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Biostructure-like Surfaces with Thermally Responsive Wettability Prepared by Temperature-Induced Phase Separation Micromolding Jian Gao, Yiliu Liu, Huaping Xu,* Zhiqiang Wang, and Xi Zhang* Key Lab of Organic Optoelectronics & Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing, 100084, P. R. China Received January 19, 2010. Revised Manuscript Received February 9, 2010 We present a very efficient and convenient approach to obtain smart biosurfaces by directly replicating biological surface structures. It is realized by a two-step replication process combining regular replica molding and tempetatureinduced phase separation micromolding (PSμM). The negative replicas of biological surface structures using poly(dimethylsiloxane) as the replication material are durable molds for further replication. The positive replicas of biological surface structures are obtained by the second step replication using PSμM of poly(N-isopropylacrylamide) aqueous solution, which can be easily carried out just by adjusting temperature. With cold water as good solvent and hot water as nonsolvent, an environmentally friendly PSμM process is successfully achieved, and organic solvents for PSμM are completely avoided. Our study has demonstrated that the micro- and nanostructures of the lotus leaf and rice leaf can be well replicated using this two-step replication process, and the replicated artificial lotus leaf and rice leaf using poly(N-isopropylacrylamide) exhibit good thermally responsive wettability.
Introduction Biological surfaces in nature usually exhibit many fascinating functionalities owing to their unique morphologies and physicochemical properties, such as the self-cleaning ability of lotus leaves,1 the anisotropic wettability of rice leaves,2 and the antireflection property of cicada wings,3 etc. Inspired by the biofunctionalities in the nature, many efforts have been done to directly replicate the biological surface structures, which is known as “bioreplication”. For example, the surface structures of butterfly wing, cicada wing, and rose petal have been well replicated by several groups.4-6 Poly(dimethylsiloxane) (PDMS) has been widely used in regular replica molding (REM) for its simple operation and chemical stability.7 Recently PDMS has also been used in the direct replication of lotus leaf and butterfly wing.8-11 Usually, biology can adapt the changes of environment based on the smart surface structures. Learning from nature, many fundamental researches about the fabrication of surfaces with stimuli-responsive wettability have been done, e.g., thermally controlled wettability of a liquid crystalline polymer,12 reversibly *Corresponding authors. E-mail:
[email protected] (H.X.);
[email protected] (X.Z.). (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) Huang, J. Y.; Wang, X. D.; Wang, Z. L. Nano Lett. 2006, 6, 2325. (5) Zhang, G. M.; Zhang, J.; Xie, G. Y.; Liu, Z. F.; Shao, H. B. Small 2006, 2, 1440. (6) Xi, J. M.; Jiang, L. Ind. Eng. Chem. Res. 2008, 47, 6354. (7) Xia, Y. N.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 550. (8) Sun, M. H.; Luo, C. X.; Xu, L. P.; Ji, H.; Qi, O. Y.; Yu, D. P.; Chen, Y. Langmuir 2005, 21, 8978. (9) Liu, B.; He, Y. N.; Fan, Y.; Wang, X. G. Macromol. Rapid Commun. 2006, 27, 1859. (10) Saison, T.; Peroz, C.; Chauveau, V.; Berthier, S.; Sondergard, E.; Arribart, H. Bioinspiration Biomimetics 2008, 3, 046004. (11) Bhushan, B.; Jung, Y. C.; Koch, K. Langmuir 2009, 25, 3240. (12) de Crevoisier, G.; Fabre, P.; Corpart, J. M.; Leibler, L. Science 1999, 285, 1246. (13) Lahann, J.; Mitragotri, S.; Tran, T.; Kaido, H.; Sundaram, J.; Choi, I. S.; Hoffer, S.; Somorjai, G. A.; Langer, R. Science 2003, 299, 371. (14) Prins, M.W. J.; Welters, W. J. J.; Weekamp, J. W. Science 2001, 291, 277.
