Superhydrophilic Cotton Fabrics

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Asymmetric Superhydrophobic/Superhydrophilic Cotton Fabrics Designed by Spraying Polymer and Nanoparticles Kaichi Sasaki, Mizuki Tenjimbayashi, Kengo Manabe, and Seimei Shiratori ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b09782 • Publication Date (Web): 23 Nov 2015 Downloaded from http://pubs.acs.org on November 30, 2015

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ACS Applied Materials & Interfaces

Asymmetric

Superhydrophobic/Superhydrophilic

Cotton

Fabrics Designed by Spraying Polymer and Nanoparticles

Kaichi Sasaki, Mizuki Tenjimbayashi, Kengo Manabe, Seimei Shiratori*

Center for Material Design Science, School of Integrated Design Engineering, Graduate School of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522 Japan *[email protected]

KEYWORDS: superhydrophobicity, spray, Janus membrane, mechanical durability, cotton fabrics

ACS Paragon Plus Environment


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ABSTRACT Inspired by the special wettability of certain natural life forms, such as the high water repellency of lotus leaves, many researchers have attempted to impart superhydrophobic properties to fabrics in academic and industrial contexts. Recently, a new switching system of wettability has inspired a strong demand for advanced coatings, even though their fabrication remains complex and costly. Here, cotton fabrics with asymmetric wettability (one face with natural superhydrophilicity and one face with superhydrophobicity) were fabricated by one-step spraying of a mixture of biocompatible commercial materials, hydrophobic SiO2 nanoparticles and ethyl-alphacyanoacrylate superglue. Our approach involves controlling the permeation of the fabric coatings by changing the distance between the fabric and the sprayer, to make one side superhydrophobic and the other side naturally superhydrophilic. As a result, the superhydrophobic side, with its high mechanical durability, exhibited a water contact angle of 154° and sliding angle of 16°, which meets the requirement for self-cleaning ability of surfaces. The opposite side exhibited high water absorption ability owing to the natural superhydrophilic property of the fabric. In addition, the designed cotton fabrics had blood absorption and clotting abilities on the superhydrophilic side, while the superhydrophobic side prevented water and blood permeation without losing the natural breathability of the cotton. These functions may be useful in the design of multifunctional fabrics for medical applications.


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INTRODUCTION Synthetic materials inspired by the characteristic abilities of biological species in the natural world have attracted tremendous attention in the fields of materials science and engineering.1–3 In particular, mimicking the micro or nano architectures of biomaterial interfaces by controlling the surface morphology of materials has led biomimetic advances over the past two decades.4,5 Lotus leaves, sharkskin, moth-eyes, geckoes and nepenthes pitcher plants demonstrate useful properties such as self-cleaning,6–9 anti-fouling,10–12 antireflection,13,14 strong adhesion,15,16 and liquid repellency,17–20 respectively. Among these, there has been great discussion about selfcleaning inspired by lotus leaves both for pure scientific interest and commercial applications. Lotus leaves demonstrate a high water contact angle (CA) and low sliding angle (SA). Superhydrophobic surfaces that have a CA of greater than 150° can be achieved by combining proper surface roughness with micro or nano hierarchical structures and materials with low surface energy.21,22 Producing superhydrophobic fabrics by meeting these conditions has enabled the possibility of applying superhydrophobicity to the field of multifunctional textiles, such as selfcleaning materials,23,24 anti-fouling clothes,25–27 water collection in deserts,28 and oil/water separation.29–32 Here, we have focused on cotton fabrics as the coating substrate. Cotton, which consists of biodegradable cellulose fibers, is one of the most practical fabrics for clothes and textiles.33 Cotton fabrics are soft, breathable, and do not irritate human skin. Moreover, cotton has moisture and water absorbing abilities owing to its micro-scale porous structure, which makes it suitable as an absorbent material for various applications, such as surgical gauze, medical dressings, hemostatic covers, napkins, and hospital sheets.34,35 However, cotton fabrics can be easily stained or contaminated by complex liquids like blood, and suffer from bacterial growth because of



