The flexible sensor film (transparent) can measure the temperature of a heating plate (white). In red is a gripping device. Photograph by Raffaele di Giacomo, courtesy of ETF Zurich.
A team of engineers and scientists at California Institute of Technology (Caltech) and Eidgenössische Technische Hochschule Zurich (ETH Zurich), Switzerland, have developed a sensitive and flexible temperature sensor using a mechanism similar to the one used by the organ that allows rattlesnakes and pit vipers to sense their prey. The material could be applied to first-aid bandages to alert health professionals of a temperature increase-a sign of infection-in wounds, or could be grafted onto prosthetic limbs to restore temperature sensing. A paper about the new material will be published in Science Robotics on February 1.
While fabricating synthetic wood in a petri dish, an ETF Zurich team led by Chiara Daraio, PhD, who is now the chair of mechanics and materials at Caltech, created a material that exhibited an electrical response to temperature changes in the lab. 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, who is also a professor of mechanical engineering and applied physics in the Caltech Division of Engineering and Applied Science.
Experiments on tree branches, whose cell walls contain pectin, had shown that their electrical conductivity depended strongly on temperature. To investigate the mechanism behind that dependence, the researchers at ETF Zurich, where Daraio was working in the Department of Mechanical and Process Engineering, created an artificial wood made from pectin and carbon nanotubes. By measuring the electrical resistance at different temperatures, they found that calcium ions trapped at the contact points between two sugar molecules were responsible for the sensing mechanism. The higher the temperature, the more free calcium ions were present in the artificial wood, and hence the better it conducted electrical current.
The team ultimately created a thin, transparent, flexible film of pectin and water, which can be as little as 20 micrometers thick (equivalent to the diameter of a human hair). The film senses temperature using a mechanism similar to the pit organs in vipers, which allow the snakes to sense warm prey in the dark by detecting radiated heat, even at a meter away. 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.
Measurements revealed that the scientists at ETH Zurich had found a temperature sensor that is comparable to the sensitivity of the pit organ and twice as sensitive as human skin: The pectin film can measure temperatures between 10 to 50 degrees Celsius within a hundredth of a degree. Daraio’s team would like to boost that up to 90 degrees Celsius (194 degrees Fahrenheit). This would make pectin sensors useful for industrial applications, such as thermal sensors in consumer electronics or robotic skins to augment human-robot interactions. To do so, they will need to change the fabrication process they now use to create the material, as that process leads to the presence of water, which tends to bubble or evaporate at high temperatures.
Editor’s note: This story was adapted from materials provided by Caltech and ETF Zurich.