A Biocompatible Slippery Surface Based on a Boehmite Nanostructure

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A Biocompatible Slippery Surface based on a Boehmite Nanostructure with Omniphobicity for Hot Liquids and Boiling Stability Ryo Togasawa, Fumiya Ohnuki, and Seimei Shiratori ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00199 • Publication Date (Web): 02 Apr 2018 Downloaded from http://pubs.acs.org on April 2, 2018

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ACS Applied Nano Materials

A Biocompatible Slippery Surface based on a Boehmite Nanostructure with Omniphobicity for Hot Liquids and Boiling Stability Ryo Togasawa, †Fumiya Ohnuki, † and Seimei Shiratori*,†

†

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

University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan

[email protected]*

KEYWORDS Slippery surface, Biocompatibility, Omniphobicity, Hot-liquid repellency, Boiling Stability, Nanostructure

ABSTRACT Omniphobic surfaces have attracted much attention and have a lot of applications in

ACS Paragon Plus Environment 1

ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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various fields. In particular, the application of omniphobicity to containers of pre-packaged foods such as curry is extremely important for reducing food waste because they are especially liable to remain in the container owing to their high viscosity. To realize such application, the surface must be biocompatible to prevent health damage to the human body. However, few omniphobic surfaces with biocompatibility have been developed. Here, we present a silicone-oil-immobilized biocompatible slippery surface with omniphobicity. A biocompatible base layer to immobilize the silicone oil as a lubricant was fabricated by combining a boehmite nanostructure and silicone-oil grafting through thermal treatment. The slippery surface demonstrated omniphobicity not only for room-temperature liquids but also for high-temperature liquids including hot liquid foods. In addition, the surface exhibited long-term stability against boiling in hot water. Moreover, the surface could be fabricated within an hour. We believe that because of its excellent biocompatibility and omniphobicity, this surface will be applied to not only food containers but also various other applications.

INTRODUCTION


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Omniphobic surfaces have recently attracted a lot of attention and have various applications,

including

self-cleaning,1,2

drag

reduction,1

anti-corrosion,1

anti-biofouling,3 and food packaging.4 In particular, the application of omniphobicity to the inside of a food container is extremely important for reducing food waste, as the residual amount of liquid food in a container can be up to 15%.5 High-temperature liquid foods such as curry are especially liable to remain in a container, which results in food waste and hinders recycling of the containers. Moreover, to realize repellency for liquid foods, in addition to omniphobicity for high temperature droplets, sufficient biocompatibility is also required. This is because, in the unlikely event that components of the coating are mixed in the foods and end up in the human body, it is necessary to prevent the components from having a negative influence on the human body. To the best of our knowledge, however, no liquid-repellent surfaces with both omniphobicity and biocompatibility have been reported. To date, numerous superhydrophobic surfaces have been developed, inspired by the surface structure of lotus leaves in nature.6–10 Although repelling liquids with a low surface tension on superhydrophobic surfaces is difficult, in recent years, superomniphobic surfaces that can repel both water and organic liquids have been achieved through the fabrication of special rough structures, such as re-entrant



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structures.1,11–13 In addition, the formation of the surface structure improved the repellency to hot water, which was limited on conventional superhydrophobic surfaces probably owing to the influence of the vapor of hot water.14–19 Moreover, there are some superhydrophobic surfaces which are fabricated with only FDA-approved materials.5,20 However, to realize superomniphobicity, the rough structures need to be modified with a long-chain fluorine material, which is concerned unsafe for the human body and the environment.1,21–23 Therefore, it is difficult to fabricate a superomniphobic surface that has sufficient biocompatibility for use in food containers. As another approach, liquid-like surfaces can achieve omniphobicity without fluorine materials.24–28 The liquid-like surfaces are lyophobic-molecule-modified smooth slippery surfaces, which can be fabricated through a sol-gel method, layer-by-layer method, and other methods.24–30 Because of the mobility of the surface molecular chains, they can slide water and organic liquids. However, in most cases, a silane coupling agent is used as a molecular chain, which remains a concern from the viewpoint of biocompatibility. In contrast to the superomniphobic surfaces and liquid-like surfaces, liquid immobilized surfaces (LIS) inspired by the Nepenthes pitcher plant could overcome the problem of biocompatibility.31–34 In general, LIS are obtained by impregnating a lubricating oil that is immiscible with the target liquids into an uneven structure such as


