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Aqueous zinc compounds as residual antimicrobial agents for textiles Brandon Holt, Shawn Gregory, Todd A. Sulchek, Shannon Yee, and Mark D Losego ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15871 • Publication Date (Web): 13 Feb 2018 Downloaded from http://pubs.acs.org on February 16, 2018
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ACS Applied Materials & Interfaces
Aqueous zinc compounds as residual antimicrobial agents for textiles Brandon Alexander Holt1,#, Shawn Alan Gregory2,3,#, Todd Sulchek3,4, Shannon Yee3, and Mark D. Losego2,* 1. Wallace H. Coulter Department of Biomedical Engineering at the Georgia Institute of Technology and Emory University, Atlanta, GA, USA 2. School of Materials Science and Engineering at the Georgia Institute of Technology, Atlanta, GA, USA 3. The G. W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA, USA 4. The Parker H. Petit Institute for Bioengineering and Bioscience, Georgia Institute of Technology, Atlanta, GA, USA
#
These authors contributed equally to this work
* Corresponding Author E-mail:
[email protected]
Keywords: Antimicrobial coating, Medical textiles, Zinc salts, Residual antimicrobials, End-User disinfectants
Supporting Information Contents: Figure S1: Zone of Inhibition analysis example. Figure S2: Live/Dead staining of bacteria exposed to antimicrobial-treated textiles. Figure S3: Zone of Inhibition measurements for various zinc compounds. Figure S4: Zone of Inhibition variation as a function of time. Figure S5: Longevity efficacy comparison between zinc treated fabrics and alcohol-treated fabrics. Figure S6: Differential Scanning Calorimetry of zinc treated fabrics. Figure S7: Color Fastness and Hand Feel of zinc treated textiles. Figure S8: Moisture breathability of zinc treated textiles. Figure S9: Tensile testing of zinc treated textiles.
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Abstract
Textiles, especially those worn by patients and medical professionals, serve as vectors for proliferating pathogens. Upstream manufacturing techniques and end user practices, such as transition metal embedment in textile fibers or alcohol-based disinfectants, can mitigate pathogen growth, but both techniques have their shortcomings. Fiber embedment requires complete replacement of all fabrics in a facility, and the effects of embedded nanoparticles on human health remain unknown. Alcohol-based, end-user disinfectants are short-lived because they quickly volatilize. In this work, common zinc salts are explored as an end-user residual antimicrobial agent. Zinc salts show cost-effective and long-lasting antimicrobial efficacy when solution deposited on common textiles such as nylon, polyester, and cotton. Unlike common alcohol-based disinfectants, these zinc salt treated textiles mitigate microbial growth for more than 30 days and withstand commercial drying. Polyester fabrics treated with ZnO and ZnCl2 were further explored because of their commercial ubiquity and likelihood for rapid commercialization. ZnCl2 treated textiles were found to retain their antimicrobial coating through abrasive testing, while ZnO treated textiles did not. SEM, FTIR, and DSC analyses suggest that ZnCl2 likely hydrolyzes and reacts with portions of the polyester fiber, chemically attaching to the fiber, while colloidal ZnO simply sediments and binds with weaker physical interactions.
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I.