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switching surface using an electric field,13,14 temperature-induced reversible wettability of poly(N-isopropylacrylamide) (PNIPAAm) grafted surface,15,16 light-driven motion of liquids on a photoresponsive surface,17 and stimuli-responsive surface controlled by pH and UV.18-23 However, the directly replicated biological surface structures which show stimuli-responsive wettability were rarely reported. Phase separation micromolding (PSμM) was first introduced by Wessling et al. as a microfabrication technique to structure a broad range of polymers.24 It relies on the phase separation of a polymer solution between good solvent and nonsolvent while contacting with the structured mold.24,25 In our previous work, we have used a two-step PSμM to replicate the biology surface (15) Liang, L.; Feng, X. D.; Liu, J.; Rieke, P. C.; Fryxell, G. E. Macromolecules 1998, 31, 7845. (16) Sun, T. L.; Wang, G. J.; Feng, L.; Liu, B. Q.; Ma, Y. M.; Jiang, L.; Zhu, D. B. Angew. Chem., Int. Ed. 2004, 43, 357. (17) Ichimura, K.; Oh, S. K.; Nakagawa, M. Science 2002, 298, 1624. (18) Feng, C.; Zhang, Y. J.; Jin, J.; Song, Y. L.; Xie, L. Y.; Qu, G. R.; Jiang, L.; Zhu, D. B. Langmuir 2001, 17, 4593. (19) Feng, X.; Feng, L.; Jin, M.; Zhai, J.; Jiang, L.; Zhu, D. J. Am. Chem. Soc. 2004, 126, 62. (20) (a) Lim, H. S.; Han, J. T.; Kwak, D.; Jin, M.; Cho, K. J. Am. Chem. Soc. 2006, 128, 14458. (b) Lim, H. S.; Kwak, D.; Lee, D. Y.; Lee, S. G.; Cho, K. J. Am. Chem. Soc. 2007, 129, 4128. (21) Wang, S. T.; Liu, H. J.; Liu, D. S.; Ma, X. Y.; Fang, X. H.; Jiang, L. Angew. Chem., Int. Ed. 2007, 46, 3915. (22) (a) Jiang, Y. G.; Wang, Z. Q.; Yu, X.; Shi, F.; Xu, H. P.; Zhang, X.; Smet, M.; Dehaen, W. Langmuir 2005, 21, 1986. (b) Yu, X.; Wang, Z. Q.; Jiang, Y. G.; Shi, F.; Zhang, X. Adv. Mater. 2005, 17, 1289. (c) Jiang, Y. G.; Wang, Z. Q.; Xu, H. P.; Chen, H.; Zhang, X.; Smet, M.; Dehaen, W.; Hirano, Y.; Ozaki, Y. Langmuir 2006, 22, 3715. (d) Wan, P. B.; Jiang, Y. G.; Wang, Y. P.; Wang, Z. Q.; Zhang, X. Chem. Commun. 2008, 44, 5710. (e) Jiang, Y. G.; Wan, P. B.; Smet, M.; Wang, Z. Q.; Zhang, X. Adv. Mater. 2008, 20, 1972. (f) Wan, P. B.; Wang, Y. P.; Jiang, Y. G.; Xu, H. P.; Zhang, X. Adv. Mater. 2009, 21, 4362. (g) Chen, X. X.; Gao, J.; Song, B.; Smet, M.; Zhang, X. Langmuir 2010, 26, 104. (23) (a) Zhang, X.; Shi, F.; Niu, J.; Jiang, Y. G.; Wang, Z. Q. J. Mater. Chem. 2008, 18, 621. (b) Gao, L. C.; McCarthy, T. J.; Zhang, X. Langmuir 2009, 25, 14100. (24) Vogelaar, L.; Barsema, J. N.; van Rijn, C. J. M.; Nijdam, W.; Wessling, M. Adv. Mater. 2003, 15, 1385. (25) (a) Vogelaar, L.; Lammertink, R. G. H.; Barsema, J. N.; Nijdam, W.; Bolhuis-Versteeg, L. A. M.; van Rijn, C. J. M.; Wessling, M. Small 2005, 1, 645. (b) Vogelaar, L.; Lammertink, R. G. H.; Wessling, M. Langmuir 2006, 22, 3125. (c) 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 02/17/2010
DOI: 10.1021/la100256b
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Scheme 1. Schematic Representation of a Two-Step Replication Process: (I) Negative Replication of Rice Leaf by PDMS; (II) Positive Replication of PDMS Template by PNIPAAm in Hot Water; (III) Detachment of the Replicated PNIPAAm Film from the Template
structures.26 Not only the surface structures of lotus and rice leaves have been well replicated but also the superhydrophobicity and anisotropic wetting of the resulting replicas have been well realized. In this work, we report on the successful fabrication of the biostructure-like surfaces with thermally responsive wettability via a two-step replication process that combines REM and temperature-induced PSμM. First, PDMS is used to replicate the biology surface structures. As a negative replica, the PDMS template can be recycled in the following PSμM process. Second, a new PSμM method with only temperature variation of water is developed to replicate the negative PDMS replicas using PNIPAAm. The obtained PNIPAAm films with lotus and rice leaves structures show thermally responsive wettability.
