The main objective of this research was to better understand, predict and control of the periodic precipitation process and to apply such programmed periodic precipitation to the design of a pulsatile delivery system. In the first part of this study, a generalized model taking into account both nucleation, particle growth, and ripening process was refined and solved under various new concentration boundary conditions not previously investigated. The results clearly delineate the key differences between boundary conditions of infinite versus finite supply of inner electrolyte. When the inner electrolyte boundary concentration was allowed to increase exponentially with time, equidistant periodic precipitation was predicted and subsequently confirmed experimentally. In addition, the effects of product solubility and reaction rate constant were also shown to be important in determining the band number and band spacing. In the second part of this study, the effects of gel crosslinking and gel charge density on the periodic precipitation were investigated. The results indicate that by increasing either the gel crosslinking or decreasing the gel charge density will reduce the diffusion rate of the reactants resulting in closely spaced bands. In addition, a new and improved rotating disk method for characterizing polyelectrolyte gels with ion-penetrable soft surfaces has been established by taking into account the effect of surface conductivity which is usually ignored for ion-impenetrable hard surfaces. In the third part of this work, periodic precipitation formed in multi-component systems has been shown to be governed by a heterogeneous nucleation mechanism. Using this approach, periodic precipitation of an insulin mimetic compound VO2+ in gelatin gel, which cannot form alone in a single reaction system, was induced by the periodic precipitation of Mg(OH)2 in a multi-component system. Pulsatile release of VO2+ from the resulting multi-layered structure of VO(OH)2 via a surface erosion mechanism was subsequently demonstrated.

The main objective of this research was to better understand, predict and control of the periodic precipitation process and to apply such programmed periodic precipitation to the design of a pulsatile delivery system. In the first part of this study, a generalized model taking into account both nucleation, particle growth, and ripening process was refined and solved under various new concentration boundary conditions not previously investigated. The results clearly delineate the key differences between boundary conditions of infinite versus finite supply of inner electrolyte. When the inner electrolyte boundary concentration was allowed to increase exponentially with time, equidistant periodic precipitation was predicted and subsequently confirmed experimentally. In addition, the effects of product solubility and reaction rate constant were also shown to be important in determining the band number and band spacing. In the second part of this study, the effects of gel crosslinking and gel charge density on the periodic precipitation were investigated. The results indicate that by increasing either the gel crosslinking or decreasing the gel charge density will reduce the diffusion rate of the reactants resulting in closely spaced bands. In addition, a new and improved rotating disk method for characterizing polyelectrolyte gels with ion-penetrable soft surfaces has been established by taking into account the effect of surface conductivity which is usually ignored for ion-impenetrable hard surfaces. In the third part of this work, periodic precipitation formed in multi-component systems has been shown to be governed by a heterogeneous nucleation mechanism. Using this approach, periodic precipitation of an insulin mimetic compound VO2+ in gelatin gel, which cannot form alone in a single reaction system, was induced by the periodic precipitation of Mg(OH)2 in a multi-component system. Pulsatile release of VO2+ from the resulting multi-layered structure of VO(OH)2 via a surface erosion mechanism was subsequently demonstrated.

The aim of this study is to elucidate factors that effect growth of Sarcina lutea and Bacillus subtilis, exposed to the growth inhibitor SDS (Sodiumdodecylsulfat). Agar diffusion experiments revealed repeated, concentric zones of inhibition and stimulation upon exposure to Sodiumdodecylsulphate or to Amoxicillin. Temperature, nutrient concentration and inhibitor concentration were controlled. Formation of successively repeated zones of inhibition, stimulation, inhibition and stimulation is discussed: •The extension of the primary inhibition zone is due to the concentration of applied Sodium dodecyl sulphate.•Immediately outside the primary inhibition zone the bacteria have access to diffusing nutrients that have not been consumed in the primary inhabitation zone.•In zones of dense bacterial growth the bacteria may produce inhibiting substances, affecting growth of bacteria in adjacent zones.•In zones of dense bacterial growth the nutrients will soon become depleted, thus affecting bacteria in adjacent zones.