Complex mechanical design of bio-inspired model transient network hydrogels

Abstract

The mechanical properties of viscoelastic soft materials are strongly time-dependent, such that we must describe their mechanical properties with material functions. This is an inherently difficult problem for materials scientists: typically,we define structure-property relationships in terms of scalar material properties, such that modifying a material's structure affects a target material property. However, if the property of interest is function-valued, modifying the material's structure may affect different parts of the material function in undesirable ways. The increased dimensionality of the target material property therefore renders the overall materials design problem for soft materials significantly more difficult. Recently, transient interactions have been shown to vastly improve the mechanical properties of soft materials by providing increased energy dissipation through the dissociation of the reversible bonds. However, there is a wide variety of transient interactions to choose from, varying widely in binding strength, kinetics, specificity, and stoichiometry of the groups that form the association. More research needs to be done to identify what physical laws apply universally across the types of transient associations, and what differentiates the abilities of different types of interactions to control material mechanics. In this thesis,we show how transient metal-coordinate bonds inspired by the chemistry of the mussel byssal threads can be used to engineer viscoelastic material functions in an intuitive and facile manner. We show that intelligent understanding of the thermodynamics and kinetics of metal-coordinate complexes allows quasi-independent control over different regimes of the viscoelastic material function. We draw from classical polymer physics and metal-coordinate chemistry to show that our 4-arm polyethylene glycol-based hydrogels crosslinked with transient histidine:metal bonds represent a uniquely ideal system for probing fundamental questions in how the properties of transient interactions affect viscoelastic material functions. In the final part of this thesis, we extend our control over the viscoelastic material functions of hydrogels by exploiting the redox-sensitivity of histidine:metal crosslinks. In this way, we show how histidine:metal interactions are perhaps more versatile than other types of transient interactions, promising a facile way to examine structure-property relationships in transient networks.