key: cord-0824921-akffx93g authors: Parlin, A. F.; Stratton, S. M.; Culley, T. M.; Guerra, P. A. title: Silk fabric as a protective barrier for personal protective equipment and as a functional material for face coverings during the COVID-19 pandemic date: 2020-06-29 journal: nan DOI: 10.1101/2020.06.25.20136424 sha: da7c1c4ea5531ba3741326c72dff05506e21140f doc_id: 824921 cord_uid: akffx93g Background The worldwide shortage of single-use N95 respirators and surgical masks due to the COVID-19 pandemic has forced many health care personnel to prolong the use of their existing equipment as much as possible. In many cases, workers cover respirators with available masks in an attempt to extend their effectiveness against the virus. Due to low mask supplies, many people instead are using face coverings improvised from common fabrics. Our goal was to determine what fabrics would be most effective in both practices. Methods and findings We examined the hydrophobicity of fabrics (silk, cotton, polyester), as measured by their resistance to the penetration of small and aerosolized water droplets, an important transmission avenue for the virus causing COVID-19. We also examined the breathability of these fabrics and their ability to maintain hydrophobicity despite undergoing repeated cleaning. Tests were done when fabrics were fashioned as an overlaying barrier and also when constructed as do-it-yourself face coverings. As a protective barrier and face covering, silk is more effective at impeding the penetration and absorption of droplets due to its greater hydrophobicity relative to other tested fabrics. Silk face coverings repelled droplets as well as masks, but unlike masks they are hydrophobic and can be readily sterilized for immediate reuse. Conclusions Silk is an effective hydrophobic barrier to droplets, more breathable than other fabrics that trap humidity, and are readily re-useable via cleaning. Therefore, silk can serve as an effective material for protecting respirators under clinical conditions and as a material for face coverings. instead are using face coverings improvised from common fabrics. Our goal was to determine 23 what fabrics would be most effective in both practices. Methods and findings 26 We examined the hydrophobicity of fabrics (silk, cotton, polyester), as measured by their 27 resistance to the penetration of small and aerosolized water droplets, an important transmission 28 avenue for the virus causing COVID-19. We also examined the breathability of these fabrics and 29 their ability to maintain hydrophobicity despite undergoing repeated cleaning. Tests were done 30 when fabrics were fashioned as an overlaying barrier and also when constructed as do-it-yourself 31 face coverings. As a protective barrier and face covering, silk is more effective at impeding the 32 penetration and absorption of droplets due to its greater hydrophobicity relative to other tested 33 fabrics. Silk face coverings repelled droplets as well as masks, but unlike masks they are 34 hydrophobic and can be readily sterilized for immediate reuse. All rights reserved. No reuse allowed without permission. (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. The copyright holder for this preprint this version posted June 29, 2020. shortages of these items will likely continue in many locations for the foreseeable future. 45 Although respirators and masks used by health care providers (HCP) and essential workers (EW) 46 form part of the critical armament against COVID-19, a significant drawback of PPE are that 47 they are purposed for only single use. Sterilization of PPE, especially respirators, has been 48 implemented to enable their continued and repeated use, but this approach reduces the ability of 49 respirators to effectively block particles, can induce damage, or may render the equipment unsafe 50 for further usage [1] . 51 In some cases, HCPs and EWs may only have a single respirator provided to them at their 52 workplace and must reuse them indefinitely under hazardous work conditions. To prolong the 53 life of respirators, many HCPs have adopted the clinical practice of wearing multiple pieces of 54 PPE simultaneously, e.g., a mask on top of a respirator (Fig 1A) [2] [3] [4] . This strategy is 55 unsustainable as increased thickness hampers breathing [3] and increases moisture near the 56 wearer's face, thus becoming a conduit for viral transmission [5, 6] . Masks also cannot be 57 adequately cleaned without compromising their protective properties. In many cases, HCPs and 58 All rights reserved. No reuse allowed without permission. (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. The copyright holder for this preprint this version posted June 29, 2020. . https://doi.org/10.1101/2020.06.25.20136424 doi: medRxiv preprint 4 EWs remain vulnerable as they have resorted to using (and reusing) less efficient masks on their 59 own when respirators are unavailable, leaving them at greater risk to viral transmission. to prolong and preserve N95 respirators is to layer a surgical mask over it [4] . (B) A 64 commercially available, task-specific surgical mask (top) compared to a silk face covering 65 (bottom) made according to design specifications by the Center for Disease Control and 66 Prevention (CDC) [7] . Photo in (A) was taken by Elaine Thompson (Associated Press) and used 67 with permission. 