Stanford engineers have created a plastic “skin” that can detect how hard it is being pressed and generate an electric signal to deliver this sensory input directly to a living brain cell.
“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,” says Zhenan Bao, the professor of chemical engineering who led the 17-person research team responsible for the achievement.
The new “skin” is a two-ply plastic construct. The top layer acts as the sensor that can detect pressure over the same range as human skin, from a light finger tap to a firm handshake. The bottom layer sends electrical signals and translates them into biochemical stimuli compatible with nerve cells. The top layer in the new work features a sensor.
Five years ago, Bao’s team members described how to use plastics and rubbers as pressure sensors by measuring the natural springiness of their molecular structures. They increased material’s sensitivity to pressure sensitivity by imprinting a waffle pattern into the thin plastic, which further compresses the plastic’s molecular springs.
To exploit this pressure-sensing capability electronically, the team scattered billions of carbon nanotubes through the waffled plastic. Putting pressure on the plastic squeezes the nanotubes closer together and lets them conduct electricity. This lets the plastic sensor mimic human skin, which transmits pressure information to the brain as short pulses of electricity, similar to Morse code.
Putting more pressure on the waffled nanotubes squeezes them even closer together, letting more electricity flow through the sensor. Reduce the pressure and the flow of pulses relaxes, indicating light touch. Remove all pressure and the pulses cease entirely.
The team 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.
Bao and her team have been developing flexible electronics that can bend without breaking. 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. Being able to cover large surfaces is important in making artificial skin practical, and the PARC collaboration offers that capability.
The team then proved that the electronic signals could be recognized by a biological neuron by translating the electronic pressure signals from the artificial skin into light pulses, which activated the genetically engineered neurons, proving that the artificial skin could generate a sensory output compatible with nerve cells.
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. Fortunately, the 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.