monodisperse , polymeric microspheres are formed by injecting uniformly shaped droplets of radiation polymerizable monomers , preferably a biocompatible monomer , having covalent binding sites such as hydroxyethylmethacrylate , into a zone , impressing a like charge on the droplet so that they mutually repel each other , spheroidizing the droplets within the zone and collecting the droplets in a pool of cryogenic liquid . as the droplets enter the liquid , they freeze into solid , glassy microspheres , which vaporizes a portion of the cryogenic liquid to form a layer . the like - charged microspheres , suspended within the layer , move to the edge of the vessel holding the pool , are discharged , fall and are collected . the collected microspheres are irradiated while frozen in the cryogenic liquid to form latent free radicals . the frozen microspheres are then slowly thawed to activate the free radicals which polymerize the monomer to form evenly - sized , evenly - shaped , monodisperse polymeric microspheres .