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˚C 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.

Advanced Concepts in Rheology for the Evaluation of Hyaluronic Acid–Based Soft Tissue Fillers

“…This discrepancy likely arises from how G′ is measured: at low stresses or deformations that are nearly static conditions. 20 Such conditions are not representative of the harsher conditions to which implants are submitted once implanted in situ , which may explain why G′ is poorly predictive of filler mechanical resistance and their macroscopic behaviors. Overall, the results confirm that G′ values are only partially informative 20 and should be accompanied by the LVER that characterizes how far, in terms of stress or deformation, the G′ value is maintained.…”

“… 20 Such conditions are not representative of the harsher conditions to which implants are submitted once implanted in situ , which may explain why G′ is poorly predictive of filler mechanical resistance and their macroscopic behaviors. Overall, the results confirm that G′ values are only partially informative 20 and should be accompanied by the LVER that characterizes how far, in terms of stress or deformation, the G′ value is maintained. The Strength scores was specifically designed to reflect the ability of a gel to maintain its mechanical properties and viscoelasticity, or its G′, over a wide range of stresses (or deformations), namely the LVER.…”

“…Traditionally, analysis of HA gels is limited to the elastic modulus G′, the viscous modulus G″, the phase angle δ (or tan δ), being connected to the former quantities (tan δ = G″/G′), and the complex viscosity η*. 9 , 10 , 18 , 19 Such data are obtained with oscillatory dynamic rheology and is typically measured in nearly static conditions 20 : a few Pascals (1–10 Pa) or few percentages of deformation (0.1%–1%). This fall within the linear viscoelastic region (LVER), where G′, G″, and δ are constant.…”

An Objective, Quantitative Assessment of Flexible Hyaluronic Acid Fillers in Lip and Perioral Enhancement

“…These fillers achieve distributed tissue integration, and provide flexibility and support to maintain natural movement and dynamic expression. 6 , 10 – 13 HA RK, a filler specifically designed for lip augmentation, has been approved in various countries outside the US since 2010 and was recently FDA-approved. 10 , 14 HA RR and HA RD are used as add-on treatments for subjects who also require nasolabial fold (NLF) or marionette line (ML) correction to optimize aesthetic results.…”

Complications after cosmetic periocular filler: prevention and management

“…Tear trough filler is particularly prone to this complication and it [HA]: concentration of hyaluronic acid, G': elastic modulus. Values were obtained from manufacturer documentation and published sources [56][57][58][59][60] . Exact G' values varied between sources, but the relative differences between listed products were similar is therefore important to select a product with low water affinity such as Restylane-L (Galderma, Lausanne, Switzerland).…”