Synchronization of Self-Oscillation in Polymer Chains and the Cross

Chapter 3

Downloaded by UNIV MASSACHUSETTS AMHERST on September 13, 2012 | http://pubs.acs.org Publication Date: November 18, 2003 | doi: 10.1021/bk-2004-0869.ch003

Synchronization of Self-Oscillation in Polymer Chains and the Cross-Linked Network 1,*

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Ryo Yoshida , Takamasa Sakai , Shoji Ito , and Tomohiko Yamaguchi 1

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1

Department of Materials Engineering, Graduate School of Engineering, The University of Tokyo, Tokyo 113-8656, Japan Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology, AIST Central 5-2, 1-1-1 Higashi, Tsukuba 305-8565, Japan 2

Novel biomimetic gel that undergoes autonomous swelling -deswelling oscillations without on-off switching of external stimuli was developed. The mechanical oscillation was produced via the Belousov-Zhabotinsky reaction. The gel consists of the cross-linked poly(N-isopropylacrylamide) to which ruthenium tris(2,2'-bipyridine) was covalently bonded. The redox oscillation of the catalyst site is converted into the mechanical oscillation of the polymer network. The oscillating behaviors were controlled by changing the substrate concentration, gel size or geometry, photo-illumination, etc. For the analysis of synchronization process in the gel, linear polymer chain and gel particles with submicrometer size were prepared. For linear polymer, soluble-insoluble changes were realized. Effects of polymerization and crosslinking were investigated through the analysis of oscillation for these systems.

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© 2004 American Chemical Society

In Nonlinear Dynamics in Polymeric Systems; Pojman, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.


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Stimuli-responsive polymer gels undergo either swelling or deswelling transition when varying external conditions surrounding the gels such as a change in solvent composition (7,2), pH (3,4), temperature (5,6), electric field (7,8), specific chemicals (9-11), etc. For example, thermo-sensitive hydrogels consisting of N-isopropylacrylamide (NIPAAm) swell by cooling and deswell by heating (5,6). Many kinds of stimuli responsive gels have been extensively investigated and their ability to swell and deswell according to conditions makes them an interesting proposition for use in intelligent materials (12-17). In these systems utilizing stimuli-responsive polymers, the response of polymer is temporary; that is, the polymer provides only one unique action of either expanding or coflapsing toward a stable equilibrium state. Therefore the on-off switching of external stimuli is essential to instigate the action of the polymer. On the other hand, many physiological systems maintain rhythmical oscillations under constant environmental conditions, and act in a dynamic nonequilibrium state, as represented by the autonomic heartbeat, brain waves, periodic hormone secretion. If such self-oscillation could be achieved for gels, possibilities would emerge for new biomimetic intelligent materials that exhibit rhythmical motion. Recently, we have developed such a self-oscillating gel (1823). It spontaneously exhibits cyclic swelling and deswelling under constant conditions, requiring no switching of external stimuli. Its action is similar to that of a beating heart muscle. In this paper, the se If-oscillating behaviors of the gel have been discussed.

Design of Self-Oscillating Gel The mechanical oscillation is driven by the Belousov-Zhabotinsky (BZ) reaction occurring in the gel (Figure 1). We prepared a copolymer gel which consists of N-isopropylacrylamide (NIPAAm) and ruthenium(II) tris(2,2'bipyridine) (Ru(bpy) ). Ru(bpy)3, acting as a catalyst for the BZ reaction, is pendent to the polymer chains of NIPAAm. Homopolymer gels of NIPAAm have theimosensitivity and undergo an abrupt volume-collapse (phase transition) when heated at around 32°C (5,6). The oxidation of the Ru(bpy) moiety caused not only an increase in the swelling degree of the gel, but also a rise in the transition temperature. These characteristics may be interpreted by considering an increase in hydrophilicity of the polymer chains due to the oxidation of Ru(II) to Ru(III) in the Ru(bpy) moiety. As a result, we may expect that our gel undergoes a cyclic swelling-deswelling alteration when the Ru(bpy) moiety is periodically oxidized and reduced under constant temperature. 3

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Figure 1. Self-oscillation ofpoly(NIPAAm-co-Ru(bpy)s) gel coupled with the Belousov-Zhabotinsky reaction.


