Introduction

Clothing is perceived to be second skin to the human body since it is in close contact with the human skin most of the times. The textile products right from common clothing garments to high sensitive healthcare hospital linen materials was subjected to various processing starting from the raw material to finished product [1, 2]. The need for appropriate disinfection and sterilization techniques is of paramount importance [3]. There has been a continuous demand for novel sterilization techniques appropriate for use on various textile materials as the existing sterilization techniques suffer from various technical and economical drawbacks. This review paper emphasizes necessity for sterilization, essentials of sterilization, mechanism of plasma sterilization and the parameters influencing it.

Need and Necessity for Sterilization/Decontamination

Perhaps the most critical concern for healthcare linen facilities is preventing the spread of drug resistant strains of highly infectious bacteria including Staphylococcus aureus or “Staph” and fecal-borne spores and cysts including Clostridium difficile, or “C. diff” spores, Giardia cysts and Cryptosporidium oocysts [4]. Incomplete sterilization of linens can facilitate the spread of bacteria and provide a means of infection in both healthy people and patients with compromised immune systems. Hospital related infections are crucial problems for patient, health personnel, and society and health budget. According to the data of World Health Organization (WHO), one out of ten patients in hospitals suffers from cross- infection due to improper sterilization and reported as first of the ten reasons of deaths in developed countries [5]. Hospitals, nursing homes, acute care facilities and other healthcare operations produce a variety of dirty linens with a wide range of soil loading. Sheets with blood, urine, feces and other bodily fluids are routinely washed with other less contaminated linens.

Although incontinent pads are generally washed apart from other types of linens, they too pose a great risk for infection as each pad will inevitably end up on a new patient after each cleaning. To ensure a consistently clean and sanitized finished product, stringent procedures must be in place. Also detergent and softening agent residues remaining on the textile products might pose damage to the sensitive skins and babies. Generally, the linens are washed and ironed prior to wearing in order to eliminate harmful factors. However, this is not enough to remove harmful chemical or germs on the clothes.

Further, due to the inherent properties of textile fibres, they readily support the growth of microorganisms, and must undergo preventative treatments. If infestation of microbes does occur, staining and foul odours may form, not to mention the loss of performance properties. Sterilization involves the complete destruction of all forms of microbial life including bacteria, viruses, and spores. To be effective, sterilization must be preceded by meticulous cleaning (mechanical or manual) to remove all foreign material from objects prior to undergoing sterilization [6].

Sterilization

Basics of Sterilization

The Association for the Advancement of Medical Instrumentation (AAMI) defines sterilization as a process designed to remove or destroy all viable forms of microbial life, including bacterial spores, to achieve an acceptable sterility assurance level. Sterilization can be achieved by physical, chemical and physiochemical means. Chemicals used as sterilizing agents are called chemisterilants. Disinfection is the process of elimination of most pathogenic microorganisms (excluding bacterial spores) on inanimate objects. Disinfection can be achieved by physical or chemical methods. Chemicals used in disinfection are called disinfectants. Different disinfectants have different target ranges, not all disinfectants can kill all microorganisms. Some methods of disinfection such as filtration do not kill bacteria, they separate them out. Decontamination is the process of removal of contaminating pathogenic microorganisms from the articles by a process of sterilization or disinfection.

Methods of Sterilization/Disinfection

Various methods of sterilization/disinfection are shown in Fig. 1 [7–9]. Spectrum of activity of sterilizing agents is shown in Table 1.

Fig. 1
figure 1

Methods of sterilization [7]

Full size image
Table 1 Spectrum of activity of sterilizing agents [8]
Full size table

Merits, demerits and applications of different methods of sterilization are shown in Table 2 [9]. To overcome the merits and demerits of basic sterilization, other sterilization technologies have been introduced. However these (autoclaves, ovens, and chemical like ethylene oxide) rely on irreversible metabolic inactivation or breakdown of vital structural components of microorganisms. Since heat and steam are not suitable for use on heat-sensitive materials, hydrogen peroxide and ethylene oxide are commonly used as low-temperature sterilization techniques.

