Biobased Wrinkled Surfaces Induced by Wood Mimetic Skins upon

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Biobased Wrinkled Surfaces Induced by Wood Mimetic Skins upon Drying: Effect of Mechanical Properties on Wrinkle Morphology Hironori Izawa,* Noriko Okuda, Arisu Moriyama, Yuka Miyazaki, Shinsuke Ifuku, Minoru Morimoto, and Hiroyuki Saimoto* Graduate School of Engineering, Tottori University, 4-101 Koyama-Minami, Tottori 680-8550, Japan S Supporting Information *

ABSTRACT: We previously developed biobased wrinkled surfaces based on wood mimetic skins in which microscopic wrinkles were fabricated on a chitosan film by immersion in a phenolic acid solution, horseradish peroxidase-catalyzed surface reaction, and drying. Here, we prepared a diverse range of wrinkled films by immersion treatment at 30, 40, 50, and 60 °C in p-coumaric acid and then investigated the correlation between wrinkle morphology and mechanical properties. Wrinkle wavelengths gradually decreased as the immersion temperature increased as well as the previous report. In order to clarify the mechanisms responsible for the different wrinkle morphologies, the films were subjected to elastic moduli measurement and GPC analysis after immersion treatment. These experiments provided evidence that the chitosan around the film surface decomposed along with the immersion process. The decomposition was accelerated by higher immersion temperature, suggesting that higher temperatures led to the formation of softer skins, inducing smaller wrinkles. In fact, wrinkle morphologies with this system were predominately determined by the hardness of the wood mimetic skins. This phenomenon is consistent with the fundamentals of surface wrinkling in nature. This study is the first to demonstrate that artificial wrinkling triggered by water evaporation can be controlled by precise control of the surface hardness of soft material.

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films. Wrinkle wavelength and amplitude could be controlled by the choice of a phenolic acid (ferulic acid, FE, or caffeic acid, CA) and immersion temperature. In common wrinkled surfaces, the wavelength (λ) of the wrinkle is dependent on skin thickness (d) and the mechanical properties of the film are described as follows4,19

INTRODUCTION There is a growing and urgent need for sustainable materials and technologies due to environmental concerns and depletion of fossil resources.1 Biomimetic systems that imitate the designs of nature are key technologies in the progress toward environmentally benign and high-performance materials.2,3 Surface-wrinkling is a ubiquitous physical process that creates macro/microscopic wrinkles in nature.4,5 This spontaneous process is the result of inhomogeneous changes triggered by internal stresses and swelling/shrinking of tissue layers possessing different elastic moduli.5 Nature-mimetic surface designs with surface-property gradients have been used for nano/microscopic wrinkled surfaces in optical6 and electronic devices,7 the realization of tunable wettability8 and adhesion,9 and the synthesis of cell culture scaffolds.10 Basically, a skin layer is fabricated on a soft support via dry processing methods, including chemical vapor deposition,11 photo-cross-linking,12 and UV/O3 oxidation.13 The wrinkling event can be caused/ controlled by mechanical stress,13 thermal expansion,11 and/or swelling−shrinking.10,14,15 Wrinkling/buckling upon drying is ubiquitous in nature.5,16 However, few studies have reported wrinkled surfaces produced by self-organization of a polymeric material upon drying.14,15 We previously developed a wood-inspired surface wrinkling system (Figure 1).17 Wood mimetic skins were fabricated by immersing chitosan (CS) film in phenolic acid (PH) solutions, and a subsequent horseradish peroxidase (HRP)-catalyzed surface reaction.18 The wrinkles appeared upon drying the © 2016 American Chemical Society

⎛ Es̅ ⎞1/3 λ = 2πd⎜ ⎟ ⎝ 3Ef̅ ⎠

(1)

where E̅ is the plane-strain modulus given by E/(1 − ν ), the subscripts s and f refer to the skin layer and the foundation (substrate), respectively, E is the elastic modulus, and ν is Poisson’s ratio. In addition, the amplitude (A) of the wrinkle is described as follows19 2

⎛ε ⎞1/2 A = d⎜ − 1⎟ ⎝ εc ⎠

(2)

where ε and εc are the applied and critical strain, respectively. In addition, εc is dependent on E̅ as follows:19 εc =

2/3 1 ⎛ 3Ef̅ ⎞ ⎜ ⎟ 4 ⎝ Es̅ ⎠

(3)

Received: September 9, 2016 Revised: October 20, 2016 Published: November 8, 2016 12799

