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ReseaRch aRticle

Anisotropically Fatigue-Resistant Hydrogels

Xiangyu Liang, Guangda Chen, Shaoting Lin, Jiajun Zhang, Liu Wang, Pei Zhang, Zeyu Wang, Zongbao Wang, Yang Lan, Qi Ge, and Ji Liu*

DOI: 10.1002/adma.202102011

still susceptible to fatigue fracture during multiple-cycle mechanical loads, exhibi-ting fatigue threshold (i.e., the minimal fracture energy required for crack propaga-tion under cyclic loads) below 100 J m−2.[5–7] Therefore, the long-term reliability has substantially hampered the in practical utility of hydrogels and hydrogel-based devices, and remains a key challenge in these fields.

On the contrary, biological tissues, such as skeletal muscles, tendon and car-tilage, are well known for not only their superior strength, modulus, toughness, but also long-term robustness.[8–10] For example, skeletal muscles can sustain a high stress (i.e., 1 MPa) over millions cycles per year without fracture, exhibiting fatigue thresholds (i.e., the minimal frac-ture energy required for crack propaga-tion under cyclic loads) over 1000 J m−2,

despite their high water content (≈80%).[8,11] Such unrivalled fatigue-resistance originates from their hierarchically-arranged collagen fibrillar micro/nanostructures.[10] Despite bioinspired construction of structural materials has been promising for the design of fatigue-resistant hydrogels,[12–17] how to produce hydrogel materials with unprecedented fatigue-resistance in a universal and viable manner still remains an open issue. More recently, fatigue-resistant hydrogels have been fabricated by engineering the crystalline domains,[12–14] fibril structures,[15,16] or mesoscale phase separation.[17] Ice-templated freeze-casting strategy has been utilized as a powerful technology to impart

Nature builds biological materials from limited ingredients, however, with unparalleled mechanical performances compared to artificial materials, by harnessing inherent structures across multi-length-scales. In contrast, synthetic material design overwhelmingly focuses on developing new com-pounds, and fails to reproduce the mechanical properties of natural coun-terparts, such as fatigue resistance. Here, a simple yet general strategy to engineer conventional hydrogels with a more than 100-fold increase in fatigue thresholds is reported. This strategy is proven to be universally applicable to various species of hydrogel materials, including polysaccharides (i.e., alginate, cellulose), proteins (i.e., gelatin), synthetic polymers (i.e., poly(vinyl alcohol)s), as well as corresponding polymer composites. These fatigue-resistant hydrogels exhibit a record-high fatigue threshold over most syn-thetic soft materials, making them low-cost, high-performance, and durable alternatives to soft materials used in those circumstances including robotics, artificial muscles, etc.

1. Introduction

As a polymer network infiltrated with water, hydrogel has emerged as the critical engineering material for soft robotics, tissue engineering, bioelectronics, wearable and implant-able electronics, etc.[1,2] The past decades have witnessed the advancement in hydrogel innovation with unprece-dented mechanical properties, for example, hydrogel mate-rials could be facilely engineered with tissue-like toughness (i.e., 1000 J m−2), thus capable to resist crack propagation under dramatic deformation.[3,4] However, these tough hydrogels are

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.202102011.

Dr. X. Liang, G. Chen, J. Zhang, P. Zhang, Prof. Q. Ge, Prof. J. LiuDepartment of Mechanical and Energy EngineeringSouthern University of Science and TechnologyShenzhen 518055, ChinaE-mail: liuj9@sustech.edu.cnDr. S. LinDepartment of Mechanical EngineeringMassachusetts Institute of TechnologyCambridge, MA 02139, USADr. L. WangDepartment of Material Science and EngineeringSouthern University of Science and TechnologyShenzhen 518055, China

Z. Wang, Prof. Z. WangNingbo Key Laboratory of Specialty Polymers Faculty of Materials Science and Chemical EngineeringNingbo UniversityNingbo 315211, ChinaDr. Y. LanDepartment of Chemical EngineeringUniversity College LondonLondon WC1E 7JE, UKProf. J. LiuShenzhen Key Laboratory of Biomimetic Robotics and Intelligent SystemsDepartment of Mechanical and Energy EngineeringSouthern University of Science and TechnologyShenzhen 518055, ChinaProf. J. LiuGuangdong Provincial Key Laboratory of Human-Augmentation and Rehabilitation Robotics in UniversitiesSouthern University of Science and TechnologyShenzhen 518055, China

