Chitosan Composite Sponge with

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Stearic acid-modified starch/chitosan composite sponge with asymmetric and gradient wettability for wound dressing Chunping Su, Huiping Zhao, Hao Yang, and Rong Chen ACS Appl. Bio Mater., Just Accepted Manuscript • DOI: 10.1021/acsabm.8b00508 • Publication Date (Web): 27 Dec 2018 Downloaded from http://pubs.acs.org on January 2, 2019

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ACS Applied Bio Materials

Stearic acid-modified starch/chitosan composite sponge with asymmetric and gradient wettability for wound dressing

Chunping Su, Huiping Zhao, Hao Yang, Rong Chen*

School of Chemistry and Environmental Engineering, Key Laboratory for Green Chemical Process of Ministry of Education and Hubei Novel Reactor & Green Chemical Technology Key Laboratory, Wuhan Institute of Technology, Xiongchu Avenue, Wuhan, 430073, PR China

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ABSTRACT In this work, we developed a facile approach for the fabrication of stearic acid-modified starch/chitosan composite sponge with asymmetric wettability and gradient wettability via mediating the amount of starch. The starch/chitosan composite sponge exhibited heterogeneous wetting properties on its two surfaces after stearic acid modification. The superhydrophobic top surface with a water contact angle of 150° could prevent water, blood and bacterial permeation without losing the breathability of the sponge. While the superhydrophilic bottom surface possessed optimal water and blood absorption and clotting abilities. The asymmetric wettability of starch/chitosan composite sponge was attributed to the synergistic contribution of gradient distribution of starch, rough three-dimensional skeleton structure induced capillary effect and the non-uniform surface distribution of hydrophilic/hydrophobic

groups.

Furthermore,

the

sponge

also

possessed

good

biodegradability and non-cytotoxicity to the human normal hepatocyte. The results indicated that the stearic acid-modified starch/chitosan composite sponge with ideal liquid absorption, vapor transmission rate, self-cleaning and stain-repellent ability could be used as a promising candidate for wound dressing. KEYWORDS: Chitosan sponge, asymmetric wettability, wound dressing, self-cleaning,

biodegradability

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INTRODUCTION The wetting behavior of solid surface is very important in their practical applications1, 2. Special wettability including superhydrophobic/superoleophilic3,

4,

superhydrophilic/

superoleophobic5, superamphiphobic6, and asymmetric superhydrophobic/superhydrophilic7, 8 are demonstrated with oil/water separation, self-cleaning, photocatalytic, antibacterial, self-healing and UV-blocking performance. For example, asymmetric wettability materials with heterogeneous superhydrophobic and superhydrophilic side showed great potential application in many field owing to its special wettability performance9-11. An ideal wound dressing should exhibit good water absorption and retention properties, thus it could absorb excrescent wound exudate and retain a moist environment to accelerate wound healing. Simultaneously, the wound dressing also should allow gaseous exchange and act as a barrier to prevent contamination from external liquid. Therefore, the dressing with asymmetric surface wettability is of great interest in wound healing/engineering. The hydrophobic surface could effectively prevent external liquid including water, blood, beverages and bacteria, to contaminate the dressing. And the hydrophilic surface could preserve the comfortable, moisture environment to promote the wound healing12,

13.

In addition, the asymmetric

dressing with gradient wettability could influence the directional transportation of liquids within the materials, which can affect the water vapor transmission rate of wound dressings and further influence the healing process of the wound14-16. It is well known that the surface wettability could be rationally mediated by their surface microstructures and chemical compositions2. Much effort has been made to achieve the asymmetric surface via different strategies. For example, one hydrophilic surface was

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selectively hydrophobized via coating a low surface energy layer17. Single-faced superhydrophobicity cotton fabrics by finishing the inherent cotton with fluoropolymer composite foam18, inorganic-organic single-faced superhydrophobic thiol-ene coatings on different substrates by spray-deposition19, and 2D Janus fabrics with anisotropic wettability by coating the silica nanoparticles-heptadecafluorononanoic-fluoroalkyl silane via selective electrospraying20 were also reported. Another strategy is that one side of prepared double-side hydrophobic materials was selectively hydrophilized by external stimulus, such as light irradiation, acid or alkali treatment8, 9, 14, 16, 21-26. For instance, Kong et al. reported TiO2 nanoparticle-coated fabric with superhydrophilic/hydrophobic properties through one-step fabric treatment and controlled exposure to ‘‘light’’ and ‘‘dark’’25. The asymmetric coatings were also obtained on polyester fabric substrate via a fluoropolymer (PHFDMA) deposition method23, and cotton fabric substrate via plasma-enhanced chemical vapor deposition21. However, these methods still suffered the following drawbacks such as (1) the requirment of materials with natural superhydrophilicity, (2) strongly dependence of the characteristics of the porous substrate, the health problems cases by the photogenerated reactive oxygen species (ROS), (3) the complicated preparation process and expensive reagents, (4) the cytotoxicity, bioaccumulation and environmental toxicity of modification agent, which was not suitable for wound dressing27-29. In addition, the reported materials with asymmetric wettability were mainly filmwise materials (i.e. cotton fabric, polymer film), which possessed relatively low liquid absorbency. Although porous aerogels or sponges with three-dimensional (3D) architecture possessed highly improved liquid adsorption ability compared to filmwise materials30-33, they cannot avoid the infiltration of pathogenic

