ROS from Physical Plasmas: Redox Chemistry for Biomedical Therapy

Abstract

Physical plasmas generate unique mixes of reactive oxygen and nitrogen species (RONS or ROS). Only a bit more than a decade ago, these plasmas, operating at body temperature, started to be considered for medical therapy with considerably little mechanistic redox chemistry or biomedical research existing on that topic at that time. Today, a vast body of evidence is available on physical plasma-derived ROS, from their spatiotemporal resolution in the plasma gas phase to sophisticated chemical and biochemical analysis of these species once dissolved in liquids. Data from in silico analysis dissected potential reaction pathways of plasma-derived reactive species with biological membranes, and in vitro and in vivo experiments in cell and animal disease models identified molecular mechanisms and potential therapeutic benefits of physical plasmas. In 2013, the first medical plasma systems entered the European market as class IIa devices and have proven to be a valuable resource in dermatology, especially for supporting the healing of chronic wounds. The first results in cancer patients treated with plasma are promising, too. Due to the many potentials of this blooming new field ahead, there is a need to highlight the main concepts distilled from plasma research in chemistry and biology that serve as a mechanistic link between plasma physics (how and which plasma-derived ROS are produced) and therapy (what is the medical benefit). This inevitably puts cellular membranes in focus, as these are the natural interphase between ROS produced by plasmas and translation of their chemical reactivity into distinct biological responses.

Figure 1

Schematic of two categories of commonly used plasma devices for medical application: dielectric barrier discharges and plasma jets. In dielectric barrier discharges, plasma is generated in atmospheric air directly onto the biological target (a), while in plasma jets, plasma is generated inside the device and delivered to the target via a flow of gas (b).

Figure 2

Heat map of the current state of knowledge of cold plasmas for biomedicine. Blue: known and well-characterized commercial plasma sources (left) and reported effects of plasma therapies in in vivo models and human patients (right). Yellow: many biologically relevant plasma-generated ROS in air or in liquids have been described (left); however, it is still a challenge to tune the setups to deliver specific ROS mixes for different biomedical applications. In the same way, multiple effects of plasma in cells have been reported, yet the mechanisms of action of plasma-generated ROS in cells has not been fully unraveled (right). Red: the current bottleneck in the field is the little information available on how to use plasma to activate specific signalling pathways and evoke a desired effect in cells to design better and more effective therapies.

Figure 3

Scheme of hormetic responses. In the concept of hormesis, small concentrations of a given substance or molecules (including ROS) can have opposing effects between small and large concentrations.

Figure 4

Models for the study of the penetration of plasma-generated ROS into tissue. (a) In vitro approach for the analysis of ROS penetration using 0.02% methyl red as a reporter of ROS in 0.5% agarose gel. The treatment applied with Ar/O₂ (1%) kINPen MED at 4 mm distance demonstrates that the penetration depth is directly proportional to the treatment time (unpublished/original data). (b) Proposed mechanisms of action of plasma ROS and concomitant effects in tissues. The primary effect is exerted in the first layers of cells that directly interact with the short-lived ROS. At this level, oxidative damage is induced in the extracellular matrix, cell membranes, and intracellular components of cells located in the outermost region of the tissue. The long-lived ROS able to penetrate into deeper regions of the tissue elicit a secondary oxidative effect in cells. However, the effect of plasma extends to more profound regions of the tissue due to the oxidation of redox-sensitive cysteine and thiols in proteins with paracrine effects and via cell-to-cell communication.

Figure 5

Overview of cold plasma-mediated signalling pathways, including oxidative stress (Nrf2), mitogen-activated protein (MAP) kinase, p53, Wnt/β-catenin, cytoskeletal, cell adhesion or growth factor (GF) signalling, and differentiation.

Figure 6

The cell membrane is the key compartment that plasma-derived ROS need to penetrate or interact with to elicit biological responses. While some ROS are able to penetrate cellular membranes (e.g., ozone, nitric oxide and atomic oxygen), other more polar ROS cannot (e.g., singlet delta oxygen, nitrite, hydroxyl radical, superoxide anion, hydrogen, and peroxynitrite). Hydrogen peroxide is actively transported into the cells via transporters such as aquaporins.

Box 1

Current challenges in the field of plasma medicine.

Box 2

Current opportunities in the field of plasma medicine.