Self-oscillating gel as novel biomimetic materials

A self-oscillating gel system with complex dynamic behavior based on a time delay between the oscillations

The time delay existing between the chemical oscillation and mechanical oscillation (C-M delay) in a self-oscillating gel (SOG) system is observable in previous experimental studies. However, how the C-M delay affects the dynamic behavior of a large anisotropic SOG has not been quantified or reported systematically. In this study, we observed that the oscillation period increases with a decrease in the cross-linking density of the anisotropic SOG, and this determined whether regular mechanical oscillation occurs. Unlike before, the disrupted mechanical oscillations interestingly tend to be regular and periodic under visible light, which is an inhibitor for the B-Z reaction incorporating the Ru complex as a catalyst (Ru-BZ reaction). Moreover, the study of the C-M delay at different scales has far-reaching implications for intelligent soft actuators.

A macroporous smart gel based on a pH-sensitive polyacrylic polymer for the development of large size artificial muscles with linear contraction

The physics of soft matter can contribute to the revolution in robotics and medical prostheses. These two fields require the development of artificial muscles with behavior close to biological muscles. Today, artificial muscles rely mostly on active materials, which can deform reversibly. Nevertheless transport kinetics is the major limit for all of these materials. These actuators are only made of a thin layer of active material and using a large thickness dramatically reduces the actuation time. In this article, we demonstrate that a porous material reduces the limit of transport and enables the use of a large volume of active material. We synthesize a new active material: a macroporous gel, which is based on polyacrylic acid. This gel shows very large swelling when we increase the pH and the macroporosity dramatically reduces the swelling time of centimetric samples from one day to 100 s. We characterize the mechanical properties and swelling kinetics of this new material. This material is well adapted for soft robotics because of its large swelling ratio (300%) and its capacity to apply a pressure of 150 mbar during swelling. We demonstrate finally that this material can be used in a McKibben muscle producing linear contraction, which is particularly adapted for robotics. The muscle contracts by 9% of its initial length within 100 s, which corresponds to the gel swelling time.

Chemical micro-oscillators based on the Belousov–Zhabotinsky reaction

The results of studies on the development of micro-oscillators (MOs) based on the Belousov –Zhabotinsky (BZ) oscillatory chemical reaction are integrated and systematized. The mechanisms of the BZ reaction and the methods of immobilization of the catalyst of the BZ reaction in micro-volumes are briefly discussed. Methods for creating BZ MOs based on water microdroplets in the oil phase and organic and inorganic polymer microspheres are considered. Methods of control and management of the dynamics of BZ MO networks are described, including methods of MO synchronization. The prospects for the design of neural networks of MOs with intelligent-like behaviour are outlined. Such networks present a new area of nonlinear chemistry, including, in particular, the creation of a chemical ‘computer’.

The bibliography includes 250 references.

Towards synergistic oscillations in enzymatically active hydrogel spheres

Stimuli-responsive polymers are capable of reacting to an external trigger. We report self-regulated, enzymatically active, and pH-responsive hydrogels that show dynamic behavior without an external trigger. This is enabled by a feedback loop between the enzymatic conversion of glucose into gluconic acid and the pH-induced volume phase transition that leads to a modulation in glucose permeability. The synthesized hydrogel spheres combine all required properties for sustained oscillation including enzymatic activity, switchable reactivity, hysteresis in volume phase transition and feedback between the reaction and permeation. A simple model of the system identified possible operating points where sustained oscillations are possible. Experiments at these operating points revealed that the system is able to perform a self-regulated oscillation cycle under a constant nutrient supply. A sensitivity analysis showed that the system is especially sensitive around the point of oscillation, so that precise control of the process parameters is essential to achieve sustained oscillations.

The Belousov-Zhabotinsky Reaction in Thermoresponsive Core-Shell Hydrogel Microspheres with a Tris(2,2'-bipyridyl)ruthenium Catalyst in the Core

