Enhanced Mechanophore Activation in Hydrogel Networks Driven by Swollen Network Pretension

There is often a discrepancy between the strain required to activate mechanophores and incorporation in bulk materials, inhibiting these force sensors from many practical, commercial, and biological uses. The difference is particularly pronounced for biomimetic networks such as viscoelastic hydrogels, in which the distribution of strain is unclear due to dissipation. Here, we show that the activation of spiropyran mechanophores in alginate networks is related to cross-linking characteristics by comparing ionic and covalent bonds. Through a simple shear force setup using syringes and Bernoulli's principle, we observe higher activation of spiropyran in ionic gels, regardless of mechanical versus ultraviolet light stimulus. This may be predominantly driven by differences in network pretension due to swelling, as the ring-closing reaction of merocyanine to spiropyran was similarly affected. These insights shed new light on understanding force propagation in complex networks, leading to higher mechanophore sensitivities in biologically similar materials.