A group of researchers has developed a soft prosthetic electronic skin (e-skin) patch that can transmit electrical signals to the brain to “feel” sensations like touch, pressure and temperature.  

Skin is considered one of the most important organs due to its double role as a first line of defense against external threats and because it mediates the sense of touch. When the nervous receptors embedded in our skin detect external stimuli (pressure and temperature) they transmit electrical signals to the brain, which process the information and operate the muscles generating a response. Therefore, this sensory feedback is not also essential to detect changes in our environment, but also for motor functions. As a matter of fact, patients suffering from skin damage struggle even with simple tasks such as object grasping

Current prosthetic limbs based on human-machine interfaces can partially help to restore motor functions, but they cannot perform fine movements, partially due to the lack of sensory feedback and tissue compliance. A group of scientists of Stanford University has recently created an electronic skin (or e-skin) that fulfills essential requirements (flexible, wearable and sensory) for the integration of signals into the nervous system of a living body that could improve life quality of patients by complementing prosthesis in the future.

Building artificial skin

The first step to develop the e-skin was the choice of an elastic material with sensors and integrated circuits. Existing flexible electronic systems are made of rigid semiconductors that operate at high voltage, which result in safety and power consumption concerns.

With this challenge in mind, the researchers combined three materials (nitrile-butadiene rubber, styrene ethylene-butylene-styrene and octadecyltrimethoxysilane) to develop an elastic and high-permittivity dielectric: a sensor layer that determines the strength of the signal and the voltage required to run an electronic device. The result was a stretchable and smooth like-skin material that could be used at low voltages, allowing high charge-carrier mobility in a nanoconfined space.

In order to emulate natural perception of the skin, where skin receptors transform stimuli of different amplitudes into electrical train pulses with a constant amplitude for high-fidelity signal transmission, the authors developed a circuit system with “sensing inverters”. These sensing inverters eliminate large variations in the oscillation amplitudes and enable a wide dynamic range of frequency changes. The fully-developed e-skin patch integrated several of these sensing inverters with different oscillation frequency ranges to distinguish information from multiple sensors. Thus, the resulting elastic e-skin was able to detect stimuli of different amplitudes and from distinct sources, comparable to natural skin. 

Emulating sensory perception in rats and future perspectives

To complement e-skin, the authors made another device able to transmit electrical signals from nerves to muscles, mimicking the connections in the nervous system - the synapses. As a result, the e-skin combined with the synaptic device completed a sensory feedback loop system able to mediate bidirectional signal communication between perception and actuation

Finally, the group tested the biological utility of this artificial sensory system in a live rat model. The e-skin was connected through a wire to the somatosensory cortex- the area of the brain that processes the sense of touch- to reproduce cutaneous sensations. In the experiment, the authors triggered the e-skin by touch, which successfully sent the signal tpo the brain and stimulated the electrical activity of somatosensory neurons. To complete the sensory feedback loop, the brain transmitted the signal through the artificial synapse to the sciatic nerve in the animal’s leg, causing the limb to twitch.

Although this novel device looks promising for paving the way for prosthetic limbs with a sense of touch and the restoration of sensation in individuals with damaged skin,  at present, the e-skin must still be wired to an external power source. Moreover, to have a skin that covers all the fingers of the hand and responds to touch, temperature and pressure, will require much more development according to the authors. Combining all individual technologies developed in the field of prosthetic research with findings like this will enable someday to create artificial skin that resembles more to natural touch. 

Original article

Wang W, Jiang Y, Zhong D, et al. Neuromorphic sensorimotor loop embodied by monolithically integrated, low-voltage, soft e-skin. Science. May 19 2023;380(6646):735-742. doi:10.1126/science.ade0086