Viscoelasticity: how a gel behaves
Quick facts
- Definition
- A property of a material that behaves, at the same time, as an elastic solid and as a viscous liquid.
- Measurement
- Elastic modulus (G') and viscous modulus (G'').
- Reading
- A high G' indicates a firmer gel; a high G'' indicates a more fluid gel.
- Field of study
- Rheology: the science of the flow and deformation of materials.
- Relevance in HOF
- Helps explain why different products behave differently in the tissue.
Viscoelasticity is the property of a material that behaves, at the same time, as an elastic solid (it returns to its shape when the force stops) and as a viscous liquid (it flows and dissipates energy). The hyaluronic acid gels used in orofacial harmonization (HOF) are viscoelastic materials, and understanding this property helps explain why two products with the same base can behave very differently in the tissue.
How is viscoelasticity measured?
The science that studies the flow and deformation of materials is rheology. In a rheometer, a small oscillating force is applied to the gel and two values are measured:
| Measure | What it represents | Practical reading |
|---|---|---|
| G' (G-prime) | Elastic modulus: resistance to deformation | Higher, firmer gel that holds its shape |
| G'' | Viscous modulus: tendency to flow | Higher, more fluid gel |
| Tan delta | G''/G' ratio | Lower, more elastic behavior; higher, more viscous |
These values depend on the frequency and the test conditions, so comparing numbers between manufacturers calls for caution. The information in commercial materials does not always come with the same method.
Other parameters that add up
Viscoelasticity is not the only characteristic of a gel. Other concepts come up in discussions:
- Cohesivity: how well the gel stays together without breaking into particles.
- Viscosity: resistance to flow.
- Pseudoplasticity: the gel becomes more fluid under force and firmer at rest.
- Extrusion force: the force needed to push the product through the syringe and the instrument.
Behavior in the tissue results from this combination, and also from concentration, degree of cross-linking and the injection site.
Why does this matter to the professional?
A very elastic gel tends to hold volume and resist deformation, while a more fluid gel tends to settle and integrate. Products with different behaviors may suit different areas. This understanding helps the professional choose with judgment, read the package insert and explain to the patient why different products exist, without resorting to brand names.
Watch out for marketing
Avoid repeating manufacturer slogans about "the most natural gel" or "the one that lasts longest". Those terms promise results and can mislead the patient. Prefer to talk about general characteristics and indication, in plain language.
In Harmy
Harmy does not model product properties. The projection shows an illustrative image of the volume marked per area, in ml, without representing the firmness or behavior of any specific gel.
Also called: rheological behavior, viscoelastic properties.
Frequently asked questions
Is a gel with a higher G' always better?
No. Firmness is not a synonym for quality. The choice depends on the area, the goal and the tissue, and G' is only one of the relevant parameters.