Am. J. Trop. Med. Hyg., 77(1), 2007, pp. 52-57
Copyright © 2007 by The American Society of Tropical Medicine and Hygiene
N, N-diethyl-m-toluamide–Containing Microcapsules for Bio-Cloth Finishing
Bin Fei AND
John H. Xin*
Institute of Textiles and Clothing, Hong Kong Polytechnic University, Hung Hom, Hong Kong
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ABSTRACT
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To obtain long-duration protection from mosquitoes using insect repellent N, N-diethyl-m-toluamide (DEET), this compound was incapsulated in situ during the graft copolymerization of butyl acrylate onto chitosan in an aqueous solution. Morphology of microcapsules was characterized by scanning electron microscopy, scanning probe microscopy, and transmission electron microscopy. This morphology supported successful encapsulation of DEET into polymer capsules. The encapsulation ratio of DEET was greater than 33%, as estimated from thermo-gravimetric results. The aqueous emulsions were applied to cotton textiles by spraying. Treated cloth showed high bactericidal activity against Staphylococcus aureus. Mosquito repellency of the bio-cloth was evaluated with Aedes albopictus. The 90% effective dose of emulsions on textiles was compared with that of DEET in ethanol. A time profile showed that the repellency of an optimized emulsion was 100% after eight hours, and partially preserved even after exposure to air for 48 hours.
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INTRODUCTION
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N, N-diethyl-m-toluamide (DEET) is a broad-spectrum insect repellent that is used extensively against both human and animal pests to prevent many infectious diseases. It has been shown to be effective against mosquitoes, bugs, ticks, and mites.1 A study showed that DEET was also highly effective in killing Schistosoma cercariae in the skin.2 However, the mosquito repellency of DEET cannot be maintained for a long duration because of its high volatility and migration rate. Commercial DEET repellents are effective only for 4-8 hours. Although DEET in ethanol commercial products provide a complete protection time (CPT) longer than other natural and synthetic repellents and the CPT increases with DEET concentration up to 30%, the longest CPT obtained from a formulation containing 23.8% DEET had a mean CPT of only 301.5 minutes.3 In some cases, DEET may induce skin irritation when directly applied.3 To achieve long-term protection against mosquitoes, novel DEET analogs and combinations of DEET with insecticides are being developed.4,5
Encapsulation of DEET is of great interest because of its additional increase in water resistance6 and reduction of skin absorption.7 Many organic polymers have been used as capsules to control the release of mosquito repellents. For example, a polymer cream formulation (33% DEET) and a microparticle formulation (42% DEET) tested against Aedes aegypti, Ae. taeniorhynchus, Anopheles albimanus, and An. stephensi were superior to a 75% DEET lotion.8 The effect against Ae. aegypti and An. albimanus with nine microcapsule formulations was significantly greater than simple (unformulated) DEET at the same concentration for periods up to 24 hours, and the best results were obtained with three micro-capsule formulations containing lanolin, arabic gum, gelatin, tannic acid, stearic acid, polypropylene glycol, and water.9 When 10–30% N,N-diethyl phenyl acetamide (DEPA) was mixed with polysiloxanes, the protection time was 1.0–7.5 hours at DEPA concentrations of 1.0 mg/cm2 for different polysiloxane- DEPA formulations, and a maximum protection time of 7.5 hours was obtained from a poly(dimethylvi-nylmethyl siloxane)–DEPA formulation.10 However, mosquito repellent protection longer than 24 hours has not been reported for DEET.
Generally, controlled release capsules with a core-shell structure are synthesized by interfacial polymerization, self-assembly of lipids, and phase separation in mixed solvents.11–13 Recently, we prepared polymer nanospheres by graft copolymerization of acrylates from amino groups of water-soluble polymer chains.14,15 During the polymerization, in situ formed amphiphilic copolymer acted as an emulsifier and wrapped the water-insoluble acrylate monomers to form stable emulsions. This synthesis route can be conveniently applied in many areas, including agriculture and textile industry, where its attractive features will be fully exploited. First, chitosan, one of the water-soluble polymers used, is a biodegradable and biocompatible polysaccharide industrially produced from abundant natural resources (shells of crabs and shrimp). This polymer is widely used in medical, food, and textile materials and shows superior antibacterial activity and mechanical properties. Second, core acrylates of the nano-spheres are also biocompatible and can be structured to have adjustable properties by selecting different monomers. Third, synthesis can be used to encapsulate hydrophobic organics with special functions, without addition of common surfactants.