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a porous structure with a low surface energy. Because of the immiscibility of the lubricant and the target liquids, liquids can slide off LIS. Depending on the purpose of use, LIS can realize various combinations of a base layer and a lubricant layer. For example, Wong et al. fabricated an omniphobic slippery surface by using a fluorine-based lubricant.31 In addition, some LIS with hot water repellency have been reported.35–37 Because the outermost surface of the LIS has smoothness owing to lubricating liquid, they are hardly affected by the vapor of hot water. Therefore, it is relatively easy to prepare a surface with hot water repellency. Some silicone oils have sufficient biocompatibility to be used for food packages. For instance, silicone oils with a dimethylpolysiloxane structure and a viscosity of 500 cS or more are registered on the FDA’s approved list and can be added to foods as deforming agents.38 Therefore, these silicone oils are promising materials for the fabrication of a biocompatible slippery surface. Indeed, Wu et al. fabricated an omniphobic slippery surface that immobilized a silicone oil in a colloidal template with a re-entrant structure.39 Zhang et al. reported excellent thermal stability in a silicone-oil-infused

slippery

surface.36

However,

materials

with

insufficient

biocompatibility were utilized in the fabrication process of the base layer and the fabrication processes were time-consuming. Wooh et al. utilized a photocatalytic



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reaction to graft a silicone oil to a mesoporous TiO2 of a base layer.40 This method was efficient and the slippery surface prepared through this process was superior in terms of biocompatibility. However, because TiO2 was used for the base layer, it is difficult to manufacture at low cost. There are also LIS that do not have a rough base layer and immobilize a silicone oil only by intermolecular interactions.41,42 Although the LIS can be fabricated in a fast process, because the base layers do not have the rough structure, there is a concern about the loss of the lubricant under liquid conditions and the durability against the shear stress may not be sufficient. As mentioned above, no liquid-immobilized omniphobic surfaces have been developed which have enough biocompatibility and long-term stability. In addition, few paper have reported hot liquid repellency and its sliding behavior on LIS. Herein, we fabricated a biocompatible slippery surface with omniphobicity for high temperature liquid and stability under various environments, by combining a boehmite nanostructure and silicone-oil grafting through thermal treatment. Aluminum is often used for the sealed pouch of hot food. Boehmite nanostructure is formed just by boiling an aluminum substrate in hot water and no other chemical compounds are needed.43 Therefore, from the point of biocompatibility, boehmite is one of the suitable nanostructure for the fabrication of a base layer.44 Moreover, a silicone oil with a


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dimethylpolysiloxane structure and a viscosity of 100 cS was utilized for surface treatment of the boehmite and as a lubricant, because this silicone oil is registered in the approved list of the Japan Hygienic Olefin and Styrene Plastics Association which authorizes a use of the silicone oil for food packages. The fabricated slippery surface exhibited omniphobicity not only for room-temperature liquids but also for high-temperature liquids including hot liquid foods. In addition, the surface demonstrated stability against boiling in hot water and long-term immersion under water thanks to the nanostructure. Moreover, this surface was fabricated within an hour and at low cost. We believe this surface will be applicable for practical food packaging and utilized in various fields.

