High-Strength, Tough, Fatigue Resistant, and Self-Healing Hydrogel Based on Dual Physically Cross-Linked Network

Abstract

Hydrogels usually suffer from low mechanical strength, which largely limit their application in many fields. In this Research Article, we prepared a dual physically cross-linked hydrogel composed of poly(acrylamide-co-acrylic acid) (PAM-co-PAA) and poly(vinyl alcohol) (PVA) by simple two-steps methods of copolymerization and freezing/thawing. The hydrogen bond-associated entanglement of copolymer chains formed as cross-linking points to construct the first network. After being subjected to the freezing/thawing treatment, PVA crystalline domains were formed to serve as knots of the second network. The hydrogels were demonstrated to integrate strength and toughness (1230 ± 90 kPa and 1250 ± 50 kJ/m(3)) by the introduction of second physically cross-linked network. What̀s more, the hydrogels exhibited rapid recovery, excellent fatigue resistance, and self-healing property. The dynamic property of the dual physically cross-linked network contributes to the excellent energy dissipation and self-healing property. Therefore, this work provides a new route to understand the toughness mechanism of dual physically cross-linked hydrogels, hopefully promoting current hydrogel research and expanding their applications.

Tensile Fatigue of Poly(Vinyl Alcohol) Hydrogels with Bio‐Friendly Toughening Agents

“…Toughness was selected as the key metric for screening additives due to its practical applications in the realm of articular cartilage when considering the ability of a hydrogel to dissipate energy, which is a direct relationship to toughness. [ 71–74 ] The results can be seen in Figure a in which toughness is shown for each additive. With these results in view, the top four performing additives on toughness enhancement alone were agar, xanthan gum, starch, and cotton flock.…”

“…The intensity of the diffraction pattern for the samples can be correlated to the presence of microcrystalline domains that formed in the network structure. [ 83 ] Dried out PVA gives way to sharp diffraction peak that can occur anywhere from 18° to 21° for a 10 reflection of a typical PVA crystalline domain [ 71 ] even though this peak was difficult to isolate from Figure 7a. Prior work has shown the importance of the freezing–thawing process in forming enhanced microcrystalline domains.…”

“…In the hydrogel research community, cyclic studies that utilize loading and unloading of hydrogels under tensile conditions are important, but they could be improved with 1) systematically varying the applied load or displacement level as well as 2) increasing the number of cycles applied in a test to support the claim that a material is “fatigue resistant” or “anti‐fatigue.” [ 4,56,57,59,71,94 ] If a material is deemed to be “fatigue resistant,” then researchers need a greater number of cycles to validate this claim. As such, it is difficult to assert that a material is “fatigue resistant” or “anti‐fatigue” simply because it is tougher or stronger in terms of its monotonic tensile properties.…”

Ultrastretchable, Super Tough, and Rapidly Recoverable Nanocomposite Double‐Network Hydrogels by Dual Physically Hydrogen Bond and Vinyl‐Functionalized Silica Nanoparticles Macro‐Crosslinking

“…The first category is slide ring gels and tetra‐PEG gels with homogeneously designed network structure to prevent stress localization . The second approach is double‐network (DN) hydrogels and single network hydrogels (SN) which resist the growth of a crack by dissipating energy through scission of chemical or supramolecular sacrificial bonds . The third category of tough hydrogels is to incorporate multifunctional crosslinker to uniformly distribute stress and dissipate mechanical energy, such as nanocomposite hydrogels (NC) and macromolecular microsphere composite hydrogels …”

Tough and recoverable triple‐network hydrogels based on multiple pairs of toughing mechanisms with excellent ionic conductivity as stable strain sensors

“…In recent years, one breakthrough to design tough hydrogels is by integrating sacrificial bonds to dissipate mechanical energy such as fracture of short polymer chains and reversible crosslinks (hydrogen bond, crystallization, and ionic interaction) into polymer networks . The chemical and physical hybrid crosslinked double‐network (DN) gels or single‐network gels demonstrate not only tough but also self‐recovery abilities from fatigue damage .…”