Textiles play an important role in the daily lives of humans, and the demand for various quality attributes of them are based on enhancing the properties through proper finishing. Fabrics that are fire resistant, wrinkle resistant, and stain repellent are already in the market while other property enhancing treatments, such as UV protective and antimicrobial, are being researched. Antimicrobial textiles have been developed for use in the medical industry for some time. Currently, the antimicrobial textiles used in the health care industry are disposable nonwoven and wound dressing materials. Some of the treatments used are harmful to our environment not only because of the chemicals used in the treatments but also because the treated textiles are not reusable.
Textile goods are excellent substrate for growing microorganisms. For the last 50 years, the prevention of microbial attack on textile materials has become increasingly important to consumers and textile producers. Currently, there is growing concern in protecting health care workers from diseases that might be carried out by patients especially from surgical gowns. There is an urgent need to protect medical staff from infection by blood borne pathogens such as HIV and HBV2. In addition the textile used in hotels, transportation and biological institution needs antimicrobial textiles. Now-a-days, nonwoven fabrics are the most commonly used textiles for surgical gowns, patient drapes, laboratory coats, coveralls, and other kinds of protective clothing(4). Polyester is a preferred textile fibre in many durable applications of nonwoven for its ease of use and compatibility with other fibres. Although, Polyester has excellent mechanical strength and good stability but end use capacity is limited due to problems to impart the functional finish because of lack of polar groups on the surface and poor wettability(5,6). Different antimicrobial agents have been applied to obtain antimicrobial properties to textile(7,8). Among them, the quaternary ammonium salts of cationic surfactants are widely used in antimicrobial finishing of textile9. Quaternary ammonium salts exhibit marked antimicrobial activity against a wide range of bacteria, fungi, and viruses(10,11).
Antimicrobial textiles - A necessity
Antimicrobial textiles have been in use since World War II. One of the first antimicrobial textile finishes used during World War II, was made to prevent cotton textiles, such as tents, tarpaulins, and vehicle covers, from rotting(12,13) especially, due to focus on synthetic fibres. As knowledge of functional finishes and man-made fibres evolved, the concern on health and safety also grew. It soon became more important for antimicrobials finished textiles to protect the wearer from bacteria than it was to simply protect the garment from fibre degradation(13). All textiles provide a growing environment for these micro-organisms. In fact, some finishes accelerate the growth of microbes (12,14). Natural fibres, such as cotton and wool, are susceptible to microbial growth and even dust mites because they retain oxygen, water, and nutrients (12,14,15). Micro-organisms can embed themselves in clothes in a closet, curtains, carpets, bed, bath, and kitchen linens, and even pillows and mattresses. Many bacteria also live on the skin while dust mites live on shed human skin cells that have been deposited on items such as sheets, towels, and clothing(12,15).
Current medical protective wear, such as gloves, masks, and gowns are insufficient in protecting the wearer against both air-borne pathogens and blood-borne viruses, like HIV/AIDS and hepatitis B. It is reported that outbreaks of severe acute respiratory syndrome (SARS) in hospitals is due to the inadequacy of this protective gear(16). The majority of these microorganisms are passed from person to person by various textiles(17,18). More recently, threats of biological warfare, like anthrax, have increased health concerns for both militaries and citizens19. While developing antimicrobial finishes to protect against chemical warfare is a life saving strategy, other industries require similar finishes to simply cater to their customers.
"Micro-organisms metabolize nutrients, such as sweat and soil present in textile products, producing odour causing intermediates that cause irritation"(14). Controlling moisture is also a major concern for many manufacturing companies because microorganisms only attack fibres when they are damp(14). Functional textiles include everything from antimicrobial finished textiles, to durable, or permanent press finished garment, to textiles with self-cleaning properties, and also textiles with nanotechnology (20).
In view of the above, antimicrobial treatment for textile materials is necessary to fulfill the following objectives:
- To avoid cross infection by pathogenic micro-organisms.
- To control the infestation by microbes.
- To arrest metabolism in microbes in order to reduce the formation odour.
- To safeguard the textile products from staining, discolouration and quality deterioration.
Microorganisms are small forms of life that generally cannot be seen by the human eye. They consist of single or multiple levels of cells and can be found in very cold or hot climates worldwide(21). Microorganisms repeat their structure through gene transfer and their rapid multiplication allows them to grow colonies at extreme rates overwhelming larger areas quickly(22). Some examples of Bacteria that cause disease include: Escherichia coli, Staphylococcus aureus, and Klebsiella pneumoniae(22). Bacteria attach to surfaces through many different mechanisms. In a liquid medium, multiple forces act to bring or repel the bacteria from a solid surface.
Lifshitz-van der Waals forces act to attract the bacteria at greater than 50 nm away from a solid surface. As the bacteria come closer to the solid surface, an interfacial water barrier exists. The bacteria then remove this barrier by hydrophilic interactions, bringing the bacteria close to the surface of the solid. In a dry environment bacteria attract to solids solely through charge with most surfaces being positively charged and bacteria being negatively charged(23).
How to overcome the menace of microbes?
Growth of microorganisms is known to be very rapid depending on the media on which they grow. Bacteria that are associated with disease reproduce at a rate that doubles every 18 to 38 minutes at 37°C. This rate means that with the majority of bacteria, billions can be produced in less than a day (22,24).
