Silk fabric as a protective barrier for personal ... · 6/25/2020 · 65 commercially available, task-specific surgical mask (top) compared to a silk face covering 66 (bottom) made

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Silk fabric as a protective barrier for personal protective equipment and as a functional material 1

for face coverings during the COVID-19 pandemic 2

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Adam F. Parlin1, Samuel M. Stratton1, Theresa M. Culley1, Patrick A. Guerra1* 4

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1Department of Biological Sciences, University of Cincinnati, Cincinnati, Ohio, United States of 6

America 7

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*Corresponding author: [email protected] 9

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Abbreviations: EW, essential worker; HCP, health care provider; PPE, personal protective 13

equipment 14

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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 (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.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 preprintthis version posted June 29, 2020. ; https://doi.org/10.1101/2020.06.25.20136424doi: medRxiv preprint

NOTE: This preprint reports new research that has not been certified by peer review and should not be used to guide clinical practice.

mailto:[email protected]

https://doi.org/10.1101/2020.06.25.20136424

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Abstract 17

Background 18

The worldwide shortage of single-use N95 respirators and surgical masks due to the COVID-19

19 pandemic has forced many health care personnel to prolong the use of their existing 20

equipment as much as possible. In many cases, workers cover respirators with available masks in 21

an attempt to extend their effectiveness against the virus. Due to low mask supplies, many people 22

instead are using face coverings improvised from common fabrics. Our goal was to determine 23

what fabrics would be most effective in both practices. 24

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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. 35

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Conclusions 37

Silk is an effective hydrophobic barrier to droplets, more breathable than other fabrics that 38

trap humidity, and are readily re-useable via cleaning. Therefore, silk can serve as an effective 39

material for protecting respirators under clinical conditions and as a material for face coverings. 40

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Introduction 42

Personal protective equipment (PPE), specifically N95 respirators and surgical masks, are 43

vital to protect against viral transmission during the current COVID-19 pandemic, yet global 44

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) [2-4]. 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



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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. 60

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Fig 1. Recommended clinical practice with personal protective equipment (PPE) and a 62

comparison of PPE with a handmade face covering. (A) One commonly used clinical method 63

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]. Cloth, such as cotton, has 76

been suggested as a suitable material for face coverings [14,15], but it remains an open question 77

as to what material possesses the best suite of characteristics to block droplets and viral particles, 78

as well as what material best facilitates comfort, wearability, and reuse of face coverings. 79

To help combat the PPE shortage amid the COVID-19 pandemic, we examined what 80

materials can serve as immediate solutions for (a) fashioning effective, protective barrier layers 81



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that serve to increase the longevity of respirators and the effectiveness of masks under clinical 82

conditions, and (b) for the construction of face coverings to be worn according to current public 83

health guidelines when standard PPE is not available. We focused on the properties of silk, a 84

material commonly used in the garment industry, which is a natural, fibrous biomaterial 85

produced by animals such as moths and spiders. For example, silk moth caterpillars, such as 86

those of the domesticated silk moth, Bombyx mori, and of the Robin moth, Hyalophora cecropia, 87

produce and use silk for spinning their cocoons [16-18]; these structures consist of hydrophobic 88

and semi-impermeable membranes [19,20] that protect the developing moth residing inside from 89

harsh environmental conditions [20-22]. In addition, protein fibers in silk have been shown to 90

have antimicrobial, antibacterial, and antiviral properties [23-26]. Silk is already used in 91

biomedical applications such as surgical sutures [26], and current research on silk has examined 92

its utility as a biomaterial for many biomedical and human health applications [27-28]. Previous 93

work examining the use of commercially available fabrics for improvised face coverings has also 94

shown that silk possesses some capacity as an antimicrobial barrier when used alone for the 95

fabrication of face coverings [29] and has increased filtration efficiency with more layers [13]. 96

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Methods 98

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



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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



