This study presents a set of experiments designed to study the stability over time of the conducting polymer poly(3,4-ethylene dioxythiophene) (PEDOT), under simulated physiological conditions. Especially, the influence of switching the counter ion used in electropolymerisation, from surfactant polystyrene sulphonate (PSS) to heparin, was investigated. Electropolymerised PEDOT was exposed to different solutions at 37 °C over a 5-6 weeks study period. Two methods were used to study changes over time, spectroscopy and cyclic voltammetry. Phosphate buffer solution (PBS) and diluted hydrogen peroxide (H2O2) (0.01 M) were used to simulate in vivo environment. Some PEDOT electrodes in PBS were also subject to voltage pulsing to further stress the material.
The vast part of the samples of both types lost both electroactivity and optical absorbance within the study period, when exposed to H2O2. An overall slightly higher stability of PEDOT:PSS compared to PEDOT:heparin could be seen. The time dependence of the decline also differed, with a linear decrease of electroactivity for PEDOT:heparin while for PEDOT:PSS a comparably stable appearance initially, followed by a marked decrease after 8-15 days.
Polymers were relatively stable in PBS throughout the study period, with around 80% of electroactivity remaining after five weeks. Disregarding a slight drop in electroactivity during the first day, voltage pulsing in PBS did not increase degradation (tested over 11 days). Delamination of PEDOT exposed to PBS was however a significant problem, especially for polymer on ITO substrates.
PEDOT is sensitive to oxidising agents, also in the dilute concentrations used here, and counter ion influences the time course of degradation. Even without oxidising agents, some decline in electroactivity can be expected and it is unclear whether this decrease will continue over time, or if the polymer will stabilise. Such stabilisation was however not seen within the five weeks studied here. Delamination of polymer is likely to be a problem on implantation, especially with unwisely chosen substrates, and might be an even more serious threat to long term applications than degradation in biological fluids.