Effect of First Network Topology on the Toughness of Double Network Hydrogels

Xin, Hai; Saricilar, Süreyya; Brown, Hugh R.; Whitten, Philip G.; Spinks, Geoffrey M.

doi:10.1021/ma400892g

Cited by 52 publications

(51 citation statements)

References 29 publications

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“…14 The results shown in Figures 1-3 clearly illustrate the effect of MBAA and Ca 2+ concentration on tensile properties and fracture energy of the fully swollen PAAm-alginate hybrid gel. The observations mirror the findings for double network gels where toughness enhancement is produced by the presence of a tight network that is reinforced with a much larger quantity of a loose network [19,21]. In the hybrid gels studied here (and previously in [9,18]), a densely-crosslinked alginate and a loosely-crosslinked PAAm network are necessary conditions for toughness enhancement.…”

Section: Tensile Properties and Fracture Energysupporting

confidence: 86%

“…While slight differences in preparation methods may contribute to the lower toughness values reported here, the main contribution to the lower toughness is the higher swelling ratio (9-24) of the gels studied here in comparison to the previous work where the gels were tested in their as-synthesised state with swelling ratio of ~7. Increased swelling reduces the density of network chains thereby decreasing gel toughness as it has been previously shown that gel toughness is proportional to the density of network strands in the tight network [19]. 14 The results shown in Figures 1-3 clearly illustrate the effect of MBAA and Ca 2+ concentration on tensile properties and fracture energy of the fully swollen PAAm-alginate hybrid gel.…”

Section: Tensile Properties and Fracture Energymentioning

confidence: 75%

“…When a hybrid gel is subject to external force the crosslinks are "unzipped", dissipating dissociation energy of the ionic bonds and entropic energy of the loaded network strands. High toughness of the hybrid gel is obtained when the alginate network is highly-crosslinked and the PAAm network is loosely-crosslinked [9,[14][15][16][17][18], which mirrors the topologies known to maximize toughness in DN gels [19]. The ionic bonds that are pulled apart during the hybrid gel deformation are re-formed at zero-stress at a temperature-induced network recovery process to largely retrieve the mechanical properties of the virgin gel [9].…”

mentioning

confidence: 87%

“…The multiple damage zones that develop in the 1 st network are held together by the longer and more prevalent 2 nd network chains so that catastrophic failure is prevented. Therefore, the remarkable toughness of DN gels is obtained when the 1 st network is highly-crosslinked and the 2 nd network is loosely crosslinked [19,[21][22][23][24]. The former condition guarantees a large sum of short polymer strands whose entropic energy is dissipated during crack development.…”

Section: Tensile Properties and Fracture Energymentioning

confidence: 99%

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Time-dependent mechanical properties of tough ionic-covalent hybrid hydrogels

Xin

Brown

Naficy

et al. 2015

Polymer

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Hybrid gels featuring interpenetrating covalent and ionic crosslinked networks have recently been shown to exhibit both high toughness and recoverability of strain-induced network damage. The high toughness results from the energy dissipated as entropically strained network strands are released by the dissociation of ionic crosslinks. As in the so-called double network hydrogels, the toughening process is inherently linked to network damage. This damage, however, can be recovered to a large degree in hybrid gels due to the reformation of ionic associations when the gel is unloaded. The stability of the ionic network under load is here investigated and it is shown that these networks show large stress relaxation at constant strain, time dependent stress-strain behaviour and rate-dependent toughness. A double exponential model is invoked to mathematically describe the stress relaxation of the hybrid gels indicating at least two relaxation mechanisms. The rate-dependent toughness and the relaxation behaviour of the hybrid gels are attributed to the labile unzipping of the ionic crosslinks which is assumed to be load and time dependent. AbstractHybrid gels featuring interpenetrating covalent and ionic crosslinked networks have recently been shown to exhibit both high toughness and recoverability of strain-induced network damage. The high toughness results from the energy dissipated as entropically strained network strands are released by the dissociation of ionic crosslinks. As in the socalled double network hydrogels, the toughening process is inherently linked to network damage. This damage, however, can be recovered to a large degree in hybrid gels due to the reformation of ionic associations when the gel is unloaded. The stability of the ionic network under load is here investigated and it is shown that these networks show large stress relaxation at constant strain, time dependent stress-strain behaviour and rate-dependent toughness. A double exponential model is invoked to mathematically describe the stress relaxation of the hybrid gels indicating at least two relaxation mechanisms. The ratedependent toughness and the relaxation behaviour of the hybrid gels are attributed to the labile unzipping of the ionic crosslinks which is assumed to be load and time dependent.