In this study, copolymers of chitosan and polyacrylates were used to encapsulate DEET in situ. The microcapsules obtained were characterized for structure and composition. Because novel functional textiles are of growing interest in textile industry, and health care is becoming a prominent concern,16 DEET-containing microcapsules were applied to cotton textiles. The antibacterial and mosquito repellency of the bio-cloth were then tested.
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MATERIALS AND METHODS
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Materials.
N,N-diethyl-m-toluamide, chitosan (medium molecular weight, 77% deacetylation determined by elemental analysis), acetic acid, butyl acrylate (BA), and tert-butyl hydroperoxide (70% solution in water) were obtained from the Aldrich Chemical Company (St. Louis, MO). All materials, except BA, were used without further purification. Butyl acrylate was purified by vacuum distillation to remove phenolic inhibitor. Freshly deionized water was used as the dispersion medium. Plain cotton textiles (15 tex bleached woven with a weight density of P = 133 g/m2) were provided by the China Dye Holdings Ltd. (Hong Kong, People’s Republic of China). This textile is less than 0.4 mm thick and suitable for biting by mosquitoes, as shown in Figure 1a
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Preparation of emulsion particles.
In a glycerol-jacketed flask equipped with a condenser, a magnetic stirrer, and a nitrogen inlet, the chitosan solution (0.5% [w/v], 100 mL) with acetic acid (0.6% [w/v]) was purged with nitrogen for 30 minutes with stirring. A mixture of DEET (1.0–4.0 mL) and BA monomer (0.5–2.0 mL) was added at 80°C. After stirring for 10 minutes, an appropriate amount of tert-butyl hydroperoxide was added and the reaction under nitrogen was incubated at 80°C for 4 hours. Stable white emulsions were obtained from this process. Emulsions were applied to the plain cotton textile by spraying at a dose of 0–10 mg/cm2.
Characterization of emulsion particles.
The morphologies of the emulsion particles were observed by scanning electron microscopy (SEM) (Stereoscan 440 operating at 20 kV; Leica, Wetzlar, Germany). Multimode scanning probe microscopy (SPM) (Nanoscope IV; Veeco Instruments Inc., Woodbury, NY) was used to scan the heights and sizes of separate emulsion particles in the tapping mode. Transmission electron microscopy (TEM) (model 20, 120 kV; Philips, Amsterdam, The Netherlands) was used to observe separated evaporated emulsion particles. To determine emulsion particle size, generally 20–50 typical particles were measured, and the average value was recorded. Thermogravimetric (TG) curves of emulsion cast films and DEET were recorded on an American SDT-2960 thermal analyzer (TA Instruments, New Castle, DE) in an atmosphere of air and at a heating rate of 10°C/ minute.
Staphylococcus aureus (ATCC 6538), a gram-positive bacteria, was used to test the antibacterial activity of textiles treated with emulsions by using a standard method developed by the Dow Corning Corporation.17 A bacterial culture (25 µL) was diluted with 900 mL of 0.5 mM phosphate-buffered saline, pH 5.00. Cotton textile (1.0 gram) sprayed with emulsion was cut into small pieces and placed in 50 mL of bacterial solution and mixed at 150 rpm. The amount of bacteria in solution was evaluated by monitoring the optical absorbance of solution at 600 nm. The antibacterial activity of sample was evaluated by the reduction of bacteria in culture with time.
