Protective coatings on extensible biofibres

Formulating effective coatings for use in nano- and biotechnology poses considerable technical challenges 1 . If they are to provide abrasion resistance, coatings must be hard and adhere well to the underlying substrate 2 . High hardness, however, comes at the expense of extensibility 3 , 4 . This property trade-off makes the design of coatings for even moderately compliant substrates problematic, because substrate deformation easily exceeds the strain limit of the coating 5 . Although the highest strain capacity of synthetic fibre coatings is less than 10%, deformable coatings are ubiquitous in biological systems 3 , 6 . With an eye to heeding the lessons of nature, the cuticular coatings of byssal threads from two species of marine mussels, Mytilus galloprovincialis and Perna canaliculus , have been investigated. Consistent with their function to protect collagenous fibres in the byssal-thread core, these coatings show hardness and stiffness comparable to those of engineering plastics and yet are surprisingly extensible; the tensile failure strain of P. canaliculus cuticle is about 30% and that of M. galloprovincialis is a remarkable 70%. The difference in extensibility is attributable to the presence of deformable microphase-separated granules within the cuticle of M. galloprovincialis . The results have important implications in the design of bio-inspired extensible coatings.