Bio-inspired Edible Superhydrophobic Interface for Reducing

Feb 14, 2018 - The bio-inspired edible superhydrophobic interface showed multiscale structures, which were similar to that of a lotus leaf surface. ...
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Bio-inspired edible super-hydrophobic interface for reducing residual liquid food Yao Li, Jingran Bi, Siqi Wang, Tan Zhang, Xiaomeng Xu, Haitao Wang, Shasha Cheng, Bei-Wei Zhu, and Mingqian Tan J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.7b05915 • Publication Date (Web): 14 Feb 2018 Downloaded from http://pubs.acs.org on February 15, 2018

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Journal of Agricultural and Food Chemistry

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Bio-inspired

edible

super-hydrophobic

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reducing residual liquid food

interface

for

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Yao Li,abc Jingran Bi,ab Siqi Wang,ab Tan Zhang,ab Xiaomeng Xu,ab Haitao Wang,ab Shasha Cheng,ab

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Bei-Wei Zhuabc* and Mingqian Tanab*

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a

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District, Dalian, Liaoning 116034, People’s Republic of China

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b

School of Food Science and Technology, Dalian Polytechnic University, Qinggongyuan1, Ganjingzi

National Engineering Research Center of Seafood, Dalian, Liaoning 116034, People’s Republic of

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China

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c

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266000, People’s Republic of China

State Key Laboratory of Bioactive Seaweed Substances, Huangdao district, Qingdao, Shandong,

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*Corresponding author (Tel& Fax: +86-411-86318657, E-mail: [email protected];

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[email protected], ORCID: 0000000275350035).

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Abstract:

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Significant wastage of residual liquid food, such as milk, yogurt, and honey, in food container has

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attracted great attention. In this work, a bio-inspired edible super-hydrophobic interface was fabricated

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using the FDA-approved and edible honeycomb wax, arabic gum, and gelatin by a simple and low-cost

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method. The bio-inspired edible super-hydrophobic interface showed multiscale structures, which were

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similar to that of lotus leaf surface. This bio-inspired edible super-hydrophobic interface displayed high

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contact angles for a variety of liquid foods and the residue of liquid foods could be effectively reduced

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using the bio-inspired interface. To improve the adhesive force of the super-hydrophobic interface, a

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flexible edible elastic film was fabricated between the interface and substrate materials. After being

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repeatedly folded and long-time flushed, the interface still maintained excellent super-hydrophobic

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property. The bio-inspired edible super-hydrophobic interface showed a good biocompatibility which

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may have potential applications as a functional packaging interface material.

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1. Introduction

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In recent years, great efforts have been made to alleviate the global food crisis.1-2 Nevertheless,

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there is a significant waste of residual liquid foods such as milk, yogurt, and honey adhered to the

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inside of food container when they are poured out. If the residual liquid food in the container can be

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reduced, it would be helpful to save food. One strategy to reduce the residual liquid food is to employ

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bio-inspired super-hydrophobic films, which are extremely repellent to water and can reduce the

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adhesion between the container and liquid food products. 3-5

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The inspiration of bio-inspired super-hydrophobic film comes from the natural world. In the long

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evolutionary process, certain plant leaf like lotus (Nelumbo nucifera) exhibits special wettability, to

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avoid the adhesion of residual water.6-7 On the surface of lotus leaf, water droplets are almost spherical

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and can roll freely. The special wettability of lotus leaf is due to two main factors: chemical

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composition to impart low solid surface energy, and multiscale structures to enhance hydrophobicity on

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the lotus leaf surface. The cooperation of the chemical composition and multiscale structures result in

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an excellent super-hydrophobicity, resulting in a high water contact angle and a small sliding angle.8-15

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For special wettability, many strategies have been developed to construct the super-hydrophobic

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surfaces by an electrostatic spinning technique to manufacture micro/nanoscale structures using

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hydrophobic polymer.16-19 However, the electrostatic spinning technique is prohibitively expensive,

