Polymers for biodegradable medical devices. IX: Microencapsulation studies; effects of polymer composition and process parameters on poly-hydroxybutyrate-hydroxyvalerate microcapsule morphology.

Reservoir-type microcapsules were prepared using a double emulsion solvent evaporation technique from a series of nine different poly-beta-hydroxybutyrate (PHB)-based polymers in which both molecular weight and hydroxyvalerate content were varied. Particles prepared from a low molecular weight (MW 43,000) homopolymer had a shrivelled appearance, but were not porous. When the molecular weight of the fabricating homopolymer was increased to 540,000, however, these features disappeared and non-wrinkled particles with microporous surfaces were observed. Microcapsules prepared from a high molecular weight copolymer of PHB with 10.8 per cent hydroxyvalerate (HV) had a similar appearance, but particles prepared from a high molecular weight 20.1 per cent HV copolymer had much smoother and less porous surfaces. Lowering the molecular weight of the copolymer had the effect of producing particles that were generally distorted in shape and had highly irregular, macroporous, surface morphologies. Increasing the double emulsion temperature to 40 degrees C at 2 min after phase combination typically generated smoother and/or less porous particles and improved the batch yield. The numerous factors potentially responsible for the development of morphological characteristics of the products are discussed in the light of these observations.