The skin of cephalopods is characterized by dermal erector muscles called papillae that can be protruded by several millimeters to change the skin texture and overall shape for camouflage purposes or control of the hydrodynamic drag. We developed Twisted Spiral Artificial Muscles (TSAMs) to emulate the working mechanism of cephalopods papillae and design self-morphing smart skins. TSAMs are Archimedean spirals made from highly twisted nylon monofilaments, coated with an electrically conductive layer. With a small electrical input (only 0.02 V/mm), the filament experiences an anisotropic thermal expansion which causes the untwisting of the fiber, and a vertical displacement ranging from a few microns to several millimeters. In recent work, we developed a static theoretical model to relate the volume expansion of the fiber to the output displacement. In this work, we propose a physics-based dynamic model to describe the time-varying actuation of TSAMs under a specific electrothermal input. We demonstrate that the actuation is directly caused by the variation of the stiffness constant of the system during Joule heating. The model is experimentally validated on TSAMs with different dimensions and conductive coatings. These results represent a useful tool to tailor the actuation of TSAMs, as well as for the design and control of TSAMs-based self-morphing devices and 3D displays.

Artificial papillae for self-morphing skin: a dynamic model

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