A New Approach for Characterizing the Thixotropic Properties of Gel Formulations as Sprayable Agents Based on Rheological Analysis

The present study evaluated the rheological properties of gel formulations composed of the thixotropic peptide amphiphile, palmitoyl-glycine-histidine (Pal-GH), and the thickener, propylene glycol alginate (PGA), to propose a proper approach to design sprayable gel formulations with good spray performance and high retention of a therapeutic agent. The hysteresis loop area (HLA), a conventional index of thixotropy, was determined from the relationship between the shear stress and shear rate of various formulations with different amounts of Pal-GH and PGA. In addition, a new assessment method for characterizing the thixotropy using the initial structure recovery speed was determined based on the time course of the complex modulus (G*) after structural breakdown of the gel formulations. The G* values increased with the increase in the amount of Pal-GH and PGA, indicating that the formulations were not deformable. Additionally, high HLA and high initial structure recovery speed are preferable when selecting a formulation with good spray performance and high retention. As suitable combinations of Pal-GH and PGA could exhibit both high HLA and high initial structure recovery speed, they are promising components for gel formulations to be used as sprayable agents with good spray performance and high retention. The results also suggested that the initial structure recovery speed would reflect the thixotropy for the formulation more appropriately than HLA. Thus, the initial structure recovery speed could be a useful scale for the preparation of sprayable gel formulations.