Surfaces Have Long-Range Impact. Adv. Colloid Interface Sci. 2006, 127, 19– 27, DOI: 10.1016/j.cis.2006.07.002
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  Surfaces and interfacial water: Evidence that hydrophilic surfaces have long-range impact
  Zheng, Jian-ming; Chin, Wei-Chun; Khijniak, Eugene; Khijniak, Eugene; Pollack, Gerald H.
  Advances in Colloid and Interface Science (2006), 127 (1), 19-27CODEN: ACISB9; ISSN:0001-8686. (Elsevier B.V.)
  It is generally thought that the impact of surfaces on the contiguous aq. phase extends to a distance of no more than a few H2O-mol. layers. Older studies, however, suggest a more extensive impact. The authors report here that colloidal and mol. solutes suspended in aq. soln. are profoundly and extensively excluded from the vicinity of various hydrophilic surfaces. The width of the solute-free zone is typically several hundred microns. Such large exclusion zones were obsd. in the vicinity of many types of surface including artificial and natural hydrogels, biol. tissues, hydrophilic polymers, monolayers, and ion-exchange beads, as well as with a variety of solutes. Using microscopic observations, as well as measurements of elec. potential and UV-visible absorption-spectra, IR imaging, and NMR imaging, the solute-free zone is a phys. distinct and less mobile phase of H2O that can coexist indefinitely with the contiguous solute-contg. phase. The extensiveness of this modified zone is impressive, and carries broad implication for surface-mol. interactions in many realms, including cellular recognition, biomaterial-surface antifouling, biosepn. technologies, and other areas of biol., physics and chem.
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30. 30
  Zheng, J.-m.; Pollack, G. H. Long-Range Forces Extending from Polymer-Gel Surfaces. Phys. Rev. E 2003, 68 DOI: 10.1103/physreve.68.031408 .
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  Chai, B.; Pollack, G. H. Solute-Free Interfacial Zones in Polar Liquids. J. Phys. Chem. B 2010, 114, 5371– 5375, DOI: 10.1021/jp100200y
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  Solute-Free Interfacial Zones in Polar Liquids
  Chai, Binghua; Pollack, Gerald H.
  Journal of Physical Chemistry B (2010), 114 (16), 5371-5375CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
  Large, solute-free interfacial zones have recently been described in aq. solns. Found next to hydrophilic surfaces, these "exclusion zones" are commonly several hundred micrometers wide and represent regions of water that appear to be more ordered than bulk water. We report here that other polar solvents including methanol, ethanol, isopropanol, acetic acid, D2O, and DMSO show similar near-surface exclusion zones, albeit of smaller magnitude. Microelectrode measurements show that these zones are neg. charged and grow in response to incident IR radiation, similar to exclusion zones in aq. solns. Hence, near-surface exclusion zones appear to be features characteristic not only of water but of other polar liqs. as well.
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32. 32
  Florea, D.; Musa, S.; Huyghe, J.; Wyss, H. Long-Range Repulsion of Colloids Driven by Ion Exchange and Diffusiophoresis. Proc. Natl. Acad. Sci. U.S.A. 2014, 111, 6554– 6559, DOI: 10.1073/pnas.1322857111
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  Long-range repulsion of colloids driven by ion exchange and diffusiophoresis
  Florea, Daniel; Musa, Sami; Huyghe, Jacques M. R.; Wyss, Hans M.
  Proceedings of the National Academy of Sciences of the United States of America (2014), 111 (18), 6554-6559CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
  Interactions between surfaces and particles in aq. suspension are usually limited to distances smaller than 1 μm. However, in a range of studies from different disciplines, repulsion of particles was obsd. over distances of up to hundreds of micrometers, in the absence of any addnl. external fields. Although a range of hypotheses were suggested to account for such behavior, the phys. mechanisms responsible for the phenomenon still remain unclear. To identify and isolate these mechanisms, we perform detailed expts. on a well-defined exptl. system, using a setup that minimizes the effects of gravity and convection. The obsd. long-range repulsion is driven by a combination of ion exchange, ion diffusion, and diffusiophoresis. A simple model was developed that accounts for this data; this description is expected to be directly applicable to a wide range of systems exhibiting similar long-range forces.
