Stimuli-responsive membranes may act as "on-off switches" or "permeability valves", producing patterns of pulsatile release, where the period and rate of mass transfer can be controlled by external or environmental triggers (e.g. pH, temperature, electric field), In this work, composite-heterogeneous polyelectrolyte gel (composite-HPG) membranes consisting of polymethacrylic acid (PMAA) gel particles dispersed within a poiydimethylsiloxane (PDMS) network were developed and evaluated as pH-responsive membranes. The mechanism of permeability control for caffeine and vitamin B-12 through composite-MPG membranes was determined to be a synergistic function of membrane hydration and the percolating volume fraction of PMAA gel. Larger changes in permeation as a function of pH were achieved when both hydration and percolation effects occurred together than when either of these effects occurred on their own. Vitamin B-12 permeation was observed when the hydrated gel volume fraction was above approximately 0.38, but not below. Furthermore, the percolating fraction of composite-MPG membranes containing 28% (dry basis) PMAA gel particles was manipulated via pH to fall above (pH 7) or below (pH 3) this transition in permeability, resulting in membranes that delivered solutes of high molecular weight (vitamin B12) With large on/off delivery ratios (160).