The science of silks
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Introduction In silks, proteins are the structural components, and water is the solvent. Protein and water combine and separate—under ambient pressures and temperatures—to make a surprisingly tough thread (Figure 1). Spider silk is a case in point for the superb mechanical performance that can be achieved with this process. This, and the ability to collect the material under highly controlled conditions, makes spider drag-line silk an excellent starting material in the quest to unravel animal silks and their many trade secrets. Good silk research requires comprehensive biological understanding as well as the application of the fullest possible range of tools and techniques developed in the polymer sciences. This feature, in addition to its enormous diversity, makes silk a powerful model for inspiration to guide the design of novel polymeric materials and matter. Importantly, silk already has a venerable history as well as a strong following in our present society, which happily spends a hundred billion dollars annually on silk products. In particular, the silk threads of the Bombyx silkworm, a mulberry leaf-eating moth larva, have for millennia formed the basis of a hugely lucrative textile industry and attracted extensive industrial research and development (R&D).1,2 The silk threads produced by spiders, on the other hand, have (so far) not led to any important commercial or technological advances, in spite of being intrinsically stronger and tougher than their silkworm counterparts.3 A few basic generalizations may be
useful to highlight some key differences and similarities of the two types of fiber. Both silk types share many important features (such as fundamental molecular composition and flow characteristics of the spinning feedstock) despite a totally independent origin and evolution.4–6 However, millions of years of selection/natural R&D have resulted in spiders yielding silks specialized for application in lightweight net structures (webs), while moths use silks optimized for solid multilayer non-woven mesh composites (cocoons).7,8 Fundamentally, moth silks are all of one basic design-type, while spider silks cover a huge range of types. Moreover, moths have only one set of glands, each producing one set of silk, while each individual spider is able to produce several different silk types, each originating in a dedicated gland. This overview will avoid speculation and hype about the wonders of spider silk and consider rationally what we have learned from studying the different silks as a biomaterial and what further study and technological application might have in store. In particular, we start from the position that research on spider silks has already resulted in a wide range of fundamental insights into biopolymer production, processing, properties, and functions that would have been impossible to derive solely from the study of moth silks.9–12 This is due to the massive diversity and apparently more attractive mechanical properties of spider silks because of the wider range of properties required in w
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