Communication Cite This: Chem. Mater. XXXX, XXX, XXX−XXX
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Reversible and Directional Control of Chemical Wave Propagation in a Hydrogel by Magnetic Migration through Liquid Interfaces Eunjong Lee,† Youn Soo Kim,*,†,‡ Aya Mizutani Akimoto,† and Ryo Yoshida*,† †
Chem. Mater. Downloaded from pubs.acs.org by EASTERN KENTUCKY UNIV on 09/02/18. For personal use only.
Department of Materials Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ‡ Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), 77 Cheongam-Ro, Nam-Gu, Pohang, Gyeongbuk 37673, South Korea S Supporting Information *
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the hydrogel because of its environmentally sensitive nature.21,22 Therefore, it is a significant challenge to control the propagation direction of the chemical wave. To date, research efforts into control over wave propagation in BZ reaction systems using hydrogels have been mainly based on regulation of the physical environment of hydrogels by the following methods: (1) variation in the hydrogel shape,21 (2) preparation of a hydrogel with a gradient structure,22,23 (3) construction of a hydrogel array system composed of small hydrogel patches with the same shapes,24−26 and (4) local irradiation with light.27 These methods can lead to the unidirectional propagation of chemical waves within the hydrogel. However, they only cause a single direction of propagation during the BZ reaction because the propagation direction is determined at the start of the BZ reaction and is an inherent property of the system. Therefore, a feasible method for switching the propagation direction within the hydrogel to a desired direction at a desired time is still an unexplored area. In this study, we introduce a new approach to control and switch the propagation direction of chemical waves through magnetic migration of a self-oscillating hydrogel containing magnetic nanoparticles in a liquid−liquid system composed of the aqueous BZ solution and an organic solvent such as hexane or dichloromethane (DCM). By magnetic migration of the selfoscillating hydrogel in the liquid layer system, we could control not only the propagation direction but also the dynamic propagation behavior, such as a pause in the propagation and the reversal of the propagation direction. This study may also provide a new concept for the regulation of functional materials: the dynamic chemical environment surrounding the material can control the reversible signal transmission within the material. First, we investigated the aspects of chemical wave propagation within 2D self-oscillating hydrogel sheets when they are in reaction environments composed of an aqueous BZ solution and organic solvents immiscible in water. The higher density liquid was placed on the surface of the hydrogel sheet as a droplet. The lower density liquid occupied extra space excluding the droplet. Thus, the reaction environments consisted of (a) a DCM droplet/BZ solution and (b) a BZ solution droplet/hexane, as shown in Figure 1. Chemical waves
or a wide range of biological events to occur, chemical signals must spread through space and time. The basic mechanism by which molecules spread through space is simple diffusion; that is, random movement of molecules due to thermal energy. However, for larger signal transmission distances, diffusion is too slow to be the conduit for biological events occurring within a few seconds or minutes. Furthermore, diffusion tends to attenuate signals, and spatiotemporal coordination of biological events often requires propagation of unimpeded signals. These limitations can be overcome by traveling chemical waves that can propagate signals in spacetime. By taking advantage of chemical waves, biochemical signals can be transmitted over long distances or can induce rapid activation in adjacent areas1−4 through positive feedback. Typical examples of chemical waves in biological systems are the propagation of action potentials in neurons, and calcium waves during cardiac contraction.5−7 Furthermore, chemical waves play important roles in mobility, mass transport, and signaling in living systems, as shown in the peristaltic motion of organisms,8,9 cell migration through steady treadmilling of actin arrays,10 the auditory pathway of the cochlea,11 etc. Therefore, introduction of chemical waves into man-made materials is essential to mimic such biological systems. As an example of a chemical wave that can be artificially induced, traveling waves evolving in the media of the Belousov− Zhabotinsky (BZ) reaction are well-known. So far, we have developed a self-oscillating hydrogel that shows autonomous swelling/deswelling without the on/off switching of external stimuli by employing the BZ reaction.12−16 During the course of the BZ reaction, ruthenium tris (2,2′-bipyridine) (abbreviated as Ru(bpy)3), which is copolymerized with the hydrogel networks and acts as a reaction catalyst, cycles between oxidized and