Introduction
Hospital acquired infections (HAIs) are prevalent and a growing problem in the United States. Approximately 4% of all hospitalized patients contract an HAI, affecting nearly 650,000 individuals annually in the United States.1 These infections have a mortality rate approaching 12%, and an annual economic burden of approximately $33 billion.1-2 To combat HAIs, hospitals attempt to reduce surface bioburdens by frequently applying alcohol-based disinfectants to tables, counters, and human body-parts (e.g., hands using sanitizers). However, these disinfectants are rarely applied to textiles within the hospital environment (sheets, clothes, drapery, etc.), which makes textiles particularly prone to harboring microbes and facilitating disease transmission.3 For example, Ohl et al. found that after one week, 92% of hospital curtains contained either staphylococcus aureus or a drug resistant pathogen, and Wiener-Well found that nearly 60% of healthcare worker’s fabrics contain potentially harmful bacteria.3-5 Consequently, finding easily adoptable technologies that reduce this bioburden is an urgent need for the medical community. Industry and academia alike are solving this problem by imbedding fibers and coatings with transition metals. Dow Chemical’s silver-based Silvadur™ and Microban’s zinc pyriththionebased ZPTech™ enable industrial manufacturers to impart antimicrobial agents into raw polymer blends and/or apply them to directly to fibers. Academic research efforts have featured the use of Ag nanoparticles on textiles (Kim), CuO-cotton nanocomposites (Perelshtein), physically deposited zinc-chitosan nanocomplexes via sonication (Petkova), quaternary amines deposited with and without zinc (Xu, Zhao), and PEG engineering (Gon, Xu, Wang) .6-13 However, these prior industrial and academic approaches are not end-user friendly. Advanced manufacturing costs hinder market penetration and efficacy of embedded textiles, while the use of nanomaterials is restrictive because the effects of nanoparticles on human health is not well
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understood. In contrast, alcohol-based disinfectants are easily applied by end-users but fail to provide long-lasting protection. Herein, we present simple zinc-based salt and colloidal solutions as antimicrobial treatments for textiles that are potentially both end-user friendly and long-lasting. These treatments are found to be effective against various types of pathogens (gram positive, gram negative, and fungi) while not harming human cells. Treatments with ZnCl2 solutions are particularly robust against mechanical wear, and we attribute this enhanced residuality to chemical reactions between the salt and the polyester fibers. In contrast to traditional alcohol disinfectants, these fabrics can even retain antimicrobial effectiveness through an industrial drier.
II.
Methods
A. Solution Deposition and Materials Commercially available polyester t-shirts were purchased from Walmart and were used as our base fabric. We focus on polyester terephthalate (PET or just “polyester”) fabrics because of their ubiquity in medicine, carpeting, upholstery, and athletic attire. The t-shirts were labeled 100% polyester, and this composition was confirmed by spectroscopy in our lab. The fabric had a single jersey knit structure with ~15 µm width x ~5 µm thickness fibers.
Fabric
squares (3 cm x 3 cm) were cut from the shirt body. Each square had a nominal mass of 100 mg, yielding an apparent density of 100 g / m2. In limited testing, we also examine cotton and nylon fabrics purchased from Walmart. Zinc formulations consisted of 10 mL of deionized water and their appropriately massed salts or colloidal particles, ranging from 0.05 to 0.5 M. Active agents tested include zinc oxide, zinc chloride, zinc sulfate, zinc nitrate, and zinc citrate. Samples resided in solution for five minutes at room temperature and were then air dried in a fume hood on a polyethylene surface
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for at least 24 hours. Before and after solution treatment, fabric samples were weighted to quantify inorganic loading.
ZnCl2 solutions and ZnO colloidal dispersions were studied in
greater detail due to their inclusion on the FDA’s Generally Regarded As Safe (GRAS) for food items list and their abundance on the EPA’s registered pesticide list. These designations improve the likelihood for their rapid introduction to the commercial market.
B. Antimicrobial Zone of Inhibition Testing Standard zone of inhibition (ZoI) testing was implemented to quantify the antimicrobial efficacy of different formulations in various textile environments. To provide topical insight on the treated fabric’s antimicrobial efficacy on gram positive and negative species and fungi, three microbial strains were applied: Escherichia coli (DH5α and O128), Staphylococcus epidermidis, and Saccharomyces cerevisiae. Microbes in Luria-Bertani (LB) broth were grown to a concentration of approximately 50x106 bacteria/mL and 50 µL of this solution was plated on 100 cm diameter LB-agar plates. Approximately 3 days after treatment, the textiles were further fragmented into 1 cm x 1 cm square pieces and placed on top of the liquid bacterial film. The cultures were grown for 16 hours at 37 °C in contact with the fabrics. The diameter of the circular zone of inhibition was measured from photographs of the plate using FIJI software (Fig. S1).
