Secondary Ion Mass Spectrometry (SIMS) employing an SF5+ polyatomic primary ion source for sputtering and Bi3+ primary ions for analysis was used to depth profile through DES coatings obtained from Medtronic (DES manufacturer) at variable temperatures. PLGA/Rapamycin films were prepared by casting solutions of 2% w/v poly(lactic-co-glycolic acid) (PLGA) containing 5% w/w rapamycin (~6 µm) onto steel substrates, and a SIMS depth profile was acquired at room temperature. The signal was observed to decay at SF5+ primary ion doses of ~1.3 x 10^15 SF5+ ions/cm2 and the steel substrate was never reached. However, the depth profile stability was dramatically improved at low temperatures (-100 degrees C), as indicated by the relatively constant signal up through SF5+ primary ion doses of ~1.5 x 10^16 SF5+ ions/cm2. At this dose the secondary ions characteristic of the DES coating decreased while the corresponding steel substrate intensities increased, indicating that the entire film was eroded. This result shows that using low temperatures can extend the utility of SF5+ to characterize thicker polymeric materials (6.0 µm as opposed to 0.2 µm).
Cluster SIMS at low temperatures has also be used to elucidate the 3-dimensional structure in these DES coatings, as illustrated in the Figure, which shows secondary ion image overlays of m/z = 99 (fragment characteristic of PLGA) and m/z = 84 (fragment characteristic of Rapamycin) in a PLGA film containing 25% rapamycin. These images were acquired as a function of increasing sputter time or depth, and thus give detailed information on the heterogeneity in the surface and near surface region in these systems as compared to the bulk. To our knowledge, these images represent the first demonstration of successful 3-D SIMS imaging in real world drug delivery devices.