Scientists have developed an artificial skin that can detect temperature changes, an advance that may have varied robotic and biomedical applications. The material could be grafted onto prosthetic limbs to restore temperature sensing in amputees. It could also be applied to first-aid bandages to alert health professionals of a temperature increase - a sign of infection - in wounds.
While fabricating synthetic woods in a petri dish, a team led by Chiara Daraio from California Institute of Technology in the US created a material that exhibited an electrical response to temperature changes in the lab. It turned out that the component responsible for the temperature sensitivity was pectin, a long-chain molecule present in plant cell walls. "Pectin is widely used in the food industry as a jellifying agent; it is what you use to make jam. So it is easy to obtain and also very cheap," said Daraio.
Intrigued, the team shifted its attention to pectin and ultimately created a thin, transparent flexible film of pectin and water, which can be as little as 20 micrometres thick (equivalent to the diameter of a human hair). Pectin molecules in the film have a weakly bonded double-strand structure that contains calcium ions. As temperature increases, these bonds break down and the double strands "unzip," releasing the positively charged calcium ions.
Either the increased concentration of free calcium ions or their increased mobility results in a decrease in the electrical resistance throughout the material, which can be detected with a multimeter connected to electrodes embedded in the film.
The film senses temperature using a mechanism similar - but not identical - to the pit organs in vipers, which allow the snakes to sense warm prey in the dark by detecting radiated heat. In those organs, ion channels in the cell membrane of sensory nerve fibers expand as temperature increases. This dilation allows calcium ions to flow, triggering electrical impulses.
Existing electronic skins can sense temperature changes of less than a tenth of a degree Celsius across a five-degree temperature range. The new skin can sense changes that are an order of magnitude smaller and have a responsivity that is two orders of magnitude larger than those of other electronic skins over a 45-degree temperature range. So far, the skin is capable of detecting these tiny changes across a range of temperatures roughly between five to 50 degrees Celsius, which is useful for robotics and biomedical applications. The study appears in the journal Science Robotics.