Evaluation of Rheometry Amplitude Sweep Cross-Over Point as an Index of Flexibility for HA Fillers

Abstract

Background : The majority of studies published on rheology of hyaluronic acid (HA) fillers have focused on results from frequency sweeps within the linear viscoelastic region (LVR), i.e. at nearly static conditions. To study the properties and behavior of HA fillers at more dynamic conditions, it is necessary to go outside the LVR. Objective : Evaluation of the G’/G’’ cross-over point from the amplitude sweep is suggested as a manner to estimate the flexibility of HA fillers. Materials and methods : Rheological properties of 7 HA fillers were measured in an amplitude sweep from 0.1 % to 10000% strain at 1 Hz, using an Anton Paar MCR 301, a PP-25 measuring system with a gap of 1 mm at 25 ℃ and a 30-min relaxation time. The cross-over point was evaluated for stress, strain and G (G’ and G’’ identical), the values denoted xStrain, xStress and xG. Results : The xStrain values spanned from below 1000 % to above 2000% for the products based on the Optimal Balance Technology (OBT) ? (in the US, XpresHAn Technology ? ), compared to below 100% for the products based on the NASHA ? technology. Conclusions : Measurement of the flexibility provides a more complete picture of the rheological properties of HA fillers as a complement to firmness measured as G’. The test results show that the Restylane family of products covers a large range in flexibility, and that the flexibility can be estimated using xStrain derived from the amplitude sweep.