Skin of sea cucumbers: the smart connective tissue that alters mechanical properties in response to external stimuli

—Catch connective tissue (CCT) is the connective tissue that shows large stiffness changes in response to stimulation under nervous control. The dermis of sea cucumbers is a typical example of CCT. Mechanical properties of the dermis are determined by the extracellular materials that are made of collagen fibrils embedded in a hydrogel of proteoglycans. The dermis takes 3 mechanical states soft (S a ), standard ( S b ) and stiff (S c ). Different molecular mechanisms of stiffening have been found in the transition S a → S b and in the transition S b → S c . In this article I will review my works on this intelligent material.