Hydrogels have emerged as promising scaffolds in regenerative medicine for the delivery of biomolecules to promote healing. However, increasing evidence suggests that the context that biomolecules are presented to cells (e.g., as soluble verses tethered signals) can influence their bioactivity. A common approach to deliver biomolecules in hydrogels involves physically entrapping them within the network, such that they diffuse out over time to the surrounding tissues. While simple and versatile, the release profiles in such system are highly dependent on the molecular weight of the entrapped molecule relative to the network structure, and it can be difficult to control the release of two different signals at independent rates. In some cases, supraphysiologically high loadings are used to achieve therapeutic local concentrations, but uncontrolled release can then cause deleterious off-target side effects. In vivo, many growth factors and cytokines are stored in the extracellular matrix (ECM) and released on demand as needed during development, growth, and wound healing. Thus, emerging strategies in biomaterial chemistry have focused on ways to tether or sequester biological signals and engineer these bioactive scaffolds to signal to delivered cells or endogenous cells. While many strategies exist to achieve tethering of peptides, protein, and small molecules, this review focuses on photochemical methods, and their usefulness as a mild reaction that proceeds with fast kinetics in aqueous solutions and at physiological conditions. Photo-click and photo-caging methods are particularly useful because one can direct light to specific regions of the hydrogel to achieve spatial patterning. Recent methods have even demonstrated reversible introduction of biomolecules to mimic the dynamic changes of native ECM, enabling researchers to explore how the spatial and dynamic context of biomolecular signals influences important cell functions. This review will highlight how two photochemical methods have led to important advances in the tissue regeneration community, namely the thiol-ene photo-click reaction for bioconjugation and photocleavage reactions that allow for the removal of protecting groups. Specific examples will be highlighted where these methodologies have been used to engineer hydrogels that control and direct cell function with the aim of inspiring their use in regenerative medicine.

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Photodriven click reactions have emerged as versatile tools for biomaterial synthesis that can recapitulate critical spatial and temporal changes of extracellular matrix (ECM) microenvironments in vitro. In this article, we report on the synthesis of poly(ethylene glycol) (PEG) hydrogels using photodriven step-growth polymerization, where one of the reactive functionalities is formed by a photocleavage reaction. Upon photocleavage, an aldehyde functionality is generated that rapidly reacts with hydrazinefunctionalized PEGs; the gelation kinetics and final material modulus are distinctly controlled by variations in the light intensity. This light-driven aldehyde generation is further exploited to install biochemical ligands in the hydrazone-based hydrogels with precise spatial control. We expect that userdirected spatial and temporal control over both biophysical and biochemical gel properties through photochemical reactions and photopatterning, respectively, should provide newfound opportunities to probe and understand dynamic cell−matrix interactions.