Cold plasma is an excited gas. The first curiosity is that it is the fourth state of matter. When we were taught states of matter at school, until recently it was said that there were three: solid, liquid and gas. But the reality is that there is a fourth state which is the plasma state. It is a gas that contains a lot of energy and is ionized. It contains a large number of particles and these are the ones that are used for what you point out in your question, medical applications.
Traditionally, cold plasma was used to treat surfaces. The TV screens are plasma or also fluorescent. But where plasma is most common is in nature. The Sun, for example, is a very, very hot plasma. Or the northern lights that appear when the gas in the atmosphere is ionized and their colors depend on which gas is ionized. When nitrogen does, we see green, if oxygen is ionized we see purple or pink.
Until about fifteen years ago it was difficult to generate cold plasma in the laboratory at atmospheric pressure because it required very specific conditions, but then small devices began to be designed, with a head similar to a pen, that can work in atmospheric conditions, that is say at room temperature and pressure. The beauty of this in contrast, for example, to the plasma of the Sun is that the plasmas that we can generate with these medical devices are cold. When we say that they are cold, it means that they are at body temperature. And this allows us to use them to treat biological tissues.
The medical areas in which they are beginning to be used are for the treatment of wounds and to treat tumors. More intense research is being carried out in these two areas because there are very promising results. And there are already companies around the world that are developing these cold plasma medical devices for these two therapeutic areas.
Plasma contains a large number of active species and in fact one of the most active agents it contains are reactive oxygen and nitrogen species. Reactive species are very small molecules that are highly reactive (very capable of reacting in the presence of other molecules) because they have a shell of electrons. The tool we play with is oxidative stress. Oxidative stress is caused by an imbalance between the production of reactive oxygen species and the ability of a biological system to repair the damage that production causes. It is related to many diseases. And what it causes is a reduction in cell power that in severe cases can lead to cell death.
What happens with oxidative stress is that if we give it at low doses with these cold plasma devices for the treatment of wounds, it helps to stimulate the regeneration of healthy tissues. It helps the tissue of the wound to regenerate and at the same time, for example, in chronic wounds such as diabetic ulcers that are very difficult to heal, it is capable of attacking the bacteria in that wound. And this double effect helps make it very effective.
In the case of tumors, what is done is to give higher doses to selectively attack cancer cells. If we compare a healthy cell with a cancerous one, we see that the latter reproduce much faster, so they have a very high metabolic state and contain more reactive species than healthy cells. When we give the tumor a significant dose of cold plasma that contains reactive species, since cancer cells already have a greater amount of their own oxidative stress from the outset, we increase their levels to a limit at which we induce cell death. But the healthy cells around the tumor start with a lower amount of reactive species, so they do not reach that limit and are able to compensate for the oxidative stress that we cause, so they survive.
There are currently many teams around the world investigating the treatment of tumors with cold plasma. There are several clinical studies underway. In Europe there is a clinical study led by Germany that is investigating head and neck tumors. And in the United States there is another clinical trial to test a very powerful team in various types of solid tumors.
Cristina Canal She is a Full Professor and directs the laboratory PlasmaMED of Plasmas for biomedical applications at the Barcelona East School of Engineering of the Polytechnic University of Catalonia and is president of PlasTHERa research network formed by 24 countries to study the therapeutic applications of cold plasma.
Question sent via email by Jesus Lopez
Coordination and writing:Victoria Bull
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