68 69 PPE shortages are now affecting the general population, especially employees instructed to 70 wear masks in the workplace as well as people in public places where mask wearing is 71 mandatory or strongly recommended as part of public health policy [7, 8] . As a result, the 72 majority of the general public has been reduced to using improvised face coverings constructed 73 from commercially available materials ( Fig 1B) [9, 10] . Although the primary purpose of face 74 coverings is to minimize potential viral transmission from the wearer to others [11] , they can also 75 provide some protection to the wearer from external sources [12, 13] (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. Experimental design 99 We tested silk material relative to other commercially available fabrics as a protective barrier 100 layer over existing PPE (Fig 1A) and as a material for the construction of face coverings (Fig 101 1B) . We evaluated five types of material that consisted of animal-derived silk that was natural 102 and unmanipulated (i.e., cocoon walls of B. mori and H. cecropia) or processed (unwashed and 103 washed 100% mulberry silk from pillowcases), processed plant-derived (100% cotton) and 104 All rights reserved. No reuse allowed without permission. (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. The copyright holder for this preprint this version posted June 29, 2020. . https://doi.org/10.1101/2020.06.25.20136424 doi: medRxiv preprint 6 synthetic (polyester) fabrics, as well as a water-absorbent material as a positive control (paper 105 towels, see S1 File for material details). These processed fabrics (silk, cotton, and polyester) 106 represent commonly available materials that can be readily used for making protective layers and 107 face coverings. For processed silk, we tested both washed and unwashed silk to examine if the 108 material properties of silk might be altered by standard cleaning techniques (i.e., washing). 109 We compared the different fabric groups in their level of hydrophobicity, functionally 110 characterized by their ability to block either water or aerosolized droplets (from spray), vehicles 111 for the transmission of the virus underlying COVID-19 [30]. Greater hydrophobicity was defined 112 as the starting contact angles of droplets being greater than 90°, which produces increased 113 resistance to the penetration of droplets into the fabric. We assessed hydrophobicity by first 114 measuring the contact angle behavior of an individual water droplet (5 µL and 2 µL volumes) 115 deposited onto the surface of these materials using the sessile drop technique. In these tests, 116 greater contact angles that are more consistent over time indicate greater hydrophobicity. We 117 also measured the saturation propensity of a droplet (2 µL) and the rate of gas exchange over a 118 24-hour period through the material to examine the ability of water (liquid and vapor) to 119 penetrate through the material. Gas exchange rates are a measure of porosity and therefore 120 breathability. We also compared the performance of single and multi-layered silk in saturation 121 trials. Finally, we compared the different fabric types and commercially available surgical 122 masks, in terms of penetration of aerosolized droplets delivered as spray through the material, via 123 a modified custom apparatus [31] . We also tested vertical aerosolized spray after sterilization 124 where face coverings were sterilized a total of five times using a dry-heat oven at 70 °C. In all 125 experiments, we tested three different sources for each material type and performed three 126 technical replicates for each source of fabric. Thickness measurements were made in three 127 All rights reserved. No reuse allowed without permission. (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. The copyright holder for this preprint this version posted June 29, 2020. . https://doi.org/10.1101/2020.06.25.20136424 doi: medRxiv preprint separate locations on the material and fabric then averaged. More details on these methods, tests, 128 and sterilization are found in S1 File. with material thickness as a covariate. Gas exchange data were first log10-transformed to meet 141 assumptions of normality, and then compared among material types using a one-way ANOVA. Comparisons of the percentage of samples that were penetrated by a 2 µL water droplet for 143 either single or multilayered silk fabric layers were analyzed using a Fisher's Exact omnibus test, 144 which was then followed by pairwise Fisher's Exact tests with Bonferroni correction (α = 0.016). Relative to when no face covering was present