USP drafts on viscoelastic properties of semisolids

The United States Pharmacopeia (USP) is requesting early input from stakeholders on two new proposed General Chapters:

  •  <915> Measurement of Structural Strength of Semisolds by Penetromety,
  •  <1912> Measurement of Hardness of Semisolids

The chapters are estimated to be published for comments in Pharmacopeial Forum (PF) 43(2) [Mar.–Apr. 2017].

The USP posted this announcement on February 1, 2017. Input/Comment Deadline is May 31, 2017, which is the comment deadline for PF 43(2) [Mar.–Apr. 2017]). Suppliers and manufactures of drug substance, drug products, excipients, dietary supplements and drug testing organizations and drug products related regulatory agencies are encouraged to submit input or comments.

The proposed chapter <915> describes empirical methods of measuring the structural strength or consistency (the term consistency may be confused with uniformity or homogeneity, therefore the term structural strength is now preferred) of a semisolid raw material or dosage form with a penetrometer. In addition, this chapter outlines a method for performing the gravity-driven penetrometry measurements of semisolids, as well as an alternate method for using computer-controlled instruments to perform penetrometry measurements to achieve results that are comparable to the traditional gravity-driven method.

Preliminary outline of chapter <915>: 

  • Procedure
  • Sample Preparation
  • Determination of Penetration
  • Method I (gravity-driven measurement of structural strength)
  • Method II (constant-speed measurement of structural strength)

Expression of Results

Characterizing and monitoring the viscoelastic properties of semisolids is not straightforward, because the properties can be dominated by either solid- or liquid-like behavior depending on how much stress is applied to the material. Therefore, the proposed new chapter <1912> summarizes the mathematical models used to quantify the viscoelastic properties of semisolids (raw material or formulation), as well as the most common experimental methods for assessing the viscoelastic properties and determining the apparent yield stress for semisolids.

If the quality of a raw material or dosage form is primarily dependent on the behavior under high-shear conditions, then the viscosity may be an appropriate parameter to monitor. In this case, efforts should be made to avoid wall slip and make measurements where the applied shear stress is much greater than the apparent yield stress. If the properties of the raw material or dosage form are critical to quality at low shear or at rest (e.g., uniformity of an ointment suspension or residence time at site of application), then measurement of the apparent yield stress (Pa) may be required.

Preliminary outline of <1912>: 

  • Hooke’s Law, Newton’s Law, and Viscoelastic Models
  • Herschel-Bulkley Equation
  • Oscillatory Measurement of Viscoelastic Properties

Experimental Methods

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


The proposed texts are available in advance of their publication in PF on the USP General Announcements homepage.

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