microsphere

A microsphere is a tiny solid ball of polymer, typically a few to a few hundred micrometres across (so a powder of them feels like very fine sand), with drug dispersed throughout it. As the sphere slowly releases or breaks down, it lets the drug out gradually.

Microspheres are commonly made from biodegradable polymers such as PLGA (a copolymer of lactic and glycolic acids), in which the drug is mixed uniformly through the polymer matrix rather than sitting in a single hollow core. Drug escapes both by diffusing through the polymer and as the polymer slowly erodes by hydrolysis, and by tuning the polymer composition and particle size the release can be stretched over weeks to months. They are most often injected to form a long-acting depot, but can also be given by other routes.

The big attractions are long, sustained release from a single dose and the use of polymers that safely degrade into natural metabolites. The difficulties are real: an initial burst of drug can be released too fast in the first hours, manufacturing sterile, uniform microspheres reproducibly is demanding, and once injected the dose cannot be retrieved if problems arise. Microspheres differ from nanoscale carriers chiefly in size, being far too large to leave the bloodstream or be taken into cells.