Injectable solid hydrogel: mechanism of shear-thinning and immediate recovery of injectable β-hairpin peptide hydrogelsThis paper is part of a joint Soft Matter and Journal of Materials Chemistry themed issue on Tissue Engineering. Guest editors: Molly Stevens and Ali Khademhosseini.Electronic supplementary information (ESI) available: Additional rheology results (gelation kinetics and gel restoration kinetics) and small-angle neutron scattering measurements. See DOI: 10.1039/c0sm00642d

β-Hairpin peptide-based hydrogels are a class of injectable hydrogel solids with significant potential use in injectable therapies. β-hairpin peptide hydrogels can be injected as preformed solids, because the solid gel can shear-thin and consequently flow under a proper shear stress but immediately recover back into a solid on removal of the stress. In this work, hydrogel behavior during and after flow was studied in order to facilitate fundamental understanding of how the gels flow during shear-thinning and how they quickly recover mechanically and morphologically relative to their original, pre-flow properties. While all studied β-hairpin hydrogels shear-thin and recover, the duration of shear and the strain rate affected both the gel stiffness immediately recovered after flow and the ultimate stiffness obtained after complete rehealing of the gel. Results of structural analysis during flow were related to bulk rheological behavior and indicated gel network fracture into large (>200 nm) hydrogel domains during flow. After cessation of flow the large hydrogel domains are immediately percolated which immediately reforms the solid hydrogel. The underlying mechanisms of the gel shear-thinning and healing processes are discussed relative to other shear-responsive networks like colloidal gels and micellar solutions. Hydrogel behavior during and after flow was studied to elucidate the underlying mechanisms of the gel shear-thinning and healing processes.