RheoTack evaluation of detaching behavior of silicone-based pressure sensitive adhesives for transdermal therapeutic systems

Abstract

This study focuses on applying the RheoTack method to assess the detaching behavior of transdermal therapeutic systems (TTS), which represent skin-applied medications comprising a drug-loaded pressure-sensitive adhesive (PSA) and a flexible backing layer. The RheoTack method provides detailed force-retraction displacement-curves (F-h-curves) that reveal the influence of chemical structure and resin content on PSA deformation and fibril formation. To compare various rod geometries (flat rods with diameters of 5 mm and 8 mm, and a spherically rounded rod with a contact area of 5 mm2), the force-retraction displacement curves were normalized to account for the effective contact areas. The flat and spherical rods led to completely different F-h-curves as well as different failure and tack behaviors. Furthermore, the adhesion formation between the TTS with flexible backing layers and rods during the dwell phase occurs in a different manner compared with rigid plates, particularly for flat rods, where maximum compression stresses occur at the edges and not uniformly over the cross-section. Measurements of F-h-curves were performed with retraction speeds of 0.01, 0.1, and 1 mm/s. The increase in retraction speed increased the stiffness from 250 to 1200 N/m for non-amine-compatible PSA measured with a rod of 8 mm. RheoTack measurements were performed with a dwell time of 1 s, which is consistent with ASTM D2949. However, the TTS was in the adhesion-establishing compression phase for 3 s at 1 mm/s and for 30 s at 0.01 mm/s. Thus, the approach to follow ASTM D2949 has to be reconsidered for testing TTS materials.