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microorganisms and wound dehydration15. Therefore, the exploration of safety and ecofriendly dressing materials with good biological compatibility, biodegradability, optimal vapor transmission rate and liquid absorption ability, self-cleaning and stain-repellent properties is highly desirable. As a widely used wound dressing, chitosan (CS) has the properties of biocompatible, biodegradable, non-toxic, low-cost, and moisture retentive. In our previous work, single face superhydrophobic silver nanoparticles (AgNPs) /chitosan composite dressing with three-dimensional porous structure has been reported34. Unfortunately, in order to achieve asymmetric wettability, the stearic acid modification of AgNPs/chitosan composite dressing should be performed on the special side of AgNPs/chitosan composite dressing under freezing. Thus, a simple method of preparing asymmetric wettability chitosan-based materials needs to be explored. Chitosan is a cationic nature polysaccharide owing to the presence of free amino groups in chitosan molecules. Its cationic nature could lead to a strong interaction with lipids with an opposite charge. Parra-Barraza et al. reported the possibility of specific sites of electrostatic bonds between chitosan chains and stearic acid molecules by simulating the interaction of chitosan chains with stearic acid molecules35. Compared to the cationic nature of chitosan, starch is a neutral polysaccharide, which interaction with stearic acid is weaker than that of chitosan. Therefore, we hope to use this feature to achieve asymmetric wettability of chitosan-based materials. Herein, we demonstrated the preparation of chitosan/starch composite sponge with asymmetric wettability via a simple vacuum freeze drying and stearic acid modification method by using chitosan and starch as raw materials. The 3D porous structure and the

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property of one side superhydrophobicity and another side superhydrophilicity endowed chitosan/starch sponge with optimal vapor transmission rate and liquid absorption ability, good self-cleaning and stain-repellent properties. To the best of our knowledge, the asymmetric and gradient wettability of chitosan sponge mediated by the amount of starch has never been reported. EXPERIMENTAL SECTION Materials Chitosan (CS, deacetylation degree ≥ 95%), corn starch (ST), alpha-amylase, and stearic acid (SA, CH3(CH2)16COOH) were purchased from Aladdin (Shanghai, China). Sodium chloride (NaCl), acetic acid (CH3COOH), sodium hydroxide (NaOH), absolute ethanol (C2H5OH), lysozyme and dimethyl sulfoxide (DMSO) was purchased from Sinopharm Chemical

Reagent

Co.

Ltd

(Shanghai,

China).

3-(4,5-dimethylthiazol-2-yl)-2,5

-diphenyltetrazolium bromide (MTT, C18H16BrN5S) were purchased from Sigma-Aldrich. All reagents were analytically pure and used without further purification. The bacteria strains of Escherichia coli (E. coli, CCTCC AB 208270) and human normal hepatocyte (L02) were obtained from China Center for Type Culture Collection (CCTCC). The bacteria strains of Staphylococcus aureus (S. aureus, ATCC 9118) were obtained from Life Sciences College of Central China Normal University. The bacteria were incubated in nutrient broth solution at 37 oC

under shaking. The bacteria were incubated and stored in the Luria-Bertani (LB) medium.

The cells were cultured in the Dulbecco’s minimum essential medium (DMEM). Preparation 1.0 g chitosan and 1.0 g corn starch (CS/ST=1:1, m/m) were added into acetic acid solution

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(50 mL, 1%, v/v) under continuous stirring at 60 oC. Then the mixture solution was poured into glass culture dish and kept at -20°C overnight to obtain the pre-frozen sample, which was lyophilized at -54 oC for 18 h by using the vacuum freeze dryer (LGJ-10) to obtain the primary chitosan/starch composite sponges. After that, the primary chitosan/starch composite sponges were immersed in 2% NaOH (w/w) solution for 2 h to neutralize the residual acetic acid, followed by rinsing with deionized water to neutral. Finally, the neutral chitosan/starch composite sponges were lyophilized again to obtain the chitosan/starch composite sponge (CS-ST1). Chitosan and other chitosan/starch composite sponges with different content of starch were prepared via the same method under the identical conditions by using different amount of starch (0.1 g, 0.5 g, 2 g), which was labelled as CS, CS-ST0.1, CS-ST0.5 and CS-ST2, respectively. The surface modified chitosan/starch composite sponges were prepared by immersing in alcohol solution of stearic acid (20 mmol/L) at room temperature for 30 min. Then the modified sponges were air-dried at room temperature to obtain the SA-modified chitosan/starch composite sponges, which was labeled as CS-S, CS-ST0.1-S, CS-ST0.5-S, CS-ST1-S and CS-ST2-S, respectively. The detailed schematic illustration of the fabrication of chitosan/starch sponges with different surface wettability was elucidated in Scheme 1.