over a testing surface, we compared the capability 146 of face coverings (cotton, silk, and polyester) and surgical masks to repel aerosolized droplets 147 (i.e., resist penetration and saturation by aerosol droplets delivered via spray) using a one-way (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. Testing the performance of material for use as protective barriers and face coverings 153 We found that material groups differed significantly in starting contact angles for both 154 droplet volumes tested (5 µL -ANCOVA: F6,55=16.88, P<0.001; η 2 =0.62, ηp 2 =0.64; 2 µL -155 ANCOVA: F6,55=20.36, P<0.001; η 2 =0.68, ηp 2 =0.69). In all trials, silk-based materials (B. mori 156 and H. cecropia cocoons, unwashed and washed silk) were found to be hydrophobic, as they had 157 starting contact angles approximately or greater than 90° (S1 Table) . In contrast, cotton, 158 polyester, and paper towel materials were classified as hydrophilic as starting angles were far 159 below 90° or had immediate droplet absorption (S1 Table) . Thickness was significantly related Table) . (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. The copyright holder for this preprint this version posted June 29, 2020. Table) . The magnitude of change from start to final contact angles was significantly different across Table) . The saturation propensity of a 2 µL water droplet significantly differed between material 189 groups (ANCOVA: F6,49=55.875, P<0.001; η 2 =0.74, ηp 2 =0.87), with cotton and paper towel 190 having the largest droplet spread followed by the remaining groups (Table 1) (Table 1) . 196 All rights reserved. No reuse allowed without permission. (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. The copyright holder for this preprint this version posted June 29, 2020. To test the resistance of silk layers, we compared the ability of a 2 µL water droplet to 208 penetrate single and multiple fabric layers. We found that droplet penetration of silk fabric 209 significantly decreased as the layers of silk increased from a single layer to either double or triple 210 layers (Fisher's Exact, P<0.001), but two and three layers of silk did not differ from each other 211 All rights reserved. No reuse allowed without permission. (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. The copyright holder for this preprint this version posted June 29, 2020. . https://doi.org/10.1101/2020.06.25.20136424 doi: medRxiv preprint (S3 Table) . As the public typically wears face coverings with one or two layers, we compared the and standard PPE such as surgical masks. Our results demonstrate that the greater 234 All rights reserved. No reuse allowed without permission. (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. Silk performs similarly to surgical masks when layered over respirators, as they would occur 238 in clinical settings (Fig 1A) , yet has the added advantages of greater hydrophobicity and the 239 ability to be easily cleaned through washing for multiple uses. Recent work has also aimed at (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. Currently, public health recommendations focus on cotton material for face coverings [14] . 258 We found that cotton materials are hydrophilic, and readily allow droplets to rapidly penetrate 259 and saturate the fabric like a sponge. Therefore, face coverings made out of these materials may 260 quickly become reservoirs of virus and act as conduits for viral transmission when worn, even 261 after a short time [5, 6] . Face coverings made out of polyester face these same limitations, as it is 262 hydrophilic like cotton. Therefore, cloth and polyester face coverings appear to be more suitable (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. The copyright holder for this preprint this version posted June 29, 2020. . https://doi.org/10.1101/2020.06.25.20136424 doi: medRxiv preprint 14 non-specialized PPE require copper particles to be infused during the manufacturing process 281 [39], an expensive process that could be circumvented by using natural silk fibers. In summary, 282 we suggest that silk has untapped potential for use during the current shortage of PPE in the All relevant data are found within the paper and its Supporting information files. Competing interests 295 The authors declare that they have no competing interests. 