Scientists want to develop biomimetic materials both for practical design purposes and as simple model systems to improve their understanding of more complex biological phenomena. Promising stimuli-responsive materials have been synthesized with entangled gels or cross-linked polymer networks swollen with varying degrees of solvent. These responsive gels react to external environmental changes such as in temperature, pH (a measure of the acidity or basicity of a solution), or electric field.1,2 Typically, the response involves swelling, collapse, or reorientation of the polymer network, leading to notable deformation or release of particles originally trapped inside. Applications range from drug delivery to mechanical actuators in microfluidic devices.
If the gel's response could be autonomous, it will be spontaneous as a result of internal stimuli. Classical examples of automatic behavior are the visceral nervous system that affects our internal organs and the beating heart. Responsive gels with this characteristic are beginning to become a reality. For example, such gels driven by the Belousov-Zhabotinsky (BZ) reaction3 display autonomous, rhythmic chemical and mechanical swell-deswell oscillations without needing an external stimulus.4 In the BZ gel reaction, a metal catalyst immobilized in the network is involved in both rapid autocatalytic and reset steps. In the former, the metal loses an electron (oxidized) and during the reset step it gains an electron (reduced). The primary effect is that the gel oscillates between less hydrophilic (reduced) and more hydrophilic (oxidized) states. As a result, it deswells and swells when the metal is reduced and oxidized, respectively. A color change also accompanies the catalyst reduction-oxidation, providing an easy way to see the chemical changes (see Figure 1). We can easily observe traveling waves associated with millimeter-scale gel systems. Uniform swell-deswell oscillations occur for samples smaller than the chemical wavelength (hundreds of micrometers).4
Figure 1. Color change in a Belousov-Zhabotinsky reaction gelatin film (color enhanced).
These materials are attractive for their ability to reversibly couple mechanical and chemical behavior and their inherent biomimetic qualities. Indeed, the pulsing motion of self-oscillating gels brings to mind rhythmic beating of biological tissue. Potential applications include autonomous micro-actuators, device transport, drug delivery, information processing, autonomously modulating optics, and sensors that convert mechanical stimuli into chemical signals.
The majority of work to date has centered on BZ gels composed of poly(N-isopropylacrylamide) (NIPAAm). The metal catalyst, a ruthenium vinyl bipyridine complex—Ru(vbpy)—is immobilized in the polymer network as a co-monomer. This architecture has various biomimetic structures related to mobility and transport.5 Recently, other researchers examined self-oscillating gels from a theoretical perspective with several interesting predicted results, such as synchronized coupled oscillators, conversion of mechanical impact to chemical waves, gels with changing elasticity, and photo-directed motion.6–8 Overall, BZ autonomic gels have further enriched the investigation of nonlinear chemical oscillators, complexity, and pattern emergence.9
To realize the potential of these materials and design functional autonomous devices, further investigations into the relationship between chemomechanical response and material performance limits are needed. Numerical parameters used in the theoretical predictions require further validation to tackle the inverse design-optimization problem. And new gel chemistries should be developed to explore the entire design space. In addition, the precise effects of network morphology on material strength and durability, chemomechanical response, and potential-force production need to be understood.
We recently developed two new, water-based chemistries to address these goals by expanding processing alternatives. The first system binds a ruthenium succinimide bipyridine complex—Ru(sbpy)—to the biopolymer gelatin using a succinimide-amine coupling reaction10 (see Figure 2). The second is based on polyacrylamide (PAAm). Ru(vpby) can be introduced during polymerization, as is done with NIPAAm. Alternatively, Ru(sbpy) can also be used to bind the ruthenium complex to the primary amines present in PAAm. The main advantage of using Ru(sbpy) is that it makes hydrogel post-functionalization possible. As a result, a film with no metal catalyst can be manufactured and subsequently patterned through the succinimide-amine coupling reaction.
Figure 2. The succinimide-amine coupling reaction enables post-functionalization and gel patterning. The dark curve attached to the primary amine (NH2) represents a polymer chain. Ru(sbpy): Ruthenium succinimide bipyridine.
To characterize the chemomechanical properties of the BZ gelatin system, we measured the chemical wave-period dependence on the BZ reactants' initial concentrations. The BZ gelatin's chemical response is quite robust with a repeatable and well-defined range of wave periods over various reactant concentrations. We saw significant swell-deswell oscillations in the PAAm system. Investigations of the chemical and mechanical responses of these new BZ gels is ongoing, including of mechanical swell-deswell amplitudes and mechanical-force production related to the polymer-network architecture, as well as creation of devices with patterned autonomous regions for response amplification and information processing.
Self-oscillating gels display many attractive biomimetic qualities, such as autonomous mechanical motion and mechanical-to-chemical signal transduction. Experimental and theoretical efforts must continue to determine the performance limits of these materials to increase speed, stability, and compatibility with conventional device-fabrication methods. We have developed two new systems based on gelatin and polyacrylamide, both with potential for patterning through post-functionalization, that provide new opportunities to address these critical challenges. In the future, we plan to explore chemomechanical oscillations in patterned gels and the relationship between gel morphology and optimal mechanical actuation.
Matthew Smith and Kevin Heitfeld gratefully acknowledge support from the National Research Council.
Richard A. Vaia, Matthew Smith
Air Force Research Laboratory
Wright-Patterson Air Force Base, OH
Renegade Materials Corp.
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