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Journal Club for July 2020: Fatigue-resistant hydrogels: Principles, Experiments, and Applications

linst06's picture



Fatigue-resistant hydrogels: Principles, Experiments, and Applications

Shaoting Lin, Xuanhe Zhao

1.     Introduction

The use of hydrogels in devices and machines requires them to maintain robustness under cyclic mechanical loads [1, 2]. Hydrogels have been made tough to resist crack propagation under a single cycle of mechanical load [3, 4]. In general, the toughening of hydrogels is achieved by integrating mechanisms for dissipating mechanical energy such as the fracture of short polymer chains and reversible cross-links into stretchy polymer networks [5, 6].

Figure 1. Fatigue threshold of A), conventional polymer networks for elastomers and hydrogels and B), biological hydrogels with intrinsically high-energy phases such as nanocrystalline domains and nanofibers.

Despite the progress of tough hydrogels, existing tough hydrogels suffer from fatigue fracture under multiple cycles of mechanical loads revealed by Tang, Bai, and Suo et al [7-9](Please refer to the Journal club of Fatigue of hydrogels in May 2019 by Ruobing Bai for details.) Their experimental findings conclude that 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) (Fig. 1A), which is unaffected by the additional dissipation mechanisms introduced in tough hydrogels. These findings are reminiscent of fatigue of elastomers, which has been intensively studied since 1958 by Lake, Thomas, and Lindley [10-12]. 

Following the classical Lake-Thomas theory [10], the fatigue resistance of hydrogels with conventional polymer networks can be estimated by

where  n*sqrt(N)*b/(λs)^2 is the number of polymer chains per unit area with n being the number density of active polymer chains per unit volume in the dry state, N being the number of Kuhn monomer per polymer chain, b being the length of each Kuhn monomer, and λbeing the swelling stretch of polymer chains, N*Uf is the energy required to fracture a polymer chain with Uf being the energy required to fracture a single Kuhn monomer. For hydrogels with conventional networks, the reasonable ranges for these parameters are n~10^25-10^26 m-3, N~100-1000, b~0.3 nm, λs~2-10, and Uf~10^6 J/mol [8, 13-16]. By substituting the values of these parameters into Eq. (1), we can evaluate the typical range of the fatigue resistance of hydrogels with conventional polymer networks on the order of 1-100 J/m^2, which is far below fracture energies of tough hydrogels on the order of 1,000 J/m^2 [5]. Strategies toward the design of anti-fatigue-fracture hydrogels have remained critical needs and central challenges for long-term applications of hydrogels in devices and machines. 

In contrast to synthetic hydrogels, biological tissues such as cartilages, tendons, muscles, and heart valves show extraordinary anti-fatigue properties. The anti-fatigue property of biological tissues possibly arises from its inherent highly ordered and partially crystalline structures of collagen fibers in the tissues. For instance, the adhesions of tendons, ligaments, and cartilages to bones are commonly achieved through a transitional interface (Fig. 1B), from the uncalcified collagen nanofibrils (i) to the calcified collagen nanofibrils (ii) to the bones (iii). At the interface (ii), nanostructured composites of aligned collagen nanofibrils and ordered hydroxyapatite nanocrystals are anchored on the bones, leading to fatigue-resistant adhesions of tendons, ligaments, and cartilages to the bones [17,18]. The cartilage-bone interface in the human knee joint can sustain compressive stresses of 1 MPa along with an interfacial toughness around 800 J/m2 over 1 million cycles of loading per year.

Taking the opportunity of this journal club, we would like to discuss i) design principle for fatigue-resistant hydrogels, ii) symptoms of interfacial fatigue of hydrogel adhesions, iii) design principle for fatigue-resistant hydrogel adhesions, and iv) applications of fatigue-resistant hydrogels and hydrogel adhesions.

2.     Fatigue-resistant hydrogels

To address the challenge of fatigue failures in conventional hydrogels, we and others propose the general design principle for fatigue-resistant hydrogels: Make the fatigue crack pinned by intrinsically high-energy phases [19], such as nanocrystals [20], micro-/nano-fibers [21], and macro-fibers [22] in hydrogels. In what follows, we will summarize the recently reported strategies to implement the design principle. 

Our experimental finding first suggests 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. 2A) [20]. 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. To test the hypothesis, we select PVA as a model hydrogel with tunable crystallinity. We find that the increase of crystallinity can greatly enhance the fatigue thresholds of PVA hydrogels due to the decrease of the average distance between adjacent crystalline domains and the increase of the average size of the crystalline domains. In particular, the fatigue threshold can exceed 1000 J/m2 when the crystallinity of PVA in the swollen state reaches 18.9 wt % (Fig. 2A).  