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separate locations on the material and fabric then averaged. More details on these methods, tests, 128

and sterilization are found in S1 File. 129

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Data analysis 131

For contact angle trials (both larger 5 µL and smaller 2 µL droplets), we compared the 132

different material types in terms of their starting, dynamic (i.e., change over time), and final 133

contact angles during trials, and the magnitude change in contact angle between the start and 134

final measurements. We analyzed starting and dynamic contact angles, and the magnitude 135

change in contact angle, using a two-way ANCOVA with material thickness as a covariate. 136

Dynamic contact angle data were analyzed using a linear mixed-effect model with group and 137

time as a fixed effect interaction, and fabric sample as the random effect. Individual models were 138

compared against a null using a likelihood ratio test, and the conditional and marginal r2 are 139

reported for each model [32]. We analyzed saturation propensity using a two-way ANCOVA 140

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. 142

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). 145

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 148

ANOVA. All data were analyzed in R [33]. Significance was set to α = 0.05 except when 149

adjusted for multiple pairwise comparisons. 150



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151

Results 152

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; η2=0.62, ηp2=0.64; 2 µL – 155

ANCOVA: F6,55=20.36, P<0.001; η2=0.68, ηp2=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 160

to the starting contact angle for both droplet types (5 µL – ANCOVA: F1,55=4.47, P=0.039; 161

η2=0.03, ηp2=0.08; 2 µL – ANCOVA: F1,55=6.87, P<0.05; η2=0.04, ηp

2=0.11). 162

In all trials, there was a significant interaction between material group and time for dynamic 163

contact angles (5 µL – χ2 = 778.58, df=13, P<0.001; marginal r2=0.62, conditional r2=0.94; 2 µL 164

– χ2 =549.18, df=13, P<0.001; marginal r2=0.46, conditional r2=0.93). Hydrophilic materials 165

(cotton, polyester, paper towel), in combination with a lower starting contact angle, had a faster 166

change in contact angle as the droplet was almost immediately absorbed. The droplet stayed on 167

longer, resulting in a gradual change over time, for hydrophobic materials (Fig 2, S2 Table). 168

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Fig 2. Mean dynamic contact angle (°) of a 5 µL (black) and 2 µL (orange) water droplet 170

for each material group over a 2-minute duration. The positive control (paper towel) is not 171

shown since the water droplet was immediately absorbed and therefore no contact angle could be 172

measured in all trials. For 5 µL droplets, B. mori, H. cecropia, washed, and unwashed silk all had 173



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starting contact angles above 90° which indicated a hydrophobic surface, while the other fabric 174

types had contact angles less than 90°, indicating a hydrophilic surface. For 2 µL droplets, B. 175

mori, washed silk, and unwashed silk all had mean starting contact angles above 90°, which 176

indicates a hydrophobic surface. The remaining fabric groups had contact angles below 90° 177

indicating a hydrophilic surface. 178

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Final contact angles also differed significantly between groups for both droplet volumes 180

tested (5 µL – ANCOVA: F6,55=13.02, P<0.001; η2=0.62, ηp2=0.64; 2 µL – ANCOVA: 181

F6,55=8.72, P<0.001; η2=0.52, ηp2=0.56). Thickness was significantly related to the final contact 182

angle for all droplet trials (5 µL – ANCOVA: F1,55=25.04, P<0.001; η2=0.16, ηp2=0.31; 2 µL – 183

ANCOVA: F1,55=19.43, P<0.001; η2=0.15, ηp2=0.26; S1 Table). 184

The magnitude of change from start to final contact angles was significantly different across 185

material groups for all trials (5 µL – ANOVA: F6,56=3.48, P<0.01; η2=0.27; 2 µL – ANOVA: 186

F6,56=3.93, P<0.01; η2=0.30). Post hoc pairwise comparisons, however, indicated only significant 187

differences involving paper towel with each other material group (S1 Table). 188

The saturation propensity of a 2 µL water droplet significantly differed between material 189

groups (ANCOVA: F6,49=55.875, P<0.001; η2=0.74, ηp2=0.87), with cotton and paper towel 190

having the largest droplet spread followed by the remaining groups (Table 1). Thickness was 191

significantly related to droplet area (ANCOVA: F1,49=7.14.884, P<0.001; η2=0.03, ηp2=0.23), 192

with a significant interaction between thickness and fabric type (ANCOVA: F6,49=9.772, 193

