Autonomous chemomechanical oscillations in a hydrogel/enzyme system driven by glucose

An autonomous chemomechanical oscillator, driven by membrane-controlled enzymatic conversion of a physiological substance, glucose, to hydrogen ion, has been constructed. The oscillator consists of a pH-sensitive, hydrophobic polyelectrolyte hydrogel membrane based on poly(N-isopropylacrylamide-co-methacrylic acid), and the enzyme glucose oxidase. The system is configured as a transport cell, with the membrane separating two compartments. A solution containing glucose at constant concentration flows through one compartment (Cell I). Glucose permeates the membrane into the other compartment (Cell II), containing glucose oxidase, which converts glucose to hydrogen ion. Hydrogen ions in turn regulate membrane charge, swelling, and glucose permeability, establishing a negative feedback loop. The membrane'S response to hydrogen ion exhibits hysteresis, and under proper conditions a feedback instability is created, leading to oscillations in membrane swelling and permeability, and in pH measured in Cell II. The range over which pH oscillates is shifted in the alkaline direction by reducing methacrylic acid content. Period of oscillations increases with time, and ultimately oscillations cease. Both of these phenomena appear to be due to the buildup of gluconate ion in Cell II, which buffers and slows down pH variations.