Membrane rate control

Coating polymer membrane (rate controlling) porous/non-porous... 

Alza Corp. has also developed an iatrauteriae device, Progestasert, designed to release progesterone [57-83-0] by diffusion through a rate-controlling membrane for up to one year. The dmg reservoir is built into a T-shaped device that is inserted intravaginaHy (15). 

Membra.ne Diffusiona.1 Systems. Membrane diffusional systems are not as simple to formulate as matrix systems, but they offer much more precisely controlled and uniform dmg release. In membrane-controlled dmg deUvery, the dmg reservoir is intimately surrounded by a polymeric membrane that controls the dmg release rate. Dmg release is governed by the thermodynamic energy derived from the concentration gradient between the saturated dmg solution in the system s reservoir and the lower concentration in the receptor. The dmg moves toward the lower concentration at a nearly constant rate determined by the concentration gradient and diffusivity in the membrane (33). 

Fouling is the term used to describe the loss of throughput of a membrane device as it becomes chemically or physically changed by the process fluid (often by a minor component or a contaminant). A manifestation of fouling in cross-flow UF is that the membrane becomes unresponsive to the hydrodynamic mass transfer which is rate-controlling for most UF. Fouling is different from concentration polarization. Both reduce output, and their resistances are additive. Raising the flow rate in a cross-flow UF will increase flux, as in Eq. 

FIGURE 14 In vitro rate of release of testosterone from a PCL capsule (reservoir device), illustrating rate control by drug dissolution when the polymer membrane thickness is small. (From Ref. 68.)... 

It has been demonstrated that a variety of different polyphosphazenes can be developed as biomaterials, membranes or hydrogels, bioactive polymers, and bioerodible polymers. As with most new areas of polymer chemistry and biomaterials science, molecular design forms the basis of most new advances, but the rate-controlling step is the testing and evaluation of the materials in both in vitro and in vivo environments. This is particularly true for polyphosphazenes where the availability of research quantities only has limited the... 

Because neither nor are strong functions of the physical-chemical properties of the analytes (see below), the issue of which phase controls the uptake rate is primarily governed by the membrane-water partition coefficient, which varies between compounds by many orders of magnitude (Reynolds et al., 1990 Lefkovitz et al., 1996 Booij et al., 2003a). With increasing log Ko , there always will be a critical log Kov, value where the uptake rates will be controlled by the WBL instead of by the membrane. Next to /fmw (which is a compound specific property) it is important to note that rate control is also dependent on the magnitude of kw, which is determined by the hydrodynamical conditions prevailing at the membrane-water... 

Buccal dosage forms can be of the reservoir or the matrix type. Formulations of the reservoir type are surrounded by a polymeric membrane, which controls the release rate. Reservoir systems present a constant release profile provided (1) that the polymeric membrane is rate limiting, and (2) that an excess amoimt of drug is present in the reservoir. Condition (1) may be achieved with a thicker membrane (i.e., rate controlling) and lower diffusivity in which case the rate of drug release is directly proportional to the polymer solubility and membrane diffusivity, and inversely proportional to membrane thickness. Condition (2) may be achieved, if the intrinsic thermodynamic activity of the drug is very low and the device has a thick hydrodynamic diffusion layer. In this case the release rate of the drug is directly proportional to solution solubility and solution diffusivity, and inversely proportional to the thickness of the hydrodynamic diffusion layer. 

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