DOI: 10.1021/acs.langmuir.6b03330 Langmuir 2016, 32, 12799−12804

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Figure 1. Illustration of the wood-inspired surface wrinkling system used in this study. Preparation of the CS Film. CS (2.0 g) was dissolved in 100 mL of an acidic aqueous solution containing 0.5 mL of acetic acid. Then, 10 mL of the CS solution was added to a Teflon Petri dish (ϕ = 50 mm) and degassed under reduced pressure. The CS solution was heated at 50 °C for 24 h to yield a CS film after evaporation. This was subsequently heated at 50 °C under reduced pressure for 12 h. The inhomogeneous edge of the films was cut down with scissors. The weight and thickness of the CS film were ca. 0.15 g and 106 ± 12 μm, respectively. Surface Wrinkling of CS Films. In a typical experiment, a CS film was immersed in 20 mL of methanol containing 0.05 g/mL CO at 30 °C for 24 h. The resulting films are hereafter referred to as CO/CS film. The CO/CS film was removed and subsequently soaked in 10 mL of water, followed by prompt addition of the HRP enzyme (1 mL, 137 U) and H2O2 (200 μL, 30% concentration). The system was kept at 30 °C for 12 h, after which the film was removed and dried at 40 °C, 50% relative humidity, for 12 h. Film Drying under Stress. The skin-CO/CS film prepared through the immersion process at 30 °C was cut into a rectangular shape (4.5 cm × 1.0 cm). The film was then clamped, a weight (12.5, 32.5, or 62.5 g) was added to one end, and the film was hung for 12 h in air (50% relative humidity) at 40 °C. Tensile Tests of the PH/CS Films. The CO/CS films were dried under reduced pressure at 25 °C. Stress−strain curves were measured for rectangular shape samples (4.5 cm × 1.0 cm) at a cross-head speed of 1 mm min−1 with a gage length of 3.0 cm. The elastic moduli (stress/strain) were calculated from the straight-line portions of the stress−strain curves.

The change in Poisson’s ratio by cross-linking of a polymeric material is not large.20,21 The thickness of wood mimetic skins produced by the HRP-catalyzed reaction does not change by the conditions of the immersion process because the thickness of the skin is determined by the diffusion depth of HRP from the film surface. Thus, the differences in molecular structure of the phenolic acid and the immersion temperature create the significant difference in mechanical properties. However, the relationship between the mechanical properties and wrinkle morphologies in our system has not been explored. Here, we show the effect of the mechanical properties of the films on surface wrinkling induced by the mimetic skin system. A range of wrinkled films were prepared by immersion treatments at 30, 40, 50, and 60 °C with p-coumaric acid (CO). We then investigated the correlation between the elastic moduli of the CO/CS films and the wrinkle wavelengths and amplitudes. In addition, wrinkle-CO/CS films dried under diverse uniaxial external stresses were prepared, and the effect of the drying stress on the wrinkle wavelengths and amplitudes was investigated.

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MATERIALS AND METHODS

Materials. CS (Mn, 6.4 × 104; Mw/Mn, 2.03; GPC analysis with Pullulan standards) was supplied from Koyo Chemical Co., Ltd., Tottori, Japan) with an undeacetylated 23.5% fraction of CS (elemental analysis). CO was purchased from Tokyo Chemical Industry Co., Ltd., Tokyo, Japan. HRP (274 U/mg) was purchased from Toyobo Co., Ltd., Osaka, Japan. Other reagents used were commercial grade and used without further purification. Instrumentation. SEM micrographs were recorded by a JSM6700F (JEOL, Japan). The sample was coated with an approximately 2 nm layer of Pt by an ion sputter coater. The wrinkle amplitudes of the wrinkle-CO/CS films were obtained with a NanoCute-NanoNavi IIs (Seiko Instruments, Japan). Elastic moduli were measured with a universal testing instrument (AG-10KNX; Shimadzu, Japan). Mn and Mw/Mn of CS and CO/CS films were measured by gel permutation chromatography (GPC) at 40 °C in acetate buffer solution eluent: Asahipak GS-220 HQ, Asahipak GS-320 HQ, Asahipak GS-520 HQ, and Asahipak GS-2G 7B (Shodex, Japan), a pump L-2130, and an RIdetector L-2490 (Hitachi, Japan). The flow rate was 0.5 mL/min.

