Nerves that run from the spinal cord to the skin allow us to sense and interact with the world around us through touch. But there’s no reliable way to restore tactile sensation when such nerves have been damaged. In a new study, scientists have created tiny devices that, when implanted under the skin, harness electricity from tactile pressure to power up damaged nerves (ACS Nano 2021, DOI: 10.1021/acsnano.0c10141). Tested in animals, the devices restored a sense of touch after injury.
Rubbing two different materials together, such as a balloon against your hair, causes friction that results in the two materials taking on a mild electrical charge, a phenomenon known as the triboelectric effect. Over the past decade, researchers have attempted to harness this electricity to create devices called triboelectric nanogenerators, or TENGs, to charge cell phones, sensors, or other electronics.
Ben M. Maoz, a biomedical engineer at Tel Aviv University who led the new work, wondered whether the technology might solve a vexing biomedical problem. An old friend, Amir Arami, a hand surgeon at Sheba Medical Center and coauthor on the study, had described to Maoz the injuries he observed in the clinic and lamented the limited tools available for restoring nerve function. A TENG could potentially detect tactile pressure against the skin and power a nerve prosthetic, Maoz realized.
Maoz and his colleagues created a TENG about 25 mm2 in size by layering together two flexible and biocompatible dielectric materials: cellulose and polydimethylsiloxane. When this sheet is compressed, friction between the two materials drives an exchange of electrons, creating a small voltage of up to 2 V. Thin layers of gold above and below this sandwich convey the current via a wire to a cuff electrode that wraps around the injured nerve just above the damaged section, stimulating it. “To put it crudely, it’s a bypass,” Maoz says.
The researchers first showed that the device activated mouse sensory neurons in a dish and that the amount of pressure exerted on the TENG relates linearly to the voltage it produces. The linearity means that the electrical signal the nerve conveys accurately reflects the tactile experience. “The brain is adaptive and can learn, and this linearity enables the brain to develop the ability to restore sensation for different forces,” Maoz explains.
They then severed a sensory nerve in the hind footpad of rats and implanted a TENG to compensate for the lost sensation, surgically positioning the cuff around the nerve. Using a pressure test, the researchers found that the device restores the animals’ ability to respond to touch.
In tests outside the body, they also found that the device continues to work after 500,000 touches, suggesting it is durable. He and his colleagues plan to optimize the device and replicate the study in more animals before exploring its use in people. Because it is cheap to produce, such a tactile TENG could provide a widely accessible treatment for restoring peripheral nerve function, Maoz says.
“As a first step, definitely it’s great,” says Ramakrishna Podila, a physicist at Clemson University who is exploring TENG technologies. He cautions, though, that its durability might be hampered by blood clotting and other biological factors.
Additionally, Podila says, one issue with TENG devices is that the friction they rely on can change the surface roughness of the materials, and indeed, the researchers report that the cellulose became rougher with use. That, in turn, can change how much voltage a given touch intensity produces, making the link between a tactile stimulus and the nerve response it produces unpredictable.