Rheological Investigation as Tool to Assess Physicochemical Stability of a Hyaluronic Acid Dermal Filler Cross-Linked with Polyethylene Glycol Diglycidyl Ether and Containing Calcium Hydroxyapatite, Glycine and L-Proline.

(1) Background: Dermal fillers are commonly used in aesthetic practice and their rheological characterization is of much interest today, as well as the stability study of the finished formula against external stimuli of a different nature (biological and physicochemical). Rheological tools have been exploited to characterize the physiochemical behaviour of a hyaluronic acid (HA) based dermal filler subjected to different thermal conditions over time. The collected results provide an index of its rheological stability. (2) Methods: After a preliminary Amplitude sweep test, the Frequency sweep test was performed in order to study the stability of a HA dermal filler cross-linked with Polyethylene Glycol Diglycidyl Ether (PEGDE) and containing Calcium Hydroxyapatite (CaHA), Glycine and L-Proline subjected to different conditions. Also, a shear rate ramp test was performed in order to investigate the filler's flow behavior. (3) Results and Conclusions: G' (elastic modulus), G'' (viscous modulus) and consequentially tan δ (tangent of the phase angle) show a similar trend at different thermal conditions, underlining that the product is not affected by the storage conditions. The viscosity of the dermal filler decreases with an increasing shear rate, so a non-Newtonian shear thinning pseudoplastic behavior was demonstrated in all tested conditions.