reduced states. Consequently, the hydrogel shows autonomous swelling and deswelling in response to increased and decreased hydrophilicity, respectively.12,17 The micrometer-sized selfoscillating hydrogel generates entirely synchronized swelling and deswelling like a cardiac muscle cell. However, when the self-oscillating hydrogel is larger than the wavelength of the chemical wave over subcentimeter length scales, chemical wave propagation with peristaltic motion appears within the hydrogel. By using the chemical wave propagation and the consequent peristaltic motion, autonomous intestine-like mass transport systems have been reported.18−20 However, it remains difficult to predict the exact direction of chemical wave propagation in © XXXX American Chemical Society
Received: July 17, 2018 Revised: August 27, 2018 Published: August 27, 2018 A
DOI: 10.1021/acs.chemmater.8b03029 Chem. Mater. XXXX, XXX, XXX−XXX
Communication
Chemistry of Materials
poly(N-isopropylacrylamide-random-N-aminopropylmethacrylamide) (P(NIPAAm-r-NAPMAm)) were synthesized via reversible addition−fragmentation chain transfer (RAFT) random copolymerization. Ru(bpy)3, which is the catalyst for the BZ reaction, was postmodified into P(NIPAAm-rNAPMAm) copolymers by a coupling reaction between the primary amine groups of NAPMAm and the ester groups of Nhydroxysuccinimidyl Ru(bpy)3 (abbreviated as NHS-Ru(bpy)3), generating P(NIPAAm-r-NAPMAmRu(bpy)3-r-NAPMAm). P(DMAAm-r-NAS) was synthesized via free radical polymerization and was used for cross-linking. Detailed formation procedures and characterizations are described in the Supporting Information. The self-oscillating hydrogel containing magnetic nanoparticles was synthesized by post cross-linking between P(NIPAAm-r-NAPMAmRu(bpy) 3 -r-NAPMAm) and P(DMAAm-r-NAS) in an aqueous dispersion of magnetic nanoparticles, as shown in Figure 2a. Typically, pristine magnetic nanoparticles without additional treatments such as surface modification by hydrophilic functional groups28 rapidly precipitate in aqueous media. If such bare magnetic nanoparticles were introduced directly into the hydrogel via a typical gelation reaction such as free radical polymerization using a low molecular weight cross-linker, which requires at least several minutes for gelation, a heterogeneous hydrogel would be produced, in which most of the magnetic nanoparticles would be found as a precipitate at a bottom of the hydrogel. However, in our method using post cross-linking between the two copolymers, gelation can be completed within 10 s. Therefore, we were able to obtain the hydrogel without sedimentation of magnetic nanoparticles. The coupling reaction kinetics between NAS and the primary amine groups can be controlled through the pH of the reaction solution, i.e., a higher pH value accelerates the coupling reaction kinetics.29 In our synthesis, the pH of the pregel solution was adjusted to pH 9 to cause rapid gelation. After gelation, we investigated how the magnetic nanoparticles are distributed within the hydrogel networks using scanning electron microscopy (SEM) to investigate a lyophilized hydrogel (Figure S4). If the magnetic nanoparticles were positioned sufficiently closely, applying a magnetic field could influence hydrogel networks to form an aligned structure along the direction of the magnetic field.30 According to a report,31 the critical distance between magnetic nanoparticles required to cause such network transformation can be approximately calculated. In the present study, a calculation via this method revealed that the critical distance was about 50 μm (Supporting Information, p. 6). However, the SEM images showed that the distance between the aggregated magnetic nanoparticles was around 60 μm, which is greater than the critical distance. It was therefore possible to attach the prepared self-oscillating hydrogel to a magnet without causing deformation of the hydrogel networks, as shown in Figure 2b. To investigate aspects of chemical wave propagation in our hydrogels during the BZ reaction, a cylindrical self-oscillating hydrogel containing magnetic nanoparticles with 10 mm length and 1 mm diameter was prepared. The cylindrical hydrogel was then dipped in a cell container filled with the BZ solution and organic solvents, as shown in Figure 2c; the 10 mm length was much longer than the wavelength of a chemical wave of the BZ reaction. First, the cell container was filled with two liquid layers, where the BZ solution and hexane formed the lower and upper layers, respectively (Figure 3a). When the hydrogel was vertically fixed at the hexane/BZ-solution interface by a
Figure 1. Time-resolved images of chemical wave propagation within 2D self-oscillating hydrogel sheets in the aqueous BZ solution and water-immiscible organic solvents reaction medium. (a) DCM droplet/ BZ solution and (b) BZ solution droplet/hexane. The red dashed circular outlines indicate liquid interfaces, and the red arrows indicate propagation directions (scale bar: 2 mm).
