Current Gas Plasma Sterilization Procedure and Its Future Trends

Abstract

Ninety-nine per cent of the material in the universe is in the plasma state. Physicists call plasma the fourth state of matter, after solid, liquid, and gaseous states. Basically, plasma is composed of gas molecules that have been dissociated by an input of energy. Low-temperature gas plasmas are generated when certain gases are stimulated at atmospheric pressure (AP) or relatively low pressure (LP) with pulsed energy, radio-frequency waves or microwave energy. The plasmas of several different gases such as argon, helium, oxygen, nitrogen or their mixtures have sporicidal activity. Many modern medical devices are thermo-and hydro-sensitive. Given the drawbacks and limitations of other low temperature sterilization procedures, low temperature gas plasma sterilization could represent a useful alternative. The first practical application of gas plasma sterilization was developed in 1972. Since then several kinds of gas plasma applications have been designed to sterilize the bioburden of various products, and many researchers have studied the subject as well as the mechanism of gas plasma sterilization. The bioburden defines the type and number of viable microorganisms in/on a product (ISO 14161, 11138-1). Low temperature gas plasmas, used for surface modification and organic cleaning, are ionized gases generated at pressures between 0.1 and 2 torr. These types of plasmas work within a vacuum chamber from which atmospheric gases have been evacuated, typically below 0.1 torr. Low pressure allows for a relatively long free path of accelerated radicals and metastable. Neutral particles such as radicals and metastable can be produced at or near ambient temperatures, and undergo relatively few collisions with molecules at this temperature. Although both radicals and metastable are neutral compounds, their flight distances differ significantly. Whereas the flight distance of a radical is ~0.003 cm that of a metastable is ~ 144 cm. There are three traditional states of matter: 1iquid gas and solid. Plasma may be considered the fourth state in which the molecules of a gas are excited to become plasma when the gas atoms lose their electrons and generate a highly excited mixture of charged nuclei and free electrons. True plasma is actually considered to consist of positively and negatively charged particles in approximately equal concentrations. Plasma can be generated by the application of sufficient energy, in the form of heat or an electromagnetic field, to a gas. Plasma can be subsequently formed by further energy absorption by the gas, which fragments the gas atoms and molecules to produce negative ions, positive ions, electrons, and other short-lived or long-lived reactive species. It should be remembered that an atom of any element consists of a central nucleus (made up of positively charged protons and neutrons) that is surrounded by negatively charged and paired electrons, which are organized in defined orbitals depending on their energy levels. In this state, each atom is balanced, with an overall neutral charge produced by an equal number of electrons and protons. As energy is applied to the atoms/molecules in a gas, the molecules and atoms fragment to produce positive ions (as they now have a higher number of protons) and free, negatively charged electrons. In some cases, the electrons react with other atoms, thereby gaining an overall negative charge (negative ions). Further unstable species are also generated including ozone (in the case of oxygen plasmas; ozone causes a serious etching phenomenon) and other free radicals, metastable, photons, UV and VUV. The free radicals