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33. 33
  Huszár, I.; Mártonfalvi, Z.; Laki, A.; Iván, K.; Kellermayer, M. Exclusion-Zone Dynamics Explored with Microfluidics and Optical Tweezers. Entropy 2014, 16, 4322– 4337, DOI: 10.3390/e16084322
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  Exclusion-zone dynamics explored with microfluidics and optical tweezers
  Huszar, Istvan N.; Martonfalvi, Zsolt; Laki, Andras Jozsef; Ivan, Kristof; Kellermayer, Miklos
  Entropy (2014), 16 (8), 4322-4337CODEN: ENTRFG; ISSN:1099-4300. (MDPI AG)
  The exclusion zone (EZ) is a boundary region devoid of macromols. and microscopic particles formed spontaneously in the vicinity of hydrophilic surfaces. The exact mechanisms behind this remarkable phenomenon are still not fully understood and are debated. We measured the short- and long-time-scale kinetics of EZ formation around a Nafion gel embedded in specially designed microfluidic devices. The time-dependent kinetics of EZ formation follow a power law with an exponent of 0.6 that is strikingly close to the value of 0.5 expected for a diffusion-driven process. By using optical tweezers we show that exclusion forces, which are estd. to fall in the sub-pN regime, persist within the fully-developed EZ, suggesting that EZ formation is not a quasi-static but rather an irreversible process. Accordingly, the EZ-forming capacity of the Nafion gel could be exhausted with time, on a scale of hours in the presence of 1 mM Na2HPO4. EZ formation may thus be a non-equil. thermodn. cross-effect coupled to a diffusion-driven transport process. Such phenomena might be particularly important in the living cell by providing mech. cues within the complex cytoplasmic environment.
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34. 34
  Tamagawa, H.; Ikeda, K. Another Interpretation of the Goldman–Hodgkin–Katz Equation Based on Ling’s Adsorption Theory. Eur. Biophys. J. 2018, 47, 869– 879, DOI: 10.1007/s00249-018-1332-0
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  Another interpretation of the Goldman-Hodgkin-Katz equation based on Ling's adsorption theory
  Tamagawa, Hirohisa; Ikeda, Kota
  European Biophysics Journal (2018), 47 (8), 869-879CODEN: EBJOE8; ISSN:0175-7571. (Springer)
  According to std. membrane theory, the generation of membrane potential is attributed to transmembrane ion transport. However, there have been a no. of reports of membrane behavior in conflict with the membrane theory of cellular potential. Putting aside the membrane theory, we scrutinized the generation mechanism of membrane potential from the view of the long-dismissed adsorption theory of Ling. Ling's adsorption theory attributes the membrane potential generation to mobile ion adsorption. Although Ling's adsorption theory conflicts with the broadly accepted membrane theory, we found that it well reproduces exptl. obsd. membrane potential behavior. Our theor. anal. finds that the potential formula based on the GHK eq., which is a fundamental concept of membrane theory, coincides with the potential formula based on Ling's adsorption theory. Reinterpreting the permeability coeff. in the GHK eq. as the assocn. const. between the mobile ion and adsorption site, the GHK eq. turns into the potential formula from Ling's adsorption theory. We conclude that the membrane potential is generated by ion adsorption as Ling's adsorption theory states and that the membrane theory of cellular potential should be amended even if not discarded.
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35. 35
  Tamagawa, H. Mathematical Expression of Membrane Potential Based on Ling’s Adsorption Theory is Approximately the Same as the Goldman–Hodgkin–Katz Equation. J. Biol. Phys. 2018, 45, 13– 30, DOI: 10.1007/s10867-018-9512-9
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36. 36
  Ritos, K.; Borg, M. K.; Mottram, N. J.; Reese, J. M. Electric Fields Can Control the Transport of Water in Carbon Nanotubes. Philos. Trans. R. Soc., A 2016, 374, 20150025 DOI: 10.1098/rsta.2015.0025
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  Electric fields can control the transport of water in carbon nanotubes
  Ritos, Konstantinos; Borg, Matthew K.; Mottram, Nigel J.; Reese, Jason M.
  Philosophical Transactions of the Royal Society, A: Mathematical, Physical & Engineering Sciences (2016), 374 (2060), 20150025/1-20150025/19CODEN: PTRMAD; ISSN:1364-503X. (Royal Society)
