Cold atmospheric plasma jet (CAPJ) is one of the latest and most promising techniques for potential cancer treatment and other biomedical applications. Due to the direct contact of air, the jet is sensitive to the parameters of the local environment such as relative humidity (RH). In a RH controlled chamber, the electron density of a helium CAPJ is measured using the Rayleigh microwave scattering method, and its optical emission spectroscopy (OES) is recorded using a spectrometer. A decreasing electron density along with the increasing RH is thus revealed, while the humidity effect on OES at a high discharge voltage is increased. These trends imply a shift of electron energy distribution function (EEDF) due to extra attachments of electrons as the physics behind such a phenomenon. This hypothesis is supported by a computation of the mean electron temperature and EEDF based on the electron density we measured and a plasma chemistry model. Therefore, this report is a basis of future CAPJ stabilization development, which is a necessity of reliable biomedical applications, such as an active control loop to make CAPJ immune to any accident environmental disturbance during a plasma-based cancer surgery.

The electron temperature of small streamer plasmas with low ionization degree is difficult to measure using existing diagnostic technologies such as Langmuir probe and laser Thomson scattering. In this report, we introduce a method of average electron temperature estimation based on the electron continuity equation. The equation includes a temporal derivative of electron density as the summation of diffusion current, drift current, and electron gain/loss events. In the considered example, the electron density is measured using the Rayleigh scattering while the currents and the rate coefficients of those events are functions of the electric field, electron collision frequency, and electron temperature. Therefore, once the electric field and collision frequency are either measured or estimated, the only unknown in the equation is the average electron temperature which can be solved. It was estimated that electron temperature in the streamer peaks at about 3.9 eV for the given example.

Cold atmospheric plasma (CAP) jet exhibits remarkable properties that trigger cell death in cancer cells. The effect of CAP on cancer cells is influenced by several factors including plasma jet discharge voltages, gas composition and cancer cell type. Consequently in clinics it becomes challenging to plan plasma cancer treatments for a particular cancer types. To address this, we present preliminary results for an in vitro model which includes an optimal feedback control scheme that can adjust treatment conditions based on the actual cancer cell response. Translation to an in vivo model will be the next objective of the presented project. First, a mathematical model is presented for the dynamic response of cancer cells to CAP jets based on experimental data that provide temporal measurements of cancer cell viability after CAP treatments. A differential equation is developed to model the influence of CAP on the viability of two cancer cell lines, U-87 MG and MDA-MB-231, under varying treatment duration and plasma discharge voltages. Subsequently, a control scheme is presented to determine CAP treatment conditions in an optimal fashion by reducing cancer cell viability less than a prescribed goal while minimizing a weighted sum of the treatment duration and the discharge voltage. This is further extended to a model predictive control framework such that a pre-planned CAP treatment schedule is revised according to the actual cancer cell response. The efficacy of the proposed approach is illustrated by numerical simulations based on experimental data.