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Scheme 1. Illustration of the fabrication of chitosan/starch sponges with different surface wettability. Characterization Scanning electron microscopy (SEM, JSM-5510LV) were utilized to characterize the morphology structure of the prepared sponges after gold sputtering. The surface chemical composition of the prepared sponges were characterized by Fourier transform infrared spectroscopy (FTIR, Nicolet Impact 420) spectra, using an attenuated total reflectance (ATR) system. The surface element of the prepared sponges were analyzed by X-ray photoelectron spectra (XPS, VG Multilab2000 spectrometer), using Al K α (1486.6 eV) radiation as the source. The static contact angles (CAs) were measured by Dataphysics OCA 20 (Germany) contact angle measuring instrument at an ambient temperature. The average CA value was obtained by measuring several different positions of the same sample. Measurement of water vapor transmission rate The water vapor transmission rate (WVTR) was measured according to ASTM E96 procedure36. The sponge was placed tightly over a cup of water and fastened with Teflon tape 8


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across the edges to prevent any water vapor loss through the boundary. The water vapor transmission rate was determined by measuring the mass loss of water in the cup after 24 h. For asymmetric wettability sponges, the vapor transmission rate of both hydrophilic and hydrophobic surface were measured by the same procedure. The WVTR (g/m2/day) was calculated with the following equation: 𝑊𝑉𝑇𝑅 =

𝑊𝑖 ― 𝑊𝑡 𝐴

g/𝑚2/day

(1)

where A, Wi and Wt is the area of the cup mouth (m2), the weight of the cup containing water and the weight of the cup containing water after 24 h, respectively. The water vapor transmission rate of the sponges after blood clotting (100 μL/cm2 blood dropped) was determined by using the same method to investigate its breathability. Measurement of water and blood absorption ability The water and blood absorption ability was measured by the weight variation of the test sponges before and after being immersed into the beaker contained with water or blood at room temperature until they were saturated with liquid. The water or blood absorption ability (Qa) was determined according to the formula (2)： Q𝑎 =

𝑚𝑎 ― 𝑚𝑜 𝑚𝑜

× 100%

(2)

where ma and mo is the mass of the sponge after water or blood absorption and the mass of the dried sponge, respectively. Blood clotting study Blood clotting investigation was performed according to the reported literature37, 38. Blood was collected from human ulnar vein and stored in anticoagulated Vacutainer containing sodium citrate (1:9, v/v). The test sponges were placed into flat-bottom bottles and

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prewarmed at 37 oC. Then 100 μL blood was dropped onto the surface of sponge, which was further incubated at 37 °C for 10 min. After that, 30 mL of deionized water was slowly pouring into the blood sponge contained bottles to avoid the interference of the clotting blood. Next, the supernatant after the centrifugation at 1000 rpm for 1 min was collected and incubated at 37 °C for 1 h. Then the absorbance of the supernatant at 540 nm was measured by utilizing an UV-vis spectrophotometer (Mapada UV-6100s, Shanghai, China). As a reference, the absorbance of 100 μL of blood in 30 mL of distilled water at 540 nm was also measured. The blood clotting index (BCI) of sponge was calculated by the equation (3). BCI =

𝑎𝑏𝑠𝑜𝑟𝑏𝑎𝑛𝑐𝑒 𝑜𝑓 𝑏𝑙𝑜𝑜𝑑 𝑤ℎ𝑖𝑐ℎ ℎ𝑎𝑑 𝑏𝑒𝑒𝑛 𝑖𝑛 𝑐𝑜𝑛𝑡𝑎𝑐𝑡 𝑤𝑖𝑡ℎ 𝑠𝑎𝑚𝑝𝑙𝑒𝑠 𝑎𝑡 540 𝑛𝑚 absorbance of 100 μL of blood in 30 mL of water at 540 nm

× 100%

(3)

Bacteria infiltration evaluation S.aureus and E. coli were used to evaluate the bacteria infiltration of prepared sponges. The sterile sponge (1 cm × 1 cm) was placed on an LB agar plate with the test surface up. Next, S.aureus and E. coli suspension (100 μL, 1×106 CFU/mL) was dropped on the surface of the tested sponge, respectively. Then the plate was incubated at 37°C for 24 h. Finally, the bacterial permeability of the sponge was evaluated by observing the growth of bacteria on the surface of the plate. In vitro cytotoxicity study In vitro cytotoxicity of the sponge was estimated by MTT assay34. The human normal hepatocyte (L02) was chosen as the study subject to evaluate the in vitro toxicity of materials according to the reported literatures33, 39, 40. L02 cells were cultured in DMEM with 10% (v/v) fetal bovine serum (FBS) at 37 °C in a humidified atmosphere with 5% CO2. The sterile sponge with a diameter of 0.5 cm and a height of 0.5 cm was placed in a well of a 12-well 10


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plate. Then L02 cells were seeded into 12-well plate at a seeding concentration of 1 × 105 cells/well. After incubation at 37 °C for 48 h, 0.5 mL, 5 mg/mL of MTT was added into the well and incubated with cells for 4 h. Subsequently, 1 mL of dimethylsulfoxide (DMSO) was added to the well and further incubated for 10 min to dissolve the formazan crystal. The cell viability was measured by the absorbance of formazan at 490 nm by using a microplate reader (Multiskan MK3). Cells free of sponge were served as the control. All experiments were done in parallel three times. Biodegradation test The biodegradation test of the CS-ST1-S sponge was performed according to the procedure described in our previous work3. Briefly, the CS-ST1-S sponges were incubated in phosphate-buffered saline (PBS, pH=7.4) solution with lysozyme and alpha-amylase (10000 U/ mL) at 37 °C, respectively. After 5, 10, and 15 days, the CS-ST1-S sponge was taken out and washed with deionized water to remove the adsorbed ions on surface. After freeze-drying, the sponge was weighed. The degradation rate (D) of CS-ST1-S sponge was calculated according to the equation (4): D = (w𝑡 ― 𝑤𝑜)/𝑤𝑜 × 100%

