Inside the brain, many neurons fire so that the body will perform a single action like picking up a cup or kicking a ball. Unfortunately for amputees with missing limbs, this brain activity is for naught. Now, engineers at the University of Southampton say they’ve shown that low-power devices known as memristors might be more energy efficient than today’s experimental neural interfaces that help relay signals from the brain to prosthetic limbs. 

Themis Prodromakis, who studies nanoelectronics at the University of Southampton, in England, is exploring one of the building blocks of brain and computer interfaces for medical applications. His early research supports the development of special neuronal brain-chips: neural implants that communicate with prosthetic limbs when neurons fire.

Monitoring thousands of recording sites in the brain and transmitting all that data is a very difficult problem, he says. He told IEEE Spectrum that adding memristors to a system with integrated circuits could allow researchers to monitor activity “from potentially millions of neurons.”

Memristors, the fourth type of basic electronic device—along with resistors, capacitors, and inductors—are special because they have a memory. When current flows, their resistance changes, but the changes remain even after the current is shut off. There’s a scientific movement aimed at using memristors to mimic how the brain learns and processes information.

In the new research, described in a paper published in Nature Communications on 26 September, Prodromakis and his team used memristors for another purpose: recording neuron activity. Recording neuron activity isn’t a new concept; an approach in 2005, for example, used CMOS devices. But it required offline data processing.

In the new experiment, the memristors work in a way reminiscent of the function of the retina in human eyes. Not everything our retina sees gets to the human brain as “it would overload,” Prodomakis says.

Memristors have set voltage thresholds. This lets a memristor, like the human retina, preprocess input signals locally and filter out everything but the important events—in this case, neuron firing activity.

The researchers cultured neurons and hooked them up to an electronic system including memristors. After the system did some preprocessing of the signals, the memristors successfully detected neuron spikes. The memristors then locally compressed the spike amplitude and firing rate.

The ultimate goal, Prodomakis, says, is that the low-power memristors would be hooked up with other devices as an implant to control prosthetic limbs. The next step, he says, is to classify the neuronal activity into different categories.

“This technology is very interesting as a processing tool,” says Max Ortiz Catalan, a biomedical engineer at the Chalmers University of Technology in Göteborg, Sweden. “However, the real clinical application is not very clear or straightforward.”

Ortiz Catalan, who was not involved in the study, works on creating implants for controlling artificial limbs. He notes that the “real bottleneck” in the research field is not processing signals, but finding a way to interface between the brain and prosthetics with long-term stability and high resolution.

“Generally speaking, brain-machine interfaces would benefit from
miniaturization and low-power computational systems,” says Paul Nuyujukian, an engineer at Stanford University who works on brain-computer interfaces for prosthetics. Nuyujukian, who was also not involved in the study, added that, “Work towards that goal is, of course, encouraging to see.”

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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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