3D Printing of Anisotropic Ultra‐Soft Hydrogels via In Situ Crystallization

ABSTRACT 3D printing of hydrogels is widely applied in regenerative medicine and flexible electronics. However, due to their ultra‐soft properties, they are susceptible to deformation under gravity, reducing printing fidelity. Herein, we propose a novel strategy for printing such hydrogels by temporarily enhancing their mechanical properties via supersaturation‐driven in situ crystallization, followed by photo‐cross‐linking and crystal removal through washing to restore their inherent softness. To achieve this, a supersaturation phase‐change additive, sodium acetate trihydrate(SAT), is incorporated into hydrogel inks. Upon extrusion, hydrogel inks undergo instantaneous crystallization, providing sufficient mechanical support for layer‐by‐layer construction. Interestingly, due to the intrinsic supersaturation and needle‐like crystal habit of SAT, the crystal alignment—radial, unidirectional, or multi‐directional—is effectively controlled by pre‐printed seeding templates (point, linear, polygonal, and planar) and printing paths. Using this method, high‐fidelity, large‐scale, and anisotropic ultra‐soft hydrogels with a minimum Young's modulus of 243 Pa are achieved, representing a 12 345‐fold reduction compared to that before washing. After lyophilization, hydrogels are converted into aerogels with unidirectional aligned pore structure, resulting in anisotropic ultralow thermal conductivity (15.9 mW/(m·K) transverse to alignment) and excellent thermal insulation performance. This study offers a versatile platform for precise printing of ultra‐soft hydrogels, highlighting its promising potential for next‐generation soft devices.