C. Confocal Microscopy for Suspension Testing Suspension testing was performed with a confocal microscopy to validate the efficacy of these antimicrobial textiles in a liquid environment. Textiles treated with 0.1 M ZnCl2 were suspended in bacteria solution containing E. coli for 3 seconds, then a 7 µL sample of the solution was stained and imaged. A live/dead bacterial stain was added using the LIVE/DEAD BacLight Bacterial Viability Kit (ThermoFisher Scientific, Waltham, MA) at 3 µL per sample. The solutions were then immediately deposited on glass slides in 5 µL droplets. The identification of 3 ACS Paragon Plus Environment
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live and dead bacteria was determined by confocal microscopy (Zeiss Elyra PS.1 / LSM 780, Zeiss, Oberkochen, Germany).
D. Human Dermal Fibroblast Viability Testing Viability testing was performed on human skin cells to observe cytotoxic effects of these formulations at these concentrations. Approximately 100,000 human dermal fibroblasts were cultured in a 6-well plate in complete culture media overnight. Complete media consisted of DMEM (Dulbecco’s Modified Eagle Medium) with 10% FBS (fetal bovin serum) and 1% penicillin-streptomycin. 90% of the media was removed, and treated/untreated textiles were placed directly on top of the cells for 10 minutes. Treated textiles consisted of polyester treated with
0.25
M ZnCl2.
To remove
the cells
for
counting,
1 mL
of
Trypsin/EDTA
(Ethylenediaminetetraacetic acid) solution was added. Cells were then counted using a hemocytometer. Percent viability for each condition was normalized to the cells only control counts.
E. Abrasion Testing Abrasion testing was performed on control and treated textiles at varying concentrations of zinc chloride and zinc oxide. Test swatches were weighted with 100 mL of vialed water, yielding a normal pressure of approximately 25 kPa, and dragged along a clean lab benchtop for approximately 1 meter. The mass of the swatch was recorded before and after frictional loading. The mass loss was then used to quantify the abrasive removal of zinc salt.
F. Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray (EDX) Analysis A Phenom ProX SEM was used to characterize the fiber microstructure before and after solution treatment. In electron microscopy mode, energy dispersive x-ray (EDX) spectroscopy data was collected for compositional analysis. Fabric samples were prepared via slicing into 4 ACS Paragon Plus Environment
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specimens with a diameter less than 5 mm. Samples were then edge-mounted onto electrically conductive carbon tape. A high-pressure charge compensation holder was used to avoid the need to sputter deposit a conductive coating on the fabric. Data was collected from each fabric and averaged from 5 separate locations.
G. Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR) A Nicolet iS Fourier-Transform Infrared Spectrometer with a Pike Attenuated Total Reflectance (ATR) attachment (diamond crystal) was utilized to characterize fabric composition. Before each sample set, a background scan of an untreated fabric, comprised of ten measurements, was performed. This background spectrum was then subtracted from the experimental signal to calculate the final spectra reported.
H. Differential Scanning Calorimetry (DSC) A TA Instruments DSC Q200 was used to characterize the enthalpy of melting and relative polymer crystallinity between control and treated samples. Standard aluminum pans were used to hold ~5 mg of fabric. DSC scans were performed under a nitrogen purge of 50 µL/min from 30 °C to 300 °C at a heating and cooling ramp rate of 10 °C/min.
III.
Results and Discussion
Solution deposition on fabric substrates The loading of zinc agents onto fabric materials as a function of formulation concentration was initially assessed using gravimetric measurements and SEM imaging of fiber microstructure. Fig. 1a plots fabric mass gain as a function of solution concentration. Inorganic
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loading increases linearly with homogenous salt solution concentration (ZnCl2), while colloidal dispersions of ZnO show a non-linear increase in zinc loading that appears to somewhat saturate above 0.1 M. Figs. 1(b, c, d) provide a representative summary of our electron microscopy analysis. Zinc chloride treated fabrics show little change in fiber microstructure, while zinc oxide treated fabrics reveal aggregated deposits of presumably ZnO colloids. In Fig. 1e, it is clear that ZnCl2 treated samples provide a more homogeneous, though diffuse, distribution of zinc while ZnO treated samples (Fig. 1f) have more localized, aggregated zinc deposits. This difference is likely a result of ZnCl2 being in solution while ZnO being a dispersion. Additionally, this difference may also explain why fabrics treated with homogenous and well-dispersing zinc chloride solutions show linear mass gains with increasing solution concentration, while ZnO mass gain is limited by where colloidal particles and mixtures can reach and easily wet fibers, respectively. As discussed subsequently, these differences in structure play an important role in the treatment’s effectiveness and robustness.