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cotton’s imbibition and natural superhydrophilic properties36. Such staining is not desirable for clothes and textiles used in clinics or hospitals. Therefore, cotton fabrics are expected to possess unique functions of superhydrophobicity. Despite considerable efforts to develop cotton fabrics with superhydrophobicity, only limited success has been achieved in terms of durability, eco-friendliness, flexibility, and breathability.37 The weak attachment between the low surface energy materials used and the cotton fibers has resulted in coatings that easily lose their superhydrophobic ability through mechanical abrasion such as friction from fingers and clothes.38–40 Lin et al.41 reported that a crosslinked elastomeric thin coating with a nano-micro composite structure and low free-energy surface endowed fabrics with highly durable superhydrophobicity. In the case of fabrics, however, such a surface treatment can easily fill the interspace between fibers with low surface energy nanomaterial, which should cause the loss of airflow and stretchability. To solve these challenges, Liu et al.42 applied the superhydrophobic coating on only a single face of the cotton fabrics to achieve fabrics with asymmetric wettability (one face with natural superhydrophilicity and the other with superhydrophobicity). The superhydrophilic side maintained its natural breathability and flexibility and the superhydrophobic side, which was derived from fluorine materials, repelled water and exhibited self-cleaning ability after the addition of mechanical durability via another step. Although many researchers have fabricated superhydrophobic surfaces using fluorinated hydrocarbons as the low surface energy material, fluorine components are non-biodegradable and expensive, and can easily react with other materials, resulting in environmental contamination that can impede nerve growth in children.43 Here, we report single-faced superhydrophobicity on cotton fabrics using biodegradable materials via a one-step spraying method. Ethyl-alpha-cyanoacrylate as adhesive resin and


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hydrophobized SiO2 nanoparticles were mixed to fabricate a superhydrophobic coating with high mechanical durability. These materials are fluorine free and biocompatible.44,45 By just spraying this mixture on a fabric, the coating would easily penetrate to the opposite side because of its natural superhydrophilicity. This would make both sides of the fabric superhydrophobic, causing it to lose its breathability and water absorbing properties. Therefore, the spray distance between the fibers and the sprayer and the amount of sprayed solution were optimized to produce singlefaced superhydrophobicity with breathability and flexibility. Furthermore, the modified cotton fabrics maintained the blood clotting and absorption abilities arising from their natural superhydrophilicity, while the sprayed side prevented water and blood from permeating to the other side of the fabric. This study presents new insights into the development of functional textiles with asymmetric wettability that can absorb and repel water and blood in medical applications.

EXPERIMENTAL SECTION

Materials. Cotton fabric (M-3II, Asahi Kasei Fiber CO., Ltd., Osaka, Japan) with an average fiber diameter of approximately 12 μm was used as the substrate. Ethyl-alpha-cyanoacrylate and hydrophobic SiO2 nanoparticles (RX200, modified with hexamethyldisilazane, average primary particle diameter of 12 nm, Aerosil, Evonik Industries, Germany) were used to impart single-faced superhydrophobicity to the cotton fabrics. These materials were selected to achieve fluorine-free fabrication. Pig blood (with 0.3 wt% citric acid, Tokyo Shibaura Zouki CO., Tokyo, Japan) was



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used to evaluate the blood repellency, blood absorption ability, and blood clotting index of the fabric samples.

Cotton fabric with single-faced superhydrophobicity produced by spray coating. The cotton fabric were cleaned in ethanol solution before use. 2.0 g of SiO2 nanoparticles was added to 46.0 g of acetone and stirred for 10 min, after which 2.0 g of ethyl-alpha-cyanoacrylate was added and the mixture was stirred for a further 180 min. Acetone was used to dissolve the ethyl-alphacyanoacrylate and disperse the SiO2 nanoparticles. Ethyl-alpha-cyanoacrylate was used to bond the SiO2 nanoparticles with the cotton fabric and enhance the mechanical durability of the coating. After stirring for 180 min, the mixture was sprayed onto the cotton fabric pieces using a spray gun (nozzle diameter: 0.6 mm, XP7; Airtex CO., Ltd., Tokyo, Japan), and dried at room temperature. The spraying pressure of the spray gun was 0.3 MPa; the amount of solution sprayed was 0.3 mL/cm2; and the “spraying distance” between the spray gun and cotton fabric was varied from 10 cm to 60 cm. Double-sided superhydrophobic cotton fabrics were produced to compare their water and blood absorption abilities and vapor transmission rate with those of the single-faced superhydrophobic cotton fabrics. These samples were prepared by dipping the cotton fabric into the coating mixture and leaving it to dry at room temperature.

Characterization. Field emission scanning electron microscopy (FE-SEM; JSM-7600F, JEOL Ltd, Akishima, Japan) was carried out at an accelerating voltage of 5 kV to characterize the surface morphologies of the fabrics. A commercial contact angle system (FACE; Kyowa Interface Science CO.,Ltd., Niiza, Japan) was used to measure the CAs and SAs of the fabrics at room temperature. Distilled water (10 μL, with a surface tension of γlv = 72.8 mN/m) was used as the probe liquid.