While the introduction of crystalline domains can greatly enhance the fatigue threshold, it increases the modulus and decreases the water content of the hydrogel. Thereby, we come up with the other strategy via mechanical training to enhance hydrogels’ fatigue resistance while maintaining its low modulus [21]. The strategy first involves growing compliant nanofibrils in PVA hydrogels by forming two separated phases via repeated cycles of freezing and thawing [23]: (i) high concentration of polymer chains in the form of nanofibrils cross-linked by nanocrystalline domains and (ii) low concentration of amorphous. The pristine freeze-thawed hydrogels with randomly distributed nanofibrils are exposed to repeated prestretches in a water bath as mechanical training, to form aligned nanofibrillar structures (Fig. 2B). The fatigue threshold of the trained PVA hydrogel measured along the aligned nanofibrils reaches a record-high value of 1,250 J/m2 (Fig. 2B). By contrast, the fatigue threshold perpendicular to the aligned nanofibrils is 233 J/m2, which is on the same order as that of the pristine freeze-thawed PVA hydrogel (i.e., 310 J/m2), but still much larger than that of the chemically cross-linked PVA hydrogel (i.e., 10 J/m2).

Figure 2 Strategies for fatigue-resistant hydrogels

More recently, Wang and Suo et al. [22] reported a new principle to enhance elastomer’s fatigue resistance by introducing unidirectional fibers in the soft matrix (Fig. 2C). Both the fibers or the matrix have a low fatigue threshold. The combination of the fibers and the matrix shows a remarkable high fatigue threshold, because the soft matrix deconcentrates stress in the fiber. They demonstrate the strategy by embedding fibers of a polydimethylsiloxane (PDMS) elastomer into a matrix of a polyacrylamide (PAAm) hydrogel. No damage is observed when the composite is stretched for 30000 cycles with an amplitude of the energy release rate of 1290 J/m2. By harnessing the capability of 3D printing, Tang and Suo et al. further expanded the principle into two-dimensional elastomeric lattices, reaching the fatigue threshold beyond 500 J/m2 in multiple directions [24] 

We would like to also highlight recent work by Li and Gong et al. [25]. In this work, they study antifatigue properties of soft materials with hierarchical structures and reveal the mechanism of the hierarchical structures on suppressing crack advance under reciprocating movement (Fig. 2D). Using polyampholyte hydrogels (PA gels) as a simple model system with a hierarchical structure, they demonstrate a high fatigue resistance through a synergistic effect between different scales. Particularly, they find that the polymer network at the 10-nm scale determines the threshold of energy release rate G0 above which the crack grows, while the bicontinuous phase networks at the 100-nm scale significantly decelerate the crack advance until a transition Gtran far above G0. Such understanding may shed light on designing synthetic hydrogels that can truly replicate load-bearing biological tissues, composed of exquisite hierarchical structures. 

3.     Fatigue-resistant hydrogel adhesions

3.1 Symptoms of interfacial fatigue of hydrogel adhesions

Tough adhesions between hydrogels and engineering materials have been achieved by covalently anchoring polymer chains of tough hydrogels on solid surfaces [26]. When the hydrogel is peeled from the solid under a single cycle of the mechanical load, the energy required for fracturing anchored polymer chains and the energy dissipated in deforming the bulk hydrogel synergistically give an interfacial toughness over 1000 J/m2. We recently characterized fatigue properties of tough hydrogel adhesion to glass substrates. As shown in Fig. 3A-C, the interfacial fatigue threshold of tough hydrogel adhesion (PAAm-alginate) is as low as 68 J/m2, similar to that of common hydrogel adhesion (9 J/m2 for PAA and 32 J/m2 for PAAm), and comparable with the energy required for fracturing a layer of amorphous polymer chains (1-100 J/m2). When the tough hydrogel adhesion is subjected to an energy release rate of 200 J/m2 over 5000 cycles, substantial propagation of the interfacial cohesive crack can be identified. 

 Figure 3 Symptoms of interfacial fatigue of hydrogel adhesion

Similarly, Ni and Li et al. [27] observed the interfacial fatigue of hydrogel adhesions to biological tissues (Fig. 3D-E). They performed modified lap-shear tests with three types of loading (e.g., monotonic, static, and cyclic loads). They observed shakedown of the load-displacement curves during cycling and two interfacial fracture phenomena: fast debonding and interfacial fatigue fracture. They further confirmed the existence of a fatigue threshold (24.4 J/m2) for the tough hydrogel-tissue interface, below which the interface is immune against prolonged cyclic deformation. 

3.2 Design principle for fatigue-resistant hydrogel adhesions

We recently proposed a bioinspired strategy to achieve fatigue-resistant adhesions of synthetic hydrogels by anchoring ordered nanostructures (e.g., nanocrystalline domains) on engineering materials, since the ordered nanostructures require a much higher energy for fatigue-crack propagation than the corresponding amorphous polymer chains (Fig. 4A-B). We chose poly(vinyl alcohol) (PVA) hydrogels as a model material system, which can readily form nanostructures (e.g., nanocrystalline domains and nanofibrils) with tunable crystallinity. The anchorage of nanocrystalline domains on solid substrates through dry-annealing treatment gives a remarkable fatigue-resistant adhesion between hydrogels and substrates, with an interfacial fatigue threshold of 700 J/m^2 (Fig. 4C). This method is applicable to diverse engineering solids including glass, ceramics, Titanium, aluminum, stainless steel, and even elastomers such as PU and PDMS (Fig. 4D). To understand the high interfacial fatigue threshold by ordered crystalline domains at the interface, we further carried out all-atom molecular dynamics (MD) simulations to compare the energies required to pull out a PVA polymer chain (30 nm in contour length) from a nanocrystalline domain and to fracture an amorphous PVA polymer chain of the same contour length (Fig. 4E). As shown in Fig. 4F-G, pulling out of a PVA chain between the nanocrystal–glass interface requires a higher energy (~70,000 kJ/mol, PVA–SiO2) than that out of a standalone nanocrystal (~50,000 kJ/mol), which is consistent with our observation that the bulk hydrogel fractures during the peeling test, other than interfacial detachment. Therefore, introducing nanocrystalline domains on the interface and within the bulk hydrogel synergistically ensures a hydrogel–solid interface with extremely high fatigue resistance.

