A temperature sensor for artificial skin

Researchers at ETH Zurich have developed a highly sensitive and flexible temperature sensor that could soon be integrated into artificial skin on prosthetic limbs or robotic arms. Their breakthrough, published in the journal Science Robotics, could restore amputees’ capacity to detect temperature changes — an important function of human skin.

The creation of the sensor began in a bit of an unusual place, when researcher Raffaele Di Giacomo stumbled upon a peculiarity of the vegetable material pectin (often used as a gelling agent for puddings and marmalades). But Di Giacomo was interested in a different property of the substance, with experiments on tree branches (whose cell walls contain pectin) showing that their electrical conductivity depended strongly on temperature.

To investigate the mechanism behind this dependence, Di Giacomo and his team created an artificial ‘cyberwood’ made from pectin and carbon nanotube. 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 the better it conducted electrical current.

“In this way we had found an ideal material for making a highly sensitive temperature sensor, but it was as stiff as wood and hence rather unsuitable for use in an artificial skin,” said Di Giacomo.

The breakthrough came when Di Giacomo and doctoral student Luca Bonanomi produced a thin film of only 100 micrometres from a simple pectin gel laced with calcium solution. At two ends of the film they attached electrodes that allowed them to measure the electrical conductivity. Like the cyberwood, this simple film conducted electrical current more easily with increasing temperature. The difference was that it was wafer-thin and transparent — and could be bent and deformed almost arbitrarily.

More precise measurements revealed that the temperature sensor that could match the performance of the highly sensitive pit organ found in snakes, which allows them to detect the warm body of a mammal even at a metre’s distance. From 10–50°C, the film can measure temperatures with a precision of a hundredth of a degree. That is comparable to the sensitivity of the pit organ and twice as sensitive as human skin.

The researchers have additionally demonstrated the film’s ability to function when strongly bent — as would be required in prosthetic limbs — and to localise a temperature variation at a specific position on a larger film. “What’s more,” said Di Giacomo, “our pectin film is extremely easy to make and it contains no transistors or other electronic components. That means it’s far more robust and less prone to interference than existing flexible temperature sensors.”

The scientists are currently looking to improve the computer algorithms used for analysing the film’s electrode signals, as well as its electrical contacts. They say the temperature sensor should soon be ready for a field trial in robotics or prosthetics.

Image caption: The flexible sensor film (transparent) can measure the temperature of a heating plate (white). In red is a gripping device. Image credit: Raffaele di Giacomo/ETH Zurich.