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Fig. 1: Analysis of Zn loading on textiles using ZnO and ZnCl2. (a) Plot of percent mass gain versus concentration for both ZnCl2 solutions and ZnO dispersions; (b) Representative SEM micrograph of a control (untreated) PET fabric; (c & d) SEM micrograph of fabric after treatment with (c) 50 mM zinc chloride solution and (d) 50 mM ZnO dispersion; (e & f) Elemental mapping of treated fibers using SEM/EDX showing Zn concentration in pink on fabrics treated with (e) 50 mM zinc chloride solution and (f) 50 mM ZnO dispersion.
Antimicrobial efficacy in controlled environments Antimicrobial performance of the treated textiles was quantified with zone of inhibition testing. Five primary formulations, comprising of four different 0.1 M zinc ion solutions (citrate, nitrate, sulfate, and chloride) and a colloidal dispersion (ZnO) were tested against three major groups of microbes: gram negative bacteria (E. coli), gram positive (S. epidermidis), and yeast (S Cerevisiae). Results from ZoI testing, at a neutral pH, are reported in Fig. 2. 7 ACS Paragon Plus Environment
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Fig. 2: Plot of the diameter of the zone of inhibition (ZoI) for five different zinc formulations (0.1 M) and a control. Each condition is tested against three strains of microbes: Escherichia coli, Staphylococcus epidermidis, and Saccharomyces cerevisiae
Efficacy varied with counter ion and microbial strain, but all treated fabrics showed inhibition of microbial growth. Highest efficacy was seen with zinc chloride, nitrate, and sulfate against gram-positive bacteria. The zinc oxide treatment, while more cytotoxic than the control, was noticeably less effective than the zinc salts tested. It is reported that zinc ions accelerate cell lysis.14 This appears to be consistent with our findings because the aforementioned salts are more ionic relative to zinc oxide and citrate. To be exact, we performed a one-way anova and a tukey multiple comparisons test to compare each compound within each microbial strain. When comparing zinc oxide to the other 4 treatments, we found the differences to be significant in 2-4 of the comparisons, depending on the strain (S. cerevisiae, 2/4; E. coli, 2/4; S. epidermidis, 4/4). While the variance in measurements makes is difficult to confirm some comparisons in strains where the mean ZOIs are smaller, we can assert that the zinc oxide was less effective than all 4 zinc salts tested in S. epidermidis. 8 ACS Paragon Plus Environment
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To test instantaneous efficacy, bacteria and yeast in liquid cultures were exposed to a 0.1 M ZnCl2 solutions. This treatment proved to be almost entirely cytotoxic (See Supplemental Fig. S2). Solution optimization was performed by varying zinc concentrations. Concentration had a positive influence on cytotoxicity, but in the range tested, increasing concentrations appear to display a diminishing return in antimicrobial efficacy (See Fig. S3). Consequently, concentrations above 0.3 M were not further evaluated. Various fabric materials were also tested, including nylon, cotton, and polyester at solution pHs of 3 and pH 6. These results for E. coli ZoI are presented in Fig. 3. Effectiveness did not change significantly amongst the three materials tested except for a reduction in cytotoxicity for the pH 3 formulation applied to nylon, which is possible because the amide protonation may inhibit zinc coordination with the polymer (vida infre, Scheme 1).
Fig. 3: Plot of the diameter of the zone of inhibition (ZoI) for three material substrates (polyester, nylon, and cotton) tested against E. coli at varying pH values for a constant 0.1 M ZnCl2 treatment. Two-way ANOVA yielded that the effect of material substrate was not significant. A Sidak's multiple comparisons test yielded that the effect of pH was only significant on the nylon substrate.
Residuality of Antimicrobial Treatments
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Residuality, or the long-term effectiveness of an antimicrobial treatment, is critical if longlasting protection is desired for an end-user product. Because our formulation relies on inorganic antimicrobial agents, we expect them to, unlike alcohol-based disinfectants, retain their effectiveness with time.