 Figure 4 Anchoring ordered nanocrystalline domains at the interface

Lu and Suo et al. [28] reported a different approach for the design of fatigue-resistant hydrogel adhesion by using a particularly simple kind of elastic dissipater: long-chain polymers (Fig. 5A). As a proof of concept, they used polyacrylamide hydrogels to adhere two pieces of polyester cloth through topological entanglement. The measured fatigue threshold of the adhesion is linearly proportional to the square root of the chain length and reaches 300 J/m2 (Fig. 5B).

Figure 5 Long-chain polymers as elastic dissipaters 

4.     Applications 

Fatigue-resistant hydrogels open up many opportunities for applications in biomedical areas. A nascent field named hydrogel machines has rapidly evolved, suggesting potentials for using fatigue-resistant hydrogels to complement or even replace many conventional machines based on rigid materials [2]. 

4.1 Ingestible gastro-retentive device

Devices that interact with living organisms are typically made of metals, silicon, ceramics, and plastics. Implantation of such devices for long-term monitoring or treatment generally requires invasive procedures. Hydrogels offer new opportunities for human-machine interactions due to their superior mechanical compliance and biocompatibility. Additionally, oral administration, coupled with gastric residency, serves as a non-invasive alternative to implantation (Fig. 6A). Achieving gastric residency with hydrogels requires the hydrogels to swell very rapidly and to withstand gastric mechanical forces over time (Fig. 6B). However, high swelling ratio, high swelling speed, and long-term robustness do not coexist in existing hydrogels. 

Based on the fatigue-resistant hydrogels we developed, we invented a hydrogel device that can be ingested as a standard-sized pill, swell rapidly into a large soft sphere, and maintain robustness under repeated mechanical loads in the stomach for up to one month. The hydrogel device consists of superabsorbent hydrogel particles that enable the device to quickly imbibe water (instead of diffusion) encapsulated in a soft yet anti-fatigue hydrogel membrane that maintains long-term robustness of the device. The hydrogel device can be ingested as a standard-sized pill (diameter of 1–1.5 cm), rapidly imbibe water and inflate (up to 100 times in volume within 10 min) into a large soft sphere (diameter of up to 6 cm, Video 1) (Fig. 6C), and maintain robustness under repeated mechanical loads over a long time (more than 26,000 cycles of 20 N force over 2 weeks in vitro) (Fig. 6D). In comparison, we show that the hydrogel device made of common tough hydrogels (i.e., PAAm-agar) degrades within 8 hours on the 1st day of testing. Owing to the high fatigue resistance of the hydrogel encapsulation membrane, we successfully demonstrated gastric retention of the hydrogel device for up to 30 days in a large porcine model. Such a platform could bring a number of societal impacts including but not limited to long-term measurements of biosignals (Fig. 6E), visualizing GI tract disorders, inducing satiety to control obesity, and prolonged drug delivery. Other recent works in this area include Liu, Giovanni, and Langer et al. [29], Raman and Giovanni, et al. [30].

Figure 6 Ingestible hydrogel pills as a gastro-retentive device

4.2 Robust coatings of biomedical implants

Materials and devices with complex shapes can be exposed to repeated mechanical loads, which pose a challenge to their coating materials in terms of fabrication method and long-term robustness. We demonstrate that the fatigue-resistant hydrogel adhesion can potentially provide a facile and versatile solution towards this challenge. We first present a set of materials and devices, including a stainless-steel spring, a glass optical fiber, a glass tube, and a ball-and-socket metallic joint, which are coated with a fatigue-resistant hydrogel coating layer (Fig. 7A). The thin (~20 μm in thickness) and uniform hydrogel coatings are applicable to devices with various feature sizes (from 200 μm to 35 mm), curvatures (convex surface, concave surface, and inner and outer surfaces), and diverse materials (glass, stainless steel, and silicone elastomer). We further adopt the ball-on-flat sliding test to evaluate the adhesion, friction, and wear performances of our fatigue-resistant hydrogel coating on a stainless-steel surface over cyclic mechanical loading (Fig. 7B-C). The fatigue-resistant hydrogel coating remains mechanically robust and adheres on the substrate over 5000 cycles of reciprocating motion (Video 2). As shown in Fig. 7D, the friction coefficient of the fatigue-resistant hydrogel coating increases from 0.006 to 0.02 and maintains around 0.02 until the end of the test (5000 cycles). In comparison, the friction coefficient of the bare metal surface increases from 0.045 to as high as 0.3 as the cycle number increases from 1 to 300 (under a compression force of 100 N), and remain constant afterwards.