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The CAs and SAs reported in this study are the averages of measurements obtained at five different points on the surface of each sample, with each static contact angle measurement performed using water droplets of 10 μL in volume. A high speed camera (HAS-D3, Ditect, Tokyo, Japan) was used to monitor the behavior of the sprayed mixture deposited on the fabric surface. The chemical composition of the surfaces was measured by X-ray photoelectron spectroscopy (XPS, JPS9010TR; JEOL Ltd, Akishima, Japan). The mechanical durability of the surfaces was determined using an abrasion device (Tribogear Type 18 L; Shinto Scientific CO., Ltd., Tokyo, Japan). Sandpaper (#10000; Sankyo Rikagaku Co., Ltd., Saitama, Japan) was used as the abrasive material, and the abrasion pressure was set at 5 kPa or 49 kPa. The water and blood absorption abilities of untreated cotton fabric, single-faced superhydrophobic cotton fabric produced using a spray distance of 30 cm, and double-sided superhydrophobic cotton fabrics were measured as follows (Figure S1a). First, all samples were immersed in distilled water or pig blood for 10 min. The samples were then hung to dry at room temperature for 10 min to remove residual liquid adhered to the surface. The masses of the samples before (w0) and after the immersion (wa) were measured, and the water or blood absorption ability (w%) of the sample was calculated according to the following equation. 𝑤% =

𝑤𝑎 −𝑤0 𝑤0

× 100%

(1)

The vapor transmission rate of the untreated cotton fabrics, single-faced superhydrophobic cotton fabrics produced using a spray distance of 30 cm, and double-sided superhydrophobic cotton fabrics was measured using the following method (Figure S1b). A glass container filled with distilled water was covered with the cotton fabric sample, and then the water was boiled for 6 h by heating at 200 °C. The mass of water in the container before (V0) and after heating (Va) was



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measured and the Vapor transmission rate (V%) was calculated according to the following equation. 𝑉% =

𝑉𝑎 −𝑉0 𝑉0

× 100%

(2)

Additionally, pig blood (100 µL/cm2) was dropped onto the untreated, single-faced superhydrophobic fabric produced using a spray distance of 30 cm, and double-sided superhydrophobic cotton fabrics and dried at 20 °C. The blood was dropped onto the superhydrophilic side of the sprayed fabric in consideration of the intended practical application. After this treatment, the vapor transmission rate of the samples was calculated using the same method as that described above to investigate the breathability of the fabrics after blood clotting. Blood clotting studies were carried out based on the previous literature (Figure S1c).46 The untreated cotton fabric, single-faced superhydrophobic fabric produced using a spray distance of 30 cm, and double-sided superhydrophobic cotton fabrics were cut into samples measuring 1 cm2 and placed in a 100 mL bottle. The samples were first incubated at 37 °C for 10 min and then 100 μL pig blood was slowly dropped onto each sample. The blood was dropped onto the superhydrophilic side of the sprayed fabric in consideration of the intended practical application. The bottles containing the samples were then further incubated at 37 °C for 15 min, after which 50 mL of distilled water was added without disturbing the clotting blood. Subsequently, 10 mL of solution was taken from each bottle and centrifuged at 1000 rpm for 1 min. The supernatant was collected and incubated at 37 °C for 1 h. After these steps, the optical density of the supernatant was measured at 540 nm using an ultraviolet-visible (UV-vis) spectrophotometer (UV mini-1240; Shimadzu, Kyoto, Japan). The absorbance of 100 μL pig blood in 50 mL distilled water at 540 nm was measured immediately after addition as a reference; and the blood clotting index (BCI) was


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calculated using the following equation. A higher BCI value indicates a lower amount of blood clotting. 𝐵𝐶𝐼 =

𝐴𝑏𝑠𝑜𝑟𝑏𝑎𝑛𝑐𝑒 𝑜𝑓 𝑏𝑙𝑜𝑜𝑑 𝑤ℎ𝑖𝑐ℎ ℎ𝑎𝑑 𝑏𝑒𝑒𝑛 𝑖𝑛 𝑐𝑜𝑛𝑡𝑎𝑐𝑡 𝑤𝑖𝑡ℎ 𝑠𝑎𝑚𝑝𝑙𝑒𝑠 𝑎𝑡 540 𝑛𝑚 𝐴𝑏𝑠𝑜𝑟𝑏𝑎𝑛𝑐𝑒 𝑜𝑓 100 𝜇𝐿 𝑝𝑖𝑔 𝑏𝑙𝑜𝑜𝑑 𝑖𝑛 50 𝑚𝐿 𝑤𝑎𝑡𝑒𝑟 𝑎𝑡 540 𝑛𝑚

