(Invited) Numerical Simulation of a CAP Needle and Its Prospective Therapeutical Applications
Authors: Schroeder Max , Ochoa Angel , Breitkopf Cornelia 
Published: February 2020

Plasma medicine is an emerging field where plasma physics is used for therapeutical applications. Plasmas seem to have healing effects on different skin diseases, and are also promising on cancer treatment.One of their capabilities is the increase of antimicrobial activity without damaging surrounding tissue, combined with a controlled stimulation of tissue regeneration. A plasma discharge can be applied directly on biological tissues. The so-called cold atmospheric plasmas or CAPs are used for these purposes. They can be generated by different sources, namely, barrier discharges (BDs), plasma jets, and corona discharges.
In order to characterize the plasma chemistry, some experimental methods, such as mass spectrometry, can be used. However, these methods suffer from inaccuracies and it is difficult to obtain a complete understanding of the plasmas by just using them. Therefore, mathematical modeling and numerical simulations are essential tools that can be used for an improved interpretation of experimental diagnostics and better understanding of these plasma processes and their optimization.
In our group at the TU Dresden, we are currently working on finite-element simulations of CAPs. We are performing simulations of two main devices: plasma jets and plasma needles. Both of them are well known and they are starting to be widely used in the discipline of plasma medicine. These devices are able to operate with different gases, being the most common argon and helium. In order to simplify simulations, literature generally starts with more simplified plasma chemistries. We also did, however, extended this to more complex chemistries also. The plasma discharge is simulated using a fluid model. Drift-diffusion equations are used in order to calculate electron densities, electron energies and heavy particles transport. Poisson´s equation is used for calculating the electric potential. Boltzmann equation has also been investigated in order to calculate the electron energy distribution function eedf (depending on the electron mean energy) and the electron transport coefficients.
Temperature is an important factor to take into account for the applications of plasma to biological systems. During the treatment of a tissue, its temperature could increase to critical values. The model developed by our group to simulate the plasma needle is capable of predicting skin temperature during a treatment with a radio frequency driven plasma needle, as it was shown in [1], assuming helium to be the gas which is forming the plasma discharge. The results provide maximum application times for different power depositions in order to avoid reaching critical skin temperatures. To achieve this, a discharge model was coupled to a heat transfer and fluid flow model. The detailed procedure, it´s advantages and disadvantages will be discussed in the presentation.
Moreover, some simulations using this plasma needle have been performed considering argon plasma chemistry. The results show very low power depositions in comparison with experimental results. Including a more complex argon chemistry does not change this fact. One of the possible reasons is that, in actual devices, argon is mixed with other species. Therefore the plasma power deposition may change significantly in comparison with pure argon plasma discharges. We built a 1D plasma discharge model with a complex chemistry of argon and oxygen which proved that the presence of oxygen increases importantly the power deposition. This could be the explanation for our “low” power depositions when using just argon in a plasma needle.
Further results will be discussed in detail as for instance simulated plasma jets and complete loops of plasma discharge, heat transfer and fluid flow to the plasma jet.
All model simulations will be evaluated with respect to literature data and possible applications in medical treatment.

[1]
M. Schröder, A. Ochoa Brezmes und C. Breitkopf, „Numerical simulation of an atmospheric pressure RF-driven plasma needle and heat transfer to adjacent human skin using COMSOL,“ Biointerphases, Nr. 10, p. 029508, 2015.