Figure 7 Robust coatings of biomedical implants

5.     Challenge and Opportunity

The area of fatigue-resistant hydrogels faces a number of challenges and opportunities. Efforts in the field of experimental mechanics, multi-scale modeling, material synthesis, advanced fabrications, and translational science are particularly welcome for pushing both the fundamental understanding and translational application.

Experimental mechanics. Advanced techniques for probing chain fracture [31], mapping stress [32], and quantifying topological defects [33] are highly desirable for the in-depth understanding of fatigue of hydrogels.

Related Journal Club

1. Aug 2017, Fracture mechanics of soft dissipative materials, Rong Long

2. April 2019, Self-healing soft materials: from theoretical modeling to additive manufacturing, Qiming Wang

Modeling. Multi-scale modeling that accounts for biomimetic hierarchical and heterogeneous structures [34, 35] may open a new avenue for the next generation of tissue-like hydrogels.  

Chemistry. Structural design of hydrogels (e.g. ideal network hydrogel [36]) provides model material systems for fundamental investigations. 

Translational science. The ultimate goal is to transform the fundamental understanding of synthetic hydrogels for translational applications. Representative examples include recent work of wet tissue adhesives [37].

 Related Journal Club:

1. June 2020, Mechanically instructive biomaterials: a synergy of mechanics, materials and biology, Jianyu Li

2. Dec 2018, Bonding hydrophilic and hydrophobic soft materials for functional soft devices, Qihan Liu 

6. References 

1.    Yang, C. and Z. Suo, Hydrogel ionotronics. Nature Reviews Materials, 2018. 3(6):p. 125.

2.    Liu, X., et al., Hydrogel machines. Materials Today, 2020.

3.    Gong, J.P., et al., Double‐network hydrogels with extremely high mechanical strength. Advanced materials, 2003. 15(14): p. 1155-1158.

4.    Sun, J.-Y., et al., Highly stretchable and tough hydrogels. Nature, 2012. 489(7414): p. 133-136.

5.    Zhao, X., Multi-scale multi-mechanism design of tough hydrogels: building dissipation into stretchy networks. Soft matter, 2014. 10(5): p. 672-687.

6.    Gong, J.P., Why are double network hydrogels so tough? Soft Matter, 2010. 6(12): p. 2583-2590.

7.    Bai, R., J. Yang, and Z. Suo, Fatigue of hydrogels. European Journal of Mechanics-A/Solids, 2019. 74: p. 337-370.

8.    Tang, J., et al., Fatigue fracture of hydrogels. Extreme Mechanics Letters, 2017. 10: p. 24-31.

9.    Bai, R., et al., Fatigue fracture of tough hydrogels. Extreme Mechanics Letters, 2017. 15: p. 91-96.

10.   Lake, G. and A. Thomas, The strength of highly elastic materials. Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences, 1967. 300(1460): p. 108-119.

11.   Thomas, A., Rupture of rubber. V. Cut growth in natural rubber vulcanizates. Journal of Polymer Science, 1958. 31(123): p. 467-480.

12.   Lake, G. and P. Lindley, The mechanical fatigue limit for rubber. Journal of Applied Polymer Science, 1965. 9(4): p. 1233-1251.

13.   Zhao, X., A theory for large deformation and damage of interpenetrating polymer networks. Journal of the Mechanics and Physics of Solids, 2012. 60(2): p. 319-332.

14.   Lin, S., Tissue-like hydrogels by design. 2019, Massachusetts Institute of Technology.

15.   Mao, Y., B. Talamini, and L. Anand, Rupture of polymers by chain scission. Extreme Mechanics Letters, 2017. 13: p. 17-24.

16.   Akagi, Y., et al., Fracture energy of polymer gels with controlled network structures. The Journal of chemical physics, 2013. 139(14): p. 144905.

17.   Genin, G.M. and S. Thomopoulos, The tendon-to-bone attachment: Unification through disarray. Nature materials, 2017. 16(6): p. 607-608.

18.   Rossetti, L., et al., The microstructure and micromechanics of the tendon–bone insertion. Nature materials, 2017. 16(6): p. 664-670.

19.   Zhao, X., EML webinar overview: Extreme mechanics of soft materials for merging human-machine intelligence. Extreme Mechanics Letters, 2020: p. 100784.

20.   Lin, S., et al., Anti-fatigue-fracture hydrogels. Science advances, 2019. 5(1): p. eaau8528.

21.   Lin, S., et al., Muscle-like fatigue-resistant hydrogels by mechanical training. Proceedings of the National Academy of Sciences, 2019. 116(21): p. 10244-10249.

22.   Xiang, C., et al., Stretchable and fatigue-resistant materials. Materials Today, 2019.

23.   Hassan, C.M. and N.A. Peppas, Structure and morphology of freeze/thawed PVA hydrogels. Macromolecules, 2000. 33(7): p. 2472-2479.

24.   Li, C., et al., Fatigue-resistant elastomers. Journal of the Mechanics and Physics of Solids, 2020. 134: p. 103751.

25.   Li, X., et al., Mesoscale bicontinuous networks in self-healing hydrogels delay fatigue fracture. Proceedings of the National Academy of Sciences, 2020. 117(14): p. 7606-7612.

26.   Yuk, H., et al., Tough bonding of hydrogels to diverse non-porous surfaces. Nature materials, 2016. 15(2): p. 190-196.

