Chemistry for Everyone edited by
Products of Chemistry
George B. Kauffman California State University Fresno, CA 93740
Chemistry of Durable and Regenerable Biocidal Textiles Gang Sun* Division of Textiles and Clothing, University of California, Davis, CA 95616; *
[email protected] S. Dave Worley Department of Chemistry, Auburn University, Auburn, AL 36849
Need for Protective Textiles In recent years, protection of healthcare workers from cross-transmission of infectious diseases, particularly bloodborne viruses such as HIV and hepatitis B, has become extremely urgent and important to medical professionals (1, 2). Medical protective gear for doctors and nurses—including gowns, masks, and gloves—are currently serving as barriers to the diseases and are insufficient in preventing the transmissions of the diseases. Researchers have revealed that textiles are advantageous media for hosting microorganisms and, therefore, are potentially responsible for the disease transmission (3). Moreover, the spread of multidrug-resistant bacteria in healthcare facilities is threatening not only to the safety of healthcare workers but also to the public. One drug-resistant microorganism, methicillin-resistant Staphylococcus aureus (MRSA), was found not only existing, but also surviving for long periods of time, on all of the textile materials in a hospital environment (3–5). Recent outbreaks of severe acute respiratory syndrome (SARS) in hospitals further demonstrate inadequate protection of the current medical protective gear. Textile materials may be responsible for disease transmission and the spread of new strains of diseases from the main sources to elsewhere (3). On the other hand, textile materials, as necessary materials for clothing and daily life, are possible means for prevention of infectious diseases and pathogens, if they become antimicrobial. Thus, the research and development of antimicrobial textiles, particularly the fabrics for healthcare providers and patients, are important and necessary. Antimicrobial Textiles Antimicrobial textiles can be categorized into two groups, biocidal and biostatic materials, according to their functions. Biostatic functions refer to inhibiting growth of microorgan-
isms on textiles and preventing the materials from biodegradation. By definition this involves only protecting the textile materials. Biocidal materials are able to kill microorganisms, thus eliminating their growth, sterilizing the textile, and possibly protecting wearers of the textiles from biological attacks. (Biocidal materials are often termed antibacterial materials.) Biostatic functions are usually employed in preservation of textile art in museums or for odor control of materials, but cannot prevent transmission of diseases owing to the limitation of the functions. The term “antimicrobial” is relatively broad and nonspecific. Therefore, due to its overlapping uses, the antimicrobial term, especially the various claims of antimicrobial functions, has created some confusion to the public. In general, for protective purposes, biocidal functions that provide rapid and efficient inactivation of a broad spectrum of microorganisms are required. The biostatic or simple antimicrobial functions are insufficient protection in these applications, particularly in areas such as medical-use textiles or protective clothing for occupational uses. Antimicrobial textile material was first developed in 1867 by Lister who demonstrated the relationship between fibrous materials and diseases (6). Since then, many innovative antimicrobial materials have been developed. In general, antimicrobial properties of textile materials can be obtained by two different approaches, that is, chemically or physically incorporating antimicrobial agents into fibers or fabrics. The antimicrobial agents can be antibiotics, formaldehyde, heavy metal ions (silver, copper), quaternary ammonium salts (with long hydrocarbon chains), phenols, and oxidizing agents such as chlorine, chloramines, hydrogen peroxide, iodine, and ozone (Table 1). These agents inactivate microorganisms via different mechanisms and therefore have different limitations. Antibiotics kill bacteria by inhibiting their reproductive enzymes, which can create problems for the environment. En-
Table 1. Common Antimicrobial Agents Used in Textile Applications Compound
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Function
Limitation
Example
Halogens (Cl2, Br2, I2)
Oxidizing
Toxicity and skin irritation
Chlorine bleach
H2O2/sodium perborate
Oxidizing
Toxicity and skin irritation
Color-safe bleach
Quaternary ammonium salts
Affecting permeability
Less effective and skin irritation
Softeners, disinfectants
Phenols
Affecting permeability
Less effective and skin absorption
Triclosan
Heavy metals
Sulfhydryl binding
Not effective against spores, water pollution
Silver-containing materials
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zymes can easily develop resistance to their selective inhibitors. Thus, overuse of the antibiotics will stimulate resistance from bacteria and generate more negative consequences to the environment. Quaternary ammonium salts and phenolic compounds damage bacterial cells by affecting permeable properties of microorganisms that usually results in slow action. Oxidizing agents can rapidly inactivate microorganisms by chemical reactions with some functional groups of microorganisms, but most of these are toxic and skin irritating (Table 1). However, oxidative inactivation of microbes is rapid, nonselective, and nonmutable to microorganisms. A means by which textiles can be made antimicrobial by harnessing the disinfecting power of oxidative chlorine, thus avoiding the limitations caused by the use of free chlorine, is discussed below. This can be employed as a classroom example of how corrosive chlorine can serve a useful purpose without the further environmental endangerment that use of elemental chlorine would pose. The advantages of Nhalamine-treated textiles over other existing biocidal textile materials are also discussed.
