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.