Mimicking Biological Structured Surfaces by Phase-Separation

Mar 25, 2009 - In this letter, we present a very convenient and efficient technique of direct replication of biological structures via a two-step phas...
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Mimicking Biological Structured Surfaces by Phase-Separation Micromolding Jian Gao, Yiliu Liu, Huaping Xu,* Zhiqiang Wang, and Xi Zhang* Key Laboratory of Organic Optoelectronics & Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, PR China Received March 5, 2009. Revised Manuscript Received March 17, 2009 In this letter, we present a very convenient and efficient technique of direct replication of biological structures via a two-step phase-separation micromolding process (PSμM). Our study has demonstrated that PSμM can be used to replicate the surface structure of a lotus leaf. On one hand, the micro/nanostructures of the lotus leaf are well replicated after a two-step PSμM. On the other hand, the replicated artificial lotus leaf shows good superhydrophobicity, similar to that of the natural lotus leaf. In addition, we have also applied the same technique to replicate a rice leaf and have confirmed that replicated artificial rice leaves can exhibit not only a very similar structure of the natural rice leaf but also surface anisotropic wetting. It is greatly anticipated that this PSμM can be extended to mimic many other biostructures, therefore opening new avenues for surface molecular engineering.

Introduction Biological materials found in nature exhibit many fascinating biofunctionalities owing to their unique morphologies and physicochemical properties. These special biofunctionalities include the self-cleaning ability of lotus leaves,1 the anisotropic wetting properties of rice leaves,2 and the antireflective property of cicada wings,3 which are attributed to the arranged micro/nanostructures on their surfaces. Inspired by these biofunctionalities from the nature, many approaches have been developed to fabricate artificial biostructures with specific functionality.1-22 Jiang et al. fabricated a series of surfaces based on aligned carbon tubes that possessed superhydrophobility similar to that of lotus or rice leaves.2,4 Fuchs and Chi et al. demonstrated the anisotropic wetting properties of surfaces prepared by Langmuir-Blodgett *Corresponding authors. E-mail: [email protected], xi@ mail.tsinghua.edu.cn. (1) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1. (2) Feng, L.; Li, S. H.; Li, Y. S.; Li, H. J.; Zhang, L. J.; Zhai, J.; Song, Y. L.; Liu, B. Q.; Jiang, L.; Zhu, D. B. Adv. Mater. 2002, 14, 1857. (3) Watson, G. S.; Watson, J. A. Appl. Surf. Sci. 2004, 235, 139. (4) Sun, T. L.; Feng, L.; Gao, X. F.; Jiang, L. Acc. Chem. Res. 2005, 38, 644. (5) Gleiche, M.; Chi, L. F.; Fuchs, H. Nature (London) 2000, 403, 173. (6) Gleiche, M.; Chi, L. F.; Gedig, E.; Fuchs, H. ChemPhysChem 2001, 2, 187. (7) Qu, L. T.; Dai, L. M.; Stone, M.; Xia, Z. H.; Wang, Z. L. Science 2008, 322, 238. (8) Shi, F.; Wang, Z. Q.; Zhang, X. Adv. Mater. 2005, 17, 1005. (9) Shi, F.; Niu, J.; Liu, J. L.; Liu, F.; Wang, Z. Q.; Feng, X. Q.; Zhang, X. Adv. Mater. 2007, 19, 2257. (10) Zhang, X.; Shi, F.; Yu, X.; Liu, H.; Fu, Y.; Wang, Z. Q.; Jiang, L.; Li, X. Y. J. Am. Chem. Soc. 2004, 126, 3064. (11) Vukusic, P.; Hooper, I. Science 2005, 310, 1151. (12) Ma, Y.; Sun, J. Q. Chem. Mater. 2009, 21, 898. (13) Sanchez, C.; Arribart, H.; Guille, M. M. G. Nat. Mater. 2005, 4, 277. (14) Zhang, X.; Shi, F.; Niu, J.; Jiang, Y. G.; Wang, Z. Q. J. Mater. Chem. 2008, 18, 621. (15) Mitragotri, S.; Lahann, J. Nat. Mater. 2009, 8, 15. (16) Sotiropoulou, S.; Sierra-Sastre, Y.; Mark, S. S.; Batt, C. A. Chem. Mater. 2008, 20, 821. (17) Huang, J. Y.; Wang, X. D.; Wang, Z. L. Nano Lett. 2006, 6, 2325. (18) Zhang, G. M.; Zhang, J.; Xie, G. Y.; Liu, Z. F.; Shao, H. B. Small 2006, 2, 1440. (19) (a) Sun, M. H.; Luo, C. X.; Xu, L. P.; Ji, H.; Qi, O. Y.; Yu, D. P.; Chen, Y. Langmuir 2005, 21, 8978. (b) Liu, B.; He, Y. N.; Fan, Y.; Wang, X. G. Macromol. Rapid Commun. 2006, 27, 1859. (20) Xi, J. M.; Jiang, L. Ind. Eng. Chem. Res. 2008, 47, 6354. (21) Saison, T.; Peroz, C.; Chauveau, V.; Berthier, S.; Sondergard, E.; Arribart, H. Bioinsp. Biomim. 2008, 3, 046004. (22) Bhushan, B.; Jung, Y. C.; Koch, K. Langmuir 2009, 25, 3240.