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high-voltage and uncontrollable in large-area fabrication. The other attractive approach is the use of

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long-chain fluorocarbon materials with low surface energy.20-21 However, the long-chain fluorocarbon

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material has safety concern to humans due to its bio-accumulative and toxic effects. Therefore, an ideal

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super-hydrophobic surface material should be non-toxic and edible, thus eliminating toxicity of the film

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materials during food applications.22-26 In addition, the preparation method of super-hydrophobic

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surface should be simple, low cost and controllable in large-area fabrication for potential commercial

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applications. The traditional preparation methods of super-hydrophobic surface are not suitable for the

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flexible surface of food applications.27-29 Therefore, it is a great challenge to design and develop an

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ideal super-hydrophobic interface on a flexible surface in food packing.

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In this work, edible honeycomb wax, a U.S. food and Drug Administration (FDA)-approved and

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edible material, was used as main matrix material to fabricate super-hydrophobic interface by a simple

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microemulsion spraying method.30 A unique microstructure of honeycomb wax was formed via the

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simple, low-cost and scalable spray-coating process. The formed honeycomb wax interface possesses

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super-hydrophobic property on which water and liquid food products including Lipton green tea,

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Gatorade, Coca-Cola, orange juice, whole milk, coffee and honey can roll freely in all directions.

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Moreover, an edible elastic film was fabricated between the honeycomb wax and matrix material, such

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as plastic film, to enhance the adhesion of super-hydrophobic interface for the application in flexible

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substrates. In vitro cytotoxicity test was carried out to investigate the viability of cells upon contacting

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the super-hydrophobic interface. This is a significant attempt to apply natural food as a functional

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packaging interface material.

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2. Experimental

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2.1 Materials

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Lipton green tea, Gatorade, Coca-Cola, orange juice, whole milk, coffee (Nescafe® Smoovlatte

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containing water, milk, instant coffee, sugar, thickener 460i and 466, stabilizer 500 ii, acidity regulator

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331iii and emulsifier 473, flavouring essence) and honey were purchased from local grocery market

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(Dalian, China). Arabic gum, gelatin (from porcine skin), honeycomb wax and 3-(4,

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5-Dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide (MTT) were purchased from Aldrich

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Chemical Co. (Shanghai, China). Mouse osteoblast cell line was purchased from the Cell Bank of Type

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Culture Collection of the Chinese Academy of Sciences (Shanghai, China). Unless otherwise stated,

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acetone and other chemicals were purchased from commercial vendors and used without further

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purification.

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2.2 Preparation of edible elastic film

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The edible super-hydrophobic interface was prepared as outlined in Scheme 1. Arabic gum (0.6 g)

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and 8 mL water were mixed in a 50 mL round-bottom flask. After the Arabic gum was dissolved,

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gelatin (0.2 g), glycerine (0.4 g) and 8 mL water were added. The mixture was mixed and heated at

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50 °C for 12 h and filtered to remove the undissolved solid. The matrix material (7.5 cm x 7.5 cm)

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plastic (or glass) was thoroughly rinsed with ethanol and deionized water and subsequently dried using

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nitrogen gas. The mixture (0.5 mL) was added dropwise to the surface of the plastic while it was hot. A

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thin uniform film was formed after swing lightly. After cooling at room temperature overnight, the

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edible elastic film was obtained.

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2.3 Preparation of edible super-hydrophobic interface

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Honeycomb wax (1 mg), acetone (8 g) and n-hexane (1 g) were mixed in a 50 ml round-bottom

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flask. The mixture was heated to reflux and the temperature was maintained for 1 h. After the reaction

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was completed, the mixture was colorless and transparent. Ethanol (11 g) was added dropwise to the

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mixture within half an hour. The mixture was then stirred for 1 h to obtain honeycomb wax emulsion

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upon cooling to room temperature. And then, the honeycomb wax emulsions were sprayed onto the

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surface of the edible elastic film, which were held at a distance of ~12 cm from the spray coater nozzle.

