Apparent diffusion coefficient of sodium phenobarbitone in ethylcellulose microcapsules: effects of capsule size

Analysis of film coating thickness and surface area of pharmaceutical pellets using fluorescence microscopy and image analysis

A method is presented which enables geometrical characterisation of pharmaceutical pellets and their film coating. It provides a high level of details on the single pellet level. Image analysis was used to determine the coating thickness (h) applied on the pellets and the surface area (A) of the pellet cores. Different definitions of A and h are evaluated. Hierarchical analysis of variance was used to resolve different sources contributing to the total variance. The variance within pellets and the variance between pellets were found as significant sources of variation. Special emphasis was put on evaluation of A/h due to its influence on the release rate of an active drug substance from the pellet core. The pellet images were thus used to predict variations in the release rate using a mathematical model as a link between the image data and the release rate. General aspects of image analysis are discussed. The method would be useful in calibration of near infrared spectra to h in process analytical chemistry.

Preparation of ethylcellulose microcapsules containing theophylline by using emulsion non-solvent addition method

A new technique in which ethylcellulose microcapsules containing theophylline (a water-soluble drug), prepared using the O/W emulsion non-solvent addition method, was developed. Toluene-cyclohexane was chosen as the solvent-nonsolvent system. The effects of four process variables, polymer concentration, species and concentration of emulsifier, and core to wall ratio, on the micromeritic properties and release behaviour of microcapsules were investigated. The results indicated that theophylline can be microencapsulated with a high yield (low drug loss) by using the O/W emulsion non-solvent addition method with the toluene-cyclohexane system. The particle size and drug content of the microcapsules were influenced by these process variables. The morphology of microcapsules was also affected by the core to wall ratio. The release pattern of the microcapsules was found to have similar properties to the release of a drug from a spherical homogeneous matrix. The effective diffusion coefficient increased with increasing core to wall ratio.

Studies on the properties of ethylcellulose microcapsules prepared by emulsion non-solvent addition method in the presence of non-solvent in polymer solution

Ethylcellulose microcapsules containing theophylline were prepared by the O/W emulsion non-solvent addition method. Toluene-cyclohexane was chosen as a solvent-non-solvent pair. The effect of the non-solvent added to the polymer solution on the properties of microcapsules was investigated. The results indicated that the size distribution and drug content of microcapsules were slightly affected by the amount of non-solvent in polymer solution. However, the internal conformation of microcapsules was directly related to the amount of non-solvent added to the polymer solution. The greater the non-solvent to solvent ratio in polymer solution the larger the percentage of microcapsules having a hollow conformation. Dissolution studies showed that the release rate of theophylline from microcapsules increased with increasing amount of non-solvent added to the polymer solution, but the release pattern of microcapsules was not obviously changed.

Evaluation of the properties of ethylcellulose-cellulose triacetate microcapsules containing theophylline prepared by different microencapsulation techniques

Using cellulose triacetate as an added complementary coating material in preparing sustained-release ethylcellulose-cellulose triacetate microcapsules of theophylline, three microencapsulation techniques were investigated. Ethylcellulose-cellulose triacetate composite microcapsules, ethylcellulose-cellulose triacetate dual-walled microcapsules and ethylcellulose microcapsules containing cellulose triacetate matrices were prepared using the non-solvent addition phase separation method. The effects of cellulose triacetate on the release of theophylline from the different ethylcellulose-cellulose triacetate microcapsules were obtained from dissolution studies. The results showed that the release rates of ethylcellulose-cellulose triacetate microcapsules were slower than those obtained from the ethylcellulose microcapsules prepared with similar core to wall ratios. The ethylcellulose microcapsules containing cellulose triacetate matrices had longer release half-times and smaller surface areas than the other capsule preparation. The release patterns of theophylline from the different ethylcellulose-cellulose triacetate microcapsules fitted first-order kinetics. Scanning electron micrographs showed that the surfaces of various ethylcellulose-cellulose triacetate microcapsules were different from those of theophylline, cellulose triacetate matrices of cellulose triacetate microcapsules, and that the surface morphology of ethylcellulose-cellulose triacetate microcapsules was affected by the preparative method.

Effect of the solvent-non-solvent pairs on the surface morphology and release behaviour of ethylcellulose microcapsules prepared by non-solvent-addition phase separation method

Four solvent-non-solvent pairs (ethyl-acetate-cyclohexane, dichloromethane-cyclohexane, acetone-cyclohexane and dichloromethane-n-hexane) with different solubility parameter differences were chosen to prepare ethylcellulose microcapsules containing theophylline by using non-solvent-addition phase separation method. The results showed that the surface morphology and release behaviour of microcapsules were greatly affected by different solvent-non-solvent pairs. The surface of the microcapsules prepared from the system of high solubility parameter difference was more smooth than those from the systems of low solubility parameter difference. The release rate of the drug from microcapsules decreased with increasing solubility parameter difference of the preparative system. The determination of the wall thickness and porosity of the microcapsules could reasonably explain the release characteristics. The porosity of the microcapsules decreased with the increase of solubility parameter difference of the preparative system, but the wall thickness of the microcapsules showed a corresponding increase. The release of the drug from various ethylcellulose microcapsules fitted first-order kinetics with biphasic release profiles.

