| Amphiphilic diblock copolypeptides that are 200 amino acids in length with a hydrophilic lysine (K) block and a hydrophobic leucine (L) block assemble into stiff, porous hydrogels at low volume fractions of polymer (<0.5 wt%) and neutral pH. When assembled quickly in pure water, the polypeptides form rigid hydrogels with bulk mechanical properties that are tunable based on molecular design, i.e. chain length, choice of amino acid, and hydrophilic to hydrophobic block ratio, and solution conditions such as salt concentration. Hydrogels assemble via the formation of membranes on the nanoscale due to hydrophobic interactions that interconnect to create an innately porous network on both the nano- and microscale. The porous morphology, high modulus, and peptidic foundation of these hydrogels make them intriguing candidates for biomaterials applications. Biocompatibility studies of these peptide-based materials revealed that anionic polypeptide hydrogels are not cytotoxic to mammalian cells.; In an effort to understand the overall assembly mechanism, polypeptides were assembled from miscible organic/water mixtures to slow the kinetics of assembly. Prior suspension of the block copolypeptides in organic cosolvent enabled greater copolypeptide chain mobility that, after water addition and subsequent removal of the organic via evaporation, provides for a more regular packing of the copolypeptide molecules and, more importantly, for the controllability of the assembled morphology. At the fastest organic evaporation rate (∼6 hours), the copolypeptides assembled into weak hydrogels with a homogeneous microstructure. Slowing the evaporation rate to ∼24 hours, and hence the kinetics of assembly, produced regular, non-interconnected fibrils. Longer evaporation times, ∼2 days, led to the formation of hexagonal platelets. Interestingly, copolypeptides with a racemic copolymer leucine block did not assemble into any regular nanostructures, indicating that the secondary structure of the leucine block is critical for assembly. In all assemblies, regardless of the resulting nanostructure, the secondary structure of the hydrophobic block remains an alpha-helix.; Characterization of the hydrogels, fibrils, and platelets using laser scanning confocal, atomic force, and transmission electron, and cryo-transmission electron microscopies, as well as neutron scattering, rheology, and circular dichroism, indicate that assembly is dictated by the association of the hydrophobic block having a helical conformation with the hierarchical microscale morphology controlled through a specific assembly pathway. |