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lithography.5,6 Recently, Dai and Wang et al. reported carbon nanotube arrays that mimic gecko feet with a shear adhesive force of close to 100 N 3 cm-2, almost 10 times higher than that of the foot of a gecko.7 Our group has developed a method for the fabrication of superhydrophobic surfaces on gold threads by combining the layer-by-layer assembly of polyelectrolytes with the electrodeposition of gold to mimic the legs of water striders.8-10 Most of the above-mentioned approaches focused on using fabrication techniques and synthetic nanomaterials to mimic the functionality of biological materials. However, there are only a few successful examples of the direct replication of biological structures, also known as “bioreplication”. Wang et al. reported the controlled replication of butterfly wings to achieve tunable photonic properties using a low-temperature atomic layer deposition technique.17 Liu et al. presented the fabrication of cicada winglike structure based on nanoimprint lithography.18 However, these reports often rely on expensive equipment to replicate the biological structures. Moreover, it will be helpful if the method allows for the regeneration of the biotemplates. Therefore, it is a significant challenge to develop a simple approach for the direct replication of natural biological structures and the realization of their biofunctionality. Wessling et al. first introduced phase-separation micromolding (PSμM) as a microfabrication technique to replicate the patterns from their templates.23 It relies on the phase separation of a polymer solution when in contact with a structured surface.22-26 Compared with regular replica molding (REM), which normally uses poly(dimethylsiloxane) (PDMS) as the replication material,19 PSμM is a convenient and versatile technique that can be used to structure a broad range of polymers, including block copolymers and biodegradable and conductive polymers, without the need for expensive facilities. Herein we present a convenient (23) Vogelaar, L.; Barsema, J. N.; van Rijn, C. J. M.; Nijdam, W.; Wessling, M. Adv. Mater. 2003, 15, 1385. (24) Vogelaar, L.; Lammertink, R. G. H.; Barsema, J. N.; Nijdam, W.; BolhuisVersteeg, L. A. M.; van Rijn, C. J. M.; Wessling, M. Small 2005, 1, 645. (25) Vogelaar, L.; Lammertink, R. G. H.; Wessling, M. Langmuir 2006, 22, 3125. (26) Xu, H. P.; Ling, X. Y.; van Bennekom, J.; Duan, X. X.; Ludden, M. J. W.; Reinhoudt, D. N.; Wessling, M.; Lammertink, R. G. H.; Huskens, J. J. Am. Chem. Soc. 2009, 131, 797.

Published on Web 3/25/2009

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technique for the direct replication of biostructures via the PSμM process. Our technique features a combination of (i) low fabrication costs, with no specific equipments are needed; (ii) the possibility to recycle the biotemplates for multiple use; and (iii) an ambient processing environment to replicate biological structures directly from natural materials. The morphological replications of lotus and rice leaves are demonstrated by a two-step PSμM process. In addition, the biofunctionality of such artificial lotus and rice leaves (i.e., their respective superhydrophobicity and anisotropic wetting properties) is examined.

