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.