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Hollow biodegradable poly(DL-lactide) (PLA) particles with porous shell walls were prepared by freeze drying small droplets of PLA solution formed by emulsification or spraying. The hollow freeze-dried particles were dispersed in water, and the resulting aqueous suspensions were exposed to plasticizing solvents, either dichloromethane or compressed carbon dioxide. The plasticizing solvent causes the pores in the shell wall to close, forming microcapsules surrounding an aqueous core. A water soluble drug, procaine hydrochloride, was successfully encapsulated in the microcapsule core. The encapsulation efficiency is affected by the hollow particle morphology, amount of solvent used, solvent exposure time, surfactant, and method of dispersing the freeze-dried particles in water. The encapsulation process is explained in terms of interfacial free energy of the hollow particles and mobility of the plasticized polymer. Controlled release of procaine hydrochloride from the microcapsules into phosphate buffer solution was observed. The microcapsules had a small burst release, with the remainder of encapsulated drug slowly released over 9 days. The novel hollow PLA particles produced by emulsification/freeze drying and spray/freeze drying can potentially be used as vehicles for controlled release.

Ionic dyes were successfully encapsulated in colloidal polymer particles by a CO2-based microencapsulation technique. The water-soluble dyes were made hydrophobic by forming ion pairs with several types of alkyl quaternary ammonium cations. The hydrophobic ion pairs were then encapsulated in preexisting size monodisperse polystyrene particles dispersed in water. High-pressure carbon dioxide swelled and plasticized polystyrene and thus facilitated mass transport of the dye into the particles. The results show that the particles maintain their size and morphology after exposure to CO2, and that ion-paired dyes have significantly higher loading in the polymer particles than the original dyes. The effects of cosolvent and surfactant are also discussed.

A new procedure is described for dyeing polymer beads using liquid carbon dioxide as a plasticizer to facilitate transport of dye into the particle phase. Aqueous latexes consisting of monodisperse polystyrene particles with surface-grafted poly(N-vinylpyrrolidone) were dyed with Sudan Red 7B using CO{sub 2} at 25 C and 310 bar. Adding CO{sub 2} to the headspace above the latex resulted in some dyeing of the polymer, but better results were obtained by forming an emulsion of CO{sub 2} in the aqueous latex phase. Emulsions were formed with both a fluorinated and a hydrocarbon-based surfactant. It was found that the carbon dioxide emulsion greatly enhances the transfer of dye into the polystyrene without altering the size or morphology of the particles.

An organic solvent-free method for encapsulating progesterone at high loadings within micron-sized inert latex polymer beads is reported. This approach makes use of a polymeric surfactant to emulsify carbon dioxide into an aqueous latex suspension. In this way, preformed approximately 4 microm polystyrene (PS) microparticles surface-grafted with poly(N-vinylpyrrolidone) (PVP) were plasticized and swollen followed by rapid partitioning of progesterone into the polymer matrix. The as-prepared polystyrene beads incorporated over 10% progesterone by weight while maintaining their initial size and morphological uniformity. Dissolution experiments were also carried out to obtain the release profile of progesterone entrapped within the PVP/PS particles.