Atmospheric pressure plasma-substrate interactions: H2O2 diffusion in hydrogels and plasma induced surface charging.

Abstract

Low temperature plasmas (LTP) can produce chemically rich environments at close to room temperature at ambient pressures. LTP is able to deliver reactive oxygen and nitrogen species (RONS) in a non-destructive, beneficial way to even extremely heat sensitive surfaces. This provides a unique condition that enables plasma generated RONS to regulate key biological pathways by causing chemical and physical changes in living matter and has the potential to underpin new medical therapies for a range of illnesses. The goal of my project is (i) to study the production and diffusion of hydrogen peroxide species, a key RONS with biological impact, generated by interaction of an atmospheric pressure plasma jet with a gelatin gel used as a skin tissue model in this work and (ii) study the surface charge deposition by a plasma jet on a dielectric target. Surface charges are an important parameter to monitor since it influences the dynamics of the discharge propagation on the surface and charged species at biological interfaces are suggested to play a possible role in plasma induced biological effects. Plasma – Hydrogel interaction The interaction of He atmospheric pressure plasma with Gelatin hydrogel (10% w/v), a model for skin is investigated and in particular generation of reactive oxygen species (ROS) and penetration of ROS in the hydrogel. The diffusion of ROS (in particular H2O2) in gelatin hydrogel is analyzed by implementing two diffusion analysis approaches; diffusion cell experiments and experiments in which a droplet containing H2O2 is placed on top of a hydrogel. Furthermore, the flux of plasma generated H2O2 species both in liquid medium (distilled water) and gelatin hydrogel are determined in this study. KI – Starch is used as an indicator to observe and quantify the diffusion of H2O2 in the gel. Diffusion profiles of H2O2 droplets are directly compared to those generated by interaction of plasma. H2O2 droplet diffuses through the gelatin sample over time, giving rise to a deep blue colored diffusion profile. On the other hand, the helium generated plasma produces a surface level color change. In order to understand the surface level color change patterns experiments were performed by using ‘He + H2O’ and ‘He + O2’ as the feed gas in our plasma jet to investigate the influence of plasma produced H2O2, OH and O3 species respectively. The flux of plasma (He + 0.81% H2O) generated H2O2 species in gelatin hydrogel is found to be 1013-1014 molecules/(s·mm2). This is smaller than for the same plasma interacting with liquid H2O. The flux of plasma generated H2O2 species in humid helium distilled water was found to be (5.1 ± 0.3) × 1014 molecules/(s·mm2) which is 2 times higher than (2.5 ± 0.3) × 1014 molecules/(s·mm2) as measured for dry helium. The penetration depth for plasma (~ 2 mm) in gelatin hydrogel on a 20 min time scale mainly due to H2O2 diffusion, while other species like O3 and OH lead to interfacial effects. Plasma induced electric field and surface charge distribution The aim of this study was to determine the surface charge distribution induced by an RF driven atmospheric pressure plasma jet impinging on a glass substrate. The goal of my work was to augment the experimental data previously obtained with simulations, and calibration measurements. This study includes a validation measurement of the used approach in a Laplacian electrostatic field in a similar geometry as in the plasma experiment. A good correspondence of the experimental and simulation results for both negative (t = 9.48 µs) peak and positive (t = 9.52 µs) peak of the RF pulse were obtained by assuming a Gaussian surface charge distribution function. The instantaneous positive surface charge distribution had a peak density of σpos = 0.05 nC cm−2 and the corresponding full width at half maximum (FWHM) of the surface charge distribution was found to be ~1.5 mm. The instantaneous negative surface charge distribution had a peak density of σneg = 0.06 nC cm−2 and the FWHM of the surface charge distribution was found to be ~2 mm. The diameter is similar to the nozzle diameter of the jet and suggest minimal spreading of the jet on the glass surface. The results obtained in this work contribute to a better understanding of the interaction of plasma jets with biological and dielectric substrates and will help in the development of LTP applications that would benefit by using plasma to effectively ‘drive’ ROS into biological substrates and assess the role of surface charging on plasma-induced biological effects.