Reversibly Cross-Linkable Thermoresponsive Self-Folding Hydrogel

Mar 27, 2015 - ranging from simple tubes to complex centipede-like structures. The demonstrated approach opens new perspectives for the design of 3D...
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Reversibly Cross-Linkable Thermoresponsive Self-Folding Hydrogel Films Yaoming Zhang and Leonid Ionov* Leibniz Institute of Polymer Research Dresden, Hohe Str. 6. D-01069 Dresden, Germany ABSTRACT: This paper reports a novel approach for the design of self-folding films using reversibly cross-linkable thermoresponsive polymers with coumarin groups: poly(N-isopropylacrylamide-co-7-(2methacryloyloxyethoxy)-4-methylcoumarin). We demonstrated that, depending on the structure of the films and the conditions of crosslinking/de-cross-linking, one can fabricate a variety of different forms ranging from simple tubes to complex centipede-like structures. The demonstrated approach opens new perspectives for the design of 3D self-assembling materials.

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INTRODUCTION Self-folding polymer films are a special kind of polymeric actuators,1−4 which are able to considerably change their shape in response to environmental signals. In other words, selffolding films are able to fold and unfold when, for example, temperature or pH is changed.5−9 Self-folding films offer a number of advantages and were recently used for various applications, including controlled encapsulation and release of particles and cells,10 microfabrication,11−15 design of biomaterials,16,17 and swimmers.18 The main prerequisite for the folding behavior is inhomogeneity of the polymeric film. The films can have a bilayer structure or/and be patterned.10,19 Bilayers are typically prepared by sequential deposition of polymers with different properties. The bilayers are commonly cross-linked by UV light, and the shape of the resulting structures is determined by the 2D shape of the film.11 Patterned films are prepared by multistep procedures.15 In one approach, polymer films with defined shape were first prepared by photolithography. In the next step, the crosslinking density of the polymer was locally increased by additional illumination with UV light through another photomask.15 In another approach, homogeneous hydrogel film was prepared by cutting of a cross-linked hydrogel. The film was swollen in another monomer with addition of photoinitiator. UV illumination through a photomask resulted in localized generating of an interpenetrating network of hydrogel with another polymer.14,20 Typically, permanent photo-cross-linking of polymers was used for preparation of structured self-folding films. In this paper we make a step forward and develop self-folding films based on reversibly cross-linkable polymers. For our approach we used reversible coupling of coumarin units (Figure 1), which occurs upon irradiation with light at 365 nm. The decoupling occurs upon irradiation with 254 nm UV light.21−23 This reversible reaction was already successfully applied for the design of self-healing polymers, shape memory polymers, and drug delivery.21−27 The use of reversible cross-linking, as it is shown in this work, allows for © 2015 American Chemical Society

Figure 1. Scheme of synthesis of photo-cross-linkable monomer 7-(hydroxyethoxy)-4-methylcoumarin (MAEMC).

the fabrication of various kinds of self-folding films with different folding behavior.

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EXPERIMENT

Materials. 7-Hydroxyl-4-methylcoumarin (HMC), ethylene carbonate, triethylamine (TEA), acryloyl chloride, N,N-dimethylformamide (DMF), potassium carbonate, and methyl methacrylate (MMA) (Sigma-Aldrich) were used without purification. N-Isopropylacrylamide (NIPAM) was recrystallized from hexane. 2,2′-Azobis(2methylpropionitrile) (AIBN, Fluka) was recrystallized from acetone. Synthesis of the Monomers and the Polymers. The coumarin monomer was synthesized as described in refs 25 and 28 (Figure 1). First, 7-hydroxy-4-methylcoumarin (0.05 mol) and ethylene carbonate (0.05 mol) were dissolved in 40 mL of DMF. Potassium carbonate (0.1 mol) was added to the mixture. Then, the mixture was stirred for 10 h at 100 °C in an argon atmosphere. The product was precipitated in cold water and was recrystallized twice from ethyl acetate. As a result, pure 7-(hydroxyethoxy)-4-methylcoumarin (0.043 mol, HMC) was obtained. Second, HMC (0.01 mol) and TEA (0.02 mol) were dissolved in DMF (20 mL) and cooled in ice bath. A solution of acryloyl chloride Received: January 23, 2015 Revised: March 5, 2015 Published: March 27, 2015 4552