triclosan take more than 10 hours of contact time to exhibit their maximum functions, which therefore limits their applications in medical-protective clothing (10, 12). Besides the slow action against microorganisms, triclosan was found to cause mutations of drug-resistant strains in microorganisms (13), which is also a major concern to the use of antibiotics in textile treatment. Beyond all of the problems related to the slow-releasing treatment, the durability is limited, and the function is nonregenerable as well (Figure 2). To prepare durable protective antimicrobial textile materials, the slowreleasing mechanism is not the best method and novel approaches are needed.
Cl
NH NH
Cl
Slow-Releasing Functions There are some successful examples of antimicrobial textiles developed based on the slow-releasing concept. Durable antibacterial cellulose and its blend fabrics were prepared by incorporating a zinc peroxide polymer (7). Recently, in a similar approach, Vigo used magnesium peroxide in the finishing of fabrics and achieved durable antimicrobial properties on the materials (8). Polymers of hexamethylene biguanide hydrochloride (9), quaternary ammonium salts (10), and silver ions (11) have been employed in treatment of textile products. Triclosan, a phenolic compound, has found applications in liquid or solid soaps, plastic products, and fibers as a bacterial inhibiting agent (12). Nevertheless, the antimicrobial properties of these materials (Figure 1) are insufficient to execute a quick and complete inactivation of organisms upon contact, which is critically needed for the protection of medical workers. For example, quaternary ammonium salts and www.JCE.DivCHED.org
NH NH
chlorohexidine
Durability of Antimicrobial Textiles The greatest challenge to functional textiles, especially clothing materials, is durability of the antimicrobial functions; that is, the washfastness of the functions subjected to repeated laundering. The durability of antimicrobial functions of textile materials can be grouped into two categories: temporary or durable functions. Temporary biocidal properties of fabrics are easy to achieve in finishing, but readily lost in laundering and thus is useful only for disposable materials. Durable antimicrobial functions, generally considered as functions that can survive at least 50 machine washes, have been achieved by using a common technology, a slow-releasing method on certain textiles, mainly for preservation of the materials from biodegradation or for odor reduction. According to this method, sufficient antimicrobial agent should be incorporated into fibers or fabrics in a wet-finishing process to provide prolonged usage. The fabrics inactivate bacteria by slowly releasing the agents from the surface of the materials. However, the bactericides eventually vanish completely since they are impregnated in the materials without permanent covalent bonding.
•
Cl
NHCNHCNH(CH2)6NHCNHCNH
Cl
HO O
Cl
triclosan
N Ag H2N
SO2NH
N silver sulfadiazine
H3CO
OCH3
CH3
Si
N
CH2CH2CH2
OCH3
ⴙ
C18H37 Clⴚ
CH3
quaternary ammonium salt
Figure 1. Examples of antimicrobial agents used on textiles.
chemical modification
+ cellulose
biocide
cellulose + biocide
kill bacteria
+ cellulose + biocide
cellulose
biocide
Figure 2. Slow-releasing mechanism (reproduced from ref 19, copyright American Association of Textile Chemists and Colorists, AATCC).