Experimental Section Materials. PDMS silicone rubber SYLGARD-184 and curing reagent were obtained from Dow Chemical. PNIPAAm was a commercial product of Polysciences, Inc. Lotus leaves were picked from the lotus pond of Tsinghua University. Rice leaves were picked from Yixing, Jiangsu Province, China. Method. The two-step replication process of the biological surface structure is shown in Scheme 1, taking the replication of rice leaf as an example. To begin, a natural rice leaf was used as the original biotemplate in the first step replication. The mixture of PDMS and curing agent (100:15, mass proportion) was casted onto the surface of the lotus leaf. After solidification at 70 °C for 12 h, the PDMS negative replica was peeled off. And then, the PDMS replica was used as the negative template in the second step replication. A concentrated PNIPAAm aqueous solution (25 wt %, 20 °C) was casted onto the PDMS negative replica of biological surface structures. Subsequently, the PNIPAAm film was solidified by immersing it into hot water (55 °C). After a few minutes of immersion, the PNIPAAm film together with PDMS mold were taken out from the hot water and then dried in an oven at 55 °C for 24 h under reduced pressure. During the drying process, the PNIPAAm film was detached from PDMS mold. Thus, a positive PNIPAAm replica of rice leaf was obtained. Characterization. The surface structures of the replicas were observed by a field emission scanning electron microscope (JSM7401F, JEOL) at 3.0 kV. Static contact angles were measured on a heating stage by an optical contact angle measuring device (OCA 20, Dataphysics Instruments GmbH). A water droplet of 2 μL was used. The temperature of heating stage was controlled by a (26) Gao, J.; Liu, Y. L.; Xu, H. P.; Wang, Z. Q.; Zhang, X. Langmuir 2009, 25, 4365.
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Figure 1. SEM images of the surface structures of natural lotus leaf (A), negative PDMS replica of lotus leaf (B), natural rice leaf (C), and negative PDMS replica of rice leaf (D). digital temperature controller (PolySciene). When detecting the temperature-dependent wettability of the replicas, the PNIPAAm films were heated in the air in situ.
Results and Discussion Natural lotus leaf is well-known for its self-cleaning property, as presented by Barthlott and Neinhuis et al.1 To demonstrate our idea, natural lotus leaf was first selected for our biological replication study. The experimental process of a two-step replication to replicate the biology surface structures is shown in Scheme 1. To begin, PDMS, a kind of transparent and elastic silicon-based polymer, was employed to replicate a natural lotus leaf. PDMS is usually used in REM to replicate the surface structure. Before cross-linking, PDMS prepolymer and its curing agent are viscous liquids. After the cross-link reaction, PDMS is solidified and the surface structures of the biotemplate can be easily replicated. The surface structures of natural lotus leaf are shown in Figure 1 A. As seen from the image, there are many micropapillae with the diameter of about several micrometers randomly distributed on the leaf surface. And some branchlike structures about hundreds of nanometers grow on the papillae. After the first step replication, a large amount of randomly distributed micropits can be clearly seen on the surface of negative PDMS replica of lotus leaf, as shown in Figure 1B, which indicates that the PDMS replica has successfully maintained the negative surface structure of the lotus leaf during the solidification process. By comparing SEM images of the natural lotus leaf with its negative PDMS replica by REM process, one may conclude that the resulting negative replicated structures agree well with the original surface structures of the lotus leaf. To demonstrate the versatility of this technique in our biological replication study, natural rice leaf was selected as another biotemplate owing to its anisotropic wetting property. Figure 1C shows the morphology of a rice leaf, from which microprotrusions are orderly arranged in the parallel direction of the grooves of the rice leaf while irregularly distributed in the perpendicular direction. After using PDMS to replicate the rice leaf in the first step replication, the negative structures of the rice leaf are obtained, as shown in Figure 1D. There are lots of orderly arranged micropits along the grooves of the surface of PDMS replica, which agree well with the original surface structures of the rice leaf. Hence, the negative structures of the lotus leaf rice leaf have been successfully obtained by the first step replication of REM process. Langmuir 2010, 26(12), 9673–9676
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Figure 2. SEM images of the surface structures of the negative PDMS replica of lotus leaf with different scales (A-C) and the positive PNIPAAm replica of lotus leaf with different scales (D-F).
