Evolution has produced some remarkable biological materials, and materials scientists try to cherry-pick the best properties to produce ‘biomimetic’ composites that transcend the originals. To this end, David Kaplan and colleagues have created a fusion protein that combines spider silk protein and a peptide (R5) that helps to deposit the silicate skeletons of microoorganisms called diatoms (Proc. Natl Acad. Sci. USA doi:10.1073/pnas.0601096103).
In diatom biosilicates, the organic protein component improves the toughness of the inorganic silicon, and allows fine control of the microstructure — giving complex skeletons that are produced under mild aqueous conditions. Spider silk, on the other hand, is renowned for its tensile strength and, of particular importance for this study, can assemble itself from its constituent units. Silk assembly can be manipulated experimentally to produce many structures, from the threads seen in nature, to films, gels and porous matrices. Silk studded with cell-binding domains or other biologically active molecules can even be made, to encourage the formation of tissues such as bone or cartilage.
Kaplan and colleagues cast their silk–R5 fusion protein into films or spun it into fibres, depositing silica by adding a silicon solution during or after the process. They obtained spherical silica structures 0.5–2 μm in diameter (the picture shows silica deposited on a silk–R5 film; scale bar 2 μm). The strength and resilience of the silk–silica structures have not been tested, but the work shows in principle that they can be produced in different forms, for instance as silk fibres coated entirely in silica. The authors hope that their fusion strategy could produce a variety of composite materials. By adjusting the proteins involved, it might be possible to direct the deposition of titanium dioxide rather than silica, say.
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