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Durable and Regenerable Functions In 1962 (14) Gagliardi proposed a model for making antimicrobial textiles named the regeneration principle. Although the model was presented over 30 years ago, there has been little reported success in textiles and other related materials until recently. However, this principle has provided an important role in the design of this innovative functional finishing. Before discussing the technology, it is necessary to understand the requirements for durable and regenerable antibacterial textiles. It is commonly believed that the ideal biocidal textile materials should possess these features: (i) rapid inactivation of a broad spectrum of microorganisms; (ii) nonselective and nonmutable to pathogens; (iii) nontoxic and environmentally friendly; (iv) durable to repeated washes; and (v) capable of being recharged in laundering. If the functions are rechargeable, the recharging agents should be nontoxic, readily available, and compatible with laundering chemicals such as detergents or bleaching agents. If regenerable properties are considered in selection of biocidal agents, only oxidative biocidal agents (halogens and hydrogen peroxide in Table 1) fit well into the requirements since redox reactions are reversible or regenerable. Chlorine bleach is a registered biocide and has been used as a disinfectant for decades without any reported resistance generated from any microorganisms. Unfortunately, it is quite corrosive and toxic, particularly of concern is producing carcinogens (such as CHCl3) in water (15). However, some of the chlorine derivatives, for example, halamine compounds, though possessing biocidal properties similar to chlorine, are more environmentally friendly and thus widely used in swimming pools (16–18). Halamines inactivate microorganisms by oxidation mechanisms rather than biological functions,
O
O H2C
N
N
H2C
H
N
N
Cl
kill bacteria
H3C CH3
O
H3C
O
Halamines Halamines that can achieve this durable and regenerable antimicrobial function are chlorinated products of 5,5-dimethylhydantoin and 2,2,5,5-tetramethyl-4-imidazolidinone (Figure 4). Monomethylol (MDMH) or dimethylol derivatives (DMDMH) of 5,5-dimethylhydantoin, and 2,2,5,5tetramethyl-4-imidazolidinone (Figure 4) can be employed in grafting the heterocyclic ring to cellulose (19–22). When
O
O
[bleach]
and wide usage of them could result in less concern about drug-resistance of diseases. Since the halamine compounds can be covalently connected to polymers, a reversible redox reaction can then be implemented on solid materials according to Scheme I. The design of cellulose modification, activation or regeneration of halamine structures, and inactivation of microorganisms is expressed by the regeneration principle (Figure 3) (19–22). According to the mechanism of the biocidal function and regeneration process, diluted chlorine bleach solutions serve as activation and regeneration agents of the biocidal functions. By using the chlorine bleaching process, the potential biocidal groups grafted on cellulose, for example, amide or imide N⫺H bonds in hydantoin rings, can be converted to biocidal halamine structures, allowing the textile materials to be sterilized. This provides a convenient way for activation and regeneration of biocidal functions, and is the best fit for medical-use textiles since they are commercially laundered with chlorine bleach. Many of these halamine structures have been reviewed and investigated for water disinfection purposes (17). Recent development of halamine polymers has introduced many applications of the chemical reactions (Scheme I) (18).
CH3
O
Scheme I. Chlorination of cellulose grafted with dimethylhydantoin structure.
O
O
H2 C N
H
N
H
[Cl]
O
H2 C N H
H
H
chemical modification
− HCl
+ cellulose + potential biocide (covalent bond)
potential biocide
O activation
cellulose + potential biocide (covalent bond)
rearrange
N cellulose + biocide (covalent bond)
Figure 3. Regeneration principle (reproduced from ref 19, copyright AATCC).
Journal of Chemical Education
O
O
H2 C
kill bacteria
62
Cl
OH OH
OH OH
cellulose
N
•
N
H
O
H2 C
H
N
N
H
OH
O
OH
OH
Scheme II. Chlorination and reactions of cellulose grafted with DHEU.
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a chlorine atom replaces hydrogen on the N⫺H moiety, the N⫺Cl bond is formed, which is stabilized by the vicinal methyl or carbonyl groups on the grafted dimethylhydantoin ring (Scheme I). This is completely different from another similar compound, dihydroxylethyleneurea (DHEU): its derivative, dimethyloldihydroxylethyleneurea, has been employed in wrinkle-free treatment of cotton fabrics. Grafted DHEU will result in an unstable halamine structure when chlorinated, distinctly different from the compounds shown in Figure 4, since it has a vicinal C⫺H group next to the N⫺Cl, which can quickly result in an elimination of HCl and the formation of a C⫽N bond (Scheme II). The resulting C⫽N bond and hydroxyl group are in enol form, which is less stable than its keto structure, a hydantoin ring. Such a reaction not only causes loss of antimicrobial functions, but also generates HCl that can damage cotton cellulose. Structural characteristics of the stable halamine compounds have been summarized by Worley and Williams (17). The stability of N⫺Cl bonds on halamines contributes to the durability and stability of the antimicrobial properties on the fabrics, with evidence that the bleached fabrics can retain the antimicrobial properties for more than six months in a conditioning room (at 21 ⬚C and 65% relative humidCH3
