Air-filled polymeric microcapsules from emulsions containing different organic phases

Air-filled polymeric microcapsules for use as a contrast agent in ultrasono graphy have been prepared by the freeze-drying of different oil-in-water em ulsions. The water phases consisted of a block copolymer in water. The orga nic phases consisted of a biodegradable polyester dissolved in (-)-camphene , cyclooctane, cyclohexane or tricyclene, which were relatively poor solven ts for the polyester. A polymeric wall was, therefore, precipitated at the droplet surface early in the process, i.e. during freezing. Removing the so lvent during freeze-drying, resulted in air-filled microcapsules. The micro capsules were suspended in saline after freeze-drying. All the suspensions contained echogenic microcapsules with a volume mean diameter of similar to 5-7 mum. Microscopic investigations showed that the microcapsules were sphe rical and hollow. Tricyclene and, to some degree, (-)- camphene were found unsuitable for industrial production due to melting points above 30 degrees C. Cyclooctane and cyclohexane were investigated as replacements for the in itially chosen (-)-camphene, since they are liquids over a wider temperatur e range. These solvents gave improved yields, measured both as particle vol ume concentration per amount of polymer in suspension and acoustic attenuat ion at 3.5 MHz per amount of polymer in suspension, although the freeze-dry ing cycle was not optimized for these systems.