(45a) A Hydrogel-Based Cell Culture Platform with Reversible Stiffening Via an Azobenzene-Containing Cross-Linker

The mechanical stiffness of the extracellular matrix is a dynamic property that changes during wound healing, disease progression, aging, etc.  For example, tissue stiffness increases during fibrosis, and increased stiffness may contribute to a positive feedback loop that further accelerates the disease progression. Until recently, many cell culture platforms exhibited static material properties, which required replated cells to study the effects of different elastic moduli on cell phenotype. While much progress has been made in the development of substrates that can either increase or decrease in stiffness in situ, there are fewer examples of substrates that can both stiffen and soften, which may be important for simulating the effects of disease progression and resolution. Here, we present a hydrogel-based system that can reversibly stiffen and soften using an azobenzene photoisomer in the crosslinker. To form the hydrogel, poly(ethylene glycol) macromers were crosslinked with an azobenzene-containing peptide using Michael addition chemistry in phosphate buffered saline. We find that upon irradiation with cytocompatible doses of UV light (365 nm, 10 mW/cm², 2 min), the photoisomer induces a length change in the crosslinker and a concomitant stiffening of the hydrogel by 100-200 Pa. The resulting change in gel properties is maintained for days due to the long half-life of the photoisomer. The initial modulus of the gel can be recovered upon irradiation with visible light (400-500 nm, 10 mW/cm², 2 min). The developed hydrogels should enable noninvasive control of substrate modulus independent of changes in the network connectivity, allowing us to probe questions about the effect of dynamic changes in matrix stiffness on cell behavior.