Letter pubs.acs.org/Langmuir
One-Step Microfluidic Synthesis of Janus Microhydrogels with Anisotropic Thermo-Responsive Behavior and Organophilic/ Hydrophilic Loading Capability Kyoung Duck Seo,† Junsang Doh,†,‡ and Dong Sung Kim*,† †
Department of Mechanical Engineering, Pohang University of Science and Technology (POSTECH), 77 Cheongam-ro, Nam-gu, Pohang, Gyungbuk 790-784, South Korea ‡ School of Interdisciplinary Bioscience and Bioengineering (I-Bio), Pohang University of Science and Technology (POSTECH), 77 Cheongam-ro, Nam-gu, Pohang, Gyungbuk 790-784, South Korea S Supporting Information *
ABSTRACT: We report one-step microfluidic synthesis and characterization of novel Janus microhydrogels composed entirely of the same base material, N-isopropylacrylamide (NIPAAm). The microhydrogels were fabricated by the microfluidic generation of Janus monomer microdroplets based on separation of a supersaturated aqueous NIPAAm solution into NIPAAm-rich and -poor phases followed by UV irradiation. The resulting Janus microhydrogels exhibited tunable anisotropic thermo-responsive behavior and organophilic/hydrophilic loading capability.
1. INTRODUCTION Janus particles exhibit two or more distinct properties, such as surface chemistry and polarity,1−4 within a single-particle platform. As such, they have been recognized as a new class of materials and have been employed in various applications, including as an actuator that responds to electric and magnetic fields, as surfactants, or as building blocks for self-assembly.5−12 Recently, a microfluidic approach has been demonstrated as a means of fabricating uniform microparticles with complex structures.13−15 Microscale miniaturization results in low Reynolds numbers, which enables the production of micrometer-sized particles with low polydispersity and highly controllable size and shape.16−19 Despite the advantages, few microfluidic approaches have been introduced to fabricate Janus microparticles constituted of the same material with clearly compartmentalized structures. When two or more fluids composed of the same base material are used in the microfluidic fabrication of Janus microparticles, the internal morphology of those microparticles is generally mixed due to diffusion between the fluids and perturbations induced by the flow of continuous phase.5 Few strategies have been able to circumvent this intrinsic limitation. Weitz and co-workers20 introduced functionalized polymer precursors as a means of separating prepolymerized microdroplet formation as a discrete step in the overall synthesis. They also suggested a multistep synthetic method for creating hierarchal Janus microparticles using heat-induced phase separation of colloidal nanoparticles.21 Lone et al.22 used UV-directed phase separation © 2013 American Chemical Society
to synthesize Janus microparticles using a light-sensitive prepolymer. However, to the best of our knowledge, no microfluidic synthetic approach has been reported that describes the generation of clearly compartmentalized Janus microparticles composed entirely of the same base material but with each side exhibiting different physical or chemical characteristics. Poly(N-isopropylacrylamide) (PNIPAAm) is a common biocompatible and thermo-responsive material. Recently, Sasaki et al.23 reported that supersaturated N-isopropylacrylamide (NIPAAm) monomer solution undergoes intriguing liquid− liquid phase separation if the temperature is above 25 °C (details in Figure S1 of the Supporting Information), whereas dilute NIPAAm solutions are homogeneous under the same conditions. Thus, a supersaturated NIPAAm monomer solution will separate into two immiscible phases, one NIPAAm-rich (Nrich) which has lower density and the other NIPAAm-poor (Npoor). Importantly, the liquid−liquid phase-separated solution is metastable at a temperature below 25 °C, meaning that in the absence of strong perturbation, liquid−liquid phase separation can be maintained at a temperature below 25 °C. The current report describes a simple and robust approach for synthesizing Janus microhydrogels composed entirely of the same base material. Our approach relies on this liquid−liquid Received: August 5, 2013 Revised: November 13, 2013 Published: November 26, 2013 15137
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compartmentalized Janus microdroplets are formed, the final Janus microhydrogels can be polymerized and cross-linked by UV irradiation. Polymerization was performed within 1 s in the presence of covalent cross-linker N,N′-methylenebisacrylamide (MBAAm), so after cross-linking, no further changes including phase separation and precipitation of the microhydrogels can occur. The different characteristics of the discrete N-rich and N-poor phases in the same microhydrogel result in an anisotropic thermo-responsive behavior based on differential swelling and organophilic/hydrophilic loading capability due to differences in solubility between the two NIPAAm monomer phases, as demonstrated below. The proportional volume of N-rich and N-poor phases in the microdroplets could be controlled by changing the volumetric flow rates of the two monomer solutions. Optical micrographs of monodisperse Janus microdroplets with various volume ratios of N-rich and N-poor phases are shown in Figure 2a. The
phase separation of supersaturated aqueous NIPAAm monomer solution. This approach prevents cross-mixing of the two separated streams of the dispersed phase, thereby allowing the formation of Janus microhydrogels boasting distinct compartmentalized structures in a single synthetic step. In addition, the anisotropic thermo-responsive behavior and organophilic/ hydrophilic loading capability of the fabricated Janus microhydrogels are also demonstrated.
