A team of engineers from Stanford University has 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," explained Zhenan Bao, professor of chemical engineering at Stanford.
The heart of the technique 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.
Bao 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 I want to create a flexible electronic fabric embedded with sensors that could cover a prosthetic limb and replicate some of skin's sensory functions," she said.
Bao's work, reported in the journal Science, takes another step toward her goal by replicating one aspect of touch - the sensory mechanism that enables us to distinguish the pressure difference between a limp handshake and a firm grip.
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, which further compresses the plastic's molecular springs.
To exploit this capability electronically, the team 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.
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. However, this will take some time. 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.
"We have a lot of work to take this from experimental to practical applications. But after spending many years in this work, I now see a clear path where we can take our artificial skin," Bao concluded.