Fatigue fracture of tough hydrogels
TL;DR: In this article, the authors studied the fatigue fracture of a polyacrylamide-alginate hydrogel and found that the stress-stretch curve changes cycle by cycle, and reaches a steady state after thousands of cycles.
About: This article is published in Extreme Mechanics Letters.The article was published on 2017-09-01 and is currently open access. It has received 177 citations till now. The article focuses on the topics: Fracture mechanics & Self-healing hydrogels.
Figures (7)
Fig. 1 Shakedown after prolonged cycling. (a) A sample of tough hydrogel is cyclically stretched in the pure shear test. (b) The loading profile of the test. Shakedown of stress-stretch curves was observed in tests with different loading frequencies, and shows no significant rate dependence: The loading frequency is (c) 0.25 Hz, (d) 0.125 Hz. 
Fig. 4 Crack growth as a function of the number of cycles N, under different maximum stretches. The crack growth reaches the steady state after thousands of cycles, and becomes approximately a linear function of N. 
Fig. 7 Comparison between a polyacrylamide hydrogel and a tough hydrogel in fatigue fracture. (a) The crack growth of the polyacrylamide hydrogel is faster (steeper slope) than that of the tough hydrogel, even under a less critical condition (smaller and G per cycle). (b) The fatigue threshold of the tough hydrogel is higher than that of the polyacrylamide hydrogel. The slope of dc/dN vs. G in the tough hydrogel is much lower than that in the polyacrylamide hydrogel. Both factors show that tough hydrogels have better fatigue fracture resistance than polyacrylamide hydrogels. 
Fig. 6 (a) A polyacrylamide hydrogel maintains stable stress-stretch loops over thousands of cycles. (b) The maximum stress in each cycle remains nearly constant over cycles. 
Fig. 2 (a) The maximum stress in each cycle decreases and approaches a steady state. (b) The stress-stretch curves after 2000 cycles for different maximum stretches . 
Fig. 5 (a) The energy release rate is calculated by integrating the stress-stretch curve in the steady state of the material far ahead of the crack tip. (b) In the steady state, the extension of crack per cycle, dc/dN, is plotted as a function of energy release rate G. A threshold is found, below which no crack propagation in the steady state occurs. Close above the threshold, dc/dN is found as a linear function of G. dc/dN then increases rapidly with G. 
Fig. 3 Fatigue fracture. (a) A notched sample is cyclically stretched. The sample and the grips are enclosed in a chamber with water droplets on its inner surface to control the humidity of the environment. (b) The applied loading profile over cycles. (c) The crack propagates cycle by cycle, and the evolution is recorded by photos. The crack propagation can be measured from the metric papers on the grips.
Citations
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444 citations
TL;DR: In this paper, a review aimed at synergistically reporting: (i) general design principles for hydrogels to achieve extreme mechanical and physical properties, (ii) implementation strategies for the design principles using unconventional polymer networks, and (iii) future directions for the orthogonal design of hydrogel to achieve multiple combined mechanical, physical, chemical, and biological properties.
Abstract: Hydrogels are polymer networks infiltrated with water. Many biological hydrogels in animal bodies such as muscles, heart valves, cartilages, and tendons possess extreme mechanical properties including being extremely tough, strong, resilient, adhesive, and fatigue-resistant. These mechanical properties are also critical for hydrogels' diverse applications ranging from drug delivery, tissue engineering, medical implants, wound dressings, and contact lenses to sensors, actuators, electronic devices, optical devices, batteries, water harvesters, and soft robots. Whereas numerous hydrogels have been developed over the last few decades, a set of general principles that can rationally guide the design of hydrogels using different materials and fabrication methods for various applications remain a central need in the field of soft materials. This review is aimed at synergistically reporting: (i) general design principles for hydrogels to achieve extreme mechanical and physical properties, (ii) implementation strategies for the design principles using unconventional polymer networks, and (iii) future directions for the orthogonal design of hydrogels to achieve multiple combined mechanical, physical, chemical, and biological properties. Because these design principles and implementation strategies are based on generic polymer networks, they are also applicable to other soft materials including elastomers and organogels. Overall, the review will not only provide comprehensive and systematic guidelines on the rational design of soft materials, but also provoke interdisciplinary discussions on a fundamental question: why does nature select soft materials with unconventional polymer networks to constitute the major parts of animal bodies?
312 citations
TL;DR: In this article, a new class of nature-inspired ionic conductors based on supramolecular sodium alginate (SA) nanofibrillar double network (DN) hydrogels with complex shapes by injection is demonstrated.
