Photoinitiator-Free, Visible Light-Crosslinking Hydrogel with Tuneable Properties for 3D Bioprinting

Abstract Photo-crosslinkable hydrogels are widely employed in biofabrication and tissue engineering as they provide spatiotemporal control over gelation. Most conventional photo-curing hydrogel systems rely upon small-molecule photoinitiators which, upon activation by (ultra)violet irradiation, generate free radicals to initiate polymerization. Such an approach can induce oxidative stress and DNA damage, significantly limiting their use in sensitive biological applications. Here, we present a photoinitiator-free, gelatin-based hydrogel system functionalized with acrylamidylpyrene groups (Gel-Pyr), which undergoes photocrosslinking via a visible-light-induced [2+2] cycloaddition reaction. Gel-Pyr exhibits rapid gelation kinetics, tuneable mechanical properties, facile temporal control over photocrosslinking, and long-term structural stability (>30 days) in cell culture conditions. Rheological analyses reveal pronounced shear-thinning behaviour at room temperature, enabling extrusion-based 3D bioprinting of multilayered constructs with high structural fidelity. Fine strand resolution (<400 µm) is achieved in bioprinted crosshatch structures, enabling sufficient nutrient diffusion for cell support. Encapsulation of human mesenchymal stem cells (hMSCs) within both bulk and printed constructs maintains >80% viability over 7 days, demonstrating robust cytocompatibility. By eliminating UV exposure and free radicals, this visible-light-responsive hydrogel platform offers a facile and cytoprotective alternative to other hydrogel systems. Table of Content A visible light-crosslinkable, initiator-free gelatin-based hydrogel (Gel-Pyr) is developed using acrylamidylpyrene functionalization. This radical-free system enables rapid crosslinking under cytocompatible reaction conditions and offers excellent printability, tuneable mechanics, and long-term stability. Gel-Pyr supports high cell viability and precision bioprinting, positioning it as a promising platform for tissue engineering and in vitro biofabrication. Competing Interest Statement The authors have declared no competing interest.