ABSTRACT

Several protein materials, including spider silk, [1-4] have recently been highlighted for their remarkable mechanical properties such as high stiffness, high toughness, and superelasticity, albeit they are composed of weak constituents °exible protein domains. One may raise the following question: What makes protein materials that are composed of weak constituents tougher than any other man-made composite? The fundamental insights that can resolve such a question will lead us to establish physical principles, which provide how weak proteins can achieve excellent mechanical properties suitable for performing mechanical functions. For instance, spider silk protein performs notable mechanical functions due to its remarkable mechanical properties such as superelasticity and high toughness. In particular, spider silk has been found to have unique mechanical properties: it possesses both high extensibility and high fracture toughness, which cannot be achieved with engineered materials. Specišcally, dragline silk has the elastic modulus of 10 GPa, implying that it is a soft material, while the yield strength is equal to 1.1. GPa, comparable to that of high-tensile steel, and the fracture toughness is equal to 160 MJ/m3, which is two times larger than that of a strong composite material such as Kevlar [2]. Moreover, spider silk is able to perform its biological functions through its remarkable mechanical properties. The ability of spider silk to capture °ying prey is attributed to the superelasticity and high extensibility of spider silk, which enable the conversion of the kinetic energy of the °ying prey into heat dissipation, resulting in the capture of the °ying prey in the spider silk. This sheds light on the fundamental understanding of the mechanical properties and behaviors of protein materials, which can help in gaining deep insights into the biological functions of protein materials and provide materials scientists with a novel design concept to develop biomimetic materials [3,4].