(4)

where wt and w0 is the weight of CS-ST1-S sponge before and after enzyme degradation. Cell attachment studies The cell attachment of the sponge was evaluated using mouse embryonic fibroblasts NIH 3T3 cells41, 42. Cells were cultured in DMEM with 10% FBS supplemented with penicillin (100 U/mL) and streptomycin (0.1 mg/mL), which was kept in T-25 flasks at 37oC in a humidified environment with 5% CO2. The sponges were seeded with NIH 3T3 cells in a

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6-well plate at a concentration of 1 × 105 cells/well. After 24 h of incubation, the sponges were washed with PBS and fixed with 2.5% glutaraldehyde for 4 h. The glutaraldehyde fixed sponges were thoroughly washed with PBS, and sequentially dehydrated in a graded ethanol series (30%, 50%, 70%, 90%, and 100%), air-dried and gold sputtered in vacuum for the scanning electron microscope characterization. RESULTS AND DISCUSSION Surface wettability of starch/chitosan sponge The wetting behaviors of the top and bottom surfaces of different chitosan/starch sponges before and after stearic acid modification were evaluated by contact angle measurements. All of the starch/chitosan sponges were superhydrophilic before stearic acid modification (Figure S1, supporting information). However, it was found that the stearic acid-modified starch/chitosan sponges exhibited different surface wettability with the variation of starch content. Figure 1a showed the surface wettability and the water contact angle (WCA) of the different sponges after stearic acid modification. In the absence of starch or in the presence of small amount of starch, the surface of both side of the prepared sponge presented hydrophobic wettability. For the CS-S sponge without starch, the water contact angle of top and bottom side is 146o and 143o, respectively. The wettability of the sponge had no obvious change with the addition of small amount of starch (0.1g), and the water contact angle of top and bottom side was 148o and 140o, respectively (CS-ST0.1-S sponge). Further increasing the amount of starch, the sponge with asymmetric wettability (one side hydrophobic and other side hydrophilic) was obtained. The water contact angle of top side of sponge was still 148o (CS-ST0.5-S) and 150o (CS-ST1-S). But the contact angle of bottom side of these two sponge 12


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dramatically decreased to 0o. Nevertheless, when the amount of starch increased to 2 g, the water contact angle of top side of the CS-ST2-S sponge was only 125o, indicative of the decrease of hydrophobic wettability, although the contact angle of bottom side was still 0o. In order to further understand the different hydrophilic wettability of bottom side, the time evolution of water absorption on the bottom side of CS-ST0.5-S, CS-ST1-S and CS-ST2-S sponges were investigated. As shown in Figure1b, the time of the water droplet being completely absorbed into the sponge was gradually shorten with the increase of starch content. It illustrated that the amount of starch play a crucial role in the asymmetric wettability of chitosan sponge. It was proposed that the addition of starch would affect the structure and surface composition of the starch/chitosan composite sponge. The gradient distribution of starch in the starch/chitosan sponge and the weaker interaction between starch and stearic acid lead to more distribution of stearic acid on the top surface of the starch/chitosan composite sponge, which resulted in the superhydrophobic wettability of top surface of the sponge. The superhydrophilic wettability of bottom surface was attributed to the synergistic contribution of the gradient distribution of starch, the capillary effect induced by rough three-dimensional skeleton structure and the non-uniform surface distribution of hydrophilic/hydrophobic groups.

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Figure 1. Water contact angle of the top and bottom side of different chitosan sponges (a), photographs of time-dependent water absorption on the bottom side of CS-ST0.5-S, CS-ST1-S and CS-ST2-S sponges (b). The surface wettability of double-side hydrophilic CS-ST1 sponge and hydrophilic/ hydrophobic asymmetric wettability CS-ST1-S sponge was further evaluated by the spreading behaviors of water droplets on the top side and bottom side of sponge, respectively. Figure 2a showed the water droplet placed on both side of the CS-ST1 sponge. A water droplet could wet and spread on both side of the CS-ST1 sponge. However, for the CS-ST1-S sponge, the water droplet could only wet and spread on the hydrophilic bottom side of sponge surface (Figure 2b). A spherical bead with a high contact angle (∼150°) stood on the hydrophobic top side surface of the CS-ST1-S sponge (Figure 2b). Moreover, the water droplet could roll off easily from the slightly tilted hydrophobic surface of CS-ST1-S sponge (Figure 2c and Video S1 in supporting information), indicative of the good water repellency of CS-ST1-S sponge. It demonstrated that the chitosan/starch composite sponge with suitable 14


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amount of starch could exhibit superhydrophobic/ superhydrophilic asymmetric wettability after modification. Besides the water wettability, the blood repellency of the CS-ST1-S sponge was also investigated. Figure 2d demonstrated the behavior of the blood dropped on the top (hydrophobic) and bottom (hydrophilic) surface of CS-ST1-S sponge. It was observed that the top surface of CS-ST1-S sponge showed high blood-repellent property. The blood droplet remained spherical on the hydrophobic surface with a blood CA of 136.2°, while it was absorbed on the hydrophilic surface. It indicated a blood-repelling self-cleaning ability of the CS-ST1-S sponge, which was arisen from its superhydrophobic property.

