Ionic Control of Crack Propagation in Biopolymer Hydrogels

Alginate hydrogels stabilized by ionic interactions, fracture via crosslink unzipping and chain pull-out. It is found that, allowing non-binding ions to diffuse from a drop of brine into the crack tip region, leads to a strong acceleration of the crack. This is ascribed to the exchange between binding (Ca2+) cations and non-binding ones, which facilitates the opening of unit chelating cages. The resulting lowering of the effective energy barrier is found merely entropic. The ion-exchange can be modelled as a rate-limited kinetic process, the order of which is fixed by the electroneutrality requirement, as checked by comparing the effect of monovalent and divalent cations. Although the embrittlement induced by a ionic shock could hinder the use of alginate gels in physiological environments where structural integrity and load-bearing capacity are required, it however can be thought as a powerful analytic tool for studying the nature and spatial extent of the dissipative mechanism at work when fracturing ultra-tough double network (including a ionic one) gels.