33 Self-Oscillation of the Miniature Bulk Gel The poly(NlPAA m-eo-Ru(bpy) ) gel was cut into a cubic shape (each length of about 0.5mm) in pure water, and then immersed into an aqueous solution containing malonic acid (MA), sodium brornate (NaBr0 ), and nitric acid (HN0 ) at constant temperature (20°C). This outer solution comprised the reactants of the BZ reaction, with the exception of the catalyst. Therefore the redox oscillation does not take place in this solution. However, as it penetrates into the gel, the BZ reaction is induced within the gel by the Ru(bpy) copolymerized as a catalyst on the polymer chains. Under reaction, the Ru(bpy) in the gel network periodically changes between 2+ and 3+ states. In the miniature gel whose size is smaller enough than the wavelength of chemical wave (typically several mm), the redox change of ruthenium catalyst can be regardai to occur homogeneously without pattern formation. We observed the oscillation behavior under a microscope equipped with a CCD camera and video recorder. Color changes of the gel accompanied with redox oscillations (orange: reduced state, light green: the oxidized state) were converted to 8-bit grayscale changes (dark: reduced, light: oxidized) by image processing. Due to the redox oscillation of the immobilized Ru(bpy) , mechanical s we 11 ing-de s wel 1 ing oscillation of the gel autonomously occurs with the same period as for the redox oscillation (Figure 2). The volume change is isotropic and the gel beats as a whole, like a heart muscle cell. The chemical and mechanical oscillations are synchronized without a phase difference (i.e., the gel exhibits swelling during the oxidized state and deswelling during the reduced state). In order to enhance the amplitude of swelling-deswelling oscillations of the gel, we attempted to change the period and amplitude of the redox oscillation by varying the initial concentration of substrates. It is a general tendency that the oscillation period increases with the decrease in concentration of substrates. For the bulk solution consisting of MA, NaBrcoRu(bpy) ) gel. 3

It is well known that the period of oscillation is affected by light illumination for the Ru(bpy) -catalysed BZ reaction. The excited state of the catalyst (Ru(bpy) *) causes new reaction process: production of activator (reaction (1)), or production of mhibitor (reaction (2)), which depends on the solute compositions (24). 2+

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2+

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2+

2+

+

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Ru(bpy) * + Ru(bpy) + Br0 " + 3H -> HBr0 + 2Ru(bpy) + H 0 (1) Ru(bpy) * + BrMA + H -* Br + Ru(bpy) + products (2) 3

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37 Therefore, (i) we can intentionally make a pacemaker with a desired period (or wavelength) by local illumination of laser beam on the gel, or (ii) we can change the period (or wavelength) by local illumination on the pacemaker which has already existed in the gel. In the rectangular gel, the comer often becomes a pacemaker from which chemical waves start to propagate. Therefore, the self-oscillating behaviors of the gel can be controlled by irradiating laser light locally to the pacemaker site of gels. Figure 5 shows the effect of laser irradiation (488nm) on the pacemaker under the condition that photo-illumination produces activator. The size of the pacemaker was controlled by the illuminating beam, whose diameter was determined by a pinhole placed in the light path. It was found that the wavelength of traveling waves in the gel decreased as the size of pacemaker increased. The results gave a good agreement with a theoretical model simulation. This result means that we can control the macroscopic swellingdeswelling hebavior of the gel by local perturbation, i.e., small signal can be amplified to macroscopic change.

0

0.5 1.0 Diameter of pacemaker [mm]

Figure 5. Relation between the diameter ofpacemaker and the wavelength of chemical waves.

