Microstructure and elution of tetracycline from block copolymer coatings

A critical metrology issue for pharmaceutical industries is the application of analytical techniques for the characterization of drug delivery systems to address interrelationships between processing, structure, and drug release. In this study, cast coatings were formed from solutions of poly(styrene-b-isobutylene-b-styrene) (SIBS) and tetracycline in tetrahydrofuran (THF). These coatings were characterized by several imaging modalities, including time-of-flight secondary ion mass spectrometry (TOF-SIMS) for chemical imaging and analysis, atomic force microscopy (AFM) for determination of surface structure and morphology, and laser scanning confocal microscopy (LSCM), which was used to characterize the three-dimensional structure beneath the surface. The results showed phase separation between the drug and copolymer regions. The size of the tetracycline phase in the polymer matrix ranged from hundreds of nanometers to tens of microns, depending on coating composition. The mass of drug released was not found to be proportional to drug loading, because the size and spatial distribution of the drug phase varied with drug loading and solvent evaporation rate, which in turn affected the amount of drug released.