Restoring lost foot sensations with sensory prosthesis

Rarely, papers on devices to treat pain and sensory loss in the feet get accepted for publication in Nature. But this one did - and it IS quite a breakthrough.

Peripheral neuropathy is a condition that leads to sensory loss, particularly in the feet. In a recent study, researchers presented a non-invasive wearable neuroprosthesis to restore lost foot sensation.

The device uses neurostimulation to activate partially healthy nerves at the ankle, where they remain accessible despite the degeneration in the more distant areas of the foot, like in the toes.

It works by stimulating damaged nerves in the ankle area using a specialized neuroprosthetic sock with multiple electrodes. The electrodes are placed to target three key lower-limb nerves, ensuring optimal stimulation.

The study
14 participants underwent a personalized calibration process to determine the most effective combination of electrodes and electrical charge for restoring sensations in the desired areas of sensory loss.

A custom-designed app helped adjust settings, allowing the system to be fine-tuned based on real-time feedback from the participants.

The device works in a closed-loop system, where pressure sensors in the insole detect foot movements and activate specific electrodes in the sock to match the sensations with the areas of the foot being stimulated. This system operates quickly, enabling real-time sensory feedback during walking.

The study showed that the device successfully restored sensations in participants with advanced disease, improving gait and reducing the risk of falls.

After just one day of use, participants also reported a reduction in neuropathic pain, which could be explained by the co-stimulation of larger sensory fibers that inhibit pain signals.

Improved function
The researchers investigated how well the restored sensations matched the body's natural sensory system and found that while most sensations felt like tingling or vibration, a few described the sensation as more similar to natural touch.

Moreover, the new device improved functional tasks, such as walking and cadence, contributing to better daily functioning and reduced metabolic and cardiovascular risks.

The advantage of non-invasive devices is that people avoid the risks of surgery, including infections and complications, which are particularly high in individuals with diabetic neuropathy, one of the main causes of lost sensation in the feet.

Brain activation and sensory feedback
The study employed brain imaging to understand how the restored sensations were processed in the brain.

The brain scans showed that the stimulation produced cortical activity in areas that corresponded closely to the sensations experienced when the foot was directly stimulated.

This finding supports the use of simulation as a viable approach to restore lost foot sensations, offering a more accurate and natural solution. Although the results varied among individuals, the study demonstrated the potential of meaningful sensory restoration even in cases of advanced neuropathy.

Long-term potential
The device is promising because it targets the proximal nerves that remain partially healthy in individuals with peripheral neuropathy.

This strategy contrasts with traditional methods that attempt to stimulate the distal nerves of the foot directly - which is not effective for individuals with severe nerve damage.

The study also confirmed that the device could work for several years before the degeneration of nerve fibers becomes too advanced, as long as the nerve conduction velocity remains above a certain threshold. As the disease progresses, however, the effectiveness of the device may diminish.

About the scientific paper:

First author: Noemi Gozzi, Switzerland
Published Nature Communications, December 2024
Link to paper: https://www.nature.com/articles/s41467-024-55152-7