POROSITY-CONTROLLED ETHYLCELLULOSE FILM COATING .2. SPONTANEOUS POROUS FILM FORMATION IN THE SPRAYING PROCESS AND ITS SOLUTE PERMEABILITY

A new, simple porous film formation technique for the coating of capsule-type controlled release dosage forms was investigated. When an ethylcellulose-ethanol-water ternary mixture was sprayed, a porous film was spontaneously formed during the spraying process on the basis of the phase separation principle. Various factors influencing the porosity of the resultant sprayed film were examined. The film porosity increased considerably with decreasing ethanolic concentration, whereas the polymer concentration of the spraying solution had only a slight effect and the molecular weight of the polymer even less influence. Temperature and relative humidity also apparently affected the porosity of the resultant film. To assess quantitatively the effect of film porosity on solute permeability, permeation studies were performed using five model drugs with different lipophilicities; potassium chloride, theophylline, salicylic acid, sodium salicylate and diltiazem hydrochloride. The permeation rate increased considerably with increasing film porosity. An apparent relationship between film porosity and permeability could be expressed by a power function. These results suggested that solutes predominantly permeate through micro-pores of the films, and hence that the permeation rate depends on the film structure rather than on the physicochemical properties of the solute.