Although measurement of flexibility is a well-established rheology method, xStrain as an indicator of flexibility for HA fillers was first applied to the OBT and NASHA gels.11,26-28 Using the xStrain method, an amplitude sweep is conducted where the level of deformation (or % strain) is increased until the yield point at the end of the linear viscoelastic region (LVR) is reached, when the gel can no longer return to its original shape.11 More flexible HA formulations can withstand high levels of strain before yielding.11 The current study examined strength/firmness (G prime) and flexibility (xStrain) for the full range of NASHA and OBT HA fillers. In addition, this paper aimed to link these gel properties with clinical performance.
MATERIALS AND METHODS
G prime and xStrain
G prime (strength/firmness) and xStrain (flexibility) were measured for Restylane, R. Silk, and R. Lyft (NASHA gels) and R. Refyne, R. Kysse, R. Volyme, and R. Defyne (OBT gels) and performed in sequence, including a relaxation time of 30 minutes. A frequency sweep from 10 Hz to 0.1 Hz at 0.1% strain was followed by an amplitude sweep from 0.1% to 10,000% (0.001 to 100) strain at 1 Hz. The gap was 1 mm using a PP25 rheometric measuring system at 25 degrees C. The frequency sweep was evaluated for G prime (G'), G double prime (G"), G*, and tan delta at 0.1 Hz.
The amplitude sweep was first evaluated at 0.1% strain to verify that the applied frequency sweep strain was within the LVR. The strain was then evaluated at the crossover point of the amplitude sweep (where G prime and G double prime had the same value). This value denoted the xStrain.
RESULTS
HA fillers produced with NASHA technology demonstrated the highest G primes (strength/firmness). Restylane, R. Silk, and R. Lyft had G primes of 701, 416, and 799 Pa, respectively. Across the OBT formulations, G primes were 70 (R. Refyne), 160 (R.