(3)

RESULTS AND DISCUSSION Influence

of

spray

distance

on

surface

wettability

and

single-faced

superhydrophobicity. To achieve cotton fabrics with asymmetric wettability, the hydrophobic SiO2 nanoparticles must fully cover the top layer of cellulose fibers in the cotton fabric while the bottom layers keep their natural shape. We attempted to meet this requirement by optimizing the spray method used to apply the coating. Spraying is a widely used fabrication method because it can be used to coat large areas easily under a wet process.47 Exploiting these advantages, our group has previously developed spray methods for the design of many different functional surfaces, such as an automatic layer-by-layer spray method for antireflection films,48 and a movable spray method to form gradient structures in a one-step process.45 In this report, we attempted to simultaneously control both the permeation of the sprayed mixture through the fabric and the surface coverage of the fabric by only changing the spray distance and fixing the other conditions, to meet the requirements for single-faced super-hydrophobicity (Figure 1). Certainly, changing parameters such as the spraying pressure, amount of mixture sprayed, or polymer content of the mixture could also control both the permeation and coverage of the mixture. However, if the spraying pressure is too high, the sprayed droplets will be small and may increase the soaking depth of the mixture into the fabric. According to the Navier–Stokes equation, there is a correlation between the viscosity of a sprayed mixture and the spray distance. Thus, changing the concentration of ethyl-



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alpha-cyanoacrylate would change the velocity and behavior of the mixture. Therefore, we chose spray distance as the only variable parameter and fixed the other conditions. To optimize the spray distance for single-faced superhydrophobicity, we sprayed the mixture on glass substrates and measured the film thickness and the amount per unit area of sprayed mixture that reached the surface of the glass (Figure S2). From these results, we confirmed that both the amount and thickness of the mixture deposited on the substrate increased as the spray distance was shortened. This finding supported our attempt to impart single-faced superhydrophobicity to cotton fabrics. Next, we sprayed the mixture on fabric and determined the surface wettability and morphology of the modified fabric samples. The surface wettability was evaluated by checking the behavior of water dropped on the surface. A photograph of cotton fabric samples sprayed from 30 cm (Figure 2a) demonstrates the behavior observed when a water droplet (stained blue in Figure 2a) was placed on each side of the sprayed fabric. On the sprayed side of the fabric, the water curled into a droplet and permeation of the fabric was not observed. The contact angle was 154°, indicating superhydrophobicity (Figure 2b); and the sliding angle was 16°, indicating a high water repellence. In contrast, water was immediately absorbed by the fabric when it was placed on the untreated side (Figure 2b). These results show that the cotton fabric sprayed at 30 cm exhibited asymmetric wettability of its surfaces, achieving single-faced superhydrophobicity (supporting information, Movie S1.). Furthermore, all the fabrics sprayed at distances of 10 cm to 60 cm showed this characteristic of asymmetric wettability: one face repelled water and the other side immediately absorbed water. To confirm the presence of an interface between the superhydrophobic side and superhydrophilic side, cross-sectional FE-SEM images of the cotton fabric sprayed at a distance of 30 cm were observed (Figure 3a). A hierarchical structure formed by the SiO2 nanoparticles


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was observed on the sprayed side (Figure 3b), while pristine cellulose fibers remained on the untreated side (Figure 3c). This result further confirms the single-faced hydrophobization of this cotton fabric sample, and supports the observed behavior of water droplets (10 μL) applied to each surface.

Figure 1. Illustration of sprayed cotton fabrics. By varying spray distance, the coverage and permeability of the mixture can be controlled simultaneously. When the distance is too short for the acetone to evaporate, too much of the coating mixture reaches the substrate and it fills the top layer and penetrates to the opposite side of the fabric. This results in superhydrophobicity on both sides. When the distance is too far, little of the mixture reaches the fabric, and it hardly covers the top layer or penetrates the fabrics, resulting in both sides remaining superhydrophilic. Therefore, controlling the spray distance ensures that only the top of the fabric is fully covered with hydrophobic SiO2 nanoparticles and constricts its permeation to the opposite side, allowing singlefaced coating to be achieved.



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Figure 2. Photographs of water droplets on cotton fabric samples with single-faced superhydrophobicity (sprayed from 30 cm). (a) Blue-colored water droplets placed on the sprayed side are spherical and easily roll off, while those placed on the untreated side absorb into the fabric. (b) Water droplet placed on the sprayed side exhibiting CA greater than 150°. (c) Water droplet placed on the back side.

