Over about the last two or three decades, many kinds of stimuli-responsive polymer gels, which respond to changes in their surroundings such as temperature, pH, and supply of electric field, etc., have been developed. They have attracted much attention as smart (or biomimetic) materials, and several applications to actuator (artificial muscle), biosensor, drug delivery systems, purification or separation systems, tissue engineering, etc. are extensively studied [1].

Several mechanical devices using gels were devised and demonstrated in the late 80s and early 90s when gels first began to attract attention as a functional material [2]. For example, devices driven by a change in temperature or giving an electric field [3] (artificial muscles or robot hands that lift or grasp something, artificial fish that swim with repetitive bending motions, artificial looper that walks on rails [4], etc.) attracted much attention. In addition, several chemomechanical gels, e.g., “biochemo-mechanical” gel that contracts when a substrate for enzymatic reaction is added in outside solution, were demonstrated [5].

Recently, electrically stimulated systems using polyelectrolyte gels, organogels [6], gels consisting of carbon nanotube and ionic liquid [7], etc. have become popular and some materials have already come to near practical use. For example, the ion conductive polymer actuator obtained by plating a golden electrode on both sides of a perfluoro carboxylic acid film causes bending motion and biomimetic motion by electric field addition. By using the gels, hobby products such as an artificial fish (has already beencommercialized), medical operation devices (catheter, etc.), etc. are devised [8] also. In addition, gel use as actuating systems driven by a magnetic field was studied by Zrinyi et al. [9]. They prepared PVA gel containing magnetite (Fe₃O₄) particles (ferrogel) and demonstrated that the gel showed dynamic motion in response to a magnetic field.

As one of the characteristic behaviors in living systems, autonomous oscillation - that is, spontaneous changes with temporal periodicity (called “temporal structure”) such as heartbeat, brain waves, pulsatile secretion of hormone, cell cycle, biorhythm - can be exemplified. From the standpoint of biomimetics, several stimuli-responsive polymer systems have been studied, but the polymer systems undergoing self-oscillation under constant condition without any on-off switching of external stimuli are still undeveloped. If such autonomous polymer systems like a living organism can be realized by using completely synthetic polymers, then unprecedented biomimetic materials will be created.

We attempted to develop a novel gel that provides mechanical oscillation by itself without external control in a completely closed solution. We succeeded in developing such a self-oscillating polymer and gels by incorporating oscillating chemical reactions in polymer network, i.e., by constructing a built-in circuit of energy conversion cycle producing mechanical oscillation within the polymer network itself. Under the coexistence of the reactants in a closed and constant-conditioned solution, the polymer undergoes spontaneous cyclic soluble-insoluble changes or swelling-deswelling changes (in the case of gel) without any on-off switching by external stimuli, thus differently from conventional stimuli-responsive gels. Since it was first reported in 1996 as “self-oscillating gel” [10], we have been systematically studying the self-oscillating polymer and gel as well as their applications to biomimetic or smart materials [11–13]. In this review, these recent progress on the self-oscillating polymer gels and the design of autonomic and biomimetic actuators are summarized.

In order to realize the autonomous polymer system by tailor-made molecular design, we focused on the Belousov-Zhabotinsky (BZ) reaction [14,15], which is well-known for exhibiting temporal and spatiotemporal oscillating phenomena. The BZ reaction is often analogically compared with the TCA cycle (Krebs cycle), which is a key metabolic process taking place in the living body. The overall process of the BZ reaction is the oxidation of an organic substrate, such as malonic acid (MA) or citric acid, by an oxidizing agent (bromate ion), in the presence of a strong acid and a metal catalyst. In the course of the reaction, the catalyst undergoes spontaneous redox oscillation. When the solution is homogeneously stirred, the color of the solution periodically changes, like a neon sign, based on the redox changes of the metal catalyst. When the solution is placed as a thin film in stationary conditions, concentric or spiral wave patterns develop in the solution. The wave of oxidized state propagating in the medium at a constant speed is called a “chemical wave”.

