A hybrid structure based inspired by the skin color-changing talents of cephalopods has been developed by researchers in the USA. The material is comprised of a rigid film with a low thermal emissivity and a substrate with a high thermal emissivity, with a stretchable heater that can generate microscopic cracks in the surface. The thermal emissivity can be changed reversibly and instantaneously. Such a biomimetic material might be useful in motion sensing, specifically finger motion on a touch screen, in information encryption, multiplexing displays, and thermal, infrared, camouflage.
Earlier work on creating tuneable thermal emissive materials of this kind have often been limited by low response rates, high working temperatures, or simply being difficult to fabricate. Songshan Zeng of the University of Connecticut in Storrs, and colleagues there and at Dartmouth College in Hanover, New Hampshire, turned to biological systems as inspiration they hoped would circumvent these and other problems. [Zeng, S. et al., Mater. Today (2020); DOI: j.mattod.2020.12.001]
Cephalopods, such as cuttlefish, squid, and octopuses, have sophisticated systems in their skin that allow them to change color, generate dynamic patterns, and produce surface-mimicking camouflage very effectively and efficiently. There are usually two types of functional skin cells in the active layer that each have different optical properties. The first type of cells is the iridocytes, which contain periodic protein building blocks and spaces. This presents a Bragg stacking geometry for actively controlling the scattering, refraction, and reflection of light. The second class is the chromatophores. These contain pigments surrounded by tissues that can block or reveal the underlying colors.
Other researchers have focused on mimicking the iridocytes with some degree of success. Commonly, however, there are issues with this approach such as low response rates. Zeng and colleagues have looked at mechanical strain as an alternative approach that nevertheless mimics the mechanical changes in the cephalopod skin, and as such takes its lead from the way in which chromatophores work to hide and reveal pigments using a mechanical mechanism.
The team's system has a mirror chrome coating containing thin aluminum ?akes) - this is the low emissivity layer. This is on top of a polyvinyl alcohol (PVA)/laponite composite transition layer bonded to a stretchable substrate - the high emissivity layer. The stretchable heater layer is made of a serpentine patterned conductive thread sandwiched by stretchable double-sided tape beneath the substrate. Applying strain generates microscopic cracks. When these microscopic cracks are opened or closed they reveal or hide the high emissivity layer underlying the film.
The team has demonstrated how this composite film might be used as a wearable finger motion sensor. They have also demonstrated a mechanical responsive information encryption device. The same composite film also has potential in the construction of thermographic display arrays and dynamic thermal camouflage that can adapt to a changing thermal environment to hide something from an infrared camera or detector, for instance.
"This work is expected to facilitate the creation of the next-generation thermal modulation devices with autonomous, on-demand, and wide-range control," the team concludes.
David Bradley also writes at Sciencebase Science Blog and tweets @sciencebase. His popular science book Deceived Wisdom is now available.