  The properties of water confined inside nanotubes are of considerable scientific and technol. interest. We use mol. dynamics to investigate the structure and av. orientation of water flowing within a carbon nanotube. We find that water exhibits biaxial paranematic liq. crystal ordering both within the nanotube and close to its ends. This preferred mol. ordering is enhanced when an axial elec. field is applied, affecting the water flow rate through the nanotube. A spatially patterned elec. field can minimize nanotube entrance effects and significantly increase the flow rate.
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37. 37
  Abdul Kadir, L.; Stacey, M.; Barrett-Jolley, R. Emerging Roles of the Membrane Potential: Action Beyond the Action Potential. Front. Physiol. 2018, 9 DOI: 10.3389/fphys.2018.01661 .
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  Marbach, S.; Bocquet, L. Osmosis, From Molecular Insights to Large-Scale Applications. Chem. Soc. Rev. 2019, 48, 3102– 3144, DOI: 10.1039/C8CS00420J
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  Osmosis, from molecular insights to large-scale applications
  Marbach, Sophie; Bocquet, Lyderic
  Chemical Society Reviews (2019), 48 (11), 3102-3144CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
  Osmosis is a universal phenomenon occurring in a broad variety of processes and fields. It is the archetype of entropic forces, both trivial in its fundamental expression - the van 't Hoff perfect gas law - and highly subtle in its phys. roots. While osmosis is intimately linked with transport across membranes, it also manifests itself as an interfacial transport phenomenon: the so-called diffusio-osmosis and -phoresis, whose consequences are presently actively explored for example for the manipulation of colloidal suspensions or the development of active colloidal swimmers. Here we give a global and unifying view of the phenomenon of osmosis and its consequences with a multi-disciplinary perspective. Pushing the fundamental understanding of osmosis allows one to propose new perspectives for different fields and we highlight a no. of examples along these lines, for example introducing the concepts of osmotic diodes, active sepn. and far from equil. osmosis, raising in turn fundamental questions in the thermodn. of sepn. The applications of osmosis are also obviously considerable and span very diverse fields. Here we discuss a selection of phenomena and applications where osmosis shows great promises: osmotic phenomena in membrane science (with recent developments in sepn., desalination, reverse osmosis for water purifn. thanks in particular to the emergence of new nanomaterials); applications in biol. and health (in particular discussing the kidney filtration process); osmosis and energy harvesting (in particular, osmotic power and blue energy as well as capacitive mixing); applications in detergency and cleaning, as well as for oil recovery in porous media.
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39. 39
  Hibino, H.; Takai, M.; Noguchi, H.; Sawamura, S.; Takahashi, Y.; Sakai, H.; Shiku, H. An Approach to the Research on Ion and Water Properties in the Interphase Between the Plasma Membrane and Bulk Extracellular Solution. J. Physiol. Sci. 2017, 67, 439– 445, DOI: 10.1007/s12576-017-0530-3
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  An approach to the research on ion and water properties in the interphase between the plasma membrane and bulk extracellular solution
  Hibino, Hiroshi; Takai, Madoka; Noguchi, Hidenori; Sawamura, Seishiro; Takahashi, Yasufumi; Sakai, Hideki; Shiku, Hitoshi
  Journal of Physiological Sciences (2017), 67 (4), 439-445CODEN: JPSOE2; ISSN:1880-6546. (Springer Japan)
