Abstract
Neural electrodes are the best medical technique to alleviate the symptoms of neurodegenerative diseases and to explore of our brains. However, their efficiency is limited by several constraints: their small dimensions lead to high impedance and their rigid and metallic nature coupled with the destructive insertion procedure leads to inflammatory responses in the body that further increases the impedance. Conducting polymers are soft and organic materials that possess a mixed electronic-ionic conductivity, and are ideal candidates for biotic-abiotic interfaces. They are regularly used as coatings for neural electrodes due to their enhancement of electrochemical properties and their believed biocompatibility. The dominant technique to deposit conducting polymers on neural microelectrodes is electropolymerization. Extensive work as being performed on the optimization of the parameters of this deposition method. Still, electrodeposited conducting polymers coatings suffer from poor adhesion on most of their substrates. Besides this issue, there is a lack of long-term in vivostudies confronting conducting polymer coatings to electrical stimulation conditions used in medical applications.
In this work, we investigated the influence of the processing solvent for electropolymerization on the stability and the adhesion of conducting polymer coatings. After having defined a precise deposition procedure providing us with stable coatings, we investigated the usefulness of these coatings for in vivodeep brain stimulations.
Poly(3,4-ethylenedioxithiophene) (PEDOT) was electropolymerized in three different solvent: acetonitrile, propylene carbonate and water, on platinum-iridium microelectrodes. The coated microelectrodes were subjected to different stability tests: sonication, passive ageing, steam sterilization, and electrical stimulations in vitro. We found out that acetonitrile and propylene carbonate provided the most resistant PEDOT coatings. Still, all of PEDOT coatings processed in the different solvents were stable enough to be used in a medical context. We therefore implanted PEDOT-coated stimulating microelectrodes in rats and applied daily stimulation all the while monitoring the impedance of the microelectrodes. We observed that electrical stimulations decreased the impedance of both the PEDOT-coated microelectrodes and the uncoated control microelectrodes. The decrease in impedance was more important for the control microelectrodes than for the PEDOT-coated ones, which questions the relevance of PEDOT coatings for deep brain stimulation purposes and indicates that more work is required to obtain a performant conducting polymer coating for stimulating neural electrodes.