The mosquito repellency of treated cotton textile was tested according to the modified Chinese standard GB/T 17322.10-1998. Informed consent was provided by the persons exposed to the mosquitoes. This part of the study was reviewed and approved by the appropriate authorities. Briefly, the test textile (> 4 x 4 cm2) was placed over the back of a person’s hand that was covered with a rubber glove that had a square cut of 4 x 4 cm2 for textile exposure, as shown in Figure 1b
. The textile was carefully handled to closely adhere to the skin. The covered arm was inserted into a cage (40 x 30 x 30 cm3) containing approximately 300 hungry female mosquitoes (Ae. albopictus) that were reared in a sterilized environment (26°C and a relative humidity of 65%). During the time (2 minutes) that the textile was exposed to mosquitoes, the biting occurrence was reported by the test subjects. Fabric with no repellent and then progressively higher doses of repellent was sequentially exposed to mosquitoes immediately after spraying the fabric with repellent to obtain a dose profile for each emulsion in comparison with 4% DEET in ethanol. The number of bites at the end of exposure was counted and recorded. Probit analysis was used to calculate the 90% effective dose (ED90). The percentage of repellency was defined as the difference between the number of bites on control (untreated) textile and treated textile. By applying a high dose of repellent (10 mg/cm2), which showed 100% repellency, the textile on the arm was re-exposed hourly to observe the repellency decrease profile in comparison with counts for untreated textile and textile sprayed with 4% DEET in ethanol. Fifteen test subjects identified as attractive to mosquitoes were selected to be exposed to mosquitoes. Each test subject generally used one cage for their test. Thus, the same mosquitoes can be used multiple times and their continued hunger can be checked by exposure to an untreated arm.
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RESULTS
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Preparation and characterization of microcapsules.
Four aqueous emulsions were prepared (Table 1). These emulsions were stable for 12 months at room temperature, as judged by latex appearance. The primary E0 emulsion without DEET was first analyzed. After cast into film, homopolymer poly-(butyl acrylate) (PBA) and graft copolymer chitosan-g-PBA were isolated by Soxhlet extraction with chloroform for 48 hours. After the chitosan backbone of chitosan-g-PBA was degraded by refluxing in 6 N HCl for 24 hours, the PBA graft was obtained. Synthesis parameters were calculated as previously described14 (monomer conversion = 87 weight% and graft efficiency =52 weight%). The number average molecular weights of PBA graft and PBA homopolymer were determined by gel permeation chromatography to be approximately 280,000 and 310,000.
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TABLE 1 Emulsions prepared with different compositions and their ED90 against Aedes albopictus on cotton textile*
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The emulsion particles were observed by SEM after diluting 1,000x and casting on silica wafers. Because the PBA is a soft polymer with a low glass transition temperature (Tg approximately –40°C), the original sphere particles in emulsion collapsed to a flat cake after casting. As shown in Figure 2 by side-glance at an angle of 30°, the representative emulsion particles from E0 appeared as circle cakes with a diameter of 300 nm.
When DEET was added to the synthesis, it was spontaneously wrapped with BA monomer by in situ–produced am-phiphilic copolymers and formed stable droplets. The BA monomers in the droplets further polymerized into PBA spheres. Because of the good compatibility between DEET and BA monomer and that between DEET and PBA, the DEET dissolved PBA and formed a homogeneous phase as a core covered by a shell of amphiphilic copolymer chitosan-g-PBA. This emulsion was diluted 1,000x and cast on silica wafer. Separated particles were observed by SEM by side glance at an angle of 30°, as shown in Figure 2. These particles show irregular shapes and polydisperse sizes. Most particles have diameters of 0.5–1.5 & mu;m, which are bigger than those of E0. This difference in diameter indicates encapsulation of DEET.
The three-dimension structures of these particles were analyzed by SPM. The SPM images of typical emulsion particles from E0 and E1 are shown in Figure 3. Most particles from E0 have a height less than 100 nm; most particles from E1 have a height less than 80 nm. Particles from E1 behaved were softer than those from E0. This softness of E1 particles is attributed to the encapsulation of liquid DEET. To directly observe the separate microcapsules, diluted emulsion E1 was placed on a copper grid. After evaporation at 70°C for 3 days, separated microcapsules with light cores and dark shells were observed by TEM, as shown in Figure 4. This light core was caused by the lower density after the escape of DEET from these microcapsules.