In a basic assessment of residuality, a temporal study was
conducted over the course of a day to determine if these zinc-treated fabrics show signs of degradation with time. Supplemental Fig. S4 plots the zones of inhibition as a function of time for various formulations. No loss of cytotoxicity is observed for any of the formulations tested over the time range explored (up to 24hrs). Dishes were also cultured for a month postexperimentation and zones of inhibition remained present.
In comparison, a Lysol® treated
fabric was found to show no antimicrobial properties 12 hours after its initial treatment (Fig. S5). We then conducted more aggressive residuality testing by assessing the robustness of these zinc treatments against frictional abrasion testing. Applying our abrasion test to a control fabric (no treatment applied) yielded no mass loss within an error of +/- 3%. Based on this control experiment, we attribute all of the observed mass loss in the treated fabrics to removal of the zinc material. Fig. 4 plots this mass loss as a function of treatment formulation ranging from pure ZnCl2 to pure ZnO. Pure ZnCl2 treatments lose near zero mass while pure ZnO treatments lose nearly 30% mass, with mixtures having nearly linear variations in mass loss between these two extremes. These results suggest that the ZnCl2 is making a stronger, likely chemical, attachment to the fibers while ZnO is weakly bound.
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Fig. 4: Plot of the amount of zinc compound mass loss during abrasion tests for polyester fabrics treated with varying formulation chemistries ranging from pure 0.1 M ZnCl2 to pure 0.1 M ZnO. Chemical and Physical Analysis of ZnO and ZnCl2Treated PET Fabrics To better understand variation in attachment modalities for ZnO and ZnCl2 treated PET fabrics, the chemical and physical structure of these materials were investigated with several methods including FTIR and DSC. Fig. 5 plots the IR absorption spectra for pure PET fabric, for ZnO treated (Fig. 5a) and ZnCl2 treated (Fig. 5b) fabrics of varying concentrations. Significant peaks are found at 1093, 1240, 1618, 1711, and 3428 cm-1, corresponding to C-O, C-O, C=O, C=O, and O-H absorbances respectively, and are comparable to polyester peaks found in other literature reports.15-17 When treated with ZnO and ZnCl2, these spectra change considerably, and these changes are distinctly different depending on the treatment chemistry and concentration.
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Fig. 5: ATR-FTIR absorbance spectra of fabrics treated with (a) ZnO dispersions of varying concentration and (b) ZnCl2 solutions of varying concentration. Each plot shows the original absorbance spectrum for an untreated polyester fabric at the bottom in black. Above the dotted line are spectra for each treatment Spectra are offset for clarity.
For ZnO treated fabrics, the -OH stretch at 3428 cm-1 is nearly eliminated, suggesting the removal of this functional group. This -OH stretch can be plausibly associated with the polyester’s terminal alcohol. Surface hydroxyls on colloidal ZnO likely reacted with these acidic OH moieties, resulting in diminished hydroxyl concentrations, yielding water and polymer chains assembled on the colloidal ZnO surface, which is similar to mechanisms proposed in selfassembled monolayer and surface chemistry communities (Scheme 1a).18 Because ZnO is deposited via evaporation, equilibrium would drive the forward reaction of Scheme 1a as water is removed.
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In contrast, ZnCl2 treated fabrics initially show a reduction in the –OH absorbance, but at higher solution concentration, the –OH absorbance increases. At the same time, the carbonyl (C=O) stretch at 1618 cm-1 appears to reduce and then possibly red-shift at higher ZnCl2 concentrations. We hypothesize that the eventual increase in -OH moieties at higher ZnCl2 concentrations is the result of Lewis acid mediated ester hydrolysis facilitated by the Zn2+ ions (Scheme 1b). This mechanism parallels the scheme reported by Kita et al. whom used 5 µM ZnCl2 to promote amide hydrolysis in an alcohol solvent.19 If true, this hydrolytic chain cleavage would increase the number of -OH groups in the polymer (new terminations). At high ZnCl2 concentrations, the –OH stretch also red shifts from 3428 cm-1 to 3399 cm-1. We attribute this shift to zinc coordination that reduces the O-H bond strength. Lastly, the carbonyl shoulder formation at 1618 cm-1 (redshifted from the original 1711 cm-1 peak) is likely the result of stiffer ester carbonyl bonds transforming into weaker acidic carbonyl bonds as a result of the chain
cleavage.