PNIPAAm has been widely studied for its thermal responsibility below and above the lower critical solution temperature (LCST) about 31-33 °C. It shows apparent variation in solubility below and above LCST.27 It is soluble in cold water below the LCST due to the expansion of the polymer chains with the existence of intermolecular hydrogen bond between CdO or N-H and H2O. Above the LCST, the polymer chains are shrinkaged due to the intramolecular hydrogen bond between CdO and N-H of PNIPAAm itself. PNIPAAm changes to be insoluble in hot water above the LCST since the hydrophobic groups are facing outside to contact with the surrounding water molecules. Therefore, with cold water as good solvent and hot water as nonsolvent, it is hoped that PNIPAAm replicas of the biological surface structures with reversible wetting property can be successfully fabricated by the PSμM process. To prove our idea, the resulting negative PDMS replicas of the lotus leaf and rice were used as templates in the second step replication of PSμM process. A concentrated PNIPAAm aqueous solution (25 wt %) was directly casted onto the surface of PDMS replica of biological surface structures. Subsequently, the PNIPAAm film with PDMS replica was immersed into hot water (55 °C), which can be considered as a nonsolvent for PNIPAAm. Through solidification in hot water, PNIPAAm can replicate the surface structures of negative PDMS replica of the biotemplate. Thus, a positive replica of biological surface structures using PNIPAAm can be obtained. The replication of PDMS negative replica of lotus leaf using PNIPAAm was first carried out. As shown in Figure 2D-F, there are many micropapillae with diameters about several micrometers randomly distributing on the positive PNIPAAm replica of lotus leaf. It should be noted that all of the SEM images were collected randomly from different areas, and some nanostructures were also observed on the papillae. By comparing with the surface structures of negative PDMS replica in Figure 2A-C, it can be clearly seen that the surface structures of the replicated artificial lotus leaf agree very well with the negative PDMS template. Therefore, we have obtained a PNIPAAm film with lotus-leaf-like surface structures. This environmental friendly process demonstrates a very good example of temperature-induced PSμM which combines the good solvent and nonsolvent as one solvent by utilizing the reversible solubility property of PNIPAAm. (27) (a) Heskins, M.; Guillet, J. E.; James, E. J. Macromol. Sci., Chem. 1968, A2, 1441. (b) Okubo, M.; Ahmad, H. Colloid Polym. Sci. 1995, 73, 817. (c) Lin, S.; Chen, K.; Liang, R. Polymer 1999, 40, 2619. (d) Masaro, L.; Zhu, X. X. Prog. Polym. Sci. 1999, 24, 731. (e) Percot, A.; Zhu, X. X.; Lafleur, M. J. Polym. Sci., Polym. Phys. 2000, 38, 907.
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Figure 3. Static contact angles of water droplets on a replicated artificial lotus leaf using PNIPAAm (A: 50 °C; B: 20 °C) and a flat PNIPAAm surface (C: 50 °C; D: 20 °C).