296 All rights reserved. No reuse allowed without permission. (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. The copyright holder for this preprint this version posted June 29, 2020. (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. The copyright holder for this preprint this version posted June 29, 2020. (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. The copyright holder for this preprint this version posted June 29, 2020. (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. The copyright holder for this preprint this version posted June 29, 2020. (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. The copyright holder for this preprint this version posted June 29, 2020. (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. The copyright holder for this preprint this version posted June 29, 2020. All rights reserved. No reuse allowed without permission. (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. The copyright holder for this preprint this version posted June 29, 2020. . https://doi.org/10.1101/2020.06.25.20136424 doi: medRxiv preprint (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. The copyright holder for this preprint this version posted June 29, 2020. . https://doi.org/10.1101/2020.06.25.20136424 doi: medRxiv preprint 1 S1 File. Protocol for experiments. We tested five material groups for contact angle, saturation propensity, and gas exchange rates, then tested three fabric groups for droplet penetration of single and multiple layers, and for resistance to aerosolized spray. For animal-derived silk that was natural and unmanipulated, we took Bombyx mori cocoon samples from our current laboratory colony (3 rd we tested a 100% cotton handkerchief (thickness: 0.158 ± 0.010 mm), 100% cotton fabric (thickness: 0.199 ± 0.006 mm), and a 100% Egyptian cotton pillowcase (thickness: 0.163 ± 0.005mm). Synthetic materials tested included a pillowcase that was a blend of 88% polyester -12% nylon (thickness: 0.152 ± 0.002 mm), a 100% polyester pillowcase (thickness: 0.103 ± 0.001 mm), and a 100% polyester drawstring bag (thickness: 0.088 ± 0.005 mm). The 100% Egyptian cotton pillowcase, 100% cotton handkerchief, 100% cotton fabric, 100% polyester pillowcase, 88% polyester-12% nylon pillowcase, and 100% polyester drawstring bag were purchased two-years prior from various retailers (Walmart, USA; JoAnn Fabrics, USA). Positive All rights reserved. No reuse allowed without permission. (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. Contact angle data for 5 µL and 2 µL water droplet trials were collected using an experimental setup based on those used in previous work [2] . The droplet volumes were based on the range of values previously used to test natural materials and fabrics [3, 4] . The contact angle of a water droplet deposited onto the material surface (using the sessile drop technique; see below) was used to determine the hydrophobicity of the test material based on the angle produced by the edge of a water droplet contacting the material surface. We vertically deposited the water droplet (5 or 2 µL) onto the fabric piece using a pipette. We avoided any effects of kinetic energy on the contact angle formed by the droplet by ensuring the droplet was in contact with both the pipet tip and the surface of the material swatch prior to final deposition [5] . We used a high-resolution digital camera (Micro 4/3 Lumix SLR, Panasonic Corporation) to capture trial images. During all trials, the camera was kept level with the water droplet and test material. We performed trials on a plastic platform that was positioned horizontally and leveled using a leveler (Bullseye Surface Level, Empire Level). For each trial, we obtained three mean contact All rights reserved. No reuse allowed without permission. (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. The copyright holder for this preprint this version posted June 29, 2020. . https://doi.org/10.1101/2020.06.25.20136424 doi: medRxiv preprint 3 angle measurements (mean angle of the contact angle of the left and right sides of the droplet as seen in images): the initial contact angle (time = 0 s, the first image that the pipette tip was completely out of frame), the dynamic contact angle (mean contact angle, sampled every five seconds, and averaged at the end of the trial), and the final (defined as the last reliable image in which the contact angle could be determined or at t = 120s). We tested the contact angle of a 5 µL and 2 µL water droplets separately. Images were sampled every second for a total duration of two minutes and then uploaded to ImageJ 1.52a (http://rsb.info.nih.gov/ij/) for analysis. The two points of contact were then identified as the outer most points at which the droplet touched the material surface. A straight line was then drawn with the angle tool connecting the two points of contact, parallel to the plane of the material, and the angle line was drawn tangential to the point of contact between the droplet and the material. This technique was done for both the right and left side of the droplet and then averaged to obtain the mean contact angle [6] . A contact angle measurement was determined unreliable if either of the two points of contact or the curvature of the droplet could not be determined. Saturation propensity was measured as the absorption of a water droplet and was used to test the permeability of the test material. For each trial, we applied a 2 µL water droplet and waited 1-minute before taking an image of the material to measure the total area that the water droplet had spread within the material. Images were analyzed using ImageJ. If the water droplet was not