Table 2 Merits, Demerits and Applications of Different Methods of Sterilization [9]
Full size table

The carcinogenic property of ethylene oxide residues requires that the time needed for complete ventilation be longer than the actual time taken to sterilize the materials [10, 11]. Another interesting sterilization technology is gamma irradiation for heat-sensitive materials, but it is costly and its safe operation requires an isolated site [12]. Various limitations of the existing sterilization techniques have stimulated the development of alternative techniques that are cheap, effective, and do not generate toxic residues. Recently, irradiation with electron beams, high electric field, high-voltage pulse, pulsed light irradiation, and negative and positive ions have been applied as sterilization systems [13–17]. However, the efficiencies of these new sterilization systems were guaranteed only at high treatment temperatures. In addition to these sterilization systems, substantial amount of research works have been focused on sterilization using plasma in recent years [18]. Different sterilization methods suitable for various types of textile materials are shown in Table 3.

Table 3 Types of textile materials and sterilization methods [18]
Full size table

Plasma Sterilization

Basics of Plasma

Plasma is defined as an ionized gas containing both charged and neutral species, including free electrons, positive and/or negative ions, atoms, and molecules [19]. Plasma is a gaseous state of matter that contains excited species such as ions, free electrons and large amounts of visible, UV and IR radiations [20].

Plasma state can be generated by:

  • electrical energy (electric discharges)

  • nuclear energy (fission and fusion)

  • thermal energy (intense redox reactions e.g., flames)

  • mechanical energy (shock waves)

  • radiant energy (electromagnetic radiation and particle radiation)

Plasmas differ in many respects that include pressure, charged particle density, temperature and the presence of external electric and/or magnetic fields. The overall state of plasma is considered neutral with the density of electrons and negative ions being equal to the density of the positively charged ions, known as plasma quasi-neutrality [21].

Advantages of Plasma Sterilization

The plasma state is most commonly divided into two main categories such as hot and cold. Hot plasmas are associated with electric arcs, thermonuclear reactions and laser-induced reactions. Cold plasmas are associated with low pressure electrical discharges that permit chemical synthesis and surface modifications of materials. Nowadays the wide spectrums of sterilization/decontamination methods are used for the inactivation of microorganisms on various materials and subjects [22]. The serious disadvantage of the conventional decontaminations methods is stressing of the exposed material by heat or chemicals [23–25]. Plasma sterilization is the alternative method, which is friendlier and more effective on the wide spectrum of prokaryotic and eukaryotic microorganisms [26]. Basically, the main inactivation factors for cells exposed to plasma are heat, UV radiation and various reactive species [27–29].

Plasma exposure can kill micro-organisms on a surface in addition to removing adsorbed monolayer of surface contaminants. Advantages of plasma surface treatment are removal of contaminants from the surface, change in the surface energy and sterilization of the surface. Plasma sterilization aims to kill and/or remove all micro-organisms (bacteria—microscopic plants, some of which causes diseases in human, fungi plant like micro-organism which lack chlorophyll, spores—a dormant seed-like phase of many microorganisms usually difficult to kill, and viruses—submicroscopic micro-organism, attacking cells, invaliding and replicating them) which may cause infection in humans or animals, or which can spoil foods or other goods. Types of removal of microorganisms are cleaning (removal of dirt and contamination which support growth of micro-organisms), antisepsis (killing or removal of significant fraction of microorganisms and removal of their food), disinfection (killing or removal of all but a few individual microorganisms that cause infection) and sterilization (complete removal or killing of all micro-organisms).

Basic Mechanism of Plasma Sterilization

Basic mechanism of plasma sterilization is analyzed in the work carried out by the scientists [30]. As per their study, there are three phases in the process of plasma sterilization (Fig. 2):

  • I phase—DNA destruction by direct UV irradiation of isolated spores

  • II phase—erosion of inactivated spores and debris of all kinds standing on the top of living spores (primary killing mechanism is caused by UV)

  • III phase—destruction of the last monolayer of spores by UV.

