Characterization of film-coated aerosol canisters using electrochemical impedance spectroscopy

Drug adhesion to the walls of an aerosol canister can be prevented/reduced by coating the canister with a hydrophobic polymer (e.g., a fluoropolymer). In this study, three batches of fluoropolymer-coated canisters were investigated by electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM). The EIS technique showed that only one of the batches presented as a monolithic, non-porous film. The other two batches were either partially porous or highly porous. Scanning electron micrographs showed evidence of cracks, within the films, but could not alone establish the porous nature of these defects. For the non-porous and partly porous films it was possible to use the EIS data to determine the approximate film thickness. Estimates of 2-4 microm were obtained for the mean film thickness. These values compared favorably with micrometer estimates obtained following acid dissolution of the aluminum canister. It remains to be seen whether the properties of the films (i.e., the porosity and film thickness, determined by EIS) translate to differences in drug adhesion. Nevertheless, the EIS technique was shown to be a powerful, non-destructive method that lends itself to the rapid analysis of batch-to-batch variation in film-coated canisters.