In situ Formation of Slippery-Liquid-Infused ... - ACS Publications

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Interfaces, Optics, and Electronics

In situ Formation of Slippery-Liquid-Infused Nanofibrous Surface for a Transparent Antifouling Endoscope Lens Mizuki Tenjimbayashi, Jun-Yong Park, Jun Muto, Yuta Kobayashi, Ryohei Yoshikawa, Yasuaki Monnai, and Seimei Shiratori ACS Biomater. Sci. Eng., Just Accepted Manuscript • DOI: 10.1021/acsbiomaterials.8b00134 • Publication Date (Web): 16 Apr 2018 Downloaded from http://pubs.acs.org on April 16, 2018

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ACS Biomaterials Science & Engineering

In situ Formation of Slippery-Liquid-Infused Nanofibrous Surface for a Transparent Antifouling Endoscope Lens Mizuki Tenjimbayashi,1† Jun-Yong Park,1‡ Jun Muto,2‡ Yuta Kobayashi,1 Ryohei Yoshikawa,1 Yasuaki Monnai,3 and Seimei Shiratori13* 1

Center for Material Design Science, School of Integrated Design Engineering, Keio University,

3-14-1 Hiyoshi, Yokohama, 223-8522, Japan. 2

Department of Neurosurgical Surgery, School of Medicine, Keio University, 35 Shinano-

machi, Shinjuku-ku, Tokyo, 160-8582, Japan. 3

Center for Applied Physics and Physico-Informatics, School of Fundamental Science and

Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan. Email: [email protected] KEYWORDS. Slippery surface; nanofiber; electrospinning; medical application; hydrophobicity

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ABSTRACT

Slippery-liquid-infused porous surfaces (SLIPS) are state-of-the-art materials owing to their excellent properties derived from their fluidity (e.g. dynamic omniphobicity, and self-healing function). Although SLIPS have been multifunctionalized and developed for various applications, the fabrication process is not well advanced because it is time consuming and requires multiple steps. Here, a versatile method is reported for the instant formation of slippery surfaces in situ. A lubricated fiber-filled porous sheet was designed, and a coating was formed simply by sticking a surface to the sheet. This sheet can be used as a “disposable instant coating kit” and be made available for instant and repeated coating of SLIPS. The technique is applied to a transparent antifouling endoscope lens as a proof-of-concept. This work improves the fabrication process of SLIPS and contributes to the practical use of SLIPS.


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INTRODUCTION Biomimetic materials provide enormous benefits and improve our daily life and have various applications in industry and medicine.1,2 For many years, researchers have tried to find new natural functional materials, design bioinspired functional materials, improve their properties, broaden their application, simplify the process, and decreasing their cost.3-6 Recently, nature has inspired us to develop liquid/air/solid interfacial systems.7 For example, lotus, rose, and rice leaves inspired us to design a superhydrophobic/superaerophilic surface by introducing a hydrocarbon-modified hierarchical structured surface.8-10 The surfaces of a fish, snail scales, and seaweed are superhydrophilic and superoleophobic underwater.11-13 Cactus spines, morpho wings, spider webs, and desert beetles have taught us an efficient way to collect water by their anisotropic wettability properties.14-18 Such special wettability contributed to the development of functional materials in areas ranging from industry, agriculture systems to biomedical devices.19 In biosystems inspired materials,20,21 the pitcher-plant analogues: it makes insects slide down their leaves into their digestive juices by lubricant offered us an idea of slippery liquid infused porous surfaces (SLIPS or slippery surfaces); which are a promising approach for the design of transparent anti-fouling coating.22 SLIPS are prepared by introducing a lubricating liquid layer in a nanometer- to micrometer-sized porous structured solid. Because of the existence of a lubricating liquid layer, SLIPS can repel a liquid that is immiscible with the lubricating liquid (e.g. water vs. oil). Furthermore, the fluidic property of SLIPS has a potential to add special functional features such as self-healing properties and pressure stability, in contrast to conventional solid materials with special wettability. The SLIPS technology has been used to achieved extremely weak adhesion of water (contact angle (CA) hysteresis fiber diameter) as shown Figure 2F. These analyses indicated that SiO2 NPs were not present on the fiber surface because the Si2p peak was not observed in any of the fibers composed of PVDF-HFP and SiO2 NPs. Instead, siloxane bonding was observed by FTIR analysis, which indicated that the SiO2 NPs were embedded inside the fiber and influenced the fiber nanostructure. That is because the existence of PVDF-HFP with a lower surface energy on the surface rather than SiO2 NPs is more thermodynamically stable. The resultant fiber structures are illustrated in Figure 2D. Any interactions between PVDF-HFP and SiO2 NPs were not present from FT-IR analysis in Figure 2F. Wettability analysis and stability analysis of the lubricant layer on fiber sheets The surface wettabilities on three types of fiber sheets were investigated, as shown in Figure 3. In the static wettability analysis (Figure 3A and 3B), all the fiber sheets displayed superoleophilicity (static contact angle: CA ~0˚) against silicone oil. This indicated that all the fiber sheets could infuse the silicon oil slippery liquid. The water CA increased with the increase of the SiO2 NPs content owing to the morphological change of the fiber surface. However, after lubrication, the water CA was almost constant regardless of the SiO2 NPs ratio. That was because after lubrication the surface structure and chemistry of the three types of fibers become almost the same. The CAs depend on two factors, that is, the surface structure and surface


