DOUBLE-WALLED POLYMER MICROSPHERES FOR CONTROLLED DRUG-RELEASE

ONE approach to the controlled release of drugs involves incorporation of the drug molecules into the matrix of microscopic polymer spheres or capsules1-10. Existing methods for preparing such microparticles do not, however, always guarantee a constant release rate, for example b ecause drug molecules may be trapped preferentially at the surface, be cause they have to diffuse through an increasing thickness of polymer when the particles are non-eroding or because the surface area changes for eroding particles. In other situations pulsed release may be requ ired-an application to which simple polymer microspheres do not readil y lend themselves. Multi-walled microspheres might solve some of these problems. Here we describe a one-step process for preparing double-wa lled polymer microspheres with diameters ranging from about 20 to 1,00 0 micrometres. Our technique11 involves the phase separation of a poly mer mixture owing to solvent evaporation: with an appropriate choice o f interfacial tensions and evaporation rate, a spherical droplet of on e polymer becomes coated with a highly uniform layer of the other. Thi s process, which might be adapted to yield multi-walled microspheres, should make possible the engineering of highly specific drug-release p roperties.