A team of scientists and engineers at ETH Zurich and Caltech has developed an artificial skin that can detect temperature changes by using a mechanism comparable to the one used by the organ that allows pit vipers to sense their prey.
It is possible to graft the material onto prosthetic limbs, thereby restoring temperature sensing in amputees. The material can also be attached to first aid bandages. This will alert health professionals of a temperature increase in wounds, which is often a sign of infection.
A team led by Caltech’s Chiara Daraio created a material that showed an electrical response to temperature changes in the lab, while they were creating synthetic woods in a petri dish. Further investigation revealed that the component responsible for the sensitivity to temperature was pectin, which is a long chain molecule that is found in plant cell walls.
Daraio, a professor of mechanical engineering and applied physics in the Division of Engineering and Applied Science, explained that pectin is very cheap and easy to procure, as it is widely used in the food industry as a jellifying agent to make jam.
The team was intrigued by this unexpected result and shifted their attention to pectin. They eventually created a thin, transparent film of pectin and water that is flexible. The film is about 20 micrometers thick, approximately the same as the diameter of a human hair. The weakly bonded double strand structure of the pectin molecules in the film contains calcium ions. As the temperature rises, these bonds break down, “unzipping” the double strands and releasing the positively charged calcium ions.
The researchers are not sure whether the decrease in the electrical resistance throughout the material is a result of the increased concentration of free calcium ions, or their increased mobility. They do however speculate that both factors are likely to have an effect. The change in resistance can be measured when a multi-meter is connected to electrodes that are embedded in the film.
Vipers are able to sense warm prey in the dark by detecting radiated heat through their pit organs. These pit organs contain ion channels in the cell membrane of sensory nerve fibers, which expand as temperature increases. This enlargement allows calcium ions to flow, resulting in electrical impulses being triggered. The film senses temperature using a similar mechanism.
Current electronic skins can detect temperature changes of smaller than a tenth of a degree Celsius across a 5-degree temperature range. Not only can the new skin sense changes that are an order of magnitude smaller, but it also has a responsivity that is a full two orders of magnitude larger over a temperature range of 45 degrees.
At this point, the new skin can detect these minute changes across a range of temperatures between 5 and 50 degrees Celsius (about 41 to 158 degrees Fahrenheit). This is useful for biomedical and robotics applications.
Daraio’s team would next like to boost that up to 90 °C (194 °F). This would make pectin sensors practical for use in industrial applications, such as robotic skins to augment human-robot interactions, or thermal sensors in consumer electronics. To achieve this, they will need to modify the fabrication process currently used to manufacture the material. The existing process creates the presence of water, which tends to evaporate or bubble at high temperatures.