Scheme 1. Schematic Representation of a Two-Step PSμM Processa

Experimental Section Materials. Poly(etherimide) (PEI) (Ultem 1000) was a commercial product of General Electrics. Hyflon AD 80X (a copolymer of 80 mol % 2,2,4-trifluoromethoxy-1,3-dioxole and 20 mol % tetrafluorethylene) was obtained from Solvay Solexis. SV 110, a good solvent for Hyflon AD 80X, was also from Solvay Solexis. NMP and DMF were purchased from Sinopharm Chemical Reagent Co., Ltd. Ethanol was obtained from Beijing Chemical Reagents Company, and n-pentane was purchased from Beijing Modern Eastern Fine Chemical Co., Ltd. Lotus leaves were picked from the lotus pond at Tinghua University. Rice leaves were from Yixing, Jiangsu Province, China. Method. Scheme 1 shows the two-step PSμM process. To begin, a natural biomaterial (e.g., a lotus leaf and/or a rice leaf) was used as a biotemplate. A concentrated PEI (20 wt %, unless otherwise stated) solution in NMP was cast onto a lotus leaf. Subsequently, the PEI film was solidified by immersing it in water. After a few minutes of immersion, the PEI film with negative replica of a lotus leaf was spontaneously released from the natural biotemplate. After drying, 17.5 wt % Hyflon AD 80X in SV 110 was cast onto the negative replicated PEI film. Subsequently, the precipitation-hardening of Hyflon AD 80X was achieved by immersion in n-pentane and ethanol. After detaching the Hyflon AD 80X film from the negative template, the resulting film was dried in air for further study. A positively replicated artificial lotus leaf was obtained. Characterization. The surface structures were imaged with a JEOL JSM-7401F field-emission scanning electron microscope (SEM) at 1.0 kV. Static, advancing, and receding contact angles were measured at room temperature by an optical contact angle measuring device (OCA 20, Dataphysics Instruments GmbH). A water droplet of 2 μL was used.

Results and Discussions Natural lotus leaves are the first subject of our biological replication study owing to their self-cleaning properties, as revealed by Barthlott and Neinhuis et al.1 Scheme 1 shows the experimental strategy of using a two-step PSμM process to replicate the surface structures of a lotus leaf. To begin, a concentrated poly(etherimide) (PEI) solution in 1-methyl-2-pyrrolidinone (NMP) was directly cast onto the surface of a lotus leaf attached to a glass substrate. Subsequently, the entire substrate was immediately immersed in water, a nonsolvent of PEI. By the solvent exchange between NMP and water, the structured PEI film was easily detached from the lotus leaf, giving rise to a PEI structure with a negative replica of a lotus leaf. In the second step of the PSμM process, a secondary polymer solution;Hyflon AD80 (a polymer with low surface energy) in SV 110;was cast onto the structured PEI film stuck to a glass substrate. By immersing the substrate in n-pentane (a nonsolvent of Hyflon AD80), the Hyflon AD80 film was detached from the PEI film; therefore, a positive replica structure of a lotus leaf was obtained. The natural lotus leaves, as shown in Figure 1A,B, consist of randomly distributed micropapillae with diameters of several 4366

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a (i) Negative replication of a lotus leaf by PEI, followed by PEI film detachment from the lotus leaf in water. (ii) Positive replication of a lotus leaf by casting a film of Hyflon AD 80X onto the negatively replicated PEI film as the template and immersion of the entire substrate in n-pentane. (iii) Detachment of the replicated Hyflon AD 80X film from its template.

Figure 1. SEM images of the original surface structures of a natural lotus leaf (A, B) and a PEI film with a negative replica of a lotus leaf prepared by a one-step PSμM process (C, D).

micrometers. Moreover, there are also some branchlike nanostructures on the papillae on the scale of hundreds of nanometers. As shown in Figure 1C,D, a large area of randomly distributed micropits can be clearly observed on the surface of the replicated PEI film after the first PSμM step. This showed that during the solidification process the replicated PEI has maintained the negative surface structure with respect to that of the lotus leaf template. By comparing SEM images of the natural lotus leaf and their negative replica by the PSμM process in Figure 1, it is confirmed that the resulting replicated structures agree well with the negative replication of the original surface structures of the lotus leaves. Hence, the negative structures of a lotus leaf have been successfully obtained by the first step in the PSμM process. It is also noted during the first-step PSμM process that the solvent exchange between NMP and water was so fast and efficient that the PEI film can be detached from the original lotus leaf in less than 3 min, which prevented the destruction of the lotus leaf by NMP. For the convenience of fabrication and cost-saving purposes, it is important that the biotemplates for bioreplication can be recycled for multiple replications. Hence, the original lotus leaf was repeatedly used for five replications, and its contact angle after each replication process was examined. Table 1 shows the respective static, advancing, and receding contact angles (CAs) of Langmuir 2009, 25(8), 4365–4369