It was attempted to convert the chemical oscillation of the BZ reaction to the mechanical changes of gels and generate an autonomic swelling-deswelling oscillation under nonoscillatory outer conditions. A copolymer gel, which consists of NIPAAm and ruthenium tris (2,2′-bipyridine) (Ru(bpy)₃²⁺) was prepared. Ru(bpy)₃²⁺, acting as a catalyst for the BZ reaction, is pendent to the polymer chains of NIPAAm (Figure 1). The poly(NIPAAm-co-Ru(bpy)₃²⁺) gel has a phase transition temperature because of the themosensitive constituent NIPAAm. The oxidation of the Ru(bpy)₃²⁺ moiety caused not only an increase in the degree of swelling 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)₃ moiety. As a result, it is expected that the gel undergoes a cyclic swelling-deswelling alteration when the Ru(bpy)₃ moiety is periodically oxidized and reduced under constant temperature. When the gel is immersed in an aqueous solution containing the substrates of the BZ reaction (MA, acid, and oxidant) except for the catalyst, the substrates penetrates into the polymer network and the BZ reaction occurs in the gel. Consequently, periodical redox changes induced by the BZ reaction produce periodical swelling-deswelling changes of the gel (Figure 1).

Figure 2 shows the oscillating behavior observed under a microscope for the miniature cubic poly (NIPAAm-co-Ru(bpy)₃²⁺) gel (each with a length of about 0.5 mm). In miniature gels sufficiently smaller than the wavelength of the chemical wave (typically several mm), the redox change of the ruthenium catalyst can be seen to occur homogeneously without pattern formation [16]. Due to the redox oscillation of the immobilized Ru(bpy)₃²⁺, mechanical swelling-deswelling oscillation of the gel autonomously occurs with the same period as for the redox oscillation. 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).

Typically, the oscillation period increases with a decrease in the initial concentration of substrates. The swelling-deswelling amplitude of the gel increases with an increase in the period and amplitude of the redox changes. Therefore the swelling-deswelling amplitude of the gel is controllable by changing the initial concentration of substrates. As an inherent behavior of the BZ reaction, the abrupt transition from steady state (non-oscillating state) to oscillating state occurs with a change in controlling parameters such as chemical composition, light, etc. By utilizing this characteristic, reversible on-off regulation of self-beating triggered by addition and removal of MA was successfully achieved [17]. In addition, as the gel is thermosensitive due to the NIPAAm component, the beating rhythm can be also controlled by temperature [18].

When the gel size is larger than the chemical wavelength, the chemical wave propagated in the gel is coupled with diffusion of intermediates [19–21], resulting in creation of peristaltic motion of the gel. Figure 3 shows the cylindrical gel, which is immersed in an aqueous solution containing the three reactants of the BZ reaction. The chemical waves propagate in the gel at a constant speed in the direction of the gel length. Considering the orange (Ru(II)) and green (Ru(III)) zones represent simply the shrunken and swollen parts, respectively, the locally swollen and shrunken parts move with the chemical wave, like the peristaltic motion of living worms (Figure 3) [22]. The tensile force of the cylindrical gel with oscillation was also measured [23,24].

It is well known that the period of oscillation is affected by light illumination for the Ru(bpy)₃²⁺-catalyzed BZ reaction [25]. Therefore, it is possible to intentionally make a pacemaker with a desired period (or wavelength) by local illumination of laser beam to the gel, or change the period (or wavelength) by local illumination to a pacemaker that already exists in the gel [26]. Chemical and optical control of the worm-like peristaltic motion of a structural colored porous gel were demonstrated [27–29].

In self-oscillating gel, redox changes of Ru(bpy)₃²⁺ catalyst are converted to conformational changes of polymer chain by polymerization. These periodic changes of linear and uncrosslinked 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 [38]. The conformational changes are amplified to macroscopic swelling-deswelling changes of the polymer network by crosslinking. Further, when the gel size is larger than the chemical wavelength, the chemical wave propagates in the gel by coupling with diffusion. In this case, peristaltic motion of the gel is created. In this manner, a synchronization process exists from the microscopic to the macroscopic level in the self-oscillating gel (Figure 8).

These self-oscillating gels may be useful in a number of important applications to autonomous actuator as mentioned in this review. Furthermore, in order to realize nano-actuator exhibiting autonomous oscillation (nano-oscillator), the linear polymer chain and the submicrometer-sized gel beads were actually prepared [39–43]. The self-oscillating behaviors were analyzed, and the crosslinking effects on the inter- and intrapolymer interaction under the micro-environment have been discussed. By grafting the polymers or arraying the gel beads on the surface of substrates, we have attempted to design self-oscillating surfaces as nano-conveyers to transport microparticles, etc. with the spontaneous propagation of chemical waves [44,45]. For applications in biomaterials, it is necessary to cause the self-oscillation under biological conditions without using non-biorelated BZ substrates. We attempted to introduce a pH-control site and an oxidant-supplying site into the polymer [46–49]. By using the polymer, self-oscillation was achieved only in the presence of biorelated organic acid. Applications in biomimetic and autonomous micro- or nano-actuators are expected.