Filler rheology reference: what the measurements mean
What G prime, viscosity, cohesivity and hydrophilicity are measuring, and why published figures cannot be compared directly between manufacturers.
These notes concern the measurements themselves — what each parameter is derived from, and how much confidence a published number deserves. The clinical translation is covered separately; the point here is that a figure quoted on a product sheet is the output of a test method, and the method conditions how the figure should be read.
Storage modulus
G′ is the storage modulus, obtained by oscillatory rheometry: the gel is subjected to oscillating shear and the response is separated into an elastic component stored by the material (G′) and a viscous component dissipated as flow (G″). G′ is the portion that describes how far the gel resists being deformed and returns to shape afterwards.
Two caveats attach to the number. It is frequency-dependent and temperature-dependent, so values measured under different test conditions are not interchangeable. And it correlates strongly with hyaluronic acid concentration and cross-linking, which means it partly restates information already given elsewhere on the product sheet rather than adding an independent dimension.
Viscosity
Viscosity — more precisely the complex viscosity, or dynamic viscosity under specified shear — describes resistance to flow. It is the parameter most closely connected to what the injector feels: extrusion force through a given needle or cannula, and how far the gel travels from the point of deposition.
Hyaluronic acid gels are shear-thinning, so viscosity falls as shear rate rises. A gel is therefore less viscous while passing through the needle than it is once it has come to rest. This is a useful property, and it is also why a single quoted viscosity value describes one point on a curve rather than the material as a whole.
Cohesivity
Cohesivity describes the tendency of a gel to hold together as a single mass rather than separate. It has clinical relevance to how a deposit integrates and distributes lift, and it is the least settled of the parameters discussed here.
The difficulty is methodological. The literature notes that cohesion has no standardised measurement method: different groups use different approaches, including compression tests, dispersion or drop tests and various shear-based measures, and these do not produce values on a common scale. The practical consequence is that published cohesivity figures are not comparable between manufacturers. A number quoted for one product cannot be ranked against a number quoted for another unless both were produced by the same method in the same study. Comparative papers that measure several products under a single protocol are the exception and are the only sound basis for such comparison.
Hydrophilicity
Hyaluronic acid binds water, and products differ in how much they take up after placement. Comparative work measuring swelling behaviour across products shows meaningful variation between gels. The parameter deserves separate attention because its effect appears after the appointment rather than during it, and because it is regionally consequential — sustained post-treatment oedema is most visible where the tissue is thin and lymphatic drainage limited.
Reading a product sheet
Three habits follow. Check that a figure states its test conditions, since one without them is uninterpretable. Compare within a study rather than across product literature. And treat cohesivity claims as directional statements about product family rather than as measurements on a shared scale.