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The pressure during spray coating was held constant at 30 psi. After being placed at room temperature

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for 24 h to remove the solvent, the edible super-hydrophobic interface was obtained.

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(Scheme 1) 2.4 Toxicity of edible super-hydrophobic interface

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Mouse osteoblasts were cultured in serum-free medium(K-SFM) containing 10% fetal bovine

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serum (FBS), at 37 °C, 5% CO2, and 95% humidity. The potential cytotoxicity of edible

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super-hydrophobic interface was evaluated by a MTT assay. Mouse osteoblasts were treated with

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different quality (0.2, 0.4, 0.8, 1.6 and 3.2 mg) honeycomb wax, and were incubated for 24 hours. Then,

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the cells were treated with 0.02 mL of freshly prepared 2.5 mg/mL MTT, and incubated for 4 hours.

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The supernatant was carefully removed, and dimethylsulfoxide solution was added. The cell viability

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was calculated based on the absorbance measured at 570 nm.

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2.5 Characterization

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FT-IR spectra were recorded on a PerkinElmer Spectrum 10 FT-IR infrared spectrometer

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(PerkinElmer, Norwalk, USA) in the wavelength range of 4000 to 1000 cm−1. Scanning electron

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microscopy (SEM) was conducted on a JSM 7800F electron microscope (JEOL, Tokyo ,Japan) with the

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primary electron energy of 15 kV. The contact angles of the cured samples were measured at 20 °C

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using a sessile drop method on a dynamic contact angle measurement instrument JC2000C

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(POWEREACH, Shanghai, China). The mechanical properties were characterized by Instron tension

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tester (Model 5569, Norwood, USA) at a speed of 5 mm/min and compression test at a speed of 2

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mm/min. Volatile organic solvents from the edible super-hydrophobic interface were analyzed using an

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Agilent 7890A gas chromatography/ 5977A mass spectrometer (GC–MS) system (Palo Alto, CA, USA)

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equipped with an Agilent HP-5-MS capillary column (30m×0.25 mm, 0.25 µm)

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3. Result and Discussion

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The wettability of a liquid can be characterized by the contact angle and contact angle hysteresis. For a smooth surface, the contact angle θ is given by Young’s formula as cos θ =

γ − γ γ 

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The contact angle θ of a liquid on a smooth surface is determined by the solid surface energy (γS),

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liquid surface tension (γL) and solid liquid interfacial tension (γSL). A liquid displays the apparent

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contact angle θ* and adopts model of the two possible states: Wenzel state or Cassie state as it contacts

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a surface with multiscale structures. In the Wenzel state, the liquid penetrates multiscale structures and

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wets surface completely, which is positive for a super-hydrophilic surface.31-32 In the Cassie state, air is

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trapped between multiscale structures and liquid, which is positive to gain designing

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super-hydrophobic surfaces due to the reduced solid-liquid contact area by the air pockets.33-35 As a

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result in Cassie state, liquid shows high apparent advancing contact angles θ*adv, high apparent

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receding contact angles θ*rec, and low contact angle hysteresis ∆θ*, which are necessary to reduce the

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adhesion of liquid (when θ* ≥ 150° and ∆θ* ≤ 10°).

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Figure 1a shows a large-scale SEM image of the surface of bio-inspired edible super-hydrophobic

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interface. Many botryoid structures, uniformly distributed and arranged in a direction opposite to the

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airflow direction of the spray coating, were found on the surface with diameters ranging from 60 to 90

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µm. Figure 1b displayed a SEM image of the surface of botryoid structure. Many papillae were found

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on the surface of the botryoidal in the size of 9 to 18 µm. The inset of Figure 1b shows a

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high-resolution SEM image of the fold structures in nanometer scale on the surface of a papilla

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structure. This result illustrated that the surface of the bio-inspired edible super-hydrophobic interface

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was composed of multiscale structures, including the micron scale, sub-micron scale and nanometer

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scale. These multiscale structures were the similar as the lotus leaf surface (Figure S1) which possesses

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super-hydrophobic property with large water contact angle θ*.