2. RESULTS AND DISCUSSION The one-step synthesis of Janus microhydrogels was performed with a hydrodynamic focusing microfluidic device (HFMD).24 The HFMD was fabricated using a conventional polydimethylsiloxane (PDMS) replica molding technique (details in Figure S2 of the Supporting Information).25 The method is shown schematically in Figure 1a. The N-rich and N-poor
Figure 1. (a) Schematic shows the basic principles of the hydrodynamic focusing microfluidic device (HFMD) used to generate Janus NIPAAm monomer microdroplets. (b) Optical micrograph shows the clearly compartmentalized Janus microdroplets composed of N-rich and N-poor phases generated in HFMD. Scale bar is 200 μm.
phase monomer solutions were separately injected as two distinct streams into the HFMD using two syringe pumps each operating at a prescribed volumetric flow rate (KDS 270, KD Scientific). The injected two different NIPAAm monomer streams were merged and broken up into Janus NIPAAm monomer microdroplets by a continuous oil phase at the orifice of the HFMD due to capillary instability.24 The immiscibility of the N-rich and N-poor phases solves the problem of mixing due to perturbations induced by the continuous phase flow during the generation of the Janus microdroplets. The Janus NIPAAm monomer microdroplets were monodispersely generated at a frequency of 19.4 ± 0.55 Hz, as shown in Figure 1b. The average diameter of the microdroplets was 190 μm with a coefficient of variation (CV) less than 2% (details in Figure S3 and Video S1 of the Supporting Information). It should also be emphasized that diffusion between the N-rich and N-poor phases is almost negligible, even though the base material on each side is the same (i.e., NIPAAm monomer). Once these
Figure 2. (a) Janus NIPAAm monomer microdroplets were monodispersely formed at various volume ratios by changing the volumetric flow rates of the N-rich and N-poor phase solutions. Scale bar is 200 μm. Comparison of experimental (left column) and theoretical (middle column) shapes of Janus monomer microdroplets is shown in (b). The right column shows a superposition of theoretical and experimental particles. Scale bar is 100 μm. 15138
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Figure 3. The anisotropic, thermo-responsive behavior of Janus PNIPAAm microhydrogels is shown in (a). The N-rich phase PNIPAAm hemisphere consistently exhibited a larger volumetric change relative to that of the N-poor phase hemisphere upon a temperature increase to the lower critical solution temperature, 32 °C. The first and second columns show cases of low and high cross-linker concentration, respectively. The scale bar is 200 μm. The circumscribing diameters of N-rich and N-poor phases PNIPAAm hemispheres are plotted in (b) in response to changes in temperature. Significant differences in swelling and shrinkage of Janus PNIPAAm microhydrogel formed at the N-rich and N-poor phases are probably due to differences in the monomer to cross-linker ratio. (c) The volume change ratio (Rv) is given for each hemisphere of the Janus microhydrogels.