When emulsion E1 was directly cast into film on a silica wafer, a rough film with some pores was observed by SEM (Figure 5). A continuous film could be formed by coalescence of soft polymer particles. Because some DEET was not encapsulated during synthesis, this DEET evaporated and resulted in some delves (depressed or lower positions) on the cast film, as shown in Figure 5. An open pore was observed at the bottom of the delve that was due to a break in the thin shell membrane by evaporation of DEET. The pore size ranged from 100 to 700 nm, which was less than that of corresponding emulsion particles. Two small pores were observed as a slight break of the chitosan shell membrane (upper center of Figure 5a). These results showed the presence of encapsulated DEET in the cast film. When the emulsion was applied onto textile, water evaporated and was partly absorbed by cloth fibers. A thin polymer film was formed on each fiber surface, where a change of fiber morphology was not observed. Therefore, the fiber surface image was not present. However, the results in Figure 5 show that DEET was retained in the thin film and slowly released.
The encapsulation ratio of emulsion E1 was calculated by TG measurement. Pure DEET-cast films from E0 and E1 were measured (Figure 6). Pure DEET begins to lose weight at 130°C and it completely lost below 260°C; the E0 film begins to lose weight at 241°C. Therefore, in the curve of E1, the weight loss between 130°C and 241°C should be due only to loss of DEET. Its fraction is 9.2 weight% of the total weight. The DEET content in the original emulsion E1 is 28.5 weight% of total components. Therefore, the encapsulation ratio of DEET should be higher than 33%. The actual encapsulation ratio could be underestimated because weight loss of DEET at 241°C in the TG measurement was neglected and because of loss of encapsulated DEET during casting.
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FIGURE 6. Thermogravimetric curves of pure N, N-diethyl-m-toluamide cast films from the primary emulsions E0 and E1.
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Anti-bacterial and mosquito repellent functions of bio-cloth.
Cotton textiles treated with E0 and E1 at a dose 2.5 mg/cm2 were tested with S. aureus. Both emulsions showed high bactericidal activity (Table 2). Bacteria in solution were nearly completely killed in 60 minutes. Viable cell counts were higher on pristine cotton textile because cellulosic materials are good media for the growth of bacteria.18 This bactericidal activity of treated cotton was attributed to the chi-tosan shell component, and little change was induced by addition of DEET.
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TABLE 2 Antibacterial measurement of cotton textiles treated with emulsions E0 and E1 of N,N-dimethyl-m-toluamide at a dose of 2.5 mg/cm2 against Staphylococcus aureus
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Cotton textiles treated with different emulsions of increasing doses were also assayed with Ae. albopictus in comparison with untreated textile and textile treated with 4% DEET in ethanol to determine ED90 values. The assay was conducted immediately after textile treatment. As shown in Figure 7, 4% DEET showed the lowest ED90 value (approximately 1.5 mg/ cm2), which corresponded to a DEET concentration of 0.060 mg/cm2. This value was higher than values for tests on human arms against Ae. aegypti (0.021 mg/cm2) reported by Bandolo and others,19 and much lower than values for tests on collagen membrane against Ae. aegypti (0.79 mg/cm2) reported by Cockroft and others.20 This result can be explained by the absorbing surface of cotton textile that reduced the repellency strength of DEET, which is consistent with that reported for DEPA on absorbing and nonabsorbing surfaces.21 The ED90 values for emulsions E1, E2, and E3 were 8.0, 7.0, and 1.9 mg/cm2 (Table 1). These values showed a correlation with DEET concentration and corresponded to DEET concentrations of 0.080, 0.070, and 0.076 mg/cm2, respectively. These values were consistent and higher than those obtained with DEET in ethanol because of suppression of release of DEET by the microcapsule composed of PBA and chitosan. This suppression of release by microcapsules on absorbing surface was different from that reported by Prasad and Kalyana-sundaram21 because our capsule material had a different composition. The E0 sample showed nearly zero repellency.