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RESULTS AND DISCUSSION Figure 2A shows plane-view SEM images of the surface of the wrinkled films. Detailed characterization of the wrinkles formed is provided in Figure 2B. Wrinkling under treatments at 30, 40, and 50 °C was similar to the FE/CS systems;17 the mean wrinkle wavelength and amplitude were 1.53 ± 0.19, 0.90 ± 0.21, and 0.39 ± 0.10 μm, respectively, and 0.30 ± 0.03, 0.23 ± 0.06, and 0.17 ± 0.04 μm, respectively. Interestingly, nanoscale dimples were observed on the treatment at 60 °C. The mean dimple wavelength and amplitude were 59 ± 21 and 59 ± 4 nm, respectively. Under treatment at 60 °C, the wavelength was overlapped with the amplitude where the dimple was formed. 12800

DOI: 10.1021/acs.langmuir.6b03330 Langmuir 2016, 32, 12799−12804

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Figure 2. Plane-view SEM images of wrinkled films obtained with CO (the wrinkle-CO/CS, films) using different immersion temperatures (from left, 30, 40, 50, and 60 °C) (A) and the mean wrinkle wavelength (□) and amplitude (△) of wrinkles (B).

Figure 3. Elastic moduli of the CO/CS films (A) and their correlation with wrinkle wavelength (B) and amplitude (C).

Similar morphological changes from a wrinkle to a dimple have been reported.22,23 The conditions of the immersion process, including choice of temperature, strongly affect wrinkle wavelength and amplitude. Thus, changes in the mechanical properties might occur before skin fabrication. Therefore, the elastic moduli of the CO/CS films were measured by tensile test. Figure 3A shows the elastic moduli of the CO/CS films treated at 30, 40, 50, and 60 °C. The elastic moduli of the films gradually decreased with the increase in immersion temperature. Parts B and C of Figure 3 show correlations between the elastic moduli and wrinkle wavelength and amplitude, respectively. Both wavelength and amplitude clearly decreased with a decrease in elastic moduli, strongly suggesting that the phenomenon causing the decreased elastic moduli involved decreased wrinkle wavelength and amplitude. This correlation was also shown in the previously reported FE/CS and CA/CS systems (Figure S1). It was considered that the decreased elastic moduli of the CO/CS films were caused by the decomposition of CS during the immersion in CO−methanol solutions because of its carboxyl group. The decomposition of the CO/CS film was confirmed by GPC analysis. Note that the CO/CS films were completely dissolved in acetate buffer solution (eluent). Figure 4A shows the GPC chromatograms of the CS film and the CO/ CS films treated at 30 and 60 °C. The chromatograms of the CO/CS films were not substantially different from those of the CS film, suggesting that decomposition of the CS film occurred only around the surfaces. Indeed, the elastic moduli of polymeric materials estimated by the tensile tests were clearly decreased by surface decomposition.24−27 Therefore, surface

samples were collected from the film surfaces by scratching with a razor (ca. 10 μm depth), and these were applied to the measurement (Figure 4B). The retention time of the surface sample of the CS film was ca. 15.5 min (Mn = 32000, Mw/Mn = 4.2), which was not substantially different than that for the CS film. In contrast, the retention times of the surface sample of the CO/CS films treated at 30 and 60 °C were delayed at ca. 17.1 min (Mn = 17000, Mw/Mn = 2.2) and ca. 17.8 min (Mn = 9500, Mw/Mn = 2.9), respectively. These results clearly indicate that there was remarkable decomposition of the film surfaces during the immersion process, and that decomposition was accelerated at higher immersion temperatures. We further analyzed a deeper scratched sample (20 μm) (Figure S2), in which an undecomposed CS fraction was observed, indicating the surface decomposition occurred at less than 20 μm depth around the film surface. Thus, we considered that the treatments at higher temperatures likely led to the formation of softer skins to induce smaller wrinkles (Figure 5). Although the surface hardness of the wrinkle-CO/CS films could not be evaluated, formation of the softer skins was confirmed by using the CA/CS system (Figure S3) in which the surface hardnesses of the wrinkle-CA/CS films were gradually decreased as the immersion temperature was increased. Softer skins generally induce smaller wrinkles because of eqs 1, 2, and 3. This is consistent with our results. On the other hand, the dimple formation shown on the immersion treatment at 60 °C can be explained by the decreased internal stress on the softened skin. The wrinkles are observed at high overstress described as follows22 12801

