Conventional neural electrodes can have operational times of hours or even years but all of them trigger an initial in inflammatory response once they are inserted into the brain. After that, the brain settles into a chronic, wound-healing process, which can isolate the electrode from surrounding neurons as new tissue encapsulates it. Metal electrodes are obviously conducting but lack the biocompatibility that precludes this chronic wound-healing problem.

Now, nanotubes coated with the polymer poly(3,4-ethylenedioxythiophene) (PEDOT) have been used to record neural signals [Abidian et al., Adv. Mater. (2009) 21, 3764-3770]. Mohammad Reza Abidian and colleagues explain that PEDOT is not only electrically conductive but is also biocompatible, which they hoped would prevent encapsulation.

“Microelectrodes implanted in the brain are increasingly being used to treat neurological disorders,” explains Abidian. “Moreover, these electrodes enable neuroprosthetic devices, which hold the promise to return functionality to individuals with spinal cord injuries and neurodegenerative diseases.” The challenge is to find ways to make robust and reliable neural electrodes that could monitor symptoms and perhaps even deliver the appropriate medication automatically. The researchers have demonstrated the ability of PEDOT nanotubes to act as drug carriers previously.

The team has now tested their PEDOT nanotube electrodes in laboratory rats. In the experiment, they implanted two neural microelectrodes in the brains of three rats. They then monitored the electrical impedance of the recording sites and measured the quality of recording signals over a seven-week period. They found that not only was the signal-to-noise ratio of the coated electrodes 30% higher than with uncoated electrodes, but the coated electrodes operated with less electrical resistance. This meant that the electrodes could communicate more clearly with individual surrounding neurons.

“Conducting polymers are biocompatible and have both electronic and ionic conductivity,” Abidian explains. “Therefore, these materials are good candidates for biomedical applications such as neural interfaces, biosensors and drug delivery systems.” He adds that, “This study paves the way for smart recording electrodes that can deliver drugs to alleviate the immune response of encapsulation.”