Shape-morphing photo-crosslinked alginate hydrogels via digital light processing 4D printing

The development of smart biomaterials capable of dynamic shape transformation is a key challenge in advancing 4D printing for biomedical applications. In particular, hydrogel systems must combine biocompatibility with precise control over volumetric changes to enable functionalities such as tissue scaffolds, responsive implants, or drug delivery devices. In this study, methacrylated alginate was prepared for 3D photo-crosslinked hydrogels using digital light processing (DLP). Hydrogels were fabricated with either uniform isotropic or heterogeneous anisotropic photo-crosslinking patterns. A secondary level of physical crosslinking was introduced through coordination of alginate guluronate blocks with Ca²⁺ ions, resulting in controlled hydrogel contraction. Volumetric changes, water uptake, and compression properties were first investigated on isotropic hydrogels with varying degrees of photo-crosslinking. Results showed that increasing the degree of photo-crosslinking restricted swelling in phosphate buffered saline (PBS) and contraction in calcium solution, due to reduced mesh sizes within the hydrogel network. Furthermore, anisotropic photo-crosslinking enabled spatially programmed volumetric deformations, allowing tailored shape morphing upon swelling or contraction. This effect was demonstrated in 3D-printed films with gradient crosslinking, which exhibited bidirectional bending when alternately exposed to PBS and CaCl₂ solution. More complex deformations were achieved in tubular structures and porous cubes by designing specific crosslinking gradients. Finally, cytocompatibility assays performed on fibroblast NIH 3T3 cells confirmed the non-cytotoxicity of the 4D-printed devices, highlighting their potential for biomedical applications.