DOI: 10.1021/acs.langmuir.5b00277 Langmuir 2015, 31, 4552−4557

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Table 1. Composition According to Ratio between Monomers in Polymerization Mixture and NMR Results; LCST of the P(NIPAM-MAEMC) Copolymers mole ratio of monomer

1 H NMRa

LCST (°C)

P(NIPAM-MAEMC)-1 P(NIPAM-MAEMC)-2 P(NIPAM-MAEMC)-3 P(NIPAM-MAEMC)-4

1:100 1:75 1:50 1:40

1:85.05 1:59.13 1:42.27 1:36.51

29.5−30.0 28.0−28.8 28.0 23.0−26.0

RESULTS AND DISCUSSION

We incorporated coumarin in poly(N-isopropylacrylamide) (PNIPAM) and obtained reversibly photo-cross-linkable poly(N-isopropylacrylamide-co-7-(2-methacryloyloxyethoxy)-4methylcoumarin) (PNIPAM-MAEMC) with different compositions (Table 1). The presence of peaks at δ = 7.50, 6.88, and 6.12 ppm in the 1H NMR spectrum confirmed the successful synthesis of the P(NIPAM-MAEMC) copolymers. The real composition of the polymers, as it was revealed by NMR, is slightly different from the feed ratio of monomers (Table 1) which is most probably because of different copolymerization constants. The typical molecular weight of the obtained polymers was around Mn = 46 000 (PDI = 2.85) as measured by GPC. It is very important to note that the thermoresponsive properties of P(NIPAM-MAEMC) copolymers depend on their composition and that the LCST (lower critical solution temperature) decreases with the increasing amount of coumarin fragments (Table 1). This observation is fully consistent with our previous findings, demonstrating that the introduction of hydrophobic moieties in the polymer chain decreases the LCST.31−33 Next we investigated the photoresponsive properties of the obtained polymers in aqueous solutions. As shown in Figure 2a, the absorbance peak at 320 nm, which corresponds to the pyrone ring, decreased gradually with the increasing time of irradiation with 365 nm UV light. This observation indicates the dimerization of side coumarin groups that leads to crosslinking of the polymer chains. The equilibrium state is achieved approximately after 60 min of irradiation. Exposure of the polymer solutions to 254 nm UV light results in the increase of the absorbance peak at 320 nm (Figure 2b) which indicates decoupling of the dimerized coumarin fragments and de-crosslinking of the polymer. The equilibrium is achieved after 40 min. We repeated the irradiation of polymers with 365 and 254 nm UV light many times and observed reversible change of the absorption spectra (not shown). The reversible change of the absorbance spectra upon irradiation with UV light with different wavelengths thus indicates the reversibility of the photoreaction that provides a possibility for fabrication of reversibly cross-linkable polymer films. Next we investigated the reversibility of photo-cross-linking of polymer films caused by the coupling/decoupling of coumarin groups. Apparently, films of cross-linked polymers are stable in water both above and below LCST because of the insolubility of the polymers at these conditions. The films of un-cross-linked polymers are expected to be stable in water only above LCST because the polymers are insoluble at these

