Research ArticleENGINEERING

Anti-fatigue-fracture hydrogels

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Science Advances  25 Jan 2019:
Vol. 5, no. 1, eaau8528
DOI: 10.1126/sciadv.aau8528
  • Fig. 1 Design principle for anti-fatigue-fracture hydrogels.

    (A) Illustration of fatigue crack propagation in an amorphous hydrogel and in hydrogels with low and high crystallinities under cyclic loads. The yellow areas represent crystalline domains, and the blue areas denote amorphous domains. In the amorphous hydrogel and the hydrogel with low crystallinity, the fatigue threshold can be attributed to the energy required to fracture a single layer of polymer chains per unit area. In the hydrogel with high crystallinity, the fatigue crack propagation requires fracture of crystalline domains. (B) Illustration of measuring nominal stress S versus stretch λ curves over N cycles of the applied stretch λA. The stress-stretch curve reaches steady state as N reaches a critical value N*. (C) Illustration of measuring crack extension per cycle dc/dN versus energy release rate G curves. By linearly extrapolating the curve to intercept with the abscissa, we can approximately obtain the critical energy release rate Gc, below which the fatigue crack will not propagate under infinite cycles of loads. By definition, the fatigue threshold Γ0 is equal to the critical energy release rate Gc.

  • Fig. 2 Characterization of crystalline domains in PVA hydrogels.

    (A) Representative DSC thermographs of chemically cross-linked (Ch), freeze-thawed (FT), and dry-annealed PVA with annealing times of 0, 3, 10, and 90 min. (B) Water contents of chemically cross-linked, freeze-thawed, and dry-annealed PVA with annealing times of 0, 1, 3, 5, 10, and 90 min. (C) Measured crystallinities in the dry and swollen states of chemically cross-linked, freeze-thawed, and dry-annealed PVA with annealing times of 0, 1, 3, 5, 10, and 90 min. (D) Representative SAXS profiles of freeze-thawed and dry-annealed PVA, with annealing times of 0, 10, and 90 min. (E) Representative WAXS profiles of dry-annealed PVA with dry-annealing times of 0, 3, 10, and 90 min. a.u., arbitrary units. (F) SAXS profiles of 90-min dry-annealed PVA in the dry state and swollen state. The insets illustrate the increase of the distance between adjacent crystalline domains due to swelling of amorphous polymer chains. (G) Estimated average distance between adjacent crystalline domains L and average crystalline domain size D of dry-annealed PVA with annealing times of 0, 1, 3, 5, 10, and 90 min. (H) AFM phase images of dry-annealed PVA with annealing times of 0 and 90 min. Data in (B), (C), and (G) are means ± SD, n = 3.

  • Fig. 3 Measurement of fatigue thresholds of PVA hydrogels.

    Nominal stress S versus stretch λ curves over cyclic loads for (A) chemically cross-linked hydrogel at an applied stretch of λA = 1.6, (B) freeze-thawed hydrogel at an applied stretch of λA = 2.2, and (C) 90-min dry-annealed hydrogel at an applied stretch of λA = 2.0. Crack extension per cycle dc/dN versus applied energy release rate G for (D) chemically cross-linked hydrogel, (E) freeze-thawed hydrogel, and (F) dry-annealed hydrogel with annealing time of 90 min. (G) The fatigue threshold increases with the crystallinity of the hydrogel in the swollen state. (H) Validation of fatigue threshold as high as 1000 J/m2 in 90-min dry-annealed hydrogel using the single-notch test. Data in (G) are means ± SD, n = 3. Scale bars, 5 mm (H, left) and 1 mm (H, right).

  • Fig. 4 Young’s moduli, tensile strengths, and water contents of PVA hydrogels.

    (A) Young’s modulus versus crystallinity in the swollen state. (B) Tensile strength versus crystallinity in the swollen state. (C) Water content versus crystallinity in the swollen state. Data in (A) to (C) are means ± SD, n = 3.

  • Fig. 5 Patterning highly crystalline regions in PVA hydrogels.