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chemistry.51 After lubrication, the difference in the microscopic surface of the fibers was lost because the surfaces were covered by the lubricant (see Figure 4D). In terms of surface chemistry, their surfaces must be composed of PVDF-HFP and silicone oil, not SiO2 NPs. Thus, their water CA after lubrication becomes constant. Thus, the dynamic wettability of the siliconeoil-infused fiber sheet (Figure 3C) was also the same between the three fiber sheets, as well as the water adhesion force () within a range of 40-50 µN, calculated according to the following equation: = cos − cos

(1)

in which and are the width of the drop and the liquid surface tension, respectively, and and are the advancing and receding CA, respectively.34,43 We confirmed the thermodynamic stability of the silicone-oil-infused fiber sheets. We compared the stability of the three configurations: roughened solids (fiber sheets) that were completely wetted by the targeting liquid (water) (configuration A), or lubricating liquid (silicone oil) with (configuration 1) or without (configuration 2) a fully wetted targeting liquid floating on top of it, as shown in Figure 4A.20 To form a stable lubricating layer of silicone oil in contact with air and water, the total interfacial energies of configuration 1 and 2 (E1 and E2) must be smaller than that of configuration A (EA): ∆E = E − E = R cos − cos − > 0

(2)

and ∆E = E − E = R cos − cos + − > 0

(3)


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in which , , and are the interfacial energy between oil-air, water-air, and oil-water, respectively; and are the contact angle of oil and water, respectively, on a smooth solid surface. R is the roughness factor of the under layer, which is defined as the ratio of the solidliquid contact area to its projection on a flat area. In the Wenzel state, this value is almost equal to the ratio of the contact angle on a textured surface to that on a smooth surface. is calculated using the Föwkes equation:21,27 = + − 2 ! ! ".$

(4)

in which ! and ! are the dispersion components of the oil-air and water-air interfacial tensions, respectively. For nonpolar materials, ! ~ and ! is 21.8 mN/m. The calculated values are shown in Figure 4B and indicate that all the designed materials fulfill the criteria for stable lubrication. The direct observation of a lubricated fiber sheet by low-vacuum SEM is shown in Figure 4C-E. The formation of a lubricant on the fiber sheets was owing to the capillary force as well as the hydrophobic interactions of the lubricant trapped between several fibers, as shown in Figure 4E. Influence of lubrication on fiber tensile strength For the sticking process, the elasticity of the fiber is very important. Because the fiber tensile strength should change through lubrication, we analyzed the mechanism. A summary of the tensile strength and elastic properties of the fiber sheets with/without a lubricant is shown in Figure 5A. By adding SiO2 nanoparticles to PVDF-HFP, the tensile strength and elastic properties drastically increase because the nanoparticles interact with the polymers and stabilize the sheet.52 Pristine PVDF-HFP fibers were flexible but easily fractured with small extension. Interestingly, as shown in Figure 5A and 5B, the tensile strength of the fiber sheets decreased


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after lubrication, even though the extension rate was not very different. That indicated that the capillary force driven between the lubricating liquid and the fiber was stronger than the liquid cohesion forces, as shown in Figure 5C, in which the measured tensile strength (black arrow in Figure 5C) is equal to the sum of the fiber elasticity, liquid cohesion force, and negative capillary force.53 Optical properties analysis of (lubricated) fiber sheet Similarly, the optical properties of the fiber before and after lubrication were investigated, as shown in Figure 6. A summary of the optical values of the fiber sheets with/without a lubricant is shown in Figure 6A. Without lubricant, all of the fiber sheets were opaque owing to the surface nanometer to micrometer-scale roughness causing Mie scattering;28 however, after lubrication the samples become transparent (see Figure 6B and 6C) because the lubricant layer covers the rough fiber structure, as shown in Figure 4D. After lubrication, the transparency of all fiber sheets exceeded 80% in the visible range. Application of lubricated fiber sheet to an endoscope lens Finally, as proof-of-concept, we coated the lubricated fiber sheet on to an endoscope lens,25, 51 as shown in Figure 7A. We used a lubricated PVDF-HFP fiber sheet because all the samples fulfilled the criteria for slippery surface formation (Figure 4). Figure 7B shows the coating process; the sheet after sticking is shown in Figure 4C. For medical application, the materials must undergo a sterilization process by heating at 115-130 ˚C, which is defined in the Guideline for Sterility Assurance in Healthcare Setting by the Japanese Society of Medical Instrumentation (http://www.jsmi.gr.jp). Thus, we confirmed the thermal stability of all fiber samples using thermogravimeter, as shown in Figure 7C. The loss in the fiber weight of all the samples was less


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than 1% after they were heated to 130 ˚C. Then, we confirmed the vision through the endoscope with the slippery liquid infused fiber sheet after casting a water droplet, as shown in Figure 7D. The vision through the uncoated endoscope was lost owing to water adhesion (Figure 7E), whereas the visibility through the coated lens was retained after casting a water droplet at least 70 times using plastic syringe (Figure 7F1), as well as underwater condition (Figure 7F2). We also confirmed the vapor adhesion resistance of the slippery coated lens, as shown in Figure 7G. The monitor view in Figure 7H indicated that the visibility was retained while the monitoring object (bell pepper) was covered with a mist. In terms of mechanical stability, the coating was weak because the adhesion between the coating and the lens was only through Van deer Waals forces. However, for practical applications, this weakness can be solved by simply attaching a cover on the lens tip to support the outer edge of the lens (see Figure S5). Also, this coating can be used as a “disposable instant coating kit”.


