Thickness of film coat Nature of coating material

Hydrophobicity of film coat (i.e. is the coat wetted?)

Rate of coat hydration or dissolution

Nature of contacting surface

Geometry of contact with tissue; tablet size and shape

Rheology of solution of film coating material

Surface tension of film coating material in solution

Elongational characteristics and critical rupture thickness of filament

Forces leading to separation

Figure 12.15 Diagrammatic representation of the possible sequence of events in the adhesion of a tablet to the mucosal surface and its subsequent separation, with a partial listing of the many variables in the process.

Reproduced from A. T. Florence and E. G. Salole, Formulation Factors in Adverse Reactions, Wright, London, 1990.

contact, when the angle of approach, together with the tablet size and shape, will determine the opportunity for adhesion. Stages V to VIII represent the detachment process initiated, for example, by swallowing food or water or dry swallowing; here the rheological and elonga-tional characteristics of the adhesive material are important. The process is complex and is the subject of much research.

12.6 Analysis of particle size distribution in aerosols

Analysis of particle size distribution of aerosol formulations during formulation, development and clinical trial or after storage is of obvious clinical relevance (see section 9.9). Aerosols are not easy to size, primarily because they are dynamic and inherently unstable systems. Methods of sampling may be divided into techniques which utilise a cloud, and dynamic methods in which particles are carried in a stream of gas. In cloud methods, sedimentation techniques based on Stokes' law are applied and the usual detection system is photometric. The Royco sizer is a commercially available instrument which measures individual particles in a cloud (it is used to monitor the air of 'clean rooms'). This instrument can be used to size particles in aerosol clouds provided that the particle size distribution does not change during the time of the analysis either by preferential settling of larger particles or by coagulation. Dynamic methods depend on the properties of particles related to their mass. Instruments utilise both sedimentation and inertial forces.

Probably the most widely used instrument in categorising airborne particles is the cascade impactor, in which large particles leave the airstream and impinge on baffles or on glass microscope slides. The airstream is then accelerated at a nozzle, providing a second range of smaller sized particles on the next baffle and so on (Fig. 12.16). Progressively finer particles are collected at the successive stages of impingement owing to jet velocity and decreasing jet dimension. Shearing action of the jets may lead to the break-up of aggregates.

For more routine examination of medicinal aerosols, 'artificial throat' devices are useful. With these, comparative studies of the behaviour of aerosols can be carried out. In these devices the particles are segregated according to size. Analysis of the collecting layers at the several levels of the device allows the monitoring of changes in released particle size. In devices with an artificial mouth, this


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