By Pat Anson, PNN Editor

Researchers at the Massachusetts Institute of Technology have developed experimental fiber implants that could potentially be used to deliver pulses of light from inside the body to inhibit nerve pain. Unlike other implants, the hydrogel fibers are flexible and stretch with the body during movement.     

“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,” said co-author Siyuan Rao, PhD, now an assistant professor of biomedical engineering at the University of Massachusetts at Amherst. “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.”

In tests on laboratory mice with genetically modified nerves, researchers used the fiber implants to deliver blue light to the sciatic nerve, which activated the animals’ hind limb muscles.

When pulses of yellow light were used, the light inhibited neuropathic pain in the mice.

For now, MIT engineers see the fibers primarily as a research tool that can help them study the causes and potential treatments of peripheral nerve disorders in animals.

Neuropathic pain occurs when peripheral nerves are damaged, resulting in tingling, numbness and stinging sensations in the hands and feet. About 20 million Americans suffer from peripheral neuropathy, which can be caused by diabetes, chemotherapy, lupus, HIV, Lyme disease, celiac disease and many other disorders.

“Now, people have a tool to study the diseases related to the peripheral nervous system, in very dynamic, natural, and unconstrained conditions,” said co-author Xinyue Liu, PhD, who is now an assistant professor at Michigan State University. 

The MIT team’s study, recently published in the journal Nature Methods, grew out of a desire to expand the use of optogenetics -- a technique in which nerves are genetically engineered to respond to light. Exposure to specific light waves can either activate or inhibit a nerve, giving scientists a new way to study how nerves work. 

Scientists have used optogenetics in animals to trace nerves involved in a range of brain disorders, including addiction, Parkinson’s disease, and mood and sleep disorders.

Until now, optogenetics has primarily been used in the brain, an organ that lacks pain receptors, which allows for the relatively painless implantation of rigid devices. The MIT team wondered if optogenetics could be expanded to nerves outside the brain to study peripheral nerve pain.  

Because peripheral nerves undergo constant pushing and pulling from adjoining muscles and tissues, they needed more flexible devices that would not constrain movement or cause tissue damage.  

The new optical fibers are made with hydrogel — a rubbery mix of polymers and water – that is soft and flexible. The fiber has two layers; a core and an outer shell that funnel light through the fiber without escaping or scattering. 

When implanted in mice, researchers say the animals were still able to run freely on an exercise wheel. After two months, the fiber was still resistant to fatigue and could transmit light efficiently to trigger muscle contractions. 

“We are focusing on the fiber as a new neuroscience technology,” Liu says. “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.”