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the lotus leaf template after each replication process. The results showed a negligible change in all CAs in all of the replication processes. After the fifth replication by PEI, the original lotus leaf template still exhibited good hydrophobicity, a vital feature contributing to the easy detachment of the PEI film from the lotus leaf during the replication process. In addition, in comparisons between the replicas made by the same lotus leaf template, there is no obvious change in the morphology, in particular, the micropit structure of the negative replica of the lotus leaf template (Figure 2). It should be noted that all of the SEM images above were collected randomly from different areas each time. All of the results highlight the PSμM technique as a convenient and recyclable method for replicating the biological structures. Table 1. Static and Dynamic Water Contact Angles of a Natural Lotus Leaf Biotemplate for Each PSμM Replication Cycle replication cycle

static CA (deg)

advancing CA (deg)

receding CA (deg)

0 1 2 3 4 5

152 151 151 152 147 148

152 152 149 150 150 149

150 148 148 147 148 147

To optimize the conditions for the PSμM process, we used a silicon template with 20 μm lines as a standard template. Several experimental parameters that may influence the phaseseparation process, including the good solvent and nonsolvent used during the replication process and the polymer solution concentration, have been investigated. Water was used as the nonsolvent for the PSμM process because of its biocompatibility with lotus leaves. NMP was used as a solvent to dissolve PEI while remaining miscible with water. Dimethylformamide (DMF), which is qualified as a solvent for PEI, has also been used during the negative replication process. However, the replicated structures obtained by using DMF were not very satisfactory: the corners of the protrusions are round instead of sharp, as seen in Figure 3A. However, a series of PEI solutions (dissolved in NMP) with different weight concentrations were used to replicate the silicon template. For PEI concentrations of 5, 10, and 15 wt %, defects or large shrinkage of the resulting films was found (Figure 3B-D). Although the replica of the 25 wt % solution was nearly perfect, the high viscosity and long dissolving time of the PEI solution make it an unsatisfactory recipe for practical use. We conclude that only when the percentage of PEI solution is around 20 wt % are good replication and an easy, efficient PSμM process guaranteed, as shown in Figure 3E.

Figure 2. (A-F) SEM images of the surface structures of the negative PEI replicas of a lotus leaf prepared repeatedly from the same lotus leaf template showing the replicas from the first-sixth replication cycles.

Figure 3. Negative replica of a silicon template using 18 wt % PEI in DMF solution (A), negative replicas of a silicon mold by using 5 (B), 10 (C), 15 (D), 20 (E), and 25 wt % (F) PEI in NMP solution. Langmuir 2009, 25(8), 4365–4369

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Figure 4. SEM images of the surface structures of the Hyflon AD 80X film with positive replica of a lotus leaf via a two-step PSμM process.