P<0.001; η2=0.13, ηp2=0.54). Gas exchange, a proxy for porosity, significantly differed between 194

groups (ANOVA: F6,56=16.643, P<0.001, η2=0.64). B. mori cocoons and cotton material had the 195

highest mean gas exchange rates relative to the other groups (Table 1). 196



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Table 1. Saturation (mm2) from a 2 µL droplet for material groups where absorption was 198

present (100% cotton, positive control, unwashed silk, synthetic) and not present (B. mori, 199

H. cecropia, washed silk) after 60 seconds, and gas exchange rates after a 24-hour period. 200

Material Group Saturation Area (mm2) ± SE Permeability (g/m · s ·Pa) ± SE

B. mori cocoon silk layer 1.59 ± 0.12c 1.92-09 ± 1.81-10 , a

H. cecropia cocoon silk layer 4.98 ± 1.10c 7.03-10 ± 7.58-11 c,d

Silk (Unwashed) 11.96 ± 7.23b,c 8.74-10 ± 1.03-11 c,d

Silk (Washed) 5.06 ± 1.47c 9.45-10 ± 1.45 -11 c,d

100% Cotton 86.52 ± 17.67a 1.35-09 ± 6.85-11 a,b

Synthetic 26.26 ± 10.94b 8.44-10 ± 1.06-11 b,c

Positive Control (paper towel) 69.69 ± 22.82a 7.00-10 ± 2.58-10 d

There were significant differences in saturation area between the material groups. The covariate, 201

thickness (mm), was significantly related to the saturation area of each material. For gas 202

exchange rates, silk is as porous as synthetic materials and less porous than 100% cotton. A one-203

way ANOVA test indicated significant difference between material groups. Groups that share the 204

same letters are not statistically different from each other (Tukey HSD post-hoc tests). 205

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Single versus multilayered face coverings 207

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



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(S3 Table). As the public typically wears face coverings with one or two layers, we compared the 212

capability of single and double layers for silk (washed and unwashed), cotton, and polyester 213

fabrics to resist penetration by aerosolized droplets. Vertical spray tests revealed significant 214

differences between each of the fabric groups and the control of no face covering, when face 215

coverings had one-layer (ANOVA: F5,42=18.66, P<0.001; η2=0.69) or two-layers (ANOVA: 216

F5,42=29.50, P<0.001; η2=0.78). There were no differences between any fabric groups, however. 217

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Exposure of face coverings and masks to vertical aerosolized spray after sterilization 219

To examine the effects of sterilization, we compared face coverings made from our different 220

test fabrics using recommendations from the CDC [7] with surgical masks. Discoloration of the 221

test surface from the aerosolized spray remained the same for all tested groups with no 222

sterilization (ANOVA: F4,49=0.99, p=0.42), one sterilization (F4,49=0.98, p=0.43), and five 223

sterilizations (F4,49=1.702, p=0.17). This occurred despite significant differences in the thickness 224

of the different face coverings and masks (ANOVA: F3,41=713, p<0.001; η2=0.98). Cotton face 225

coverings were the thickest (0.367 ± 0.004 mm), followed by masks (0.341 ± 0.008), silk (0.306 226

± 0.005 mm) and then polyester (0.216 ± 0.008 mm). 227

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Discussion 229

Protective layers and face coverings made from 100% silk, a naturally produced commonly 230

available material, are hydrophobic and can effectively impede the penetration and absorption of 231

both liquid and aerosolized water droplets. The hydrophobicity of silk fabric is further enhanced 232

when used in multiple layers, which when combined, are still thinner than most cotton materials 233

and standard PPE such as surgical masks. Our results demonstrate that the greater 234



