Filler viscosity: what it is and how it differs from G prime
Quick facts
- Definition
- A material's resistance to flow: the more viscous, the more it resists flowing.
- How it is measured
- With a rheometer; in filler gels, usually as complex viscosity, in oscillatory tests.
- Difference from G prime
- G prime measures firmness (resistance to deformation); viscosity measures resistance to flow.
- Typical behavior
- Hyaluronic acid gels become less viscous when forced to flow, which makes it easier to pass through the needle.
- Relevance in HOF
- Influences injection force, flow control and how much the gel spreads in the tissue.
Viscosity is a material's resistance to flow. In a hyaluronic acid filler, it describes how much the gel resists flowing when it is pushed through the needle or cannula and when it is molded in the tissue. A more viscous gel flows with more difficulty and tends to stay where it was placed; a less viscous gel flows and spreads more easily. It is one of the measures of rheology and should not be confused with firmness.
How does viscosity differ from G prime and cohesivity?
The three measures appear together in technical data sheets, but they answer different questions:
| Measure | Question it answers | Practical reading |
|---|---|---|
| Viscosity | How much does the gel resist flowing? | Injection force, flow control, spreading |
| G prime | How much does the gel resist being deformed and return to shape? | Firmness, support, projection capacity |
| Cohesivity | How well does the gel stay together? | Integration, tendency to stay in a block or fragment |
In oscillatory tests, viscosity is usually reported as complex viscosity, calculated from G prime and G double prime (the viscous modulus). That is why firmer gels often also have higher complex viscosity, but the relationship is not fixed between different products.
What does "the gel thins when injected" mean?
Hyaluronic acid gels are pseudoplastic: viscosity drops when the gel is forced to flow quickly and rises again when it stops. In practice, the gel passes through the needle with less resistance than its viscosity at rest suggests and recovers its consistency once deposited. This property helps explain why firm gels can still be injected with fine needles.
The force needed to inject does not depend on viscosity alone. The gauge and length of the needle or cannula, particle size (in biphasic gels), the presence of free hyaluronic acid as a lubricant and temperature also weigh in.
Why does it matter in HOF?
- Control during injection: very high or irregular extrusion force makes it hard to deposit small amounts precisely.
- Spreading in the tissue: less viscous gels tend to distribute more; more viscous ones tend to stay localized.
- Choice by area: thin-skinned areas usually call for gels that integrate smoothly; structural areas usually call for gels that hold their shape.
How does this information show up in routine practice?
In practice, viscosity is felt at the plunger: more viscous gels need more force and allow less flow. When changing product or needle gauge, it is worth testing the flow before the procedure. The full picture of the properties is in hyaluronic acid by area.
Also called: complex viscosity, eta star (η*), resistance to flow.
Frequently asked questions
Are viscosity and G prime the same thing?
No. G prime describes the firmness of the gel, how much it resists being deformed and returns to shape. Viscosity describes resistance to flow. The two often go together, but a gel can be firm and flow relatively easily, or the other way around.
Does a more viscous filler last longer?
Not necessarily. Durability depends mainly on cross-linking, concentration, the area and the patient's metabolism. Viscosity influences injection and spreading more than duration.