absorbed at the end of 1-minute, we measured the area of the water droplet. The water droplet All rights reserved. No reuse allowed without permission. (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. The copyright holder for this preprint this version posted June 29, 2020. . https://doi.org/10.1101/2020.06.25.20136424 doi: medRxiv preprint 4 was applied using a similar technique as above by ensuring the droplet was in contact with the material first before depositing the droplet. We tested the rate of gas exchange for each material by using methods that were modified from previous studies [7, 8] . First, we built an airtight holder for material swatches through which only water vapor was allowed to evaporate. The apparatus was created from a 0.3 mL micro reaction vessel with a hole in the rubber seal to keep the vessel airtight. Each micro reaction vessel was filled with water (300 µL), covered with the material swatch and airtight cap, and then placed on an electronic balance in the room to obtain the initial weight and to measure the weight change after a 24-hour period. We recorded the ambient temperature and humidity of the room for the duration of these tests to correct for the water vapor transfer rate [9] . We determined how increasing the number of layers of silk affects the ability of silk to be an effective barrier by comparing the ability of a 2 µL water droplet to penetrate one, two, and three layers of silk fabric for washed and unwashed silk groups. For each trial, we placed a 7.62 cm by 12.70 cm notecard on a Styrofoam mannequin head (Fig. S1a) , which covered the nose, mouth, and upper cheek areas (left and right) while the mannequin head was in a horizontal position (Fig. S1b) . Each trial was completed when the 2 µL droplet was no longer present on the surface of the silk, either through absorption or evaporation. Images were scanned using a flatbed scanner (Canon MG2220, Canon, Inc.), then uploaded and processed in ImageJ v1.52a. Blank index cards (Walmart Inc., AR, USA) were used to identify possible potential discoloration in the card from the manufacturing process that would create a false positive detection during All rights reserved. No reuse allowed without permission. (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. The copyright holder for this preprint this version posted June 29, 2020. . https://doi.org/10.1101/2020.06.25.20136424 doi: medRxiv preprint 5 image analysis. These were identified as small dark points on the card that differed from discoloration caused by the droplet. The velocity of the spray was determined through the relationship of flow rate and velocity using the following equations for flow rate (m 3 /s): = where Q is the flow rate (m 3 /s), v is the volume (m 3 ), and t is time (s). The relationship between flow rate and velocity is as follows: = where Q is flow rate (m 3 /s), A is the cross-sectional area of the cylinder (m 2 ), and V is the velocity (m/s). We solved for velocity by first calculating the flow rate (Q) from equation (1) and then rearranging equation (2) . We recorded each spray using a camera (Logitech HD Pro C920) and weighed the apparatus before and after each spray. The aerosolized spray had an average velocity of 0.88 ± 0.04 m/s with each spray containing 0.125 ± 0.05 mL of liquid. Although a real human cough has an extreme amount of variability in droplet size, cough plume, and other characteristics [10] , our device based on a similar experimental design [11] represents an extreme case in which a patient openly coughs in close proximity without any protective barrier. (1) (2) All rights reserved. No reuse allowed without permission. (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. The copyright holder for this preprint this version posted June 29, 2020. . https://doi.org/10.1101/2020.06.25.20136424 doi: medRxiv preprint We compared 100% cotton, polyester, and 100% silk (washed and unwashed) as a single layer and double layer in the aerosolized spray test. We modified an aerosol can with a standard valve, and added 150 ml of black-dyed water (10ml black dye, 140ml water; McCormick, MD, USA). Prior to each trial, the aerosol can was filled to 82 kPa with an air pump and checked using a tire-pressure gauge. The Styrofoam mannequin head had a piece of fabric on top of a notecard pinned to the face (Fig. S1a) , and which was placed 0.66m [10] from the aerosol can ( Fig. S2) . A control group (no mask) was sprayed to provide a baseline of discoloration for comparison. The aerosolized droplets were of a random distribution in size with the speed and total volume consistent across trials. Face coverings were made according to the CDC guidelines for sewn pleated face coverings [12] , and were made with a single material that consisted of either polyester, cotton, or silk (see above). We made three face coverings for each material group and included two brands of surgical masks for comparison in the aerosolized spray test. Initially, these coverings were tested