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Indeed, polysaccharide materials having higher elastic moduli cause stronger aggregation upon drying.28,29 The skin-CO/CS film under treatment at 30 °C was clamped, and a weight was added to one end; the film was then hung for 12 h in air at 40 °C. The drying stress by weight was 0.19, 0.47, or 0.89 MPa, respectively. Figure 6A shows plane-view SEM images of the obtained films. Detailed characterization of the wrinkles formed is provided in Figure 6B. In all cases, anisotropic wrinkles were observed corresponding with the direction of the applied stress, indicating that the direction of the internal compression stress generated upon drying was controlled by application of the uniaxial external stress (Figure 6C). Higher ordered wrinkles were obtained with application of 0.47 and 0.89 MPa than that with 0.19 MPa. The mean wrinkle wavelengths under 0, 0.19, and 0.47 MPa of drying stress were 1.53 ± 0.19, 1.13 ± 0.11, and 0.90 ± 0.05 μm, respectively; thus, wavelength was slightly decreased with increases in stress. On the other hand, the mean wrinkle amplitudes under 0, 0.19, and 0.47 MPa of drying stress were 0.30 ± 0.03, 0.32 ± 0.05, and 0.35 ± 0.07 μm, respectively, slightly increased with increases in stress. We conjecture that the generated internal compression stress was increased under increased drying stress. In contrast, the mean wrinkle wavelength under 0.89 MPa was increased to 1.25 ± 0.16 μm, but the amplitude was decreased to 0.24 ± 0.03 μm as compared to that of 0.35 ± 0.07 μm under 0.47 MPa. This was due to the plastic deformation of the film by the larger drying stress because the yield stress of the film was 0.53 MPa (Figure S4). Although the wavelength and amplitude were changed by varied drying stress, the degree of variation was small compared to the changes caused by immersion temperature, indicating that wrinkle wavelength and amplitude were predominately determined by the mechanical properties of the films even under drying stress.

Figure 4. GPC chromatograms of the CS film and CO/CS films under 30 and 60 °C immersion (A), and their surface samples (B).

σ ≫1 σc

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(4)

where σ and σc are the applied and critical stress, respectively. In contrast, the dimples are observed at low overstress described as follows:22 σ ≥1 σc

(5)

σc is described as follows:19,22 σc =

2/3 Es̅ ⎛ 3Ef̅ ⎞ ⎜ ⎟ 4 ⎝ Es̅ ⎠

CONCLUSIONS

We investigated the effect of mechanical properties on biobased wrinkled surfaces induced by wood mimetic skins. Diverse wrinkles were fabricated on the surface of chitosan (CS) films with an easy procedure that involved immersion in p-coumaric acid (CO) solution at 30, 40, 50, or 60 °C, a horseradish peroxidase (HRP)-catalyzed reaction, and drying. The wrinkle wavelengths and amplitudes of the wrinkle-CO/CS systems gradually decreased with treatment at increased immersion temperatures. We determined that this phenomenon was caused by the difference in the hardness of the skin. The CS film surface was decomposed during the immersion process, and film surfaces that were more decomposed under higher immersion temperatures formed softer skin by the HRPcatalyzed reaction. We also investigated the effect of drying stress on wrinkle morphology. The wrinkle wavelengths and amplitudes did not show a drastic change upon drying stress, even though the wrinkle direction was highly controlled. These results signified that the wrinkle wavelength and amplitude in this system are predominately determined by skin hardness.

(6)

σc is decreased on the softer skin because of eq 6. However, the decreased σc does not lead to the dimple formation because of eqs 4 and 5. Therefore, we suppose that the generated internal stress upon drying (σ) was decreased as the skin was softened to form the dimple on the immersion treatment at 60 °C, and that σ/σc was predominately determined by σ in this system.

Figure 5. Illustration of plausible mechanisms for the production of smaller wrinkles under treatment at higher immersion temperatures. 12802

