Towards synergistic oscillations in enzymatically active hydrogel spheres.

Stimuli-responsive polymers are capable of reacting to an external trigger. We report self-regulated, enzymatically active, and pH-responsive hydrogels that show dynamic behavior without an external trigger. This is enabled by a feedback loop between the enzymatic conversion of glucose into gluconic acid and the pH-induced volume phase transition that leads to a modulation in glucose permeability. The synthesized hydrogel spheres combine all required properties for sustained oscillation including enzymatic activity, switchable reactivity, hysteresis in volume phase transition and feedback between the reaction and permeation. A simple model of the system identified possible operating points where sustained oscillations are possible. Experiments at these operating points revealed that the system is able to perform a self-regulated oscillation cycle under a constant nutrient supply. A sensitivity analysis showed that the system is especially sensitive around the point of oscillation, so that precise control of the process parameters is essential to achieve sustained oscillations.