In dissolution testing, the U.S. pharmacopeia allows, as a variation to Apparatus 2, a non-reactive stainless steel wire helix as a 'sinker' to be typically used to sink dosage forms that would otherwise float. The objective of this investigation was to determine if other sinker shapes will influence the rate, extent, or variability of dissolution. Criteria for the optimal sinker were defined. Various new sinker designs were fabricated,
tested, and classified.
Four classes of sinker shapes were defined:Longitudinal
Longitudinal sinkers contact the dosage forms on the long axis. Lateral sinkers either wrap around or contact capsule dosage forms in the middle, such as the line where the top and bottom halves of a capsule shell come together. Screen enclosures are of two types: either a wire cage, which holds the capsule, or a circular piece of wire screen placed on top of the capsule. Internal weights consist of two steel ball bearings, one inserted into each end of the capsule.
The choice of sinker will depend on the tablet or capsule. In general there are some guidelines that can be followed however:The sinker should ideally not touch the tablet as this can affect the dissolution rate.
They are available in different sizes to accommodate most capsule/tablet dimensions.
A minimum amount of weight is preferable, just enough to have the desired affect.
Coated sinkers can be used where there may be a reaction between steel and the tablet.
Basket type sinkers can be useful but care should be taken that the outer coating of the tablet does not clog the mesh.
Wire spirals should be as wide as practically possible to avoid clogging which will affect the dissolution rate.
Japanese Sinkers are specified by the JP and have strict dimensions.