Figure 3. Cross-sectional FE-SEM images of (a) cotton fabric with single-faced superhydrophobicity (sprayed from 30 cm), (b) top view of sprayed side in (a), and (c) top view of untreated side in (a).


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Chemical composition and surface structure. XPS measurements were carried out to determine the change in the composition of the material coated on the fabric with variations in the spray distance. All the survey spectra (Figure 4a) of the sprayed side of the cotton fabric samples clearly revealed the presence of Si, C, and N elements. The Si 2p and Si 2s peaks from the hydrophobic SiO2 nanoparticles were observed at a binding energy of 102 eV (theoretical value 99 eV) and 157 eV (theoretical value 151 eV), respectively, while the N-1s peak from the cyano-group of ethylalpha-cyanoacrylate was observed at 399 eV (theoretical value 398 eV). These results confirmed the existence of both the binding material and hydrophobic nanoparticles intended to enhance the water repellency of the cotton fabric on the surface of all the sprayed fabric samples. The narrow scan around the C 1s peak (Figure 4b, 4c, and 4d) was resolved into four components: C-C (284.7 eV), C-O (286.5 eV), O-C-O (287.9 eV), and COO (288.8 eV). The pristine cotton fabric and fabric samples sprayed from 40, 50, and 60 cm showed the presence of COO, which is derived from cellulose. Therefore, uncoated fibers remained on the surface when the spray distance was set to 40, 50, or 60 cm. FE-SEM images of the sprayed side of the cotton fabric samples (Figure 5) revealed that the surface morphology changed with the spray distance. The fibers of the samples sprayed at distances of 10 cm and 20 cm could not be seen, and the interfiber spaces were fully filled with the ethyl-alpha-cyanoacrylate and SiO2 nanoparticle composite coating. The composite coating only covered the top surface and fibers when the spray distance was 30 cm. In contrast, the fabric sprayed from 40 cm, 50 cm, and 60 cm were not well covered with the composite coating and pristine fibers remained on the top surface. Furthermore, high magnification FE-SEM images indicated that the surface roughness increased with the distance between the sprayer and fabric.



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The aggregation mechanism of the sprayed mixture after leaving the sprayer, taken from our previous reports45,49, is shown in Figure 6. Note that the acetone immediately evaporated at room temperature, the ratio of acetone in the sprayed mixture reaching the fabric decreased with longer spray distance. It is assumed that the droplets of sprayed mixture became widely dispersed in the air but maintained their spherical form owing to their surface tension, and hydrophobized nanoparticles covered the spheres because of their low affinity to the acetone (Figure 6a). Conversely, the ethyl-alpha-cyanoacrylate moved to the inside of the acetone spheres because of its high affinity for acetone. The ethyl-alpha-cyanoacrylate monomers in contact with water in the air polymerized instantly, while those in the sprayed mixture started to polymerize when the acetone evaporated and exposed them to the air. Observation using a high speed camera revealed the behavior of the sprayed mixture after reaching the surface (Figure 6). When the spraying distance was short (10 cm and 20 cm), the mixture reached the fibers with abundant acetone, consistently accumulated before the evaporation of the solvent, and both covered the fibers and filled the interfiber spaces. After the acetone evaporated, the ethyl-alpha-cyanoacrylate polymerized and filled the interfiber spaces, which led to a comparatively flat surface. In contrast, when the spraying distance was long (30 cm, 40 cm, 50 cm, and 60 cm), the acetone in the mixture evaporated and the hydrophobized nanoparticles covered the mixture while it was still in the air. After the mixture reached the fabrics, the acetone evaporated and the hierarchical structures were formed. The relative amount of SiO2 nanoparticles on the surface was high and the surface was rough. However, the coverage of the fibers by mixture gradually decreased with increasing spraying distance because less mixture reached the surface.


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Figure 4. XPS survey spectra of the cotton surfaces: (a) Survey spectra of the sprayed samples, (b) C 1s peak core level spectra for untreated surface, (c) C 1s peak core level spectra for surface sprayed at a distance of 30 cm, and (d) C 1s peak core level spectra for surface sprayed at a distance of 40 cm.



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Figure 5. FE-SEM images of sprayed side of cotton fabrics sprayed at different distances. Scale bars are 20 μm (×1000) and 500 nm (×35000).


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Figure 6. Illustration of the behavior of the sprayed mixture with increasing spray distance. (a) The sprayed mixture in flight. (b) Illustration and images of the mixture after reaching surfaces at different distances. Scale bars are 1 mm.