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Scheme 1: Proposed zinc mediated reaction mechanisms. a) Colloidal ZnO particles with surface hydroxyls undergo an acid-base mediated assembly with terminal carboxylic acid groups, reducing the quantity of hydroxyl groups in the polymer chain. b) Zinc ions, provided by soluble zinc chloride, act as Lewis acids and coordinate with carbonyl and ester moieties, enabling hydrolytic cleavage and forming new terminal carboxylic acid and alcohol groups.
The hydroylysis reaction with ZnCl2 proposed in Scheme 1b would also suggest a reduction in the molecular weight and a disruption of the crystallinity of the PET polymer. Thermal analysis was used to investigate these possibilities. The results from DSC experiments are reported in Table 1 with full DSC plots included in Supplemental Fig. S6. Crystallinity fraction was calculated using Equation 1:
% =
(∆ − ∆ ) ∗ 100% ∆
where ∆ is the enthalpy of melting, ∆ is the enthalpy of cold crystallization, and ∆ is the
enthalpy of melting of a perfectly crystalline polyester fabric. Relative crystallinity values for 0.1 M ZnO and ZnCl2 treated fabrics in comparison to the control polyester fabrics were calculated using Equation 2: ∆, − ∆, ∆ = ∆, − ∆, ∆, − ∆, ∆
∆, − ∆,
where “tf” and “c” subscripts reference treated fabric and control, respectively. Double melting endothermic peaks, for all samples, are likely due to the melting of two different ordered structures.20-22 14 ACS Paragon Plus Environment
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Control values for pure polyester fabrics are within the same range as prior reports with a melting temperature of 241°C and a crystallinity of 67%.23-24 When treated with ZnO, the melting temperature stays constant, but crystallinity drops to 55%. Using the ZnCl2 treatment, these values drop even more to 225 °C and 45% crystallinity. As pictured in Fig. S6, control fabrics exhibit two distinct, rounded melting peaks. ZnO treated fabrics lack a lower melting peak but display a sharp higher melting peak, while ZnCl2 treated fabrics show only a broad single melting curve. Decreased melting temperature and decreased crystallinity suggest that the zinc treatments disrupt polymer chain order in the polyester textiles. We attribute this reduced order to disruption of terminal hydrogen bonding caused by binding of the Zn species (true in both ZnO and ZnCl2 treated fabrics) and shortened chain lengths due to hydrolysis (true in ZnCl2 treated fabrics). These observations further support our hypothesis that ZnCl2 hydrolyzes the polyester chain, creating more reactive sites for strong chemical binding between organic and inorganic components.
Table 1: Thermal and structural parameters measured and calculated from differential scanning calorimetry for polyester fabrics untreated, treated with 0.1 M ZnO and 0.1 M ZnCl2.
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This model of zinc-moderated polyester chain hydrolysis also helps explain the earlier observation of zinc uniformity mapped with EDX elemental analysis in Fig. 1. While zinc oxide treatments are aggregated, the zinc chloride treatments seem to fully distribute the zinc amongst the fibers, indicating widespread disruption and integration with the polymer.
Practical efficacy and residuality Because of their proven efficacy and residuality, ZnCl2 treated fabrics were used in field testing to assess real-world viability. Polyester textiles treated with 0.05, 0.1, and 0.5 M ZnCl2 were tested against untreated controls via extended contact to various surfaces in common environments over the course of a week. These fabrics were then cultured overnight on 100 mm LB agar plates. Untreated textile controls showed extensive bacterial lawn formation (Fig. 6a), covering nearly the entire plate. In contrast, polyester treated with ZnCl2 at all concentrations showed undetectable bacterial growth (Fig. 6b).
