Artificial skin can sniff out heat as well as a pit viper

Robots are one step closer to being touchy and feely thanks to a new type of temperature-sensitive artificial skin.

Developed by researchers in the US and Switzerland, the skin is made from pectin, the plant polymer that helps jam set. And when it comes to sensing heat, the pectin skin outperforms nature’s frontrunner – the pit organ of vipers. 

Unveiled in Science Robotics, the low-cost material could be used in thermal sensors on gadgets to augmenting human-robot relationships. 

But the researchers’ initial goal wasn’t to make heat-sensitive skin. They were trying to produce synthetic wood that was also electrically conductive for the development of gesture-sensitive wooden gadgets. After testing different components, they found that pectin changed the wood’s conductivity in response to temperature fluctuations. 

And making a pectin skin was almost as simple as making jelly. They dissolved powdered pectin in water, mixed with calcium chloride, poured it onto a flat surface and dried it in an oven. The end result: a temperature-sensitive, flexible film about half the thickness of a human hair. 

Pectin is a two-stranded molecule that can be zipped together by calcium ions. During the zipping process, calcium ions become trapped in the pectin zip, just as certain parts of the anatomy can get trapped in a zipper.

When the film is exposed to heat, the strands of the pectin zipper separate, releasing trapped calcium ions and changing the electrical conductivity of the film.

Bottom line: a tiny temperature change is converted into a measurable electrical signal.

And we are talking really tiny: the film detected a temperature change of 0.001 °C. It could do this across a range from five to 50 °C – conveniently overlapping with the temperature range of a human body – and detect body heat a metre away.

Existing electronic skins sense temperature changes of around 0.1 °C over a range of less than 5 °C. 

“We realised that the response to temperature was record-breaking,” says Chiara Daraio, a materials scientist at Caltech and co-author of the study. And when they sought a comparison from nature, they encountered the pit viper.

Pit vipers have a special organ on their nose containing a heat-sensitive membrane. When the membrane heats up, it opens channels in the membrane allowing calcium ions to flow in and create an electrical signal that’s detected by nerves. Just like the pectin films, the snake’s pit organ can respond to the heat of a prey animal from up to one metre away. 

Daraio and her crew believe the pectin film is likely to find its first application in artificial skin for prosthetic limbs, providing the wearer with temperature feedback to help restore full functionality. Before that happens, though, further work is needed to protect the pectin film from corrosives such as humidity. 

Dietmar Hutmacher, a biomedical engineer from the Queensland University of Technology in Australia, says that while the pectin skin is a “great piece of work from the basic sciences”, there’s still a long way to go before it’s used near or on human skin. 

“The material will need to be protected from the aggressive protein cocktail found in sweat, which just isn’t that easy to do.”

But for Daraio, the most important aspect of the project was the potential to learn from the natural world. “Learning fundamental mechanisms from nature allows us to create materials that have unprecedented and extreme properties,” she says in the video below.