Furthermore, fast reaction rates and absence of by-product formation classify them as "click" reactions. In the past years, several examples include the use of cycloaddition reactions for the preparation of bio-and nanomaterials. Strain-promoted azide alkyne cycloaddition (SPAAC) is of particular interest as injectable collagen hydrogel crosslinked using SPAAC has been developed for in situ filling of corneal defects and demonstrated to support the regeneration of corneal epithelium in vivo. However, these hydrogels are injectable only during the early phase of gelation and not self-healing. This is attributed to the irreversible nature of SPAAC. Among other cycloadditions, the reaction between norbornene and tetrazine has been employed to prepare hydrogels from ECM-derived polymers. Similarly, these hydrogels were not injectable owing to the very fast and irreversible nature of the crosslinking reaction.