Surface Grafted Chitosan Gels. Part II. Gel Formation and Characterization
Abstract

Responsive biomaterial hydrogels attract significant attention due to their biocompatibility and degradability. In order to make chitosan based gels, we first graft one layer of chitosan to silica, and then build a chitosan/poly(acrylic acid) multilayer using the layer-by-layer approach. After cross-linking the chitosan present in the polyelectrolyte multilayer, poly(acrylic acid) is partly removed by exposing the multilayer structure to a concentrated carbonate buffer solution at a high pH, leaving a surface-grafted cross-linked gel. Chemical cross-linking enhances the gel stability against detachment and decomposition. The chemical reaction between gluteraldehyde, the cross-linking agent, and chitosan was followed in situ using total internal reflection Raman (TIRR) spectroscopy, which provided a molecular insight into the complex reaction mechanism, as well as the means to quantify the cross-linking density. The amount of poly(acrylic acid) trapped inside the surface grafted films was found to decrease with decreasing cross-linking density, as confirmed in situ using TIRR, and ex situ by Fourier transform infrared (FTIR) measurements on dried films. The responsiveness of the chitosan-based gels with respect to pH changes was probed by quartz crystal microbalance with dissipation (QCM-D) and TIRR. Highly cross-linked gels show a small and fully reversible behavior when the solution pH is switched between pH 2.7 and 5.7. In contrast, low cross-linked gels are more responsive to pH changes, but the response is fully reversible only after the first exposure to the acidic solution, once an internal restructuring of the gel has taken place. Two distinct pKa’s for both chitosan and poly(acrylic acid), were determined for the cross-linked structure using TIRR. They are associated with populations of chargeable groups displaying either a bulk like dissociation behavior or forming ionic complexes inside the hydrogel film.
1 Introduction
2 Materials and Methods
2.1 Materials
2.2 Preparation of Surface-Grafted Chitosan Gels
2.3 Characterization Methods
2.3.1 Total Internal Reflection Raman (TIRR) Spectroscopy
2.3.2 Transmission Fourier Transform Infrared Spectroscopy
2.3.3 Atomic Force Microscopy
2.3.4 Quartz Crystal Microbalance with Dissipation
(1)where C is the mass sensitivity factor which equals 17.7 ng Hz–1 cm–2 for our sensors and n is the overtone number (1, 3, 5, 7...). The acoustic shear waves associated with the different overtones penetrate the solution outside the sensor to different depths, with a decay length ranging from 140 nm (n = 3) to 65 nm (n = 13) in water at 25 °C. (25)
(2)where ρ is the film density, d is the thickness, ω is the angular frequency, and J is the viscoelastic compliance. This model is appropriate when the viscoelastic compliance is frequency independent, and in such a case the sensed mass, mj, is obtained from the intercept of the ms versus ω2 plot at zero frequency.
(4)3 Results and Discussion
3.1 Stability of the Multilayer Film Prior to Cross-Linking
3.2 Cross-Linking of the Chitosan/Poly(acrylic acid) Multilayer
Figure 1

Figure 1. Molecular structure of (a) glutaraldehyde in its monomeric form, (b) amine groups of chitosan cross-linked by a simple Schiff base on both ends of the monomeric glutaraldehyde (unstable under acidic conditions), and (c) one of the likely cross-linking reaction products between a polymeric form of glutaraldehyde and chitosan, which allow for multiple connections with chitosan moieties (note the conjugate system consisting of a Schiff base and an adjacent ethylenic double bond).
3.2.1 Cross-Linking Reaction Kinetics Obtained from TIRR
Figure 2

Figure 2. (a) Selected TIRR spectra during the cross-linking reaction of chitosan with glutaraldehyde. Contributions from the chitosan/PAA multilayer have been removed by subtracting the spectrum of the multilayer prior to the addition of glutaraldehyde. Fluorescence, which gave rise to sloping backgrounds in the early stages of the reaction, was also manually subtracted. Note the break in the spectrum between 1775 cm–1 and 2825 cm–1. (b) Relative cross-linking density (fraction of imine groups in the grafted gel) as a function of time estimated from the C–H stretching mode of the hydrogen attached to the unsaturated carbon forming the Schiff base (see text for details). Data originates from analysis of 440 spectra collected in a 24 h period.
3.3 Formation of Surface Grafted Chitosan Gels upon Removal of PAA
3.3.1 Molecular Information Obtained from TIRR
Figure 3