27.   Ni, X., C. Chen, and J. Li, Interfacial fatigue fracture of tissue adhesive hydrogels. Extreme Mechanics Letters, 2020. 34: p. 100601.

28.   Zhang, W., et al., Fatigue-resistant adhesion I. Long-chain polymers as elastic dissipaters. Extreme Mechanics Letters, 2020: p. 100813.

29.   Liu, J., et al., Triggerable tough hydrogels for gastric resident dosage forms. Nature communications, 2017. 8(1): p. 1-10.

30.   Raman, R., et al., Light-degradable hydrogels as dynamic triggers for gastrointestinal applications. Science advances, 2020. 6(3): p. eaay0065.

31.   Ducrot, E., et al., Toughening elastomers with sacrificial bonds and watching them break. Science, 2014. 344(6180): p. 186-189.

32.   Chen, Y., et al., From force-responsive molecules to quantifying and mapping stresses in soft materials. Science Advances, 2020. 6(20): p. eaaz5093.

33.   Zhong, M., et al., Quantifying the impact of molecular defects on polymer network elasticity. Science, 2016. 353(6305): p. 1264-1268.

34.   Gautieri, A., et al., Hierarchical structure and nanomechanics of collagen microfibrils from the atomistic scale up. Nano letters, 2011. 11(2): p. 757-766.

35.   Qin, Z. and M.J. Buehler, Flaw tolerance of nuclear intermediate filament lamina under extreme mechanical deformation. Acs Nano, 2011. 5(4): p. 3034-3042.

36.   Sakai, T., et al., Design and fabrication of a high-strength hydrogel with ideally homogeneous network structure from tetrahedron-like macromonomers. Macromolecules, 2008. 41(14): p. 5379-5384.

37.   Yuk, H., et al., Dry double-sided tape for adhesion of wet tissues and devices. Nature, 2019. 575(7781): p. 169-174.


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Comments's picture

Dear Shaoting,

Thanks for sharing your great progress and thoughts in fatigue of adhesion of hydrogels. It is always a wise choice to learn from nature. The idea of high energy phases is beautiful. 

We also thank you for including our work on elastic dissipators. The improvement of fatigue threshold by this method is not as high as your established benchmark. We feel that one advantage of our approach is the simplicity of hydrogel synthesis, which may open more possibilites to add more functional property to the hydrogel adhesive. 

I have a question. It seems that in both high-energy phase approach and elastic dissipator approach, the hydrogel adhesive is not in a fully swollen state. Do you think it a limitation in application under prolonged cyclic loads in tissue enviroment?



linst06's picture

Dear Tongqing,

Thanks a lot for your kind words and insightful comments. I fully agree that using a simple approach to solve unsolved challenges will give high impacts.

Regarding the experimental conditions for our fatigue tests, we actually immersed our samples in water or PBS solvents throughout the fatigue test of bulk hydrogels and interfacial fatigue tests. Maintaining hydrogels or hydrogel adhesives in a fully swollen state is crucial for the fatigue characterization of hydrogels, particularly considering hydrogels' working conditions in applications. For example, here, we also show one application of ingestible hydrogel pills staying in the stomach for 30 days at a fully swollen state under acid environment and severe dynamic loading.


Shaoting's picture

Dear Shaoting,

Sorry for my mistake. I just remembered that the water content in your fatigue-resistant hydrogels was low so I got the wrong impression that the hydrogel is not fully swollen.

Well, then swelling is not an issue for you, but an issue for our approach:)   We are figuring out how to solve this.



linst06's picture

Dear Tongqing,

No worries. The low water content is a limitation for our current design of hydrogel adhesion. I feel your approach of long-chain polymer chains shows the strength to retain high water content while increasing the fatigue resistance.



Dear Shaoting,

       Thanks for leading the discussion on the topic of fatigue resistant hydrogel and hydrogel adhesion. Your works revealed that the crystalline domains at the crack front of the hydrogel or hydrogel-substrate interface inhibit the crack propagation under fatigue loading. For natural rubber, it will also crystalize at crack front under loading. But why natural rubber still suffers fatigue fracture? How do you think the difference between the two materials?




linst06's picture

Dear Junjie,

This is a very important question. There are several intrinsic differences between the crystallinity in natural rubber and PVA hydrogels.

1, The crystallinity in natural rubber is induced by stretch and is reversible, while the crystallinity formed in PVA hydrogels are thermodynamically stable. Please refer to a previous discussion among Jingda, Ruobing, and me.

2, Due to the nature of stretch-induced crystallization, the crystalline domains are only localized at the crack tip where large deformation is generated. Such localized crystalline domains may not be sufficient to pin the fatigue crack propagation. People used XRD to show the map of crystallinity around the crack tip, confirming its localized crystalline domains. Some results are presented in the paper (S. Trabelsi, P.-A. Albouy, J. Rault, Stress-induced crystallization around a crack tip in natural rubber. Macromolecules 35, 10054-10061, 2002).

3, In addition, the crystallinity in natural rubber is typically below 10% for the extension ratio of 5 (Fig. 6 in B. Huneau, Strain-induced crystallization of natural rubber: a review of X-ray diffraction investigations. Rubber chemistry and technology 84, 425-452, 2011). In contrast, for the fatigue-resistant PVA hydrogels we reported, its crystallinity is as high as 47%. 

