Characterisation of topical film-forming systems using atomic force microscopy and Raman micro-spectroscopy

Polymeric film-forming systems for dermal drug delivery represent an advantageous alternative to more conventional topically applied formulations. Their mechanical properties and homogeneity can be characterised with atomic force microscopy (AFM), using both imaging and nanoindentation modes, and Raman micro-spectroscopy mapping. Film-forming polymers, with and without a plasticizer and/or betamethasone 17-valerate (a representative topical drug), were dissolved in absolute ethanol. Polymeric films were then cast on glass slides and examined in ambient air using AFM imaging and Raman micro-spectroscopy. Using nanoindentation, the elastic moduli of various films were determined and found to decrease with increasing plasticizer content. Films with 20% w/w plasticizer had elastic moduli close to that of skin. AFM images showed little difference in the topography of the films on incorporation of plasticizer. Raman micro-spectroscopy maps of the surface of the polymeric films, with a spatial resolution of approximately 1 μm, revealed homogenous distributions of plasticizer and drug within the films.