Natural load-bearing materials such as tendons have a high water content of about 70 per cent but are still strong and tough, even when used for over one million cycles per year, owing to the hierarchical assembly of anisotropic structures across multiple length scales1. Synthetic hydrogels have been created using methods such as electro-spinning2, extrusion3, compositing4,5, freeze-casting6,7, self-assembly8 and mechanical stretching9,10 for improved mechanical performance. However, in contrast to tendons, many hydrogels with the same high water content do not show high strength, toughness or fatigue resistance. Here we present a strategy to produce a multi-length-scale hierarchical hydrogel architecture using a freezing-assisted salting-out treatment. The produced poly(vinyl alcohol) hydrogels are highly anisotropic, comprising micrometre-scale honeycomb-like pore walls, which in turn comprise interconnected nanofibril meshes. These hydrogels have a water content of 70–95 per cent and properties that compare favourably to those of other tough hydrogels and even natural tendons; for example, an ultimate stress of 23.5 ± 2.7 megapascals, strain levels of 2,900 ± 450 per cent, toughness of 210 ± 13 megajoules per cubic metre, fracture energy of 170 ± 8 kilojoules per square metre and a fatigue threshold of 10.5 ± 1.3 kilojoules per square metre. The presented strategy is generalizable to other polymers, and could expand the applicability of structural hydrogels to conditions involving more demanding mechanical loading.
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The data that support the findings of this study are available from the corresponding author on reasonable request.
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This research was supported by NSF CAREER award 1724526, AFOSR awards FA9550-17-1-0311, FA9550-18-1-0449 and FA9550-20-1-0344, and ONR awards N000141712117 and N00014-18-1-2314. X.Z. acknowledges Shanghai Municipal Government 18JC1410800 and National Natural Science Foundation of China 51690151. This research used resources of the Advanced Photon Source, a US Department of Energy (DOE) Office of Science User Facility, operated for the DOE Office of Science by Argonne National Laboratory under contract number DE-AC02-06CH11357.
The authors declare no competing interests.
Peer review information Nature thanks Jiaxi Cui, Sylvain Deville and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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Extended data figures and tables
Extended Data Fig. 1 Micro- and nanostructures and stress–strain curves of PVA hydrogels fabricated by different methods and their corresponding fracture energies and critical strains.
a, ‘DF + Salting out’ denotes a hydrogel prepared by directional freezing of a 5% PVA solution and salting out in 1.5 M sodium citrate solution for 24 h. b, ‘F + Salting out’ represents a hydrogel prepared by non-directional freezing of a 5% PVA solution in the fridge and salting out in 1.5 M sodium citrate solution for 24 h. c, ‘DFT-3 cycle’ denotes a hydrogel prepared by directional freezing and thawing of a 5% PVA solution for three cycles. d, ‘FT-3 cycle’ indicates a hydrogel prepared by non-directional freezing in the fridge and thawing of a 5% PVA solution for three cycles. e, ‘Chemical Crosslinking’ represents a hydrogel prepared by mixing 0.5% glutaraldehyde and 0.5% hydrochloric acid into a 5% PVA solution for gelation. f, Salting out resulted in non-gelation. Only weak globules of loose and random nanofibrils were made by directly adding a 1.5 M sodium citrate solution into a 5% PVA solution. Panels a–c are the same as Fig. 3a–c.
a, Fatigue threshold of HA-20PVA. When loading above the threshold energy release rate (ε = 400%), the crack propagates slowly. N, number of cycles. b, Validation of fatigue threshold with an energy release rate slightly lower than the fatigue threshold. No crack propagation or failure was observed for 30,000 loading cycles.
a, Water content of HA-xPVA hydrogels for x = 2, 5, 10 and 20. The error bars (1 s.d. from five measured samples) were obtained from five measured samples with standard deviations of 1.72%, 2.29%, 2.69% and 2.43% for x = 2, 5, 10 and 20, respectively. b, Fracture energy of HA-xPVA hydrogels, x = 2, 5, 10 and 20, measured by pure shear tests. The error bars (1 s.d. from five measured samples) were obtained from five measured samples with standard deviations of 0.16, 4.83, 5.44 and 5.62 kJ m−2 for x = 2, 5, 10 and 20, respectively.
a, Photograph of HA-5alginate hydrogel. b, Tensile stress–strain curve of a HA-5alginate hydrogel compared to that of a regular calcium-alginate hydrogel. Scale bar, 5 mm.
This file includes a list of materials, Supplementary Text, Supplementary References, Supplementary Figs. 1–13, Supplementary Tables 1–3 and full captions for Videos 1 to 4.
Tensile loading of HA-5PVA dog-bone shaped hydrogel specimen.
Tensile loading of HA-5PVA hydrogel with pre-made crack.
Tensile loading of HA-20PVA hydrogel with pre-made crack.
Tensile loading of 5PVA hydrogel prepared by non-directional freezing and salting out with pre-made crack.
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Hua, M., Wu, S., Ma, Y. et al. Strong tough hydrogels via the synergy of freeze-casting and salting out. Nature 590, 594–599 (2021). https://doi.org/10.1038/s41586-021-03212-z
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