For qualified practitioners only
Two dermal fillers can share the same hyaluronic acid concentration and perform nothing alike in tissue. Concentration tells you how much material is present. It tells you very little about how that material will behave once it is under the skin, moving with a face that talks, chews and smiles.
What explains the difference is rheology: the study of how a material flows and deforms. For an injector, rheology is the closest thing there is to a specification sheet for behaviour.
Three measurements carry most of the story.
Viscosity: how the gel resists spreading
Viscosity describes a gel's resistance to flow. Measured at low shear, it tells you what the product does after it leaves the needle and before the tissue settles around it.
A high-viscosity gel stays broadly where it is placed. A low-viscosity gel spreads and integrates more readily into the surrounding tissue.
Neither is better. They are answers to different questions. Placing a firm gel in a fine, superficial plane risks visibility and irregularity. Placing a thin, spreadable gel where you need projection means it moves away from the point you were trying to build.
Complex modulus: how the gel resists deformation
Complex modulus, written G*, describes the gel's overall stiffness, its resistance to being deformed when force is applied to it. In the face, that force is constant: muscle movement, sleeping position, the weight of the tissue above.
A higher modulus gel holds its shape against that load and supports the tissue sitting on it. A lower modulus gel yields, moves with the face, and stays soft under the fingertips.
This is the property most closely tied to lift and projection, and the one most often reduced to a single number in marketing. It should not be read alone.
Phase angle: how elastic the gel actually is
Every viscoelastic material sits somewhere between a solid and a liquid. Phase angle, written as delta (δ), expresses where.
A low phase angle means the gel behaves mostly like an elastic solid. It stores energy and returns to shape. A high phase angle means more viscous, more fluid behaviour. It flows and dissipates energy instead.
Phase angle tells you whether a gel's stiffness comes from genuine elasticity or simply from thickness. Two gels can share a modulus and behave differently in the hand and in the tissue because their phase angles differ.
The Intraline M Series, measured
The following values come from independent testing of the Intraline M Series.
| Viscosity (Pa·s at 0.01 s⁻¹) | Complex modulus G* (Pa) | Phase angle δ (°) | |
|---|---|---|---|
| Intraline M2 Plus | 536 | 78.3 | 28.5 |
| Intraline M3 Plus | 3,880 | 210 | 13.1 |
| Intraline M4 Plus | 5,792 | 316 | 9.1 |
Read across the range and a clear progression appears. Viscosity rises roughly elevenfold from M2 Plus to M4 Plus. Complex modulus rises fourfold. Phase angle falls by about two thirds.
That is the material moving, step by step, from soft and spreadable to firm and elastic.
M2 Plus has the lowest viscosity and the lowest modulus in the range, with the highest phase angle. It is the most fluid and the most forgiving of the three, made for delicate, superficial work where integration matters more than support.
M3 Plus sits in the middle on every measure. Enough structure to hold, enough movement to look natural in an animated face.
M4 Plus has the highest viscosity and modulus and the lowest phase angle. It is the most elastic and the most resistant to deformation, the choice when the indication calls for projection and structural support.
You may also see us quote G′, the elastic part of the modulus: 37, 166 and 254 Pa for M2, M3 and M4 Plus at 1 rad/s. It tells the same story. For which product to use where, see our guide, M2, M3 or M4 Plus? How to Choose the Right Intraline Filler.
Why we publish this
Rheology is not a ranking. There is no best gel, only a gel suited to the plane, the tissue and the indication in front of you. A practitioner choosing on concentration alone, or on a single modulus figure quoted without context, is choosing on incomplete information.
We publish our rheological data because the alternative is asking you to take our word for it, and we would rather not. It is the same reason our pricing is what it is: we run a lean, family-owned team with no sales reps, and we put the difference into the product and into supporting the practitioners who use it.
Exceptional Quality. Honest Pricing. Integrity.
Methodology. Testing was performed by the Centre for Industrial Rheology on a DHR2 rotational rheometer (TA Instruments) with a 40 mm sandblasted plate geometry, 500 μm gap, at 25°C. Viscosity values are the mean of two runs at a shear rate of 0.01 s⁻¹. Complex modulus and phase angle were derived from oscillation stress sweeps at 1 Hz. Full report available on request.
For qualified practitioners only. Product selection remains a clinical decision. Rheological data describes material behaviour and does not predict outcomes in any individual patient.