MAEMC:NIPAM polymer

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a

The ratio between MAEMC and NIPAM was calculated from 1H NMR (integral of signals at2.39 and 4.0 ppm were used, respectively). (0.02 mol) in DMF (10 mL) was added dropwise into the cold mixture for 0.5 h. The mixture was further stirred at room temperature. The reaction was stopped after 12 h, and the precipitate was removed by filtration. The obtained solution was concentrated by rotary evaporation. Finally, the residue was recrystallized in ethanol twice to obtain the 7-(2-methacryloyloxyethoxy)-4-methylcoumarin (MAEMC). 1H NMR (DMSO-d6, 500 MHz): 7.67 (d, 1H), 7.00 (m, 2H), 6.34 (d, 1H), 6.22 (m, 2H), 5.98 (d, 1H), 4.48 (t, 2H), 4.37 (t, 2H), 2.39 (s, 3H). Benzophenone acrylate (BA) and copolymer of methyl methacrylate with benzophenone acrylate P(MMA-BA) 3% were prepared as described earlier.29 P(NIPAM-MAEMC) and P(MMA-BA) copolymers with different composition were synthesized by radical polymerization using AIBN as initiator. For example, PNIPAMMAEMC-3 was synthesized as follows: NIPAM (5.658 g, 50 mmol), MAEMC (0.262 g, 1 mmol), and AIBN (0.05 g, 0.3 mmol) were mixed in 25 mL of ethanol in a flask. The reaction was purged with nitrogen for 30 min. Then the reaction was carried out at 70 °C for 24 h. Following, the copolymer was precipitated by pouring the solution in diethyl ether. Lastly, the product was dried in vacuum oven at 40 °C for 20 h. Fabrication of the Polymer Films. The polymer films were prepared using combination of dip-coating and photolithography.29,30 The thickness of the polymer films was controlled by the concentration of the solution and the dip-coating speed. Bilayer films were fabricated by sequential deposition of P(NIPAM-MAEMC) (from ethanol) and P(MMA-BA) (from toluene). The obtained films were irradiated through a photomask either with 254 nm (2000 μW cm−2) or 365 nm (2300 μW cm−2) UV light for 60 min. Finally, non-crosslinked polymer was removed by rinsing in dichloromethane. Characterization. A Zeiss LSM 780 NLO microscope was used for the investigation of the self-folding behavior. The Specord M40 UV−vis spectrophotometer was used to measure the absorbance. A Herolab NU-15 UV mercury lamp with two wavelengths (254 and 365 nm) was used for the irradiations. A Multiscope-Optrel ellipsometer was used for measuring the thickness of the films. A Zeiss NEON 40 scanning electron microscope was used for investigation of the morphology.

Figure 2. Change of UV−vis adsorption spectra of the P(NIPAM-MAEMC)-3 upon the irradiation with (a) 365 nm and (b) 254 nm UV light. 4553

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the polymer remains if the amount of coumarin is minimal (≈1% of coumarin) compared to more than 70% in the case of polymer with maximal content of coumarin (≈2.5% of coumarin). Thus, irradiation with 365 nm UV light results in cross-linking of the polymers, and the cross-linking efficiency increases with the increase of the fraction of coumarin fragments. Irradiation of the film with 254 nm UV light, which was immersed in water, leads to de-cross-linking of polymers, which is soluble in the water. As a result, the thickness of the polymer films decreases. The amount of residual polymer after this step depends on the amount of coumarin. All polymers except for one with maximal content of coumarin are completely removed. The thickness of the polymer with the maximal fraction of coumarin was reduced slightly (5−10% with respect to the thickness after cross-linking). Thus, considering both the value of LCST and the reversibility of photoreaction, P(NIPAM-MAEMC)-3 was chosen for the fabrication of self-folding films: it has LCST slightly higher than room temperature, can be considerably cross-linked by 365 nm UV light, and completely de-cross-linked by irradiation with 254 nm UV light. The dependence of swelling properties on illumination conditions was investigated using in situ ellipsometry. The P(NIPAM-MAEMC)-3 with initial thickness of 126 nm in dry state swelled to 515 nm in water at room temperature. Irradiation with 254 nm UV light led to rapid increase of the thickness to 530 nm. Switching off the 254 nm light and irradiation with 365 nm UV light resulted in slowing down of the swelling. We observed that irradiation with 254 nm led always to acceleration of the swelling and switching off of the illumination, as well as illumination with 365 nm UV light, resulted in a slowdown. We explain the observed behavior by decrease of the cross-linking density after illumination with 254 nm UV light, which increases the swelling degree. The reversibly cross-linkable P(NIPAM-MAEMC)-3 allowed the fabrication of different kinds of self-folding films. First, we fabricated self-folding films from P(NIPAM-MAEMC)-3 only. For this we casted thick films (thickness is 60 μm) of the polymer on a substrate and cross-linked it by irradiation with 365 nm UV light (Figure 4a,b). Then, the film was irradiated with 254 nm UV light through a mask in order to reduce the cross-linking density and increase the swelling ratio in certain areas (Figure 4c). The patterned film was immersed in water

