Mechanical unfolding of a fluorescent protein enables self-reporting of damage in carbon-fibre-reinforced composites

Carbon- fi bre-reinforced polymer composites with an enhanced yellow fl uorescent protein (eYFP) at the interface of fi bres and resin were prepared. The protein was immobilized on the carbon fi bres by physisorption and by covalent conjugation, respectively. The immobilized eYFP fl uoresced on the carbon fi bres, in contrast to non-protein fl uorophores that were fully quenched by the carbon surface. The fi bres were embedded into epoxy resin, and the eYFP remained fl uorescent within the composite material. Micromechanical tests demonstrated that the interfacial shear strength of the material was not altered by the presence of the protein. Immunostaining of single fi bre specimen revealed that eYFP loses its fl uorescence in response to pull-out of fi bres from resin droplets. The protein was able to detect barely visible impact damage such as fi bre – resin debonding and fi bre fractures by loss of its fl uorescence. Therefore, it acts as a molecular force and stress/strain sensor at the fi bre – resin interface and renders the composite self-sensing and self-reporting of microscopic damage. The mechanoresponsive e ff ect of the eYFP did not depend on the type of eYFP immobilization. Fibres with the physisorbed protein gave similar results as fi bres to which the protein was conjugated via covalent linkers. The results show that fl uorescent proteins are compatible with carbon fi bre composites. Such mechanophores could therefore be implemented as a safety feature into composites to assure material integrity and thereby prevent catastrophic material failure.