Indeed, I also agree that an in-depth understanding of the correlation between fatigue properties and its molecular structures requires future efforts. 



canhui yang's picture

Dear Shaoting,

        Thank you for sharing such timely and comprehesive review. Fatigue of hydrogel is indeed important yet challenging for practical use. I believe the progresses made recently have laid the foundations for addressing this mission-critical issue, and the emerging of following up work is expectable.
        Most existing experiments for probing fatigue behavior utilize the tension of thin sheets of samples (pure shear tests). Such configuration makes it easier to corelate experimental results to theoretical analysis, whereas it deviates from practical scenarios to some extent. How do you think about the necessity of different designing principles, provided the fatigue behaviors under more complex stress states may be different from those observed in simple tension.



linst06's picture

Dear Canhui,

I think you are hitting a very important point when people try to translate the fundamental studies of fatigue of soft materials into a more practical deployment. 

1, First of all, as you mentioned, at the initial stage of material innovations, we first need standard characterization tests (e.g., pure-shear, single-notch tension, 90-degree peeling) so that we can obtain a material property (e.g., fatigue threshold, interfacial fatigue threshold) favorable for theoretical analysis. 

2, When we try to apply a material into a practical system, we do need characterizations that can closely mimic the real conditions including complex stress states, acidity, or even real psychological environment. For example, when designing ingestible hydrogel pills, we perform cyclic compressive tests on the spherical hydrogel balloon in acid solution, as an ex-vivo test to simulate its real dynamic loading conditions. To further evaluate the material's performance in real conditions, people even prefer in vivo tests to evaluate if the material is working when implanted or ingested. Here, we collaborate with Prof. Gio Traverso for a large animal test to show the hydrogel pill can stay in a pig stomach for 30 days.  

3, Your point of the necessity of different designing principles is indeed correct. More specifically, We first need design principles as a material-level characterization. We also need design criteria as a system-level evaluation. 

I try to tackle this broad and challenging point. I would like to hear your viewpoints as well.



Tang jingda's picture

Dear Shaoting,

Thank you very much for this wonderful review on fatigue of hydrogels. It is so comprehensive that people can get into this field easily. Since the last meeting on EASF seminar, I am waiting for this review. I want hear your opinion on  some points :

(1) The study on fatigue of hydrogels almost starts  at the same time with the study of hydrogel adhesion. Both are around 2015. Now, we can envision the huge potential of hydrogel adhesion in biomedical engineering. What will be the killer application of fatigue-resistant hydrogels?

(2) PVA hydrogel is a classical material. Thanks to your contribution, people now know it is very fatigue resistant.  Will this discovery find new applications and oppurtunities for PVA hydrogels? What is the status for the study of PVA hydrogels, especially for applications? 

(3) A detailed question: For PVA hydrogels, it is fatigue-resistant in the loading direction because of the obstruction of alligned polymer chains. How about the direction perpendicular to the loading axis? Is that one fatigue prone?

linst06's picture

Dear Jingda,

Thanks a lot for your kind words. Also, hearty congratulation on your fantastic talk in EASF! Your questions are challenging but really inspiring. I will try to answer them from my perspective. I would like to hear your thoughts as well.

(1)  You are right. The developments of the study on hydrogel adhesion and fatigue of hydrogels are in a different stage, though both fields are initiated at the same time. Hydrogel adhesion, as a leading technique for translational science, already shows its broad and immediate impacts; while the study on fatigue-resistant hydrogels is mostly focused on the fundamental investigation or material development.

One application for fatigue-resistant hydrogel which I also envision a huge potential for real translations is the ingestible hydrogel pill as a gastro-retentive device, for prolonged psychological signal monitoring, obesity control, drug delivery, and imaging of GI tract. For the groups who have the ability to conduct such kind of in vivo tests, I believe there are huge potentials for fatigue-resistant hydrogels to achieve previously inaccessible performances or functions. 

In addition to the application of ingestible gastro retentive devices, fatigue-resistant hydrogels are crucial for many existing applications of soft materials towards soft machines. The majority of the efforts in soft machines are laboratory work. However, when targeting practical deployment, their long term reliability is the killing factor for their lifetime. If we look back to the development of metals, composites, and plastics, their fatigue studies have been a hot topic and fatigue evaluation has been a standard routine before the commercialization of a product. I feel the development of soft machines has not entered that stage yet, but will bring people's attention to more realistic factors such as lifetime, corrosion, aging.

I also need to point out that many applications of hydrogels actually do not need that high fatigue resistance. For example, in an epidermal wearable device, the deformation of the relevant materials is typically small. When used in human body, on-demand degrading is also more favorable. It really depends on the specific application conditions.

(2) As I mentioned in (1), ingestible hydrogel pills are the new application of PVA hydrogels. If we track literature of PVA hydrogels, mostly are focused on cartilage replacement. I had the experience of visiting Philips, where they used PVA as an ultrasound phantom with complex shapes. Robust coatings for existing biomedical devices seem to be also the interests of doctors. 

