MIT engineers have designed a soft hydrogel optical fiber (shown illuminated) that stimulates peripheral nerves, and could help researchers in identifying the origins and treatments for nerve-related pain. [Image courtesy of the researchers/MIT]
These fibers can deliver light to major nerves throughout the body. When the nerves are genetically manipulated to respond to light, the fibers can send pulses of light to the nerves to inhibit pain. The flexible, stretchable fibers offer an experimental tool to explore peripheral nerve disorders in animal models.
The team reported the details of their findings in Nature Methods.
“Current devices used to study nerve disorders are made of stiff materials that constrain movement, so that we can’t really study spinal cord injury and recovery if pain is involved,” Siyuan Rao, assistant professor of biomedical engineering at the University of Massachusetts at Amherst, who carried out part of the work as a postdoc at MIT, said in a post on the MIT News website. “Our fibers can adapt to natural motion and do their work while not limiting the motion of the subject. That can give us more precise information.”
The researchers say they embarked on this effort to expand the use of optogenetics beyond the brain. Optogenetics — the technique that genetically modifies nerves to respond to light — can help bring information about how nerves work and interact with surroundings. To date, optegenetics primarily get employed in the brain, an area that lacks pain receptors and allows for the relatively painless implantation of rigid devices. But, the rigid devices can still damage neural tissues.
Researchers sought to expand the technique to the peripheral system, identifying the causes of peripheral nerve conditions and testing therapies to alleviate them. However, they identified motion as the main hurdle to implementing the technique beyond the brain. These types of nerves experience constant pushing and pulling from the surrounding muscles and tissues, according to MIT. Rigid silicon devices in the periphery could constrain an animal’s natural movement and potentially cause tissue damage.
The team decided to find an alternative that could work and move within the body. This led to the soft, stretchable, transparent fiber made from hydrogel. They tuned the ratio of polymers and water to create tiny, nanoscale crystals of polymers scattered throughout a gelatin-like solution. The fiber has a core layer and an outer shell called “cladding.” Each layer has a specific, different refractive index and, put together, keep light traveling through the fiber from escaping or scattering away.
Testing took place in mice who had their nerves genetically modified to respond to blue light or yellow light. The blue light would excite neural activity, while yellow would inhibit their activity. They found that mice could run freely on a wheel with the implanted fiber in place. After two months of wheel exercise, the fiber remained robust and resistant to fatigue while still capable of transmitting light efficiently to trigger muscle contraction.
Using a yellow laser run through the implanted fiber, the team found that mice were much less sensitive to pain than rodents that were not stimulated with light. The fibers were able to significantly inhibit sciatic pain in those light-stimulated mice.
The team believes the fibers could offer utility as a new tool that can help scientists identify the roots of pain and other peripheral nerve disorders.
“We are focusing on the fiber as a new neuroscience technology,” Liu said. “We hope to help dissect mechanisms underlying pain in the peripheral nervous system. With time, our technology may help identify novel mechanistic therapies for chronic pain and other debilitating conditions such as nerve degeneration or injury.”