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As shown in Figure 2a, a smooth surface with honeycomb wax has relatively low water contact

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angle θ* (around 85.0°), which is not suitable for application of super-hydrophobic interface. With a

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sufficient surface coverage of the multiscale structures, the hydrophobicity of honeycomb wax surface

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was improved significantly (θ* = 158.1°, ∆θ* = 7° for water, Figure 2b), at surface density (ρs) (weight

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of honeycomb wax of the coating per unit area) ≈ 0.55 mg/cm2. It is noteworthy that liquid contacted

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the hydrophilic edible elastic layer and resulted in the decrease of water contact angle θ* while the

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surface coverage was insufficient (ρs ≤ 0.55 mg/cm2) (Figure 2c). At the same time, the increase of ρs

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did not change the multiscale structures, and the water contact angle θ* did not change significantly (ρs

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≥ 0.75 mg/cm2). This result illustrated that multiscale structures with appropriate density on coating

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surface was necessary for super-hydrophobic property of coating interface.

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Application of the bio-inspired edible super-hydrophobic interface was to reduce the residual

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liquid food in food containers after being poured out. Liquid food has lower γL than water (γL = 72

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mN/m), such as Coca-Cola (γL = 59 mN/m), Lipton green tea (γL = 69 mN/m), whole milk (γL = 48

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mN/m), apple juice (γL = 57 mN/m), wine (γL = 53 mN/m), beer (γL = 52 mN/m), Gatorade (γL = 68

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mN/m), and coffee [γL = 46 mN m−1]. To validate the bio-inspired edible super-hydrophobic interface

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can effectively reduce the residual liquid food in food container, the wetting behavior of liquid food

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was evaluated by different γL liquid food. As shown in Figure3, the liquid foods all have large contact

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angle (θ* ≥ 150° and ∆θ* ≤ 10°), but the coffee (θ* = 139.6° and ∆θ* = 21°). The coffee is the

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commercial NESCAFÉ Smoovlatte, which is an aromatic blend of NESCAFÉ and milk, and it contains

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stabilizer and emulsifier that can change the Cassie state of multiscale structures.

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To demonstrate the super-hydrophobic property of the interface in practical application, common

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polystyrene cups were employed as the matrix for the super-hydrophobic interface. The cups were

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filled with high viscosity liquid food honey (Figure 4). Subsequently, upon tipping the liquid foods out,

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the cups with the super-hydrophobic interface facilitated removal of liquid food without residue. In

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contrast, the control cups without coating super-hydrophobic interface exhibited significant residual

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honey. This is due to the cups with the super-hydrophobic interface has an excellent super-hydrophobic

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(θ* = 142.6° for honey). In contrast, water contact angle θ* of honey on the surface of control cup was

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only 64.8°. The result suggested that the super-hydrophobic interface fabricated using edible material

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could effectively reduce the honey residue. The parameters of super-hydrophobic interface with

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different liquid foods were listed in Table 1.

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The bio-inspired edible super-hydrophobic interface with simple multiscale structures of

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honeycomb wax is not competent for the practical application. The small γS of honeycomb wax results

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in poor adhesion with substrates and the honeycomb wax is broken off easily from the substrate. The

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edible elastic film could effectively connect multiscale structures and the substrates to prevent the

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peeling of multiscale structures. As shown in Figure 5, a significant decrease of water contact angle θ*

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of the super-hydrophobic interface was found for the coating layer without edible elastic film after

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long-time flushing. In contrast, with the support of edible elastic film, the water contact angle θ* of the

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super-hydrophobic interface did not decrease significantly. This result suggested that the edible elastic

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film was effective to increase the compatibility between honeycomb wax and substrate.

