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, a team led by Caltech's Chiara Daraio 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's what you use to make jam. So it's easy to obtain and also very cheap,” said Daraio, professor of mechanical engineering and applied physics in the Division of Engineering and Applied Science.
The team then shifted its focus and created a thin, transparent flexible film of pectin and water, which can be as little as 20µm thick. 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.
This is claimed to result in a decrease in the electrical resistance throughout the material, which can be detected with a multimeter connected to electrodes embedded in the film.
Existing electronic skins can sense temperature changes of less than a tenth of a degree Celsius across a 5-degree temperature range. The researchers say their 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 5 to 50°C, which is useful for robotics and biomedical applications. Prof Daraio's team want to boost that up to 90°C, making pectin sensors useful for industrial applications, such as thermal sensors in consumer electronics or robotic skins to augment human-robot interactions.
