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To test the hypothesis, we select PVA as a model hydrogel with tunable crystallinity. The energy per unit area required to fracture crystalline domains of a polymer is much higher than that required to fracture a single layer of amorphous chains of the same polymer ( 26). Inspired by anti-fatigue biological tissues, here, we hypothesize that the increase of crystallinity in synthetic hydrogels can substantially enhance their fatigue thresholds because of the need to fracture crystalline domains for fatigue crack propagation ( Fig. The anti-fatigue property of biological tissues possibly arises from the inherent highly ordered and partially crystalline structures of collagen fibers in the tissues ( 25). For example, the knee joint of an average person needs to sustain peak stresses of 4 to 9 MPa for 1 million cycles per year, and its fracture energy after prolonged cycles of loads is above 1000 J/m 2 ( 23, 24). In contrast to synthetic hydrogels, biological tissues such as cartilages, tendons, muscles, and heart valves show extraordinary anti-fatigue properties.

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A general strategy toward the design of anti-fatigue-fracture hydrogels has remained a critical need and a central challenge for long-term applications of hydrogels in devices and machines.

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The highest fatigue threshold for hydrogels reported so far is 418 J/m 2 for a double-network hydrogel, poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS)–PAAm, which possibly can be attributed to the PAAm network with very long polymer chains, and thus, high intrinsic fracture energy ( 13, 22). The reported fatigue thresholds of various tough hydrogels are 8.4 J/m 2 for polyacrylamide (PAAm)–polyvinyl alcohol (PVA) ( 21) and 53.2 J/m 2 for PAAm-alginate ( 20), the same order as their intrinsic fracture energies. However, existing tough hydrogels still suffer from fatigue fracture under multiple cycles of mechanical loads ( 11, 20, 21), because the resistance to fatigue crack propagation after prolonged cycles of loads is the energy required to fracture a single layer of polymer chains (i.e., the intrinsic fracture energy of the hydrogel), which is unaffected by the additional dissipation mechanisms introduced in tough hydrogels ( 20, 21). The toughening of hydrogels is achieved by integrating mechanisms for dissipating mechanical energy such as fracture of short polymer chains and reversible cross-links into stretchy polymer networks ( 18, 19). ( 13), hydrogels have been made tough to resist crack propagation under a single cycle of mechanical load ( 14– 17). Following the pioneering work by Gong et al. The use of hydrogels in devices and machines requires them to maintain robustness under cyclic mechanical loads. More recently, hydrogels have been explored for applications in devices and machines, including wearable electronics ( 4, 5), stretchable optical fibers ( 6), adhesives ( 7– 9), soft robotics ( 10), and hydrogel-based soft machines ( 11, 12). Cyclic loading of the reinforced kirigami hydrogel sheet.Īs polymer networks infiltrated with water, hydrogels have been widely used as scaffolds for tissue engineering ( 1), vehicles for drug delivery ( 2), and model platforms for biological studies ( 3). Tension of the pristine notched sample and the tip-reinforced sample. Electrical circuit and thermal mapping for programmable crystalline domains. Raman spectroscopy of freeze-thawed PVA and 90-min dry-annealed PVA.įig. Mechanical characterization of PVA hydrogels with various crystallinities.įig. Validation of high fatigue threshold with pure-shear test.įig. Validation of high fatigue threshold with single-notch test.įig. Steady-state nominal stress versus stretch curves of PVA hydrogels with various crystallinities.įig. Shakedown softening of three types of PVA hydrogels.įig. Experiment method for measuring fatigue threshold.įig. Measurement of the mass of freeze-thawed PVA during air-drying and the amount of residual water in the sample after air-drying.įig.

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Crystallinities and water contents in chemically cross-linked (Ch), freeze-thawed (FT), and dry-annealed PVA with annealing times of 0, 1, 3, 5, 10, and 90 min.įig. Supplementary material for this article is available at

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