The antimicrobial agent/compound must be safe to the user, environmentally benign, and non-damaging to the treated substrate. It also should be easily applied to a material and cost competitive to replacement of untreated materials24. Lastly, the treatment should be durable to multiple launderings and continue to be effective over the duration of the garment life. There are two types of antimicrobial treatments for textile materials viz; leaching agent that releases its antimicrobial agent into the environment to proactively attack micro-organisms, bound agents that come in contact with the organism and act by disrupting the protective cell wall of the microbe thus killing the microbe.
Mechanism of antimicrobial activity
Negative effect on the vitality of the microorganisms is generally referred to as antimicrobial effect. There are two types of antimicrobials viz; biocidal and bistatic. The differentiation of antimicrobial activity is given in the diagram (Figure 1). The activity, which affects the bacteria is known as Antibacterial and that of fungi is Antimycotic.
The non-leaching type or bio-static finish shows good durability and may not provoke any health problems. For example, in the ion exchange process, the release of the active substances is at a slower rate compared to direct diffusion ad hence, has a weaker effect. Similarly, in case of antimicrobial modifications where the active substances are not released from the fibre surface, it is so less effective. They are active only when they come in contact with microorganisms. These so called new technologies have been developed by considering the medical, toxicological and ecological principles.
The antimicrobial textiles can be classified into two categories, namely, passive and active based on their activity against microorganisms. Passive materials do not contain any active substances but their surface structure (Lotus effect) produces negative effect on the living conditions of microorganisms (Anti-adhesive effect). Materials containing active antimicrobial substances act upon either in or on the cell(12).
Types of antimicrobial agents
There are broadly two types of antimicrobial agents viz; leaching type and bound type. Each one has some merits and demerits.
Leaching type antimicrobial agents
There are different leaching agents that have been used in the industry. Some have been proven to harm the environment, or humans, and have been removed from the market. These include copper naphthelate, copper-8-quinolinate, and numerous organo mercury compounds24. Other products that have not been regulated at this time include: Triclosan, dichlorophene, and Silver compounds(24).
2, 4, 4'-trichloro-2'-hydroxydiphenyl ether (Triclosan)
Triclosan is used in soaps, lotions, toothpaste, and other hygiene products. It is a bisphenol antimicrobial that has a broad range of effectiveness(25). It is a bacteriostatic at low concentrations, but higher concentrations are bactericidal26. Triclosan is used as a textile finish, but must be applied with binder and dispersing agents due to inherent low solubility(24). Triclosan works like other leaching agents and releases to the environment over time. Because of this and its applications, there are concerns that it could be harmful to humans and aquatic life(27,28). It has been found in effluent from many different sources from normal sewage to rainwater(28).
MDMH
MDMH, or monomethyl-5, 5-dimethyl hydantoin is a bifunctional compound possessing one side for reacting with cellulose and another for reacting with active chlorine that is capable of forming a halamine bond. This agent can be processed by standard pad methods, but must be activated prior to bacterial exposure. Activation can be achieved through a two-step chemical process involving: (1) Finishing under acidic conditions and (2) Rinsing with chlorine bleach.
The resulting finish is regenerable and durable to reversible oxidation reaction properties of the cyclic halamine structure (14). Zeolites
Zeolites are natural or synthetic inorganic antimicrobial additives. Typical Zeolites are mainly composed of an aluminosilicate framework of alkali or alkaline earth metal, with a three-dimensional skeletal structure consisting of tetrahedral linked SiO4 and AlO4 sharing oxygen bonds (14). AgION Technologies developed one specific Zeolites agent. This biocide has an aluminosilicate structure in which silver is incorporated into the mineral structure by means of an ion-exchange reaction. The added silver kills microbes by interacting with multiple binding sites on their surfaces(29).
Silver compounds
In the late 1900s silver was used as an eye dressing for children to prevent gonorrheal infections(29). Today uses of silver are found in products such as washing machines, wound care, water filters. Some manufacturers have incorporated silver into clothing and sportswear for higher performance requirements for their sporting and active wear apparel (30). Silver is dependent on a slow release to have the maximum effect of antimicrobial action. As the leaching of silver ions occurs the silver compound loses some of its antimicrobial effectiveness. Encapsulating silver in ceramics is one way to slow the release of the silver. This allows for the release of silver ions that are needed to impact microbe growth allowing for a prolonged effect of the treated material(29).
Bound agents
Many different types of bound antimicrobials are currently being used for textiles. Some of these include: polyhexamethylene biguanide (PHMB), BiosilTM, regenerable antimicrobial agents and chitosan. In order for the antimicrobial to be effective they must remain on a clean surface. This is sometimes difficult with textile materials, and stain repellent techniques such as the use of fluoropolymers have been incorporated to keep textile surfaces clean allowing the bound antimicrobial to work more effectively (31).
Polyhexamethylene biguanide (PHMB)
Polyhexamethylene biguanide (PHMB) is a polycationic molecule that results in a tightly adsorbed complex to negative materials (32). The use of PHMB is restricted due to its cationic nature and interactions with anionic materials in the human body that result in diminishment of its antimicrobial effectiveness(33).
BiosilTM
BiosilTM is an antimicrobial agent that joins a quaternary ammonium salt with an organosilicone. This agent is applied on the fibre by graft-polymerisation and does not leach out even after multiple launderings. Currently BiosilTM have received acceptance in underwear and other products worn directly on the skin.
Regenerable antimicrobial agents
Regenerable antimicrobial agents for cellulosics have been looked at as a novel way to impart a "permanent" renewable antimicrobial finish. The agent works by using a bifunctional compound with one side reactive to cellulose and the other to chlorine via, a halamine bond. The reaction occurs each time the fabric is washed using chlorine bleach and the antimicrobial agent "regenerates" its antimicrobial properties(31).
