Influence of dielectric coatings on pin-to-rod nanosecond-pulsed discharges in phosphate-buffered saline.

Plasma medicine is a rapidly expanding field that utilizes non-equilibrium plasma discharges at atmospheric conditions or in liquids for clinical applications. There is significant interest in the production of plasma in the liquid phase for wastewater treatment, agricultural applications, and medical purposes. However, little investigation has been done about the effects of dielectric coatings on submerged electrodes, which is of significant interest to limit electrical current flow in the liquid. This work investigates the effect of different dielectric coatings including aluminum oxide, parylene C, and bi-layer combinations, on plasma discharge characteristics in phosphate-buffered saline (σ = 18 mS/cm) from nanosecond high-voltage pulses. Observed results for aluminum oxide are consistent with past works, including micron-sized clusters of holes generated in the layer due to dielectric breakdown. A bi-layer combination of parylene C on top of aluminum oxide resulted in longer lifetime for electrodes, possibly due to the melting/solidification behavior of the polymer, which may have a "healing" effect. The use of a thick parylene C layer resulted in a different, "creeping", discharge regime, which is hypothesized to be similar to triple-gap discharge observed in space plasma physics and high-voltage insulators, in which the electric field is enhanced at the boundary of a conductor, dielectric, and a vacuum/fluid, resulting in discharge at this junction point. Temporally-resolved and high-spatial-resolution imaging are required for verification.