Figure 3. Change of thickness of the P(NIPAM-MAEMC) films with different composition after irradiation with 365 and 254 nm UV light (a). Swelling kinetics of P(NIPAM-MAEMC)-3 upon illumination with UV light of different wavelength (b).

conditions. Cooling below LCST should lead to dissolution of the polymer film. As shown in Figure 3a, the thickness of P(NIPAM-MAEMC) films decreases after irradiation with 365 nm UV light and subsequent washing with acetone. The amount of residual polymer increases with the increasing of the fraction of coumarin fragments. In particular, only 16% of

Figure 4. Schematic illustration of preparation of patterned P(NIPAM-MAEMC)-3 film and its folding in water at different temperatures: (a → b) the film was first cross-linked by irradiation with 365 nm UV light; (b → c) the cross-linked film was partially de-cross-linked by irradiation with 254 nm UV light through a photomask and as result patterned film with different cross-linking density was obtained (d); (e) the patterned film folds in water at temperature below LCST because of different swelling of areas, which were irradiated by only 365 nm UV light, and areas, which were irradiated by both 365 and 254 nm UV light. 4554

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Figure 5. Scheme of fabrication the P(MMA-BA) tubes by folding of P(NIPAM-MAEMC)-3/P(MMA-BA) bilayers: (a) the cross-linked bilayer film of P(NIPAM-MAEMC)-3/P(MMA-BA); (b) self-fold bilayer film to tube by immersion in water below LCST; (c) P(MMA-BA) single-layer tube was obtained after removing the outlayer of P(NIPAM-MAEMC) by its de-cross-linking.

Figure 6. Scheme of complex patterning of P(NIPAM-MAEMC)-3/P(MMA-BA) films and their folding: (a) irradiation of the bilayer film with 254 nm UV light through a photomask to cross-link the top P(MMA-BA) layer (b); (c) irradiation of the film with 365 nm UV light through another photomask to cross-link the bottom P(NIPAM-MAEMC)-3 layer. As a result, complexly pattered P(NIPAM-MAEMC)-3/P(MMA-BA) films were obtained (d); (e) the centipede-like structures were formed by patterned P(NIPAM-MAEMC)-3/P(MMA-BA) films folded in water.

thermoresponsive polymer forms the bottom layer and P(MMA-BA) forms the top layer. The bilayer was cross-linked by simultaneous irradiation with 254 and 365 nm UV light. Similar to previous observations, this bilayer rolls and forms a tube in water due to swelling of the P(NIPAM-MAEMC)-3 when the temperature is lower than LCST (Figure 5b). Then, the rolled tube was irradiated with 254 nm UV light for 24 h in water in order to de-cross-link and remove the outer layer of P(NIPAM-MAEMC). We found that although the P(NIPAMMAEMC) layer was removed, the tube did not unroll (Figure 5c). We also observed that the diameter of the tubes after the removal of P(NIPAM-MAEMC) was reduced from 20 to 14 μm. We explain this behavior by rigidity/plasticity of PMMA once being deformed, the PMMA film does not relax. As a confirmation of this, we have observed in our previous experiments that PNIPAM-PMMA tubes roll irreversibly.12

