Dynamic Covalent Hydrogels: Strong yet Dynamic

Abstract

Hydrogels are crosslinked polymer networks with time-dependent mechanical response. The overall mechanical properties are correlated with the dynamics of the crosslinks. Generally, hydrogels crosslinked by permanent chemical crosslinks are strong but static, while hydrogels crosslinked by physical interactions are weak but dynamic. It is highly desirable to create synthetic hydrogels that possess strong mechanical stability yet remain dynamic for various applications, such as drug delivery cargos, tissue engineering scaffolds, and shape-memory materials. Recently, with the introduction of dynamic covalent chemistry, the seemingly conflicting mechanical properties, i.e., stability and dynamics, have been successfully combined in the same hydrogels. Dynamic covalent bonds are mechanically stable yet still capable of exchanging, dissociating, or switching in response to external stimuli, empowering the hydrogels with self-healing properties, injectability and suitability for postprocessing and additive manufacturing. Here in this review, we first summarize the common dynamic covalent bonds used in hydrogel networks based on various chemical reaction mechanisms and the mechanical strength of these bonds at the single molecule level. Next, we discuss how dynamic covalent chemistry makes hydrogel materials more dynamic from the materials perspective. Furthermore, we highlight the challenges and future perspectives of dynamic covalent hydrogels.

Recent advances in double network hydrogels based on naturally-derived polymers: synthesis, properties, and biological applications

“…Focused discussions of these specific crosslink mechanisms and how Crystal K. they can be used in the preparation of hydrogels has been presented in other reviews. [35][36][37][38] Table 1 provides a summary of the different crosslink mechanisms employed in recent natural polymer-based DN hydrogels discussed in this review. The majority of these are hybrid crosslinked DN hydrogels with one polymer network stabilized by ionic interactions while the other is formed through photopolymerization.…”

“…Focused discussions of these specific crosslink mechanisms and how they can be used in the preparation of hydrogels has been presented in other reviews. 35–38…”

New Smart Bioactive and Biomimetic Chitosan-Based Hydrogels for Wounds Care Management

“…The dynamic crosslinking methods have gained more attention lately as studies suggest that using chemical interactions and versatile hydrogels may be developed [ 27 ]. These chemical interactions are capable of uncoupling and recoupling if the dynamic equilibrium of the reaction is achieved or exceeded [ 24 , 28 ].…”

“…These chemical interactions are capable of uncoupling and recoupling if the dynamic equilibrium of the reaction is achieved or exceeded [ 24 , 28 ]. For example, oxidized alginate, polyethylene glycol dibenzaldehyde, and oxidized polysaccharides (hyaluronic acid and chitosan) were investigated for their crosslinking capacity via Schiff bases linkage in order to develop innovative nontoxic self-healing hydrogels for different medical applications [ 27 , 29 ]. The presence of a Schiff bases linkage (dynamic imine bonds) allows hydrogels to modify their 3D matrix, so as to adapt perfectly to the shape and depth of a wound (self-adapting ability).…”

Recent Advances in Hydrogels for the Diagnosis and Treatment of Dry Eye Disease

“…The methods used for preparing hydrogels include chemical cross-linking (covalent bond) and physical cross-linking (non-covalent bond) [ 37 ]. Chemical cross-linking can lead to good mechanical strength in the hydrogels [ 38 ]. It is worth noting that, some small-molecule chemical crosslinkers and initiators are not environmentally friendly or are even toxic [ 39 ].…”