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Liquid Injectable Electrodes Designed for Nerve Stimulation

Kip Ludwig with syringe

Kip Ludwig holds a syringe with the injectable electrode material (Renee Meiller, University of Wisconsin – Madison)

12 Nov. 2019. In lab animal tests, an injectable silicon-metal material performs as well as conventional electrodes for electrical nerve stimulation, an emerging therapy for pain. Researchers at University of Wisconsin in Madison and Case Western Reserve University in Cleveland describe the material and test results in the 7 November issue of the journal Advanced Healthcare Materials (paid subscription required).

Stimulating nerve pathways with implanted electrodes is used increasingly as a non-drug alternative treatment for pain and other neurological conditions. Implanting electrodes, however, is an invasive procedure, usually requiring surgery, which adds cost and complexity to the treatments. Researchers led by Wisconsin biomedical engineering professor Kip Ludwig and Case Western Reserve adjunct professor Andrew Shoffstall are seeking a simpler and less expensive method for inserting electrodes into the body for electrical nerve stimulation.

The solution devised by Ludwig, Shoffstall, and colleagues is a material they call an “injectrode,” a material injected as a liquid that coats the nerves being stimulated. The team’s injectrode is a liquid, uncured bio-compatible silicon plastic, mixed with enough metallic particles, silver in this case, to attract and conduct an electric current.

The injectrode material is injected to the nerve requiring stimulation, where the silicon cures in the body and forms a conductive contact with the nerve. “Typical implants are really stiff, and so as the body moves, they wear and tear and break down,” says Ludwig in a University of Wisconsin statement. “Our liquid cures, and the result is much closer to the normal elasticity of tissue. You can actually stretch it and increase its size 150 percent to 200 percent without losing its conductivity.”

The researchers first bench-tested the material’s conductivity in the lab, and found the material performs as well as pure silver wire and commercially-available nerve stimulation electrodes. Then, to prove the concept, the team tested the injectrode with rats and pigs. For the animal tests, the team injected the material around the target nerve and retracted the syringe leaving a thin, wire-like residue that reached to a small well of injectrode under the skin’s surface. The tests used a low voltage current similar to today’s transcutaneous electrical nerve stimulation with implanted electrodes.

In the tests with pigs, animals with organs similar in size and function as humans, the researchers injected the material around the vagus nerve. The vagus nerve pathway extends from the brain stem to the abdomen, connecting other major organs including the heart, esophagus, and lungs. Results show the injectrode enables stimulation of the pig’s vagus nerve that induces changes in the animal’s heart rate.

In September, the Ludwig-Shoffstall team received a three-year, $2 million grant from National Institute of Neurological Disorders and Stroke, part of National Institutes of Health, to develop an injectrode system to stimulate the dorsal root ganglion, a complex of sensory nerve cells in the spinal cord, as a less expensive and more widely available non-opioid treatment for pain. Collaborating on the project is the company Neuronoff Inc., in Valencia, California, founded by Ludwig and Shoffstall in 2017, to commercialize the injectrode technology.

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