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hydrogel materials with preferentially-aligned microstructures, thus substantially improved fatigue-resistant performance.[18]

Herein, we describe a simple yet general strategy to engineer conventional hydrogels with over 100-fold increase in fatigue thresholds. Our two-step process involves the formation of preferentially-aligned micro/nanostructures within a hydrogel material through the ice-templated freeze-casting process,[19–23] followed by thermal annealing. Crucial to the extraordinary fatigue resistance to crack propagation of these hydrogels is the synergetic integration of hierarchically-arranged structures and well-defined crystalline domains.

2. Results and Discussion

Figure  1A depicts our consecutive fabrication process to syn-ergistically engineer the preferentially-aligned microstructures and nanocrystalline domains within a hydrogel system. This strategy is based on the freeze-casting process, a well-established routine for endowing materials with anisotropic structures through unidirectional ice-templating. [19–24] To implement our

design rationale, we choose poly(vinyl alcohol) (PVA) aqueous solution (≈146–186 kDa, 99% hydrolyzed) as the exemplary and starting material, due to its highly hydrophilic essence, well-documented strategies to engineer its crystalline structures, as well as biocompatible essence.[25] A PVA aqueous solution with weight fraction of 5 wt% is used, whereas freeze-thawed PVA (referred as FT PVA) or chemically crosslinked PVA (referred as Ch PVA) with the same weight fraction exhibits extremely low modulus, toughness and strength, making them difficult to be used as engineering materials.[12,25] Here, we transfer the PVA solution (5 wt%) into a home-designed mold, which is placed directly in contact with the cold finger (copper plate, Figure 1A). The vertical temperature gradience facilitates the ice nucleation and subsequent vertical growth in parallel.[24] During the subse-quent lyophilization process, PVA polymer chains play a critical role as the scaffold to support the microstructures upon ice sublimation through strong hydrogen-bonding, leading to hon-eycomb-like porous structures, however, with an extremely low crystallinity of ≈1.56 wt% (referred as FC PVA, Figure S1, Sup-porting Information). The PVA matrix is annealed afterward at a temperature between its glass transition (≈70 °C) and melting

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Figure 1. Fabrication of hydrogels with preferentially aligned fibril structures. A) Schematic illustration for the hydrogel fabrication by freezing a polymer solution on a copper substrate, one side of which is submersed within liquid nitrogen. The ice crystals nucleate simultaneously over the surface of copper substrate, and grow perpendicularly, resulting in honeycomb-like structures with only a single domain. The anisotropic polymer scaffolds are further annealed at high temperature (i.e., 100 °C, 90 min), leading to polymer materials with higher degree of crystallinity and larger size of crystals. Afterward, the annealed samples were soaked in water till equilibrium, resulting in the hydrogels with preferentially aligned fibril structures. B) Pres-ence of larger-size crystals and aligned fibril structures synergistically pin the crack, imparting them with astonished fatigue-resistance. C) Images of twisting, rolling and folding the as-prepared hydrogels, demonstrating their compliant, soft, and flexible essence. D) Image demonstrating the remark-ably high strength of the as-prepared hydrogels along the alignment direction, by sustaining a dumbbell (25 kg) with over 31 000× of its own weight.

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temperature (≈200 °C), that is, 100 °C for 90 min, in order to initiate further crystal growth. Final hydrogel material (referred as FC-A PVA) is obtained after swelling in deionized water till equilibrium. The FC-A PVA hydrogels possess a crystallinity of 36.6 wt% (at its dry state) and water content of 90.8 wt%, exhib-iting outstanding softness yet high strength (Figure 1C,D).