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hydrophobicity of silk relative to other fabrics, such as cotton and polyester, can make it more 235

effective at impeding droplets, which is a common transmission pathway for the virus that 236

underlies COVID-19 [30]. 237

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 240

making synthetic, reusable hydrophobic layers to layer on top of respirators [34]. The use of 241

natural silk barriers to protect PPE adds to these initiatives, but with the added benefits of silk’s 242

inherent beneficial properties and accessibility of silk for both commercial and public use. Here, 243

the sericulture, textile, and garment industries, along with their supply networks and 244

infrastructure, potentially have a direct pathway to becoming important partners against the 245

current COVID-19 pandemic and in future public health emergencies in which PPE may again 246

be in short reserve. 247

A limitation of respirators and masks, but especially face coverings, is that normal breathing 248

can be hampered when worn, and this difficulty increases with thickness. Prolonged use also 249

exposes individuals to added risks, as they increase the local humidity around the area upon 250

which it is worn (>90% relative humidity) [35], thereby creating a potential pathway for the virus 251

to travel due to the trapped moisture near the face that inadvertently increases wetness [5,36]. 252

Increased humidity underneath these items, exacerbated when worn in hot and humid 253

environments, significantly decreases their wearability because of higher friction and skin 254

moisture [37]. This creates discomfort and can result in an individual unintentionally touching 255

their face. Our results suggest that this limitation and its accompanying risks can be mitigated by 256

silk, at least when it is used in the fabrication of face coverings. 257



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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 263

for brief, one-time use. In contrast, silk’s hydrophobicity, increased filtering efficiency when 264

layered [13], and lack of capillary action [20] make it a more advantageous material for face 265

coverings that are also thin, light, and breathable. Recent recommendations by the World Health 266

Organization have also mentioned combining hydrophilic and hydrophobic layers when creating 267

face coverings [38], and our work supports the use of silk as a better hydrophobic layer for face 268

coverings that is more effective than either cotton or polyester material that are hydrophilic. 269

Furthermore, our results also suggest that using multiple layers of silk for face coverings can 270

increase filtering efficiency, yet retain and enhance its advantageous hydrophobic properties that 271

can preclude it from becoming a reservoir and conduit for the virus, while remaining breathable 272

and comfortable when worn. 273

Although N95 respirators are still the most effective form of protection against viral 274

transmission, our study highlights the practicality of using current commercially available 100% 275

silk material as a protective barrier for PPE and as a material for face coverings. Moreover, silk 276

may play a major role in the development of new PPE equipment, such as respirator inserts, that 277

capitalize on its many benefits. For example, silk possesses antimicrobial, antiviral, and 278

antibacterial properties [24,26], potentially due to the presence of copper, a compound that has 279

antiviral properties and which animals naturally incorporate into their silk [23]. Other fabrics and 280



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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 283

ongoing COVID-19 pandemic. It can be effective when used as a covering to extend the lifetime 284

of N95 respirators, when fashioned as face coverings for the general public, and as a material for 285

the development of the next generation of PPE. 286

287

Acknowledgements 288

We thank Eric J. Tepe for logistical support and helpful comments during the course of this 289

study. P.A.G., A.F.P., T.M.C. and S.M.S. were supported by funds from the University of 290

Cincinnati. 291

Data reporting 292

All relevant data are found within the paper and its Supporting information files. 293

294

Competing interests 295

The authors declare that they have no competing interests. 296



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395

396

Supporting information 397

S1 Fig. Experimental set-up and notecard attachment for the sessile horizontal droplet 398

tests. Material barriers were held to the mannequin head using pins. The dark-grey covering 399

represents the note-card placement while the light-grey represents the face covering. 400

S2 Fig. Aerosolized spray experimental set-up with mannequin head (no mask or face 401

covering during trials) and aerosolizing apparatus. Prior to each test, the apparatus was filled 402

to 82 kPa. The dark-grey covering represents the note-card placement while the light-grey 403

represents the face covering. 404

S1 Table. Contact angle metrics. Three metrics of contact angle (CA) including starting contact 405

angle, final contact angle, and the magnitude change from the start to final contact angles for 5 406