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In order to explain the strengthening effect of the edible elastic film, the FT-IR spectra of edible

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elastic film and honeycomb wax were measured (Figure 6a). A broad and intense peak at 3200-3600

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cm−1 was attributed to the –NH2 and –OH stretching. The band at 2920 and 2849 cm−1 was attributed to

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–CH2 stretching vibrations. Typical absorption peak of amide groups appeared at 1648 cm−1 in edible

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elastic film. The band at 1737 cm−1 was associated with the stretching of the ester groups in the

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honeycomb wax. The presence of lots of hydroxyl and amidogen groups in the edible elastic film can

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form the hydrogen bonding with ester groups of honeycomb wax, and thus increase the adhesion

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between them.36-40 The binding mechanism of edible elastic film under the bio-inspired

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super-hydrophobic interface was shown in Figure 6b. The honeycomb wax interacted with the edible

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elastic film by the hydrogen bond and van der Waals force via functional groups on their surface. The

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edible elastic film achieved a high compressive stress of more than 0.18 MPa and its normal

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compressive stress is more than 50% with no-damage after compression (Figure 6c). At the same time,

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an excellent tensile strength (30 KPa) was indispensable when edible elastic film was used for practical

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applications (Figure 6d). The high energy storage modulus and tensile strength of edible elastic film

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was effective to absorb and protect the impact multiscale structures.

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For a wide range of applications, the bio-inspired edible super-hydrophobic interface was prepared

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on the flexible substrate of plastic (Figure 7a). The bio-inspired edible super-hydrophobic interface

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showed a relatively big water contact angle θ* (Figure 7b). Importantly, the water contact angle θ* of

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super-hydrophobic interface was still more than 140

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super-hydrophobic interface fabricated was successfully applied to flexible substrate. Temperature, pH

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and time stability of the super-hydrophobic interfaces were shown in Figure S2, S3 and S4, respectively.

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The results showed that such film/coating was insensitive to pH value in the solution and were stable

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for as long as 30 days. However, the contact angle θ of the edible super-hydrophobic interface reduced

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significantly when heated at 60 or 90 oC for 10 minutes.

o

after repeated folding. This suggests that the

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For the food applications, the potential cytotoxicity of the super-hydrophobic interface was

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evaluated

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dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide. Figure 8 shows that the relative cell viability is

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around 100% after the cells being incubated with the material of super-hydrophobic interface at the

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concentration as high as 15 mg/mL for 72 h. This demonstrated that the material of super-hydrophobic

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interface was not toxic to mouse osteoblasts. In addition, the gas chromatography-mass spectrometry

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analysis (Figure S5) suggested that the super-hydrophobic interface didn’t contain volatile organic

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solvent residue, such as acetone and n-hexane used for the preparation of films. This is a very

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important issue for the use of this coating technique for food.

against

mouse

osteoblasts

via

the

MTT

assay

using

3-[4,5-

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This study demonstrated a simple and low-cost method to fabricate the bio-inspired edible

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super-hydrophobic interface. The super-hydrophobic interface exhibited high θ* and low ∆θ* for water

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with an appropriate coating density on surface. The result shows that most liquid testing food has large

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contact angle on the super-hydrophobic interface, except those which contain emulsifiers. For the

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application on flexible substrates, an edible elastic film was fabricated between the honeycomb wax

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and substrate material, which was proved to be effective to enhance the adhesive force of

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super-hydrophobic interface. Although the bio-inspired edible super-hydrophobic interface displayed

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reasonable mechanical durability, the interface was able to maintain good super-hydrophobic property

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after being repeatedly folded and long-time flushed. The cytotoxicity tests indicated that there was no

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negative effect on the viability of cells in vitro. This bio-inspired edible super-hydrophobic interface

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would have applications in reducing the residue of high value-added liquid foods.

