Characterisation of small changes in the physical properties of powders of significance for dry powder inhaler formulations

In this paper we address the following issue: Why is surface characterisation important? All pharmaceutical processes (with the exception of mixing two gasses) involve interfacial contact, and, consequently, it is not surprising that surface energetics play an important role in determining the outcome of all events. For a dry powder inhaler system interfacial considerations may relate to drug-drug interactions (cohesion), drug-carrier or drug-device interactions (adhesion) and deaggregation phenomena during use. As all adhesive and cohesive interactions are interfacial phenomena it is reasonable to accept that the basis of interactions within dry powder inhalers is through interfacial forces, which can be divided into apolar (Lifshitz-van der Waals) and polar (electron donor-electron acceptor) components. Further to this it can be accepted that changes in the nature of any surface within the product (the drug, the carrier or the container) can be expected to result in changes in the surface interactions involving that phase. Thus, in essence, the success or failure of a formulated inhalation device is dependent upon the nature of the surface of the materials used, and, as such, measurement of these surfaces becomes of paramount importance. In this review comparatively little effort will be taken to prove the dominant role of surface energetics in inhalation products; this is primarily because much of the proof which exists is held as confidential by manufacturers. Consequently, this review will concentrate on surface characterisation of powders with respect to determination of surface energies and changes in solid-state properties. (C) 1997 Elsevier Science B.V.