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Sensors may soon give prosthetics a lifelike sense of touch
Bao Research Group/Stanford University

Sensors may soon give prosthetics a lifelike sense of touch

Prosthetic limbs may work wonders for restoring lost function in some amputees, but one thing they can’t do is restore an accurate sense of touch. Now, researchers report that one day in the not too distant future, those artificial arms and legs may have a sense of touch closely resembling the real thing. Using a two-ply of flexible, thin plastic, scientists have created novel electronic sensors that send signals to the brain tissue of mice that closely mimic the nerve messages of touch sensors in human skin.

Multiple research teams have long worked on restoring touch to people with prosthetic limbs. 2 years ago, for example, a group at Case Western Reserve University in Cleveland, Ohio, reported giving people with prosthetic hands a sense of touch by wiring pressure sensors on the hands to peripheral nerves in their arms.

Yet although these advances have restored a rudimentary sense of touch, the sensors and signals are very different from those sent by mechanoreceptors, natural touch sensors in the skin. For starters, natural mechanoreceptors put out what amounts to a digital signal. When they sense pressure, they fire a stream of nerve impulses; the more pressure, the higher the frequency of pulses. But previous tactile sensors have been analogue devices, where more pressure produces a stronger electrical signal, rather than a more frequent stream of pulses. The electrical signals must then be sent to another processing chip that converts the strength of the signals to a digital stream of pulses that is only then sent on to peripheral nerves or brain tissue.

Inspired by natural mechanoreceptors, researchers led by Zhenan Bao, a chemical engineer at Stanford University in Palo Alto, California, set out to make sensors that churn out digital signals directly. Bao’s group started by refining sensors that they first made 5 years ago. In that earlier work, the group designed tiny rubber pillars containing electrically conductive carbon nanotubes, which were placed over a pair of electrodes side by side. When no pressure is applied, the rubber, which is an insulator, prevents current from flowing between the two electrodes. But when touched, the pressure squishes the pillars, pushing the conductive nanotubes together to make a continuous electrical path and allowing current to flow. When the pressure is removed, the rubber pillars bounce back to their original shape.

For their current work, Bao and her colleagues turned their pillars into inverted pyramids and tweaked their size so they were sensitive to a range of pressures, from a light touch to a firm handshake. They also changed the electrode setup and added another layer of flexible electronic devices, known as ring oscillators, which convert the electrical signals emerging from the touch sensitive pyramids to a stream of digital electrical pulses. The upshot was that—just like the signals from natural mechanoreceptors—when more pressure is applied, the oscillators turn out pulses at a higher frequency.

But Bao’s group didn’t stop there. The Stanford team also wanted to see if brain tissues could receive these signals. That’s typically done by inserting metal electrodes into the so-called somatosensory cortex of animals and watching their response. But metal electrodes can quickly damage natural brain tissue, making it impossible to study the transfer of signals over extended periods. So for their current study, Bao’s team decided to send the electronic pulses coming from the touch sensors to a light emitting diode, which converted them into a stream of pulses of blue light. Bao’s team then partnered with Stanford colleagues, led by Karl Deisseroth, to genetically engineer somatosensory cortex tissue of mice to absorb blue light and fire in response. They sacrificed some of the engineered mice and isolated a slice of the light-sensitive somatosensory cortex, which remained viable for several hours. Finally, they tested their touch sensors and monitored whether the mouse brain tissue received the signals and fired in response. In today’s Science they report that the brain neural tissue faithfully reproduced the firing patterns coming from the touch sensor. That raises hopes that such sensors may eventually help restore a natural sense of touch to amputees, Bao says.

“It’s great to see research moving in this direction, and this particular paper is impressive,” says John Rogers, a chemist and expert in flexible electronics at the University of Illinois, Urbana-Champaign. Both Rogers and Bao note, however, that giving amputees a natural-like sense of touch still has a ways to go. Doctors, for example, won’t be able to engineer human brain tissue to receive light signals. That means researchers will need to find other ways to pass electrical signals from a prostheses to the brain in a way that is stable and safe for long periods of time. Bao says she hopes to use flexible organic electronics for this task as well. Eventually, as these different threads of research are woven together, it’s likely to give people with prosthetic limbs a whole new feel for their surroundings. 

(Video credit: Bao Research Group)