**1. Introduction**

310 Smart Actuation and Sensing Systems – Recent Advances and Future Challenges

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Zainudeen, U. L., Careem, M. A. & Skaarup, S. (2008). PEDOT and PPy conducting polymer bilayer and trilayer actuators. *Sens. Actuat. B-Chem*, Vol. 134, No. 2, pp. 467-470, ISSN

> Intelligent materials with changing properties and functions have been studied in many fields [1-9]. In particular, stimuli-responsive polymer systems have been considerably investigated for the purpose of the many types of possible applications, such as soft actuators, microfluidics and medical devices, *etc.* due to their light weight, flexibility and low noise, *etc.* [10-18]. Polymer gels have the flexible bodies allowing volume changes and sensors properties for chemical substances at the same time. By utilizing these properties, the gel actuators have been considered to be applied for the biomimetic robots. In particular, a thermoresponsive poly(N-isopropylacrylamide) (PNIPAAm) gel have been significantly investigated especially for medical devices. [19, 20] However, in order to drive these polymer materials, external device for controlling the external stimulus must be needed. On the other hand, organic systems can generate autonomous motion without external stimuli. In recent years, self-oscillating gels and polymer chains were developed and researched in order to understand and mimic the spontaneous mechanical works. [21-27] The energy source of self-oscillating polymer systems is chemical energy like a living organism, that is, the polymer system cause the self-oscillation induced by the Belouzov-Zhabotinsky (BZ) reaction or the pH oscillating reaction. [28-43] The Self-oscillating reactions have been widely studied for their relevance to nonlinear, chaotic dynamics and biological signals. In previous investigations, the self-oscillating behaviors of polymer systems induced by the Belousov-Zhabotinsky (BZ) or the pH oscillating reactions were realized.[44-60] The BZ reaction is well known as an oscillating reaction accompanying spontaneous redox oscillations to generate a wide variety of nonlinear phenomena, e.g., a target or spiral pattern in an unstirred solution, and periodicity, multi-periodicity, or chaos in a stirred solution. The overall process of the BZ reaction is the oxidation of an organic substrate by an oxidizing agent in the presence of the catalyst under strong acidic conditions. In the BZ

© 2012 Hara et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

reaction, the metal catalyst undergoes spontaneous redox self-oscillation, and it changes the solubility from hydrophilic to hydrophobic at the same time. In order to cause the selfoscillation under temperature constant condition, the different solubility of the Ru catalyst in the reduced and oxidized state is utilized. In previous investigations, poly(Nisopropylacrylamide) (poly(NIPAAm)) was covalently bonded to (ruthenium (4 vinyl-4'-methyl-2,2-bipyridine) bis(2,2'-bipyridine)bis(hexafluorophosphate)) (Ru(bpy)3) and a negatively charged acrylamide-2-methylpropanesulfonic acid (AMPS) as a pH and solubility control site [47-50]. The AMPS-containing polymer systems caused self-oscillations originating from the different solubility of the Ru(bpy)3 moiety in the oxidized and reduced state. However, the conventional-type self-oscillating polymer system has the temperature limitation for causing the self-oscillation. That is because the conventional-type self-oscillating polymer system causes the aggregation or the contraction above the LCST. In this chapter, we introduce novel type self-oscillating polymers systems that are constituted the non-thermoresponsive and biocompatible poly-vinylpyrrolidone (PVP) as a polymer main-chain in order to expand the application fields. In our previous investigations, we first succeeded in causing the aggregation-disaggregation self-oscillation of the novel polymer chain and polymer gel under the constant condition induced by the BZ reaction. As for the VP-based self-oscillating polymer chain, the influence of the concentration of the three BZ substrates (sodium bromate, malonic acid and nitric acid) other than the metal catalyst on the waveform and period of the self-oscillation was investigated. As a result, it was demonstrated that the amplitude of the self-oscillation is hardly affected by the initial concentration of the BZ substrates. Furthermore, we firstly succeeded in causing the swelling-deswelling self-oscillation at high temperature condition by adapting the VP-main chain. We studied the influence of the initial concentration of the three BZ substrates other than the metal catalyst and the temperature on the period of the self-oscillation. As a result, it was clarified that the period of the self-oscillation for the VPbased polymer gel can be controllable by the selection of the initial concentration of the three BZ substrates (malonic acid (MA), sodium bromated (NaBrO3) and nitric acid) and the temperature. In addition, by optimizing the initial concentration of the BZ substrates and the temperature, it was demonstrated that the maximam frequency of the swellingdeswlling self-oscillation is 0.5 Hz. This value was 20 times as large as that of the conventional-type self-oscillating polymer gel (poly(NIPAAm-co-Ru(bpy)3 gel) with the thermoresponsive nature.

Next, in this chapter, we introduce the challenge of mimicking the peculiar locomotion of living organism, such as mollusks and apods, by utilizing polymer gel that cause the swelling-deswelling induced by the pH oscillating chemical reaction. Gastropods, like snails and slugs, can obtain the peristaltic movement by forming contraction waves, which is the propagation of the shrinking part of the body [61]. In this research, we focused on producing a functional gels obtaining peristaltic movements by the chemical energy directly.

In addition, we introduce a novel-tpye nanofiber gel actuator that was manufactured by the electrospining method. The nanofiber gel actuator can cause bending-streching motion synchronized with the pH oscilltion. Polymer nanofibers have been much studied for the application to various fields, such as chemical detector, scaffolds, wound dressing, and multifunctional membranes, sensors, filtration media and biomaterials, *etc.* [62-65]. That is because the nanofiber gel membranes have a high surface-to-volume ratio and a relatively small pore size compared to normal-type polymer hydrogel. The merit of the electrospining method to manufacutre nanofiber gel actutors is low cost, relatively high production rate, and applicability to many types of polymers. Moreover, by utilizing the electrospinning method, we can construct various shapes of nanofiber gel actuators because it does not require a mold to synthesize the gel. By controlling the external conditions, we can construct more complex structures such as porous fiber, core sheathe fiber, *etc.* [66-69]. Recently, some researchers succeeded in manufacturing functional nanofiber gels by utilizing electrospining method. For example, Hsieh and coworkers have reported ultrafine gel fibrous membranes that consist of poly(acrylic acid) (PAAc), poly(N-isopropylacrylamide-*co*-acrylic acid) and poly(vinyl alcohol)(PVA) and PVA/PAAc mixtures [70-72]. However, there are a few challenges to realize the nanofiber gel actuator [73,74]. In this study, in order to construct an autonomous nanofiber gel actuator, we utilized Landolt pH-oscillator based on a bromated/ sulfite/ferrocyanide reaction. By coupling with this pH-oscillator, we realized a nanofiber gel actuator that shows the bending and stretching motions with a constant period and displacement.