The Belousov-Zhabotinsky (BZ) reaction shows temporal or spatiotemporal structures such as redox oscillation of the catalyst, [ruthenium(II)tris(2,2'-bipyridine)][PF₆]₂ ([Ru(bpy)₃][PF₆]₂). In this study, autonomously oscillating hydrogel microspheres (microgels) were investigated, which show swelling/deswelling oscillation induced by the redox oscillation of the BZ reaction inside the gel. Despite the periodically and autonomously induced oscillation that does not require an external stimulus, it has not been possible to perform any manipulation of the oscillatory behavior over time. The results of the present study show that it is possible to reversibly switch the microgel oscillations from an "on" active state of the BZ reaction to an "off" inactive state by changing the temperature in combination with thermoresponsive microgels. To realize on-demand switching, the construction of double-shell structures is crucial; the thermoresponsive first shell allows the microgels to modulate the diffusion of the substrates or intermediates in the BZ reaction, while the second shell maintains colloidal stability under high temperatures and high ion concentrations. The functionalized double-shell microgels were prepared via multistep seeded precipitation polymerization. The oscillatory switching behavior of the BZ reaction was observed directly and evaluated by ultraviolet-visible (UV-vis) spectroscopy. The central concept of this study, i.e., "on-off switching" can be expected to benefit the development of advanced bioinspired materials.

Viewpoint: Homeostasis as Inspiration-Toward Interactive Materials

Homeostatic systems combine an ability to maintain integrity over time with an incredible capacity for interactive behavior. Fundamental to such systems are building blocks of "mini-homeostasis": feedback loops in which one component responds to a stimulus and another opposes the response, pushing the module to restore its original configuration. Particularly when they cross time and length scales, perturbation of these loops by external changes can generate diverse and complex phenomena. Here, it is proposed that by recognizing and implementing mini-homeostatic modules-often composed of very different physical and chemical processes-into synthetic materials, numerous interactive behaviors can be obtained, opening avenues for designing multifunctional materials. How a variety of controlled, nontrivial material responses can be evoked from even simple versions of such synthetic feedback modules is illustrated. Moreover, random events causing seemingly random responses give insights into how one can further explore, understand and control the full interaction space. Ultimately, material fabrication and exploration of interactivity become inseparable in the rational design of such materials. Homeostasis provides a lens through which one can learn how to combine and perturb coupled processes across time and length scales to conjure up exciting behaviors for new materials that are both robust and interactive.

Autonomous Movement System Induced by Synergy between pH Oscillation and a pH-Responsive Oil Droplet

A sustainable droplet motion that is driven by pH oscillation was obtained. The pH oscillation is only of a single pulse in a batch reactor. However, it shows continuous oscillation around the moving droplet, as the motion itself controls the diffusion flux in an asymmetric manner. Various types of motions that are spontaneous in nature may be obtained by a single-pulse oscillation coupled with mass transport.

Synchronization of Two Self-Oscillating Gels Based on Chemo-Mechanical Coupling

Two coupled polymer gels, showing volume oscillation caused by the Belousov-Zhabotinsky (BZ) reaction, were investigated to understand the system composed of mechanically coupled chemical oscillators. The two gels were connected with a movable plastic sheet in between and placed under constant compression. Synchronization between two identical gels occurred in a range of compression ratios. The phase difference between the two oscillating gels was not zero; instead, they showed alternate swelling-deswelling oscillations. Similar phenomena were also observed with gels of different sizes and natural oscillation periods. The experimental results suggest that a physical change in one gel can lead to a chemical change in the other and vice versa. These results were qualitatively reproduced by a mathematical model based on coupled chemical oscillators.

Kinetic modelling and bifurcation analysis of chemomechanically miniaturized gels under mechanical load

Chemomechanically responsive gels, with great potential applications in the fields of smart structures and biomedicines, present autonomously oscillatory deformation driven by the Belousov-Zhabotinsky chemical reaction. The dynamic behavior of the responsive gels is obviously affected by the external mechanical load. This approach proposed a kinetic model with an ordinary differential equation to describe the oscillatory deformation of the gels under the mechanical load. Then the periodic solutions and phase diagrams of the oscillation are obtained using the improved Runge-Kutta and shooting methods. The results demonstrated that bifurcations are typically existent in the system and the characters of the oscillatory deformation regularly depend on the mechanical load as well as the concentration of reactants and the stoichiometric coefficient of chemical reaction. This development is supposed to promote the practical applications of the chemomechanically responsive gels.

Hydrogels for delivery of bioactive agents: a historical perspective

Since 1960 when the history of modern hydrogels began significant progress has been made in the field of controlled drug delivery. In particular, recent advances in the so-called smart hydrogels have made it possible to design highly sophisticated formulations, e.g., self-regulated drug delivery systems. Despite intensive efforts, clinical applications of smart hydrogels have been limited. Smart hydrogels need to be even smarter to execute functions necessary for achieving desired clinical functions. It is necessary to develop novel hydrogels that meet the requirements of the intended, specific applications, rather than finding applications of newly developed hydrogels. Furthermore, developing smarter hydrogels that can mimic natural systems is necessary, but the fundamental differences between natural and synthetic systems need to be understood. Such understanding will allow us to develop novel hydrogels with the new, multiple functions that we are looking for.

