The Robust Hydrogel Hierarchically Assembled from a pH Sensitive Peptide Amphiphile Based on Silk Fibroin

Supramolecular polymers can be formed by self-assembly of designed subunits to yield highly ordered materials. In this paper, hierarchically structured materials, from molecules to nanofibers to macroscopical hydrogel, were fabricated by pH-induced assembly of C₁₂-GAGAGAGY, a peptide amphiphile (PA) based on silk fibroin. Due to the different acid dissociation constants of the carboxyl and phenolic hydroxyl groups on tyrosine residue (Y), the PAs showed unique pH sensitive assembly and aggregation behaviors. It was found that not only the molecular-scale assemblies of these PAs gradually changed from cylindrical nanofibers to nanoribbons with the decreasing of pH value from 11 to 8 but also most of nanoribbons aggregated into parallel bundles in such a case. Further decrease of pH value resulted in a hierarchically structured robust and plastic hydrogel, of which the rheological moduli reached around 10⁵ Pa. Moreover, noodle-like hydrogel fibers with bundles of nanoribbons aggregated parallel along the long axis in them could be steadily prepared under shear force. Taking the pH-sensitive reversible sol–gel transition, high modulus and plasticity into account, the hydrogel is believed to have significant potential applications in tissue engineering or as the biocompatible adhesives