  A review. In vivo, cells are immersed in an extracellular soln. that contains a variety of bioactive substances including ions and water. Classical electrophysiol. analyses of epithelial cells in the stomach and small intestine have revealed that within a distance of several hundred micrometers above their apical plasma membrane, lies an extracellular layer that shows ion concn. gradients undetectable in the bulk phase. This "unstirred layer", which contains stagnant solutes, may also exist between the bulk extracellular soln. and membranes of other cells in an organism and may show different properties. On the other hand, an earlier study using a bacterial planar membrane indicated that H+ released from a transporter migrates in the horizontal direction along the membrane surface much faster than it diffuses vertically toward the extracellular space. This result implies that between the membrane surface and unstirred layer, there is a "nanointerface" that has unique ionic dynamics. Advanced technologies have revealed that the nanointerface on artificial membranes possibly harbors a highly ordered assembly of water mols. In general, hydrogen bonds are involved in formation of the ordered water structure and can mediate rapid transfer of H+ between neighboring mols. This description may match the phenomenon on the bacterial membrane. A recent study has suggested that water mols. in the nanointerface regulate the gating of K+ channels. Here, the region comprising the unstirred layer and nanointerface is defined as the interphase between the plasma membrane and bulk extracellular soln. (iMES). This article briefly describes the physicochem. properties of ions and water in the iMES and their physiol. significance. We also describe the methodologies that are currently used or will be applicable to the interphase research.
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40. 40
  Endeward, V.; Gros, G. Extra- and Intracellular Unstirred Layer Effects in Measurements of CO₂ diffusion Across Membranes - A Novel Approach Applied to the Mass Spectrometric ¹⁸O Technique for Red Blood Cells. J. Physiol. 2009, 587, 1153– 1167, DOI: 10.1113/jphysiol.2008.165027
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  Extra- and intracellular unstirred layer effects in measurements of CO2 diffusion across membranes - a novel approach applied to the mass spectrometric 18O technique for red blood cells
  Endeward, Volker; Gros, Gerolf
  Journal of Physiology (Oxford, United Kingdom) (2009), 587 (6), 1153-1167CODEN: JPHYA7; ISSN:0022-3751. (Wiley-Blackwell)
  We have developed an exptl. approach that allows us to quantify unstirred layers around cells suspended in stirred solns. This technique is applicable to all types of transport measurements and was applied here to the 18O technique used to measure CO2 permeability of red cells (PCO2). We measure PCO2 in well-stirred red cell (RBC) suspensions of various viscosities adjusted by adding different amts. of 60 kDa dextran. Plotting 1/PCO2 vs. viscosity ν gives a linear relation, which can be extrapolated to ν = 0. Theor. hydrodynamics predicts that extracellular unstirred layers vanish at zero viscosity when stirring is maintained, and thus this extrapolation gives us an est. of the PCO2 free from extracellular unstirred layer artifacts. The extrapolated value is found to be 0.16 cm s-1 instead of the exptl. value in saline of 0.12 cm s-1 (+30%). This effect corresponds to an unstirred layer thickness of 0.5 μm. In addn., we present a theor. approach modeling the actual geometrical and physico-chem. conditions of 18O exchange in our expts. It confirms the role of an extracellular unstirred layer in the detn. of PCO2. Also, it allows us to quantify the contribution of the so-called intracellular unstirred layer, which results from the fact that in these transport measurements - as in all such measurements in general - the intracellular space is not stirred. The apparent thickness of this intracellular unstirred layer is about 1/4-1/3 of the maximal intracellular diffusion distance, and correction for it results in a true PCO2 of the RBC membrane of 0.20 cm s-1. Thus, the order of magnitude of this PCO2 is unaltered compared to our previous reports. Discussion of the available evidence in the light of these results confirms that CO2 channels exist in red cell and other membranes, and that PCO2 of red cell membranes in the absence of these channels is quite low.
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41. 41
  Wilson, F. A.; Dietschy, J. M. The Intestinal Unstirred Layer: Its Surface Area and Effect on Active Transport Kinetics. Biochim. Biophys. Acta 1974, 363, 112– 126, DOI: 10.1016/0005-2736(74)90010-8
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  Intestinal unstirred layer. Its surface area and effect on active transport kinetics
  Wilson, Frederick A.; Dietschy, John M.
  Biochimica et Biophysica Acta, Biomembranes (1974), 363 (1), 112-26CODEN: BBBMBS; ISSN:0005-2736.
