Scientists have been working for years to allow artificial skin to transmit what the brain would recognize as the sense of touch. For anyone who has lost a limb and gotten a prosthetic replacement, the loss of touch is reputedly one of the more difficult losses to accept. The sense of touch is also vital in robotics if the field is to expand and include activities reliant on the sense of touch, e.g., how much pressure do you use to grasp a cup; how much strength  do you apply when moving an object from one place to another?

For anyone interested in the ‘electronic skin and pursuit of touch’ story, I have a Nov. 15, 2013 posting which highlights the evolution of the research into e-skin and what was then some of the latest work.

This posting is a 2015 update of sorts featuring the latest e-skin research from Stanford University and Xerox PARC. (Dexter Johnson in an Oct. 15, 2015 posting on his Nanoclast blog (on the IEEE [Institute of Electrical and Electronics Engineering] site) provides a good research summary.) For anyone with an appetite for more, there’s this from an Oct. 15, 2015 American Association for the Advancement of Science (AAAS) news release on EurekAlert,

Using flexible organic circuits and specialized pressure sensors, researchers have created an artificial “skin” that can sense the force of static objects. Furthermore, they were able to transfer these sensory signals to the brain cells of mice in vitro using optogenetics. For the many people around the world living with prosthetics, such a system could one day allow them to feel sensation in their artificial limbs. To create the artificial skin, Benjamin Tee et al. developed a specialized circuit out of flexible, organic materials. It translates static pressure into digital signals that depend on how much mechanical force is applied. A particular challenge was creating sensors that can “feel” the same range of pressure that humans can. Thus, on the sensors, the team used carbon nanotubes molded into pyramidal microstructures, which are particularly effective at tunneling the signals from the electric field of nearby objects to the receiving electrode in a way that maximizes sensitivity. Transferring the digital signal from the artificial skin system to the cortical neurons of mice proved to be another challenge, since conventional light-sensitive proteins used in optogenetics do not stimulate neural spikes for sufficient durations for these digital signals to be sensed. Tee et al. therefore engineered new optogenetic proteins able to accommodate longer intervals of stimulation. Applying these newly engineered optogenic proteins to fast-spiking interneurons of the somatosensory cortex of mice in vitro sufficiently prolonged the stimulation interval, allowing the neurons to fire in accordance with the digital stimulation pulse. These results indicate that the system may be compatible with other fast-spiking neurons, including peripheral nerves.

And, there’s an Oct. 15, 2015 Stanford University news release on EurkeAlert describing this work from another perspective,

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.

Five years ago, Bao’s [Zhenan Bao, a professor of chemical engineering at Stanford,] 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 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.

Importing the signal

Bao’s team has 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. 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 developed by Karl Deisseroth, a fellow professor of bioengineering at Stanford who pioneered a field that combines genetics and optics, called 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.

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 has already worked with Bianxiao Cui, an associate professor of chemistry at Stanford, to show that direct stimulation of neurons with electrical pulses is possible.

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. 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.”

Here’s a link to and a citation for the paper,

A skin-inspired organic digital mechanoreceptor by Benjamin C.-K. Tee, Alex Chortos, Andre Berndt, Amanda Kim Nguyen, Ariane Tom, Allister McGuire, Ziliang Carter Lin, Kevin Tien, Won-Gyu Bae, Huiliang Wang, Ping Mei, Ho-Hsiu Chou, Bianxiao Cui, Karl Deisseroth, Tse Nga Ng, & Zhenan Bao. Science 16 October 2015 Vol. 350 no. 6258 pp. 313-316 DOI: 10.1126/science.aaa9306

This paper is behind a paywall.