Spatiotemporally programmed tissue regeneration via visible-light-responsive hydrogels: Biosafety-oriented design and...

Tissue engineering requires biomaterial systems that can be regulated with high spatiotemporal precision in complex physiological environments. Among stimulus-responsive biomaterials, photoresponsive hydrogels are especially attractive because they enable non-contact control over gelation, degradation, mechanics, and cargo release. However, most existing reviews discuss photoresponsive hydrogels as a broad class and pay limited attention to how different optical windows shape biological applicability. In particular, visible-light-responsive hydrogels have rarely been examined as a distinct category within a biosafety-oriented design framework, despite biosafety being central to their translational potential in tissue engineering. This distinction is important because light influences not only material transformation, but also cytocompatibility and in vivo feasibility. UV-responsive systems are often limited by shallow penetration and phototoxicity, whereas many near-infrared-responsive systems rely on indirect activation processes that may introduce thermal burden, formulation complexity, or reduced efficiency. By contrast, visible light offers a favorable balance between reaction efficiency, tissue accessibility, and biological tolerance, making it well suited for in situ gelation and localized regenerative therapy. In this review, we summarize the key photochemical mechanisms, material design strategies, and representative tissue engineering applications of visible-light-responsive hydrogels, and highlight current challenges and future directions from a biosafety-oriented perspective.