Figure 2. Photographs of water droplets on the top and bottom side of CS-ST1 sponge (a), CS-ST1-S sponge (b), water droplet rolling on the top side of CS-ST1-S sponge (c), and blood droplet on the top and bottom side of CS-ST1-S sponge (d). The water was labeled with RhB for clear observation. 15



Surface morphology and chemical composition As we known, the wettability is highly related to the surface morphology, topographical structure and chemical composition of materials. Hence, the surface morphology and chemical composition of the chitosan sponge before and after starch modification were investigated. The SEM images of the untreated CS, CS-ST1 sponge and SA modified CS-ST1-S sponge demonstrated that all the sponges possessed 3D porous structure for both top and bottom surface, as shown in Figure 3. It was found that the bottom side displayed a more regular morphology and porous microstructures, compared with top side of the sponge. In the absence of starch, both the top and bottom side of CS sponge possessed smooth skeleton structure. With the addition of starch, the bottom side of CS-ST1 sponge presented more rough structure than that of CS sponge (Figure 3c and 3g). As shown in Figure 3h, there are large number of crater-like structure with the width of 5-10 μm in the skeleton surface of the bottom side of CS-ST1 sponge. Figure 3i-3l showed the SEM images of the top and bottom side of SA-modified starch/chitosan sponge (CS-ST1-S). Compared with the CS-ST1 sponge, the morphology and structure of CS-ST1-S sponge did not obviously change after stearic acid modification. It still remained three dimensional porous structure with crater-like skeleton surface. Noticeably, some wrinkled laminar structure adhered to the skeleton surface of the top side of CS-ST1-S sponge, which was ascribed to the presence of stearic acid. While no obviously wrinkled laminar structure of SA layer was observed on the bottom surface. It indicated the stearic acid was mainly distributed to the upper surface of sponge. The reason could be ascribed to the degree of interaction between chitosan or starch and stearic acid, which could influence the adsorption of stearic acid in the surface of starch/chitosan

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composite

sponge.

Therefore,

it

was

proposed

that

the

rough

interconnected

three-dimensional network structure induced three-dimensional capillary effect, thus more beneficial for the capture of water molecules and the water transportation. In addition, the good moisture absorbing property of starch also could improve the hydrophilic of sponge, illustrating the synergistic contribution to the hydrophilicity of the starch-modified sponge. The water swelling ratio (SR) of different CS-based sponges was also calculated, which was summarized in Figure S2 (Supporting Information). The swelling ratio of CS-ST0.5, CS-ST1 and CS-ST2 sponges was 1872.8±51.5, 1920.4±137.4 and 1484.4±55.8%, respectively. All the swelling ratio of CS-ST sponges were much higher than that of the CS sponge (1104.2 ± 35.2%), indicating that the addition of starch also enhanced the swelling ratio of sponge.

Figure 3. SEM images of CS sponge (top side: a and b, bottom side: c and d), CS-ST1 sponge (top side: e and f, bottom side: g and h) and CS-ST1-S sponge (top side: i and j, bottom side: k and l). 17



The distribution of starch in the sponge was investigated by the color reaction of starch and iodine in different surface of the sponge. As shown in Figure 4a and 4b, when brown iodophor solution was dropped on the surface of CS sponge, no color change from the top to bottom surface of CS sponge was observed. However, when the brown iodophor solution was dropped on the surface of CS-ST1 sponge, obvious color changes could be observed from top to bottom surface through interlayer with the gradually darker blue color, indicative of the gradient distribution of starch in the CS-ST1 sponge (Figure 4c~4f). Similarly, gradual color changes also could be obviously observed from top to bottom layer when brown iodophor solution was dropped on the longitudinal section of CS-ST1 sponge (Figure S3, Supporting Information). The result further illustrated the gradient distribution of starch in the CS-ST1 sponge.

Figure 4. Photographs of color changes of the iodine dropped on different surface of the sponges: top (a) and bottom (b) surface of CS; top side (c), cross section (d, e) and bottom (f) 18