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CONCLUSIONS An in situ coating method was developed for the formation of slippery surfaces using an electrospun fiber-filled porous sheet and applied as a coating with antiwetting properties on an endoscope lens as a proof-of-concept. The technique is limited to a small coating area. However, it has an advantage over conventional slippery surface formation methods because it is a one-step coating method, there is no solvent evaporation time, it is repeatable and instantly coatable. This work contributes to the development of the processing of functional surfaces because the research focusing on small area coatings has rarely been reported. Further functionalization using this technology will promote the design of small-scale functional materials in various fields.


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FIGURES

Figure 1. An overview of the preparation procedure of the lubricated fiber coating on an endoscope lens: (I) Preparation of the fiber coating on a collector. The collector is composed of a porous PET film (size: 160 × 160 × 0.18 mm, pore: 20 mm of 5 × 5 pores with same intervals) and aluminum plate (size: 160 × 160 × 0.5 mm), in this order. These layers are adhered by a starch paste. The inserted color distribution (top right) is simulation of the z-direction electric field distribution on porous PET by finite element method. (II) Fibers deposited on the porous sheet are detached from the aluminum array to obtain fiber-coated porous PET. Then, another porous PET film with the same topography is attached to the fiber-coated surface to fit the pore area to obtain the fiber-filled sheet. (III) Lubricating liquid is cast on the fiber filled area to obtain a lubricated fiber sheet. (IV) The lubricated fiber sheet works to coat the slippery surfaces on endoscope lens (diameter: 4 mm) via Van der Waals forces simply by sticking to the sheet. Because the fiber sheet is sandwiched between two porous PETs, the slippery surface can stably be coated on the lens.


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Figure 2. Morphology, mechanical property, and chemical composition of the fiber coating deposited on the porous PET sheet. (A) Prepared solutions used in this research by changing the weight ratio of the SiO2 nanoparticles (NPs); the solutions become opaque and scattering by red laser increase. (B) scanning electron microscopy (SEM) image of electrospun fibers (PVDFHFP+ 2 wt.% SiO2 NPs) on a porous PET sheet. No apparent differences in macroscopic view are observed with other electrospun fibers. The inset is the fiber diameter distribution. (C) SEM images and (D) the schematic images of fibers from 3 different solutions in (A). The fiber content in the schematic images are obtained from the X-ray photoelectron spectrum in (E), and Fourier-transform infrared spectrum in (F).


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Figure 3. Wettability analysis of the fiber sheets. (A) Photo images of the liquid contacting behaviors of oil or water contact on 3 types of fiber sheets (green or red frames, respectively), and water contact on the silicon-oil-infused fiber sheets (blue frames). (B) Contact angles of oil or water on the fiber sheet (green or red, respectively), and apparent water contact angles on silicone oil-infused fiber sheets (blue). (C) Dynamic contact angles (red: advancing, green: receding, blue: hysteresis) and adhesion force (yellow) of water on the oil-infused fiber sheets.


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Figure 4. Stability analysis of the oil layer immobilized on the fiber sheets. (A, B) Thermodynamic analysis of the oil-infused fibrous sheets when the target liquid for repellence is water. ∆E1(2) indicates the difference of the total interfacial energy between configuration A and configuration 1 (2). When ∆E1(2) is positive, a water layer does not replace the oil layer. (C-E) Observation of the oil immobilization on the fiber sheets. (C) Photo image and (D) low-vacuum SEM image of a lubricated PVDF-HFP fiber sheet formed on porous PET. (E) Low-vacuum SEM image of partially lubricated PVDF-HFP fiber.


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Figure 5. Tensile strength analysis of electrospun fiber sheets before and after lubrication. (A) Tensile strength and elastic properties of fiber sheets before and after lubrication. (B) Photographic images of the tensile test. Samples are PVDF-HFP with 2 wt.% SiO2 NPs (Left) and the corresponding lubricated sample (Right). (C) Schematic images of the applied forces between the tensile test which indicate an influence of lubricant on tensile strength. The left one is the fiber sheet and the right one is the lubricated fiber sheet.


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Figure 6. Optical properties analysis of the (lubricated) fiber sheet. (A) Total transmittance, parallel transmittance, haze, and diffusion of fiber sheets before and after lubrication. Parallel transmittance means the degree of light that passes through a sample within a small range of angles (

In situ Formation of Slippery-Liquid-Infused ... - ACS Publications

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