Influence of total surface area of core material on yield of deposited coacervate

Influence of total surface area of core material on yield of deposited coacervate was evaluated. A simple coacervation method was employed solvent-evaporation method were used as a model core substance. A mixed polymer system--aqueous solution of gelatin and potassium salt of cellulose acetate phthalate--was used as a wall material. A sodium sulphate solution acted as a desolvating agent. Results obtained clearly show an opposite effect of the size of core material and derived total surface area on the yield of attached coacervate. While keeping the amount of core material constant, the amount of deposited coacervate increased both with the decreased size of core particles (increased total surface area), and increased concentration of desolvating agent. The proportion of wall material gradually increases with the increased concentration of sodium sulphate solution in the range from 1 to 9 per cent of the weight of microcapsules. Analysis of variance was used for the evaluation of obtained data.

Preparation and release kinetics of hydrochlorothiazide from butyl half-ester of PVM/MA microcapsules

This study describes the principle of a simple rapid method for encapsulating hydrochlorothiazide in butyl half-ester of polyvinyl methyl ether-maleic anhydride copolymer to produce a controlled release dosage form. Unplasticized microcapsules and microcapsules plasticized with Tween 80 and castor oil were prepared. Particle size distribution, flow properties and drug content of microcapsules suggested the suitability of the method for encapsulating a wide variety of materials. The in-vitro release rate was studied as a function of core: coat ratio, type and concentration of plasticizer. Various release mechanisms were considered but no single mechanism can explain all the data completely.

Preparation and release kinetics of microencapsulated cisplatin with ethylcellulose

Several kinds of microcapsules containing cisplatin were prepared by the ethylcellulose coacervation process in an attempt to administer cisplatin with chemoembolization. Microcapsules made either with mechanical stirring or with sonication showed similar properties; the chemical structure of cisplatin was not affected by the micro-encapsulation process. The release kinetics of cisplatin from ethylcellulose-walled microcapsules followed different patterns, according to the wall thickness. In each case, the release kinetics did not depend on the stirring rate of the surrounding medium. Only microcapsules with a cisplatin release ratio from 80 to 100% within 24 h were selected for later clinical use.

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.

Preparation and biopharmaceutical evaluation of microcapsules of amoxicillin

Two methods of microencapsulation of amoxicillin, an orally administered antibiotic, were studied. One is based on dispersion of gelatin-amoxicillin mixture in liquid paraffin followed by drying and hardening with formalin-isopropanol treatment; the other is based on dispersion of ethylcellulose-amoxicillin mixture in purified water containing sodium lauryl benzene sulphonate. The microcapsules were recovered as discrete, free-flowing fine granules with a particle diameter of about 250-1000 microns. Dissolution of amoxicillin from ethylcellulose microcapsules was suppressed considerably with a zero-order dissolution pattern in solutions of various pH. Gastric-emptying-controlled rabbits were used for the in vivo evaluation of gelatin and ethylcellulose microcapsules. The ethylcellulose microcapsule containing 25 per cent amoxicillin showed a significantly sustained release pattern of amoxicillin. To establish a suitable design and for the evaluation of the sustained release microcapsules, a nomogram was made using pharmacokinetic parameters obtained after administration of a conventional formulation. It is advantageous for the preparation of sustained release microcapsules to chose pharmaceuticals having over about 2 as the ratio of the elimination rate constant, k10, to the release/absorption rate constant, kr, in the rabbit.

Influence of drug partition coefficient and pH value of sink solution on permeation from porous thick-walled ethyl cellulose microcapsules

The permeation of barbitone sodium, benzoic acid, and salicylic acid from microcapsules into aqueous medium has been examined at different pH values. The apparent diffusion coefficients of drugs were linearly proportional to the ethyl cellulose/water partition coefficient of drugs, and the straight line parameters were dependent upon volume fractions of water-filled pores (i.e. capsule size), testifying to a previously proposed mechanism of drug permeation. The rate of drug permeation was also a function of the pH-value of the surrounding sink solution; the period of zero order release was longer at low pH because of the change of drug partition or solubility or both.

Volume of water-filled pores in the ethyl cellulose membrane and the permeability of microcapsules

The volume fraction of the water-filled pores in the microcapsule membranes was calculated and the values from 0.55 to 2.5% were obtained. Differential scanning calorimetry of aqueous suspensions of microcapsules showed no structured water present in the ethyl cellulose membranes. The temperature effect on the apparent diffusion coefficient of a drug was investigated and the apparent activation energy of diffusion was calculated.