Printing Double-Network Tough Hydrogels Using Temperature-Controlled Projection Stereolithography (TOPS)

We report a new method to shape double-network (DN) hydrogels into customized 3D structures that exhibit superior mechanical properties in both tension and compression. A one-pot prepolymer formulation containing photo-cross-linkable acrylamide and thermoreversible sol–gel κ-carrageenan with a suitable cross-linker and photoinitiators/absorbers is optimized. A new TOPS system is utilized to photopolymerize the primary acrylamide network into a 3D structure above the sol–gel transition of κ-carrageenan (80 °C), while cooling down generates the secondary physical κ-carrageenan network to realize tough DN hydrogel structures. 3D structures, printed with high lateral (37 μm) and vertical (180 μm) resolutions and superior 3D design freedoms (internal voids), exhibit ultimate stress and strain of 200 kPa and 2400%, respectively, under tension and simultaneously exhibit a high compression stress of 15 MPa with a strain of 95%, both with high recovery rates. The roles of swelling, necking, self-healing, cyclic loading, dehydration, and rehydration on the mechanical properties of printed structures are also investigated. To demonstrate the potential of this technology to make mechanically reconfigurable flexible devices, we print an axicon lens and show that a Bessel beam can be dynamically tuned via user-defined tensile stretching of the device. This technique can be broadly applied to other hydrogels to make novel smart multifunctional devices for a range of applications.


Dimension of the sample holder
The dimensions associated with the geometry are provided in Table 1.

Heat Distribution Simulation
It was important to maintain a critical temperature of the solution throughout the fabrication process, hence we designed a CAD model of the sample holder and studied the temperature distribution using simulations. The design of the sample holder consisted of a copper plate with a center hole embedded inside the PDMS bath ( Figure S1). Two heated rods on either side of the dish were designed to heat the copper plate and the 16mm diameter hole in the Cu plate acted as the fabrication window. To gain a better insight into the temperature distribution over the PDMS layer of the sample holder design, we performed computational fluid dynamics (CFD) simulation with conjugate heat transfer. The computational domain consisted of the designed PDMS dish and a copper plate extended on either side of the dish (Figure S1). At the top and bottom surface of the copper plate, a constant temperature boundary condition with T = 416 K (142.85°C) was applied to mimic the heater (used in an experimental study). All other surfaces of the geometry were provided with convection heat transfer to ambient temperature (T amb = 300 K (26.85°C).
The simulation was performed by discretizing the computational domain into a finite number of control volumes (or grid points) and by simultaneously solving the physical equations of continuity, momentum, and energy, in each point to obtain the spatial and temporal distribution of temperature. For the computational domain, a mesh with 160,000 grid points was utilized to obtain the temperature distribution ( Figure S2). The distribution was obtained for various time points until the steady state is attained. After the simulation study, the mesh was refined, and the simulation was repeated for meshes with 200,000, 240,000, and 280,000 grid points to investigate the grid sensitivity of the result. By comparing temperature distribution over the PDMS layer for different meshes, one with 240,000 grid points was found to be an optimum mesh which is utilized for further study here.
Finite volume methods-based commercial solver ANSYS Fluent was utilized to solve the equations. In the simulation, the entire domain was first initialized with T init = 353 K (79.85 °C), and the ambient was set at 300 K (26.85°C). The operating pressure was 1 atm. Figure S3 shows the temperature distribution over the PDMS layer at different time instants obtained using the optimum mesh. At approximately t = 90 s, the spatial distribution of temperature attains a steady state and further continuing the simulation shows no changes in the state. Figure S2. Temperature distribution over the PDMS layer at the plane corresponding to sections A-A' shown in Figure S1.

Swelling of DN gel structures
DN cylindrical stubs printed via TOPS were immersed in DI water. Results show that stub diameters and heights increased by 21% during the first hour, reached 45% in one day, and saturated after 78 hrs ( Figure S8). In terms of mass, the printed structure (right after the printing) weighed 0.5 gm and the structure absorbed 7.9 gm of water, which is 17 times the original mass.
Total water content before swelling was 81% and this increased to 98.8% after immersing the structure in water for 78 hours.

Influence of swelling on tensile and compression properties
Tensile properties of DN dogbone samples swollen for 5 min, 10 min, 4hrs, and 4 days were studied. The 4-day swollen sample was too soft to reliably handle, and hence it was omitted from this study. Results show that the ultimate stress, the ultimate strain, and the modulus were highest for samples swelled for 5 minutes as compared to samples swelled for 10 minutes and 4 hours. Longer exposure time during TOPS printing resulted in lesser swelling and therefore exhibited better mechanical properties. For instance, structures exposed for 2 minutes swelled 1.28 times of original length in 4 hours, whereas the structure exposed for 1 minute swelled 1.85 times its length at the same time. Longer light exposure during TOPS, when swelled for 4 hours withstood the larger ultimate stress of 14±2 kPa and modulus 14.95±3.7 kPa. These parameters were 5.5±0.5 kPa and 7.68±0.9 kPa for the structure exposed for 1 minute. Further, the strain was almost double (8.05±1.45) for longer exposed structures ( Figure S10). Figure S10. Stress-strain plots obtained from the swelled structures printed using different exposure times (60 seconds and 120 seconds). Structures were swelled for 4 hours in water.
Ultimate stress, ultimate strain, position, and elastic moduli obtained from swelled structures printed using different exposure times.

Effects of hydration, dehydration, and rehydration
13 The ability of the printed structure to recover after dehydration followed by rehydration was tested. As-printed DN dogbone structure, dried for 2 days using a dehumidifier, was rehydrated in water for 30 mins until the size reaches 1.3 times the size of the as-printed structure. Tensile tests showed that these samples regained their ultimate stress (24.5±1.5 kPa), ultimate strain (7.675±0.205). Similar results were obtained when the as-printed samples were first hydrated completely, dehydrated completely, and rehydrated to 1.3 times the size of as-printed samples with ultimate stress (24±0 kPa), ultimate strain (9±0.6), and modulus (11.11±0.12 kPa) ( Figure S11). Lens stretcher Figure S15. CAD design of before and after assembly of axicon lens stretching device.