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surface of CS-ST1. Moreover, FTIR-ATR spectra was utilized to analyze the surface chemical group changes of the chitosan sponges before and after starch and stearic acid modification. Figure 5 showed the FTIR-ATR spectra of top and bottom surface of CS (a), CS-ST1 (b), CS-S (c) and CS-ST1-S (d) sponges. As a control, the FTIR-ATR spectra of chitosan, starch and stearic acid were also performed, as shown in Figure S4 (Supporting Information). As shown in Figure 5a, the top and bottom surface of CS sponge presented the same functioned groups in FTIR-ATR spectra. After the introduction of starch, no other peaks and no difference in the top and bottom surface was observed in the FTIR-ATR spectra of CS-ST1 sponge (Figure 5b). However, the peak intensity displayed obvious difference in the FTIR-ATR spectra of top and bottom surface of CS-S and CS-ST1-S sponges after stearic acid modification. As shown in Figure 5c and 5d, the intensity of absorption peaks at 2915, 2848 and 1702 cm−1 in FTIR-ATR spectra of top surface of CS-S and CS-ST1-S sponge were obviously higher than that of its corresponding bottom surface after the stearic acid modification, which could be attributed to the stretching vibration of C-H and C=O groups in stearic acid43. While the peak intensity at 3360, 1147, 1070, 1029 and 987 cm-1 in the IR spectrum of the top surface of chitosan sponge were obviously lower than that of the bottom surface, which was assigned to the N-H, O-H and C-O stretching vibration in chitosan44, 45. It illustrated that the stearic acid was mainly distributed on the top surface of sponge, which affected the exposure of other groups on the chitosan sponge surface. In addition, the relative content of stearic acid in sponge was qualitatively analyzed by comparing the peak intensity of -CHn groups in FTIR-ATR spectra of different layers (top surface, cross section and bottom surface) of

19



CS-ST1-S sponge, as shown in Figure S5 (Supporting Information). The absorption peaks at 2955, 2915 and 2848 cm-1 were assigned to the asymmetric and symmetric stretching vibration of -CHn groups of stearic acid. It was observed that the peak intensity of -CHn groups gradually decreased from top surface to bottom surface via the cross section. It indicated that the amount of stearic acid in CS-ST1-S sponge gradually decreased with the increase of starch content.

Figure 5. FTIR-ATR spectra of top and bottom surface of CS (a), CS-ST1 (b) CS-S (c), and CS-ST1-S (d). XPS spectra were further employed to characterize the surface elements composition and relative content of top and bottom surface of the sponges. Figure 6 showed the XPS survey spectra of top and bottom surface of the CS-ST1 and CS-ST1-S sponges. There is no obvious difference in the total content of carbon element of the top and bottom surface of CS-ST1 sponge (Figure 6a and 6b). After stearic acid modification, it was found that the carbon content of top surface was much higher than that of bottom surface in CS-ST1-S sponge (Figure 6c and 6d), which was in accord with FTIR-ATR results. Combined with the gradient 20


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distribution of starch in the starch/chitosan composite sponge (Figure 4), the result also indicated that more stearic acid was adsorbed on the top surface of CS-ST1 sponge owing to the stronger chitosan interaction with stearic acid than that of starch. Figure 6e showed the high-resolution C 1s spectra of the top and bottom surface of CS-ST1 and CS-ST1-S sponges. The C 1s peak of CS-ST1 sponge could be deconvoluted into four peaks at 284.6, 285.8, 286.3 and 287.7 eV, which could be assigned to the C-C, C-N, C-O and O-C-O groups, respectively. While the C 1s peak of CS-ST1-S sponge was deconvoluted into five peaks at 284.6, 285.5, 286.5, 287.9 and 288.6 eV, which could be assigned to the C-C, C-N, C-O, O-C-O and O-C=O groups, respectively. The relative contents of these groups were calculated and normalized by the peak at 284.6 eV (C-C groups), which was summarized in Table 1. Among them, C-N, C-O and O-C=O groups are hydrophilic groups. After the normalization, the total proportion of hydrophilic groups (C-N, C-O and O-C=O groups) on the top surface of CS-ST1-S sponge (0.63) was much lower than that of the top surface of CS-ST1 sponge (1.25) and the bottom surface of CS-ST1-S sponge (1.93). It indicated that more stearic acid was distributed on the top surface of CS-ST1-S sponge, which reduced the exposure of surface hydrophilic groups on the top surface, resulting in the hydrophobicity of the top surface of CS-ST1-S sponge. Based on these, it was believed that the gradient distribution

of

starch,

the

rough

three-dimensional

network

structure

induced

three-dimensional capillary effect and the asymmetric distribution of hydrophilic groups and hydrophilic nature of native starches synergistically contributed to the asymmetric wettability of CS-ST1-S sponge.

21



Figure 6. XPS survey spectra of the top surface of CS-ST1 (a), the bottom surface of CS-ST1 (b), the top surface of CS-ST1-S (c), the bottom surface of CS-ST1-S (d), and the high-resolution XPS spectra of C 1s of samples (e). Table 1 C 1s component analysis

* The relative contents of these groups in C 1s spectrum is normalized by the peak area at 284.6 eV. 22


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Vapor transmissibility, liquid (water/blood) absorption and clotting ability The wound dressing with optimal vapor transmission rate and liquid absorption ability could prevent dehydration and excessive buildup of exudates, and control the water loss to create an environment with an ideal moisture level for wound healing. Figure 7a showed the vapor transmission rate of different sponges. The vapor transmission rate of the hydrophilic CS-ST1 sponge, was determined to be 2090.8 g/m2/day, indicative of high vapor transmission rate. After stearic acid modification, the vapor transmission rate of hydrophilic and hydrophobic surface of the CS-ST1-S sponge was 1526.8 and 1463.9 g/m2/day, respectively. Although the vapor transmission rate value decreased slightly after stearic acid modification, it was reported that the desired water vapor transmission rate values in the range of 1000-2500 g/m2/day would be sufficient to give adequate moisture and prevent wound dehydration36,

46, 47.

