Electrical Conduction Mechanism of Gels and Films Formed by Self-Assembled Elastin-Like Polypeptide Nanofibers: Implications for Tissue Regeneration

High Resolution Image Download MS PowerPoint Slide Electrically conductive scaffolds have been developed for effective tissue regeneration. However, most of these are composites of biocompatible polymers and conductive additives, with little attention being paid to the conductivity of the biocompatible polymers. Elastin-like polypeptides (ELPs), which can self-assemble to form nanofibers and hydrogels, are promising scaffolds for tissue engineering applications. In this study, we investigated the electrical conduction mechanism of nanofibers composed of GPPG, an ELP consisting of Gly-rich and Pro-rich sequences derived from elastin, and a terminal oligohistidine tag (His-tag). The aqueous dispersion of the GPPG nanofibers was weakly acidic, and its electrical conductivity was approximately three times higher than that of an aqueous acetic acid solution at the same pH. Dynamic viscoelasticity measurements and small-angle X-ray scattering suggested that the nanofibers formed a network (gel) with charged amino acids on their surfaces. Humidity-dependent current increase and hysteresis were observed in the current–voltage characteristics of the GPPG nanofiber films, whereas current was not observed under vacuum conditions. These results showed that proton conduction was dominant in the electrical conductivity of this system. Impedance spectroscopy revealed that the resistance of bulk nanofiber films decreased with increasing relative humidity. The resistance of the GPPG nanofiber film was ten times lower than that of the film of bovine serum albumin, regardless of the lower content of charged amino acids in GPPG. Wide-angle X-ray scattering of GPPG nanofiber films suggested that His-tags linked to Gly-rich sequences forming β-sheets were in close proximity to each other. Furthermore, the coordination of cobalt ions to His residues resulted in the loss of electrical conductivity of GPPG nanofibers. These results suggest that His residues are densely presented on the surface of the self-assembling network of GPPG, causing effective proton conduction at the nanofiber–water interface. ELP nanofibers with proton conductivity and biocompatibility are expected to be applied not only as scaffold materials, but also for wearable sensors and biofuel cells.