Researchers may be moving a step closer toward adding a sense of touch to prosthetics with their creation of an artificial skin that they say can detect pressure and send a signal directly to a living brain cell.
The team, led by Zhenan Bao, a professor of chemical engineering at Stanford University, published their work recently in Science. In this study, they replicated one aspect of touch—the sensory mechanism that enables humans to distinguish the pressure difference between a limp handshake and a firm grip, according to a media release from Stanford University.
“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,” Bao says in the release.
What makes this work is a two-ply plastic construct: the top layer creates a sensing mechanism and the bottom layer acts as the circuit to transport electrical signals and translate them into biochemical stimuli compatible with nerve cells. The top layer in the new work featured a sensor that can detect pressure over the same range as human skin, from a light finger tap to a firm handshake, the release explains.
The team then indented a waffle pattern into the thin plastic, which further compressed the plastic’s molecular springs, and then scattered billions of carbon nanotubes through the waffled plastic. Putting pressure on the plastic squeezes the nanotubes closer together and enables them to conduct electricity.
This allowed the plastic sensor to mimic human skin, which transmits pressure information as short pulses of electricity, similar to Morse code, to the brain. Increasing pressure on the waffled nanotubes squeezes them even closer together, allowing more electricity to flow through the sensor, and those varied impulses are sent as short pulses to the sensing mechanism. Remove pressure, and the flow of pulses relaxes, indicating light touch. Remove all pressure and the pulses cease entirely, the release continues.
The team then hooked this pressure-sensing mechanism to the second ply of their artificial skin, a flexible electronic circuit that could carry pulses of electricity to nerve cells.
The team worked with researchers from PARC, a Xerox company, which has a technology to deposit flexible circuits onto plastic. They also used a technique called optogenentics, developed by Karl Diesseroth, a professor of bioengineering at Stanford, to prove that the electronic signal could be recognized by a biological neuron.
In optogenetics, researchers bioengineer cells to make them sensitive to specific frequencies of light, then use light pulses to switch cells, or the processes being carried on inside them, on and off.
For this experiment the team members engineered a line of neurons to simulate a portion of the human nervous system. They translated the electronic pressure signals from the artificial skin into light pulses, which activated the neurons, proving that the artificial skin could generate a sensory output compatible with nerve cells, the release explains.
Bao’s team envisions developing different sensors to replicate, for instance, the ability to distinguish corduroy versus silk, or a cold glass of water from a hot cup of coffee. But the current two-ply approach means the team can add sensations as it develops new mechanisms. And the inkjet printing fabrication process suggests how a network of sensors could be deposited over a flexible layer and folded over a prosthetic hand, the release continues.
“We have a lot of work to take this from experimental to practical applications,” Bao states in the release. “But after spending many years in this work, I now see a clear path where we can take our artificial skin.”
[Source(s): Stanford University, Science Daily]