Diffusion-controlled drug delivery

Figure 1. Mechanisms of controlled release drug delivery. Diffusion-controlled (A), dissolution-controlled (B), osmoticSly controlled (C), ion-exchange controlled (D), and degradation-controlled (polymeric prodnig, E) systems.

Because PCL hydrolyzes slowly compared to PLA and PLGA, it is most suitable for long-term drug delivery. Capronor , a 1-year contraceptive represents such a system [106]. The release of drugs is diffusion controlled rather than erosion controlled [106]. Nitrofurantoin, an antibacterial agent used in the treatment of urinary tract infections, has been incorporated into PCL microspheres [ 105]. The drug release rate was proportional to the square-root of time, i.e., fol-... 

Levamisole, cimetidine, and chloramphenicol were used as model drugs. A comparative study on the dependence on the pH value of the release of cimetidine, levamisole, and chloramphenicol from the gel was reported [144]. The results reveal that the drug delivery is controlled by diffusion and relaxation processes, while the diffusion coefficient and relaxation time are highly dependent on the pH of the medium. Moreover, the drug solubility in water obviously has an influence on the release [ 144]. 

Shah SS, Cha Y, Pitt CG. Poly(glycolic acid-co-dl lactic acid) diffusion or degradation controlled drug delivery J Controlled Release 1992 18 261-270. 

Shah, S. S., Cha, Y., and Pitt, C. G., 1992, Poly(glycolic acid-co-DL-lactic acid) Diffusion or degradation controlled drug delivery , J. Controlled Release 18 261-270. 

The high permeability of PCL and its copolymers coupled with a controllable induction period prior to polymer weight loss (vide infra) lends itself to the development of delivery devices that are based on diffusion-controlled delivery of the drug during the induction period prior to weight loss. The subsequent biodegradation of the polymer serves the purpose of eliminating the need to recover the spent device. 

Ethylene vinyl acetate has also found major applications in drug delivery. These copolymers used in drug release normally contain 30-50 wt% of vinyl acetate. They have been commercialized by the Alza Corporation for the delivery of pilocarpine over a one-week period (Ocusert) and the delivery of progesterone for over one year in the form of an intrauterine device (Progestasert). Ethylene vinyl acetate has also been evaluated for the release of macromolecules such as proteins. The release of proteins form these polymers is by a porous diffusion and the pore structure can be used to control the rate of release (3). Similar nonbiodegradable polymers such as the polyurethanes, polyethylenes, polytetrafluoroethylene and poly(methyl methacrylate) have also been used to deliver a variety of different pharmaceutical agents usually as implants or removal devices. 

In the pharmaceutical sciences, diffusion processes are central to drug delivery. They are particularly important in the release of drugs from controlled-... 

Diffusion-controlled drug delivery, 9 77 Diffusion Deborah number, 23 101 Diffusion flames... 

Novel drug delivery systems can also have a profound effect on pharmacokinetics, even if they do not involve the use of prodrags in the classical sense. Novel polymers have permitted the development of membranes with controlled diffusion rates. For example. 

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