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Gavard Molliard et al. Plast Aesthet Res 2018;5:17 I http://dx.doi.org/10.20517/2347-9264.2018.10 Page 5 of 8

Gel viscosity Cohesive gel Non cohesive gel
Gel with low viscosity

Gel with high viscosity

Figure 1. Schematic representation of the cohesivity and viscosity of a HA gel. Impact of the cohesivity and viscosity on the shape and
behavior of the gel

for each other, is naturally important during the product distribution into the tissues of the treated area .
[7]
However, the rheological properties have also a substantial role at this beginning of the in vivo HA filler
lifetime. More specifically, among the rheological properties, the viscosity η is the key parameter at this step
of the treatment. The viscosity level of the HA filler, combined with its cohesivity profile [Figure 1], define its
capacity to remain at the injection site or to spread into the tissues. Thus, a HA gel with a poor viscosity has
a higher ability to flow and spread in the tissues in comparison to a gel with a high viscosity. Appropriate
viscosity and cohesivity of the HA filler provide the capacity to be easily moldable after injection during
massage, allowing the product to be adequately placed, distributed and homogenized within the tissues,
without fragmentation of the gel.

The consideration of the viscosity as a key rheological parameter for product distribution, and therefore
for tissue integration, is consistent with the clinical uses of the HA fillers by the physicians. Indeed, on
the contrary to high viscosity HA fillers, the products with low viscosity are often used to treat superficial
indications, where nice and homogeneous tissue integration is especially desired, as for instance for the
treatment of fine lines. These products spread easily or quite easily in the skin tissues, sometimes with the
implementation of a smooth massage by the physicians, and there are commonly injected in the dermis,
sometimes even in the superficial dermis with the “blanching injection technique”, without high risk of
nodules formation .
[14]

Step 2 of HA fillers’ lifetime: projection
The normal force F of the HA filler plays a preponderant role during all the implantation phase with an
N
essential effect on the tissues projection capacity . The rheological parameter F is defined as the force
[8]
N
applied by the gel perpendicularly to its surface when it is compressed. The force F allows the gel to push
N
over the surrounding tissues and to counter the deformation and flattening of the product due to the pressure
caused by skin tissues. Therefore, this force reflects the ability of the HA filler to project the skin tissues all
along the implantation period. The higher is the normal force F of the filler and higher is the capacity to
push the skin tissues to project them [Figure 2]. N

Step 3 of HA fillers’ lifetime: dynamic facial expression
As a HA filler is commonly administered in the face, it is essential for the patients and the physicians to have
a product with a high capacity to smoothly and naturally accompany the mechanical motions of the face
due to the dynamic facial expression. In this regard, the dynamic parameters G’ (elastic modulus in shear
stress) and E’ (elastic modulus in compression), representative of the gel firmness in dynamic conditions,
are extremely important rheological parameters during this step of the HA filler lifetime . The elastic
[8]
modulus G’ represents the ability of the HA filler to resist to dynamic shearing forces (i.e., behavior of the
gel for recovering its shape after shear deformation) while the elastic modulus E’ represents the ability of the