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BIOMIMETIC MATERIALS
Polymers with bio-inspired strength
Biopolymers, ingeniously designed by nature, can combine dierent mechanical properties and even adapt to changes in their environment. By imitating the structure of a protein, chemists have now made a strong, tough polymer that also exhibits elastic properties.
Stuart J. Rowan
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As in many other areas of science, polymer chemists have been drawing inspiration from nature since the
dawn of the eld; for example, Carothers invented nylon while trying to mimic the properties of silk. More recently, scientists have been inspired by a diverse range of animals and plants1, such as the gecko foot2, the sea cucumber dermis3 and the lotus leaf4, resulting in new materials that imitate these natural models.
In most of these cases, the synthetic materials mimic the structural characteristics of the natural systemsin an attempt to bestow them with the desired properties and/or function of these biosystems. Now, in Journal of the American Chemical Society, Zhibin Guan and his colleagues at the University of California, Irvine, introduce a new synthetic polymer inspired by the protein titin, which shows a rare combination of mechanical properties5.
Titin, a very large protein found in muscles, combines strength (how much stress the material can be exposed to beforeit breaks), toughness (how much energy the material can absorb) and elasticity (the ability to recover its original size and shape once the stress is released)6. In general, polymers do not possess such a combination of properties. For example, rigid plastic polymers (suchas polycarbonates) are generally very strong but also tend to be brittle, whereas a exible plastic such as polyethylene does not show the strength of a rigid plastic but will deform under stress, thus absorbing more energy, which makes it a tough material. Neither of these materials, however, shows signicant amounts of elasticity. So how does titin manage to combine these properties?
Its modular, multidomain structure(Fig. 1a) seems to be key to this ability7. Ata very basic level, titin consists of over 300 folded (immunoglobulin-like) domains that consist primarily of -sheet structures. If one imagines stretching such a multidomain polymer, the force on the chain willinitially rise quickly with extension, until it reaches a level that breaks the non-covalent interactions within a specic domain. At...