3D printing of hard/soft switchable hydrogels

Abstract 3D (three-dimensional) printing of soft/tough hydrogels has been widely used in flexible electronics, regenerative medicine, and other fields. However, due to their loose crosslinking, strong hydration and plasticizing effect of solvent (typically water) and susceptibility to swelling, the printed hydrogels always suffer from bearing compressive stress and shear stress. Here we report a 3D photo-printable hard/soft switchable hydrogel composite which is enabled by the phase transition (liquid/solid transition) of supercooled hydrated salt solution (solvents) within hydrogel. In hard status, it achieved a hardness of 86.5 Shore D (comparable to hard plastics), a compression strength of 81.7 MPa, and Young’s modulus of 1.2 GPa. These mechanical property parameters far exceed those of any currently 3D printed hydrogels. The most interesting thing is that the soft/hard states are easily switchable and this process can be repeated for many times. In the supercooled state, the random arrangement of liquid solvent molecules within hydrogels makes it as soft as conventional hydrogels. Upon artificial seeding of the crystal nucleus, the solvent in hydrogel undergoes rapid crystallization, resulting in the in-situ formation of numerous rigids, ordered rod-like nanoscale crystals uniformly embedded within the hydrogel matrix. This hierarchical structure remarkably enhances the Young’s modulus from kPa to GPa. Furthermore, the softness of hydrogel can be restored by heating and then cooling down to recover the supercooled state of the solvent. Taking advantage of soft/hard status switching, the hydrogel can conform to complex surface morphologies in its soft state and subsequently freeze that shape through crystallization, enabling rapid mold fabrication. Moreover, a shape fixation and recyclable smart hydrogel medical plaster bandage was also developed, capable of conforming the limb shapes and providing adequate support for the bone fracture patients after 10 min of crystallization. Our work suggests a bright future for the direct use of hard hydrogel as a robust industrial material.