EXPERIMENTAL SECTION Materials. Aluminum (1050, Hikari Co., Ltd., Osaka, Japan) was used as a substrate. Acetone (Kanto Chemical Co., Inc., Tokyo, Japan) and ethanol (Kanto Chemical Co., Inc., Tokyo, Japan) were used for cleaning the substrates. For the film fabrication and each experiment, ultrapure water (Aquarius GS-500.CPW, Advantec, Tokyo, Japan) was used. A silicone oil (100 cS, Shin-Etsu Chemical Co., Ltd., Tokyo, Japan) was utilized



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for low-surface energy treatment and as a lubricant. Oleic acid (Tokyo Chemical Industry Co., Ltd., Tokyo, Japan) was used to check oleophobicity. Methylene blue (Waldeck GmbH & Co. KG, Münster, Germany) and oil red (Wako Pure Chemical Industry Ltd., Osaka, Japan) were utilized as a dye for water and oleic acid, respectively. The surface tension of the aqueous solutions was adjusted using ethanol, according to the reference paper.45 Curry, carbonara sauce, coffee with milk, corn soup, and oshiruko (a sweet porridge made of azuki beans) were purchased from a supermarket. Hydrochloric acid (Kanto Chemical Co., Inc., Tokyo, Japan) and sodium hydroxide (Kanto Chemical Co., Inc., Tokyo, Japan) were used for pH adjustment.

Film fabrication. A schematic image of the fabrication procedure is shown Scheme 1. Aluminum substrates were cut into 2.5×2.5 cm or 5×5 cm pieces. To remove the machine oil that adhered at the time of cutting, the substrate was wiped with a kimwipe impregnated with acetone. Then, the substrates were washed by ultrasonic cleaning with ethanol for 5 min. The cleaned substrates were immersed into boiling water for 10 min to form a boehmite nanostructure and dried by air blow.46 10 µL of the silicone oil was cast per 2.5×2.5 cm on the boehmite and heat treatment was performed on a hot plate at 300°C for 5 min to graft the silicone oil to the boehmite nanostructure and form a base


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layer. Finally, 10 µL of the silicone oil was cast per 2.5×2.5 cm on the base layer by spin-coating at a speed of 1000 rpm for 5 s and then at 6000 rpm for 10 s to form a lubricant layer.

Characterization. The surface morphology was analyzed by field-emission scanning electron microscopy (FE-SEM; S-4700, Hitachi, Tokyo, Japan), atomic force microscopy (AFM5000II, Hitachi High-Tech Science Co., Tokyo, Japan), and a 3D laser scanning microscope (VK-9700, KEYENCE, Osaka, Japan). X-ray photoelectron spectroscopy (XPS; JPS-9010TR, JEOL, Tokyo, Japan) with a Mg Kα laser was used to investigate the chemical composition of the base layer. Contact angles and sliding angles of a 10 µL droplet were measured using a contact angle meter (CA-DT, Kyowa, Tokyo, Japan) and the measurements were carried out 5 times. It was possible to tilt the stage of the machine automatically and we measured the critical angles at which a droplet started moving on a slippery surface as the sliding angles. Contact angle hysteresis was measured 5 times through tilted plate method.47 The sliding speeds were measured by a high-speed camera (HAS-D3, Direct, Tokyo, Japan) and analyzed with Image J software (U. S. National Institutes of Health, Bethesda, Maryland, USA). The sliding distances were measured by carefully placing the droplets on a 5° tilted sample



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for 0.5 s and the sliding speeds were calculated by dividing the distance by the time. The sliding speed measurements were carried out at least 5 times. The error bars in the measurements of the contact angles, the sliding angles, and the sliding speeds were calculated with STDEV.P function using excel. The surface tension of the liquids was investigated by an automatic surface tensiometer (CBVP-Z, Kyowa, Tokyo, Japan). A thermal camera (PI400, Optris, United States) was used to measure the temperature of a droplet just after the droplet was placed on the sample. Temperature control of the sample was performed by placing the sample on the temperature-controlled hot plate and the temperature of the hot plate was measured using an infrared thermometer (PT-S80, OPTEX Co., Ltd, Shiga, Japan). Because aluminum was used as the substrate, it was difficult to directly measure the surface temperature of the sample; thus the temperature of the hot plate was measured instead. The temperature difference between the aluminum substrate and the hot plate was considered to be small, thanks to the high thermal conductivity of aluminum. In addition, because it was difficult to keep the temperature of the liquids and the sample constant during the sliding speed measurements and the surface tension measurements of the liquid foods, these measurements were carried out by controlling the temperature within ± 5°C from the target temperature. The repellency to the liquid foods was evaluated by dropping the