tension. It should be noted that the actual value of γR,P is strongly affected not only by the molecular interactions between the N-rich and N-poor phase monomer solutions but also by the surfactant in the mineral oil, which reduces interfacial tension. Therefore, the actual value of γR,P is likely lower than the measured value. Although an exact value of γR,P could not be obtained, it was estimated in simulations as approximately 0.65 mN m−1 without a loss of generality. The first and second columns of Figure 2b, respectively, show the experimentally obtained Janus NIPAAm monomer microdroplets that were generated at three different volume ratios and the corresponding shapes of theoretical Janus microdroplets after the total surface energy was minimized for each volume ratio. The theoretical shapes agree well with the experimental results, as shown in the merged images in the third column of Figure 2b. The final Janus poly(N-isopropylacrylamide) (PNIPAAm) microhydrogels were synthesized by polymerizing and crosslinking the Janus NIPAAm monomer microdroplets with a UV light source (LC8, Hamamatsu) in the downstream channel of the HFMD. It should be noted that after Janus PNIPAAm microhydrogels are formed, their properties will be completely different from those of monomer solution. There is slight volume shrinkage of N-poor phase during polymerization and cross-linking because NIPAAm monomer concentration of Npoor phase is low, as shown in the first row of Figure 3 (panels a and b). The Janus microhydrogels were collected and washed several times in ethanol and distilled water. After removing the mineral oil, the Janus microhydrogels swelled in distilled water
total volumetric flow rate of both phases was fixed at 4 μL min−1 in all cases. The morphology of the microdroplets can be described by a spreading coefficient (Si), which represents the interfacial tensions among the three liquid phases26 Si = γjk − (γij + γik )
(1)
where γij, γjk, and γik each represent the interfacial tension between two of the three liquid phases i, j, and k. These spreading coefficients were used to distinguish three possible two-phase droplet morphologies: complete engulfing (core− shell), partial engulfing (Janus), and nonengulfing. Interfacial tensions between the N-rich and N-poor phase monomer solutions and mineral oil with surfactant were γR,M = 2.06 mN m−1 and γP,M = 2.36 mN m−1, respectively, as measured using the pendant drop method (details in Figure S4 of the Supporting Information). The subscripts R, P, and M refer to the N-rich and N-poor phase monomer solutions and mineral oil, respectively. The interfacial tension between the N-rich and N-poor phases was γR,P = 1.47 mN m−1. On the basis of these measurements, three different spreading coefficients were calculated, SP = −1.77, SM = −2.95, and SR = −1.17, which satisfy the criteria for the formation of Janus microdroplets (details in Figure S5 of the Supporting Information). The shapes of the Janus monomer microdroplets at three different volume ratios were confirmed by entering the measured interfacial tension data into the surface evolver simulation algorithm (www.susqu.edu/brakke/evolver). The surface evolver simulation algorithm which was studied by K. Brakke27 is an interactive program for the study of surfaces shaped by surface 15139
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dye (Oil red O; Sigma-Aldrich) and a water-soluble dye (Green food dye; Edentown F&B) were selected as representative organophilic and hydrophilic materials. In the N-rich phase, the majority of water molecules are tightly associated with NIPAAm molecules by hydrogen bonding,23 so N-rich phase may behave as an organic solvent rather than water. Therefore, the dyes typically soluble in organic solvents or fats dissolved well while dyes typically soluble in water exhibited poor solubility, suggesting that N-rich phase behaved as an organic solvent rather than water. In contrast, in the N-poor phase, only a small fraction of water molecules is associated with NIPAAm molecules, so the N-poor phase would preferentially dissolve hydrophilic polar molecules. Thus, exactly opposite solubility of dyes was observed in the N-poor phase, meaning that water still had a predominant effect on solubility of the N-poor phase (details in Figure S6 of the Supporting Information). Janus NIPAAm monomer microdroplets containing both of these dyes were generated using the synthetic procedure shown in Figure 4a. Finally fabricated Janus microhydrogels show the
to dimensions greater than those of the monomer microdroplets shown in the second row of Figure 3 (panels a and b). The anisotropic, thermo-responsive behavior of the Janus microhydrogels was investigated in distilled water by increasing the temperature to the lower critical solution temperature (LCST, approximately 32 °C). A small amount of surfactant [0.005% (v/v) Tergitol NP-10; Sigma Aldrich] was added to prevent the microhydrogels from sticking to the reservoir wall. The PNIPAAm hemispheres of N-rich and N-poor phases exhibited differential swelling and shrinking in response to changes in temperature. As mentioned above, properties of PNIPAAm microhydrogels will be completely different from the NIPAAm monomer solution. Therefore, both PNIPAAm hemispheres of the N-rich and N-poor phases were fully swollen at room temperature. Significant differences in swelling of PNIPAAm hydrogel formed at N-rich and N-poor phases at room temperature is probably due to the differences of the monomer to cross-linker ratio. Although identical concentration of cross-linker was used for both phases, the amount of NIPAAm in the N-rich phase was significantly higher than that in the N-poor phase. Therefore, PNIPAAm hydrogel formed in the N-rich phase had much higher monomer to cross-linker ratio than PNIPAAm hydrogel formed in the N-poor phase, resulting in larger mesh sizes of hydrogel networks and increased swelling volume.28 In this regard, the N-rich phase PNIPAAm hemisphere of the microhydrogel underwent a larger change in diameter than did the N-poor phase hemisphere upon increasing the water temperature to LCST, 32 °C, as shown in Figure 3b. Since monomer to cross-linker ratio influenced thermo-responsive behavior of microhydrogels, the volume change ratio can be controlled by adjusting the cross-linker concentration. To demonstrate the tunability of the anisotropic thermoresponse, two types of Janus PNIPAAm microhydrogels were prepared with cross-linker concentrations of 2 and 40 mg mL−1 (details in Videos S2 and S3, respectively, of the Supporting Information). For quantitative comparisons of thermoresponse, a volume change ratio (Rv) was defined as
RV =
|Vf − Vi | Vi
Figure 4. (a) Generation of Janus monomer microdroplets containing organophilic (fat-soluble dye) and hydrophilic (water-soluble dye) materials is shown. (b) Resulting Janus microhydrogels act as dual organophilic/hydrophilic carriers. Scale bar is 200 μm.
