Hyaluronic acid cohesivity: what it means
Quick facts
- Definition
- Ability of the gel to stay together as a continuous mass, without fragmenting.
- Difference from G prime
- G prime measures resistance to deformation (firmness); cohesivity measures the internal bond of the gel.
- How it is assessed
- In the lab, through dispersion and drop-weight tests, with no single standard across manufacturers.
- Relevance in HOF
- Together with G prime, it helps predict projection, spread and how the gel integrates into the tissue.
- Point of attention
- Values from different manufacturers are not always comparable with each other.
Cohesivity is the ability of a hyaluronic acid gel to stay together, as a continuous mass, instead of breaking apart into particles. A highly cohesive gel behaves like a block that stays together when deposited; a poorly cohesive gel tends to fragment and spread more through the tissue. It is one of the rheological properties used to describe and choose fillers.
What is the difference between cohesivity and G prime?
This is the most common confusion. The two properties describe different things:
| Property | What it measures | Practical question |
|---|---|---|
| G prime (G') | Resistance of the gel to deformation, that is, firmness and elasticity | How well does the gel resist the pressure of the tissues? |
| Cohesivity | Internal bond of the gel | Does the gel stay together or spread? |
| Viscosity | Resistance to flow | How much force is needed to inject it? |
The properties combine. The ability to give projection and support depends on both firmness (G') and cohesivity: a firm but poorly cohesive gel may spread more than expected; a cohesive, soft gel may blend well in thin-skinned areas without giving as much projection.
Where does cohesivity come from?
It results from the manufacturing process: hyaluronic acid concentration, chain size, degree of crosslinking with agents such as BDDE and how the gel is prepared. In general, the literature describes two profiles:
- More monophasic or homogeneous gels: tend to be more cohesive and uniform.
- Particulate or biphasic gels: made of calibrated gel particles, usually described as less cohesive.
This division is a teaching simplification. Many products fall somewhere in between, and the manufacturer describes the gel's behavior in the technical documentation.
How is it measured?
There is no single standardized test. In the lab, dispersion tests (observing whether the gel breaks up in liquid) and drop-weight tests (measuring the weight of the drop that forms as the gel is extruded) are used. That is why numbers from different manufacturers are not always comparable, and tables that mix sources call for caution.
Why does it matter in HOF?
Cohesivity helps explain behaviors seen in the office:
- In thin-skinned areas, such as the tear trough and the lower eyelid, a gel that integrates evenly tends to leave less irregularity.
- In contour areas, such as the chin and jaw, a gel that holds its shape under muscle pressure is desired.
- In the lip, which moves all the time, the combination of malleability and cohesion influences feel and naturalness.
The choice of product for each area belongs to the qualified professional, after an assessment. For a view by area, see hyaluronic acid by area.
Also called: gel cohesion, filler cohesivity.
Frequently asked questions
What is the difference between cohesivity and G prime?
G prime measures how much the gel resists being deformed, that is, firmness. Cohesivity measures how well the gel holds together. A gel can be firm and poorly cohesive, or soft and highly cohesive.
Is a more cohesive gel better?
There is no absolute best. A more cohesive gel tends to stay in place like a block; a less cohesive gel tends to spread more. The choice depends on the area, the plane and the goal, at the professional's discretion.