Assessment of microneedle array insertion into skin using Raman spectroscopic techniques.

Microneedle (MN) arrays bypass the skin's stratum corneum barrier to deliver drugs directly into the viable tissue. The skin disposition of three types of MNs-dissolvable, degradable and hydrogel-forming, fabricated using different polymers-have been evaluated post-treatment with examples of these MN arrays and then examined by confocal Raman spectroscopy and stimulated Raman scattering (SRS) microscopy. The presence of the polymers was assessed from their characteristic Raman signals. SRS image mosaics were acquired to survey and visualise larger areas of the skin surface. After MN insertion, the skin's spectrum was acquired using confocal Raman spectroscopy at the surface, and at nominal depths of 50 µm, 100 µm, and 150 µm. For dissolvable and degradable MNs, Raman signals from the constituent polymers were detectable in the skin. However, the polymer used to form the hydrogel MNs was not detectable under the experimental conditions used. SRS confirmed that the MN arrays penetrated the skin in a reasonably uniform manner. In summary, polymeric MN insertion into the skin has been visualised using confocal Raman spectroscopy and SRS microscopy. Together, these techniques have the potential to shed light on the spatial and temporal skin disposition of the constituent MN polymers used.