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A new kind of printed 'artificial skin' turns pressure into nerve pulses

Bao Research Group, Stanford University

A new type of sensor could help pave the way for "artificial skin" that can accurately and efficiently mimic a real sense of touch. In a study published today in Science, a group of Stanford University engineers successfully created a stretchable circuit that can sense pressure and transmit the information directly to mouse brain tissue. With more research, it could be used in prosthetic limbs that are more sensitive to their surroundings — and better at navigating them.

In prosthetics, touch can be the difference between simply interacting the world and actually sensing that interaction. Over the past several years, we've gotten closer and closer to mimicking at least some of the abilities of real skin. One of the most promising developments has been flexible circuits and sensors, which can detect stretching, heat, pressure, and other inputs that we take for granted. But the issue isn't just figuring out how to capture the input that we experience as touch, it's getting the brain to register that input.

"It makes these devices more compatible with communicating with the body."

Study co-author Alex Chortos says that transmitting information from artificial skin often involves gathering analog signals, then using a computer or microcontroller to turn them into something the human body can process. But this translation can add imprecise "noise" to the data the brain gets, and it requires more energy. The team at Stanford hoped that by changing how the sensor worked, they could cut out the extra step altogether.

The sensor they created combines a stretchy printed circuit with a layer of carbon nanotubes, which conduct more electricity the more they're compressed. This creates a series of discrete electrical pulses that can be sent to the brain — "kind of like Morse code," Chortos explains, and similar to the way that our own skin captures and transmits information. The "skin" is clear plastic embedded with a pressure sensor smaller than a fingertip; they appear as small black dots on the robotic hand above.

For prosthetics attached to a living body, information can be transmitted through direct electrical impulses. This proof of concept was tested with a different system called optogenetics, where cells are genetically modified to respond to light. The sensor was hooked to an LED that flashed based on its pulses, activating mouse neurons. It doesn't tell us what the skin "feels," exactly, but it shows that the sensors can interface successfully with actual brain cells, even if they're in a petri dish instead of a skull.

Artificial Skin

The research team sees distinct benefits in taking a direct approach. "By communicating information in the same way as the human body, it makes these devices more compatible with communicating with the body," says Chortos. He warns, however, that it's too early to tell whether this will be the path prosthetics ultimately take. "Like with any technology, there are strengths and there are weaknesses," he says. Because the sensors are single-handedly capturing and transmitting information, each one is a relatively complex electronic device. And their direct link-up to the human body has its own drawbacks. "There’s sort of a challenge with wiring, in order to get a large number of sensors that are all directly connected to the nerves."

For now, the next step is to make the individual printed sensors — referred to as "pixels" — smaller, so more of them can fit on a single area. Like squeezing more pixels into a computer display, this would let the material sense pressure in more fine-grained detail, but it could also let the team add different kinds of sensors that mimic real skin's other capabilities. "Now that we have this kind of platform that can communicate with the body, we can use stretch sensors, or we can use temperature sensors, all kinds of other things," Chortos says.

Once they've overhauled the sensors, the team hopes to test the system with a live animal, not just cortical cells. Eventually, they want to create something that could be used in robotic limbs, wearable devices, or even flexible displays — a commercial-grade version of the material could potentially be ready in the next three to five years. It's not going to be a perfect substitute for our own epidermis any time soon, but it's a promising development in cracking the capabilities of the body's largest organ.