Besides utilizing the reversible solubility of PNIPAAm in water to carry out temperature-induced PSμM, we are wondering if we can use the transition between hydrophilicity and hydrophobicity of PNIPAAm to realize the thermally responsive wettability of the biostructure-like surfaces. Temperature-dependent contact angle measuring was employed to study the thermally responsive wettability of the replicated artificial lotus leaf using PNIPAAm. Static contact angle of the replicated artificial lotus leaf using PNIPAAm changes from 95 ( 5° at 50 °C to 61 ( 9° at 20 °C. One can clearly see that the replicated artificial lotus leaf of PNIPAAm indeed exhibits a good thermally responsive wettability. As a control experiment, static contact angle of a flat PNIPAAm film varies between 64 ( 9° at 50 °C and 48 ( 3° at 20 °C. This enhancement of the thermally responsive wettability can be attributed to the special rough surface structures of the replicated artificial lotus leaf using PNIPAAm. It should be noted that the surface structure of the PNIPAAm replicas is stable at high temperature, but unstable at low temperature, because the PNIPAAm can be dissolved by water at low temperature, which will flatten the surface structure of the PNIPAAm films. For such reason, the temperature-induced wettability of PNIPAAm replicas cannot be repeated many times. To demonstrate the versatility and applicability of such a temperature-induced PSμM technique for the replication of biological surface structures, the negative PDMS replica of rice leaf is employed as a template to carry out PSμM using PNIPAAm in the second step replication. We used the similar DOI: 10.1021/la100256b
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Figure 4. SEM images of the surface structures of the negative PDMS replica of rice leaf with different scales (A-C) and the positive PNIPAAm replica of rice leaf with different scales (D-F).
lotus leaf and rice leaf are uniform PNIPAAm films, with no pores generated during the solvent exchange process. One more interesting result is that the replicated artificial rice leaf exhibits good thermally responsive anisotropic wettability. As shown in Figure 5, the static contact angle of the artificial PNIPAAm rice leaf at 50 °C measured along the parallel direction of the grooves is 119 ( 10°; it decreases dramatically to 77 ( 9° at 20 °C. While in the perpendicular direction, the contact angle is 87 ( 9° at 50 °C and decreases to 49 ( 6° at 20 °C. The thermally responsive anisotropic wettability can be ascribed the cooperation effect between the thermally responsibility of PNIPAAm and the special anisotropic surface structure of the replicated artificial rice leaf.
Conclusions Figure 5. Static contact angles of water droplets on a replicated artificial rice leaf using PNIPAAm, as taken from the direction parallel to the grooves (A: 50 °C; B: 20 °C) and from the direction perpendicular to the grooves (C: 50 °C; D: 20 °C).
operation process as described for the replication of lotus leaf. Figure 4 shows the SEM images of the surface structures of the negative PDMS replica of rice leaf and the positive PNIPAAm replica of rice leaf. As shown in Figure 4D-F, several grooves in some macroscopic scale are arranged on the surface of the PNIPAAm film. In addition, higher resolution SEM images clearly show that the microprotrusions is orderly arranged in the parallel direction of the grooves and randomly distributed in the perpendicular direction. These surface structures agree with the morphologies of the negative PDMS replica of rice leaf. Therefore, the replication of the anisotropic surface structure of rice leaf is successfully demonstrated by the temperature-induced PSμM technique. It should be noted that the replicated artificial
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In summary, we have presented a simple approach to obtain the artificial lotus and rice leaves with thermally responsive wettability prepared by combining REM and temperature-induced PSμM. Negative PDMS replicas of biological surface structures can be ideal and stable templates in PSμM process. With cold water as good solvent and hot water as nonsolvent, a new PSμM method by only utilizing temperature variation has been developed. Our research indicates the artificial lotus and rice leaves using PNIPAAm have shown very interesting thermally responsive wettability. It is greatly anticipated that this temperature-induced PSμM approach can be extended to mimic more biostructures with stimuli responsivity and open new avenues for smart surface engineering. Acknowledgment. This work was funded by the Natural Science Foundation of China (20944001, 20904028), the DFGNSFC Transregio SFB (TRR61), Tsinghua University Initiative Scientific Research Program 2009THZ02230, and the National Basic Research Program of China (2007CB808000).
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