The protein matrix and hard tissues of insects, worms, ants and spiders naturally incorporates metals, such as zinc, manganese and copper. This leads to mechanical hardening of teeth, jaws, mandibles, ovipositors and to an enhancement of silk toughness. Thus, the artificial incorporation of metals, or even insulating or semiconducting materials, into these protein structures could be exploited to obtain a reinforced matrix. A number of groups reported the introduction of metals, such as zinc, titanium, aluminium, copper and lead in the protein structure of spider silk through multiple pulsed vapor-phase infiltrations. This allowed us to increase its toughness modulus from 131 MPa up to 1.5 GPa. Biomaterials with increased mechanical or conductive properties could find innovative applications in garment textiles and medical nerve regeneration. It was suggested to coat spider silks with amine – functionalized multi-wall carbon nanotubes, to produce electrically conducting fibers, or with cadmium telluride, magnetite or gold nanoparticles, for fluorescent, magnetic and electronic applications. However, to the best of our knowledge, the incorporation of materials in the inner protein structure of spider silk has not been achieved to date. Here, we report the production of silk incorporating graphene and carbon nanotubes directly by spider spinning, after spraying spiders with the corresponding aqueous dispersions. We observe a significant increment of the mechanical properties with respect to the pristine silk, in terms of fracture strength, Young’s and toughness moduli. We measure a fracture strength up to~5.4 GPa, a Young’s modulus up to ~47.8 GPa and a toughness modulus up to ~2.1 GPa, or 1567 J/g, which , to the best of our knowledge, is the highest reported to date , even when compared to the current toughest knotted fibers . This approach could be extended to other animals and plants and could lead to a new class of bionic materials for ultimate applications.
In summary, spiders placed in an environment with water solutions of nanotubes or graphene produce dragline silk with unprecedented mechanical properties, realizing the toughest achieved fibers , with a strength only comparable with that of the strongest carbon fibers or that of the limpet teeth. Knots could further increase the toughness. Spiders could spin graphene and nanotubes in the silk also as an efficient way of eliminating them from their organism. Spider natural and very efficient spinning can thus allow the collection of the most performing silk fiber when compared to synthetic recombinant silks, which represents the most promising silk material to be efficiently reinforced. This new reinforcing procedure could also be applied to other animals and plants, leading to a new class of bionic materials for ultimate applications
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