  Uptake into the intestinal mucosa is detd. by the rates of penetration of solute mols. across the unstirred H2O layer and lipid cell membrane of the mucosal cell. The presence of the unstirred H2O layer results in artifactually low permeability coeffs. for passive transport processes and high Km values for active transport processes. Appropriate correction for this unstirred layer resistance requires knowledge of the effective surface area of the diffusion barrier in the intestine. Using 3 sep. exptl. and math. approaches this surface area was found to vary from 1.02 to 14.24 cm2/100 mg dry wt. of rat intestine. These values are much lower than the 1226 and 696 cm2/100 mg area of the microvillus membrane in the jejunum and ileum, resp., and indicating that the effective surface area of the rate-limiting membrane for such highly permeant solutes as long-chain fatty acids is from 1/100 to 1/200 the actual anatomical surface area. The presence of the unstirred layer introduces major artifacts into the detn. of Km and Jmax (max. transport rate) values for active transport processes.
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  Artificial Seawater. Cold Spring Harbor Protocols ; 2012 DOI: 10.1101/pdb.rec068270 .
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  Mørch, Ý. A.; Donati, I.; Strand, B. L.; Skjåk-Bræk, G. Effect of Ca²⁺, Ba²⁺, And Sr²⁺ on Alginate Microbeads. Biomacromolecules 2006, 7, 1471– 1480, DOI: 10.1021/bm060010d
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  Effect of Ca2+, Ba2+, and Sr2+ on Alginate Microbeads
  Morch, Yrr A.; Donati, Ivan; Strand, Berit L.; Skjaak-Braek, Gudmund
  Biomacromolecules (2006), 7 (5), 1471-1480CODEN: BOMAF6; ISSN:1525-7797. (American Chemical Society)
  Microcapsules of alginate cross-linked with divalent ions are the most common system for cell immobilize metabolization. In this study, we wanted to characterize the effect of different alginates and crosslinking ions on important microcapsule properties. The dimensional stability and gel strength increased for high-G alginate gels when exchanging the traditional Ca2+ ions with Ba2+. The use of Ba2+ decreased the size of alginate beads and reduced the permeability to IgG. Strontium gave gels with characteristics lying between calcium and barium. Interestingly, high-M alginate showed an opposite behavior in combination with barium and strontium as these beads were larger than beads of calcium-alginate and tended to swell more, also resulting in increased permeability. Binding studies revealed that different block structures in the alginate bind the ions to a different extent. More specifically, Ca2+ was found to bind to G- and MG-blocks, Ba2+ to G- and M-blocks, and Sr2+ to G-blocks solely.
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44. 44
  Topuz, F.; Henke, A.; Richtering, W.; Groll, J. Magnesium Ions and Alginate do Form Hydrogels: A Rheological Study. Soft Matter 2012, 8, 4877, DOI: 10.1039/c2sm07465f
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  Magnesium ions and alginate do form hydrogels: a rheological study
  Topuz, Fuat; Henke, Artur; Richtering, Walter; Groll, Juergen
  Soft Matter (2012), 8 (18), 4877-4881CODEN: SMOABF; ISSN:1744-683X. (Royal Society of Chemistry)
  Our study shows that magnesium ions which have so far been considered as non-gelling ions for alginate do induce alginate gelation. Rheol. is used to examine effects of alginate chem. compn. as well as alginate and magnesium ion concn. Gelation in this system occurs at ca. 5-10 times higher concn. of ions than reported for calcium-based gels. Alginate network formation with magnesium ions is very slow and is typically accomplished within 2-3 h. Gelation with magnesium ions is also strongly dependent on alginate chem. compn. as the presence of long guluronic units privileges faster gel formation.
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  Vreeker, R.; Li, L.; Fang, Y.; Appelqvist, I.; Mendes, E. Drying and Rehydration of Calcium Alginate Gels. Food Sci. 2008, 3, 361– 369, DOI: 10.1007/s11483-008-9087-2
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  Fang, Y.; Li, L.; Vreeker, R.; Yao, X.; Wang, J.; Ma, Q.; Jiang, F.; Phillips, G. Rehydration of Dried Alginate Gel Beads: Effect of the Presence of Gelatin and Gum Arabic. Carbohydr. Polym. 2011, 86, 1145– 1150, DOI: 10.1016/j.carbpol.2011.06.003
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  Rehydration of dried alginate gel beads: Effect of the presence of gelatin and gum arabic