and exposed to different temperatures. We observed that the film is unfolded at elevated temperature when the polymer is not swollen (Figure 4d). Cooling led to nonhomogeneous swelling of the differently cross-linked areas and rolling (Figure 4e). On the other hand, we observed that films, which were homogeneously irradiated with UV light of different wavelength, did not bend which indicates that inhomogeneous cross-linking is the origin of folding. The shape transformation of film with patterned swelling is similar to the behavior observed by Hayward and Kumacheva et al.19,20,34 Next, we fabricated and investigated the folding of bilayer consisting of P(NIPAM-MAEMC)-3 and photo-cross-linkable poly(methyl methacrylate-co-benzophenone acrylate) P(MMA-BA), which can be cross-linked permanently by 254 nm UV light (Figure 5a). The bilayers were prepared by sequential dipcoating of P(NIPAM-MAEMC)-3 and P(MMA-BA) in a way that the 4555

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(6) Leong, T. G.; Randall, C. L.; Benson, B. R.; Bassik, N.; Stern, G. M.; Gracias, D. H. Tetherless thermobiochemically actuated microgrippers. Proc. Natl. Acad. Sci. U. S. A. 2009, 106, 703−708. (7) Bassik, N.; Brafman, A.; Zarafshar, A. M.; Jamal, M.; Luvsanjav, D.; Selaru, F. M.; Gracias, D. H. Enzymatically triggered actuation of miniaturized tools. J. Am. Chem. Soc. 2010, 132, 16314−16317. (8) Ryu, J.; D’Amato, M.; Cui, X.; Long, K. N.; Qi, H. J.; Dunn, M. L. Photo-origami—Bending and folding polymers with light. Appl. Phys. Lett. 2012, 100, 161908. (9) Kuribayashi-Shigetomi, K.; Onoe, H.; Takeuchi, S. Cell Origami: Self-folding of three-dimensional cell-laden microstructures driven by cell traction force. PLoS One 2012, 7. (10) Malachowski, K.; Breger, J.; Kwag, H. R.; Wang, M. O.; Fisher, J. P.; Selaru, F. M.; Gracias, D. H. Stimuli-responsive theragrippers for chemomechanical controlled release. Angew. Chem. 2014, 126, 8183− 8187. (11) Stoychev, G.; Turcaud, S.; Dunlop, J. W. C.; Ionov, L. Hierarchical multi-step folding of polymer bilayers. Adv. Funct. Mater. 2013, 23, 2295−2300. (12) Stoychev, G.; Zakharchenko, S.; Turcaud, S.; Dunlop, J. W. C.; Ionov, L. Shape programmed folding of stimuli-responsive polymer bilayers. ACS Nano 2012, 6, 3925−3934. (13) Thérien-Aubin, H.; Wu, Z. L.; Nie, Z.; Kumacheva, E. Multiple shape transformations of composite hydrogel sheets. J. Am. Chem. Soc. 2013, 135, 4834−4839. (14) Wu, Z. L.; Moshe, M.; Greener, J.; Therien-Aubin, H.; Nie, Z.; Sharon, E.; Kumacheva, E. Three-dimensional shape transformations of hydrogel sheets induced by small-scale modulation of internal stresses. Nat. Commun. 2013, 4, 1586. (15) Kim, J.; Hanna, J. A.; Byun, M.; Santangelo, C. D.; Hayward, R. C. Designing responsive buckled surfaces by halftone gel lithography. Science 2012, 335, 1201−1205. (16) Jamal, M.; Kadam, S. S.; Xiao, R.; Jivan, F.; Onn, T.-M.; Fernandes, R.; Nguyen, T. D.; Gracias, D. H. Bio-Origami hydrogel scaffolds composed of photocrosslinked PEG bilayers. Adv. Healthcare Mater. 2013, 2, 1066. (17) Randall, C. L.; Kalinin, Y. V.; Jamal, M.; Manohar, T.; Gracias, D. H. Three-dimensional microwell arrays for cell culture. Lab Chip 2011, 11, 127−131. (18) Magdanz, V.; Stoychev, G.; Ionov, L.; Sanchez, S.; Schmidt, O. G. Stimuli-responsive microjets with reconfigurable shape. Angew. Chem. 2014, 126, 2711−2715. (19) Kim, J.; Hanna, J. A.; Hayward, R. C.; Santangelo, C. D. Thermally responsive rolling of thin gel strips with discrete variations in swelling. Soft Matter 2012, 8, 2375−2381. (20) Thérien-Aubin, H.; Wu, Z. L.; Nie, Z.; Kumacheva, E. Multiple shape transformations of composite hydrogel sheets. J. Am. Chem. Soc. 2013, 135, 4834−4839. (21) Shao, Y.; Shi, C.; Xu, G.; Guo, D.; Luo, J. Photo and redox dual responsive reversibly cross-linked nanocarrier for efficient tumortargeted drug delivery. ACS Appl. Mater. Interfaces 2014, 6, 10381− 10392. (22) Zhu, Y.; Wang, F.; Zhang, C.; Du, J. Preparation and mechanism insight of nuclear envelope-like polymer vesicles for facile loading of biomacromolecules and enhanced biocatalytic activity. ACS Nano 2014, 8, 6644−6654. (23) Mal, N. K.; Fujiwara, M.; Tanaka, Y. Photocontrolled reversible release of guest molecules from coumarin-modified mesoporous silica. Nature 2003, 421, 350−353. (24) Azagarsamy, M. A.; McKinnon, D. D.; Alge, D. L.; Anseth, K. S. Coumarin-based photodegradable hydrogel: Design, synthesis, gelation, and degradation kinetics. ACS Macro Lett. 2014, 515−519. (25) Jiang, J.; Qi, B.; Lepage, M.; Zhao, Y. Polymer micelles stabilization on demand through reversible photo-cross-linking. Macromolecules 2007, 40, 790−792. (26) Iliopoulos, K.; Krupka, O.; Gindre, D.; Salle, M. Reversible twophoton optical data storage in coumarin-based copolymers. J. Am. Chem. Soc. 2010, 132, 14343−14345.

