Self-Healing and Self-Recovering Hydrogels

Introducing additional physical and reversible crosslinks to a chemically crosslinked hydrogel is an interesting and viable alternative to increase the toughness of a hydrogel. Yet while in general the physical crosslink points provide dissipative mechanisms, there are still many details that are unknown in particular on the role that physical crosslinks play on the large strain behavior. We explore the mechanical properties in small and large strain of two dual crosslink gels made from a random copolymer of poly(acrylamide-co-vinylimidazole) with a range of elastic moduli in the tens of kPa. The interaction between vinylimidazole groups and metal ions (Zn²⁺ and Ni²⁺) results in physical crosslink points and in a markedly stretch-rate-dependent mechanical behavior. While a main relaxation process is clearly visible in linear rheology and controls the small and intermediate strain properties, we find that the strain hardening behavior at stretches of λ > 4 and the stretch at break λ_b are controlled by an additional longer-lived physical crosslinking mechanism that could be due to a clustering of physical crosslinks.

Several strategies have been developed in the past decade for the fabrication of self-healing or self-recovery hydrogels. Because self-healing and mechanical strength are two antagonistic features, this chapter tries to answer the question “How to design both mechanically strong and self-healable hydrogels?”. Here, I show that although autonomic self-healing could not be achieved in high-strength hydrogels, a significant reversible hard-to-soft or first-order transition in cross-link domains induced by an external trigger creates self-healing function in such hydrogels. I mainly focus on the physical hydrogels prepared via hydrogen-bonding and hydrophobic interactions. High-strength H-bonded hydrogels prepared via self-complementary dual or multiple H-binding interactions between hydrophilic polymer chains having hydrophobic moieties exhibit self-healing capability at elevated temperatures. Hydrophobic interactions between hydrophobically modified hydrophilic polymers lead to physical hydrogels containing hydrophobic associations and crystalline domains acting as weak and strong cross-links, respectively. Semicrystalline self-healing hydrogels exhibit the highest mechanical strength reported so far and a high self-healing efficiency induced by heating above the melting temperature of the alkyl crystals. Research in the field of self-healing hydrogels provided several important findings not only in the field of self-healing but also in other applications, such as injectable gels and smart inks for 3D or 4D printing.

The development of high-performance polymeric materials typically involves a trade-off between desirable properties such as processability, recyclability, durability, and strength. Two common strategies in this regard are composites and reversibly cross-linked materials. Making optimal choices in the vast design spaces of these polymeric materials requires a solid understanding of the molecular-scale mechanisms that determine the relation between their structure and their mechanical properties. Over the past few years, a wide range of computational techniques has been developed and employed to model these mechanisms and build this understanding. Focusing on approaches rooted in molecular dynamics, we present and discuss these techniques, and demonstrate their use in several physical models of novel polymer-based materials, including nanocomposites, toughened gels, double network elastomers, vitrimers, and reversibly cross-linked semiflexible biopolymers.

Incorporation of dynamic, reversible bonds into the polymer network of soft gels has been exploited as a strategy to enhance fracture toughness and to enable self-healing. Gels with dynamic bonds often exhibit macroscopic viscoelasticity which can be traced back to the kinetics of bond dissociation and reformation. This chapter discusses recent efforts in developing constitutive models to connect the molecular-level bond kinetics to the continuum-level viscoelasticity. Two different modeling approaches are described using a model system, i.e., hydrogel with dynamic physical crosslinks and static chemical crosslinks. Both approaches are based on the theoretical framework of continuum mechanics and thermodynamics and aim to quantify how the total network free energy is governed by macroscopic deformation and molecular kinetics. In the first approach, the network is treated as a collection of polymer chains formed at different instants along the loading history. These chains experience different extent of deformation and thus carry different free energy. The total free energy is the sum of contributions from all chains. The second approach considers a statistical distribution of the chain end-to-end vectors, which evolves upon macroscopic deformation and reaction of dynamic bonds. The total free energy is calculated by integrating the single-chain free energy over the chain distribution space. These two approaches, capable of capturing the time-dependent mechanical behaviors of hydrogels with reversible crosslinks, can be extended to model the macroscopic mechanics induced by other molecular mechanisms such as bond exchange and chain scission.