Self-Oscialltion of Polymer Chains with Rhythmical SolubleInsoluble Changes In the self-oscillating gel, redox changes of Ru(bpy) catalyst are converted to confomational changes of polymer chain by polymerization. The conformational changes are amplified to macroscopic swelling-deswelling changes of polymer network by crosslinking. Further, when the gel size is larger 3


38 than chemical wavelength, the chemical wave propagates in the gel by coupling with diffusion. Then peristaltic motion of the gel is created. In these manners, hierarchical synchronization process exists in the self-oscillating gel. Our interests are to clarify the polymerization effect of catalyst on the oscillating behavior of the BZ reaction, as well as the effect of crosslmking the polymer chains on the synchronization of each polymer's oscillation. For this purpose, firstly, we synthesized linear poly(NIPAAm-co-Ru(bpy) ) and investigated selfoscillating behavior of polymer chain through the analysis of transmittance changes of polymer solution (25). Then the gel particles with submicronmeter size were prepared. By comparing the oscillating behaviors between them, the effect of polymerization and crosslinking were discussed. Figure 6 shows the transmittance changes of poly(NDPAAm-co-Ru(bpy) ) (5wt% and 10wt% Ru(bpy) ) solutions as a function of temperature under the different conditions of reduced Ru(II) sate and oxidized Ru(IH) state. The wavelength (570nm) at isosbestic point of reduced and oxidized states was usai to detect the optical transmittance changes based on soluble-insoluble changes of the polymer, not on the redox changes of Ru(bpy) moiety. The transmittance suddenly decreases as temperature increases, demonstrating the lower critical solution temperature (LCST). When the Ru(bpy) site is kept in an oxidized state, the LCST shifts higher than that of the reduced state. The rise in the LCST by oxidation is due to an increase in hydrophilicity of the polymer by the charge


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Temperature [°C]

Figure 6. Temperature dependence of optical transmittance for poly(N!PAAmco-Ru(bpy) ) solutions under the different conditions of reduced Ru(H) state (in Ce(lll) solution) and oxidized Ru(III) state (in Ce(IV) solution). 3

(Reproducedfromreference 25. Copyright 2002 American Chemical Society.) In Nonlinear Dynamics in Polymeric Systems; Pojman, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

39 increase of the catalyst. The difference of the LCST between reduced and oxidized states becomes larger as the Ru(bpy) content increases in the polymer. In reduced state, the effect of hydrophobicity of bipyridine ligand surrounding ruthenium ion would be predominant over the ionization effect and make the polymer more hydrophobic than PNIPAAm. Due to the hydrophobic interaction, the sharpness of transmittance change may become duller in reduced state, and the LCST decreases more significantly with the increase in Ru(bpy) content In oxidized Ru(III) state, on the other hand, lhe LCST of the copolymer is slightly higher than that of PNIPAAm. As charge number increases, the contribution of ionic effect increases, which makes the polymer more hydrophilic than PNIPAAm. From the deviation of the LCST between Ru(II) and Ru(III) states, we may expect that the polymer undergoes periodical soluble-insoluble changes when the Ru(bpy) moiety is oxidized and reduced periodically by the BZ reaction at constant temperature. Synchronized with the periodical changes between Ru(II) and Ru(III) states of the Ru(bpy) site, the polymer becomes hydrophobic and hydrophilic, and exhibits cyclic soluble-insoluble changes. These periodic changes of polymer chains can be easily observed as cyclic transparent and opaque changes for the polymer solution with color changes due to the redox oscillation of the catalyst. Figure 7 shows the oscillation profiles of transmittance for the polymer solution at constant temperatures. As temperature increases, the amplitude in transmittance increases and the period becomes short. It is a general tendency that the oscillation period of the BZ reaction decreases as temperature increases, following the Arrhenius equation (26). As observed in Figure 6, the degree of difference in transmittance between reduced and oxidized states depends on temperature because the transmittance in reduced state drops more gradually over wide temperature range while drops abruptly in oxidized state. Consequently, the difference in transmittance between two states becomes large at high temperature. This results in the large amplitude of oscillations at high temperature in Figure 7(a). Figure 7(b) shows the oscillating behavior of the polymer with higher Ru(bpy) content. The amplitude increases at each temperature. This result is attributed to the large difference of LCST between reduced and oxidized states of high Ru(bpy) content (see Figure 6). It is suggested that the hydrophilic-hydrophobic changes of polymer becomes more remarkable due to an increase in molar content of redox site. We have investigated dependence of the oscillation period on initial substrate concentration for three systems; (i)the conventional BZ solution using non-polymerized catalyst, (ii) the polymer solution using polymerized catalyst by NIPAAm, and (Hi) the suspension of submicron-sized gel beads, i.e., the crosslinked polymer network of the polymerized catalyst. The empirical equatbn, Τ = a [MA]~ [NaBr0 ] [HN0 ]" , was obtained for these three systems. It was found that the concentration dependence of period for the nonpolymerized solution (each constants in the equation are a=2.97, b=0.414, c=0.794 and d=0.743, respectively) was partly similar to the polymer solution 3


