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.

pH‐dependent changes in permeability to glucose of a poly(N‐isopropylacrylamide‐co‐methacrylic acid) hydrogel membrane were measured with the membrane clamped between two cells. The donor cell was maintained at pH 7.0 and the receptor cell was subjected to a set of ramps in pH. Starting with the membrane in a state of high permeability to glucose, a ramp of decreasing pH led ultimately to shutoff of glucose flux. With passage of time at a fixed, low pH, however, permeability was restored to an intermediate value. Upon ramping pH back to its initial value, the initial permeability was restored. Based on light microscopy, the nonstationary permeability behavior is attributed to lateral stress‐induced morphological changes that occur in a collapsed “skin” formed on the side of the membrane facing the acidic donor solution.

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The trend in granule size distribution during the experiment closely followed the predicted model with an initial increase in the weight fraction of the larger granules. This increase was possibly due to extensive breakage of weaker granules and less extensive breakage, as if by attrition, of stronger granules, accompanied by the attachment of dry powder to the cracked surfaces. Eventually, larger granules experience increased impact energy and break. When excess binder is added and, higher volumes of powder reattach to the crack surface, more large granules form leading to granule overgrowth. This model highlights the importance of the probability of impact per unit time interval (ie, the rate of impact), the strength of the granules and the volume of powder that could attach to the cracked surface in high shear granulation processes where significant granule breakage is encountered.