Hot Electrons, Cool Gas: The Energy Imbalance That Makes Physical Plasma Suitable for Medical Use

Anyone encountering physical plasma in medicine for the first time might easily underestimate what is actually happening. Physical plasma can be applied close to skin or wounds without strongly heating the tissue. In suitable medical applications, the gas temperature is often around 30 to 40 °C. At the same time, highly reactive chemical compounds are generated, molecules become excited, and chemical bonds can be broken. How can physical plasma that is barely warmer than the human body provide enough energy to initiate such processes?

The answer lies in one of the defining properties of these systems. Physical plasma used in medicine is usually non-thermal, and is therefore often referred to colloquially as cold physical plasma. However, non-thermal does not mean low in energy. It means that the different particles within physical plasma do not all have the same average energy. This state is known as non-equilibrium. It is precisely this non-equilibrium that allows physical plasma to combine a low gas temperature with pronounced chemical reactivity.

Non-thermal means non-equilibrium

To understand this, it helps to look briefly at what temperature actually represents. In a gas, atoms and molecules are constantly moving. The greater their average kinetic energy, the higher the temperature of the gas. In an ordinary gas, energy is redistributed through countless collisions between particles, allowing the system to approach thermal equilibrium.

Physical plasma contains not only neutral atoms and molecules, but also free electrons and ions. In thermal physical plasma, energy can become distributed relatively evenly among these different particle populations. Electrons, ions, and neutral particles then reach similarly high thermal energies, and the gas temperature can rise to several thousand degrees Celsius. Such a state would clearly be unsuitable for most direct medical applications.

Non-thermal physical plasma behaves very differently. It remains far from thermal equilibrium. In particular, free electrons can reach comparatively high energies, while neutral atoms, molecules, and ions retain much lower kinetic energies. The heavier particles therefore remain at temperatures that are compatible with treatment of biological tissue.

This unequal distribution of energy is not a minor side effect. It is the physical basis that makes the medical use of non-thermal physical plasma possible in the first place.

Light electrons, heavy atoms and molecules

The key difference between these particle groups is their mass. Electrons are extremely light. Atoms and molecules are much heavier, and ions have almost the same mass as their corresponding neutral atoms or molecules. This enormous difference in mass strongly influences how these particles respond to an electric field.

To generate physical plasma, electrical energy is introduced into a gas. The electric field acts particularly effectively on free electrons. Because electrons have such a small mass, they can be accelerated very rapidly. Their velocity increases, and with it their kinetic energy.

Ions also respond to electric fields because they carry an electric charge. However, they are much heavier and therefore cannot be accelerated as easily as electrons. Neutral atoms and molecules have no net electric charge and are not directly accelerated by the electric field in the same way. The result is a clear separation between two very different particle populations. The electrons are light and fast. The atoms, molecules, and ions are much heavier and move far more slowly.

This difference is fundamental for the medical application of physical plasma. The electrical energy supplied to the system is not distributed equally across the entire gas. A substantial fraction is initially transferred to the electrons.

Why this energy does not simply become heat

At this point, an obvious question arises. If the electrons take up so much energy, why do they not simply transfer it to surrounding atoms and molecules through their many collisions? If that happened efficiently, the gas should heat up very quickly.

Once again, the large difference in mass provides the explanation. During an elastic collision between a very light electron and a much heavier atom or molecule, only a small fraction of the electron's energy is transferred into the motion of the heavier particle. The electron may change its direction or speed substantially, while the heavier collision partner gains comparatively little kinetic energy.

This matters because gas temperature is mainly determined by the average kinetic energy of these heavy particles. If only a small amount of energy is transferred into their motion, the gas temperature rises only modestly.

A low gas temperature therefore does not mean that all particles in physical plasma have low energies. It mainly reflects the energetic state of the heavy components of the gas. But not every collision between an electron and another particle is elastic. This is where the chemistry that is so important for medical applications begins.

When a collision triggers chemistry

If an electron carries enough energy, a collision can transfer energy into the internal structure of an atom or molecule. In this case, the energy is not primarily converted into motion. Instead, it can change the electronic or chemical state of the collision partner.

A molecule can, for example, become excited. It then enters a higher energy state and may become more likely to participate in subsequent reactions. At higher transferred energies, ionization can occur, meaning that an electron is removed from an atom or molecule and a charged particle is formed.

