Mathematical simulation of membrane-moderated controlled release

Permeation of urea through various polyurethane membranes

BACKGROUND

Controlled-release systems using polymer membranes are very important in agriculture for labour-saving and effective delivery of pesticides and other agents. Polymer-coated granules are one of the most useful formulations, and a study of the factors for polymer design is necessary to achieve various release patterns. A permeation study using plain membranes was carried out in order to clarify parameters, and the results were compared with the release from polymer-coated granules.

RESULTS

The permeation coefficient of urea through a plain polyurethane membrane decreased significantly as the urethane and alkyl side chain content increased. The glass transition temperature and crosslink density of the polyurethanes hardly influenced its permeability. The release rate from polyurethane-coated granules was also reduced by alkyl side chains. However, it was faster than that through a plain membrane because of capsule expansion by continuous water penetration and structural changes in the membrane.

CONCLUSION

The release rate of urea through a polyurethane plain membrane and from polyurethane-coated granules can be controlled by changing the chemical properties of the membrane. In addition, physical properties such as the glass transition temperature T(g) or crosslink density should be considered to assess the release profile from polyurethane-coated granules.

Comparative study of drug release from pellets coated with HPMC or Surelease

The release of metoclopramide hydrochloride (very water soluble cationic drug) and diclofenac sodium (sparingly soluble anionic drug) from pellets coated with hydroxypropylmethylcellulose (HPMC; water-soluble polymer) or ethylcellulose aqueous dispersion (Surelease; water-insoluble polymer) at different coating loads was investigated. The release rates of either drug decreased as the coating load of HPMC increased, but overall, the release was fast, and the majority of both drugs released in about 1 hr, even at the highest coating load. The drug release mechanism for either drug was not affected by the coating load of HPMC or by the type of drug used, and it was found to be mainly diffusion controlled. Diclofenac sodium released slightly more slowly than metoclopramide hydrochloride from HPMC-coated pellets. This was attributed to the lower water solubility of the former drug. The release rate of either drug decreased greatly as the coating load of Surelease increased. The release of both drugs was sustained over 12 hr as the coating load of Surelease increased, and only about 70% of either drug was released after this period at the highest coating load (20%). The mechanism of release of metoclopramide hydrochloride was independent of coating load, and it was predominantly diffusion controlled. However, the mechanism of diclofenac sodium release was dependent on the coating load of Surelease. At low coating loads, diffusion of drug was facilitated due to the presence of more pores at the surface of the coated pellets; therefore, the rate of dissolution of the drug particles was the rate-limiting step. However, at high coating loads, drug release was mainly diffusion controlled. Despite its lower water solubility, diclofenac sodium released slightly faster than metoclopramide hydrochloride from Surelease-coated pellets at equivalent coating loads.

A mechanism on the drug release into a perfect sink from a coated planar matrix with a super-saturation loading in the core

A comprehensive model is proposed to accurately describe drug release kinetics from a coated plane sheet when drug loading in the core is above its saturation level. The general solutions are acquired in a dimensionless form by the Laplace transform and the solution for the special case-a perfect sink condition, is derived from these general ones. On the basis of the model calculations, the effects of the diffusion ratios and thickness ratios of the coatings to the core, and drug loading entrapped in the core during the release processes have been discussed over wide range of variables. To validate the model equations proposed, the coated systems, 5-fluoracilum/ethylene-vinyl alcohol copolymer (5-Fu/EVAL) core matrix coated with various polymeric materials such as EVAL, cellulose acetate (CA), poly(2-hydroxyethyl methacrylate) (PHEMA) and poly( methyl methacrylate) (PMMA), having different diffusivities, are designed, experimentally investigated and graphically and quantitatively compared with the theoretical values. The results show a good correlation between the theory and experiment.