Artificial skin returns a sense of touch
New plastic skin could one day provide feedback to the brain from prosthetic limbs. Viviane Richter explains.
As a boy, Benjamin Tee marvelled at Luke Skywalker’s touch-sensitive prosthetic hand in The Empire Strikes Back – and now, he’s on the way to making it reality. Along with biomedical engineers from Stanford University, Tee, who is now at Singapore’s Agency for Science Technology and Research, created flexible artificial skin that senses and sends touch signals to a nerve cell. The researchers described the technology, which lays groundwork for fully functioning plastic skin, in Science in October.
For years amputees have been able to pick up mugs or scratch their nose with artificial limbs, but they have not been able to send touch signals back to the brain. That feedback is necessary for dexterity. The skin on each fingertip holds 2,500 sensory receptors feeding information to some 300 neurons, which in turn guide your motor movements. “If you want to pick up, say, an egg, you want to make sure that you don’t squeeze too hard,” explains David Grayden, an electrical engineer at the University of Melbourne, who was not involved with the study.
To make prostheses more sensitive and responsive, Tee has produced an artificial skin that pings tactile information to a nerve cell. The technology, called the Digital Tactile System, or DiTact, consists of two layers housed in a flexible sheet of plastic less than a millimetre thick. The top layer – a sheet of rubber – bristles with tiny inverted pyramids that hang down like stalactites. Each pyramid, about half the width of a hair, contains billions of conductive carbon nanotubes. The layer below is a printed electric circuit powered by an 11-volt battery.
When an object touches the artificial skin, the nanotubes are squeezed, channelling a pulse of electric current through each pyramid to the circuit below. The more pressure on the skin, the more pyramids are compressed and the faster the electric pulses fire. Eventually, Tee hopes, those controllable signals could be transmitted to brain cells, which translate the pulse rate into perceived pressure, delivering an authentic sense of touch.
In Tee’s first version of DiTact, the electrical pulses built up quickly, then dropped off again, like a wave. Hard pressure pumped so many pulses through the circuit that they overlapped into one sustained blast of electricity. A nerve cell on the receiving end of this electrical onslaught would not be able to fire quickly enough and would tire out eventually. Tee knew the artificial skin could only be deemed a success if the pulses could be transmitted to animal tissue in a manner that more closely mimicked what happens in the body.
So his team hooked the skin circuit to an LED light, turning the waves of electricity into bursts of light. The light flicked on and off much more quickly and didn’t bleed into the next signal like the electrical pulses did. The team relayed the LED via optic fibre to a mouse nerve cell in a petri dish. The mouse cell was genetically modified to fire its own electrical message when exposed to light. This laboratory technique, called optogenetics, is a relatively new method of stimulating neural activity. So far, it has been widely used to control non-human brain cells, such as those from mice and fruit flies, both in lab dishes and in live animals.
The workaround paid off. The harder the pressure on the artificial skin, the faster the light flashed, and the mouse nerve cell fired in time.
Touch receptors in fingertips become accustomed to consistent pressure. They stop firing after a while so your brain isn’t overloaded with information.
Tee designed the skin to sense pressure from a light finger tap to a firm handshake. Grayden is impressed by how thin DiTact is and how little power it consumes – it only needs a small battery. “You’d be able to cover an artificial hand with it,” he says.
So how real would touching something with DiTact feel? Brief taps might feel similar to having real skin, but relaying information about sustained pressure remains a challenge.
Touch receptors in fingertips become accustomed to consistent pressure. They stop firing after a while so your brain isn’t overloaded with information – say, when you’re holding a smartphone or a book. DiTact, though, keeps the electrical circuit firing, along with the nerve cell – even if the pressure stays the same. “It would feel like you’re starting to touch something all the time – or that that the thing you’re touching is getting bigger and applying more and more pressure,” Grayden explains. While it’s a shortcoming for now, he adds, the technology could likely be refined to more closely resemble how live skin senses and relays information about prolonged pressure.
The DiTact technique isn’t ready for real patients yet. Getting it to work in a clinical setting would require genetically engineering people’s brain cells to respond to the LED light signal – a step that’s “a long way off”, Grayden says. But Tee’s team is now working on a way to stimulate nerves directly with the fast electric pulses, rather than using the LED workaround. He also wants to expand the artificial skin’s repertoire to pick up temperature and texture.
Grayden says reuniting amputees or paralysed patients with a sense of touch is more than just about function – it will give them a much better experience of reality. He points to footage of children who have a ‘bionic’ ear switched on for the first time. “It’s just magic,” he says.