If not limited to PVA hydrogels, I actually also see the potential of using hydrogels in applications beyond biomedical engineering. Hydrogels recently also bring interests to the area of sustainable water, energy harvesting, and agriculture. For example, the following paper used hydrogels for photovoltaic cooling (R. Li, Y. Shi, M. Wu, S. Hong, P. Wang, Photovoltaic panel cooling by atmospheric water sorption–evaporation cycle. Nature Sustainability, 1-8, 2020).

(3) We actually present two forms of PVA hydrogels: dry-annealed PVA hydrogel and nanofibrous PVA hydrogel. For the dry-annealed PVA hydrogel, it is isotropic, thereby it displays fatigue-resistance in any directions. However, for the nanofibrous PVA hydrogel, it is anisotropic. The fatigue resistance is high (i.e., 1250 J/m^2) when loading is along the direction of aligned fibrils, but relatively low (i.e., 233 J/m^2) when the loading is perpedicular to the aligned fibrils. Ruobing's work ( also shows a similar anisotropic property of nanofibrous PVA hydrogels using a different fabrication technique. 

To solve that, we print the nanofibrous hydrogels into mesh-like structures, so that the sample shows fatigue-resistance in both in-plane directions. I believe you show a much-advanced version of 3D printing to achieve fatigue-resistant elastomers. 



linst06's picture

Dear Jingda,

I would like to also share two review papers by my colleagues, which particularly shed light on the applications of hydrogels.

Hydrogel machine by Xinyue

Hydrogel bioelectronics by Hyunwoo and Baoyang.



Tang jingda's picture

Dear Shaoting,

    Thank you for such a deep and helpful discussion. And thank you for sharing the two reviews, I usually read each paper from your group.  I agree with you that the developments of the study on hydrogel adhesion and hydrogel fatigue are in a different stage.  In fact, hydrogel adhesive have been studied for many decades. Only strong hydrogel adhesion initiates in recent years. This makes the comparison clear. Hydrogel fatigue is a much younger field (hahaha).  

    If we suppose fatigue resistant hydrogel will be mostly used in biomedical engineering, have you ever thought about the effect of in vivo environment on the fatigue resistance of hydrogels? You did fatigue tests under water in your papers, better than what we did before. Have you ever changed the water environment to a PBS Buffer or an other biological liquid (e.g., gastric juice, very low pH)? These are my random thoughts, may not be fair questions :)   



linst06's picture

Dear Jingda,

Thanks for your great points. Yes, we conducted fatigue characterizations in different biological fluids as well.

In our ex-vivo long-term characterization of the ingestible hydrogel balloon, we used the gastric juice.

In the fatigue characterization of hydrogel adhesion using a 90-degree peeling test, we immersed the sample in a PBS buffer.

In the cartilage-on-flat dynamic fractional test, the interface between hydrogel coating and the cartilage was sprayed a thin layer of simulated synovial fluid as a lubricant to mimic the working environment of knee joints.



Ruobing Bai's picture

Dear Shaoting,

Congratulations on having such a nice overview of an on-going field of hydrogel fatigue and anti-fatigue. Since we have already discussed quite a lot online and offline, I am here with just some random questions and thoughts.

1. While people in the mechanics community usually refer fatigue to the failure under cyclic loads, the word "fatigue" in reality can mean a lot of things, as we previously summarized as "symptoms of fatigue". We have mainly focused on the one symptom, "cyclic fatigue crack growth", for a while, which is of critical importance, and is now more or less well adressed thanks to the efforts by you and other colleagues. On the other hand, there are still challenges in other symptoms of hydrogel fatigue. For example, the property of a hydrogel degrades after long-time loading. To address these challenges, it requires other complex loading conditions, sample geometries, and in vivo tests, as folks already discussed above. In addition, these problems should be related to real applications. Pursuing a "perfect" "fatigue resistant" hydrogels may be too much, and there will be an extent to which the problem becomes more engineering than scientifically interesting. What is your thought for the "next"?

2. In addition to engineering applications, I believe that fatigue of hydrogels, together with all the characterizations and designs surrounding it, forms a great platform for targeting fundamental problems. The long time study and ambiguity on "intrinsic fracture toughness" is one of such example. Viscoelasticity, poroelasticity, and complex rheology accompanying the fracture and failure of hydrogels still have numerous intersting fundamental problems to study. I know you have tried to target some of these problems for a while. What do you see as the challenges or opportunities related to these fundamental studies?

3. In fracture mechanics, many people talk about both crack nucleation and crack growth. Classical fracture mechanics addresses crack growth quite well, but crack nucleation can always be a challenging subject. In hydrogel fracture and fatigue, I think it is interesting to think about crack nucleation, or even, whether we truly need fatigue-crack-growth-resistant hydrogels or not (of course we do!). The flaw sensitivity length or cohesive zone size of a double-network tough hydrogel can be as large as 1 mm. In this case, all we have worried and tested using samples with large initial crack length may be just irrelevant to real applications. So long as we have a great tough hydrogel that resists fatigue damage (degradation over cycles/loading time), we are good to go. Of course all these phenomena are intrinsically connected, but I sometimes reflect on myself whether we are chasing some particular aspect too far.






linst06's picture

Dear Ruobing,

I like your random thoughts a lot, which are sort of brainstorming for the future pathway of this direction.

