Communication pubs.acs.org/cm
Autonomous Pulsatile Flow by Peristaltic Motion of Tubular SelfOscillating Gels Yusuke Shiraki,†,‡ Aya Mizutani Akimoto,‡ Takashi Miyata,† and Ryo Yoshida*,‡ †
Faculty of Materials, Chemical and Bioengineering, Kansai University, 3-3-35, Yamate-cho, Suita, Osaka 564-8680, Japan Department of Materials Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku 113-8656, Japan
‡
S Supporting Information *
n the field of materials chemistry, the development of biomimetic or bioinspired materials has played an important role. In particular, material design based on soft materials such as polymer and gel has been actively studied by many researchers. A wide variety of smart polymers and gels with stimuli-responsive function,1−6 self-healing function,7−10 high mechanical strength,11−14 etc. have been exploited in multiple applications. In comparison with these stimuli-responsive gels, we have developed a different type of smart gels which are referred to as “self-oscillating gels”. A self-oscillating gel undergoes autonomous swelling−deswelling oscillation under constant conditions. Since the first report,15 we have systematically studied the autonomous polymer and gel systems on several scales from the order of polymer chains to bulk gels.16−26 In addition, recently we have reported self-oscillating systems using block copolymers23,24 and the autonomous polymer systems coupled with supramolecular chemistry.25,26 There are many studies related to self-oscillating gels,27 including theoretical simulations on chemomechanical behaviors.28,29 As a basic chemical structure, the self-oscillating gel is composed of a cross-linked network made of temperatureresponsive poly(N-isopropylacrylamide) (PNIPAAm), which is copolymerized with ruthenium tris(2,2′-bipyridine) (Ru(bpy)3). In this system, the ruthenium complex is the catalyst for the Belosuov−Zhabotinsky (BZ) reaction. The BZ reaction is a chemical oscillating reaction that causes a spontaneous redox oscillation of the catalyst; the reaction solution shows periodic changes in color under stirring conditions and propagating wave patterns (chemical waves) under stationary conditions.30,31 When the poly(NIPAAm-co-Ru(bpy)3) gel is immersed in catalyst-free BZ solution containing the substrates (malonic acid, sodium bromate, and nitric or sulfuric acid), the reaction occurs in the gel. The redox changes of the polymerized catalyst induce a change in the volume phase transition temperature (VPTT) of the gel and also in the swelling ratio because the hydrophilicity of the polymer chains increases in the oxidized Ru(III) state and decreases in the reduced Ru(II) state. As a result, the gel exhibits an autonomous swelling−deswelling oscillation along with the redox oscillation in the closed solution system under constant conditions. We have evolved the self-oscillating gels as novel functional materials for application to biomimetic actuators, mass transport systems, functional fluids, etc. As a biomimetic actuator, self-walking gel,19 artificial cilia, etc. are reported. In addition, recently we fabricated a tubular self-oscillating gel
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© 2014 American Chemical Society
with autonomous intestine-like motion.22 In the tubular gel, a CO2 gas bubble adheres to the inner surface and is autonomously transported showing an intermittent motion caused by the peristaltic pumping of the gel. From this result, an autonomous pulsatile flow for a fluid inside the tubular gel is expected. If such an autonomous flow is possible, a novel autonomous gel pump and its application to microfluidic systems would be expected. In this study, we demonstrate that we can cause autonomous pulsatile flow in a tubular gel by utilizing the peristaltic motion of the tubular self-oscillating gel like an intestine (Figure 1). The autonomous flow is proved by
Figure 1. Schematic illustration of autonomous pulsatile flow by peristaltic motion of a tubular self-oscillating gel.
using latex beads as tracer particles and analyzing the motion. A new possibility of the self-oscillating gel as an autonomous chemomechanical gel pump is demonstrated. Figure 2 shows time-course images of the peristaltic motion of the tubular self-oscillation gel in the catalyst-free BZ substrate solution. The peristaltic motion synchronizes with the propagation of the chemical wave of the BZ reaction. To show this peristaltic motion more clearly, a time-series image analysis was employed. Two spatiotemporal patterns were constructed by extracting one line image along the x and y directions, respectively (Figure 3a; dashed lines), from each frame in the movie and lining them up sequentially with time (Figures 3b,c). From the slope of the stripes of the Received: August 19, 2014 Published: September 24, 2014 5441
dx.doi.org/10.1021/cm503040u | Chem. Mater. 2014, 26, 5441−5443
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velocity and the period of the redox changes were estimated to be 24.5 μm/s and 1420 s, respectively. As shown in Figure 3d, the wall thickness of the tubular gel at a fixed point changed periodically. Figure 3e shows the moving velocity of the tracer particles in the inner fluid. It was found that the velocity oscillates periodically synchronized with the peristaltic motion of the tubular gel. These two waveforms for the wall thickness and velocity changes were superposed for the first cycle in the oscillation (Figure 3f). The maximum velocity of the tracer particles was not observed at the maximum swollen state, but on the way during swelling process. This phase difference is related to the slow swelling kinetics of the gel. Figure 4 shows the distribution of the flow velocity vector
Figure 2. Time-course images of the peristaltic motion of the tubular poly(NIPAAm-co-Ru(bpy)3) gel. Light green band moves from right to left, indicating thickening of the wall thickness of the tubular gel.
