Plasmasterilization: active ingredient cocktail to fight bacteria

Interview with Prof. Katharina Stapelmann, Chair of General Electrical Engineering and Plasma Technology (AEPT), Ruhr University Bochum


Until now, plasma, the fourth state of matter,was consideredfascinatingonly to astrophysicists and science fiction fans. But at this point, it also attracts the interest of medicine because plasma can have many uses in this field. In the future, plasma sterilization could become an important component of hospital hygiene-provided that the right device is being used.

Photo: Smiling woman with long blond hair and glasses - Junior Prof. Katharina Stapelmann

Junior Prof. Katharina Stapelmann; ©Meike Klinck

In this interview with, Junior Prof. Katharina Stapelmann explains how plasma affects pathogens, how wounds can be treated with it and how easy to use a plasma sterilizer is.

Junior Prof. Stapelmann, what is plasma?

Junior Prof. Katharina Stapelmann: Plasma is a partially ionized gas. This state of matter occurswhen you add energy to a gas like hydrogen, oxygen or argon. Broadly speaking, we differentiate between hot and cold plasmas. The best-known example of hot plasma is our sun. This hot plasma contains lots of energy, the degree of ionization is high and the particles-the lightweight electrons, the heavier neutral particles, and the ions-are able to efficiently interchange energy.

In the case of cold plasma, which can be used for sterilization or wound healing, only the lightweight electrons carry energy. Due to their small mass, they are not able to efficiently transfer energy to the heavier particles. The plasma as a whole stays cold and near room temperature because the low energy heavier particles dictate the temperature.

What happens during plasma sterilization of medical equipment or surgical instruments?

Stapelmann: Plasma is an active ingredient cocktail with many different substances and modes of action. The enclosed ultraviolet radiation destroys DNA so that bacteria can no longer proliferate. Reactive particles like radicals interact with biological materials and oxidize them. Biological material can also be very easily removed with plasma by using cauterization and sputtering processes.

This cocktail attacks bacteria at different levels. It prevents the development of resistances in doing so.

How effective is plasma sterilization compared to other methods?

Stapelmann: There is no direct comparison with other methods yet. Plasma sterilization is currently still in the research stage. It is not yet approved anywhere as a process. Compared to other methods, it offers a number of advantages but not on all levels. Plasma sterilization is considerably faster than the autoclaving procedure and it does not require any hazardous materials. The only restriction I know pertains to some plastics: not all types of plastic can be sterilized with plasma.

Plasma sterilization - photo gallery

Please click on the arrows on the right or on the left of each picture to browse through the pictures. For a bigger view please click the symbol below the lower right corner of each picture.
Photo: Female researcher with short dark hair and glasses works at a large apparatus
Junior professor Dr Katharina Stapelmann has built a plasma steriliser prototype at RUB.
Photo: Female researcher with short dark hair and glasses puts a transparent drawer box into an apparatus
The researcher designed the sterilisation chamber as a convenient drawer with a surface in DIN-A4 format.
Photo: Plastic parts inside a transparent, pink glowing drawer box
Katharina Stapelmann's steriliser is suitable not only for medical instruments and aerospace components. It can also remove germs from drill templates for tooth implants - and is thus of interest for dental technicians who have not as yet had any sterilisation devices for treating sensitive metal and plastic parts at their disposal.
Photo: Large metal apparatus with a pink glowing window in the front
The steriliser at the RUB lab deploys a hydrogen plasma that is characterised by its pink glow.
Photo: Carpets of spherical bacteria in a black-and-white image
For her sterilisation experiments, Katharina Stapelmann used screws that were coated by some 25 layers of a particularly persistent bacterium. By magnifying it by the factor 1000 under the scanning electron microscope, the single spherical bacteria cells are rendered visible.

You just mentioned wound healing. Where is the common denominator with sterilization?

Stapelmann: Wound healing is not about sterilization but rather the reduction of pathogens in the wound, so the body is able to fight back again on its own. This is achievedwith pathogen inactivation, that being the destruction of its DNA.

Having said that, plasma also has a multi-layered effect in this case. By adding nitric oxide, NO, or one of its derivatives such as nitrate or nitrite into the wound, the immune system is being activated in this area. This is actually a normal process in our body but it malfunctions in chronic wounds. Plasma also decreases the pH value in the wound and makes the environment inhospitable to pathogens.

Is exposure to plasma not harmful to body cells?

Stapelmann: Generally, plasmas produce UV radiation and radicals that can have a toxic effect. This is why it is important to observe critical values and thoroughly characterize plasma before use. Having said that, because of their size, human cells are considerably more immune to the effects of plasma compared to bacterial cells.

Wound healing with plasma is already approved; there are several devices on the market and first clinical trials have also been conducted. The procedure makes a great first impression and so far there are no known side or adverse effects.

Back to the subject of sterilization: you have built a plasma sterilizer at the RUB that could someday be used in hospitals. What does this device look like?

Stapelmann: It is approximately the size of a conventional autoclave and fits on a table.The device is controlled via touchscreen. The user only needs to start the sterilization, after which the entire process runs automatically. The discharge chamber is shaped like a drawer and lockable so that it can also serve as a sterile container. Sterilized instruments can remain in there until use and are protected.

No special safety precautions need to be taken for its use. The device works with a conventional 220-volt electrical outlet. Everything is shielded from electromagnetic radiation.

How are you planning on continuing the development?

Stapelmann: The basic research of the sterilizer has been completed. To develop the device even further, we now need the assistance of an interested company. In light of this, you cannot refer to this as a prototype because the sterilizer is not ready for serial production yet.

However, we continue to study the mode of action of plasma sterilization to optimize the process, make it more efficient and understand it. In a current project, we examine Bacillus subtilis spores to identify the protective mechanismsin spores and determine how we can take advantage of them.

Photo: Timo Roth; Copyright: B. Frommann

© B. Frommann

The interview was conducted by Timo Roth and translated from German by Elena O'Meara.