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food with a temperature of approximately 70°C on a sample at room temperature tilted at 30°. In addition, the repellency to large amount of hot corn soup was evaluated on an 80° tilted sample and on a curved sample tilted at 30°. The stability under water was investigated by the contact angles and the sliding angles after leaving the sample in water at a depth of 2 cm for a maximum of ~2 months. The stability under air was investigated by the contact angles and the sliding angles after leaving the sample at ambient condition for a maximum of ~2 months. The durability against shearing stress was also investigated by the contact angles and the sliding angles after adding shear stress by spinning at a speed of ~6000 rpm for 1 min. The scratching tests were conducted to investigate the mechanical durability using a medicine spoon and a cutter.

Boiling and retort test. 70 mL of water was placed in two containers with a volume of 195 mL and heated until the water boiled. Then, the samples were immersed into the boiling water for 5 min. We defined the boiling test as a test in an open container. Conversely, the retort test was defined as a test in a sealed container to obtain a high pressure. The contact angles, sliding angles, and sliding speeds of a 10 µL droplet on the sample were measured both before and after the tests.



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Scheme 1. Schematic diagram of the preparation procedure.

RESULTS & DISCUSSION Fabrication of a base layer. First, we fabricated the base layer to immobilize the lubricant layer. The SEM images of (a) a bare aluminum substrate, (b) an aluminum substrate after immersion into boiling water for 10 min (boehmite), and (c) a silicone-oil-grafted boehmite are shown in Figure 1. The inset images show a 10 µL water droplet on each surface. As shown in Figure 1b, the boehmite nanostructure was formed after boiling an aluminum substrate in hot water for 10 min and the water contact angle decreased to approximately 9°. After the silicone oil was cast on the prepared boehmite nanostructure and heat treatment was carried out on a hot plate at 300°C for 5 min, the water contact angle increased to approximately 96° and indicated


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hydrophobicity, although no change in the surface structure was observed (Figure 1c). In addition, the root-mean-square roughness calculated by AFM was almost same between before and after the heat treatment (Figure S1). The reason why the surface became hydrophobic and the original nanostructure was maintained is probably because the silicone oil was crosslinked to the boehmite through heat treatment and a thin hydrophobic silicone layer with a nanometer-scale thickness was formed on the boehmite. In general, silicone oil is thermally stable, but when it is heated at very high temperature such as 300°C, it reacts rapidly with water and hydrolyzes.41,48 Therefore, it is considered that the silicone oil was hydrolyzed at 300°C by a slight amount of water adhered to the boehmite surface and the -OH groups generated by the hydrolysis were dehydrated and condensed with -OH groups on the boehmite surface, as shown in Figure S2.48 The XPS of the boehmite surface after heat treatment is shown in Figure 1d. The Al-O-Si bond, which was caused by crosslinking of the silicone oil on the boehmite, was confirmed.49 Therefore, the hypothesis mentioned above was supported. Incidentally, because this base layer was fabricated with an aluminum substrate, aluminum leaching from the substrate should be suppressed for biocompatibility of the surface. According to the previous study, the boehmite has a sealing effect that the boehmite layer completely covers the aluminum surface and drastically decreases the



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amount of dissolved aluminum.50,51 Therefore, the boehmite contributes to not only nanostructure formation but also biocompatibility of the surface. Moreover, the silicone layer is covalently-bonded polydimethylsiloxane. Thus, the silicone-oil-grafted biocompatible base layer was formed.


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Figure 1. SEM images of (a) a bare aluminum substrate, (b) a boehmite nanostructure, and (c) a silicone-oil-grafted boehmite. The images in the insets show a water droplet



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(10 µL) on each surface. (d) XPS analysis of O1s of the silicone-oil-grafted boehmite surface.

