A team of researchers and engineers at Caltech and ETH Zurich have created an artificial skin with the ability to detect temperature changes using a mechanism similar to the one used by the organ that enables pit vipers to sense their prey.
Grafting the material onto prosthetic limbs would repair temperature sensing in amputees. Additionally, it could be used on first-aid bandages to alert health professionals of an increase in temperature, which is an indicator of infection in wounds.
A paper describing the new material will be published on February 1 in Science Robotics.
During the fabrication of synthetic woods in a petri dish, a team headed by Caltech's Chiara Daraio developed a material that displayed an electrical response to temperature alterations in the lab. They realized that pectin was the component accountable for the temperature sensitivity. Pectin is a long-chain molecule found 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.
Chiara Daraio, Caltech
Daraio is a professor of mechanical engineering and applied physics in the Division of Engineering and Applied Science.
Curious, the team changed its focus to pectin and eventually developed a thin, transparent flexible film using pectin and water, which can measure as little as 20 μm in thickness; equal to the diameter of a human hair. Pectin molecules in the film have a poorly bonded double-strand structure that comprises of calcium ions.
As the temperature rises, these bonds collapse and the double strands "unzip," emitting the positively charged calcium ions.
Increase in mobility of free calcium ions or increase in its concentration is speculated by the researchers as the reason behind the reduction in the electrical resistance across the material. This can be detected using a multimeter connected to electrodes fixed in the film.
The film used a mechanism similar to the pit organs in vipers to sense temperature. Pit organs in vipers help the snakes to sense warm prey in the dark by detecting radiated heat. When temperature increases in those organs, ion channels in the cell membrane of sensory nerve fibers expand. This expansion allows calcium ions to flow, stimulating electrical impulses.
Electronic skins currently available can sense temperature alterations of less than a tenth of a degree Celsius across a 5°-temperature range. The new skin has the capacity to sense alterations that are an order of magnitude smaller and possess a responsivity that is two orders of magnitude larger than those of other electronic skins across a 45°-temperature range.
So far, the skin can detect these miniature alterations across a range of temperatures roughly between 5 and 50 °C (approximately 41 and 158 °F), which is practical for biomedical and robotics applications. Going forward, Daraio's team plans to increase that up to 90 °C (194 °F).
Pectin sensors can then be used in industrial applications, such as thermal sensors in robotic skins or consumer electronics to boost human-robot interactions.
To achieve that, they will have to alter the fabrication process they currently use to develop the material, as that process results in the presence of water, which tends to evaporate or bubble at high temperatures.