Fig. 2
figure 2

Three phased process of plasma sterilization [30]

Full size image

Plasma inactivation is characterized by the existence of two or three distinct phases in the survival curves of given type of spores. These phases correspond to changes in the dominating kinetics of spore inactivation as a function of exposure time. In the case of medium and low-pressure discharges, the basic process is the DNA destruction of the spore by UV irradiation, preceded in some cases by erosion of the microorganism through intrinsic photo desorption and etching (eventually enhanced by UV radiation). These elementary mechanisms clearly set plasma sterilization apart from all other sterilization methods [30].

Efficient plasma sterilization at reduced pressure in O2 containing mixtures under flowing afterglow conditions depends essentially on maximization of the UV emission intensity. Similar results are expected when the microorganisms are directly exposed to the corresponding discharge. Etching in presence of oxygen atoms provides a faster erosion of the protecting layers of microorganisms than simply photo desorption, only the latter being possible with Hg lamps. Furthermore, sterilization using UV lamps and lasers suffers from shadowing effects (which includes the difficulty to sterilize in crevices), in contrast to gas plasma sterilization where the UV photon is brought to the appropriate site by its emitting atom or molecule [30]. Parameters influencing plasma sterilization are shown in Fig. 3.

Fig. 3
figure 3

Parameters influencing plasma sterilization [30]

Full size image

Atmospheric–Pressure Plasma based Sterilization

In recent years, plasma sterilization was carried out with different pressure range and position of the sample in the discharge. Main advantage of plasma treatment over other common methods is that heat sensitive materials can be treated. Other advantage is that in all these discharge methods no toxic ingredients are required. The remaining task is to increase the efficiency (e.g. reduce the treatment time) and to control the change of the material characteristics in a positive direction. The characteristics of all these atmospheric plasma sources are in terms of plasma properties.

At atmospheric pressure, Dielectric Barrier Discharges (DBD), Resistive Barrier Discharges (RBD), Cascaded Dielectric Barrier Discharge (CDBD) or Corona Plasma Discharges (CPD) is used where the samples are in direct contact with the discharge [31–36]. Other discharges for sterilization purposes are the plasma jet or the plasma plume which have a compact setup and may be useful for cleaning wounds directly [37, 38]. Depending on the discharge (gas mixture, power and setup) and the type of micro-organism and substrate, the time necessary for a substantial reduction of living cells varies from a few seconds e.g. for B. globigii on glass while for B. atrophaeus on glass the time can be a few minutes. The mechanisms responsible for the cell death are still being analysed. In atmospheric discharges with oxygen, it seems that reactive species like atomic oxygen and ozone play a significant role. In the presence of hydrogen and oxygen the hydroxyl-radical OH also reacts with the surface of cells and leads to cell death. In low pressure plasma sterilization, it was distinguished between direct and remote treatment of samples. In the case of a remote exposure to a owing afterglow of a microwave plasma many results were presented [39–41]. A higher substrate temperature leads to higher erosion rate. In particular, the feasibility of depyrogenation is examined in a direct microwave discharge exposure [42]. Rossi et al. showed the sterilization feasibility of a low pressure discharge ICP setup for direct treatment [43]. They revealed the VUV/UV radiation as the only mechanism leading directly to spore death in the monolayer case [44].

