EFFECT OF CAPSULE DIAMETER ON THE PERMEABILITY TO HORSERADISH-PEROXIDASE OF INDIVIDUAL HEMA-MMA MICROCAPSULES

Poly (2-hydroxyethyl methacrylate-methyl methacrylate), (HEMA-MMA) mic rocapsules to be used for the transplantation of live mammalian cells were prepared by an interfacial precipitation process. ii submerged je t coextrusion technique was used to make various sizes of capsules as small as 450 mu m in outside diameter. A novel method based on an enzy me-chromogenic substrate assay was used to measure the capsule permeab ility at the individual capsule level. Capsule size, shearing frequenc y (number of capsules sheared per unit time), capsule permeability and the capsule-to-capsule variation in permeability were dependent, dire ctly or indirectly, upon the shearing force (i.e. hexadecane flow-rate ) applied to the nozzle during preparation. The average permeability c oefficients of those capsules to horseradish peroxidase, (HRP, MW 40kD a, a model protein), were in the range of 2x10(-10) to 9x10(-10) cm(2) /sec; the permeability varied from 1x10(-10) to 3x10(-10) cm(2)/s for 500 mu m capsules (the best) and 0.1x10(-10) to 6x10(-10) cm(2)/s for 660 mu m capsules (the poorest). Scanning electron microscopy illustra ted that finger-like macrovoids were formed under the external skin la yer of the capsule membrane, but that an interconnected open-cell stru cture was formed under the internal skin. Modelling the precipitation process in two dimensions (i.e. between two glass slides) suggested th at the faster precipitation of HEMA-MMA at the external surface domina ted the formation of the membrane structure, compared with the slower precipitation at the internal surface. (C) 1997 Elsevier Science B.V.