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Microcapsules containing pigment and polymer were prepared by dispersing a viscous mixture of pigment, core monomers, initiators and oil-soluble shell monomer in an aqueous solution of surfactants, forming oil-in-water droplets. Subsequently, a water-soluble shell monomer was added to these droplets, encapsulating them via interfacial (IF) polycondensation. These microcapsules were then heated for free radical (FR) polymerization of the core monomers. Effects of primary variables, such as the shearing time during particle formation, surfactant concentration, organic phase concentration, and mode of water-soluble shell monomer addition, were studied. The results indicated that polyvinylalcohol (PVOH), used as the surfactant/stabilizer, reacted with the oil-soluble shell monomers. The depletion of PVOH, especially when PVOH concentration was low, resulted in rapid growth of particle size and, eventually, suspension failure. The kinetic data revealed a particle formation mechanism which consists of two processes. The first process is the formation of an equilibrium particle size by the equilibrium process of particle breakage due to the mechanical shearing force and coalescence due to collisions among particles and surface tension forces. The second process is the reaction between PVOH and oil-soluble shell monomer which leads to the depletion of PVOH and consequently causes more coalescence of particles and a significant increase in the equilibrium particle size. The net effect of these two processes shows an optimum shearing time where the smallest particle size can be attained, and this optimum time is a function of several primary variables. Methods to prevent the reaction and therefore the depletion of PVOH are proposed.

A viscous organic phase, containing up to 65 per cent solid pigment, was dispersed into water with an emulsifier by a rotor-stator homogenizer and the droplets formed were encapsulated by interfacial polymerization. Microcapsules with volume median diameters d50 ranging from 10 to 25 microns and geometric standard deviation (GSD), from 1.25 to 1.65, were obtained depending on emulsification conditions. Larger impellers gave smaller d50 and slightly narrowed GSD; d50 decreased and GSD increased as volume fraction of dispersed phase is decreased. Higher homogenizer speed and emulsifier concentration decreased d50 but slightly increased GSD. Increasing pigment content in dispersed phase decreased d50 but had little effect on GSD. These effects were assessed quantitatively by fitting an empirical model to the data.

The objective of this pre-formulation study was to systematically investigate the effects of two surfactants (Brij 58® and Tween 80®) and change in solution pH on in vitro permeation of naltrexone HCl (NTX-HCl) across tissue engineered human buccal mucosa. For the study, 10 mg/mL solutions of Tween 80® (0.1 and 1 % w/v) and Brij 58® (1 % w/v) were prepared in standard artificial saliva buffer solution (pH 6.8). For studying pH effects, solution pH was adjusted to either 7.5 or 8.2. As controls, three concentrations of NTX-HCl (2.5, 10 and 25 mg/mL) were prepared. Using NTX standard solution (10mg/ml; pH 6.8), the permeation was observed between in vitro human and ex vivo porcine mucosa. It was observed that Brij 58® increased the permeation rates of NTX significantly. The flux of 10mg/ml solution (pH 6.8) increased from 1.9 ± 0.6 (×102) to 13.9 ± 2.2 (×102) μg/cm2/h (approximately 6 fold) in presence of 1% Brij 58®. Increasing pH of NTX-HCl solution was found to increase the drug flux from 1.9 ± 0.6 (×102) (pH 6.8) to 3.0 ± 0.6 (×102) (pH 7.4) and 8.0 ± 3.5 (×102) (pH 8.2) μg/cm2/h respectively. Histological analyses exhibited no tissue damage due to exposure of buccal tissue to Brij 58®. The mean permeability coefficients (Kp) for 2.5, 10 and 25 mg/mL solutions of NTX-HCl (pH 6.8) were 5.0 (×10−2), 1.8 (×10−2) and 3.2 (×10−2) cm/h respectively, consistent with data from published literature sources. Increase of NTX flux observed with 1% Brij 58® solution may be due to the effects of ATP. Increase in flux and the shortening of lag time observed by increasing in solution pH confirmed earlier finding that distribution coefficient (log D) of NTX is significantly affected by small increments in pH value and therefore plays an important role in NTX permeation by allowing faster diffusion across tissue engineered human buccal membranes.