Biocompatible, Electrically Conductive Polymer Coating Promises to Improve Brain Implants

brain-implants

Firing neurons (the green structures in the foreground) communicate with nanotubes in the background. Illustration courtesy of Mohammad Reza Abidian.

Brain implants that can more clearly record signals from surrounding neurons in rats have been created by scientists at the University of Michigan (U-M; Ann Arbor). Prompted by the work of Mohammad Reza Abidian, the findings could eventually lead to more-effective treatment of neurological disorders such as Parkinson's disease and paralysis.

Neural electrodes must work for periods ranging from hours to years. When they are implanted, the brain first reacts to an acute injury with an inflammatory response. Then the brain settles into a wound-healing, or chronic, response. During this secondary phase, the brain tissue starts to encapsulate the electrode, cutting it off from communication with surrounding neurons.

To prevent this phenomenon, the new brain implants are coated with nanotubes made of poly(3,4-ethylenedioxythiophene)--PEDOT--a biocompatible and electrically conductive polymer that has been shown to record neural signals better than conventional metal electrodes. PEDOT nanotubes in the coating enable the electrodes to operate with less electrical resistance than current metal electrode sites, which means they can communicate more clearly with individual neurons.U-M researchers have found that PEDOT nanotubes increase high-quality unit activity (a signal-to-noise ratio >4) by about 30% over uncoated sites. They have also discovered that based on in vivo impedance data, PEDOT nanotubes might be used as a novel method for biosensing to indicate the transition between acute and chronic responses in brain tissue.

In the experiment, the researchers implanted two neural microelectrodes in the brains of three rats. PEDOT nanotubes were fabricated on the surface of every other recording site by using a nanofiber templating method. Over the course of seven weeks, researchers monitored the electrical impedance of the recording sites and measured the quality of recording signals.

"Microelectrodes implanted in the brain are increasingly being used to treat neurological disorders," remarks Abidian, a postdoctoral researcher working with Daryl Kipke in the neural engineering laboratory at the U-M department of biomedical engineering. "Moreover, these electrodes enable neuroprosthetic devices, which hold the promise to return functionality to individuals with spinal cord injuries and neurodegenerative diseases. However, robust and reliable chronic application of neural electrodes remains a challenge."

"Conducting polymers are biocompatible and have both electronic and ionic conductivity," Abidian comments. "Therefore, these materials are good candidates for biomedical applications such as neural interfaces, biosensors, and drug delivery systems."

In previous experiments, Abidian and his colleagues have shown that PEDOT nanotubes could carry drugs to prevent encapsulation. "This study paves the way for smart recording electrodes that can deliver drugs to alleviate the immune response of encapsulation," Abidian says.

Read more about the breakthroughs that PEDOT is enabling in the development of better microelectrodes for neurological implants from other U-M researchers.