On the use of modelling antagonistic enzymes to aid in temporal programming of pH and PVA–borate gelation†

Feedback through enzyme reactions creates new possibilities for the temporal programming of material properties in bioinspired applications, such as transient adhesives; however, there have been limited attempts to model such behavior. Here, we used two antagonistic enzymes, urease in watermelon seed powder and esterase, to temporally control the gelation of a poly(vinyl alcohol)–borate hydrogel in a one-pot formulation. Urease produces base (ammonia), and esterase produces acid (acetic acid), generating a pH pulse, which was coupled with reversible complexation of PVA. For improved understanding of the pulse properties and gel lifetime, the pH profile was investigated by comparison of the experiments with kinetic simulations of the enzyme reactions and relevant equilibria. The model reproduced the general trends with the initial concentrations and was used to help identify conditions for pulse-like behaviour as the substrate concentrations were varied.