Artificial skins detect the gentlest touch
Super-sensitive materials can detect the weight of a butterfly.
Artificial electronic skins that can detect the gentlest of touches have been developed by two independent US research groups. The skins could eventually be used in prosthetics, or in touch-sensitive robotic devices.
Both systems detect pressure changes of less than a kilopascal, the same as everyday pressures felt by our fingers when typing or picking up a pen. This sensitivity is better than previous systems, which detected pressures of tens of kilopascals or more, or only detected static pressures so that once an object was sat on the skin, the device could not sense that it was still there.
The devices, both reported in Nature Materials today, work in different ways1,2. Chemist Zhenan Bao at Stanford University, California, and her colleagues used the elastic polymer polydimethylsiloxane (PDMS)1. Bao took a piece of PDMS measuring six centimetres square with pyramid-shaped chunks cut out of it at regular intervals. When the PDMS is squashed, the pyramid-shaped holes that were previously filled with air become filled with PDMS, changing the device's capacitance, or its ability to hold an electric charge.
To make it easier to detect the changes in capacitance, Bao stuck the PDMS capacitor onto an organic transistor, which can read out the differences as a change in current. The team used a grid of transistors to track pressure changes at different points across the material.
The PDMS-based skin is sensitive to the lightest of touches: Bao tested her device by placing a bluebottle fly and a butterfly on it, both of which were clearly 'felt'. Although the device is sensitive, it is still not ideal for prosthetic uses because it is not stretchy. However, Bao expects to have a stretchable skin that could be used in prototype prosthetics by the end of this year. "Making it biocompatible and allowing it to be integrated with animals or real tissues, that will be a long-term thing," she adds.
The other type of artificial skin was developed by Ali Javey at the University of California, Berkeley, and his colleagues. In a different approach, Javey used semiconductor nanowires pulled into the shape of a grid using a technique called contact printing. The grid was then laid out on a flexible pressure-sensitive rubber — a material that changes its electrical resistance under pressure2.
In the 7-centimetre-square grid, the criss-crossing nanowires act as transistors. Each transistor is like a pixel, and the pressure-induced current change at each individual position can be read out. And because it's made mainly of rubber, the device is bendy. "Because we're using very small inorganic semiconductors, the devices are very flexible," explains Javey. He has bent the sensor into a U-shape with each arm of the 'U' separated by a gap of just 5 millimetres and it still works.
However, fully working artificial skins will need to do more than detect pressure and bend. "The ultimate prosthetic skin should behave like our own skin," says Stephanie Lacour, a materials scientist from the University of Cambridge, UK. That would mean the skin being able to detect sideways shear forces — such as those produced by scratching a twig down your leg — as well as pressure. "This is one of the most difficult things to implement," she says.
Javey agrees that there are many challenges to overcome to make a fully functional artificial skin, not least integrating that skin with the brain. But applications in robotics could come much sooner, he says. The next step is to scale up production so that enough artificial skin can be made to cover an entire robot's body, Javey says.
John Boland, a chemist at the Centre for Research on Adaptive Nanostructures and Nanodevices at Trinity College Dublin, who has written an accompanying News & Views article3 about the two groups' work, says that the flexibility of Javey's device and the sensitivity to really small pressures of Bao's device might one day be exploited in a skin combining both characteristics.
Although challenges remain — not least relaying information from the skin so it is easy to interpret — Boland is impressed by the work of both groups. The skins could revolutionize prosthetic devices, he says, or might be used to help surgeons feel their way while performing minimally invasive keyhole surgery. "They've moved to flexible materials, that's the critical thing," he says, "and they both give access to low-level sensitivity."