Zhenan Bao, PhD, a professor of chemical engineering and material science at Stanford University, has spent a decade trying to develop a material that mimics skin’s ability to flex and heal, while also serving as the “sensor net” that sends touch, temperature, and pain signals to the brain. Ultimately, she wants to create a flexible electronic fabric embedded with sensors that could cover a prosthetic limb and replicate some of skin’s sensory functions. Bao’s latest work, reported October 15 in Science, takes another step toward her goal by replicating the sensory mechanism that enables people to distinguish the pressure difference between a limp handshake and a firm grip.
The device on the “golden fingertip” is the skin-like sensor developed by Stanford engineers. Photograph courtesy of Stanford University.
The heart of the technique is a two-ply plastic construct: The top layer creates a sensing mechanism that can detect pressure over the same range as human skin, and the bottom layer acts as the circuit to transport electrical signals and translate them into biochemical stimuli compatible with nerve cells.
Five years ago, Bao’s team members first described how to use plastics and rubbers as pressure sensors by measuring the natural springiness of their molecular structures. They then increased this natural pressure sensitivity by indenting a waffle pattern into the thin plastic and scattering 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 to the brain as short pulses of electricity. 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 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. For this project, team members worked with researchers from PARC, a Xerox company, which has a technology that uses an inkjet printer to deposit flexible circuits onto plastic. Covering a large surface is important to making artificial skin practical, and the PARC collaboration offered that prospect.
Finally the team had to prove that the electronic signal could be recognized by a biological neuron. It did this by adapting a technique called optogenetics, in which 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.
Optogenetics was only used as an experimental proof of concept, Bao said, and other methods of stimulating nerves are likely to be used in real prosthetic devices.
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. This will take time, however, as there are six types of biological sensing mechanisms in the human hand, and the experiment described in Science reports success in just one of them. 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.
“We have a lot of work to take this from experimental to practical applications,” Bao said. “But after spending many years in this work, I now see a clear path where we can take our artificial skin.”
Editor’s note: This story was adapted from materials provided by Stanford University.