It was believed that the 3D porous structure and the asymmetric

wettability endowed CS-ST1-S sponge with optimal water vapor transmission rate. Moreover, the water vapor transmission rate of hydrophilic and hydrophobic surface of the CS-ST1-S sponge still reached 957.1 and 1100.2 g/m2/day even after blood clotting, respectively. In the absence of starch, the vapor transmission rate value of CS-S was obviously lower than that of CS-ST1-S. It illustrated that the CS-ST1-S sponge could be an ideal candidate for the wound dressings. Figure 7b showed the water and blood absorption ability of the hydrophilic CS-ST1 sponge, hydrophilic/hydrophobic asymmetric wettability CS-ST1-S sponge, and hydrophobic CS-S sponge. The water and blood absorption ability of hydrophilic CS-ST1 sponge was determined to be 1920 ±137% and 1898 ±153%, respectively. After the stearic acid

23



modification, the water and blood absorption ability of the CS-ST1-S slightly decreased to 1532 ± 95% and 1258 ±89%, which still was suitable capacity for wound dressing. However, the hydrophobic CS-S sponge almost lost its water and blood absorption ability in the absence of starch. Such low water and blood absorption ability of CS-S sponge would lead to the accumulation of exudates, resulting in the deceleration of healing process and the risk of bacterial growth. Additionally, an ideal wound dressing should not only possess the optimal vapor transmission rate and liquid absorption ability, but also possess the hemostatic potential. Thus, the hemostatic ability of the CS-ST1-S sponge was assessed by the blood clotting test. The clotting of the blood on the hydrophilic/hydrophobic asymmetric wettability CS-ST1-S sponge was evaluated by the obtained BCI value (Figure 7b). A higher BCI value indicates a slower blood clotting rate. The CS-ST1-S sponge exhibited a BCI of 25%, which was lower than that of the hydrophobic CS-S sponge (80%), indicating that the hydrophilic/hydrophobic asymmetric wettability CS-ST1-S sponge also possessed good hemostatic ability. For comparison, the water vapor transmission rate, absorption ability, and BCI values of CS-ST1-S sponge and other reported wound dressings are listed in Table 2. The CS-ST1-S sponge not only exhibited the asymmetric wettability, but also possessed optimal vapor transmission rate and absorption ability and clotting ability, compared with other reported wound dressings materials48.

24


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Figure 7. Water vapor transmission rate of different chitosan sponges before and after blood clotting (a): CS-ST1 sponge (i), hydrophilic surface (ii) and hydrophobic surface (iii) of CS-ST1-S sponge, and CS-S sponge (iv); Water and blood absorption ability and BCI value of different chitosan sponges (b): CS-ST1 sponge (i), CS-ST1-S sponge (ii), and CS-S sponge (iii). Table 2. Comparison of properties of various wound dressings Materials

CS-ST1-S sponge SiO2 NPs/ethyl-α-cyanoacrylate sprayed cotton fabrics Fluoropolymer coated cotton fabrics Waterborne polyurethane /poly(N-vinylpyrrolidone) composite film Chitosan/polyethylene glycol fumarate blend film Chitosan/poly-L-lactic acid blend membranes

Water vapor transmission rate (g/m2/d) 1526.8 (HL) a 1463.9 (HB) 3.86 × 105 (HL) 4.25 × 105 (HB) 1866 (HL) 1963 (HB)

Absorption ability (%) water: 1532 blood: 1258 water: 979 blood: 779

BCI (%)

Whether possessing asymmetric wettability

Ref

25

yes

this work

74

yes

[7]

water: 80

—b

yes

[18]

1816-2728

water: 21-158

—

no

[36]

904-1447

water: 122-305

—

no

[46]

2637

water: 121

—

no

[48]

a: HB: hydrophobic surface, HL: hydrophilic surface. b: “—” the data was not provided in the literature.

Bacteria infiltration property In the practice application, traditional wound dressings are easily contaminated by 25



environmental liquids (such as blood, bacteria and virus) owing to its surface hydrophilicity, which usually resulted in wound infection. Therefore, the bacteria infiltration properties of the prepared CS-ST1 and CS-ST1-S sponge were evaluated. Figure 8 demonstrated the bacteria infiltration of the CS-ST1 and CS-ST1-S sponge for the Gram-negative E.coli and Gram-positive S. aureus. The bacteria suspension could remain a spherical droplet on the hydrophobic surface of the CS-ST1-S sponge when it was dropped on the surface of the sponge, whereas the suspension was quickly absorbed by the hydrophilic surface of CS-ST1 sponge. The bacterial suspension on the hydrophobic surface still could not permeate the CS-ST1-S sponge after 24 h of incubation, and no bacteria grew on the agar plate when the CS-ST1-S sponge was taken away. Nevertheless, the bacteria suspension infiltrated into the CS-ST1 sponge, resulting in the growth of large numbers of bacteria on the covered area of the agar plate. It illustrated that the CS-ST1-S sponge with asymmetric wettability possessed the excellent bacterial infiltration resistance ability.