occurred only within the partial area of the hydrogel in contact with the BZ solution, while the areas in contact with organic solvents maintained a paused state due to the blocking of the reactant. We observed a regularity in the propagation of chemical waves; they started in the BZ solution and propagated to the interface between the two liquids. This propagation behavior revealed the possibility that the propagation direction of the chemical waves could be controlled by the position of the liquid interface. However, it is still difficult to switch a propagation direction during the BZ reaction due to the stationary arrangement of its environment. If the chemical environment surrounding the self-oscillating hydrogel changed dynamically, it might modulate the propagation direction of chemical waves within the hydrogel. Therefore, we designed a self-oscillating hydrogel containing magnetic nanoparticles to freely manipulate its position using a magnet, without any physical contact. To prepare the self-oscillating hydrogel containing magnetic nanoparticles, we first synthesized two types of linear copolymers with different functional groups (Figure 2a). One
Figure 2. Schematic representation of the preparation of the selfoscillating hydrogel containing magnetic nanoparticles and observation of chemical wave propagation. (a) Prepared copolymers and magnetic nanoparticles, (b) image (upper) and chemical structure (lower) of the self-oscillating hydrogel containing magnetic nanoparticles, and (c) observation of chemical wave propagation by magnetic migration.
copolymer is a well-defined and self-oscillating material possessing Ru(bpy)3 and primary amine groups. The other copolymer is used for cross-linking, which is composed of hydrophilic N,N-dimethylacrylamide (DMAAm) and N-acrylocysuccinimide (NAS). Because NAS can react with primary amine groups forming amide bonds, the two copolymers can be readily cross-linked to produce a hydrogel. The first copolymers B
DOI: 10.1021/acs.chemmater.8b03029 Chem. Mater. XXXX, XXX, XXX−XXX
Communication
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of one-dimensional shapes like the cylindrical gel, the hydrogel has two edges with the same probability to function as an initiating point for the chemical wave. Accordingly, chemical waves should be initiated from both edges of the hydrogel and propagate inward. After the two chemical waves are generated at both edges, the surviving waves with higher frequencies determine the direction of propagation within the hydrogel.21,22 However, the oscillating behavior of the BZ reaction is wellknown for its tendency to fluctuate under minor changes in the experimental conditions. Thus, the chemical wave patterns are not stable and often change their propagation direction. For example, it was reported that the chemical wave patterns were different between the initial and later stages (after approximately 30 min) of the BZ reaction.21 Therefore, it is difficult to predict the actual chemical wave propagation direction within the hydrogel because multiple pattern formation modes with several ignition points often emerge as time passes. In contrast, our cylindrical hydrogel showed a single mode of propagation after the direction was determined to be either upward or downward, and subsequently, propagation was continuously maintained in the same direction until the reaction was finished. We investigated how the ratio of the immersed area in the BZ solution adjacent to the interface affects the time required to reach unidirectional chemical wave propagation between the edges. The hydrogels were placed at the BZ solution/DCM with different immersion-length fractions in each phase. When 51% of the hydrogel length was initially immersed in the BZ solution (the left image of Figure 4a), part of the hydrogel exhibited continuous downward chemical wave propagation up to the boundary of the BZ solution/DCM. After 1 h, all of the hydrogel was easily shifted to the BZ solution layer. Then, the hydrogel started to reveal a collision between the previously established downward waves and newly generated waves, as shown in the time-resolved images in Figure 4a.
Figure 3. Time-resolved images of chemical wave propagation when the hydrogel was fixed, at the interface (left), and in the BZ solution after magnetic migration (right). The interfaces are (a) hexane/BZ solution and (b) BZ solution/DCM (scale bar: 2 mm).
