Untethered electrode sheds light on therapeutic uses of neural stimulation

Researchers at the University of Pittsburgh have developed a method of neural stimulation that uses an untethered electrode activated by light, an advance that could mitigate damage done by implanted devices.

neural stimulation
A laser shining onto an untethered, ultrasmall carbon fiber electrode to stimulate neurons via the photoelectric effect (Pic: J. Mater. Chem. B, 2015,3, 4965-4978 – Reproduced by permission of RSC)

Neural stimulation can provide therapeutic effects in neurological disorders like Parkinson’s disease, but implanted devices deteriorate over time and can cause scarring in neural tissue.

“Typically with neural stimulation, in order to maintain the connection between mind and machine, there is a transcutaneous cable from the implanted electrode inside of the brain to a controller outside of the body,” said Takashi Kozai, an assistant professor of bioengineering at the university’s Swanson School of Engineering. “Movement of the brain or this tether leads to inflammation, scarring, and other negative side effects. We hope to reduce some of the damage by replacing this large cable with long wavelength light and an ultra small, untethered electrode.”

The photoelectric effect occurs when a photon hits an object and causes a local change in the electrical potential. Based on Einstein’s 1905 publication on this effect, Kozai’s group – the Bionic Lab – expected to see electrical photocurrents at high-energy ultraviolet wavelengths.

“When the photoelectric effect contaminated our electrophysiological recording while imaging with a near-infrared laser [low energy photons], we were a little surprised,” said Kozai. “It turned out that the original equation had to be modified in order to explain this outcome. We tried numerous strategies to eliminate this photoelectric artefact but were unsuccessful in each attempt, so we turned the ‘bug’ into a ‘feature.'”

“Our group decided to use this feature of the photoelectric effect to our advantage in neural stimulation,” said Kaylene Stocking, a senior bioengineering and computer engineering student. “We used the change in electrical potential with a near-infrared laser to activate an untethered electrode in the brain.”

The lab created a carbon fibre implant 7-8 microns in diameter, or roughly the size of a neuron. Stocking, first author of the paper detailing the research, simulated their method on a phantom brain using a two-photon microscope. She measured the properties and analysed the effects to see if the electrical potential from the photoelectric effect stimulated the cells in a similar way to traditional neural stimulation.

“We discovered that photostimulation is effective,” said Stocking. “Temperature increases were not significant, which lowers the chance of heat damage, and activated cells were closer to the electrode than in electrical stimulation under similar conditions, which indicates increased spatial precision.”

“What we didn’t expect to see was that this photoelectric method of stimulation allows us to stimulate a different and more discrete population of neurons than could be achieved with electrical stimulation.” said Kozai, “This gives researchers another tool in their toolbox to explore neural circuits in the nervous system.

“We’ve had numerous critics who did not have faith in the mathematical modifications that were made to Einstein’s original photoelectric equation, but we believed in the approach and even filed a patent application“, said Kozai. “This is a testament to Kaylene’s hard work and diligence to take a theory and turn it into a well-controlled validation of the technology.”

Kozai’s group is currently looking further into other opportunities to advance this technology, including reaching deeper tissue and wireless drug delivery.

The team’s research is detailed in a paper titled: Intracortical neural stimulation with untethered, ultra small carbon fiber electrodes mediated by the photoelectric effect. The work was done in collaboration with Alberto Vasquez, research associate professor of radiology and bioengineering at the University of Pittsburgh.