(Invited) Cold Atmospheric Plasma Jet Sources with a Flexible Matrix
Authors: Portal Sabine , Corbella Roca Carles , Lin Li , Keidar Michael 
Published: November 2022

Plasma is an ionized gas containing free electrons, ions, neutral species (atoms, molecules, radicals), photons from UV to infrared wavelengths. Its composition makes it a rich environment for reactive process, showing a collective response to external perturbations. Low temperature plasmas are hot electron gas (1-10 eV) which are not in thermal equilibrium with the other cold part of the atomic or molecular species (0.02 eV). These cold plasmas are then suitable for applications to delicate surfaces (skin) without damage, especially since RONS (Reactive Oxygen and Nitrogen Species) show healing properties. Biomedicine is facing new challenges like decontamination, wound healing, and oncotherapy. Plasma medical treatments are selective, instantaneous, tunable in comparison to lasers and chemicals. There is a big need for developing new plasma sources that can treat complex surfaces (with asperities, sharp edges, cavities, complex accessibility)

For this purpose, two types of flexible Cold Atmospheric Plasma Jet (CAPJ) sources with different geometries (planar and radial) were made and characterized through: multijet length and flow regime, electrical parameters, Optical Emission Spectroscopy (OES), performance after bending the aerogel. He (99.995%) was used in both configurations. Finite Element Method simulations were run to predict and understand the electromagnetic process in this plasma jet configuration.

In planar geometry, DBD (Dielectric Barrier Discharge) was constituted of a silica aerogel placed between two biased electrodes. Silica aerogel has a very low gas permeability and is mechanically stable (highly cross-linked fiber glass network). Plasma jets constituted of 3 aligned jets, a few mm long above the DBD region whose length is proportional to the He flow rate, flowing in laminar regime, were produced by Helium flow (flow rate: 1-4 slm) subjected to sinusoidal voltage of 5kV and 15kHz frequency. Higher flow rate produced turbulent flows. The ratio between gas velocity and discharge voltage can be adjusted: short jets (flow limited operation) are suitable for near surface treatments, longer jets (power limited operation) are appropriate for greater distance treatments. OES showed strong N2 emission. Planar DBD was studied in flat configuration then its flexibility was characterized in bent (concave, convex) configurations. Compliance of the plasma source is limited by aerogel size and thickness. There is possibility to extend the device from 1D to 2D (matrix of plasma jet lines).

In radial geometry, DBD was constituted of a soft hollow cylinder (about 5 cm external diameter, 2.5 cm internal diameter and 10 cm long), made of polyethylene (PE) foam, material selected for its flexibility and low permeability. An aluminum/dielectric coated rod electrode was placed in the core of the cylinder, copper circular electrodes were disposed on the external cylinder wall at 15 mm distance from each other surrounding 1mm holes punched in the PE cylinder, where the CAPJs were formed. In this geometry, two different operations modes were apprehended: comb mode was represented by four equidistant axial CAPJs and round brush mode was experienced when three CAPJs were aligned at constant azimuthal angle. In this case, discharges were triggered at 15 kHz AC voltage and a lower voltage than in planar configuration (3.8 kV). The jets were uniform in length and intensity. OES were recorded in the active region (inside the PE housing) and in the afterglow region. In the afterglow region, the spectrum was similar to the one obtained in planar geometry. A preponderance of hydroxyl groups (OH), nitrogen molecules (N2, N2
+), He atoms was found. The source remains stable after deformation of the PE housing.

The feasibility of these flexible plasma sources in radial and axial configurations is promising for treating any kind of surface with special topography.