neodymium magnet (500 mT), the BZ reaction occurred only in the partial section of the hydrogel that was immersed in the BZ solution (Figure 3a, left image). We note that the vertically placed hydrogel is observed in Figure 3 as a front view, and thus there was an organic solvent/BZ-solution boundary of at the hydrogel that was slightly lower than the level indicated by the front view due to the meniscus (Figure S5); we indicate such boundaries using dashed lines in Figure 3. The limited region of the hydrogel immersed in the BZ solution exhibited a chemical wave, which was generated by periodic and autonomous change of the redox state of Ru(bpy)3 in the hydrogel during the BZ reaction. The chemical waves propagated at a constant speed along the axis of the hydrogel in an upward direction from the bottom to the boundary of the hexane/BZ solution. On the other hand, the residual part of the hydrogel immersed in hexane was maintained throughout in an oxidized state and displayed a green color. This was because our hydrogel was initially oxidized in the solution containing HNO3 and NaBrO3 before being dipped in the cell container. The oxidized state of the hydrogel was maintained in the organic solvents, where it was isolated from the progressive BZ reaction. The hydrogel was kept at the hexane/BZ-solution interface for 1 h, and then magnetically shifted to the lower phase of the BZ solution so that the entire hydrogel structure was immersed in the solution. Then, after several minutes, the chemical waves started to propagate from the bottom to the top edge of the hydrogel (upward propagation), as shown in the right panels in Figure 3a. This upward propagation was maintained for several hours until the BZ reaction finished. However, when the hydrogel was vertically fixed at the BZ solution/DCM interface (left panels in Figure 3b), the generated chemical waves propagated at a constant speed along the long axis of the hydrogel in a downward direction from the top to the BZ solution/DCM boundary. After remaining at the interface for 1 h, the hydrogel was magnetically shifted to the upper phase of the BZ solution. Then, after several minutes, chemical waves propagated downward in the hydrogel from the top to the bottom edge (right panels in Figure 3b). Note that this direction was the opposite of that of the hexane/BZ solution, as shown in Figure 3a. In general, chemical wave patterns depend on the shape of the hydrogel, as the wave initiates at the edges or corners and propagates to the center of the hydrogel. Because the probability to encounter the substrates for the BZ reaction is higher at the edges and corners than at the center of the hydrogel. In the case
Figure 4. Difference in the times required to reach unidirectional propagation in the hydrogel depending on the fraction of hydrogel length immersed in the BZ solution. (a) 51% and (b) 72% of the hydrogel length was initially immersed in the BZ solution. Timeresolved images illustrate propagation behavior after shifting the hydrogels to the BZ solution. The times indicated on the figure give the time at the start of each set of five images that have a 30 s interval between each step; the zero of time is when the gel is moved to be immersed in the BZ solution. The lower set of time-resolved images in each figure represents the binarized images of the waveforms derived from analysis of the gray-level intensities corresponding to Ru3+ concentration (scale bar: 2 mm). C
DOI: 10.1021/acs.chemmater.8b03029 Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials
layer (Step 2). In accordance with our previous results, the hydrogel showed downward-wave propagation (Figure 5b). Following this, it was directly shifted and placed at the hexane/ BZ-solution interface. However, the hydrogel maintained downward propagation, even at the hexane/BZ-solution interface, where the propagation direction is expected to be upward (Figure S6a). This is because the downward-propagating fully grown waves were too stable to produce new upward propagation. Thus, the hydrogel, whose propagation direction was determined to be downward, was shifted to the hexane layer so as to be completely surrounded by the organic solvent (Pause 1). Then, chemical wave propagation in the hydrogel stopped, and it remained in the oxidized state (see Figure S6b). When the oxidized hydrogel in the hexane layer was shifted and placed at the hexane/BZ-solution interface again (Step 3), upward-wave propagation began (Figure S6c). After that, the hydrogel was placed in the BZ solution (Step 4) and upward waves propagated throughout the hydrogel. The hydrogel was sequentially shifted to the DCM layer through the BZsolution/DCM interface (Pause 2), and there the waves completely disappeared (Figure S6c). With the reverse shift of the hydrogel, downward wave propagation started when the hydrogel was at the interface (Step 5), as in Step 1. Further, the wave propagated thorough the entire hydrogel in the BZ solution (Step 6), as in Step 2. Note that by immersing the hydrogel in the organic solvent, the propagation of chemical waves in the hydrogel could be temporarily paused and resumed at the desired time by shifting it to one of the organic solvent/ BZ-solution interfaces. Moreover, the propagation direction of the chemical wave was reversibly switched between upward and downward directions by changing the position of the hydrogel. In this study, we investigated how immiscible liquid interfaces affected the propagation direction of a chemical wave of a selfoscillating hydrogel. We prepared a self-oscillating hydrogel containing magnetic nanoparticles by a rapid post-cross-linking method. Because of magnetic nanoparticles embedded in the hydrogel network, the self-oscillating hydrogel could be magnetically fixed and shifted in liquid−liquid layers consisting of the aqueous BZ solution and water-immiscible organic solvents. When the hydrogel was magnetically fixed at the liquid interfaces, chemical wave propagation occurred only in the portion of the hydrogel that was immersed in the BZ solution. On the other hand, the residual portion of the hydrogel immersed in the organic solvent was maintained at a paused state. In this state, the chemical waves started at the edge of the hydrogel immersed in the BZ solution and propagated to the liquid interface at a constant speed. Moreover, it was possible to reversibly switch the direction of the chemical wave in a targeted manner at a desired time by manipulating the position of the hydrogel between three immiscible liquid layers. We postulate that these particular features should give rise to improvement of functionalities of materials and diverse applications by allowing spatiotemporal control of intrinsic changes.