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Figure 7. Oscillating profiles of optical transmittance for poly (NIPAArn-coRu(bpy) ) (Ru(bpy) = 5 or 10wt% in feed composition) solution at constant temperatures. (Reproduced from reference 25. Copyright 2002 American Chemical Society.)

Time [s]

100

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(a=2.97, b=0.413, c=0.934 and d=0.567), but largely different from the gel beads suspension (a=5.75, b=0.506, c=0.667 and d=0.478). Compared with the period under the same substrate concentrations, the period increased as the following order; (i) < (ii) < (iii). The reason for the increases in perod can be considered as follows. In the poly(NIPAAm-co-Ru(bpy) ), charge site is fixed on the polymer chain. Due to the fixation, the increase in electrostatic repulsion between the charge sites may be suppressed. As a result, change to oxidation state is restrained. This leads to longer duration of reduced state and therefore elongation of oscillation period. In the case of crossiinked polymer network (gel beads), polymer chains are constrained to behave cooperatively. And also, diffusion limitation of substrate from the outer solution into gel phase will take place. This will result in decreasing effective concentration inside the gel phase and elongation of oscillation period. Detail mechanisms are still under investigation.


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Ciliary Motion Actuator Using Self-Oscillating Gel One of the promising fields of the MEMS is micro actuator array or distributed actuator systems. The actuators, that have a very simple actuation motion such as up and down motion, are arranged in an array form. If their motions are random, no work is extracted from this array. However, by controlling them to operate in a certain order, they can generate work as a one system. One of the typical examples of this kind of actuation array is a ciliary motion micro actuator array. There have been many reports to realize it. Although various actuation principles have been proposed, all the previous works based on the same concept that the motion of actuators were controlled by external signals. If the self-oscillating gel plate with micro projection structure array on top is realized, it is expected that the chemical wave propagates and creates dynamic rhythmic motion of the micro projection structure array. This is the structure of proposed new ciliary motion array that exhibits spontaneous dynamic propagating oscillation. The gel plate with micro projection array was fabricated by molding technique (27). First, the moving mask deep-X-ray lithography technique was utilized to fabricate the PMMA plate with truncated conical shape microstructure array. This step was followed by the evaporation of Au seed layer and subsequent electroplating of nickel to form the metal mold structure. Then, a PDMS mold structure was duplicated from the Ni metal mold structure and utilized for gel molding. The formation of gel was carried out by vacuum injection molding technique. The structure with the height of 300 μηι and bottom diameter of 100 μηι were successfully fabricated by the proposed process (Figure 8). The propagation of chemical reaction wave and dynamic ihythmic motion of the micro projection array were confirmed by chemical wave observation and displacement measurements (27). The feasibility of the new



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Figure 8 The micro projection structure array on the gel surface fabricated by X-ray lithography technique.

concept of the ciliary motion actuator made of self-oscillating polymer gel was successfully confirmed.

Conclusions Novel biomimetic polymer gels with self-oscillating function have been developed. The gel has a cyclic reaction network in itself and generates periodic mechanical energy from the chemical energy of the BZ reaction. The selfoscillating behavior can be controlled by changing the reaction condition or geometric design of the gel. The self-oscillating gel may be useful m a number of important applications such as self-walking (auto-mobile) actuators or micropumps with autonomous beating or peristaltic motion, etc. Research into the mechanism of the oscillating behavior as well as the practical applications continues.

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Synchronization of Self-Oscillation in Polymer Chains and the Cross

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