Aim: Over the last 15 years, hyaluronic acid (HA) fillers have become the most popular injectable biomaterial for soft tissue correction. With the increasing number of available HA fillers and the multiplication of facial treatments all over the world, there has been a need from physicians to better understand the HA fillers science. There is especially a growing interest in the science-based evaluation of rheological characteristics which represents an essential tool to guide physicians in the

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... selection of the most appropriate HA fillers, administration techniques and depths of injection for their clinical applications. Methods: Four key rheological parameters (viscosity η, elasticity G', normal force F N and elasticity E') are measured and discussed on five HA fillers. Results: These four key rheological parameters are demonstrated to play a pivotal role, in combination with the cohesivity, for better predicting the clinical behavior of HA fillers at different stages of their lifetime. Conclusion: This article discusses the importance of four key rheological parameters during the main steps of the clinical HA fillers' lifetime, from the product injection to the loss of clinical effects. A better knowledge of these HA fillers' rheological parameters can help the physicians to optimize their aesthetic outcomes, safety and patient satisfaction. Page 2 of 8 Gavard Molliard et al. Plast Aesthet Res 2018;5:17 I http://dx.