Temporal evolution of wave propagation was analyzed by image binarization of the green-colored signal intensity corresponding to the oxidized state in the hydrogel (lower images of Figure 4a). In the initial stage, the upper 51% of the hydrogel structure exhibited stable downward propagation of waves, whereas the residual lower part of the hydrogel remained in a static oxidized state. After 15 min, new target waves were generated in the lower part of the hydrogel. When these newly generated waves encountered downward waves, the downward waves immediately disappeared. As a result, the chemical wave patterns in the entire hydrogel took on a different phase, and the waves propagated in both the upward and downward directions simultaneously. This behavior is shown in the two middle panels in Figure 4a. After more than 2 h, unidirectional propagation in the downward direction became dominant and was maintained until the BZ reaction finished. Therefore, the intrinsic wave propagation direction reappears 2 h later, although the unidirectional propagation fades out as a consequence of destructive interference during the BZ reaction, indicating that the previously determined unidirectional propagation remains latent in the hydrogel. We also investigated the wave-propagation behavior in another condition, when 72% of the hydrogel length was initially immersed in the BZ solution (the left image of Figure 4b). After the entire hydrogel was shifted toward the BZsolution layer, there was a collision between the previously established downward waves and newly generated upward waves, as shown in the images in Figure 4b. In this case, it took only 35 min to stabilize the downward wave propagation in the hydrogel. This downward propagation was maintained until the BZ reaction finished. These results indicate that the greater area of the hydrogel immersed in the BZ solution provided more stable chemical waves, shortening the time required to reach unidirectional propagation. By taking advantage of such a straightforward method to regulate the propagation direction of chemical waves and shorten the time required to reach unidirectional propagation, we further elaborated our system for reversibly switching the propagation direction of waves in a single hydrogel. No selfoscillating hydrogel has been reported to exhibit such reversible switching of propagation direction.18−27 First, we prepared three liquid layers composed of hexane, BZ solution, and DCM (Figure 5). The hydrogel was initially placed at the BZ solution/ DCM interface (Step 1) and was then shifted to the BZ solution
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ASSOCIATED CONTENT
S Supporting Information *
Figure 5. Reversible propagation of chemical waves depending on hydrogel position. (a) Schematic image of hydrogel position modulation by magnetic migration and (b) time-resolved images of unidirectional chemical waves propagating reversibly (scale bar: 2 mm).
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b03029. D
DOI: 10.1021/acs.chemmater.8b03029 Chem. Mater. XXXX, XXX, XXX−XXX
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Descriptions of experimental procedure; characterization results of copolymers used in this study; additional figures; the description of movie files (PDF) Movie S1: Chemical wave propagation when the selfoscillating hydrogel was fixed at the hexane/BZsolution interface and when it was magnetically shifted to the BZ solution (AVI) Movie S2: Chemical wave propagation when the selfoscillating hydrogel was fixed at the BZ-solution/DCM interface and when it was magnetically shifted to the BZ solution (AVI) Movie S3: Reversible propagation of the chemical wave, depending on the hydrogel position (AVI) Movie S4: Chemical wave propagation within 2D selfoscillating hydrogel sheets in reaction media composed of the aqueous BZ solution and water-immiscible organic solvents (AVI)
AUTHOR INFORMATION
Corresponding Authors
*Y. S. Kim. E-mail:
[email protected]. *R. Yoshida. E-mail:
[email protected]. ORCID
Youn Soo Kim: 0000-0003-1531-3635 Ryo Yoshida: 0000-0002-0558-2922 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported in part by a Grant-in-Aid for Scientific Research to R.Y. from the Ministry of Education, Culture, Sports, Science, and Technology of Japan (Grant No. 15H02198) and Research Fellowships to E.L. (No. 16J09967) from the Japan Society for the Promotion of Science for Young Scientists. We thank Mr. Koji Aga and Mr. Takashi Kojima in Powdertech CO., LTD for providing magnetic nanoparticles.
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