Repeat-proteins films exhibit hierarchical anisotropic mechanical properties.

Complex hierarchical structures provide beneficial structure-property relationships that can be exploited for a variety of applications in engineering and biomedical fields. Here we report on molecular organization and resulting mechanical properties of self-assembled designed repeat-protein films. Wide-angle X-ray diffraction indicates the designed 18-repeat concensus tetratricopeptide repeat protein (CTPR18) orients normal to the casting surface, while small-angle measurements and electron microscopy show a through-plane transversely aligned laminar sheet-like morphology. Self-assembly is driven by the combination of CTPRs head-to-tail stacking and weak dipole-dipole interactions. We highlight the effect that this hierarchical structure has on the material's mechanical properties. We use nanoindentation and dynamic mechanical analysis to test the mechanical properties over multiple length scales, from the molecular level to the bulk. We find that morphology predictably affects the film's mechanics from the nano- to the macroscale, with the axial modulus values ranging from 2 to 5 GPa. The predictable nature of the structure-property relationship of CTPR proteins and their assemblies proves them a promising platform for material engineering.