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Biomedical subjects

M Donbrow

Publications and source records attributed to M Donbrow.

43 records · Page 3Linked to original sources
PubMedDate not supplied

Permeability of ethylcellulose microcapsules towards phenobarbital.

Inward permeation from the surrounding medium of phenobarbital through the wall of water-loaded ethylcellulose microcapsules was investigated as a function of capsule size under the conditions of constant total capsule volume and constant total capsule surface area. The experimental data obtained were analysed in terms of capsule wall density and drug partition coefficient. The drug permeability coefficients calculated according to an equation derived from Fick's first law of diffusion were found to increase with decreasing capsule size in both constant total capsule volume and constant total capsule surface area experiments. The wall density and the drug partition coefficient also exhibited the same trend. Based on these findings, it was concluded that the drug permeation through ethylcellulose microcapsule membrane occurs predominantly by a solution-diffusion mechanism.

Capsules↗
PubMedDate not supplied

Polyacrylate resin (Eudragit retard) microcapsules as a controlled release drug delivery system-improved non-solvent addition phase separation process.

Eudragit retard microcapsules were prepared using an improved non-solvent addition phase separation process with tetrahydrofuran as the solvent. The evolution of microcapsule wall formation was studied by direct methodology. Eudragit coacervation was effected by progressive uptake of tetrahydrofuran by the non-solvent cyclohexane in the presence of a protective colloid, polyisobutylene (PIB). The core materials had a higher affinity for the acrylic that the PIB phase, thus ensuring encapsulation. Microcapsule batch reproducibility depended mainly on the variation in particle size distribution of the recrystallized core material. All batches gave apparent first-order release profiles, confirmed by regression procedures. The release rate was decreased by raising the wall/core ratio, holding constant concentration of either the wall polymer or the core material. Increase in the non-solvent addition rate elevated the release rate, probably due to structural changes in the microcapsule wall. The velocity fell, however, with decrease in particle size of the core material, contrary to expectations. PIB concentration increase elevated the release rate by enhancing wall porosity, shown by scanning electron microscopy.

Acrylic Resins↗
PubMedDate not supplied

Phase separation modulation and aggregation prevention: mechanism of the non-solvent addition method in the presence and absence of polyisobutylene as an additive.

The mechanism by which microcapsule aggregation is diminished in the presence of low concentrations of adjunct non-walling polymers such as polyisobutylene (PIB) in organic solvents is uncertain. It is here investigated by phase separation studies employing the non-solvent addition method (cyclohexane) in parallel with microencapsulation experiments, using Eudragit wall polymers and tetrahydrofurane (THF) as solvent with and without PIB. Polymer incompatibility leads to formation of two dilute liquid phases and emulsification from the outset. Progressive solvent transfer to the PIB phase modulates the rate of wall polymer desolvation and maintains the concentrated wall polymer phase in a fluid state during the critical microencapsulation stages of wall polymer entrapment--and spreading. A three-phase system is formed when gelling ensues (L1 + L2 + gel). In the absence of PIB, phase separation leads immediately to deposition of the wall polymer in gel form (L1 + gel) leading to adhesion. Phase diagrams are presented.

Capsules↗
PubMedDate not supplied

Gradation of microcapsule wall porosity by deposition of polymer mixtures (Eudragit RL and Eudragit RS). Phase separation of polymer mixtures and effects of external media and conditions on release.

With the aim of increasing flexibility in controlling release from microcapsules, mixtures of wall polymers varying in porosity were investigated by phase separation. Eudragit RL and RS (polymethylmethacrylate linear backbone polymers) mixtures differing in polar substituent content and porosity were used as the wall material and were deposited using a non-solvent addition method. Release rates increased with polar group content of the mixtures, using theophylline, potassium dichromate or sodium chloride as model core materials. Theophylline release rate had the same relationship to polar group content as found earlier for urea permeation of cast mixed-polymer films. Release was generally accelerated in these systems when the external medium contained sodium lauryl sulphate as a wetting agent but not consistently, decreasing unexpectedly for RL-theophylline microcapsules. Localized dissolution of core substance was visible microscopically during release from single microcapsules. The release rate was sensitive to agitation intensity only at low wall to core ratios. Temperature change revealed only a single release mechanism for sodium chloride by Arrhenius equation treatment. Buffer ions penetrated coatings readily, changing theophylline release rates and providing clear evidence of diffusion via a pore-capillary mechanism.

Acrylic Resins↗