Advantages of Atmospheric–Pressure Plasma based Sterilization

Plasma sterilization enables measuring at different conditions. Various type of plasma generation can be employed, e.g. in recent years a number of papers have been published on plasma sterilization using DBD [23–25] and microwave plasma [26, 28, 38]. It is possible to use low pressure or atmospheric pressure and the plasma sterilization can be applied using different gases (nitrogen, argon, helium, air etc.) [23–26, 38, 45]. It is also possible to choose proper carrying medium (paper, polymers or glass) [18, 23, 24, 29, 45]. Though there are numerous sterilization techniques available, the textile and healthcare industry have been thriving for a sterilizing method that functions near room temperature and is effective in much shorter durations than the 12 h treatment of autoclaving and ethylene oxide exposure [46]. To explore possible applications of the OAUGDP in the sterilization of microorganisms on surfaces and fabrics and to other healthcare applications, an interdisciplinary team was formed at the UTK Plasma Sciences Laboratory and it has carried out research works to remove a variety of microorganisms on glass slides and embedded in agar, on fabrics and in sealed commercial sterilization bags [47–49].

Vojkovska et al. focused on effect of DBD operating at atmospheric pressure on bioindicator Aspergillus niger, in paper and PET foil. The plasma sterilization was influenced by plasma exposition time, plasma power density, the type of operating gas and type of the medium supporting the microorganism [50]. By using double inductively coupled low pressure plasma a method of sterilization of bio-medical materials was developed for homogeneous treatment of three-dimensional objects [51]. The design combined the positive attributes of getting the potentiality of solving the problems of common sterilization methods. The use of non-toxic gases ensures no noxious residue. Innocuous gases like argon, nitrogen, hydrogen and oxygen were used for a short duration (1 min). Short treatment time and low temperature allowed the sterilization of heat sensitive materials like ultra high molecular weight polyethylene or polyvinyl chloride, practically possible. Moreau et al. claimed a two—in the case of a pure argon discharge and three in the case of argon with added oxygen discharge step kinetics for the reduction of B. atrophaeus spores [52]. The overall slow reduction needed oxygen radicals to deactivate B. subtilis spores in addition to radiation.

The first step is related to UV radiation breaking bonds in the outer coat of the spore and additionally the UV radiation damages the Deoxyribonucleic Acid (DNA). The oxygen radicals in combination with UV radiation effect a reduction by etching the surface. With oxygen, the third phase is visible in the reduction kinetics. The oxygen enhances the deactivation of spores so that after a treatment time of 40 min the deactivation of 106 spores is enhanced from 104 remaining Colony Forming Units (CFUs) to 1 remaining CFU.

Comparison of Various Methods of Sterilization

A comparative evaluation in general on efficacies of Ethylene Oxide Gas (EOG), Hydrogen Peroxide Gas Plasma and Low Temperature Steam Formaldehyde (LTSF) sterilization methods was studied [53]. The results expressed an overview that plasma sterilization may be unsuccessful under certain conditions particularly when used for items with complex shapes and narrow lumens. Alternatively, LTSF sterilization demonstrates excellent efficacy and is comparable to EOG sterilization. LTSF could potentially act as a substitute if EOG becomes unavailable due to environmental concerns. Experimental data demonstrated the real availability of deep and fast inactivation by the atmospheric-pressure Non-Thermal Plasma (NTP) method for cold sterilization of liquids and thermal sensitive surfaces which can resist microorganisms such as spores, bio films, and fungi on the surface and in liquids [54]. These methods are based on the use of direct current (DC) gas discharge plasma sources fed with steady-state high voltage. Parameters characterizing the plasma sources used (plasma-forming gas, gas flow rate, electric power consumed etc.) were streamlined. By using corona discharge, the researchers constructed a simple device to study the decontamination effects on a set of microorganisms including gram-negative, gram-positive sporulating and nonsporulating bacteria of different cell shapes and one species of yeast [55]. The action of the corona discharge does not affect the properties of common semisolid culture media at exposures shorter than 60 min except causing medium dehydration. The results obtained in vegetative forms on culture media shows that the microbicidal effect is perceptible after a mere 1 min exposure. Its efficiency measured by the size of the growth inhibition zone, does not appreciably differ in different bacteria, rises regularly with exposure time and drops in an irregular manner with increasing inter-electrode distance.

