An introduction to injectable hydrogels.

Injectable hydrogels have emerged as intelligent and versatile materials that have been proven to possess huge potential for many biomedical applications including drug delivery, tissue engineering, and regenerative medicine. Hydrogels are a class of polymers with highly hydrated 3D networks that have microenvironmental properties such as oxygen/nutrient permeability that are similar to the native extracellular matrix. In addition to possessing the typical advantages of conventional hydrogels, injectable hydrogels offer extra unique features, enabling minimally invasive injectability and durability for irregularly shaped sites, and the possibility of processing these materials , , additive manufacturing techniques. As such, there has been a growing interest in using injectable hydrogels as scaffolds/carriers for therapeutic agents, including but not limited to drugs, cells, proteins, and bioactive molecules, targeted to treat chronic diseases including cancer, but also to facilitate the repair and regeneration of damaged organs/tissues. In this themed collection of and , we include outstanding contributions covering recent developments in this rapidly evolving field of injectable hydrogels including emerging chemistries, synthesis pathways, fabrication methods, cell-material interaction, , and performances, and subsequent targeted applications (drug delivery, tissue engineering and regenerative medicine) of injectable hydrogels.