prior to any sterilization and stretching. After the initial trials, the coverings were sterilized using dry heat (70 °C) for 1-hour each and then retested after a single sterilization and after five sterilizations. After each sterilization, face coverings were worn for approximately 5-minutes and stretched (i.e., diagonally, horizontally, and vertically) to simulate wear-and-tear, and the same masks were used across all trials. Images were processed in ImageJ 1.52a. The images were converted into 16-bit images to allow grayscale thresholding to isolate and separate pixels darkened by the aerosolizing apparatus. Using a positive control, the threshold value was determined by incrementally All rights reserved. No reuse allowed without permission. (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. The copyright holder for this preprint this version posted June 29, 2020. . https://doi.org/10.1101/2020.06.25.20136424 doi: medRxiv preprint increasing the value until both visible spots were sufficiently covered and before there was significant threshold identification on either the white of the card or on the background on which the cards were placed. From this process, we were able to obtain an area and associated identity for every contiguous threshold particle. This tool enabled us to exclude any particles that were obviously not droplets from the aerosolizing apparatus and instead resulted from the experimentation itself. These included (1) holes created by the pins securing the card to the fabric, (2) large shadowed portions of the card created by unintentional bending or creasing of the card during experimentation, and (3) large fabric remnants or other debris found on the card. After these areas were excluded, the total sum area of all the threshold particles was calculated. All rights reserved. No reuse allowed without permission. (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. The copyright holder for this preprint this version posted June 29, 2020. Dimorphic cocoons of the cecropia moth (Hyalophora cecropia): Morphological, behavioral, and biophysical differences Contact angle measurements using a simplified experimental setup Mulberry non-engineered silk gland protein vis-a-vis silk cocoon protein engineered by silkworms as biomaterial matrices Wettability gradient on the elytra in the aquatic beetle Cybister chinensis and its role in angular position of the beetle at water-air interface Contact angle and wetting properties Contact angle measurements and water drop behavior on leaf surface for several deciduous shrub and tree species from a temperate zone Water resistance and mechanical properties of biopolymer (alginate and soy protein) coated paperboards The silkmoth cocoon as humidity trap and waterproof barrier Measurement errors in water vapor permeability of highly permeable, hydrophilic edible films No reuse allowed without permission. (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. The copyright holder for this preprint this version posted Airflow dynamics of human jets: sneezing and breathing-potential sources of infectious aerosols A cough aerosol simulator for the study of disease transmission by human cough-generated aerosols Recommendation regarding the use of cloth face coverings, especially in areas of significant community-based transmission All rights reserved. No reuse allowed without permission.(which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.The copyright holder for this preprint this version posted June 29, 2020. . https://doi.org/10.1101/2020.06.25.20136424 doi: medRxiv preprint S1 Paper towel (positive control) 0.00 ± 0.00 d 0.00 ± 0.00 c 0.00 ± 0.00 b Three metrics of contact angle (CA) including starting contact angle, final contact angle, and the magnitude change from the start to final contact angles for 5 µL and 2µL water droplets (mean ± SE). Letters indicate Tukey post-hoc similarities between material group in each respective metric and water droplet test.All rights reserved. No reuse allowed without permission.(which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.The copyright holder for this preprint this version posted June 29, 2020. . https://doi.org/10.1101/2020.06.25.20136424 doi: medRxiv preprint All rights reserved. No reuse allowed without permission.(which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.The copyright holder for this preprint this version posted June 29, 2020. . https://doi.org/10.1101/2020.06.25.20136424 doi: medRxiv preprint All rights reserved. No reuse allowed without permission.(which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.The copyright holder for this preprint this version posted June 29, 2020. . https://doi.org/10.1101/2020.06.25.20136424 doi: medRxiv preprint All rights reserved. No reuse allowed without permission.(which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.The copyright holder for this preprint this version posted June 29, 2020. . https://doi.org/10.1101/2020.06.25.20136424 doi: medRxiv preprint