  Fang, Yapeng; Li, Liangbin; Vreeker, Rob; Yao, Xiaolin; Wang, Jianguo; Ma, Qing; Jiang, Fatang; Phillips, Glyn O.
  Carbohydrate Polymers (2011), 86 (3), 1145-1150CODEN: CAPOD8; ISSN:0144-8617. (Elsevier Ltd.)
  This study investigates how the phase sepn. induced by the biopolymers gelatin and gum arabic influences the microstructure of alginate beads prepd. in CaCl2 solns. and the subsequent rehydration of the air-dried beads. The extent of associative phase sepn. in mixt. gel beads can be controlled via pH. Compared with control beads, alginate/gelatin mixt. beads swelled faster at the initial stage of rehydration while slowed down at the late stage, reaching a lower equil. swelling ratio. The faster initial swelling kinetics can be attributed to the presence of gelatin which prevents the side-by-side aggregation of egg-box junctions. This conclusion was confirmed using wide angle X-ray diffraction (WAXD) measurements. The lower equil. swelling ratio was due to the gelatin network restricting alginate from further swelling. This was evidenced by temp. dependence swelling expts. and comparison with alginate/gum arabic beads where no addnl. network was formed on top of the alginate network. The varying of pH, correspondingly the change of phase sepn. extent, had a significant influence on the rehydration of mixt. gel beads. The best rehydratability was obsd. at higher pHs where no phase sepn. occurred and the components were homogeneously mixed. With decreasing pH, the associative phase sepn. between alginate and gelatin was promoted, and led to local over-concn. of alginate, which gave rise to poor rehydratability.
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  Oh, Y.; Lee, H.; Son, S.; Kim, S.; Kim, P. Capillarity Ion Concentration Polarization for Spontaneous Biomolecular Preconcentration Mechanism. Biomicrofluidics 2016, 10, 014102 DOI: 10.1063/1.4939434
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  Capillarity ion concentration polarization for spontaneous biomolecular preconcentration mechanism
  Oh, Yoonjee; Lee, Hyomin; Son, Seok Young; Kim, Sung Jae; Kim, Pilnam
  Biomicrofluidics (2016), 10 (1), 014102/1-014102/12CODEN: BIOMGB; ISSN:1932-1058. (American Institute of Physics)
  Ionic hydrogel-based ion concn. polarization devices have been demonstrated as platforms to study nanoscale ion transport and to develop engineering applications, such as protein preconcn. and ionic diodes/transistors. Using a microfluidic system composed of a perm-selective hydrogel, we demonstrated a micro/nanofluidic device for the preconcn. of biol. samples using a new class of ion concn. polarization mechanism called "capillarity ion concn. polarization" (CICP). Instead of an external elec. voltage source, the capillary force of the perm-selective hydrogel spontaneously generated an ion depletion zone in a microfluidic channel by selectively absorbing counter-ions in a sample soln. We demonstrated a reasonable preconcn. factor (∼100-fold/min) using the CICP device. Although the efficiency was lower than that of conventional electrokinetic ICP operation due to the absence of a drift ion migration, this mechanism was free from the undesirable instability caused by a local amplified elec. field inside the ion depletion zone so that the mechanism should be suitable esp. for an application where the contents were elec. sensitive. Therefore, this simple system would provide a point-of-care diagnostic device for which the sample vol. is limited and a simplified sample handling is demanded. (c) 2016 American Institute of Physics.
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Cells in New Light: Ion Concentration, Voltage, and Pressure Gradients across a Hydrogel Membrane

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Abstract

Introduction

Results and Discussion

Figure 1

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Alginate Hydrogel as a Permselective Membrane

Figure 4

Ion Exclusion Zone

Figure 5

Figure 6

Interfacial Water Ordering and Pressure Gradients

Cell-Like Features of Gels

Figure 7

Conclusions

Experimental Section

Alginate Gel Preparation

Experimental Setup

Conductivity

Electric Potential Difference

Pressure Gradient

Crystallization Experiments

Behavior of Natural Algae

Spectroscopic Experiments

Author Information

Acknowledgments

References

Cited By

Abstract

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Figure 7

References

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