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FIGURE 7. Dose profiles of mosquito repellency of textiles treated with 4% N, N-diethyl-m-toluamide in ethanol in emulsions E0, E1, E2, and E3 at formulation doses of 0–10 mg/cm2.
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To determine the duration of the microcapsule formulation on textile, E3 was evaluated in comparison with 4% DEET in ethanol, both at a formulation dose of 10 mg/cm2 (DEET concentration = 0.40 mg/cm2) against Ae. albopictus. The time profiles (Figure 8) showed a higher efficacy for E3. When both formulations were applied at a dose of 10 mg/cm2 to cotton textile, 4% DEET in ethanol provided complete protection for four hours and E3 provided complete protection for eight hours. The DEET solution did not provide protection after 12 hours and E3 retained a repellency of 16% even after 48 hours when compared with blank textile. This long repellency is essential for application to textiles. Although the active duration of these textiles is limited by the capacity of the microcapsules, this duration is still an improvement when compared with durations previously reported for DEET controlled release. In these reports, 7–10 different mosquitoes, conditions, and methods were used to evaluate the controlled release efficacy of materials that included acrylate polymers, polypropylene glycol, polyvinylpyrrolidone, silicone polymers, gelatin, gum arabic, tannic acid, fatty acids, lanolin, lipids, and waxes.
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FIGURE 8. Time profiles of mosquito repellency of textiles treated with 4% N, N-diethyl-m-toluamide DEET in ethanol and emulsion E3 at formulation doses of 10 mg/cm2.
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DISCUSSION
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N, N-diethyl-m-toluamide was reported to be effective for more than 24 hours at a dose of 0.50 mg/cm2 with microcapsule formulations containing arabic gum, gelatin, and tannic acid, which can form strong hydrogen bonds with DEET.9 In our emulsions, the amine groups on the chitosan shell also form strong hydrogen bonds with DEET. Therefore, the long repellency period of our encapsulation formulation was attributed to the chitosan amine groups. New DEET microcapsules may show increased efficacy when used with natural proteins.
Our microcapsule formulation is resistant to washing because PBA is hydrophobic and chitosan does not dissolve in neutral aqueous solution. The microcapsule fastness (adherence) on cotton textile has been confirmed in our previous study.22 However, when an acidic liquid with a pH less than 4.0 was used, the microcapsules could be washed off the textile. The dual-function bioscloth sprayed with the synthesized DEET microcapsule emulsion can be made into bed nets for residents, protective covers for hikers, and military uniforms, and will largely improve the users’ health, especially in tropical regions.
In summary, DEET was successfully encapsulated in polymer capsules during copolymerization of BA onto chitosan and a stable aqueous emulsion was obtained. The DEET-containing microcapsules showed useful antibacterial and mosquito repellency in textiles. The treated bio-cloth showed high bactericidal activity and provided better protection against Ae. albopictus than other DEET systems previously reported. The optimized formulation provided 100% repellency for eight hours and retained repellency even after 48 hours. Although only mosquito repellency was tested here, this product should be effective in repelling other pests.
Received September 29, 2006.
Accepted for publication February 21, 2007.
Acknowledgment: We thank Chao Chen (Military Medical Institute of Nanjing Command, Nanjing, People’s Republic of China) for help with the insect test.
Financial support: This study was supported by Innovative Technology Funding ZP53 from the Hong Kong Special Administrative Region government and by the postdoctoral fellowship G-YX41 from the Hong Kong Polytechnic University.
* Address correspondence to John H. Xin, Institute of Textiles and Clothing, Hong Kong Polytechnic University, Hung Hom, Hong Kong, Special Administrative Region, People’s Republic of China. E-mail: tcxinjh{at}inet.polyu.edu.hk 
Authors’ address: Bin Fei and John H. Xin, Institute of Textiles and Clothing, Hong Kong Polytechnic University, Hung Hom, Hong Kong, Special Administrative Region, People’s Republic of China, Telephone, 852-2766-6474, Fax: 852-2773-1432, E-mail: tcxinjh{at}inet.polyu.edu.hk.
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ABSTRACT
INTRODUCTION
