Hydrogels muscle their way into new territory

Abstract

Materials that change properties from one state to another in response to a stimulus such as temperature or light are often referred to as “smart,” although “obedient” might be a more accurate term. Biological materials are smart in an entirely different way, in that they undergo a series of localized responses to adapt to a new state that is tailored to meet the demands of their changing environment. Bone and skeletal muscle, for example, build mass and strength only where and when they experience sufficient load. On page 504 of this issue, Matsuda et al. ([ 1 ][1]) bring a similar level of adaptive response one step closer in a synthetic material. They show that double-network (DN) hydrogels ([ 2 ][2]), which can be thought of as having two distinct but interpenetrating networks, can heal and even strengthen after repeated mechanical stress. ![Figure][3] Intertwined gels execute a fix Matsuda et al. use double-network hydrogels to enable fixing of damage from mechanical stress. A rigid polymer (gray) executes repair while a more flexible polymer (blue) maintains shape. GRAPHIC: C. BICKEL/ SCIENCE To mimic the adaptive properties of muscle, synthetic materials must be able to use localized mechanical forces to trigger chemistry that increases their mass, strength, or both. Rather than incorporate sensors and circuitry (the synthetic equivalent of a nervous system) to fulfill the detection and transduction requirements, Matsuda et al. chose mechanochemistry, which directly couples the mechanical forces experienced at the molecular scale to the desired chemical response ([ 3 ][4]). Mechanochemical responses range from simple bond-breaking reactions that have been known for years, to an expanding range of increasingly diverse and complex chemical reactions that have been developed more recently. Some of these reactions have even been used to trigger subsequent bond formations, including polymerizations that increase mass and cross-linking that adds strength ([ 4 ][5]). These demonstrations generally have been limited by a common shortcoming; namely, that the same stresses and strains necessary to trigger the mechanochemical response also result in irreversible deformation of the bulk material. This is obviously a critical weakness, as mechanochemical adaptation can only be useful in structural materials if the materials retain their core structure throughout the events that trigger the adaptation. To solve this problem, Matsuda et al. used DN hydrogels (see the figure) and took advantage of critical differences in the primary, permanent covalent structure of their two networks. The first network comprises strands that, in the resting state of the DN gel, are already quite extended relative to a more compact random coil. The second network possesses little to no pre-extension of this type, so that the constituent strands are on average balled up in a much more compact configuration. Because extensions of the macroscopic material are mirrored in extensions at the microscopic and molecular level, the second network can be stretched several times further than the first before it reaches its structural limit. This difference in the limits of extension allows the DN gels to realize the desired adaptive behavior. When a DN gel is stretched, strands in the first network quickly reach their theoretical limit, and continued stretching causes them to break in what is known as a homolytic fashion; the newly created chain ends of bond scission are carbon-centered radicals whose unpaired electrons make them highly reactive. These radicals are well-known intermediates in free-radical polymerizations, and once generated, they can react with an available supply of monomers (such as acrylamides) to add additional mass onto the first network of the DN gel. Although the first network is supplying the necessary chemical reactivity, it is the second network that creates the opportunity for the material to adapt to mechanical stress. The second network has much greater slack in its average constituent strand, so it remains fully intact while the first network starts to break. Because the two networks are intertwined, the second network effectively holds the shape of the initial DN gel in place, even as the first network is breaking. The intertwined structure also greatly enhances the toughness of the DN gel, so that far greater mechanochemical response, and subsequent polymerization, can be realized than in a single network alone. The net mechanochemical response leads to soft materials that respond to damage to their underlying molecular scaffold by building both mass and strength at the sites of activation. As in the muscles that inspire the design, this response is localized to the regions of stress concentration, as demonstrated by stamping experiments. Looking ahead, the demonstration by Matsuda et al. offers several opportunities for broadening its scope. For example, the reactive monomer in these systems is simply included in the aqueous content of the gel, but reactive monomers could instead be supplied by flow through a vascular network akin to a circulatory system ([ 5 ][6]). If this supply were regulated, the chemical nature and properties of the added mass could be varied. In addition, the range of mechanochemical responses that is now available extends far beyond homolytic bond scission, offering the potential for even greater control of the polymerization, the range of functional responses, or both ([ 3 ][4], [ 6 ][7]). Also, because the changes in mechanical properties of the DN gels are localized, there is the possibility for engineering nonlinear adaptive behavior, in which a response in one region of the material makes it either more or less likely for a subsequent response to occur in another region. This approach would create an opportunity for complex, patterned structural evolution of a material with properties that might not have been predicted in...