Prosthetic limbs have come a long way in the past 25 years. People who lose an arm or a leg can now be fitted with sophisticated prostheses that interface with the nervous system directly, which read the brain signals related to planning movements and translate them into commands for the device, enabling the user to control their replacement appendage by merely thinking about it.

Neurally-controlled prosthetic devices can vastly improve quality of life for amputees and paralysed patients, by helping them to move and regain at least some of their independence. Ultimately, though, researchers hope to develop devices that provide sensory feedback to the user – this would not only allow for more accurate control of the prosthesis, but would also enable the user’s brain to incorporate the artificial limb into its model of the body and take full ownership of it, so that actually feels more like a part of the body than a cumbersome add-on.

Now, two independent groups of researchers in America and South Korea have made a big step in this direction, with the development of artificial electronic skin containing sensors that can detect touch and heat, discriminate between different types of touch sensations, and relay this information to brain cells.

Although we often take it for granted, skin is in fact a highly specialised organ, and our sense of touch is extremely sophisticated. The skin on your fingertips is packed with nerve endings containing receptors that are sensitive to various types of mechanical pressure, from a slow, gentle caress to a sharp, painful blow. These mechanoreceptors also enable us to perceive fine texture. When we touch an object, its surface produces tiny vibrations on the skin, and our fingerprints amplify these vibrations to aid the mechanoreceptors in detecting and processing them. Skin is also flexible, can heal itself, and is sensitive to a wide range of temperatures and noxious chemicals.

Zhenan Bao, a chemical engineer at Stanford University, has spent the last decade trying to develop electronic skin that mimics all of these properties. In 2012, her team reported that they had developed a skin-like self-healing polymer material, and earlier this year they described a chameleon-inspired electronic skin that changes colour according to how much pressure is applied to it. Now, they describe an artificial mechanoreceptor system made out of organic electronic materials.

The Digital Tactile (DiTact) System consists of a thin plastic sheet made up of two layers. The top layer contains sensors made up of carbon nanotubes arranged in microscopic pyramid-shaped structures. These are connected to a flexible electronic circuit in the layer below, which translates the pressure information into pulses of light that can be transmitted to nerve cells by light-emitting diodes.

Artificial skin with electronic mechanoreceptors. Photograph: Bao Research Group / Stanford University

The researchers used optogenetics to demonstrate this. They introduced an algal protein called channelrhodopsin into slices of mouse somatosensory cortex, the brain region containing cells that process touch information. This makes the cells sensitive to light, so that they fire off impulses when they are illuminated by light of the right wavelength. Next, they connected the brain slices artificial skin, and showed that applying pressure to the sensors elicits trains of nervous impulses that activate the somatosensory neurons.

The key to the pressure-sensing system is the size and spacing of the carbon nanotubes in the top layer. Pressing on the plastic squeezes the nanotubes closer together so that they conduct electricity, with the size of the current produced being directly proportional to the amount of pressure applied. The more pressure that is applied, the closer they are brought together, and the more electricity they conduct. Thus, the frequency of the signals increases with the amount of pressure applied to the skin, and so, too, do the responses of the neurons in the brain slices. As a result, the digital mechanoreceptors are sensitive to the same range of pressure as biological ones, detecting everything from a light touch to a firm grip.

The Korean group, led by Hyonhyub Ko of Ulsan National Institute of Science and Technology, has developed a skin-like material that can detect touch and heat simultaneously, made from a flexible ferroelectric material, which has a spontaneous electrical field that can be reversed or manipulated by application of an external electric field.

This skin also consists of two layers, but in this case, both layers contain electrodes that come into contact under pressure, and the sensors are made of a plastic and graphene polymer, which generates voltage when heated, and whose electrical conductance and resistance change when mechanical pressure is applied. The researchers fabricated this material so that it contains microscopic ridges that enhance the stimuli being detected, just like those on our fingertips.

Ko’s team show that their e-skin is sensitive enough to detect the pressure created by water droplets, the dynamic movements of hair being pulled across it, and even the tiny amounts amounts of pressure created by a single strand of human hair. It can also discriminate between a wide range of pressures and temperatures, and dynamically alters its electrical output in response to changes in both. The skin can even detect the vibrations created by soundwaves and convert them into electrical signals, too.

Ultimately, researchers hope to incorporate skin-like materials such as these into prosthetic limbs, to help amputees regain their sense of touch, but these developments are still a long way off. Meanwhile, though, the materials are likely to be applied widely in the development of wearable medical diagnostics, and both groups of researchers have already started to apply their technology towards this aim: Bao and her colleagues incorporated the DiTact system into a glove that shows how voltage from the sensors changes with pressure, while Ko’s team used their e-skin to make a wristband that monitors blood pressure.

References

Tee, B. C. - K., et al. (2015). A skin-inspired organic digital mechanoreceptor. Science, 350: 313-316 [Abstract]

Park, J., et al. (2015). Fingertip skin–inspired microstructured ferroelectric skins discriminate static/dynamic pressure and temperature stimuli. Sci. Adv., 1: e1500661 [Full text]