Sliding

properties

of

room-temperature

droplets.

After

introducing

the

biocompatible silicone oil to the fabricated base layer as a lubricant, the sliding properties of the room-temperature liquids were investigated, as shown in Figure 2. The fabricated biocompatible surface repelled both water (blue) and oleic acid (red), which indicated dynamic omniphobicity of the surface (Figure 2a). This omniphobicity resulted from immiscibility of the silicone oil with fatty acids, which are components of animal and vegetable oils. The appearance of both water and oleic acid, their contact angles, and their sliding angles on the slippery surface are shown in Figure 2b. The sliding angles of both liquids were less than 3° and the surface demonstrated low contact angle hysteresis (< 2°) as shown in Table S1. Because the surface tension of oleic acid is much lower than that of water, oleic acid had a lower contact angle. Therefore, as shown in Figure 2c, although the sliding speeds of both liquids increased with increasing droplet volume, the sliding speeds of oleic acid were lower than those of water at all volumes, because the oleic acid droplet had a larger contact area with the slippery surface. Moreover, since liquid foods such as curry are mixtures containing


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water and oil, their surface tension should be lower than that of water. Thus, we prepared the aqueous solutions with various surface tension by using ethanol and water and investigated their contact and sliding angles on the surface (Figure 2d). Although the contact angles decreased significantly with decreasing surface tension of the solution, the sliding angles were less than 3° at all surface tensions. Because the outermost surface of the slippery surface was smooth unlike rough surfaces such as superhydrophobic surfaces as shown in Figure S3, it is considered that the influence of the decreased surface tension on the sliding angle was extremely small and the sliding angle was kept constant.

Figure 2. (a) A photo showing that a water droplet (blue) and an oleic acid droplet (red)



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sliding on an omniphobic surface. (b) Appearances, contact angles, and sliding angles of both water and oleic acid droplets on the surface. (c) Sliding speeds of both water and oleic acid droplets at different droplet volumes. (d) Relationship between surface tension of the aqueous solutions and their contact or sliding angles.

Omniphobicity of high-temperature liquids and sliding properties at high temperatures. We investigated omniphobicity of hot liquids and the sliding properties at high temperatures. In general, because the surface tension of liquids decreases with increasing temperature, and in particular, water droplets become steamy at a high temperature, it is not easy to repel hot water on superhydrophobic and superomniphobic surfaces. In contrast, the slippery surface with an inclination angle of 5° repelled hot water droplets at 80°C and hot oleic acid droplets at 90°C, as shown in Figure 3a. It is considered that the hot liquid repellency resulted because of two reasons. One is that the phenomenon that the steam entered a rough structure and nucleated in the structure did not occur because of the smoothness of the outermost lubricant layer; nucleation is generally considered as the reason of limited superhydrophobicity to hot water. The other is that the surface could repel liquids with a wide range of surface tensions. As shown in Table S2, the surface tension of the oleic acid at 90°C was in the range. Figure


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S4 shows the relationship between the sliding speeds of a 10 µL water or oleic acid droplet and the surface temperature. According to a previous study, the sliding speed of droplets on LIS is inversely proportional to the viscosity of a lubricant.52 Therefore, the sliding speeds of both water and oleic acid increased with increasing surface temperature because the viscosity of the lubricant decreased. Moreover, we investigated the sliding speeds of a 10 µL water droplet when not only the surface temperature but also the droplet temperature was changed (Figure 3b). As in the previous case, the water sliding speeds increased with increasing surface temperature at all the droplet temperatures. In particular, the water droplet with a temperature of 80°C slid off at an average speed of approximately 0.6 mm/s on the surface at 80°C. Conversely, when the temperature of the water droplet was 40°C or higher than the surface temperature, the sliding speed significantly decreased. In these cases, even though the water droplets did not always slide immediately after they were placed on the surface, they all eventually slid. Conversely, when the droplet was oleic acid at 80°C, such a phenomenon was not observed and the sliding speed of the oleic acid droplet exceeded those of the water droplets under the same condition when the surface temperature was 20°C or 40°C (Figure S5). The boiling point of oleic acid is 360°C, which is much higher than that of water.53 Therefore, the reason why the sliding speed of the water droplet significantly