O
ity). After each laundering, the fabrics, especially those treated with hydantoin derivatives, should be recharged by chlorine bleaching because of the presence of predominant imide N⫺Cl bonds that can be hydrolyzed (reverse reaction in Scheme I). Adding amide or amine halamine structures can significantly improve washing durability of the antimicrobial properties based on the following stability order of halamine structures. Imide N⫺Cl < Amide N⫺Cl < Amine N⫺Cl Fabric Evaluation The antibacterial properties of the finished fabrics were evaluated with gram-positive and gram-negative bacteria, funguses, yeasts, and viruses following AATCC standard test method 100. These microorganisms represent a whole spectrum of pathogens that health care providers encounter each day. Based on characteristics of medical protection requirements, contact time of microorganisms on the surfaces of the fabrics was chosen to be two minutes, which was the shortest interval in which a microbiological test can be managed properly. Two commonly used fabrics, pure cotton (#400) and polyester兾cotton (#7409) sheets, were treated by finishing solutions containing 2% and 6% of DMDMH, respectively, and bleached subsequently in a diluted chlorine solution. The results, listed in Table 2, are reported in log
H3C N
H
N
H
Table 2. Biocidal Results of Fabrics Treated by 2% and 6% DMDMH
O 5,5-dimethylhydantoin
CH3 H3C H
Fabric
a,b
400
H
Microorganism E. coli
7409
N
7409
6
6
400
Salmonell choleraesuis
6
7
7
6
Shigella
6
6
6
7
2
6
7409
S. aureus
7409
O
400
H3C HOH2C
N
400
CH2OH
400 7409 400 7409
CH3 H3C
400
H
7409
N
8
8
8
8
Pseudomonas
6
6
aeruginosa
6
6
Methicillin-resistant
---
3
Staph. aureus
---
6
Vancomycin-resistant
---
6
Enterococcus
---
6
Fabric 400 is plain-woven, pure cotton; fabric 7409 is a plain-woven blend of polyester and cotton (65/35%). (Reproduced from ref 22, copyright American Chemical Society.)
CH3 O
b
Fabrics are exposed to the microorganisms for two minutes.
3-methylol-2,2,5,5-tetramethyl-4-imidazolidinone (MTMIO) Figure 4. Halamine precursors.
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6
Brevibacterium
a
CH3
N
6
7409
O 1,3-dimethylol-5,5-dimethylhydantoin (DMDMH)
HOH2C
Candida albicans
7409
N
6 6
400
CH3
6
6
O 2,2,5,5-tetramethyl-4-imidazolidinone
6% DMDMH
6
400
CH3
2% DMDMH 6
CH3
N
Log Reductionc of Bacterial Challenged
•
c
A 6-log reduction means a 99.9999% inactivation of the organism.
d
AATCC standard test method 100. DMDMH is 1,3-dimethylol-5,5dimethylhydantoin.
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reductions of microorganisms, with one log reduction referring to 90% kill and three log reduction meaning 99.9% kill. Compared to other antimicrobial textiles, the new biocidal fabrics exhibited superior properties as textile materials for medical workers and patients, owing to their rapid and effective inactivation of a broad range of microorganisms. The outstanding biocidal properties of the fabrics are durable and regenerable by chlorine bleaching, a process commonly used in commercial laundering of institutional textiles. The antimicrobial results for fabrics after repeated laundering and regeneration by bleaching are shown in Table 3. Apparently, active chlorine in halamines can be affected by laundering detergents. Thus, after each laundry cycle, the fabrics are recommended to be bleached in a separate cycle to recharge the antimicrobial functions. Chlorine bleaching is a required process for used textiles, and its use in medical textiles is convenient. However, when this technology is applied to apparel products, recharging after each washing becomes burdensome to consumers. Recharging after a few washes, or having permanent functional materials, would be more desirable for apparel products. More recently, durable and regenerable antimicrobial fabrics that can survive more than 50 machine washes without recharging have been developed by using the same chemistry (23). Future Directions Functional textiles are the new direction for development of fabrics and clothing in the 21st century. More environmentally friendly and functional products that can provide benefits for health and safety and improve the quality of life are expected to undergo development. Based on the needs of the functional materials, durable and refreshable antimicrobial synthetic materials, or durable and regenerable antimicrobial fabrics with multifunctional properties such as fire-resistance, anti-UV, antistatic, and waterproofing may be Table 3. Durable and Regenerable Antimicrobial Results of 2% DMDMH Treated Fabrics Washes
Fabric
Reduction of bacteria/ Log units E. coli
S. aureus 6
After 10 washes
Cotton
6
and bleach
PET/cotton
6
6
After 20 washes
Cotton
6
6
and bleach
PET/cotton
6
6
After 30 washes
Cotton
6
6
and bleach
PET/cotton
6
6
After 35 washes
Cotton
6
6
and bleach
PET/cotton
6
6
After 40 washes
Cotton
6
6
and bleach
PET/cotton
6
6
After 45 washes
Cotton
6
6
and bleach
PET/cotton
6
6
After 50 washes
Cotton
6
6
and bleach
PET/cotton
6
6
NOTES: AATCC test method 100: contact time was 30 min. A bleaching solution containing 0.01% Cl was used in activation and regeneration processes. Machine washing was performed at 160 ⬚F for 30 min with 92 g of AATCC detergent 124.