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ASSOCIATED CONTENT

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Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXXXXX.

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Water flushing test, tensile stress–strain experiment, SEM image of the lotus leaf surface,

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temperature, pH and time effects on contact angle θ of super-hydrophobic interface, gas

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chromatography-mass spectrum of the residual organic solvent analysis for super-hydrophobic

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interface.

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AUTHOR INFORMATION

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Corresponding Author

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* E-mail: [email protected] (B.-W. Z.); M. Tan, [email protected] (M. T.)

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ORCID

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Mingqian Tan: 0000000275350035

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Funding

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This work was supported by the National Natural Science Foundation of China (31601389) and the

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National Key Research and Development Project (2017YFD0400100, 2016YFD0400404).

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Notes

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The authors declare no competing financial interest.

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Figure captions:

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Scheme 1 Fabrication process of edible super-hydrophobic interface.

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Figure 1 (a) large-scale SEM images of edible super-hydrophobic interface. (b) SEM image of a papilla

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structure on the surface of botryoid structures (inset shows the HRTEM image of the surface of a

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papilla structure).

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Figure 2 Water contact angle θ* of (a) unprocessed honeycomb wax and (b) super-hydrophobic

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interface with multiscale structures. (c) Water contact angle θ* of super-hydrophobic interface with a

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different surface density of honeycomb wax.

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Figure 3 Water contact angle θ* of edible super-hydrophobic interface with the different commercial

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liquid foods, (a) Coca-Cola, (b) Lipton green tea, (c) whole milk, (d) apple juice, (e) wine, (f) beer, (g)

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Gatorade, and (h) coffee.

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Figure 4 Photographs of representative liquid food in an uncoated cup (left) and super-hydrophobic

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interface coated cup (right) before (a) and after (b) tipping honey out.

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Figure 5 Water contact angle θ of edible super-hydrophobic interface with/without the edible elastic

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film during different flushing time.

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Figure 6 (a) FT-IR spectra of honeycomb wax and edible elastic layer. (b) Enhancement mechanism of

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honeycomb wax and edible elastic film. (c) Pressure curve of the edible elastic film. Insets show the

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film before and after being pressured. (d) Tensile stress–strain curve of the edible elastic film.

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Figure 7 (a) Photograph of edible super-hydrophobic interface on a flexible substrate polypropylene. (b)

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Water contact angle θ of edible super-hydrophobic interface with a different folding frequency.

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Figure 8 Cytotoxicity of the honeycomb wax against mouse osteoblasts evaluated with MTT assay.

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Scheme 1

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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Figure 6

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Figure 7

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Cell viability (%)

120 100 80 60 40 20 0 0

5

10

15

Concentration of honeycomb wax (mg/mL) 407 408

Figure 8

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Table 1 Parameters of super-hydrophobic interface with various liquid foods.

Items

Energy Coke

Lipton tea

Whole milk

Fruit juice

Wine

Beer

Nestle coffee

Honey

drinks Contact angle θ* on the glass surface 53.4±1.8

63.5±2.1

46.3±1.8

51.0±1.1

54.5±0.9

47.8±1.3

58.1±2.4

43.7±1.1

64.8±2.1

154.2±1.5

158.4±0.8

150.6±1.2

153.9±2.5

152.1±2.3

151.4±1.6

158.1±0.9

139.6±2.4

142.6±1.9

0.878±0.10

1.180±0.24 1.483±0.317

1.066±0.092

1.181±0.103

0.881±0.107

1.327±0.132

8

5

0.007±0.00

0.038±0.01

2

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(°) Contact angle θ* on the surface of super-hydrophobic interface (°) Residual liquid food (15 g) on the glass 0.981±0.117 surface (g)

2.759±0.1 01

Residual liquid food (15 g) on the surface of super-hydrophobic interface

0.018±0.163

0.162±0.0 0.012±0.006

0.015±0.010

(g)

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0.006±0.003

0.111±0.017 39

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