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Predicting Injection Force: Theoretical Foundation

March 9, 2021

Viscosity is an important fluid property that can be used to predict performance in real world applications. One of the most common uses of viscosity is to assess the ability to deliver pharmaceutical formulations to the patient by injection. A significant component of formulation research and development is rheology modification to ensure that therapeutics can be delivered without an excessive injection force. This concern is especially important when developing high concentration antibody formulations, which can have a broad range of viscosities depending on the specific protein-protein interactions. These formulations can involve high concentration protein formulations greater than 150 mg/mL in some cases. At these high concentrations, a minor change in concentration can lead to a dramatic change in viscosity.

Relative viscosity vs concentration - nylon 66 in formic acid

In protein therapeutics, major resources are dedicated toward the development of injection formulations for:

  • Sustainable Release
  • Subcutaneous Injection

Understanding the viscosity at the shear rate inside your needle is crucial to estimating an accurate injection force, which has a direct impact on both efficacy and patient experience.

High viscosities of candidate drugs can lead to disqualification or costly development of alternative delivery methods. Because of the challenges caused by higher viscosities, drug stability in monoclonal antibody formulations must be achieved at the lowest viscosity values possible.

To tackle the problem of relating viscosity to injection force, we turn to fluid mechanics for pressure driven flow through a circular cross section. There are different equations used to calculate the viscous contribution to the injection force for Newtonian, power-law, and general non-Newtonian fluids. Although a seemingly complicated problem, the analysis can be simplified by realizing that the pressure drop across the syringe needle dominates.

injectability header

The injectability calculation process may seem like a daunting task, however it is well worth the effort to ensure that suitable therapeutic candidates are not discarded but continue to move forward in the development process. Download our application note, "Predicting Injection Force from Viscosity Data Part 1: Theoretical Foundation”, for a full description of general syringe geometry and the equations required to calculate injection force of Newtonian, power-law, and general non-Newtonian fluids.

Download Injectability Theoretical Foundation Application Note 

Written by: Eden Reid, RheoSense Senior Marketing Associate

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