Scanning electron microscopy images (SEM, Figure  2A–C) reveal unique micro/nanostructures of FC-A PVA hydrogels: undisrupted cellular structures with a compartmental size of ≈1–10 μm and wall thickness of ≈100–200 nm, as well as highly ordered cellular arrays along the ice-template direction. The preferentially aligned microstructures could also be observed in the confocal microscopic image (Figures S2 and S3, Sup-porting Information), similar to the mechanically trained PVA

hydrogels.[15] Moreover, wide-angle X-ray scattering (WAXS) and small-angle X-ray scattering (SAXS) patterns (Figure  2D and Figures S4 and S5, Supporting Information) further verify that nanocrystalline domains orient during the freeze-casting and annealing treatment. Increase in the size of nanocrystal-line domain (D), as well as decrease in the distance among nanocrystalline domains (L), corroborates the role of annealing in increasing crystal size and overall crystallinity (Figure  2E). By contrast, FT or Ch PVA hydrogel possesses random pores, while freeze-casting (without annealing) can generate prefer-entially aligned microstructured hydrogels (Figure S6, Sup-porting Information), however, with extremely small crystals (Figure S5, Supporting Information). Based on these analysis, a hierarchical network is built for the FC-A PVA hydrogel, as depicted in Figure  1A, that is, at nanoscale, PVA nanocrystals form during the freeze-casting and annealing process (first order); the nanocrystalline domains generate along the ice-growth direction, and are connected by the amorphous chains forming the microwalls (second order), while these microw-alls further assemble the preferentially aligned cellular arrays, leading to the anisotropic hydrogel network (third order).

The macroscopic mechanical behaviors of the PVA hydrogels are comparatively investigated with a tensile machine. As shown in Figure 3A,B, before annealing, the FC PVA hydrogel displays unique anisotropic mechanical properties, such as 1.5 times difference in modulus, 3.2 times in strength and 1.8 times in stretchability between the parallel and perpendicular directions. However, the strength (32 kPa) and modulus (7.4 kPa) of the FC PVA hydrogels are still too low. Upon annealing, due to the sub-stantial increase in crystallinity (from 1.56% to 36.6% in solid state, Figure  2B, and Figure S7, Supporting Information), the FC-A PVA hydrogel exhibits a remarkably enhanced strength of 2.5 MPa along the ice-growth direction (S||), 6.4 times that of FT PVA hydrogel (5 wt%, 0.39 MPa) and 94 times that of Ch PVA hydrogel (0.027 MPa, Figure S8, Supporting Infor-mation). Moreover, the anisotropic difference in mechanical performance, namely, ratio of specific parameter between the parallel and perpendicular directions, markedly increases, that is, >10 times in strength (S||/S⊥, Figure 3B).

We then investigate the fracture behaviors of the FC-A hydrogel along different directions. When a precut crack is introduced in parallel with the ice-growth direction, the crack propagates promptly through the entire sample (Figure S9–S11, Supporting Information), resulting in a low toughness of 0.5 kJ m−2. On the contrary, crack perpendicular to ice-growth direction is effectively pinned before reaching a stretch of 2.6. The characteristic crack insensitivity originates from the inher-ently aligned fibril structures, similar to those fatigue-resistant biological prototypes, that is, muscle and tendon,[9,10] as well as artificial materials with preferentially aligned structures.[15,16,18] Once the stretch further reaches 4.7, the crack branches near the interface, and rupture of the fibrils occurs afterward. Fur-ther cross-sectioned SEM images reveal distinct fracture behav-iors: delamination of fibrils for a crack propagating along the alignment direction, while both fibrils fracturing and fibrillar delamination for a crack propagating perpendicular to the alignment direction (Figure S10, Supporting Information). A toughness of 116 kJ m−2 is quantified for the parallel direc-tion, 224 times higher than that of the perpendicular direction

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Figure 2. Hierarchically multi-length scaled structures of the hydrogels. A) Schematic illustration of aligned microstructures of the hydrogels. B) SEM images of the FC-A PVA hydrogels along the ice-template direc-tion (||), with polymer wall of ≈100–200 nm. C) SEM images of the FC-A PVA hydrogels perpendicular (⊥) to the ice-template direction, with pore size of ≈1–10 μm. D) SAXS patterns for the hydrogels, including freeze-thawed (FT PVA), freeze-cast (FC PVA) and freeze-casting followed by subsequent annealing (FC-A PVA) hydrogel samples. Scale bar: 0.05 Å−1. E) Distance among crystals (L) for the hydrogels from SAXS measure-ment. Data in (F) are means ± standard deviation (SD), n = 3.

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(Figure S11, Supporting Information). In the finite-element analysis simulation diagrams, crack perpendicular to the align-ment is effectively pinned at the notched tip, featured with a large yielding zone and a butterfly-shaped cloud pattern (Figure  3C; Figure S13 and Movie S1, Supporting Informa-tion). Therefore, a higher energy is required to initiate the crack propagation, afterward, the fracture energy decreases until the whole sample is completely fractured (Figure 3D).