(2)
where Vi and Vf represent the initial and final volumes of each hemisphere of the Janus microhydrogels at room temperature and 32 °C, respectively. Vi and Vf were estimated from the mean diameters of the N-rich and N-poor phase hemispheres of the microhydrogels, measured from optical micrographs. Each diameter was determined by circumscribing a circle around each side of the Janus microhydrogels. The volume change ratio, Rv, of the N-rich phase PNIPAAm was always greater than that of the N-poor phase, regardless of cross-linker concentration as we expected above. After increasing the crosslinker concentration, Rv of both hemisphere decreased, as shown in Figure 3c. Consequently, the hemisphere of microhydrogels formed in the N-rich phase with lower concentrations of cross-linker results in greater thermoresponsivity. It should be noted that these observed volume changes were reversible. It is also possible to create the Janus microhydrogels with organophilic/hydrophilic loading capability by dissolving organophilic and hydrophilic materials into the N-rich and the N-poor monomer phases, respectively. To demonstrate the different loading capability of such microhydrogels, a fat-soluble
possibility for use as potential organophilic/hydrophilic dual material carriers, as shown Figure 4b. Organophilic/hydrophilic loading capability is noteworthy since the compartmentalized loading capability reduces the risk of cross-contamination.
3. CONCLUSIONS In summary, a novel, one-step microfluidic synthesis of Janus microhydrogels was demonstrated based on the phase separation of supersaturated aqueous NIPAAm solution. The proposed method is facile and easily scalable for the mass production of up to ∼105 Janus microhydrogels per hour. The microhydrogels exhibited the anisotropic thermo-responsive behavior and organophilic/hydrophilic loading capability that are amenable to the development of functional particles for controlling drug release rates and/or multiple-drug encapsulation in the fields of drug chemo-therapies, pharmaceuticals, and cosmetics. We believe that this strategy suggests new material platforms and promises new opportunities for the synthesis of multifunctional Janus microhydrogels. 15140
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4. EXPERIMENTAL SECTION N-Rich and N-Poor Phase Solutions Preparation. The N-rich and N-poor phase solutions were prepared by dissolving NIPAAm (Sigma Aldrich) in distilled water at a weight ratio of 1:1 using a vortex mixer (Scientific Industries). The experimental details are presented in Figure S1 of the Supporting Information. After the two phases were fully separated, 2.5 mL each of the N-rich and N-poor phases were gently extracted using a pipet. N,N′-methylenebisacrylamide (MBAAm) (Sigma Aldrich) and 2-hydroxy-1-[4-(hydroxyethoxy)phenyl]-2-methyl-1-propanone (Irgacure 2959; BASF) were added as a cross-linker and photoinitiator, respectively, into each of the Nrich and N-poor phase monomer solutions. The oil phase was mineral oil (Alfa Aesar) with 10 wt % surfactant (ABIL EM 90; Evonik Industries).
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ASSOCIATED CONTENT
S Supporting Information *
Phase separation of supersaturated aqueous N-isopropylacrylamide (NIPAAm) solution; fabrication of the hydrodynamic focusing microfluidic device (HFMD); determining the diameter of the Janus monomer microdroplets; interfacial tension measurements; Janus microdroplet morphology; organophilic/hydrophilic loading capability, and detailed experimental three video clips. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel: +82-54-279-2183. Fax: +82-54-279-5912. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (Grants 2010-0003531, 2011-0029454, and 20110030075). We appreciate helpful discussion and technical assistance from Mr. Sang Min Park and Mr. Taewan Kim at the Department of Mechanical Engineering in POSTECH.
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