Printing double network tough hydrogels using Temperature-Controlled Projection Stereolithography (TOPS)

We report a new method to shape double-network (DN) hydrogels into customized microscale 3D structures that exhibit superior mechanical properties in both tension and compression. A one-pot prepolymer formulation containing photo-cross-linkable acrylamide and thermo-reversible sol-gel {kappa}-carrageenan with a suitable crosslinker, and photo-initiator/absorbers are optimized. A new TOPS system is utilized to photo-polymerize the primary acrylamide network into a 3D structure above the sol-gel transition of {kappa}-carrageenan (80{degrees}C), while cooling down generates the secondary physical {kappa}-carrageenan network to realize tough DN hydrogel structures. 3D structures, printed with high lateral (37m) and vertical (180m) resolutions and superior 3D design freedoms (internal voids), exhibit ultimate stress and strain of 200 kPa and 2400% respectively under tension, and simultaneously exhibit 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.