The main operating principle of a paddle/basket (or vessel-based) apparatus is to provide a precise and controlled stirring and mixing mechanism at 37 C. In reality, from the operational aspect a beaker with a magnetic stirring bar may be considered equivalent to a dissolution tester if the rpm of the stirrer is precisely controlled and beaker content can be maintained at 37C. Therefore, it is important to note that operation of a dissolution tester should be as simple as any other stirring device.
To achieve time savings and consistency in results, the current dissolution apparatuses come in units of 6 or more stirrers with appropriate mechanical and electronic controls. However, operating principle remains the same whether the apparatus is based on a single or multiple stirring units.
Generally, stirrers (or stirring devices) comes with their limitations. For example, a magnetic bar may not be suitable for mixing viscous or solid materials because bars lack sufficient torque and the stirred contents are not easily move able. Similarly, basket/paddle stirrers cannot be used as mixers for solid materials (tablets/capsules) as these settle at the bottom of the vessels where flow of the liquid (medium) is low to negligible. A representation of lack of interaction between solid/liquid with such apparatuses is shown in the figure, which is known as “cone formation”. Therefore, by its nature basket/paddle stirrers (or apparatuses) cannot be used where thorough mixing (interaction) of solid/medium is required, such as what occurs in the GI tract.
Furthermore, the random settling positions of a product (tablet/capsule and their particles) at the bottom of a vessel add variability to this mixing, thus dissolution results would be erratic and unpredictable.
To sum up, the operating principle of the basket/paddle apparatuses is based on a simple stirring device. However, operational limitations of such stirrers are such that they cannot provide efficient product/medium interaction or mixing, thus would provide erratic (highly variable) and unpredictable results.