Spider silk: The toughest natural polymer

Abstract

In summary, we have reviewed the inherent properties of spider dragline silk as well as the technological advances on the possible applications of dragline silk in the biomedical and clinical fields. Spider dragline silk exhibits excellent mechanical properties in terms of breaking energy, coupled with both high strength and strain at failure. The mechanical properties could be further tuned by varying external factors such as strain rate, temperature, reeling speed and force, the medium of the forcibly draw silk, and treatment with solvents and vapors. In addition, spider silk utilizes its inherent property, supercontraction, to maintain tension in the web and restore web shape after deformation by prey capture, precipitation or wind as they become wetted with morning dew or rain. The powerful cyclic contraction exhibited by spider silk also results in a green and energy efficient mimicry of biological muscles. The multiple occurrences of supercontraction and fatigueless cyclic contraction offer possibilities for performing work in industries and clinical sciences. The outstanding mechanical properties of spider silk are determined by its unique structural characteristics formed through its incomparable dope storage and spinning process. Scientists have applied many techniques in order to study the structure of the dragline silk. SEM and AFM are employed to study the morphology and topography of the fibre, and WAXD and FTIR are used to assess the b-crystallite and total b-sheet content in dragline silk, respectively. Besides, NMR, Raman spectroscopy and SAXS are also important tools inanalyzing the secondary and tertiary structures in dragline silk. Generally speaking, dragline silk can be modeled as a semicrystalline material, in which the polyalanine nano-b-crystallites are embedded in the non-crystalline amorphous region to form a network. In particular, the model by Du et al.25 suggested that intramolecular b-sheets acting as ''molecular spindles'' addressed the molecular origin of strain hardening of spider silk filaments. Last but not least, reconstituted and recombinant silk proteins can be used to fabricate novel biomaterials for application in the biomedical and clinical fields. The proteins are processed into different structures ranging from fibres, films, gels, hydrogels and porous sponges to microcapsules. Surface functionalization through targeting carboxylic acid groups on the amino acids in the protein could be used for biosensors and influence cell and tissue functions by increasing high levels of surface decoration with enzymes. © 2012 The Royal Society of Chemistry.