Reading filler rheology: what viscosity, G* and phase angle tell you
For qualified practitioners only
Most dermal filler comparisons start with hyaluronic acid concentration and cross-linking. Both matter, but neither tells you how a gel will actually behave once it is in tissue: how far it spreads, how much it resists the movement of the face, or how it feels under the fingertips weeks later.
That behaviour is described by rheology, the science of how materials flow and deform. Many manufacturers now publish rheology data, which is good news for practitioners. The challenge is knowing how to read it, and how to compare one product’s figures with another’s without being misled.
This guide covers both.
Three numbers worth understanding
Viscosity is a gel’s resistance to flow. Measured at a very low shear rate, it approximates the gel at rest in tissue, after the injection force has gone. Lower viscosity gels spread and integrate into the surrounding tissue. Higher viscosity gels stay closer to where they were placed.
Complex modulus (G*) is overall stiffness: how strongly the gel resists being deformed. In the face, that deforming force never really stops, from muscle movement, facial expression and the weight of overlying tissue. A higher G* means more resistance to that load and more capacity to support tissue. A lower G* means a softer gel that moves with the face.
Phase angle (δ) describes the balance between elastic and viscous behaviour. Close to 0°, a gel behaves like an elastic solid and springs back to shape. Closer to 90°, it behaves more like a liquid and flows. Phase angle is what tells you whether a gel’s stiffness comes from genuine elasticity or simply from thickness.
No single figure describes a filler on its own. A practitioner choosing on G* alone, or on concentration alone, is working with part of the picture.
Comparing data between brands: read the method first
Rheology figures are only comparable when they were measured the same way. Before putting two products side by side, check:
Test frequency and shear rate. G* measured at 0.1 Hz and at 5 Hz can differ substantially for the same gel. Viscosity is strongly dependent on shear rate.
Temperature. Results at 25°C and at body temperature are not interchangeable.
Equipment and geometry. Plate type, gap size and instrument all influence the result.
Who did the testing. Independent laboratory data, with the method stated, is more useful than an unattributed figure in a brochure.
If the method is not published, treat the number as indicative rather than comparable. A reputable manufacturer should be happy to share the full report.
The Intraline M Series profile
The figures below come from independent testing of the Intraline M Series by the Centre for Industrial Rheology. The full method is listed at the end of this article.
Intraline M2 Plus: viscosity 536 Pa·s · complex modulus G* 78.3 Pa · phase angle 28.5°
Intraline M3 Plus: viscosity 3,880 Pa·s · complex modulus G* 210 Pa · phase angle 13.1°
Intraline M4 Plus: viscosity 5,792 Pa·s · complex modulus G* 316 Pa · phase angle 9.1°
Across the range, the gel changes in a consistent direction. Viscosity increases around elevenfold from M2 Plus to M4 Plus, complex modulus around fourfold, and phase angle falls by roughly two thirds. In practical terms, the range moves from soft and integrating to firm, elastic and supportive.
M2 Plus is the most fluid of the three, with the highest phase angle. It is designed for finer, more superficial work, typically placed in the subcutaneous plane, where smooth integration matters more than lift.
M3 Plus sits in the middle on every measure, with enough structure to hold shape and enough flexibility to look natural in an animated face. It is typically placed in the subcutaneous plane and deep fat.
M4 Plus is the most elastic and the most resistant to deformation. It is used where projection and structural support are needed, typically in deep fat or on bone.
For reference, the elastic modulus G′ at 1 rad/s is 37, 166 and 254 Pa for M2, M3 and M4 Plus respectively, which follows the same progression.
Why we share our data
Rheology is not a league table. There is no best gel, only the right gel for the plane, the tissue and the indication. We publish our figures, and the method behind them, so practitioners can make that judgement on evidence rather than on claims.
It reflects how we work more broadly. Intraline is family-owned and practitioner-focused, with a lean team and no sales force, and we put the difference into product quality and into supporting the practitioners who use it. The full M Series range is CE marked.
Exceptional Quality. Honest Pricing. Integrity.
Where to find Intraline
Intraline is used by practitioners across Europe, the Middle East and the UK.
European Union: order through our partner Fraya at frayamedsupply.com/brand/intraline
United Kingdom: order directly from our UK shop at weareintraline.co.uk
UAE and other markets: contact us at order@intraline.com and we will put you in touch with the right partner, or send you the full rheology report.
Methodology. Testing was carried out by the Centre for Industrial Rheology (June 2020, ref. J20200323) on a DHR2 rotational rheometer (TA Instruments) using a 40 mm sandblasted plate geometry with a 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.