Figure 4. Distribution of the velocity vector in the tubular selfoscillating gel during the autonomous peristaltic motion.
in the tubular gel. The color stands for the relative magnitude of the velocity (red, higher; blue, lower). It is demonstrated that the velocity of the fluid increases locally around the swollen region with the propagation of the chemical wave. The velocity decreased after passing the swollen region. Before the flow velocity increases, the stopping of the flow or backflow is observed transiently. These are due to negative hydrodynamic pressure caused by water uptake with swelling of the gel. Figure 5a shows the stream line in the tubular gel. The velocity shows a maximum at the center of the tube and a minimum near the inner wall with a nearly parabolic profile like Poiseuille flow. To evaluate the flow behavior in the tubular gel, we estimated the Reynolds number (Re) that is a dimensionless number defined by the ratio of inertial force and viscous force; Re = ud/ν, where u̅ is the average velocity of the tracer particle, ̅ d is the inner diameter of tubular gel, and ν is the kinematic viscosity of the catalyst-free BZ substrate solution at the room temperature. By using the following experimental values, u̅ = u/ 2 = 1.8 μm/s, d = 2r = 600 μm, ν = 2.0 × 10−6 m2/s, Re was calculated to be 5.4 × 10−4, which is much smaller than the
Figure 3. Time-series image analysis of the peristaltic motion of the tubular poly(NIPAAm-co-Ru(bpy)3) gel shown in Figure 2. (a) One frame image extracted from the movie. (b) Spatiotemporal pattern constructed by lining up the dashed x line time sequentially. (c) Spatiotemporal pattern constructed by lining up the dashed y line time sequentially. (d) Change in the thickness of the gel layer at a fixed point, which is derived from (c). (e) The velocities of the tracer particles in the tubular self-oscillating gel. (f) Waveforms of wall thickness and velocity changes for the first cycle in the oscillation.
spatiotemporal pattern in Figure 3b and the interval between the stripes of the spatiotemporal pattern in Figure 3c, the wave 5442
dx.doi.org/10.1021/cm503040u | Chem. Mater. 2014, 26, 5441−5443
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Communication
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. 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. 22245037). We acknowledge Dr. Zulma A. Jimenez for helpful suggestions.
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Figure 5. (a) Stream line in the tubular self-oscillating gel. (b) Definition of coordinate axis of the tubular self-oscillating gel. r = 0 was defined as the center position of the tube. (c) Radius positiondependent velocity change of the tracer particle in the tubular selfoscillating gel.
critical value (ca. 2300) for transition between laminar flow and turbulent flow. Moreover, we calculated the Womersley number (α) that is a dimensionless number defined by the ratio of transient inertial force to viscous force. α is often used in biofluid mechanics for estimation of periodic flow such as oscillatory flow and pulsatile flow; α = r(ω /ν)1/2, where ω is angular frequency of oscillation. By using the following experimental values, ω = 2π/ τ = 4.5 rad/s (where τ is the period of the redox oscillations, τ = 1420 s), α was estimated to be 1.4 × 10−2. The obtained value is much smaller than 1, which agrees with the result of the parabolic velocity profile shown in Figure 5. Further, the order of the value (10−2−10−3) agrees with that for arterioles, capillaries, and venules in biological systems. In conclusion, a tubular self-oscillating gel that exhibits autonomous peristaltic motion was prepared, and the autonomous fluidic behavior inside the gel was analyzed by using latex beads as tracer particles. Pulsatile flow was observed to synchronize with the peristaltic motion of the tubular gel. The velocity increased drastically with the propagation of the chemical wave and decreased slowly after the passing of the wave. The flow velocity of the tracer particle exhibited the maximum value during the swelling process. Furthermore, the dimensionless Re and α numbers were theoretically estimated, which indicated that the flow state in the tubular gel was completely laminar flow. To the best of our knowledge, this is the first example of producing an autonomous flow in a gel tube under constant condition without on−off switching of external stimuli. This was achieved by autonomous peristaltic motion of the self-oscillating tubular gel. The tubular self-oscillation gel would be useful as a novel autonomous micropump in microfluidics, etc.
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Materials and experimental procedures. This material is available free of charge via the Internet at http://pubs.acs.org. 5443
dx.doi.org/10.1021/cm503040u | Chem. Mater. 2014, 26, 5441−5443