It would be of more interest to researchers and industry if the negative structures could be converted into the positive replicated structure, resembling that of the natural lotus leaf. To realize positive replication by using the PSμM process, we performed a second PSμM step using the formed negative structure prepared by the one-step PSμM process as the subsequent template. Because the low-surface-energy surface is one of the key factors in the self-cleaning properties of lotus leaves, perfluoropolymer Hyflon AD 80X was selected for the second replication step. A good solvent for Hyflon AD 80X is SV110, a perfluorinated solvent that does not dissolve PEI. The replication of the second step is similar to that of the first step. Immersing the film of negative structures cast with Hyflon AD 80X in n-pentane and ethanol resulted in the detachment of the Hyflon AD 80X film from the substrate. As shown in Figure 4A-C, the positively replicated Hyflon AD 80X film consists of randomly distributed micropapillae with diameters of several micrometers. In addition, nanostructures were also observed on the papillae. Therefore, we have obtained a Hyflon AD 80X film with lotus-leaf-like surface structures. The superhydrophobicity of such a replicated artificial lotus leaf is examined by contact angle measurements. Shown in Figure 5A is the 151° static CA of the replicated artificial lotus leaf, which is indicative of good superhydrophobicity. More importantly, the hysteresis between the surface advancing CA (152°) and receding CA (150°) is very small, suggesting that a uniform structured surface has been successfully fabricated. Considering that the surface wettability of the replicated artificial lotus leaf is almost the same as that of the natural lotus leaf, we conclude that we have successfully mimicked the superhydrophobicity of the natural lotus leaf by the two-step PSμM process. The superhydrophobicity of the above replicated artificial lotus leaf is further described by the Cassie-Baxter equation (i.e., cos θCB = f1 cos θ - f2, where θ is the CA on a flat surface, θCB is CA on a rough surface, and f1 and f2 are the fractions of the solid surface and air in contact with the liquid, respectively).27 The static CA of a flat Hyflon AD 80X film is 112° (Figure 5B), whereas the static CA of the replicated Hyflon AD 80X film is 151°. Therefore, the values of f1 = 0.20 and f2 = 0.80 are estimated. Our analysis showed that a large fraction of air is trapped within the interspace of the rough replicated artificial lotus leaf, which consequently enhances the apparent contact angle. To demonstrate the versatility and a broader applicability of the PSμM technique for the replication of biological structures, we have also employed a rice leaf as a template. Figure 6A,B showed the morphology of the rice leaf with orderly arranged micropapillae in the direction parallel to the edge of the rice leaf and irregularly distributed papillae in the perpendicular direction. After using PEI to replicate the rice leaf in the first-step PSμM, we have obtained the negative structures of the rice leaf, as shown in (27) Yu, X.; Wang, Z. Q.; Jiang, Y. G.; Shi, F.; Zhang, X. Adv. Mater. 2005, 17, 1289.

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Figure 5. Static contact angles of water droplets on a positively replicated artificial lotus leaf of Hyflon AD 80X (A, CA = 151°) and a flat Hyflon AD 80X surface (B, CA = 112°).

Figure 6. SEM images of the surface structures of a natural rice leaf (A, B) and its negative replica (C, D).

Figure 6C,D. One can see that there are many orderly arranged micropits in one dimension on the surface of the PEI film, which agrees well with the negative structures of the rice leaf. Similarly, we have then employed the replicated PEI film with the negative structure of the rice leaf as a template for the second step PSμM. Again, the Hyflon AD 80X was used for positive replication because of its low surface energy. As shown in Figure 7A-C, some grooves on the macroscopic scale are arranged on the surface of the Hyflon AD 80X film. In addition, the higherresolution SEM image clearly indicates the existence of microprotrusions arranged in an orderly manner in the direction parallel to the grooves and randomly in the perpendicular direction. Therefore, the PSμM technique has been successfully demonstrated for the artificial mimicking of rice leaf of anisotropic surface structure. A more interesting finding is that the replicated artificial rice leaf exhibits very good anisotropic wettability, as shown in Figure 8. The static CA measured in the direction parallel to the grooves is 160°, and that in the perpendicular direction is 138°. It should be noted that for a natural rice leaf its static CAs in the two directions are 141 and 127°, respectively. Comparing these Langmuir 2009, 25(8), 4365–4369

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Figure 7. SEM images of the surface structures of the positive replica of a rice leaf of Hyflon AD 80X via a two-step PSμM process.

static contact angles with the natural rice leaf, the PSμM process has not only successfully mimicked the surface structures of a rice leaf but the anisotropic wettability of the replicated artificial rice leaf is even better than that of the natural rice leaf. We ascribe this to the cooperation effect between the low surface energy of Hyflon AD 80X and the special anisotropic surface structure of the replicated artificial rice leaf.

Conclusions

Figure 8. Static contact angles of water droplets on a replicated artificial rice leaf of Hyflon AD 80X, as taken from the direction parallel to the grooves (A, CA = 160°) and from the direction perpendicular to the grooves (B, CA = 138°); water droplets on a natural rice leaf seen from the direction parallel to the grooves (C, CA = 141°) and from the direction perpendicular to the grooves (D, CA = 127°).

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In summary, we have presented a simple method to realize artificial lotus and rice leaves with high consistency by a two-step PSμM process. No specific equipment is needed in the PSμM process. The experiments were carried out at room temperature and normal pressure, and diverse commercially available polymers can be used for replication. It is believed that this technique can be used to replicate even smaller structures, thus the application of this technique to mimic many other biostructures is greatly anticipated. Acknowledgment. This research was funded by the National Basic Research Program of China (2007CB808000 and 2005CB724400) and the Natural Science Foundation of China (50573042).

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