1, Symptom of fatigue in real applications indeed involves more complex loading conditions, sample geometries, and chemical environments, which eventually turns to be an engineering problem. From my personal perspective, such kind of engineering designs in real applications, in turn, can inspire new scientific challenges. For instance, taking the neuro probe as one example, long-term chronic damage remains a central challenge in the field, which involves micro-motions of the human body as the boundary conditions, scar formation as the foreign body response, and increased impedance as the symptom of fatigue. This engineering challenge may inspire the design of electrically conductive fatigue-resistant materials for prolonged recording or stimulation of neuron cells. The scientific question could be what is the principle for endurant conductive materials, which falls into the design of molecular structures as well.

2, Beyond the applications, I am more interested in the fundamental implications, honestly speaking. I believe you already laid some foundation work to use fatigue characterization as a platform, showing that viscoelasticity does not contribute to the fatigue threshold. When either viscoelasticity or poroelasticity involves, the time scales should be taken into account, thereby the loading frequencies must play roles as well. I am particularly interested in understanding intrinsic fracture energy of soft materials, which has been studied since 1958 but still remains ambiguous. I believe the recent development of experimental techniques for probing chain fracture, and stress mapping may enable new opportunities for an in-depth understanding of fracture of soft materials. 

3, I totally agree with you about this comment. Some applications of hydrogels do not contain long cracks, therefore may not even suffer the issue of fracture or fatigue during its lifetime due to its flaw-insensitivity. However, as a fundamental study at initial stage, we still need standard testing methods (e.g., pure-shear samples with long cracks) to measure the value of fracture energy as its material property. A specimen with unreasonable small cracks will not give a value with a clear physical meaning, thereby it would be challenging for people to compare. 

On the other hand, I also see the strong reason why people in material science seem to be more willing to use the area of stress-strain curve to quantify a material's toughness. This toughness in the unit of J/m^3 is more relevant to real applications where samples are typically unnotched.



Li Chenghai's picture


Dear Shaoting,

Thanks for this nice review. I'm Chenghai Li, a PhD student from UCSD. I’m curious about the relationship between microscopic molecular structures and macroscopic mechanical properties. For simple microscopic structures like glass, ideal network or ceramics, pioneering people have proposed some clear understanding to relate macroscopic properties to microscopic structures. (e.g. Griffith's paper on glass fracture, Lake-Thomas model for predicting threshold of NR or toughness for a perfect network...) But generally speaking, the microscopic structures become more complex for polymers. (inhomogeneity, chain length distributions, dangling chains, loops...) As shown in Canhui and Zhigang’s JMPS paper, deviation from perfect networks can significantly affect mechanical properties of polymers (e.g. toughness, flaw sensitivity, work of rupture...). My question is that, how to qualitatively and quantitatively establish a more clear relationship between microscopic molecular structures with macroscopic properties for polymers?


7.8 磅



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Li Chenghai's picture

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linst06's picture

Dear Chenghai,

Nice to meet you in this Imechanica Journal Club. I believe your question is something missing but highly desirable in this field from the fundamental perspective.

People are actively studying the impact of topological defects on elasticity, basically correlating polymer topologies to phantom elasticity (e.g. M. Zhong, R. Wang, K. Kawamoto, B. D. Olsen, J. A. Johnson, Quantifying the impact of molecular defects on polymer network elasticity. Science 353, 1264-1268 (2016).) Such kind of study on the fracture of polymers is not well established yet, as far as I know.

If I track the literature correctly, Flory first proposed the concept of polymer topology and introduced the concept of graph theory, for understanding elasticity (Flory, P.J, Network topology and the theory of rubber elasticity. Br. Polym. J. 17, 96–102, 1985).

By tracking this work, my feeling is we need to classify defects first, then understand the impact of each type of topological defects one by one. With some theoretical understanding, we may harness existing chemical synthesis to design hydrogels with controlled defects as a model material system for experimental investigation. I am studying and working in this direction recently. Hopefully, I can talk more once I make some progress in this direction.

This is really a good question and insight.



Li Chenghai's picture

Thanks for your insightful commnets. Looking forward to your further work on this field!

Zhigang Suo's picture

I've just tweeted some ideas.

linst06's picture

Dear Zhigang,

Thanks for sharing the paper on high-strength Damascus steel. Introducing laminates of soft and hard phases seems to work pretty well for both hard materials and soft materials. The work on the design of hierarchical and heterogeneous hard materials has been a while, while the idea of engineering soft and hard phases in soft materials is just initiated recently. 

As you summarized in Twitter, the central ideal of stress de-concentration is crucial for fatigue resistance. You and Zhengjin (Z. Wang et al, PNAS, 116, 2019) have demonstrated a facile and effective approach to engineer the soft and hard phases in soft materials for achieving the stress de-concentration, from which I also learn a lot.

I actually have been interested in several length scales in a system with soft domains and hard domains. How is the size of these domains (or length of fibers) correlated with cohesive length and thereby impact on the fracture or fatigue performances?



Zhigang Suo's picture

In preparing the graduate course on fracture, I have just tweeted a thread on fatigue-resistant polymer networks.

linst06's picture

Dear Zhigang,

Sure. Thanks a lot.  It really reminds me of the great experience of taking your fracture mechanics course 3 or 4 years ago!



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