Even though there have been incredible advancements in the field of prosthetics, including some more unorthodox ones, those who are unfortunate enough to lose a body part will be unable to replicate the sense of touch with their artificial limb. A remarkable new study by a team of Stanford University engineers, published today in Science, has perhaps begun to finally address this problem: they have created a plastic skin that can “feel,” transmitting sensory information as an electric signal to the brain.
Lead researcher Zhenan Bao had previously spent a decade attempting to develop a material that mimics the skin’s ability to flex, heal, feel pain, and detect pressure and temperature changes in the external environment. This new study has achieved one of these goals: this novel artificial skin can detect pressure changes, essentially replicating touch.
“This is the first time a flexible, skin-like material has been able to detect pressure and also transmit a signal to a component of the nervous system,” said Bao in a statement.
The plastic has two distinct layers, with the upper segment containing the sensing mechanism, and the lower segment acting as the electronic circuit that transports signals to the brain. In the process, these electrical signals are converted into biochemical stimuli compatible with nerve cells.
The upper layer contains a pressure sensor that has the same range as normal human skin, meaning that a small poke and a sharp prod can be differentiated between. It is comprised of an extremely thin, springy, waffle-shaped plastic polymer, and is very sensitive to any compression. By measuring the minute molecular compression of the plastic, a very precise pressure reading can be made.
The waffle matrix contains billions and billions of carbon structures called nanotubes. These incredible little things, no bigger than a billionth of a meter (roughly 100 millionths of an inch) have extraordinary properties: they are 15 times stronger than steel or Kevlar, conduct heat more than 10 times more efficiently than copper, and conduct electricity just as efficiently. By compressing the nanotubes within the plastic, they touch each other, conducting electricity. This electric signal correlates to the amount of pressure being put on the skin, which is then communicated to the brain.
Using the work of a pioneering field known as optogenetics, the researchers engineered neurons to make them receptive to specific frequencies of light. These cells were lined up and connected to simulate a small portion of the human brain. When the skin felt pressure, the nanotubes came into contact with each other and generated an electrical signal of a certain strength. This signal was then converted into light pulses of the same strength, which activated the neurons.
This proof of concept demonstrates that the artificial skin can successfully communicate with a human nervous system, meaning that one day an artificial limb could potentially produce a genuine sense of touch, not just a poor copy of it, for its user.
There’s a lot of work to do, as Bao acknowledges, but for now, this is an astounding leap forwards.