It is revealed that tough hydrogels, such as alginate/PAAm or freeze-thawed PVA (Γ > 1000 J m−2), are susceptible to fatigue fracture under cyclic mechanical loads, since the resistance to fatigue crack propagation after prolonged cyclic loading is the energy required to fracture a single-layer polymer chains, which is unaffected by additional dissipation mechanisms.[7] However, introduction of well-defined nanocrystalline domains or prefer-entially aligned fibrillar architectures is capable of increasing the hydrogel’s fatigue threshold up to 1000 J m−2.[12,15] We

further quantify the fatigue threshold (i.e., minimal fracture energy required for crack propagation under cyclic mechanical loading, Γ0) of these hydrogel samples employing the single-notch method (Figure 4A). FT PVA (5 wt%) exhibits a fatigue threshold of 20 J m−2, comparable to the energy required to fracture a single-layer polymer chain (≈10 J m−2).[5] On the contrary, FC PVA hydrogel is featured with anisotropic fatigue resistance (Figure S14, Supporting Information), possessing a fatigue threshold of 24 J m−2 in the parallel direction (Γ0||) and 5.9 J m−2 in perpendicular direction (Γ0⊥). These relatively low fatigue thresholds could also be interpreted by its extremely low crystallinity (1.56 wt%) and high water content (95 wt%). However, these values are still comparable to that of FT PVA hydrogel (20 J m−2, 5 wt%). Since crystallinity of PVA hydrogels could be facilely tuned through multiple freeze–thaw cycles and/or annealing process,[25] we detect that after annealing, Γ0|| of FC-A PVA hydrogel remarkably increases up to 1,340 J m−2,

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Figure 3. Anisotropic mechanics of fatigue-resistant hydrogels. A) Stress–strain curves of freeze-cast (FC PVA), freeze-cast followed by annealing (FC-A PVA) and freeze-thawed (FT PVA) hydrogel samples in both parallel and perpendicular directions. The black “X” mark indicates the point where hydrogel fracture occurs. B) Summary of tensile strength (S), water content and crystallinity of the FT PVA, FC PVA and FC-A PVA hydrogels. C) Stress nephograms of FC-A|| and FC-A⊥ samples at the stage of crack blunting (stretch dc0 = 0), crack propagation (stretch dc0 = 1.6) and compete fracture (stretch dc0 = 3.2), respectively. The scale bar from red to blue indicates that the stress concentration varies from high to low. D) Plot of strain energy density against crack length. The fracture process is divided into three stages: crack initiation (pink), crack propagation (yellow), and complete fracture (green). For each sample, much higher strain energy is required to initiate the crack propagation, afterward, the strain energy gradually decreases until the sample is completely fractured. Data in (B) are means ± SD, n = 3.

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while 34 J m−2 for Γ0⊥ (Figure 4A,B). This record-high fatigue threshold of Γ0|| arises from the preferentially aligned fibril structures and remarkably high crystallinity (36.6%), whereas a low Γ0⊥ value could be interpreted by the low polymer content

among the fibrils, similar to previous report on mechanically trained PVA hydrogels with aligned structures.[15] Impressively, the water content of FC-A PVA hydrogel remains as high as 90.8 wt% (Figure 3B), while Γ0|| is orders of magnitude higher

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Figure 4. Anisotropic mechanics of fatigue-resistant hydrogels. A) Schematic diagram of pre-notch model and crack extension per cycle dc/dN versus applied energy release rate G for FC-A PVA hydrogels in both parallel (||) and perpendicular (⊥) directions, while isotropic FT PVA hydrogel is used for comparison. B) Summary of fatigue threshold (Γ0) of the FT PVA, FC PVA and FC-A PVA hydrogels in both parallel (||) and perpendicular (⊥) direc-tions. C) Validation of fatigue threshold as high as 2,740 J m−2 for FC-A PVA hydrogel sample (10 wt%) using the single-notch test at the cycle number of 1, 15 000, and 30 000. D) Summary of tensile strength (S) and fatigue threshold (Γ0) of HEC (5 wt%), alginate (3 wt%), gelatin (2.5 wt%) and PVA (10 wt%) hydrogels, as well as PVA/GO composite hydrogels, in both parallel (||) and perpendicular (⊥) directions. E) Comparison chart by plot-ting the fatigue thresholds versus water contents among tough hydrogels, (i.e., PAAm–alginate,[7] PAAm–poly(2-acrylamido-2-methylpropanesulfonic acid),[26] polyampholyte hydrogels,[17] freeze-thawed PVA,[12] and nanocrystalline hydrogel (i.e., dry-annealed PVA),[12] biogels (i.e., biological tissue),[8] composite hydrogels (i.e., PDMS filament-filled hydrogels),[27] mechanically trained PVA hydrogels,[15] polyprotein-based hydrogels,[28] single network (i.e., polyacrylamide hydrogel),[6,26] as well as hydrogel from nature polymers (i.e., HEC, gelatin, chitosan and sodium alginate). Without specification, the starting weight fraction PVA hydrogels is 5 wt%. Data in (B) and (D) are means ± SD, n = 3.

