4D Printing of a Sodium Alginate Hydrogel with Step-Wise Shape Deformation Based on Variation of Crosslinking Density

Stimuli-responsive hydrogels with tailored mechanical performances and tunable responsiveness are highly demanded in many fields, including actuators, sensors, drug delivery systems, etc. Herein, we prepared a 4D printing methacrylated alginate (SA-MA) hydrogel, and the effect of the degree of methacrylation on 3D printability, crosslinking density, and responsiveness to Ca2+/chitosan crosslinking were systematically studied. The 4D-printed SA-MA hydrogels perform a step-wise volume contraction and mechanical enhancement when sequentially immersed in Ca2+ and chitosan solutions due to the crosslinking increase. Based on this, a bilayer of SA-MA hydrogel with different chemical crosslinking densities performed an anisotropic shape change by Ca2+ soaking. Furthermore, the 4D-printed SA-MA hydrogel with gradient crosslinking density shows multiple deformations by two-step ionic immersion. This controllable shape change provides a strategy for well-defined structure manufacturing.