Purpose.
To compare the rheolological properties of four commercially available dermal fillers and use the data and photographed
microstuctures to predict product performance.

 Introduction.

 The clinical performance (i.e. perceived feel and persistence (longevity)) of dermal filler composed of hyaluronic acid is widely believed to be dependant on both its microstructure and rheological (flow) properties.
An ideal rheological profile for dermal filler would be one that exhibits a high degree of elasticity (for persistence and recovery after application) with yield like behaviour (i.e. a fairly abrupt change, at a critical stress, from a flat (water like) shear viscosity profile to a region where viscosity decreases with increasing stress/shear rate (shear thinning) above this stress to aid injection. The material should also have the ability to resist permanent destruction of the gel structure after being exposed to relatively high levels of deformation (such as those experienced during injection through a small gauge needle).

 Methods.

 The rheological study was performed on both controlled strain rate (Rheometrics ARES) and controlled stress (Rheometrics SR5) rheometers with parallel plate geometries at a test temperature of 24°C (using a gap of 2mm in all cases)  in most cases with a limited study being performed at 37°C. Parameters studied included elastic modulus, complex viscosity, shear thinning profiles, stress relaxation behaviour, strain amplitude dependency and ‘recovery coefficients’.

 Results.

 Sample Elasticity. – A guide to gel performance and persistence.

 An initial study into the strain amplitude dependence of the samples was performed at 24°C and a fixed test frequency (10 rads/s). From this analysis ‘percentage recovery’ measurements (a measure of sample elasticity - (taken for the relationship 100*G’/ (G’+G”)) were compared at different strain levels. From this analysis it is seen that the samples containing the hyaluronic acid in larger particulate form have a higher elasticity at low strain levels and a greater dependence of elasticity on strain amplitude (i.e. the region of linear elasticity is smaller) when compared with the samples with smaller gel sizes.
An alternative measure of sample elasticity can be derived from stress relaxation measurements - again the same pattern of elasticity was observed.

 Stress Dependence/Recovery Profile – A guide to injectability and gel strength.

 The samples were compared on a controlled stress rheometer whilst the time dependence and resistance of the sample structure/properties to shearing was derived from complex viscosity and modulus measurements from separate controlled strain rate sweeps. Each gel analysed showed marked shear thinning behaviour above a well defined yield point indicating that these gels would be able to be injected rapidly and in a controlled manner with the minimum of force. The recovery behaviour of all four samples was similar at lower test temperatures but at higher test temperatures the samples containing the hyaluronic acid in larger particulate form showed superior performance. These samples also exhibited a greater dependence of viscosity on both shear stress and shear rate.

 Conclusion.

The shear thinning and recovery profiles and in particular the higher measured elasticity of the samples with larger gel sizes when compared with those obtained for the smaller gel sizes would suggest that the former give a possibility of manufacturing more well defined particles.