Abstract: There is a growing demand for flexible and stretchable strain/pressure sensors for different applications. However, existing conductors usually cannot meet all the requirements for use in next-generation wearable sensors. In this work, we demonstrate a new class of nature-inspired ionic conductors based on supramolecular sodium alginate (SA) nanofibrillar double network (DN) hydrogels with complex shapes by injection. Owing to their dermis-mimicking structures, these hydrogels exhibit unique features, such as high transparency (99.6%), high tension/compression strength (0.750 MPa/4 MPa), high stretchability (3120%), high toughness (4.77 MJ m−3) and superior elasticity (100%) at high strain (1000%). In particular, the use of salts (e.g., NaCl) as triggers in supramolecular assembly combining SA makes the hydrogels ideal ionic conductors. The ionic conductors were demonstrated as strain sensors with high sensitivity to an extremely broad strain window (0.3–1800%) and a low applied voltage (down to 0.04 V), as well as with high pressure sensitivity (1.45 kPa−1). These hydrogel-based ionic sensors may find applications in sports monitoring, human/machine interfaces and soft robotics.
299 citations
TL;DR: A candid and critical overview of the current understanding of the relation between the structure and molecular architecture of polymer networks and their mechanical properties, restricting ourselves to soft networks made of flexible polymers and displaying entropic elasticity.
Abstract: Soft polymer networks have seen an explosion of recent developments motivated by new high tech applications in the biomedical field or in engineering. We present a candid and critical overview of the current understanding of the relation between the structure and molecular architecture of polymer networks and their mechanical properties, restricting ourselves to soft networks made of flexible polymers and displaying entropic elasticity. We specifically review and compare recent approaches to synthesize swollen hydrogels with enhanced toughness, resilient but tough unfilled elastomers and self-healing networks containing dynamic bonds. The purpose is less to draw a comprehensive catalogue of approaches than to identify and unify the underlying principles controlling toughening mechanisms and mechanical self-healing behavior and to point out remaining challenges.
276 citations
TL;DR: A strategy of mechanical training is proposed to achieve the aligned nanofibrillar architectures of skeletal muscles in synthetic hydrogels, resulting in the combinational muscle-like properties, which are obtained through the training-induced alignment of nan ofibrils, without additional chemical modifications or additives.
Abstract: Skeletal muscles possess the combinational properties of high fatigue resistance (1,000 J/m2), high strength (1 MPa), low Young's modulus (100 kPa), and high water content (70 to 80 wt %), which have not been achieved in synthetic hydrogels. The muscle-like properties are highly desirable for hydrogels' nascent applications in load-bearing artificial tissues and soft devices. Here, we propose a strategy of mechanical training to achieve the aligned nanofibrillar architectures of skeletal muscles in synthetic hydrogels, resulting in the combinational muscle-like properties. These properties are obtained through the training-induced alignment of nanofibrils, without additional chemical modifications or additives. In situ confocal microscopy of the hydrogels' fracturing processes reveals that the fatigue resistance results from the crack pinning by the aligned nanofibrils, which require much higher energy to fracture than the corresponding amorphous polymer chains. This strategy is particularly applicable for 3D-printed microstructures of hydrogels, in which we can achieve isotropically fatigue-resistant, strong yet compliant properties.
258 citations
References
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4,511 citations
Book•
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TL;DR: In this article, the cyclic deformation and fatigue crack initiation in polycrystalline ductile solids was studied and a total-life approach was proposed to deal with the problem.
Abstract: Preface 1. Introduction and overview Part I. Cyclic Deformation and Fatigue Crack Initiation: 2. Cyclic deformation in ductile single crystals 3. Cyclic deformation in polycrystalline ductile solids 4. Fatigue crack initiation in ductile solids 5. Cyclic deformation and crack initiation in brittle solids 6. Cyclic deformation and crack initiation in noncrystalline solids Part II. Total-Life Approaches: 7. Stress-life approach 8. Strain-life approach Part III. Damage-Tolerant Approach: 9. Fracture mechanics and its implications for fatigue 10. Fatigue crack growth in ductile solids 11. Fatigue crack growth in brittle solids 12. Fatigue crack growth in noncrystalline solids Part IV. Advanced Topics: 13. Contact fatigue: sliding, rolling and fretting 14. Retardation and transients in fatigue crack growth 15. Small fatigue cracks 16. Environmental interactions: corrosion-fatigue and creep-fatigue Appendix References Indexes.
4,158 citations
"Fatigue fracture of tough hydrogels..." refers background in this paper
...Shakedown has been extensively studied in ductile metals [26]....
[...]
