



Video: Sensing skin could be used as mouth implant

The lightest of burdens to carry (Image: University of Tokyo) As flexible as you are (Image: University of Tokyo)


A diaphanous plastic sheet peppered with sensors could lead to the development of medical implants so unobtrusive that people scarcely notice them, or provide “sensing skins” for prosthetic limbs or robots.

So say materials scientists led by Takao Someya and Martin Kaltenbrunner at the University of Tokyo in Japan. Their plastics-based circuitry is lighter than a feather and just 1 micrometre thin. It is also so flexible and tough that an implant based on it would be all but imperceptible when worn on the back of your hand, say, or lining the roof of your mouth.

“This technology will lead to biomedical sensors that cause no discomfort at all to the wearer,” says Someya. “They could measure body temperature and heart rate in a stress-free way, and their shock-resistance means they will work even during sport and exercise.”

The sensors, which have the consistency of plastic food wrap, can be bent, stretched, crumpled and placed in wet environments without affecting their ability to operate – key characteristics of artificial skin designed to sense touch or temperature.

Stretchy circuits

Super-thin sensors have been made before, including an “electronic tattoo” patch that sticks to skin and can record brain waves or even the heart rate of a fetus in the womb if worn by the mother. But making even thinner, flexible sensors can run into problems with conventional microchip manufacturing techniques. Normally, high-energy plasma ions are deposited on a surface to build circuits. But this process does not work so well on soft, skin-like materials.

“High-energy plasma treatment damages ultrathin and conformable polymer foils and creates pinholes,” says Someya. “Our method avoids this.”

Working with colleagues at the Johannes Kepler University in Linz, Austria, the team found a way to deposit a very thin layer of aluminium oxide on a sheer polymer foil at room temperature. They were then able to attach circuit components to the oxide substrate, such as transistors made from carbon-based, organic compounds and touch or temperature sensors.

Unlike traditional semiconductors, the organic components can be laid down at room temperature. For now, circuits based on organic materials cannot process information as fast as, say, silicon-based transistors, but they are well suited for sensing.

As a demonstration, the team built a circuit with organic touch sensors in a 12-by-12 array. The structure weighs only three grams per square metre and can be made super-stretchy by placing the electronic components on a pre-stretched polymer. “The electric and mechanical performance was practically unchanged even when stretched by up to 233 per cent,” says Someya.

Anti-flaking

Chris Melhuish is the lab director for Bristol Robotics Laboratory in the UK, which has developed a bio-inspired fingertip for robots that uses rubbery bumps to sense deformations caused by touching a surface. He is impressed with the new feather-light technology.

“This is a fantastic piece of work – they should be congratulated,” says Melhuish. “Robot skins will need to be as tough as human skin. You don’t want it to flake off when you touch it. So that wonderful skin-stretching performance they have achieved will be very useful.”

He cautions, though, that there is one more vital step to take before the skin sensors can make their way onto real-world limbs.

“The sensor won’t work on its own, it needs power supplied to it and data must come out of it,” says Melhuish. “So though it is as light as a feather, they need connectivity that works similarly. That’s their next challenge.”

Journal reference: Nature, DOI: 10.1038/nature12314