Diverse silk and silk-like proteins derived from terrestrial and marine organisms and their applications

Organisms develop unique systems in a given environment. In the process of adaptation, they employ materials in a clever way, which has inspired mankind extensively. Understanding the behavior and material properties of living organisms provides a way to emulate these natural systems and engineer various materials. Silk is a material that has been with human for over 5000 years, and the success of mass production of silkworm silk has realized its applications to medical, pharmaceutical, optical, and even electronic fields. Spider silk, which was characterized later, has expanded the application sectors to textile and military materials based on its tough mechanical properties. Because silk proteins are main components of these materials and there are abundant creatures producing silks that have not been studied, the introduction of new silk proteins would be a breakthrough of engineering materials to open innovative industry fields. Therefore, in this review, we present diverse silk and silk-like proteins and how they are utilized with respect to organism's survival. Here, the range of organisms are not constrained to silkworms and spiders but expanded to other insects, and even marine creatures which produce silk-like proteins that are not observed in terrestrial silks. This viewpoint broadening of silk and silk-like proteins would suggest diverse targets of engineering to design promising silk-based materials. Silk has been developed as a biomedical material due to unique mechanical and chemical properties. For decades, silks from various silkworm and spider species have been intensively studied. More recently, other silk and silk-like proteins with different sequences and structures have been reported, not only limited to terrestrial organisms (honeybee, green lacewing, caddisfly, and ant), but also from marine creatures (mussel, squid, sea anemone, and pearl oyster). Nevertheless, there has hardly been well-organized literature on silks from such organisms. Regarding the relationship among sequence-structure-properties, this review addresses how silks have been utilized with respect to organism's survival. Finally, this information aims to improve the understanding of diverse silk and silk-like proteins which can offer a significant interest to engineering fields.