Plasma decontamination of surfaces

Introduction The concept of inactivation or destruction of a population of micro-organisms is not an absolute one. This is because it is impossible to determine if and when all micro-organisms in a treated sample are destroyed (Block, 1992). It is also impossible to provide the ideal conditions to inactivate all micro-organisms: some cells can always survive under otherwise lethal conditions. Therefore, experimental investigation of the kinetics of cell inactivation is paramount in providing a reliable temporal measure of microbial destruction. Conventional methods that have been used to sterilize the surface of materials relied on high temperatures and pressures, ionizing radiation, or toxic gases/liquids. Although effective, these methods exhibit several drawbacks that make them far from being optimal approaches. For example, high temperatures can damage the materials under treatment and degrade their quality as well as shorten their lifetime. This is also true for the use of ionizing radiation, which is compounded by the possibility and risk of exposure to the operator. For polymeric materials that are widely used in medical devices and some surgical instruments, ionizing radiation is known to degrade the material. Toxic gases (such as ethylene oxide) have been used to sterilize materials under low-temperature conditions but the treatment cycle is long and residues can remain on the surface, which can lead to undesirable side effects. There have been persistent environmental concerns on the use of ethylene oxides and it is not inconceivable that current and future regulatory pressure may eventually force these to be phased out. Therefore, a need to develop an alternate method capable of inactivating bacteria on surfaces at low-temperature and without causing damage to the materials under treatment is apparent. Low-temperature plasmas have the potential to fulfill such a need. The chemistry of low-temperature plasmas can be tailored to generate controllable fluxes of specific chemically reactive species. This chemistry is controlled by the electron energy distribution function without a need to elevate the gas temperature, making plasma an ideal medium for the treatment of heat-sensitive materials such as the ones used to fabricate reusable medical instruments (catheters, endoscopes, etc.). In addition, it is possible to fine-tune the plasma to generate copious amounts of UV radiation or to minimize the emission of UV depending on what is being treated. It is for all these reasons that, in recent years, several investigators have been evaluating the potential of low-temperature plasmas as a base on which a novel sterilization technology can be developed.