Meeting Abstract
Many endoskeletal organisms have tight skins; it facilitates transfer of force from a muscular core to the body surface and then on to the external environment. Indeed, tight and well-connected tensile skins can serve as an exotendon that transmits force to terminal appendages. In highly-pressurized, soft-bodied invertebrates (e.g. nematodes), the tight skin constrains deformation and is a crucial component of the hydrostatic skeleton. The opposite is also true; many organisms and structures have loose fitting skins. Here, however, understanding the relationship between the sheath and core is less well-understood. In this study, we survey biological and engineered examples of structures that benefit from a core surrounded by a loose sheath. Biological examples include whole animals (e.g. loose hagfish and horse skin; floppy vermiform animals), parts of animals (e.g. various meiofaunal proboscides; chameleon tongues; penile foreskins and serous membranes), and human-engineered mechanisms (e.g. climbing ropes; concrete slab slip sheets; loose-tube optical cables.) We then categorize the examples to form a theoretical framework based on whether looseness was primarily adaptive to the functioning of either the core or the sheath. We found that “core adaptations” often involved mobility issues in which the sheath was loose so that the core would not be impeded (e.g. a frog leg during jumping or a hagfish body during knotting). “Sheath adaptation” functions were more varied (e.g. protection; diffusion) but included many “biotribological” examples in which skin folds served to lubricate (e.g. extension of a chameleon’s tongue.) With this categorization scheme, we hope to better understand the functional significance of loose skins in biological mechanisms and identify useful design elements for human-engineered structures.