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than that of existing tough hydrogels (≈50 J m−2).[7] The high water content could also been attributed to the microporous structures within the FC-A PVA hydrogels, which can trap more free water molecules, despite the high crystallinity for the polymer walls (see more discussion in Figure S16, Supporting Information).

Given its excellent flexibility and versatility of freeze-casting technique,[20,23] our strategy to design and fabricate anisotropic hydrogels provides a new and powerful routine to engineer conventional hydrogel materials with highly anisotropic struc-tures and mechanics. We envision that other water-soluble polymers and their nanocomposites could be engineered with similar aligned structures, giving distinct fatigue-resistant per-formance. To validate this predictable generality, we also fab-ricate freeze-cast hydrogels from other polymer systems, such as aqueous solutions of hydroxyethyl cellulose (HEC, 5 wt%), sodium alginate (SA, 3 wt%), gelatin (2.5 wt%), polyvinyl alcohol (10 wt%), or PVA/graphene oxide mixture (5 wt% PVA and 0.1 wt% GO). It is widely recognized that pristine random hydrogels of HEC, alginate or gelatin are generally brittle and fragile, exhibiting fatigue thresholds below 10 J m−2 (Figure S17, Supporting Information). Surprisingly, thanks to the charac-teristically aligned fibril structures inherited from the freeze-casting technique (Figure S20, Supporting Information), these freeze-cast hydrogels are also endowed with remarkably high fatigue thresholds (compared to their random analogues) and high anisotropic ratio (Γ0⊥/Γ0||, Figure 3F and Figures S21–S24, Supporting Information).

A long-standing challenge in engineering hydrogel design is the conflict between superior mechanical performances (i.e., high strength, modulus, and fatigue threshold) and low solid content, since these attributes are in general mutually exclu-sive. For example, in our previous report, once a freeze-thawed PVA hydrogel (10 wt%) is exposed to air-dry and annealing, crystallinity increases up to 47.3%, the fatigue threshold reaches 1000 J m−2, on the cost of low water content of 60 wt%.[12] Here, we also fabricate FC-A PVA hydrogel from a PVA precursor solution of 10 wt% (≈146–186 kDa, 99% hydrolyzed), and a record-high fatigue threshold of 2740 J m−2 (Figure S18, Sup-porting Information) is quantified (crystallinity of 46.8%), while the water content is as high as 82 wt% (Figures S25 and S26, Supporting Information). To validate this extremely high fatigue threshold, we apply Gc of 2740 J m−2 to a notched sample over 30 000 cycles (to approximate infinite cycles of loads) and observe no crack extension (Figure  3D; Figure S27 and Movie S2, Supporting Information). Accordingly, we show here that the unprecedented high fatigue threshold of a hydrogel system is not always accompanied with substantially decreased water content, highlighting the attractive merits of our strategy in constructing fatigue-resistant soft materials.