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decreased when the droplet temperature was 40°C or higher than the surface temperature was that the vapor around the hot water droplets condensed on the surface of the lubricant (Figure S6 and Scheme S1) and the condensed vapor hindered the movement of the water droplet. Thus, when the surface temperature was close to the droplet temperature of water, condensation of the vapor hardly occurred and the influence on the water sliding speed was small. Therefore, the condensation should not cause a significant slippery limitation in practical food packages, because there is almost no temperature difference between a food package and its enclosed food. Figure 3c and Movie S1 show the sliding behavior of hot liquid foods with a temperature of approximately 70°C on the 30° tilted sample. The surface tensions of these foods at 20°C and 70°C are summarized in Table S3. As shown in Figure 3c and Movie S1, the slippery surface repelled all the hot liquid foods. In addition, larger amount of hot corn soup slid on an 80° tilted sample (Movie S2) and on a curved sample tilted at 30° (Movie S3). The hot liquid-food repellency was attributed to the omniphobicity of the surface for hot liquids and the ability to repel liquids with a wide range of surface tension.


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Figure 3. (a) Photos showing hot water droplets (80°C) and oleic acid droplets (90°C) sliding on a surface. (b) Sliding speeds of water droplets on a 5° tilted surface when both the droplet temperature and the surface temperature were controlled. (c) Photos showing sliding behaviors of hot liquid foods (1: curry, 2: carbonara sauce, 3: coffee with milk, 4: corn soup, 5: oshiruko) on a 30° tilted surface.



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Boiling Stability. We investigated the stability of the surface against long-term boiling in hot water, as summarized in Figure 4. A schematic diagram of boiling and retort tests is shown in Figure 4a. Figure 4b and 4c show the contact angles and sliding angles of water (Figure 4b) and oleic acid (Figure 4c) before and after the boiling and the retort tests. Both water and oleic acid demonstrated almost no changes in the contact angles before and after the tests, and the changes in the sliding angles were less than 1°, which indicated boiling stability. In addition, the changes in the sliding speeds were less than 15% in both the water and the oleic acid (Figure 4d). These results were attributed to the fact that the lubricant layer was not lost through boiling in hot water and was remained uniform.


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Figure 4. (a) A schematic diagram of the boiling and the retort test. Contact angles and sliding angles of (b) water and (c) oleic acid before the tests, after the boiling test, and after the retort test. (d) Sliding speeds of a 10 µL water or oleic acid droplet on a 5° tilted sample before the tests, after the boiling test, and after the retort test.

Stability and Durability. A coating for pre-packaged foods requires long-term stability, because the retort foods are usually preserved for a long time. We investigated the relationship between immersion time of a sample in water and the contact and sliding angles of both water and oleic acid (Figure 5a). Even after leaving the sample in water for 2 months (1440 h), the sliding angles of the water and the oleic acid were both less than 3°. As shown in Figure S7, the sliding angles of the water and the oleic acid after



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exposing in air for 500 h were both less than 3°. However, the sliding angle of the oleic acid exceeded 5° when the exposure time in air was over 800 h. The reason for this degradation in the sliding angle was the evaporation of the “lubricant” silicone oil, not the “solid” silicone layer which was formed through heat treatment and was covalently bonded to the boehmite. Conversely, the sliding angles were maintained for 2 months in water because the evaporation of the lubricant was suppressed by the water around the sample and the silicone-oil-grafted boehmite nanostructure stably immobilized the lubricant layer. In addition, for practical use as a food container, the stability at a wide range of pH range is also important. As indicated in Figure 5b, the contact angle slightly decreased as the pH value moved away from 7. Meanwhile, the sliding angle slightly increased, but the sliding angle was less than 4° over the whole pH range. Furthermore, as shown in Figure 5c, the slippery surface maintained a sliding angle of both the water and the oleic acid of less than 3°, even after the application of a shearing stress with a spin-coater for 1 min at a speed of up to 6000 rpm. The reason for the stability against shearing stress was considered to be because of the nanostructure in the base layer, which exhibited high performance for the immobilization of the lubricant layer. In addition, the mechanical durability was investigated through scratching tests using a medicine spoon and a cutter. While no change was observed in the sliding behavior of a