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Mechanical durability of coated cotton fabric. Because mechanical stability is crucial for the practical use of superhydrophobic coatings, the abrasion durability of the spray coated cotton fabrics was evaluated. The changes in the water CAs and SAs observed after an increasing number of abrasions is shown in Figure 7, which shows that the durability of the superhydrophobic coating was improved by adding ethyl-alpha-cyanoacrylate to the spray mixture. Ethyl-alphacyanoacrylate worked as binder and strongly connected the nanoparticles and cotton fibers through polymer hydrogen bonding, van der Waals’ forces, and the anchor effect.44 After 40 cycles of abrasion with sandpaper at 5 kPa, superhydrophobic properties were still observed and water contact angles remained over 150° for fabrics sprayed from 10 cm, 20 cm, and 30 cm, but were lost when the spraying distance was 40 cm, 50 cm, or 60 cm. These results imply that the durability of the coatings decreased with increasing spraying distance. This is because the amount of ethyl-alpha-cyanoacrylate polymer, which worked as a binder between the cotton fibers and nanoparticles, on the fabric surface decreased with increasing spraying distance. XPS measurements of the sprayed fabrics after abrasion revealed changes in the chemical components of the surfaces before and after abrasion. Narrow scans around the C 1s peak revealed no chemical shift even after 40 cycles of abrasion at 5 kPa (Figure S3). The spectrum of the fabric sprayed from 40 cm, which lost its superhydrophobic properties owing to the abrasion, exhibited a stronger COO peak, which is derived from cellulose. Therefore, the superhydrophobic properties of this sample were lost after abrasion because the hydrophobized SiO2 nanoparticles had been peeled off and the hydrophilic cellulose fibers were partially exposed. Next, the mechanical stability of the sprayed coating was tested at a strong abrasion pressure of 49 kPa (Figure S4). Even after 40 cycles of abrasion, the water repellency of the coating was maintained and prevented the water from permeating through the fabric when the spray distance was 10 cm, 20 cm and 30 cm. FE-SEM


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observations of the fabric sprayed from a distance of 30 cm revealed that this was mainly because the majority of the SiO2 nanoparticles were protected by the three-dimensional structure of the fibers (Figure S5).

Figure 7. Contact angles and sliding angles with increasing number of abrasion cycles at 5 kPa.

Vapor transmissibility and water absorption ability. The superhydrophobic side of the coated cotton fabrics prevented water droplets from permeating from the sprayed side to opposite surface, and still allowed water vapor to pass through. Based on the results of the FE-SEM (Figure 5a) and mechanical durability (Figure 6) measurements, we chose the fabric sprayed from a distance of 30



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cm as the single-faced superhydrophobic fabric sample for the following section because it exhibited both high mechanical durability and unblocked interfiber spaces. The vapor transmissibility of the different fabrics was evaluated as described in the experimental section. The results are shown in Figure 8. Column (iv) shows the vapor transmission rate of the untreated cotton fabric, 4.60 × 105 g/m2/day. This high transmission was derived from the total amount of water that permeated through the membrane, was absorbed by the fabric, and then evaporated. After single-faced hydrophobization, columns (ii) and (iii), the vapor transmission rate of the fabric was 3.86 × 105 and 4.25 × 105 g/m2/day, which was 86% and 92% that of the untreated fabric, respectively. The vapor transmission rate was 6% higher when superhydrophobic side was facing the water. It is assumed that this difference was derived from a change in the capillary force. The double-sided superhydrophobic fabric exhibited a vapor transmission rate of 1.71 × 105 g/m2/day, which was 37% that of the untreated fabric. The singlefaced superhydrophobic fabric had a larger surface fiber diameter owing to the composite coating and water vapor was obstructed from passing through the sprayed side. However, water could be absorbed by the untreated side. Therefore, the single-faced superhydrophobic fabric showed a higher vapor transmissibility than the double-sided fabric. The high breathability of this sample represents a first possible step toward the practical use of superhydrophobic fabrics. The water absorption ability of untreated, single-faced superhydrophobic and double-sided superhydrophobic fabrics is shown in Figure 9. Without any treatment, the cotton fabric showed a high water absorption of 1407% after immersion in water, attributed to the natural superhydrophilicity of cellulose fibers. The single-faced superhydrophobic cotton fabric exhibited a water absorption of 797%, which was 56.6% that of the untreated fabric, and showed a moderate


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absorbance, between that of the superhydrophobic and untreated fabrics. This result reveals that water was absorbed only by the untreated region of the fabric and not the superhydrophobic side.