Fig. 6: Realistic Testing of Textiles. (a) Control Fabric and (b) 50 mM ZnCl2 treated fabrics were brought in contact with various everyday objects (toilets, door handles, desks) and then plated for 12 hours. After 12 hours, fabrics and microbial growth were photographed. A human cell viability assay was also performed to confirm the safety of these formulations at the concentrations tested. Human dermal fibroblasts were cultured overnight and exposed to polyester treated with and without ZnCl2. As shown in Fig. 7a, cells exposed to 16 ACS Paragon Plus Environment
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these treated fabrics had viabilities statistically equivalent to the cells only and untreated fabric controls. Because many textiles are laundered, we also examined the residuality of ZnCl2 treatments after laundering. Polyester swatches were washed with 0.1 M ZnCl2 and then dried in an industrial air dryer. In Fig. 7b, the bacterial cytotoxicity of these fabrics is compared to ZnCl2 treated fabrics that had not gone through the drier. Within statistical error, fabrics after a single dry cycle show no loss in antimicrobial performance. This testing demonstrates that endusers can apply the zinc treatment with their fabric softeners, and the treatment will not abrade due to tumbling or drying heat.
Fig. 7: a) Viability of human skin cells after exposure to ethanol (negative control), untreated PET fabric, and ZnCl2 (50 mM) treated PET fabric. Here the control is plated cells without any other exposure. Viability is calculated by normalizing the cell count to the control. b) Diameter of the zone of inhibition for polyester fabrics treated with ZnCl2 (100 mM) dried in air or in a tumbling industrial drier.
To ensure end-users would still want to use zinc treated garmets, color fastness, durability, and comfort were characterized. Black fabrics were treated with 0.1 M solutions of either zinc chloride or zinc oxide for six hours at elevated temperature and agitation. Averaged RGB values were assessed from digital photographs of the treated and control textiles to quantify the color on a grayscale. Only the zinc oxide treated fabric showed a measurable 17 ACS Paragon Plus Environment
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change in grayscale color, becoming 6% more “white”. The zinc chloride treated fabric showed no measurable change in grayscale color compared to the control fabric (Fig. S7). To determine fabric comfort, breathability was assessed using moisture permeability measurements (Fig. S8). All fabrics, treated and untreated, exhibited a water vapor transfer rate of 23 mg/hr. This behavior is consistent with the zinc treatments being relegated to the fabric surface rather than within the knit porosity. Lastly, mechanical testing was used to assess whether antimicrobial treatment compromised fabric strength. The jersey fabrics were preloaded until linear stressstrain behavior was reached such that fiber strength, not fabric strength, could be directly assessed. Stress-strain curves reported in Fig. S9 show only minor reductions in mechanical strength after treatment. All fabrics have a similar elastic modulus and a change in ultimate tensile strength of less than 15%. These modest reductions in mechanical strength are likely reasonable trade-offs for antimicrobial protection.
IV.
Conclusions Common zinc salts and oxides have been examined as potential residual antimicrobial
agents for fabric decontamination. These zinc-containing formulations exhibit antimicrobial efficacy on a variety of textiles and pathogens with low toxicity to human cells. Abrasion and longevity testing demonstrate that ZnCl2 solutions have significantly greater residuality than ZnO colloidal dispersions and alcohol-based treatments. Physical and chemical analysis of these zinc-treated fabrics reveal that ZnO particles are weakly bound to the textile by colloidal surface hydroxides while ZnCl2 hydrolyzes the polyester to form a more robust chemical bond to the polymer chain. While the concentrations of zinc salt formulations are about 25x higher than similarly performing nanoparticle technologies, the accepted low toxicity of these zinc
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compounds and their modest costs make them better positioned to be rapidly adopted by hospitals and other patient-care facilities.
V.
Acknowledgements Support for this work came from Georgia Tech’s President’s Undergraduate Research
Award (PURA) and a Petit Bioengineering Undergraduate Research Fellowship. A portion of this research was conducted in Georgia Tech’s Materials Innovation & Learning Laboratory (The MILL), an uncommon “make and measure” space committed to elevating undergraduate research in materials science. The authors would also like to thank Timothy Monroe, who assisted with MILL imaging and characterization equipment. S.A. Gregory and B.A. Holt have an equity interest as scientific advisors for BacOff, LLC., a company that could financially benefit from the research reported herein.
VI.
References
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Table of Contents Graphic:
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