Ostwald ripening for designing time‐dependent crystal hydrogels.

Ostwald ripening (OR), a classic solution theory describing molecular transfer from metastable crystal to stable one, is applied to design time‐dependent crystal hydrogels that can automatically change their mechanical properties. Using a system made from crosslinked polyacrylamide (PAM) and sodium acetate (NaAc), we demonstrate that metastable fibrous crystal networks of NaAc preferably form in PAM hydrogels via a polymer‐involving mismatch nucleation. These fibrous crystals would undergo OR and evolve into isolated bulk crystals, leading to a significant reduction in material rigidity (179 folds) and interfacial adhesion (20 folds). This transformation can be applied to program time‐dependent self‐recovery in shape and self‐delamination. Since OR is a ubiquitous, robust feature of various crystals, the approach reported here represents a new direction for designing advanced transient soft materials. [ABSTRACT FROM AUTHOR]