Figure 8. Vapor transmission rate of (i) double-sided superhydrophobic cotton fabric, (ii) singlefaced superhydrophobic cotton fabric with superhydrophilic side facing the liquid water, (iii) single-faced superhydrophobic cotton fabric with superhydrophobic side facing the liquid water, and (iv) untreated cotton fabrics.

Figure 9. Water and blood absorbance and BCI of (i) double-sided superhydrophobic cotton fabric, (ii) single-faced superhydrophobic cotton fabric, and (iii) untreated cotton fabrics.



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Blood repellency, absorption, and clotting ability. To test practical use in medical applications, we determined the blood repellency of the treated fabrics using pig blood. The photograph shown in Figure 10a demonstrates the behavior observed when the pig blood was placed on each side of the single-faced superhydrophobic cotton fabric sprayed from 30 cm. As observed for water droplets placed on this fabric, the sprayed side of the cotton fabric showed high blood repellency and the blood droplets became spherical, while the untreated side absorbed the blood (Figure 10c). The sprayed side exhibited a blood CA of 151.4° (Figure 10b) and a blood SA of 12.4°, indicating a blood-repelling self-cleaning ability arising from the fabric’s superhydrophobic properties. These results show that the cotton fabric sprayed from 30 cm had asymmetric wettability even for blood. Moreover, we were able to achieve single-faced superhydrophobicity without using any fluorine-containing components (supporting information, Movie S1). The asymmetric wettability of the fabric against blood influenced its blood absorption and blood clotting properties. Figure 9 shows the blood absorption ability of the three different fabrics (untreated, single-faced superhydrophobic, and double-sided superhydrophobic). The blood absorbance of the fabrics showed the same trend as that observed for water absorbance. The singlefaced superhydrophobic cotton fabric showed a blood absorption of 779%, which was derived from the absorption of blood by the superhydrophilic side and the prevention of the blood from permeating the fabric by the superhydrophobic side. Additionally, the clotting of the blood on the single-faced superhydrophobic cotton fabric was determined from the obtained BCI value (Figure 9). The single-faced superhydrophobic cotton fabrics exhibited a BCI of 74.1%, and not all of the blood placed on the fabric could be removed by rinsing. This is mainly attributed to the natural superhydrophilic properties of the untreated side of the cotton fabric. Moreover, even after blood clotting, the water vapor transmission rate of this sample was 3.56 × 105 g/m2/day when the


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superhydrophilic side was placed face-down in the liquid water (Figure 8), which was 92% that of the untreated fabric before blood clotting. Thus, the treated cotton fabric showed high blood absorption and high breathability even after blood had clotted on the superhydrophilic side, while simultaneously preventing blood permeation and exhibiting a self-cleaning ability on the sprayed side. Such highly breathable fabrics that prevent water and blood penetrating to their opposite side are expected to have a wide range of applications in various medical fields

Figure 10. Photographs of blood droplets placed on cotton fabrics with single-faced superhydrophobicity sprayed from 30 cm. (a) Blood droplets placed on the sprayed side are spherical and easily roll off the surface, but are absorbed by the untreated back side. (b) Blood droplet placed on the sprayed side exhibiting CA greater than 150°. (c) Blood droplet placed on the back side.



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CONCLUSIONS Single-faced superhydrophobic cotton fabric (one face with natural superhydrophilicity and the other with superhydrophobicity) was fabricated by one-step spraying of a mixture of biocompatible materials: ethyl-alpha-cyanoacrylate and hydrophobized SiO2 nanoparticles. Varying the spray distance allowed us to control the permeation and the coverage of the sprayed mixture on the surface of the fabric simultaneously. The sprayed face of the cotton fabric showed a high water repellency with a water CA greater than 150° and a water SA lower than 20°, which enabled the self-cleaning ability of the surface. In contrast, the opposite side of the fabric retained the natural superhydrophilicity of the cellulose fibers, which led to a higher water absorbing ability for the single-faced fabric than double-sided superhydrophobic fabric. A high mechanical durability after 40 cycles of abrasion at 5 kPa and high breathability were also achieved at the optimized distance. The obtained single-faced superhydrophobic cotton fabric exhibited a blood absorption of 779% and a blood clotting ability on the untreated surface, while the sprayed side prevented the permeation of water and blood to the superhydrophilic side. The present fabrics represent an elegant solution for medical applications that require the prevention of water containing contamination or blood permeating to the hydrophilic side of fabrics.