Figure  4E compares the fatigue thresholds and water con-tents of reported hydrogels in literatures, biological tissues and our freeze-cast hydrogels. It is shown that the FC-A hydrogel outperforms existing synthetic hydrogels, such as PAAm–algi-nate,[7] PAAm–poly(2-acrylamido-2-methylpropanesulfonic acid),[26] polyampholyte hydrogels,[17] freeze-thawed PVA,[12] and

Figure 5. Functional demonstrations as load-bearing components for underwater robots. A) Application of 1D FC-A PVA hydrogel as the load-bearing components for an assembled underwater robot. B) Snapshot images of the swimming trajectory of the underwater robot. Scale bar: 10 mm. C,D) Image of the pre-cracked (1/5 of the overall width) hydrogel belts are used as the load-bearing components before and after long-term underwater swimming. The freeze-thawed PVA hydrogel sample undergoes a fatigue-induced fracture after that is, 40 h swimming (≈197 630 cycles, C), while no crack propaga-tion is detected for the FC-A PVA hydrogel after over 250 h swimming (≈1 000 000 cycles, D) The applied energy release rate on the hydrogel materials is tuned by the maximal deformation, and 90 and 1340 J m−2 for the FT and FC-A PVA hydrogel samples, respectively. Scale bar: 5 mm.

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dry-annealed PVA,[12] biogels (i.e., biological tissue),[8] composite hydrogels,[27] mechanically trained PVA hydrogels,[15] polypro-tein-based hydrogels,[28] single network i.e., and polyacrylamide hydrogel),[6,26] in terms of fatigue thresholds, without sacri-ficing their high water contents (Figure  3C). Moreover, as far as known to us, this is the first work which successfully fab-ricates hydrogel materials with record-high fatigue threshold (2,740 J m−2), and high anisotropic ratio (Γ0⊥/Γ0||) over 40, at a high water content over 82 wt%.

The capability to engineer conventional hydrogel materials with unprecedented resistance to crack propagation during long-term cyclic loads enables applications such as load-bearing components in underwater robotics. As shown in Figure  5A, we design and fabricate an underwater swimming robot, employing an electrically driven motor to expand the arms, while our load-bearing FC-A PVA hydrogel belts contract and generate the propulsive thrust by pushing the surrounding water rearward, thus this robot could continuously swim underwater (see detailed robot design in Experimental Section, Figure 5B, Figure S28 and Movie S3, Supporting Information). During the long-term swimming, soft materials as the load-bearing components are experiencing cyclic stretching-and-contracting, similar to tendons, thus superior fatigue resistance is necessary. As a control, we employ FT PVA hydrogels, which are promising candidates for artificial tendons and cartilages thanks to its excellent elasticity and toughness.[25] By intro-ducing a small pre-crack (≈1/5 of the overall width) perpen-dicular to the stretching direction, we observe that after swim-ming for over 40 h, ≈197 630 stretching-and-contracting cycles at an energy release rate of 90 J m−2, fatigue-induced fracture occurs (Figure 5C and Figure S29A, Supporting Information). On the contrary, when the FC-A PVA hydrogels are loaded with precut crack (≈1/5 of the overall width), no crack propagation is observed over 1 000 000 stretching-and-contracting cycles at an energy release rate of 1340 J m−2 over 250 h, showing unprec-edented fatigue-resistant performance in real application cir-cumstances (Figure 5D; Figure S29B and Movie S4, Supporting Information).

3. Conclusion

We have reported a general design rationale, through freeze-casting strategy, to engineer conventional soft materials with unprecedented either anisotropic fatigue-resistance. These hydrogel materials offer advantages over existing analogues, including fatigue thresholds, ease of manufacturing, versa-tility in material choices, allowance for both isotropic and anisotropic design, etc. These fatigue resistant hydrogel mate-rials provide new opportunities for making them low-cost, high-performance and durable alternatives for soft materials used in those circumstances including robotics, artificial muscles, etc.

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsJ.L. acknowledges the research startup grant (Y01336223) by Shenzhen municipal government and the research startup grant (Y01336123) support by Southern University of Science and Technology (SUSTech), and the financial support by Centers for Mechanical Engineering Research and Education at MIT and SUSTech (MechERE Centers at MIT and SUSTech, Y01346002). This work was also supported in part by the Science, Technology and Innovation Commission of Shenzhen Municipality under grant NO. ZDSYS20200811143601004, and Basic and Applied Basic Research Foundation of Guangdong Province under grant NO. 2020A1515110288. The authors also would also like to acknowledge the technical support from SUSTech Core Research Facilities.

Conflict of InterestThe authors declare no conflict of interest.

Data Availability StatementThe data that support the findings of this study are available from the corresponding author upon reasonable request.

Keywordsanisotropy, crack propagation, fatigueresistance, freeze casting, hydrogels

Received: March 13, 2021Revised: April 2, 2021

Published online:

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