Knoevenagel Condensation Reaction-Empowered Hydrogels with pH-Tunable Dynamic Properties

Dynamic hydrogels have found extensive applications in various biomedical fields owing to their remarkable dynamic properties, such as fast stress relaxation and self-healing capabilities. In this work, we report the development of a hydrogel empowered by the reversible Knoevenagel condensation (KC) reaction featuring pH-tunable dynamic properties. By adjusting the pH, the exchange rates and quantities of the dynamic C═C bonds formed via the KC reaction can be regulated through modulation of the association rate constant (<i>k</i>₁), dissociation rate constant (<i>k</i>_-1), and equilibrium constant (<i>K</i>_eq) of the reversible KC reaction. Specifically, pH reduction decelerated both <i>k</i>₁ and <i>k</i>_-1 of the KC reaction while elevating the <i>K</i>_eq. As a result, when the pH decreased from 10 to 1, the KC reaction-formed dynamic hydrogels exhibited a progressive increase in relaxation time (τ_1/2) from 100 to over 1,000 s, accompanied by enhanced structural stability and improved mechanical performance. This study provides a new strategy to design the dynamic hydrogels with tunable dynamic properties through the pH-responsive KC reaction.