µL and 2µL water droplets (mean ± SE). Letters indicate Tukey post-hoc similarities between 407

material group in each respective metric and water droplet test. 408



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S2 Table. Summary of mixed-effect models for dynamic contact angle of a 2 µL and 5 µL 409

water droplets. 410

S3 Table. Percentage of 2 µL water droplets that penetrated single, double, or triple silk 411

layers compared with Fisher’s Exact test. Totals (n=6) are from both washed (n=3) and 412

unwashed (n=3) silk materials where four 2 µL droplets were applied per technical replicate. 413

Identical letters indicate similarities between Bonferroni-corrected pairwise comparisons. 414

S1 File. Protocol for experiments. 415

416

417



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S1 File. Protocol for experiments.

Materials and fabrics tested

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 (3rd generation reared;

Department of Biological Sciences, University of Cincinnati) and Hyalophora cecropia cocoon

samples collected outdoors from Eastern and Central, Massachusetts between 2013-2016 [1]. For

animal-derived processed silk materials, we tested black (unwashed thickness: 0.094 ± 0.002

mm; washed: 0.092 ± 0.003 mm) and white (unwashed thickness: 0.0.112 ± 0.001 mm; washed:

0.103 ± 0.004 mm) 100% silk scarves (Cyzlann, Shenzhen Yirong Technology Co. Ltd.,

Shenzhen, Guangdong, China) and a 100% mulberry silk (unwashed thickness: 0.165 ± 0.001

mm; washed: 0.169 ± 0.003 mm) pillowcase (Ravmix, Shenzhen City, Yuanmi Trade Co., Ltd.,

Guangdong, China). Subsets of the silk material were washed according to instructions outlined

by the distributor to create the silk (washed) group. For processed plant-derived material (100%),

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



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controls (i.e., paper towels) consisted of a generic brand for white paper towel (thickness: 0.129

± 0.008 mm; Proctor & Gamble, USA), brown paper towel (thickness: 0.083 ± 0.004 mm; Home

Depot, USA), and a Kimwipe (thickness: 0.073 ± 0.004mm; KimTech, USA). Fabrics used for

face coverings tested in aerosolized spray experiments were made from 100% mulberry silk

material, 100% cotton material, and 100% polyester material. Surgical masks were purchased

from local retail stores (surgical mask type 1: Huizhou Canice Health Material Co. Ltd,

Guangdong Province, China; surgical mask type 2: Jiangsu Nanfang Medical Co. Ltd,

Changzhou, Jiangsu, China).

Water droplet contact angle

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



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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 and gas exchange 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



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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].

Single and multi-layer silk water droplet absorption 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



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image analysis. These were identified as small dark points on the card that differed from

discoloration caused by the droplet.

Standardizing aerosolized spray trials

The velocity of the spray was determined through the relationship of flow rate and velocity

using the following equations for flow rate (m3/s):

𝑄 = 𝑣𝑡

where Q is the flow rate (m3/s), v is the volume (m3), and t is time (s). The relationship between

flow rate and velocity is as follows:

𝑄 = 𝐴𝑉

where Q is flow rate (m3/s), A is the cross-sectional area of the cylinder (m2), 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.

Single and multilayer barrier aerosolized spray test

(1)

(2)



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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 covering and mask aerosolized spray test 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



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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.



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of highly permeable, hydrophilic edible films. J Food Eng. 1994;21:395-409.



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(https://www.cdc.gov/coronavirus/2019-ncov/prevent-getting-sick/cloth-face-cover.html)



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S1 Table. Contact angle metrics.

