In Situ Programming of Shape-Morphing Hydrogels via Vat Photopolymerization for 4D Bioprinting.

The fabrication of complex architectures remains a central challenge in 3D bioprinting, as the low mechanical properties of hydrogels limit the range of achievable geometries. Four-dimensional (4D) bioprinting can address these limitations by enabling programmed shape-morphing behavior; however, in most approaches, this functionality is introduced after hydrogel formation, limiting the complexity of the resulting deformation. Here, a proof-of-concept strategy is presented, in which shape-morphing is directly encoded during fabrication. By modulating light exposure time layer-by-layer in vat photopolymerization, spatial variations in crosslinking density are introduced in situ within Gelatin Methacryloyl (GelMA) hydrogel constructs. Exposure times in the range of 20-70 s were investigated, enabling controlled bending of the printed structures upon immersion in aqueous media, with radii of curvature between 11 and 20 mm depending on the geometry. This approach allows deformation pathways to be programmed during printing, without requiring additional materials or post-processing steps. The morphing behavior was further supported by finite element simulations, which reproduced the experimentally observed deformation and enabled prediction of the shape change. In addition, high cell viability (>95%) was maintained after material contact and UV exposure. Overall, this study demonstrates that swelling-driven actuation can be encoded during fabrication. Although demonstrated on simplified geometries, this approach provides a versatile framework for process-driven shape-morphing and represents a step toward more spatially resolved and potentially volumetric 4D bioprinting strategies.