Another particularly important process is dissociation. Here, a chemical bond is broken, producing smaller fragments that are often much more reactive than the original molecule.

This is where the distinction between heat and chemically useful energy becomes especially important. The energy does not simply spread through the gas as random molecular motion. Instead, part of it is directed into processes that alter atoms and molecules themselves. Physical plasma can therefore initiate reactions that would not normally be expected from a gas at the same temperature.

The energy distribution of the electrons matters

The electrons themselves do not all move at the same speed, and they do not all possess the same kinetic energy. Some are relatively slow, while others are much faster. For the chemistry of physical plasma, this distribution is crucial. Many reactions require a certain minimum energy before they can occur. An electron can only break a particular chemical bond or cause ionization if it carries enough energy to cross the corresponding threshold. Not every electron therefore needs to be highly energetic. What matters is whether a sufficient fraction of the electron population reaches the energy required for a particular process.

This electron energy distribution strongly influences which excited states, ions, and chemical fragments are produced. Changes in the electrical operating conditions can therefore alter the resulting chemistry even when the measured gas temperature changes very little.

For medical applications, this distinction is important. Measuring temperature provides essential information about thermal exposure, but temperature alone cannot tell us which chemically active species a particular physical plasma will generate.

From electron energy to reactive oxygen species

This mechanism becomes especially relevant when oxygen and water are involved. Both are abundant in the environment of biological tissues and can be transformed through energetic collisions. When an oxygen molecule is excited or dissociated, highly reactive oxygen atoms can be formed. Water molecules can also be activated or split, producing reactive products such as hydroxyl radicals. These initial products can then participate in further chemical reactions.

In this way, a complex mixture of reactive oxygen species develops. These may include atomic oxygen, hydroxyl radicals, superoxide, ozone, and hydrogen peroxide. These species differ greatly in their properties. Some exist only for a very short time and react almost immediately near the site where they were generated. Others are more stable and can travel farther or enter liquids. Once in an aqueous environment, additional reaction pathways can continue the chemistry that began in the gas phase.

This is particularly important in biological applications. Cells and tissues are rarely exposed to a dry gas phase alone. Biological surfaces are covered by water, extracellular fluids, wound exudate, or other liquid layers. Reactive oxygen species can therefore interact with these interfaces and generate further reaction products. What begins as an input of electrical energy can thus develop into a complex chemical environment at the surface of biological tissue.

Why this is so relevant for medical applications

From a medical perspective, this combination is what makes non-thermal physical plasma so unusual. Under ordinary circumstances, we often associate the energy required for extensive chemical reactions with substantial heating. In non-thermal physical plasma, these two aspects are separated. Chemically active processes can therefore be initiated under thermal conditions that remain compatible with biological tissue.

This also explains why the characterization of medical physical plasma cannot rely on temperature measurements alone. A low gas temperature is an essential prerequisite for safe application to tissue, but it provides only limited information about the chemical processes occurring within and around the physical plasma. To understand its biological potential, one must also consider how electrical energy is introduced into the system, which electron energies are generated, and which reaction pathways are opened as a result. These factors influence the spectrum of reactive oxygen species that ultimately reaches biological targets.

For clinicians, laboratory personnel, and life science researchers, this is an important shift in perspective. The effect of physical plasma is not determined simply by how warm it becomes. Its biological potential depends just as much on the chemistry that can be initiated under these comparatively mild thermal conditions.

A seeming contradiction that is not a contradiction at all

The original question can therefore be answered quite clearly. Medical physical plasma can operate at gas temperatures close to body temperature while still initiating demanding chemical reactions because it is not in thermal equilibrium. The supplied energy is distributed unevenly among the different particle populations. This allows individual particles to reach energies high enough to excite molecules, ionize them, or break chemical bonds without strongly heating the gas as a whole.

These initial molecular changes open the door to the subsequent chemistry of physical plasma, including the formation of reactive oxygen species that are central to many biological effects. Perhaps this is the most useful way to think about non-thermal physical plasma. The decisive question is not simply, how warm is physical plasma? The more informative question is, where is the energy within physical plasma, and what processes does that energy drive?

That distinction is what makes non-thermal physical plasma so remarkable, and what allows it to combine tissue compatible temperatures with highly active chemistry.