Shear Rate and Sink Conditions

What is Shear Rate?

  • Shear rate is a very complex relationship but includes the interface between the surface of the solid and the rate at which fresh solvent contacts it. This is a complicated interaction that involves the interface between the solid’s surface and the rate at which fresh solvent comes into contact with it.
  • If a tablet particle were suspended in a liquid with no agitation, the liquid immediately surrounding the tablet would get saturated, and dissolution would come to a halt or stop.
Shear Rate
Shear Rate
  • As soon as the media begins to move, the saturated film is ‘washed’ away, allowing the dissolution to resume. As a result, anything that impacts the fluid dynamics or the disintegration and dissolution of a dosage form should be understood and regulated.
  • The Shear Rate depends on many variables including:
  1. Flow pattern variables,
  2. Turbulence,
  3. Viscosity,
  4. Surface tension and dissolved gasses, which are in turn effected by other system variables to do with physical parameters.
  • Before any of those can be observed however, it is essential to ensure that there is sufficient media existing to allow the free dissolution of the active ingredient (API) into solution.

What is Sink Conditions in dissolution?

  • When you put a spoonful of sugar in a beaker of water, it dissolves quickly. A second spoon will dissolve as well. However, as you continue to add spoonful, the sugar dissolves more slowly until it becomes difficult to dissolve anymore as the solution gets saturated.
  • In terms of drug dissolution, it is critical that the presence of a previously dissolved drug in solution does not interfere in any manner with the capacity of more drugs to dissolve. i.e. the drug concentration in solution should not be higher than the bottom of the drug’s saturation curve. It’s never a good idea to get too near to the saturation point.
  • If the concentration level was raised too high, the rate of new drug dissolution would be delayed, and the results would become inaccurate or reproducible.
  • To ensure that enough media is present in relation to the drug to be dissolved, a volume of media 5 to 10 times greater than the saturation point at which dissolution would slow is typically used. Sink Conditions are defined as having sufficient media to ensure complete dissolution.
  • This is why dissolution is usually done in greater quantities, such as 900mL or 1 litre. When sink conditions allow and the quantifiable level of the drug is lower, 500ml tests can be applied.
  • In recent years, the introduction of microcapsules and very low dosage levels have led to mini vessel tests in volumes as low as 100 mL or 200 mL, but in all these cases, sink conditions are maintained. Conversely, if 1000 mL is not enough volume, then larger 2000 mL vessels can be used, and above that volume, USP4 can be considered.

Ground breaking Equation:

In 1897, Noyes and Whitney projected the following ground breaking equation, allowing to quantify the dissolution rate of a solid substance (dc/dt) in its own solution (Noyes and Whitney)

dc/dt = K(cs-ct)


K is a constant;

cs denotes the solubility of the substance in the liquid

ct is the concentration of a dissolved substance at time t.

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