    (A) Illustration of introducing a highly crystalline region around crack tip. Inset: Raman spectroscopy with bright color representing low water content and dark color representing high water content (see details in Materials and Methods). (B) Comparison of crack extension per cycle dc/dN versus applied energy release rate G between the pristine sample and the tip-reinforced sample. The fatigue thresholds of the pristine sample and the tip-reinforced sample are 15 and 236 J/m2, respectively. (C) Illustration of introducing mesh-like highly crystalline regions. Inset: Digital image correlation (DIC) method shows large deformation in low-crystallinity regions and small deformation in high-crystallinity regions. (D) Crack extension per cycle dc/dN versus applied energy release rate G of the mesh-reinforced sample. The fatigue threshold of the mesh-reinforced sample is 290 J/m2. (E) Water contents of the pristine sample, the tip-reinforced sample, the mesh-reinforced sample, and the fully annealed sample. (F) Young’s moduli of the pristine sample, the tip-reinforced sample, the mesh-reinforced sample, and the fully annealed sample. (G) Illustration of introducing highly crystalline regions around cut tips in a pristine kirigami sheet. (H) Effective nominal stress versus stretch curves of the reinforced kirigami sheet under cyclic loads. Effective nominal stress versus stretch curve of the pristine kirigami sheet under a single cycle of load. (I) Images of the reinforced kirigami sheet under 1000th cycle and under 3000th cycle. (J) Comparison of fatigue thresholds and water contents among reported synthetic hydrogels (2022, 40, 41), PVA hydrogels with patterned highly crystalline regions, and biological tissues (23). (K) Comparison of fatigue thresholds and Young’s moduli among reported synthetic hydrogels, PVA hydrogels with patterned highly crystalline regions, and biological tissues. IPN in (J) and (K) represents interpenetrating polymer network. Data in (E) and (F) are means ± SD, n = 3. Scale bars, 800 μm (A), 1 mm (C), and 40 mm (I).

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/1/eaau8528/DC1

    Table S1. 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.

    Fig. S1. Measurement of the mass of freeze-thawed PVA during air-drying and the amount of residual water in the sample after air-drying.

    Fig. S2. Experiment method for measuring fatigue threshold.

    Fig. S3. Shakedown softening of three types of PVA hydrogels.

    Fig. S4. Steady-state nominal stress versus stretch curves of PVA hydrogels with various crystallinities.

    Fig. S5. Validation of high fatigue threshold with single-notch test.

    Fig. S6. Validation of high fatigue threshold with pure-shear test.

    Fig. S7. Mechanical characterization of PVA hydrogels with various crystallinities.

    Fig. S8. Raman spectroscopy of freeze-thawed PVA and 90-min dry-annealed PVA.

    Fig. S9. Electrical circuit and thermal mapping for programmable crystalline domains.

    Movie S1. Tension of the pristine notched sample and the tip-reinforced sample.

    Movie S2. Cyclic loading of the reinforced kirigami hydrogel sheet.

  • Supplementary Materials

    The PDF file includes:

    • Table S1. 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.
    • Fig. S1. Measurement of the mass of freeze-thawed PVA during air-drying and the amount of residual water in the sample after air-drying.
    • Fig. S2. Experiment method for measuring fatigue threshold.
    • Fig. S3. Shakedown softening of three types of PVA hydrogels.
    • Fig. S4. Steady-state nominal stress versus stretch curves of PVA hydrogels with various crystallinities.
    • Fig. S5. Validation of high fatigue threshold with single-notch test.
    • Fig. S6. Validation of high fatigue threshold with pure-shear test.
    • Fig. S7. Mechanical characterization of PVA hydrogels with various crystallinities.
    • Fig. S8. Raman spectroscopy of freeze-thawed PVA and 90-min dry-annealed PVA.
    • Fig. S9. Electrical circuit and thermal mapping for programmable crystalline domains.
    • Legends for movies S1 and S2

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    Other Supplementary Material for this manuscript includes the following:

    • Movie S1 (.mp4 format). Tension of the pristine notched sample and the tip-reinforced sample.
    • Movie S2 (.mp4 format). Cyclic loading of the reinforced kirigami hydrogel sheet.

    Files in this Data Supplement:

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