TL;DR: The synthesis of hydrogels from polymers forming ionically and covalently crosslinked networks is reported, finding that these gels’ toughness is attributed to the synergy of two mechanisms: crack bridging by the network of covalent crosslinks, and hysteresis by unzipping thenetwork of ionic crosslinks.
Abstract: Hydrogels with improved mechanical properties, made by combining polymer networks with ionic and covalent crosslinks, should expand the scope of applications, and may serve as model systems to explore mechanisms of deformation and energy dissipation. Hydrogels are used in flexible contact lenses, as scaffolds for tissue engineering and in drug delivery. Their poor mechanical properties have so far limited the scope of their applications, but new strong and stretchy materials reported here could take hydrogels into uncharted territories. The new system involves a double-network gel, with one network forming ionic crosslinks and the other forming covalent crosslinks. The fracture energy of these materials is very high: they can stretch to beyond 17 times their own length even when containing defects that usually initiate crack formation in hydrogels. The materials' toughness is attributed to crack bridging by the covalent network accompanied by energy dissipation through unzipping of the ionic crosslinks in the second network. Hydrogels are used as scaffolds for tissue engineering1, vehicles for drug delivery2, actuators for optics and fluidics3, and model extracellular matrices for biological studies4. The scope of hydrogel applications, however, is often severely limited by their mechanical behaviour5. Most hydrogels do not exhibit high stretchability; for example, an alginate hydrogel ruptures when stretched to about 1.2 times its original length. Some synthetic elastic hydrogels6,7 have achieved stretches in the range 10–20, but these values are markedly reduced in samples containing notches. Most hydrogels are brittle, with fracture energies of about 10 J m−2 (ref. 8), as compared with ∼1,000 J m−2 for cartilage9 and ∼10,000 J m−2 for natural rubbers10. Intense efforts are devoted to synthesizing hydrogels with improved mechanical properties11,12,13,14,15,16,17,18; certain synthetic gels have reached fracture energies of 100–1,000 J m−2 (refs 11, 14, 17). Here we report the synthesis of hydrogels from polymers forming ionically and covalently crosslinked networks. Although such gels contain ∼90% water, they can be stretched beyond 20 times their initial length, and have fracture energies of ∼9,000 J m−2. Even for samples containing notches, a stretch of 17 is demonstrated. We attribute the gels’ toughness to the synergy of two mechanisms: crack bridging by the network of covalent crosslinks, and hysteresis by unzipping the network of ionic crosslinks. Furthermore, the network of covalent crosslinks preserves the memory of the initial state, so that much of the large deformation is removed on unloading. The unzipped ionic crosslinks cause internal damage, which heals by re-zipping. These gels may serve as model systems to explore mechanisms of deformation and energy dissipation, and expand the scope of hydrogel applications.
3,856 citations
"Fatigue fracture of tough hydrogels..." refers background or methods in this paper
...Several strategies have been developed to synthesize tough hydrogels, including double network hydrogels [13, 14], nano- and micro-composite hydrogels [15-17], as well as tri-block copolymers and hydrophobic associated hydrogels [18, 19]....
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...The fatigue threshold of the tough hydrogel is about 53 J/m2, whereas the fracture toughness of the tough hydrogel under monotonic load easily reaches several thousands of J/m2....
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...We focus on a polyacrylamide-alginate tough hydrogel, where polyacrylamide forms a network of covalent crosslinks, and alginate forms a network of ionic crosslinks [14]....
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...The threshold for fatigue fracture is about 53 J/m2, much below the fracture energy (~10,000 J/m2) measured under monotonic load....
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...The re-zipping of the calcium crosslinks takes hours to days [14], and does not occur within the time period of each cycle....
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3,307 citations
"Fatigue fracture of tough hydrogels..." refers background or methods in this paper
...Several strategies have been developed to synthesize tough hydrogels, including double network hydrogels [13,14], nano- andmicro-composite hydrogels [15–17], as well as tri-block copolymers and hydrophobic associated hydrogels [18,19]....
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...High toughness of these hydrogels is achieved by breaking sacrificial bonds, such as covalent bonds in a short-chain network [13], or ionic crosslinks [14,18]....
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TL;DR: When a pharmaceutical agent is encapsulated within, or attached to, a polymer or lipid, drug safety and efficacy can be greatly improved and new therapies are possible.
Abstract: When a pharmaceutical agent is encapsulated within, or attached to, a polymer or lipid, drug safety and efficacy can be greatly improved and new therapies are possible. This has provided the impetus for active study of the design of degradable materials, intelligent delivery systems and approaches for delivery through different portals in the body.
2,195 citations