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Section: Tensile Properties and Fracture Energysupporting

confidence: 86%

Section: Tensile Properties and Fracture Energymentioning

confidence: 75%

mentioning

confidence: 87%

Section: Tensile Properties and Fracture Energymentioning

confidence: 99%

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Time-dependent mechanical properties of tough ionic-covalent hybrid hydrogels

Xin

Brown

Naficy

et al. 2015

Polymer

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“…For the chemically linked PAMPS/PAAm DN gels, [ 23,36,38 ] the toughening mechanism is mainly based on the "sacrifi cial bonds" concept. Briefl y, upon deformation, the fi rst network of the gels made of strong polyelectrolytes permanently ruptures into small clusters, and these broken clusters likely serve as sacrifi cial bonds to protect the second chemically linked network and to increase its resistance against crack propagation by forming a large damage zone.…”

Section: Toughening Mechanisms Of Dn Gelsmentioning

confidence: 99%

A Novel Design Strategy for Fully Physically Linked Double Network Hydrogels with Tough, Fatigue Resistant, and Self‐Healing Properties

Chen

Zhu

Chen

et al. 2015

Adv Funct Materials

555

376

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1 wileyonlinelibrary.com recoverability and self-healing property, due to their intrinsic structural heterogeneity and/or lack of effi cient energydissipation mechanisms, [ 13 ] which greatly limit their uses for other applications requiring highly mechanical properties such as cartilage, tendon, muscle, and blood vessel.Many efforts have been made to develop tough hydrogels with new microstructures and toughening mechanisms, such as double network hydrogels, [ 14 ] nanocomposite hydrogels, [ 15 ] sliding-ring hydrogels, [ 16 ] macromolecular microsphere composite hydrogels, [ 17 ] tetrapolyethylene glycol hydrogels, [ 18 ] hydrophobically associated hydrogels, [ 19,20 ] and dipole-dipole or hydrogen bonding enhanced hydrogels. [ 21,22 ] Among them, double network (DN) hydrogels have demonstrated their excellent mechanical properties. The existing knowledge of DN gels from synthesis methods, network structures, to toughening mechanisms mainly comes from chemically cross-linked DN gels. [ 23 ] Both networks with contrasting structures in DN gels are separately crosslinked by covalent bonds, [ 24 ] and the interpenetration of two contrasting networks makes the chemically linked DN gels both tough and soft, as evidenced by stiffness (elastic modulus of 0.1-1.0 MPa), strength (failure tensile stress of 1-10 MPa, strain 1000%-2000%, failure compressive stress 20-60 MPa, strain 90%-95%), and toughness (tearing fracture energy of 10 2 -10 3 J m −2 ). [ 23 ] Chemically linked DN gels have comparable toughness to cartilage and rubber. The toughening mechanisms are largely based on "sacrifi cial bonds" that break from the fi rst network to effectively dissipate energy, protect the second network, sustain stress, and store elastic energy, thus to reinforce the gels. However, the fracture of the fi rst network also causes irreversible and permanent bond breaks, making the gels very diffi cult to be repaired and recovered from damages and fatigues. [ 25 ] Thus, the internal fracture process of the fi rst network is considered to be critical for toughness enhancement, because relatively large damage zones formed in the fi rst network allow for more accumulated damage before macroscopic crack propagation occurs throughout whole networks. [ 26,27 ] Double network (DN) hydrogels with two strong asymmetric networks being chemically linked have demonstrated their excellent mechanical properties as the toughest hydrogels, but chemically linked DN gels often exhibit negligible fatigue resistance and poor self-healing property due to the irreversible chain breaks in covalent-linked networks. Here, a new design strategy is proposed and demonstrated to improve both fatigue resistance and self-healing property of DN gels by introducing a ductile, nonsoft gel with strong hydrophobic interactions as the second network. Based on this design strategy, a new type of fully physically cross-linked Agar/hydrophobically associated polyacrylamide (HPAAm) DN gels are synthesized by a simple one-pot method. Agar/ HPAAm DN gels exhibit excellent mech...

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Specialty Tough Hydrogels and Their Biomedical Applications

Fuchs

Shariati

2019

Adv Healthcare Materials

164

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Hydrogels have long been explored as attractive materials for biomedical applications given their outstanding biocompatibility, high water content, and versatile fabrication platforms into materials with different physiochemical properties and geometries. Nonetheless, conventional hydrogels suffer from weak mechanical properties, restricting their use in persistent load-bearing applications often required of materials used in medical settings. Thus, the fabrication of mechanically robust hydrogels that can prolong the lifetime of clinically suitable materials under uncompromising in vivo conditions is of great interest. This review focuses on design considerations and strategies to construct such tough hydrogels. Several promising advances in the proposed use of specialty tough hydrogels for soft actuators, drug delivery vehicles, adhesives, coatings, and in tissue engineering settings are highlighted.

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Effect of First Network Topology on the Toughness of Double Network Hydrogels

Cited by 52 publications

References 29 publications

Time-dependent mechanical properties of tough ionic-covalent hybrid hydrogels

Time-dependent mechanical properties of tough ionic-covalent hybrid hydrogels

A Novel Design Strategy for Fully Physically Linked Double Network Hydrogels with Tough, Fatigue Resistant, and Self‐Healing Properties

Specialty Tough Hydrogels and Their Biomedical Applications

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