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ASSOCIATED CONTENT Supporting Information Video of water and blood droplets placed on cotton fabrics with single-faced superhydrophobicity; Deposition process of the sprayed mixture sprayed from 10 cm, 30 cm and 50 cm, illustration of the experimental procedure; film thickness and the amount per unit area of sprayed mixture that reached the surface of the glass, XPS spectra of cotton fabrics subjected to 40 cycles of abrasion at 5 kPa; variation in contact and sliding angles with increasing abrasion cycle number under 49 kPa; FE-SEM images of cotton fabrics after 40 cycles of abrasion and blood absorbance. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author E-mail: [email protected]

Author Contributions K.S. conceived, designed, and carried out the experiments and analyzed the data. K.S. and K.M. wrote the paper. M.T. and K.M. provided experimental support, support in data analysis and gave scientific advice. S.S. supervised the project and commented on the manuscript.



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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by JSPS KAKENHI (grant number 26420710). We are deeply grateful to Dr. Yoshio Hotta, whose comments were valuable to our study. We appreciate very much the support from Dr. Kouji Fujimoto and Dr. Kyu-Hong Kyung, whose meticulous comments were an enormous help.


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Table of Contents only


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Figure 1. Illustration of sprayed cotton fabrics. By varying spray distance, the coverage and permeability of the mixture can be controlled simultaneously. When the distance is too short for the acetone to evaporate, too much of the coating mixture reaches the substrate and it fills the top layer and penetrates to the opposite side of the fabric. This results in superhydrophobicity on both sides. When the distance is too far, little of the mixture reaches the fabric, and it hardly covers the top layer or penetrates the fabrics, resulting in both sides remaining superhydrophilic. Therefore, controlling the spray distance ensures that only the top of the fabric is fully covered with hydrophobic SiO2 nanoparticles and constricts its permeation to the opposite side, allowing single-faced coating to be achieved. 692x379mm (150 x 150 DPI)



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Figure 2. Photographs of water droplets on cotton fabric samples with single-faced superhydrophobicity (sprayed from 30 cm). (a) Blue-colored water droplets placed on the sprayed side are spherical and easily roll off, while those placed on the untreated side absorb into the fabric. (b) Water droplet placed on the sprayed side exhibiting CA greater than 150°. (c) Water droplet placed on the back side. 220x116mm (150 x 150 DPI)


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Figure 3. Cross-sectional FE-SEM images of (a) cotton fabric with single-faced super-hydrophobicity (sprayed from 30 cm), (b) top view of sprayed side in (a), and (c) top view of untreated side in (a). 236x138mm (150 x 150 DPI)



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Figure 4. XPS survey spectra of the cotton surfaces: (a) Survey spectra of the sprayed samples, (b) C 1s peak core level spectra for untreated surface, (c) C 1s peak core level spectra for surface sprayed at a distance of 30 cm, and (d) C 1s peak core level spectra for surface sprayed at a distance of 40 cm. 408x252mm (150 x 150 DPI)


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Figure 5. FE-SEM images of sprayed side of cotton fabrics sprayed at different distances. Scale bars are 20 µm (×1000) and 500 nm (×35000). 146x262mm (150 x 150 DPI)



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Figure 6. Illustration of the behavior of the sprayed mixture with increasing spray distance. (a) The sprayed mixture in flight. (b) Illustration and images of the mixture after reaching surfaces at different distances. Scale bars are 1 mm. 586x575mm (150 x 150 DPI)


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Figure 7. Contact angles and sliding angles with increasing number of abrasion cycles at 5 kPa. 303x313mm (150 x 150 DPI)



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Figure 8. Vapor transmission rate of (i) double-sided superhydrophobic cotton fabric, (ii) single-faced superhydrophobic cotton fabric with superhydrophilic side facing the liquid water, (iii) single-faced superhydrophobic cotton fabric with superhydrophobic side facing the liquid water, and (iv) untreated cotton fabrics. 184x109mm (150 x 150 DPI)


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Figure 9. Water and blood absorbance and BCI of (i) double-sided superhydrophobic cotton fabric, (ii) single-faced superhydrophobic cotton fabric, and (iii) untreated cotton fabrics. 229x144mm (150 x 150 DPI)



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Figure 10. Photographs of blood droplets placed on cotton fabrics with single-faced superhydrophobicity sprayed from 30 cm. (a) Blood droplets placed on the sprayed side are spherical and easily roll off the surface, but are absorbed by the untreated back side. (b) Blood droplet placed on the sprayed side exhibiting CA greater than 150°. (c) Blood droplet placed on the back side. 209x102mm (150 x 150 DPI)


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355x194mm (150 x 150 DPI)


Superhydrophilic Cotton Fabrics

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