Material Group Starting CA Final CA Magnitude

5µL Water Droplet

B. mori cocoon 116.96 ± 6.36a 94.55 ± 18.86a 22.41 ± 20.34a,b

H. cecropia cocoon 92.96 ± 11.10a,b 38.69 ± 11.88b,c,d 54.26 ± 10.49a

100% Silk (washed) 107.60 ± 19.07a,b 41.69 ± 24.59b,c 65.90 ± 24.82a

100% Silk (unwashed) 120.09 ± 2.73a 69.79 ± 2762a,b 50.30 ± 26.37a,b

100% cotton 42.81 ± 37.21c,d 11.56 ± 10.01c,d 31.24 ± 27.29a,b

Polyester 61.16 ± 26.84b,c 30.66 ± 21.83c,d 30.50 ± 20.04a,b

Paper towel (positive control) 0.00 ± 0.00d 0.00 ± 0.00d 0.00 ± 0.00b

2µL Water Droplet

B. mori cocoon 102.03 ± 9.48a 64.28 ± 17.07a 37.75 ± 10.45a,b

H. cecropia cocoon 85.68 ± 14.81a,b 36.18 ± 12.12a,b,c 49.50 ± 16.96a

100% Silk (washed) 95.17 ± 18.02a 54.07 ± 27.47a,b 41.11 ± 22.37a,b

100% Silk (unwashed) 120.09 ± 8.66a 64.38 ± 25.86a 55.71 ± 21.02a

100% cotton 34.17 ± 30.61c,d 15.08 ± 13.21c 19.09 ± 16.92a,b

Polyester 46.98 ± 22.35b,c 18.38 ± 9.16b,c 28.59 ± 17.71a,b

Paper towel (positive control) 0.00 ± 0.00d 0.00 ± 0.00c 0.00 ± 0.00b

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.



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S2 Table. Summary of mixed-effect models for dynamic contact angle of a 2 µL and 5 µL water droplets.

2µL Contact Angle 5µL Contact Angle

Predictors Estimates CI P Estimates CI P

(Intercept) 87.72 66.49 – 108.96 <0.001 114.13 95.10 – 133.16 <0.001

Time -0.16 -0.20 – -0.12 <0.001 -0.07 -0.11 – -0.02 0.004

H. cecropia -29.88 -59.91 – 0.15 0.051 -54.74 -81.66 – -27.82 <0.001

100% Cotton -79.31 -109.34 – -49.28 <0.001 -106.58 -133.50 – -79.66 <0.001

Paper Towel -87.72 -117.76 – -57.69 <0.001 -114.13 -141.05 – -87.21 <0.001

100% Silk (unwashed) 16.02 -14.01 – 46.06 0.296 6.24 -20.68 – 33.16 0.649

100% Silk (washed) -31.64 -61.67 – -1.61 0.039 -51.63 -78.55 – -24.71 <0.001

Polyester (synthetic) -58.49 -88.52 – -28.45 <0.001 -81.81 -108.73 – -54.89 <0.001

Time * H. cecropia -0.16 -0.23 – -0.10 <0.001 -0.24 -0.31 – -0.18 <0.001

Time * 100% Cotton 0.06 -0.00 – 0.12 0.069 -0.03 -0.09 – 0.04 0.443

Time * Paper Towel 0.16 0.10 – 0.22 <0.001 0.07 0.00 – 0.13 0.043

Time * 100% Silk (unwashed) -0.18 -0.24 – -0.11 <0.001 -0.45 -0.51 – -0.38 <0.001

Time * 100% Silk (washed) -0.04 -0.10 – 0.03 0.248 -0.26 -0.32 – -0.19 <0.001

Time * Polyester 0.02 -0.05 – 0.08 0.591 -0.06 -0.13 – 0.00 0.052

Marginal R2 / Conditional R2 0.463 / 0.932 0.619 / 0.938



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S3 Table. Percentage of 2 µL water droplets that penetrated single, double, or triple silk

layers compared with Fisher’s Exact test.

Thickness Penetrated Total Droplets Percent (%)

1-Layer 34 72 47.2a

2-Layer 2 72 2.78b

3-Layer 1 72 1.39b

Totals (n=6) are from both washed (n=3) and unwashed (n=3) silk materials where four 2 µL

droplets were applied per technical replicate. Identical letters indicate similarities between

Bonferroni-corrected pairwise comparisons.



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S1 Figure



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S2 Figure



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Silk fabric as a protective barrier for personal ... · 6/25/2020 · 65 commercially available, task-specific surgical mask (top) compared to a silk face covering 66 (bottom) made

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