Figure 3. TIRR spectra of (a) the multilayer film consisting of 11 layers of chitosan and 10 of PAA prior to cross-linking, (b) LC-gel equivalent to a cross-linking density of 40%, and (c) HC-gel with a 100% relative cross-linking density measured in 30 mM NaCl at pH 5.7. Note the break in the spectra between 1825 cm–1 and 2650 cm–1. For ease of comparison, the spectra at low and high frequency ranges are reported using two different absolute scales.
3.3.2 Quantification of the Amount of PAA Trapped in the Surface Grafted Gels Using FTIR
Figure 4

Figure 4. FTIR spectra of (a) a dried LC-gel and (b) a dried HC-gel. The upper spectra are measured before carbonate buffer rinsing, and the bottom spectra are measured after carbonate buffer rinsing. (c) Fraction of PAA removed by exposing the cross-linked multilayer structure to 0.2 M carbonate buffer at pH 9.2 as a function of cross-linking density.
(3)where Γ is the surface excess, c is the concentration, A is the absorbance, ε is the extinction coefficient, and l is the path length. Since the absorbance is proportional to the peak area, IA, the relative amount of PAA in the gel can be estimated from eq 4, using the reported extinction coefficient ratio between the asymmetric carboxylate and carboxylic bands, ε(νas(COO–))/ε(ν(C═O)), of 2.4. (38)
(4)3.3.3 Mass and Water Content of Gel Films
| mdry (mg/m2) | mwet,S (mg/m2) | mwet,J (mg/m2) | mwet,V (mg/m2) | Xwa | |
|---|---|---|---|---|---|
| LC-gel | 57 ± 5 | 112 ± 5 | 117 ± 5 | 127 ± 5 | 0.55 ± 0.02 |
| HC-gel | 83 ± 5 | 134 ± 5 | 143 ± 5 | 155 ± 5 | 0.47 ± 0.02 |
The water content, Xw was calculated based on the sensed mass obtained with the Voigt model.
3.3.4 Topography of Surface Grafted Gels
Figure 5

Figure 5. AFM topography images (10 × 10 μm2) of the LC-gel (left) and HC-gel (right) determined in air. The insets show zoomed in sections of 2 × 2 μm2, and height scan lines over the regions marked with a straight line are also provided. Samples were studied ex situ.
3.4 Stimuli Responses
3.4.1 Responses to pH Changes
Figure 6

Figure 6. Frequency and dissipation change as a function of pH during repeated changes between pH 5.7 and 2.7 measured in 30 mM NaCl. Data for the LC-gel (a) and for the HC-gel (b). Black points represent frequency, and blue open squares represent dissipation.
Figure 7

Figure 7. TIRR spectra of the LC-gel during the first pH cycle: 5.7 in black →2.7 in red →5.7 in green. Background electrolyte: 30 mM NaCl solution. Note the break in the spectra between 1850 cm–1 and 2600 cm–1. Contributions from the silica substrate and water molecules in the evanescent field have been subtracted using as reference the spectrum of the first adsorbed layer of chitosan.
3.4.2 Determining the pKa Values of the LC-Gel Using TIRR
(5)where, β1 and β2 are the fraction of chargeable groups having a pKa1 and a pKa2 value, respectively. The determined parameters are summarized in Table 2.Figure 8

Figure 8. Relative proportion of (a) amine (NH2) groups in chitosan and (b) carbonyl (blue triangles) and carboxylate (black squares) groups in PAA as a function of pH (see text for details). Solid curves are fits to the data calculated using eq 5.
| chitosan | PAA | ||
|---|---|---|---|
| pKa1 | 6.2 ± 0.3 | pKa1 | 2.8 ± 0.3 |
| β1(pKa1) | 0.20 ± 0.03 | β2(pKa1) | 0.5 ± 0.1 |
| pKa2 | 9.0 ± 0.1 | pKa2 | 5.3 ± 0.3 |
| β2(pKa2) | 0.80 ± 0.03 | β2(pKa2) | 0.5 ± 0.1 |
The fraction of chargeable groups, βx (pKax), having a specific pKax are also shown in the table. The values for PAA are obtained by averaging the individual results from the carbonyl and carboxylate bands.
3.4.3 Response to Ionic Strength Changes
Figure 9