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suitable for different applications. Indeed, fabrics and clothing are able to provide numerous benefits to human kind. There are always new challenges to textile scientists and more innovative methods and theories are anticipated for future research in the development of new products. Acknowledgments Gang Sun is grateful to the National Science Foundation for a CAREER award (DMI 9733981) and to VansonHaloSource Corporation for sponsoring research on antimicrobial textiles. The National Textile Center and Department of Textile Engineering at Auburn University provided partial financial support. The authors acknowledge continued support from R. Broughton, Jr. and J. F. Williams, and are in debt to Xiangjing Xu and Lei Qian for their experimental work. Literature Cited 1. Binder, S.; Levitt, A. M.; Sacks, J. J.; Hughes, J. M. Science 1999, 284, 1311–1313. 2. Morse, S. S. Emerging Infectious Diseases 1995, 1, 7–15. 3. Neely, A. N.; Maley, M. P. J. Clin. Microbiol. 2000, 38, 724– 726. 4. Scudeller, L.; Leoncini, O.; Boni, S.; Navarra, A.; Rezzani, A.; Verdirosi, S.; Maserati, R. J. Hosp. Inf. 2000, 46, 222–229. 5. MacKinnon, M. M; Allen, K. D. J. Hosp. Inf. 2000, 46, 216– 221. 6. Lister J. Lancet 1867, 2, 353–355, 668–669. 7. Vigo T. L.; Benjaminson, M. A. Textile Res. J. 1981, 51, 454– 465. 8. Vigo T. L.; Danna, G. F.; Goynes, W. R. Textile Chem. Color. 1999, 31, 29–33. 9. Payne, J. D.; Kudner, D. W. Textile Chem. Color. 1996, 28, 28–30. 10. Isquith, A. J.; Abbot, A.; Walters, P. A. Appl. Microbiol. 1972, 24, 859–863. 11. Tweden, K. S.; Cameron, J. D.; Razzouk, A. J.; Bianco, R. W.; Holmberg, W. R.; Bricault, R. J.; Barry, J. E.; Tobin, E. ASAIO Journal 1997, 43, M475–M481. 12. Cutter, C. N. J. Food Protection 1999, 62, 474–479. 13. McMurry, L. M.; Oethinger, M.; Levy, S. B. Fems Microbiology Letters 1998, 166, 305–309. 14. Gagliardi, D. D. American Dyestuff Reporter 1962, 11, 123. 15. Olson, T. M.; Gonzalez, A. C.; Vasquez, V. R. J. Chem. Educ. 2001, 78, 1231. 16. Pinto, G.; Rohrig, B. J. Chem. Educ. 2003, 80, 41. 17. Worley, S. D.; Williams, D. E. CRC Critical Reviews in Environmental Control 1988, 18, 133. 18. Worley, S. D.; Sun, G. Trends Poly. Sci. 1996, 4, 364–370. 19. Sun, G.; Xu, X. Textile Chem. Color. 1998, 30 (6), 26–30. 20. Sun, G.; Xu, X. Textile Chem. Color. 1999, 31 (5), 31–35. 21. Sun, G.; Xu, X. Textile Chem. Color. 1999, 31 (1), 21–24. 22. Sun, G.; Xu, X.; Bickert, J. R.; Williams, J. F. Ind. Eng. Chem. Res. 2001, 41, 1016–1021. 23. Qian, L.; Sun, G. J. Appl. Polym. Sci. 2003, 89, 2418–2425.
Editor’s Note Structures for a number of the molecules discussed in this paper are available in fully manipulable Chime format as JCE Featured Molecules in JCE Online. See page 171 of this issue for more details and images of these molecules.
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