Crosslink density can be controlled by degradative or constructive molecular events, which depend on the desired application and can be controlled by triggers such as (UV) light, pH, enzymatic activity or reactive oxygen species (ROS). Degradative processes such as triggered crosslinker cleavage , are typically employed to release (bio) molecules from the hydrogel matrix. In contrast, in constructive molecular events the crosslink density is increased, through sequential or stepwise crosslink strategies such as secondary radical-mediated crosslinking or by sequential photo-induced crosslinking .