Conventional neuroprosthetic devices that aim to help patients bypass nerve damage are often rigid and power hungry. Now scientists have developed stretchable artificial nerves that helped paralyzed mice run on a treadmill and kick a ball while consuming less than one-hundredth of the power of a typical microprocessor. The scientists suggest these artificial nerves may one day be used in the human body.

To help restore movement to patients who have suffered nerve damage from injuries or diseases, scientists are researching neuroprosthetic devices that can help relay signals from the brain to muscles or nerves. However, these systems often face a number of critical limitations, says study co–senior author Tae-Woo Lee, a materials scientist at Seoul National University.

For example, conventional neuroprosthetic systems often depend on power-hungry external computing systems. They also typically stimulate the body with electric pulses of constant strength that abruptly increase and decrease in magnitude, “which cause drastic contraction of muscles that make patients uncomfortable,” Lee says.

To help generate more natural, comfortable movements, conventional neuroprosthetic systems may add voltage ramping during the start and end of electrical stimulation. However, this involves additional devices known as function generators that are typically rigid and bulky, Lee says, making them a poor fit for the human body.

In the new study, the researchers developed highly stretchable electronic nerves that mimic real nerves. Like real nerves, these artificial nerves can deliver electric signals that gradually ramp up and down in strength. These artificial neuroprosthetics also consume only about 1/150 of the power of a typical microprocessor.

The new device consists of a stretchable organic semiconducting nanowire transistor that electrically stimulates muscles using soft, elastic hydrogel electrodes. This bioinspired or “biomimetic” device acts like an artificial synapse, a junction that links neurons together in the human body.

The device is coupled via an ion gel to a carbon nanotube sensor that detects strain. This serves as an artificial version of a proprioceptor, a sensor that receives signals from within the body to help it keep track of its position and movements. The researchers used this artificial proprioceptor to give real-time feedback to the electronic nerve. This helped keep the artificial nerve from overstimulating and overstraining mouse leg muscles, all without the need of external computers to control the movements.

In the new study, the researchers experimented with mice they anesthetized to paralyze their muscles in order to mimic injuries or diseases targeting nerves. They found they could use their artificial nerves and proprioceptors to generate coordinated smooth leg movements, including walking and running on a treadmill or kicking a ball. They also showed they could use the neuroprosthetics to move the legs of the mice with electrical signals recorded from the rodents’ brains.

“Our work is the first example of delivering biological neural signals through biomimetic electronic nerves to biological organs,” Lee says. “Through this, it seems possible to present new solutions and strategies for nerve damage in humans such as spinal-cord injury, peripheral nerve damage, and neurological damage such as Lou Gehrig’s, Parkinson’s, and Huntington’s disease.”

In addition to potential medical applications, “the source technology of the stretchable artificial nerve may be applied to various medical wearable technologies,” says study co–senior author Zhenan Bao, a materials chemist at Stanford University, in California.

The scientists detailed their findings online 15 August in the journal Nature Biomedical Engineering.

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Restoring Hearing With Beams of Light

Gene therapy and optoelectronics could radically upgrade hearing for millions of people

13 min read
A computer graphic shows a gray structure that’s curled like a snail’s shell. A big purple line runs through it. Many clusters of smaller red lines are scattered throughout the curled structure.

Human hearing depends on the cochlea, a snail-shaped structure in the inner ear. A new kind of cochlear implant for people with disabling hearing loss would use beams of light to stimulate the cochlear nerve.

Lakshay Khurana and Daniel Keppeler

There’s a popular misconception that cochlear implants restore natural hearing. In fact, these marvels of engineering give people a new kind of “electric hearing” that they must learn how to use.

Natural hearing results from vibrations hitting tiny structures called hair cells within the cochlea in the inner ear. A cochlear implant bypasses the damaged or dysfunctional parts of the ear and uses electrodes to directly stimulate the cochlear nerve, which sends signals to the brain. When my hearing-impaired patients have their cochlear implants turned on for the first time, they often report that voices sound flat and robotic and that background noises blur together and drown out voices. Although users can have many sessions with technicians to “tune” and adjust their implants’ settings to make sounds more pleasant and helpful, there’s a limit to what can be achieved with today’s technology.

8 channels

64 channels

Since optogenetic therapies are just beginning to be tested in clinical trials, there’s still some uncertainty about how best to make the technique work in humans. We’re still thinking about how to get the viral vector to deliver the necessary genes to the correct neurons in the cochlea. The viral vector we’ve used in experiments thus far, an adeno-associated virus, is a harmless virus that has already been approved for use in several gene therapies, and we’re using some genetic tricks and local administration to target cochlear neurons specifically. We’ve already begun gathering data about the stability of the optogenetically altered cells and whether they’ll need repeated injections of the channelrhodopsin genes to stay responsive to light.

Our roadmap to clinical trials is very ambitious. We’re working now to finalize and freeze the design of the device, and we have ongoing preclinical studies in animals to check for phototoxicity and prove the efficacy of the basic idea. We aim to begin our first-in-human study in 2026, in which we’ll find the safest dose for the gene therapy. We hope to launch a large phase 3 clinical trial in 2028 to collect data that we’ll use in submitting the device for regulatory approval, which we could win in the early 2030s.

We foresee a future in which beams of light can bring rich soundscapes to people with profound hearing loss or deafness. We hope that the optical cochlear implant will enable them to pick out voices in a busy meeting, appreciate the subtleties of their favorite songs, and take in the full spectrum of sound—from trilling birdsongs to booming bass notes. We think this technology has the potential to illuminate their auditory worlds.

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