Skin is pretty unique stuff. It stretches when we flex it, heals when we cut it, and is sensitive enough to tell the difference between cat fur and dog hair, between hot and cold.
Now, scientists are a big step closer to mimicking it, according to a new report in the journal Science. Zhenan Bao, professor of chemical engineering at Stanford, and her colleagues have developed a skin-like plastic sensor that can actually feel pressure and transmit a touch signal to nerve cells.
Scientists hope to develop a sheet of skin embedded with lots of these tiny sensors. Covering a prosthetic with this artificial skin may give users the ability to feel, possibly even alleviating phantom limb pain from the loss of a limb.
Bao and her team figured out how to imitate pressure by embedding carbon nanotubes into flexible waffled plastic. The more pressure is applied, the closer the tubes are squeezed together, which conducts electricity. That triggers the sensor to act like human skin and transmit electric pulses as a pressure stimulus.
Getting that stimulus to communicate with the brain, though, was another challenge. Bao didn't want to use a computer chip to process the signal and then connect it to the brain. Instead, she and her team wanted to figure out how to transmit it directly. "We touch something, and the signal is processed right at the fingertip and then transmitted to the nerve system," Bao says. "This way, the signal can be processed quickly and sent over long distance without losing the integrity of the signal." In the new work, Bao and her team connected it to a flexible plastic electronic circuit that could carry the pulses to nerve cells, and using a simulated human nervous system and the brain cells of mice, they were able to stimulate the neurons that respond to touch.
"Currently what we have done is to demonstrate the concept, and the possibility to do so, with plastic material," she says. Plastic is promising because of its versatility. "We can engineer plastic materials so they have the ability to respond to pressure or external force, but also we can engineer them to be stretchable, or biocompatible, and maybe even self-healing," she says. "With plastic materials we can potentially imitate those kinds of properties."
Skin, highly complex as it is, won't be complete with just the ability to feel. Touch only accounts for one of the properties of skin sensation; skin can also feel pressure and temperature, all at the same time, she says.
Will it ever feel like real skin?
"That's our dream," she says, though it's probably years away. "This is a field at the very, very beginning."