Inactivation of Bacteria by Non-Thermal Plasmas

Abstract

In physical sciences, “plasma” refers to the forth state of matter, while in medicine and biology, plasma is known as the non-cellular component of blood (Fridman et al., 2008). Interestingly, the term “plasma” has been coined by Irving Langmuir to emphasize that the characteristics of ionic liquids ubiquitous in biology and medicine are analogous to plasma in the physical sciences (Fridman et al., 2008, Langmuir, 1928). Despite this historical connection, plasmas are mainly associated with the solid-state processing technology (Stoffels et al., 2003), while being rarely used in biomedical applications directly. This situation is however rapidly changing and multiple plasma applications in life sciences are recently emerging (Daeschlein et al., 2010, Vandamme et al., 2010, Kalghatgi et al., 2010, Kong et al., 2009, Nie et al., 2009, Kalghatgi et al., 2007). The plasma state can be considered to be a gaseous mixture of oppositely-charged particles with a roughly zero net electrical charge (Denes&Manolache, 2004). Besides charged particles, plasmas also contain neutral atoms and molecules, excited atoms and molecules, radicals and UV photons. Generally, plasmas can be subdivided into 2 categories: thermal plasmas and non-thermal (or cold) plasmas (Denes&Manolache, 2004, Fridman et al., 2008, Bogaerts et al., 2002). Thermal plasmas are characterized by very high temperatures of electrons and heavy particles, both charged and neutral. In contrast, non-thermal plasmas are composed of low temperature particles (charged and neutral molecular and atomic species) and relatively high temperature electrons (Bogaerts et al., 2002, Denes&Manolache, 2004). Because the ions and the neutrals remain relatively cold, a non-thermal plasma does not cause any thermal damage to articles it comes in contact with. This characteristic opened up the possibility to use these non-thermal plasmas for the treatment of heat-sensitive materials including biological matter such as cells and tissues (Laroussi, 2009). Non-thermal plasmas are already routinely used in material processing applications, such as etching, activation and deposition (Borcia et al., 2006, Bruce et al., 2010, De Geyter et al., 2008, De Geyter et al., 2009, Morent et al., 2009a, Morent et al., 2009b, Maruyama et al., 2010). More recently, the biological and medical applications of these plasmas have witnessed a great interest from both plasma as well as medical research communities. This review paper focuses on one specific fascinating application of non-thermal plasmas in biomedical science, namely the inactivation of bacteria, also called plasma sterilization