Zwitterionic hydrogels have been explored for applications in electrochemical devices very recently due to their high water retention ability and interesting electrochemical properties. The use of zwitterionic hydrogels in devices requires them tough and recoverable or healable from fatigue damage. Herein, a double network zwitterionic hydrogel contains a reversible noncovalent interaction crosslinked polyvinyl alcohol (PVA) first network, together with a covalent/noncovalent hybrid crosslinked acrylamide and sulfobetaine methacrylate copolymer (P(AM-co-SBMA)) second network, was fabricated by a simple two-steps methods of copolymerization and freezing/thawing. The reversible hydrogen bonds, crystalline domain, and electrostatic interactions in the double networks work as sacrificial bonds to dissipate energy and toughen the materials when hydrogel deforms. The broken bonds can reform upon unloading endowing the recovery of hydrogels' properties with the assistance of the elastic covalent network. The optimal hydrogels are highly stretchable (fracture strain 970%), tough (fracture toughness 693 kJ m −3 ), rapidly recoverable (65% toughness recovery and 75% stiffness recovery after resting 5 min at room temperature) and with widely tunable mechanical properties by multibond crosslinking. Meanwhile, the zwitterionic counterions of SBMA moieties endow the tough and recoverable hydrogels extremely high intrinsic ionic conductivities (7.49 S m −1 ) at room temperature. This work not only provides a simple strategy for fabricating tough and recoverable zwitterionic hydrogels but also demonstrates multifunctional properties of the zwitterionic hydrogels, which possess a great potential to fulfill flexible devices applications.

It remains a challenge to develop tough hydrogels with recoverable or healable properties after damage. Herein, a new nanocomposite double‐network hydrogel (NC‐DN) consisting of first agar network and a homogeneous vinyl‐functionalized silica nanoparticles (VSNPs) macro‐crosslinked polyacrylamide (PAM) second network is reported. VSNPs are prepared via sol‐gel process using vinyltriethoxysilane as a silicon source. Then, Agar/PAM‐SiO2 NC‐DN hydrogels are fabricated by dual physically hydrogen bonds and VSNPs macro‐crosslinking. Under deformation, the reversible hydrogen bonds in agar network and PAM nanocomposite network successively break to dissipate energy and then recombine to recover the network, while VSNPs in the second network could effectively transfer stress to the network chains grafted on their surfaces and maintain the gel network. As a result, the optimal NC‐DN hydrogels exhibit ultrastretchable (fracture strain 7822%), super tough (fracture toughness 18.22 MJ m‐3, tensile strength 431 kPa), rapidly recoverable (≈92% toughness recovery after 5 min resting at room temperature), and self‐healable (can be stretched to 1331% after healing) properties. The newly designed Agar/PAM‐SiO2 NC‐DN hydrogels with tunable network structure and mechanical properties by multi‐bond crosslinking provide a new avenue to better understand the fundamental structure‐property relationship of DN hydrogels and broaden the current hydrogel research and applications.

The emerging applications of hydrogels in flexible devices require it possess multifunctional properties including stable mechanical and functions under various deformations or external environments. Herein, a multifunctional polyvinyl alcohol/M‐alginate/PAM hydrogel with very excellent mechanical properties and sensing functions was fabricated by introducing multiple pairs of toughing mechanisms into triple network (TN). The multiple supramolecular physical networks work as sacrificial networks to toughen the materials when hydrogel deforms. The broken bonds can reform upon unloading endowing the recovery of hydrogels' properties and functions with the assistance of the elastic covalent network. The optimal TN hydrogels are extremely tough (a fracture strength of 512 kPa, a fracture toughness of 3 MJ/m3) and recoverable from fatigue damage (~77% toughness recovery after 5 min resting at room temperature). The presence of abundant ionic species endows the tough and recoverable TN hydrogels high ionic conductivity and high sensitivity as strain sensors. Moreover, such TN hydrogels with multi‐bond crosslinking in three networks can potentially guarantee stable mechanical and sensor functions under various deformations or external environments compared to the DN candidates. This work provides a simple strategy for fabricating multifunctional hydrogels with high stability to fulfill its flexible devices applications. POLYM. ENG. SCI., 59:1657–1666 2019. © 2019 Society of Plastics Engineers

Constructing double network (DN) or triple network (TN) with two or three asymmetric independent polymer networks are very useful structural platforms to integrate different mechanisms for dissipating energy and maintaining elasticity for designing tough hydrogels. To reveal the roles of different cross-linked networks with hierarchical bond energy across multiple length scales in the gel network, here we fabricate chemical-physical hybrid cross-linked polyacrylamide/Agar/poly(vinyl alcohol) (PAM/Agar/PVA) TN hydrogels through multi-network and multi-bond toughening mechanism design, and compare these TN gels with single network and DN gels. Under deformation, the covalently cross-linked PAM first network remain intact to maintain their original configuration, while the dynamic and reversible nanoscale hydrogen bonds in Agar second network and microcrystalline domain in PVA third network successively break to gradually dissipate energy and then recombine to recover the network, which lead to excellent mechanical strength (compression strength 2.6 MPa, tensile strength 197 kPa), toughness (energy dissipation up to 374 kJ/m 3 at 200% strain), excellent recoverability (~87% toughness recovery after 15 min resting at room temperature), and self-healing properties. The combination of high-strength, self-recovery, and self-healing properties makes PAM/Agar/PVA TN gels promising candidates for further functional application.

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