Hydrophobically associating hydrogels based on copolymers of a water-soluble monomer with a fluoroacrylate or fluoromethacrylate possess microphase-separated morphologies that provide unique properties. Physical crosslinks in these hydrogels involve hydrophobic bonds between fluoro(meth)acrylate groups that associate into 2–6-nm-diameter core–shell nanodomains that represent multifunctional crosslinks. These hydrogels exhibit exceptional mechanical properties and fracture toughness values approaching 10⁴ J/m², are extrudable, and show self-healing behavior of the microstructure. This chapter reviews the characteristics of these microphase-separated, hydrophobically associating hydrogels and discusses potential applications of these materials as injectable in situ forming hydrogels, electrospun fiber mats suitable for tissue scaffolds, controlled drug release, antifreeze materials, and shape memory hydrogels.

This chapter reviews the preparation, structures, properties, and applications of tough and responsive hydrogels crosslinked by triblock copolymer micelles. An amphiphilic triblock copolymer, Pluronic F127, is functionalized with reactive groups. The functional F127 chains form polyfunctional micelles in aqueous solution, which are used for reaction with hydrophilic monomers or polymer chains to form hydrophobically crosslinked network. Free radical polymerization, host-guest interaction, and dynamic bonds have been utilized to connect the polymer chains with the reactive micelle coronae. The obtained polymer hydrogels show outstanding strength and toughness, with the micelles serving as energy dissipation centers. The structural evolution and energy dissipation mechanisms have been investigated in detail. By incorporating functional monomers into the network, a series of functional hydrogels responsive to pH, salt, electric field, and temperature have been developed. The hydrogels have been utilized as building blocks to fabricate soft actuators, shape-morphing devices, and biomaterials for tissue repair.

Over the past decade, the design of hydrogels has transformed from fabrication of static cross-linked materials to dynamic systems, which upon damage can potentially self-heal to their original state, either autonomously or through the help of external stimuli. This chapter highlights, through examples, the synthesis and self-healing properties of hydrogels based on the various types of dynamic chemistries available for their fabrication. While the main focus of the chapter is on the chemistry of cross-linking and the conditions under which self-healing was achieved, a brief discussion on the method utilized to ascertain the extent of self-healing through mechanical or rheological data, as well as possible applications of such materials, is explored.

Polyampholyte (PA) hydrogels have attracted great attention as an innovative material having tough, self-healing, and viscoelastic behavior. PA hydrogels are synthesized using one-step radical polymerization of equal amounts of oppositely charged ionic monomers at a very high monomer concentration. They have 50–70 wt% of water at an equilibrium state, much lower than that of conventional hydrogels that usually have a high-water content (>80%). They are strongly viscoelastic and have a high toughness (fracture energy of 1,000–4,000 J/m²), a high modulus (0.01–8 MPa), a high failure strain (150–1,500%), and a failure stress (0.1–2 MPa), together with 100% self-recovery and a high self-healing efficiency behavior. These excellent mechanical performances are comparable to that of rubbers, most tough double-network hydrogels, and soft bio-tissues. The extensive experimental studies show that these multiple mechanical properties are related to the formation of dynamical features of inter- and intra-chain ionic bonds, and this study opens a common strategy to develop tough and self-healing hydrogels.

Cellulose-based hydrogels have emerged as promising materials in a wide range of applications owing to their inherently renewable, biocompatible, and biodegradable characteristics. The present chapter addresses advances in the synthesis methods of cellulose-based hydrogels from native cellulose, cellulose derivatives, or composites and focuses on the design and preparation of reversible cross-linked cellulose-based hydrogels featured with self-healing or dynamic stimuli response. Dynamic chemistry, including reversibly chemical and physical cross-linking methods, provides a fascinating strategy for the fabrication of cellulose-based hydrogels through the formation of reversible dynamic covalent bonds or non-covalent interactions, respectively. Moreover, we provide the future outlook for the guidance of fruitful explorations of hydrogels based on cellulose and their derivatives to expand their further advancement.

Hydrogels derived from biopolymers, also called biohydrogels, have shown potential for brain injury therapy due to their tunable physical, chemical, and biological properties. Among different biohydrogels, those made from collagen type I are very promising candidates for the reparation of nervous tissues due to its biocompatibility, noncytotoxic properties, injectability, and self-healing ability. Moreover, although collagen does not naturally occur in the brain, it has been demonstrated that collagen type I, which resides in the basal lamina of the subventricular zone in adults, supports neural cell attachment, axonal growth, and cell proliferation due to its intrinsic content of specific cell-signaling domains. This chapter summarizes the most relevant results obtained from both in vitro and in vivo studies using self-healing biohydrogels based on collagen type I as key component in the field of neuroregeneration.