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Figure 6. Plane-view SEM images of the wrinkle-CO/CS film (30 °C immersion) dried under external stress (0.19, 0.47, and 0.89 MPa from left) (A) and the correlation between the drying stress and the mean wrinkle wavelengths (□) or amplitudes (△) (B). Illustration of surface wrinkling under drying stress (C). (4) Genzer, J.; Groenewold, J. Soft matter with hard skin: From skin wrinkles to templating and material characterization. Soft Matter 2006, 2, 310−323. (5) Ionov, L. Biomimetic 3D self-assembling biomicroconstructs by spontaneous deformation of thin polymer films. J. Mater. Chem. 2012, 22, 19366−19375. (6) Ohzono, T.; Suzuki, K.; Yamaguchi, T.; Fukuda, N. Tunable Optical Diffuser Based on Deformable Wrinkles. Adv. Opt. Mater. 2013, 1, 374−380. (7) Lee, S. G.; Kim, H.; Choi, H. H.; Bong, H.; Park, Y. D.; Lee, W. H.; Cho, K. Evaporation-Induced Self-Alignment and Transfer of Semiconductor Nanowires by Wrinkled Elastomeric Templates. Adv. Mater. 2013, 25, 2162−2166. (8) Li, Y. Y.; Dai, S. X.; John, J.; Carter, K. R. Superhydrophobic Surfaces from Hierarchically Structured Wrinkled Polymers. ACS Appl. Mater. Interfaces 2013, 5, 11066−11073. (9) Davis, C. S.; Martina, D.; Creton, C.; Lindner, A.; Crosby, A. J. Enhanced Adhesion of Elastic Materials to Small-Scale Wrinkles. Langmuir 2012, 28, 14899−14908. (10) Zhao, Z. Q.; Gu, J. J.; Zhao, Y. N.; Guan, Y.; Zhu, X. X.; Zhang, Y. J. Hydrogel Thin Film with Swelling-Induced Wrinkling Patterns for High-Throughput Generation of Multicellular Spheroids. Biomacromolecules 2014, 15, 3306−3312. (11) Bowden, N.; Brittain, S.; Evans, A. G.; Hutchinson, J. W.; Whitesides, G. M. Spontaneous formation of ordered structures in thin films of metals supported on an elastomeric polymer. Nature 1998, 393, 146−149. (12) Chen, C. M.; Reed, J. C.; Yang, S. Guided wrinkling in swollen, pre-patterned photoresist thin films with a crosslinking gradient. Soft Matter 2013, 9, 11007−11013. (13) Efimenko, K.; Rackaitis, M.; Manias, E.; Vaziri, A.; Mahadevan, L.; Genzer, J. Nested self-similar wrinkling patterns in skins. Nat. Mater. 2005, 4, 293−297. (14) Huraux, K.; Narita, T.; Bresson, B.; Fretigny, C.; Lequeux, F. Wrinkling of a nanometric glassy skin/crust induced by drying in poly(vinyl alcohol) gels. Soft Matter 2012, 8, 8075−8081. (15) Rizzieri, R.; Mahadevan, L.; Vaziri, A.; Donald, A. Superficial wrinkles in stretched, drying gelatin films. Langmuir 2006, 22, 3622− 3626. (16) Xiao, H.; Chen, X. Modeling and simulation of curled dry leaves. Soft Matter 2011, 7, 10794−10802. (17) Izawa, H.; Okuda, N.; Ifuku, S.; Morimoto, M.; Saimoto, H.; Rojas, O. J. Bio-based Wrinkled Surfaces Harnessed from Biological Design Principles of Wood and Peroxidase Activity. ChemSusChem 2015, 8, 3892−3896. (18) Izawa, H.; Miyazaki, Y.; Ifuku, S.; Morimoto, M.; Saimoto, H. Fully Biobased Oligophenolic Nanoparticle Prepared by Horseradish Peroxidase-catalyzed Polymerization. Chem. Lett. 2016, 45, 631−633. (19) Chung, J. Y.; Nolte, A. J.; Stafford, C. M. Surface Wrinkling: A Versatile Platform for Measuring Thin-Film Properties. Adv. Mater. 2011, 23, 349−368.

This is consistent with the theory of surface wrinkling in nature.4 This study is the first to demonstrate that artificial wrinkling triggered by water evaporation can be controlled by precise control of the surface hardness of soft material. We found surface wrinkling upon drying is an effective method for fabricating wrinkled materials from biopolymers. The control of the surface hardness of biopolymeric materials by chemical and/or supramolecular approaches30 will provide various wrinkled biomaterials applicable for cell culture scaffolds and biological adhesives.31

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b03330. Additional information about the correlation with wrinkle wavelength and amplitudes in the FE/CS and CA/CS films, GPC analysis of the CO/CS film, hardness of the wrinkled films, and stress−strain curve of the skinCO/CS film (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by MEXT KAKENHI Grant Number 16K05916. Professor Hiroki Sakaguchi, Professor Hiroyuki Usui, and Professor Yasuhiro Domi at Tottori University are acknowledged for their provision of the VK9710 and DUH-211S systems and their assistance with the measurements.

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REFERENCES

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DOI: 10.1021/acs.langmuir.6b03330 Langmuir 2016, 32, 12799−12804