Post-synthesis modification of polymers for the development of functional dynamic covalent hydrogels

Abstract

The development of hydrogels to perform specific functions in complex environments requires a detailed refinement of their physicochemical properties and responsiveness to environmental changes. Accordingly, the conventional hydrogel synthesis methods have found an insurmountable barrier when the achievement of fully-functional structures at different scales is intended.Thus, in response to the growing requirements, post-synthesis modifications appear as essential strategies to achieve the desired characteristics. Due to the impulse provided by these methods, progress has been made in materials engineering to allow the development of smart hydrogels that predictably change their properties in response to environmental stimuli. Consequently, in addition to changes in the swelling behavior, the range of possible intelligent response has tremendously expanded to self-healing capacity, shape memory, the release of a bioactive, and/or on-demand degradation in biological tissues, to name a few.In the last years, the incorporation of reversible covalent bonds, supramolecular bonds, or other types of reversible interactions has prompted the study of the self-healing capacity and the shape memory property. The presence of dynamic/reversible bonds into a hydrogel provides it the particularity to self-repair after mechanical damage. Besides, shape-memory makes it possible to modify the hydrogel morphology through external mechanical forces and temporarily fix the new desired geometry by making use of the reversible bonds. Subsequently, the material can return to its original form applying a specific external stimulus. Furthermore, the growing interest in the biological applications of hydrogels has driven efforts in the adjustment of the mechanical properties by controlling polymer-polymer interactions and the degree and type of cross-linking. Another highly required characteristic is the insertion of functional groups in situ, with low toxicity and in the absence of side reactions. Numerous works aimed to yield macrogels with dynamic covalent bonds, the formation of gels with sol-gel behavior, self-curing properties, and/or shape memory and gelation under adequate physiological conditions. In this context, post-synthesis modifications of polymers to the development of functional dynamic covalent hydrogels are presented in this chapter. The polymer composition is usually selected to allow post-synthetic modification to achieve control over morphology, mechanical and swelling properties, and chemical functionality. Herein, we focused on the obtainment of Schiff base crosslinked dynamic covalent networks as a robust strategy to obtain smart materials by bio-orthogonal reactions. Different presented methodologies constitute useful tools for the development of functional materials that could be applied in various lines of research: from the production of devices for the controlled release of drugs to the development of soft actuators, surface modification, the anchoring of biomolecules in chromatographic supports, among others.