Princeton researchers study plasma sterilization

December 18, 2003

Hundreds of billions of plastic food and beverage containers are manufactured each year in the U.S. All of these packages must undergo sterilization, which at present is done using high temperatures or chemicals. Both of these methods have drawbacks. Chemicals often leave a residue that can affect the safety and taste of the product, and produce undesirable waste. Heat is effective and sufficiently rapid, but necessitates the use of costly heat-resistant plastics that can withstand sterilization temperatures. What if a new method could be found that eliminated the need for chemicals or heat-resistant plastics?

Plasma just might be the answer. At the U.S. Department of Energy's Princeton Plasma Physics Laboratory (PPPL), a team is conducting a small-scale research project studying plasma sterilization. This method, if successful, could be used to sterilize food and beverage containers, leading to an enormous savings - potentially hundreds of millions of dollars annually for a large soft drink manufacturer.

"We have experiments indicating it is possible to kill microbes using a new plasma approach," noted John Schmidt, lead scientist of PPPL's Plasma Sterilization project. Schmidt cautioned, however, that the research is preliminary. "These experiments need to be published, peer reviewed, and repeated by other researchers to assure reliability. Physics research will be followed by considerable development work to arrive at a practical system for assembly line use," said Schmidt, who has been awarded a patent for a plasma sterilization system [see apparatus shown in sketch at right]. Working with Schmidt are PPPL Technology Transfer Head Lewis Meixler, physicist Doug Darrow, engineer Nevell Greenough, and technicians Gary D'Amico and Jim Taylor.

To get started, PPPL researchers modified old equipment that had once been used to study radio-frequency (RF) waves for fusion applications. It consisted of a vacuum chamber equipped with an RF source. A metal sphere measuring one inch in diameter was mounted at the center of the chamber. In preparation for experiments, the sphere is removed and sent to a commercial biological testing laboratory in Hightstown where a known number of spores of bacillus subtilis, a non-toxic microbe commonly used as a standard in lab testing, are placed on its surface. Following an experiment, the sphere is returned to Hightstown where technicians determine the number of spores killed in the process.

Fusion experiments at PPPL have generated plasmas with temperatures in the hundreds of millions of degrees centigrade. For killing spores, the PPPL researchers start with "low-temperature" hydrogen plasmas in the range of 50,000 degrees centigrade. At that temperature, the hydrogen ions are moving much too slowly to kill spores quickly. Rapidly pulsing a 50-kilovolt potential between the sphere and the vacuum chamber solves the problem. The sphere is charged negatively and the vessel is at ground. Under these circumstances, the positively-charged hydrogen ions accelerate toward the sphere in pulses energetic enough for the ions to pierce the hard outer shell and soft inner core of the spore. Recent experiments employed 4,000 10-microsecond pulses, which reduced the population of live spores by a factor of 100-1000 - the kill ratio.

In the real world, equipment and processes suitable for the assembly line of a packaging plant would be needed. In such a situation, sterilization time is precious. RF generates a low-temperature hydrogen plasma inside the evacuated container, which is held in place by a surrounding conducting shell. An electrode is inserted into the container. The plasma is then subjected to a pulsed differential of 50 kilovolts, with the electrode pulsed positively and the conducting shell grounded. This causes energetic pulses of hydrogen ions to accelerate away from the electrode toward the conducting shell. On the way, they collide with spores present on the inner surface of the container. The hydrogen ions are energetic enough to penetrate the durable proteinaceous outer cover of the spores.

"These high-energy hydrogen ions stop very quickly and consequently deposit all their energy over a very small distance, a few microns, which, as it turns out, is the size of the spores. So relatively modest currents of energetic hydrogen ions can do a large amount of damage inside the spores by messing up their DNA," said Schmidt. He estimates that a sufficient kill ratio could be attained by 10-microsecond pulses every millisecond for a few seconds. Further experimentation is needed to confirm the number of 10-microsecond pulses necessary to reach the required kill ratio. A few seconds' processing time per container would make the system feasible for the assembly line.

The effectiveness of the hydrogen ions can be compared with that of gamma rays or X-rays used to sterilize bulk materials. Gamma and X-rays have long penetration depths, so they don't do as much damage per unit length as the hydrogen ions. "Textbooks contain the radiation damage coefficients that are required to kill the relevant microbes. I am confident that we will be able to attain these," said Schmidt.

A small business has been started to do the development work leading to a potential commercial application.
-end-


DOE/Princeton Plasma Physics Laboratory

Related Plasma Articles from Brightsurf:

Plasma treatments quickly kill coronavirus on surfaces
Researchers from UCLA believe using plasma could promise a significant breakthrough in the fight against the spread of COVID-19.

Fighting pandemics with plasma
Scientists have long known that ionized gases can kill pathogenic bacteria, viruses, and some fungi.

Topological waves may help in understanding plasma systems
A research team has predicted the presence of 'topologically protected' electromagnetic waves that propagate on the surface of plasmas, which may help in designing new plasma systems like fusion reactors.

Plasma electrons can be used to produce metallic films
Computers, mobile phones and all other electronic devices contain thousands of transistors, linked together by thin films of metal.

Plasma-driven biocatalysis
Compared with traditional chemical methods, enzyme catalysis has numerous advantages.

How bacteria protect themselves from plasma treatment
Considering the ever-growing percentage of bacteria that are resistant to antibiotics, interest in medical use of plasma is increasing.

A breakthrough in the study of laser/plasma interactions
Researchers from Lawrence Berkeley National Laboratory and CEA Saclay have developed a particle-in-cell simulation tool that is enabling cutting-edge simulations of laser/plasma coupling mechanisms.

Researchers turn liquid metal into a plasma
For the first time, researchers at the University of Rochester's Laboratory for Laser Energetics (LLE) have found a way to turn a liquid metal into a plasma and to observe the temperature where a liquid under high-density conditions crosses over to a plasma state.

How black holes power plasma jets
Cosmic robbery powers the jets streaming from a black hole, new simulations reveal.

Give it the plasma treatment: strong adhesion without adhesives
A Japanese research team at Osaka University used plasma treatment to make fluoropolymers and silicone resin adhere without any adhesives.

Read More: Plasma News and Plasma Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.