But on inert materials, the killing effect is perceptible after more than 1 min. Its efficiency, measured by the number of colonies, differs depending on whether the colonies are directly exposed to the drift region of the discharge or are shielded from it. The killing effect on spores is perceptible only after an exposure lasting at least 8 min. Its efficiency, measured as the size of the inhibition zone, is several-fold less than in vegetative forms [56]. Successful attempts were made to sterilize surfaces in low pressure RF glow discharge plasmas operating below one torr as early as the 1960’s [57]. The use of glow discharge plasma systems has been viewed by industry as a promising approach to achieving low temperature and short duration sterilization, since the technology of low pressure (vacuum) glow discharge plasmas has been highly developed over a period of at least 150 years. Usually low-pressure glow discharge plasma is employed in combination with a working gas such as hydrogen peroxide or per acetic acid vapor. Glow discharges formed from both working gases are effective against a broad range of bacteria and bacterial spores, and kill these microorganisms by generating oxygen, hydroxyl free radicals, and presumably other active species. Dimitrievska et al. compared the effects of ethylene oxide and Low Temperature Plasma (LTP) sterilization on PET surface chemistry and biocompatibility [58]. LTP sterilization led to an increase in overall oxygen content and the creation of new hydroxyl groups. Results of surface modification suggest that the LTP approach is more suitable for the sterilization of non-woven PET fibers. However, this did not translate into a clear in vivo biocompatibility advantage when scaffolds were implanted in mice.

The one atmosphere uniform glow discharge plasma developed at the University of Tennessee is a technique that can generate glow discharge plasma at one atmosphere using different gases such as helium, carbon dioxide and air. This plasma contains active species and has been used to modify the surface properties of polymeric fibers and films [59, 60]. In subsequent research, microorganisms were characterized and the active species were elucidated with the One Atmosphere Uniform Glow Discharge Plasma (OAUGDP) and used it to sterilize and increase the surface energy, wettability and wick ability of nonwoven fabrics.

The OAUGDP was capable of sterilizing both porous and nonporous surfaces at standard pressure and temperature. This technology kills bacterial vegetative cells, bacterial endospores, viruses and fungi (yeast) by a relatively simple, safe and rapid process. The OAUGDP technology offers several advantages over standard sterilization techniques. This technology does not require a vacuum system with added chemicals or gases, does not expose materials to high temperature and pressure as in steam sterilization, and does not use radiation or toxic chemicals such as ethylene oxide.

The sterilization process takes place at room temperature and the sterilized sample is not damaged by the plasma, which makes this technology very attractive for the sterilization of heat sensitive fabrics or webs. With an extensive experimental setup, on using Atmospheric-Pressure Cold Plasma (APCP) using helium/oxygen sterilization had been activated on nitrocellulose filter membrane [45]. The suitable test method with appropriate power and seconds to destroy Escherichia coli, Staphylococcus aureus, and Saccharomyces cerevisiae were optimized. It was found after plasma treatment, nitrocellulose filter membranes were overlaid on fresh solid media and CFUs were counted after incubating overnight. When treated cells were observed in a scanning electron microscope, E. coli was more heavily damaged than S. aureus, S. cerevisiae exhibited peeling and B. subtilis spores exhibited shrunken morphology.

Results showed that APCP with helium/oxygen has many advantages as a sterilization method, especially in a clinical environment with conditions such as stable temperature, unlimited sample size, and no harmful gas production. Atmospheric pressure plasmas possess the ability to reduce smell effectively. Siegfried et al. investigated cleaning of cotton fabrics using DBD under normal pressure [62]. In their study, significant reduction of microorganism population and smell reduction were observed in some of the samples tested.

Conclusion

There has been a continued demand for novel sterilization techniques appropriate for use on various textile materials used in public places such as hospitals, hotels, healthcare gymnasium, studios, clinics and toilets, where the infections can easily attack. Owing to the greater advantages in atmospheric pressure plasma applications in textile materials, particularly in sterilization of textile materials plasma technology offers promising future for further research and development in the field of medical textiles.