Dual cross-linked hydrogels as promising materials for drug delivery

Hydrogels have long found their place as versatile materials for drug delivery due to their biocompatibility, tunable properties and tissue-like mechanical behavior. Despite these advantages they suffer from a multitude of drawbacks, ranging from lacking injectability and low mechanical strength to short drug release times. These factors limit their potential clinical applications. Dual cross-link hydrogels are a versatile tool to address these shortcomings. By combining covalent with dynamic-covalent and physical cross-linking they create smarter, stronger and more versatile hydrogels, broadening their applicability. This review aims to bridge the gap between traditional hydrogels and emerging dual cross-linked hydrogels by providing an overview of the chemistries involved. We discuss the most popular cross-linking strategies and how these can be used to address specific challenges in hydrogel design. Furthermore we highlight how recent research has utilized these promising materials to enable the application of hydrogels in ever more demanding environments.