A smart hydrogel-based time bomb triggers drug release mediated by pH-jump reaction

We demonstrate a timed explosive drug release from smart pH-responsive hydrogels by utilizing a phototriggered spatial pH-jump reaction. A photoinitiated proton-releasing reaction of o-nitrobenzaldehyde (o-NBA) was integrated into poly(N-isopropylacrylamide-co-2-carboxyisopropylacrylamide) (P(NIPAAm-co-CIPAAm)) hydrogels. o-NBA-hydrogels demonstrated the rapid release of protons upon UV irradiation, allowing the pH inside the gel to decrease to below the pK_a value of P(NIPAAm-co-CIPAAm). The generated protons diffused gradually toward the non-illuminated area, and the diffusion kinetics could be controlled by adjusting the UV irradiation time and intensity. After irradiation, we observed the enhanced release of entrapped L-3,4-dihydroxyphenylalanine (DOPA) from the gels, which was driven by the dissociation of DOPA from CIPAAm. Local UV irradiation also triggered the release of DOPA from the non-illuminated area in the gel via the diffusion of protons. Conventional systems can activate only the illuminated region, and their response is discontinuous when the light is turned off. The ability of the proposed pH-jump system to permit gradual activation via proton diffusion may be beneficial for the design of predictive and programmable devices for drug delivery.

Imaging the Belousov-Zhabotinsky reaction in real time using an ion sensitive array

We show how an array of ion-sensitive-field-effect-transistors can be used to both spatially and temporally image the oscillating pH/ion waves produced by the Belousov-Zhabotinsky (BZ) reaction with high resolution.

Self-driven gel conveyer: effect of interactions between loaded cargo and self-oscillating gel surface

A "self-oscillating" gel that swells and deswells periodically under constant conditions is developed as a novel biomimetic gel differing from conventional stimuli-responsive polymer gels. By utilizing the peristaltic motion of the self-oscillating gel, autonomous mass-transport systems can be realized. With the propagation of the chemical wave, the loaded gel cargo is autonomously transported by rotating on the surface. To apply the self-driven gel conveyer to a wider range of uses, it is important to investigate the influence of the physical interaction between the self-oscillating gel and the loaded cargo on its transporting ability. Here, the effect of the interaction is evaluated by using several kinds of gel cargo with varying charge states, hydrophilicities, and surface roughnesses.

Application of smart nanostructures in medicine

Smart nanostructures are sensitive to various environmental or biological parameters. They offer great potential for numerous biomedical applications such as monitoring, diagnoses, repair and treatment of human biological systems. The present work introduces smart nanostructures for biomedical applications. In addition to drug delivery, which has been extensively reported and reviewed, increasing interest has been observed in using smart nanostructures to develop various novel techniques of sensing, imaging, tissue engineering, biofabrication, nanodevices and nanorobots for the improvement of healthcare.

Self-oscillating gels driven by the Belousov-Zhabotinsky reaction as novel smart materials

So far stimuli-responsive polymer gels and their application to smart materials have been widely studied; this research has contributed to progress in gel science and engineering. For their development as a novel biomimetic polymer, studies of polymers with an autonomous self-oscillating function have been carried out since the first reports in 1996. The development of novel self-oscillating polymers and gels have been successful utilizing the oscillating reaction, called the Belousov-Zhabotinsky (BZ) reaction, which is recognized as a chemical model for understanding several autonomous phenomena in biological systems. The self-oscillating polymer is composed of a poly(N-isopropylacrylamide) network in which the catalyst for the BZ reaction is covalently immobilized. In the presence of the reactants, the polymer undergoes spontaneous cyclic soluble-insoluble changes or swelling-deswelling changes (in the case of gel) without any on-off switching of external stimuli. Potential applications of the self-socillating polymers and gels include several kinds of functional material systems, such as biomimetic actuators and mass transport surface.

Design of self-oscillating gels and application to biomimetic actuators

As a novel biomimetic polymer, we have developed polymer gels with an autonomous self-oscillating function. This was achieved by utilizing oscillating chemical reactions, called the Belousov-Zhabotinsky (BZ) reaction, which is recognized as a chemical model for understanding several autonomous phenomena in biological systems. Under the coexistence of the reactants, the polymer gel undergoes spontaneous swelling-deswelling changes without any on-off switching by external stimuli. In this review, our recent studies on the self-oscillating polymer gels and application to biomimetic actuators are summarized.