Engineers at Stanford University in the US have developed a plastic “skin” that can detect how hard it is being pressed and then send this sensory information directly to a living brain cell.
The work, which is reported in the journal Science, was led by Zhenan Bao, a professor of chemical engineering at Stanford who has spent a decade working on the development of artificial skin. Prof Bao hopes one day to create a flexible electronic fabric embedded with sensors that could cover a prosthetic limb and replicate some of skin’s sensory functions.
At 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 features a sensor that can detect pressure over the same range as human skin from a light finger tap to a firm handshake.
Bao’s team has previously demonstrated 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 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 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 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.
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 cell are engineers 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.