Figure 9. Changes in frequency and dissipation as a result of changes in ionic strength for a LC-gel layer (a) and a HC-gel layer (b) at pH 2.7. The ionic strength was regulated by addition of NaCl. Black points represent frequency, and blue points represent dissipation.
4 Summary and Concluding Remarks
Supporting Information
Bulk Raman spectra of glutaraldehyde, determination of the dry and wet mass of the surface grafted gels using QCM, estimating the gel pKa’s using TIRR, and swelling behavior of the LC-gel at basic pHs. This material is available free of charge via the Internet at http://pubs.acs.org/.
Terms & Conditions
Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.
Acknowledgment
All authors acknowledge individual financial support from the Swedish Research Council, VR. E.Th. and E.Ty. also acknowledge support from the Swedish Foundation for Strategic Research (SSF) through the programs “Microstructure, Corrosion and Friction Control” and “Future Research Leaders-5”, respectively.
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Abstract

Figure 1

Figure 1. Molecular structure of (a) glutaraldehyde in its monomeric form, (b) amine groups of chitosan cross-linked by a simple Schiff base on both ends of the monomeric glutaraldehyde (unstable under acidic conditions), and (c) one of the likely cross-linking reaction products between a polymeric form of glutaraldehyde and chitosan, which allow for multiple connections with chitosan moieties (note the conjugate system consisting of a Schiff base and an adjacent ethylenic double bond).
Figure 2

Figure 2. (a) Selected TIRR spectra during the cross-linking reaction of chitosan with glutaraldehyde. Contributions from the chitosan/PAA multilayer have been removed by subtracting the spectrum of the multilayer prior to the addition of glutaraldehyde. Fluorescence, which gave rise to sloping backgrounds in the early stages of the reaction, was also manually subtracted. Note the break in the spectrum between 1775 cm–1 and 2825 cm–1. (b) Relative cross-linking density (fraction of imine groups in the grafted gel) as a function of time estimated from the C–H stretching mode of the hydrogen attached to the unsaturated carbon forming the Schiff base (see text for details). Data originates from analysis of 440 spectra collected in a 24 h period.
Figure 3

Figure 3. TIRR spectra of (a) the multilayer film consisting of 11 layers of chitosan and 10 of PAA prior to cross-linking, (b) LC-gel equivalent to a cross-linking density of 40%, and (c) HC-gel with a 100% relative cross-linking density measured in 30 mM NaCl at pH 5.7. Note the break in the spectra between 1825 cm–1 and 2650 cm–1. For ease of comparison, the spectra at low and high frequency ranges are reported using two different absolute scales.
Figure 4

Figure 4. FTIR spectra of (a) a dried LC-gel and (b) a dried HC-gel. The upper spectra are measured before carbonate buffer rinsing, and the bottom spectra are measured after carbonate buffer rinsing. (c) Fraction of PAA removed by exposing the cross-linked multilayer structure to 0.2 M carbonate buffer at pH 9.2 as a function of cross-linking density.
Figure 5

Figure 5. AFM topography images (10 × 10 μm2) of the LC-gel (left) and HC-gel (right) determined in air. The insets show zoomed in sections of 2 × 2 μm2, and height scan lines over the regions marked with a straight line are also provided. Samples were studied ex situ.
Figure 6

Figure 6. Frequency and dissipation change as a function of pH during repeated changes between pH 5.7 and 2.7 measured in 30 mM NaCl. Data for the LC-gel (a) and for the HC-gel (b). Black points represent frequency, and blue open squares represent dissipation.
Figure 7

Figure 7. TIRR spectra of the LC-gel during the first pH cycle: 5.7 in black →2.7 in red →5.7 in green. Background electrolyte: 30 mM NaCl solution. Note the break in the spectra between 1850 cm–1 and 2600 cm–1. Contributions from the silica substrate and water molecules in the evanescent field have been subtracted using as reference the spectrum of the first adsorbed layer of chitosan.
Figure 8

Figure 8. Relative proportion of (a) amine (NH2) groups in chitosan and (b) carbonyl (blue triangles) and carboxylate (black squares) groups in PAA as a function of pH (see text for details). Solid curves are fits to the data calculated using eq 5.
Figure 9

Figure 9. Changes in frequency and dissipation as a result of changes in ionic strength for a LC-gel layer (a) and a HC-gel layer (b) at pH 2.7. The ionic strength was regulated by addition of NaCl. Black points represent frequency, and blue points represent dissipation.
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Supporting Information
Supporting Information
ARTICLE SECTIONSBulk Raman spectra of glutaraldehyde, determination of the dry and wet mass of the surface grafted gels using QCM, estimating the gel pKa’s using TIRR, and swelling behavior of the LC-gel at basic pHs. This material is available free of charge via the Internet at http://pubs.acs.org/.
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