Figure 8. Photographs of bacteria infiltration properties of CS-ST1and CS-ST1-S sponges before and after 24 h incubation with E.coli (a) and S.aureus (b). 26


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Cytotoxicity and biodegradability Moreover, the human normal hepatocyte cells (L02) viability incubated with the prepared CS-ST1 and CS-ST1-S sponge was also evaluated, as shown in Figure 9a. After 48 h incubation, the cell viability of CS-ST1 and CS-ST1-S sponge was 98.5 % and 93.2 % for L02 cell grows, respectively. It indicated that the starch/chitosan composite sponge before and after stearic acid modification exhibited no cytotoxicity toward L02 cells. In addition, we also investigated the biodegradability of CS-ST1-S sponge. Figure 9b showed that 12% and 25% of the CS-ST1-S sponge was degraded after immersing in lysozyme and alpha-amylase solution for five days, respectively. With further prolonging degradation time, the degradation rate of the CS-ST1-S sponge in the enzyme solution also gradually increased, which could reach to 25% in lysozyme solution and 47% in alpha-amylase solution after 15 days. It indicated that CS-ST1-S sponge was an environmental-friendly biomaterial due to its renewable resource, facile preparation approach and good biodegradability in practice application.

Figure 9. Cytotoxicity toward human normal hepatocyte cells (a) and biodegradability (b) of CS-ST-S sponges.

27



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Cell attachment studies Generally, an ideal wound dressing material should retain a moist environment at the wound interface and allow gaseous exchange to accelerate wound healing, as well as act as a barrier to prevent contamination from microorganisms and remove excess exudates. Simultaneously, it also should be non-toxic, non-allergenic, nonadherent in the sense that it can be easily removed without trauma49. It was reported that the superhydrophilic and superhydrophobic surfaces were not conducive to cell attachment50,

51.

Therefore, the cell

attachment of the CS-ST1-S sponge was evaluated, which showed that the cells were attached on the scaffolds of sponge with an ellipsoidal morphology after 24 h of incubation (Figure 10). The ellipsoidal morphology indicated the moderate or weak attachment between cell and sponge, which would lower the risk of wound adhesion40. Hence, it was believed that the CS-ST1-S sponge was an ideal material for wound dressing.

Figure 10. SEM images of cell attachment on the bottom side of CS-ST1-S sponge (a, b).

CONCLUSIONS In conclusion, a starch/chitosan composite sponge was prepared via a facile approach by mediating the amount of starch, which possessed asymmetric wettability after stearic

28


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acid-modified modification. The starch/chitosan composite sponge exhibited heterogeneous wetting behaviors on its superhydrophobic top surface and superhydrophilic bottom surface after modification. The superhydrophilic surface showed high water and blood absorption ability and optimal vapor transmission rate even after blood clotting, and the superhydrophobic surface could prevent blood and bacterial permeation and exhibit a self-cleaning and strain- repellent ability. It was found that the gradient distribution of starch, the rough three-dimensional network structure induced three-dimensional capillary effect and the nonuniform distribution of hydrophilic/hydrophobic groups caused by stearic acid modification synergistically contributed to the asymmetric wettability of the starch/chitosan composite sponge. Furthermore, the asymmetric starch/chitosan composite sponge possessed good biodegradability and non-cytotoxicity to the human normal hepatocyte cells. It is believed that the stearic acid-modified starch/chitosan composite sponges with asymmetric wettability are expected to have a wide range of applications in wound dressing.

ASSOCIATED CONTENT Supporting Information Additional detail information including water contact angle of the top and bottom side of different chitosan sponges before stearic acid modification; the swelling ratio of different chitosan based sponges; photograph of color changes of the iodine dropped on the longitudinal section of CS-ST1 sponge; the FTIR-ATR spectra of chitosan, starch, stearic acid and the different surfaces of CS-ST1-S sponge. Video S1: water droplets roll on the hydrophobized surface of CS-ST1-S sponge (AVI). This material is available free of charge

29



via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Corresponding author: Prof. R. Chen, E-mail: [email protected]; Tel: (+86)13659815698; fax: (+86)2787195680 ORCID： Rong Chen: 0000-0003-1455-5093 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by Department of Education of Hubei Province under the project of Science and Technology Innovation Team of Outstanding Young and Middle-aged Scientists (T201606) and the National Natural Science Foundation of China (21601141). REFERENCES (1) Feng, X. J.; Jiang, L. Design and Creation of Superwetting/Antiwetting Surfaces. Adv. Mater. 2006, 18, 3063-3078. (2) Wen, L.; Tian, Y.; Jiang, L. Bioinspired Super-Wettability from Fundamental Research to Practical Applications. Angew. Chem. Int. Ed. 2015, 54, 3387-3399. (3) Su, C. P.; Yang, H.; Zhao, H. P.; Liu, Y. L.; Chen, R. Recyclable and Biodegradable Superhydrophobic and Superoleophilic Chitosan Sponge for the Effective Removal of Oily Pollutants from Water. Chem. Eng. J. 2017, 330, 423-432. (4) Su, C.; Lu, Z.; Zhao, H.; Yang, H.; Chen, R. Photoinduced Switchable Wettability of 30


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Chitosan Composite Sponge with

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