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water droplet after rubbing lightly with the spoon (Movie S4), a water droplet was pinned after scratching strongly by the cutter (Movie S5), probably because the thin silicone layer was destroyed and the hydrophilic aluminum substrate was exposed. Therefore, although stronger mechanical durability will be required to be used for a wide range of applications, the surface can be applied to inside of sealed containers.



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Figure 5. (a) Relationship between immersion time in water and water contact angle (WCA), water sliding angle (WSA), oil contact angle (OCA), and oil sliding angle (OSA). (b) Relationship between pH value and contact or sliding angle. (c) Variations of


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WCA, WSA, OCA, and OSA after the application of shearing stress up to 6000 rpm with a spin-coater for 1 min.

CONCLUSIONS We developed a biocompatible slippery surface with omniphobicity using a silicone oil as a lubricant. A biocompatible base layer was fabricated by combining a boehmite nanostructure and silicone-oil grafting through heat treatment. The fabricated slippery surface demonstrated omniphobicity for hot liquids including hot liquid foods. In addition, we investigated the sliding speed of a water droplet at various droplet temperatures

and

surface

temperatures.

Unlike

superhydrophobic

surfaces,

liquid-immobilized surfaces are thought to be less affected by the vapor of hot water because their outermost surfaces are smooth, but if the temperature of the water droplet was 40°C or higher than the surface temperature, condensation of the vapor on the lubricant layer was likely to hinder the movement of the water droplet. However, if the surface temperature was close to the droplet temperature or higher, the effect of the condensation was much smaller or not observed. In addition, the slippery surface demonstrated excellent stability against boiling in hot water and long-term immersion in



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water owing to the nanostructure. The surface can be applied to other various types of substrates by combining with sol-gel alumina coating method. We believe the biocompatible and omniphobic slippery surface will provide new insights into omniphobic liquid-immobilized surfaces and can be applied to not only food containers but other applications.

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Seven figures showing AFM images of a boehmite nanostructure and a silicone-oil-grafted boehmite, mechanism of silicone-oil-grafting, laser microscope image, relationship between sliding speeds of a 10 µL water or oleic acid droplet and surface temperature on a 5° tilted surface, relationship between sliding speeds of a 10 µL oleic acid droplet with a temperature of 80°C and surface temperature on a 5° tilted surface, condensed vapor around a hot water on a sample, relationship between immersion time of a sample in water and WCA, WSA, OCA, or OSA; three tables listing

contact angle hysteresis, surface tension of oleic acid, and surface tension of liquids foods; One schemes showing condensed vapor around a hot water on a sample (PDF)


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

Present Addresses †

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

University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522 Japan

Author Contributions R.T. proposed the research, designed the experiments, collected and analysed the data, and wrote the paper. F.O. collected the data. S.S. supervised the project, provided scientific advice, and commented on the manuscript. Funding Sources A part of this work was supported by Grant-in-Aid for Scientific Research (C) from Japan Society for the Promotion of Science (JSPS), No. 17K04992 (2017).

Notes The authors declare no competing financial interest.



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ACKNOWLEDGEMENTS We are deeply grateful to Mr. Ryohei Yoshikawa for giving us support in drawing schemes.

ABBREVIATIONS LIS, liquid immobilized surfaces; WCA, water contact angle; WSA, water sliding angle; OCA, oil contact angle; OSA, oil sliding angle.

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“Table of contents only”



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A Biocompatible Slippery Surface Based on a Boehmite Nanostructure

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