Figure 7. Reversible control of matrix mechanics.

a | Hydrogel stiffening can be achieved by increasing the persistence length of the crosslinker using complexation (for example, with DNA hybridization), and subsequent softening can be achieved by removing the hybridizing molecule with a competing strand. b | Conformational changes in hydrogel crosslinkers, such as protein folding and unfolding in response to binding ligands or environmental changes (for example, redox state), can also be used to alter the crosslinking density (and therefore the modulus) over many cycles. c | The number of elastically active crosslinks can be modulated using photoreversible host–guest chemistry. d | Reversible crosslinking between chains can also be modulated with the introduction or removal of ion coordination (for example, calcium ions in alginate networks).