Measuring adhesive performance in transdermal delivery systems

The therapeutic performance of a transdermal delivery system (TDS) can be affected by the quality of contact between the patch and the skin. The adhesion of a TDS to the skin is obtained by using pressure-sensitive adhesives (PSAs), which are defined as adhesives capable of bonding to surfaces with the application of light pressure. The adhesive properties of a TDS can only be fully and correctly characterized by considering the following factors: (i) the property that enables an adhesive to form a bond with the surface of another material upon brief contact and under light pressure (tack); (ii) the resistance of the matrix to flow that could be considered as an expression of the cohesiveness of the matrix itself (creep resistance); and (iii) the force required to peel away a patch from a surface (peel adhesion). In this paper, attention is focused on the most widely used methods for the measurement of TDS adhesive properties in development studies and in the quality control of TDSs. The most critical formulative variables in the development of TDSs are the type and concentration of additives used, the drug loaded, the PSA thickness, the composition and thickness of the backing layer, and the solvent residue. There is a lack of evidence for a relationship between the results obtained in in vitro adhesion tests and the in vivo adhesion performance of TDSs. Therefore, an analysis of the percentage of TDSs that lifted and/or detached during pharmacokinetic and clinical studies should be performed during development studies. No official tests for adhesive strength are currently included in the TDS monographs of the most authoritative pharmacopeias. At present, the medical application of TDS relies on tests standardized by the adhesive tape industry or prescribed in the adhesive tape monographs of US and Japanese Pharmacopoeias, or adaptations of these. The development of new methods is recommended to meet the specific requirements of evaluating TDSs, and to improve the correlation between in vitro tests and in vivo performance.