Finally, complexly patterned P(NIPAM-MAEMC)-3/P(MMA-BA) bilayer self-folding polymer films were prepared. The films were prepared by sequential deposition of the polymers by dipcoating in a way that the thermoresponsive layer is the bottom layer and the hydrophobic polymer is the top layer. The bilayer was first irradiated with 254 nm UV light through a photomask in order to cross-link the P(MMA-BA). Non-crosslinked (PMMA-BA) was removed by washing in toluene. Then, we irradiated the same film with 365 nm UV light through a second photomask in order to cross-link P(NIPAM-MAEMC)-3 (Figure 6c). As a result, complexly patterned P(NIPAM-MAEMC)3/P(MMA-BA) films were obtained (Figure 6d). Exposure of such film to water at room temperature led to swelling of the thermoresponsive layer and formation of centipede-like structures.

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CONCLUSIONS In this paper we demonstrated several approaches for the design of self-folding films using reversibly cross-linkable polymers based on coumarin derivatives. In one of the approaches, we used only reversibly cross-linked polymer, which was photolithographically patterned into crease locations with different swelling properties. In a second approach, we used traditional bilayer consisting of permanently cross-linked hydrophobic polymer and reversibly cross-linked stimuliresponsive polymer. The bilayer rolls and forms a tube. Then, the stimuli-responsive polymer was removed by second irradiation with UV light leaving a rolled tube consisting of one polymer. In a third approach, we patterned both the hydrophobic and reversibly cross-linked stimuli-responsive polymers that allowed the fabrication of a complex folded shape. We strongly believe that the use of reversibly crosslinkable polymers opens new perspective for fabrication of selffolding films with very complex and diverse folding behavior.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (L.I.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by DFG (IO 68/1-1 and IO 68/1-3). We acknowledge Alexander von Humboldt Foundation for the postdoctoral AvH fellowship. The authors are thankful to Vladislav Stroganov and Manfred Stamm for fruitful discussions as well as Svetalana Zakharchenko for assistance with experiments.

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REFERENCES

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DOI: 10.1021/acs.langmuir.5b00277 Langmuir 2015, 31, 4552−4557