How to Characterize Viscoelastic Properties of Semisolids

The USP General Chapters - Physical Analysis (GCPA) Expert Committee proposed a revision of USP general chapter <1912> Measurement of Hardness of Semisolids. The proposal has been published for comment in Pharmacopeial Forum (PF) 47(2) with a comment deadline until 31 May 2021.

What is "Yield stress" ?

The viscoelastic properties of semisolids can be dominated by either solid-like or liquid-like behavior depending on how much stress is applied. If the quality of a semisolid material or preparation is primarily dependent on the behavior under high-shear conditions, then viscosity may be an appropriate parameter to monitor. However, if the properties are critical to quality at low shear or at rest (e.g. residence time at site of application), then the measurement of the apparent yield stress using the methods described in chapter <1912> may be required (apparent yield stress indicates that the result is dependent on the measurement method used).

The chapter <1912> describes the following measurement methods:

• Strain Ramp
• Shear Rate Ramp
• Oscillation Amplitude Sweep
• Penetrometry

The current proposal is based on chapter <1912> to be official on May 1, 2021 (the draft chapter was published for comment in PF 46(1), January 2020). A change in the title of the chapter (Measurement of Yield Stress of Semisolids) is proposed together with alignments in terminology and tolerances in measurements to be consistent with USP chapter <915> Measurement of Structural Strength of Semisolids by Penetrometry. For more clarity, the following definition for "yield stress" has been added:

"Yield stress for a raw material or dosage form is the applied stress at which a change in the viscoelastic properties of a semisolid is observed: below the yield stress the material response is dominated by elastic deformation, whereas above the yield stress the material response is dominated by viscous flow."

The revised chapter <1912> Measurement of Yield Stress of Semisolids can be viewed after registration for the Pharmacopeial Forum.