(721a) Nanostructure-Driven Fatigue Resistance and Dynamic Recovery in Thermoplastic Elastomer Hydrogel Networks

Creative efforts in network design have led to numerous impactful improvements in hydrogel mechanics. Our group has been focused on the synthesis of hydrogel networks capable of rapid and complete elastic recovery coupled with a high intrinsic resistance to mechanical fatigue under continuous cyclic loading. Much of our motivation is derived from a desire to synthesize biphasic materials capable of replicating the biomechanical demands of musculoskeletal soft tissues, which has continued to present a formidable materials science challenge. Reduction to practice has been plagued by the gap in mechanical performance that continues to differentiate biological and synthetic systems. The most prominent advances in hydrogel design have employed the inclusion of interpenetrating networks, hydrophobic interactions,^{2, 3} ionic interactions,^{4, 5, 6} deformable domain structures,⁷ sliding junction points,^{8, 9, 10} and multifunctional macromers of prescribed structure^{2, 11, 12} to produce intriguing network designs pushing the boundaries of mechanical achievement. These designs are responsible for stunning demonstrations of modulus, ^{4, 6} stretchability, ^{3, 5, 6, 8, 9} ultimate strength,^{1, 2, 11} and overall toughness ^{3, 5, 6, 8, 9} that were inconceivable less than two decades ago. However, most of these systems are plagued by combinations of limited moduli at small strain, unacceptably high levels of energy dissipation and fatigue, or slow and incomplete elastic recovery that are incompatible with the cyclic biomechanical loading profiles of most musculoskeletal soft tissues. Intrinsic susceptibility to permanent covalent bond rupture and significant plastic deformation preclude a network from returning elastically to its original configuration, while recovery dynamics in the minutes to hours range^{5, 6} preclude it from doing so at physiologically relevant (often sub-second) time scales. Unfortunately, none of these advanced systems would yet be acceptable, for example, as substitutes for the fibrocartilage comprising menisci of the knee, the annulus fibrosis of the intervertebral disc, or connective tissue of the cardiovascular system, which rely on high levels of elasticity to rapidly absorb and transfer (not dissipate) strain energy into efficient body movement or pulsatile blood flow.^{13, 14, 15}

Here, we report a new paradigm in hydrogel design based on prefabrication of an efficient nanoscale network architecture using the melt-state self-assembly of amphiphilic block copolymers. Rigorous characterization and mechanical testing reveal that swelling of these preformed networks produces hydrogels with physiologically relevant moduli and water compositions, negligible hysteresis, sub-second elastic recovery rates, and unprecedented resistance to fatigue over hundreds of thousands of compression cycles. Furthermore, by relying only on simple thermoplastic processing to form these nanostructured networks, the synthetic complexities common to most solution-based hydrogel fabrication strategies are completely avoided. We use melt blends of sphere morphology forming AB and ABA diblock copolymers to form a network of tethered micelle units. During melt-state self-assembly, A blocks form spherical aggregates while the B blocks form the corona or matrix in which the spherical A domains sit. The triblock copolymer acts as a bridge, mechanically linking adjacent spherical domains. Hydrogel formation relies on two important phenomena that dictate the choice of A and B blocks. One, the A block must be able to become a vitrified solid upon cooling. We use polystyrene (PS) with a glass transition temperature near 80 ËšC. Two, the B block must be selectively soluble in the swelling media, while the A block remains impervious to it. We use poly(ethylene oxide) (PEO) as the water soluble B block.

In terms of network structure this method of hydrogel fabrication produces key features we believe are critical for eliminating fatigue, producing a high modulus, and ensuring rapid recovery. First we counter the effects of strand length inhomogeneity by keeping the density of fixed junction points (ρ_x-link, glassy spherical domains) low, and the strand molecular weight adjoining them high (M_x-link,180kDa) and narrowly distributed (PDI < 1.06). This would normally favor soft, weak materials, but we use extremely high junction point functionalities (f ~ 230, block copolymer chains per spherical aggregate) in the 200 – 300 range. Thus even moderate triblock copolymer compositions produce a high numbers of strands emanating from each sphere. Finally, because assembly takes place in the melt, the density of topologically trapped entanglements among these bridging (and looping) triblock copolymers is intrinsically quite high. In the model PS-PEO systems used to develop our preliminary systems, we estimate the number of entanglements per fixed junction point numbers in the thousands. These entanglements act much like the sliding junction points introduced by Ito in his slide ring-gels, and provide the network with both a significant modulus and an ability to rapidly redistribute stress, the latter being the keystone to the unusual fatigue resistance exhibited by these systems.

References

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