Rheology and Physical Properties

Rheology describes how a gel responds under a defined laboratory deformation; it does not prescribe where a filler should be placed. A useful result identifies the exact product, instrument, test geometry, temperature, strain, frequency, preparation, and reported endpoint.

Without that context, a number can look more precise than the comparison actually is.

Common measurements

Measurement	What it describes in the stated test	What it does not prove alone
`G′` — storage or elastic modulus	Energy stored and elastic response during oscillatory shear	Clinical lift, ideal plane, safety, or duration
`G″` — loss or viscous modulus	Energy dissipated through viscous response	Injectability or tissue spread by itself
`G*` — complex modulus	Combined magnitude of elastic and viscous response	Overall clinical `strength` across unlike methods
`tan δ` — `G″/G′`	Relative viscous-to-elastic contribution under the test condition	A universal soft-versus-firm product category
Complex viscosity	Resistance to oscillatory deformation under specified conditions	Force required through a particular syringe and needle
Cohesivity	Internal association assessed by a defined assay	Integration, migration, or projection without other evidence
Swelling or water uptake	Change under a stated hydration protocol	Post-treatment edema or in-vivo volume change
Extrusion force	Force measured through a specified syringe, tool, rate, and setup	Ease, accuracy, tissue trauma, or safety in clinical injection

The modulus definitions and their oscillatory measurement basis are described in filler-characterization literature; a rheometer typically deforms a sample between defined geometries and measures stress relative to strain (HA filler characterization critique 🔗).

Why the test conditions must travel with the number

HA gels are viscoelastic and their measured response can change with frequency, strain, temperature, geometry, sample handling, relaxation time, and the point selected from a sweep.

Method field	Why record it
Instrument and geometry	Plate size, gap, and surface can affect the test
Temperature	Material response is temperature-dependent
Strain or stress	Measurements should be interpreted within the tested deformation regime
Frequency or angular frequency	The same gel can respond differently across a sweep
Relaxation and loading	Sample history can influence the result
Replicates and variation	A single value hides measurement spread
Product lot and expiry	The tested sample must be identifiable

An amplitude or strain sweep is commonly used to identify the linear viscoelastic region (LVER), within which the measured response remains proportional to the applied deformation. A frequency sweep interpreted outside the established LVER may describe a network already being disrupted rather than its small-deformation response (HA filler characterization critique 🔗).

Keep frequency units explicit. Frequency in hertz and angular frequency in radians per second are related by ω = 2πf; 1 Hz and 1 rad/s are not the same test point.

One study measured eight fillers under defined settings and found published G′ differences of 1.6- to 7.4-fold for the same product, attributing much of the variation to rheometer settings (factors affecting rheological measurement 🔗). Another independent laboratory study found that G′, G″, G*, and tan δ changed across the tested angular-frequency range for all 35 products (frequency-spectrum study 🔗).

For this reason, Injectable Index will not combine isolated values from unrelated studies into a single ranking table.

Read `G′` as a measurement, not a verdict

G′ is the most commonly reported HA filler rheology value. Under oscillatory shear, it describes the elastic component of the response at the stated test conditions.

A higher reported G′ does not independently establish:

better lifting or projection in vivo
a required deep placement plane
poor suitability for a mobile area
greater longevity
lower migration risk
greater or lower hyaluronidase susceptibility
superior clinical performance

Clinical behavior also depends on the rest of the formulation, amount and pattern placed, tissue environment, anatomy, technique, and the endpoint being assessed. Even ex-vivo relationships among G′, swelling, and cohesivity became more consistent in one comparative study only when products shared concentration and crosslinking technology (18-product comparative study 🔗).

`G″`, `G*`, and `tan δ` add context

G″ describes the viscous portion of the oscillatory response. G* combines the magnitudes of G′ and G″, while tan δ is their ratio, G″/G′. The phase angle is δ; tan δ is the dimensionless tangent of that angle, not the angle itself.

These values help show whether the tested response is more elastically or viscously dominated and how that balance changes across the test. They still remain frequency- and method-dependent. A single tan δ cannot assign an exact anatomy, depth, movement profile, or outcome to a product.

Whenever possible, prefer the full frequency or amplitude sweep over one value extracted at an arbitrary point. If only one value is available, keep the chosen frequency and strain visible beside it.

Cohesivity is assay-dependent

Cohesivity is intended to describe how a gel holds together, but it has been evaluated by different methods, including visual dispersion scales, compression or stretching approaches, and drop-weight methods. These assays do not necessarily share units or measure an identical construct.

The Gavard–Sundaram scale, for example, was developed as a five-point visual grading method after dyed filler was dispersed in stirred water. Its values should remain tied to that assay rather than being merged with compression-derived measurements (cohesivity assay development study 🔗).

Avoid translating high cohesivity directly into better tissue integration, lower migration, more projection, or a preferred injection plane without product-specific clinical evidence.

Swelling and water uptake are not post-treatment edema

Laboratory swelling depends on the medium, exposure time, starting hydration state, temperature, and calculation method. A filler that takes up water in an assay is not a direct model of the magnitude or timing of clinical edema.

Record whether a source reports:

equilibrium swelling
swelling factor or ratio
water uptake by mass or volume
change from the product’s packaged state
free or unbound liquid after incubation

Do not compare results reported with different definitions as though they were one unit.

Extrusion force is a system measurement

Extrusion force depends on more than the gel. The syringe, plunger, needle or cannula, internal diameter, length, connection, extrusion speed, temperature, and test apparatus all matter.

It can help characterize a specified product–device setup. It does not by itself prove easier injection, finer control, lower complication risk, or interchangeability with another supplied system.

Minimum standard for a comparison table

Include a measurement only when the source supports these fields:

Required field	Record
Product	Exact name, variant, market, and lot if reported
Measurement	`G′`, `G″`, `G*`, `tan δ`, cohesivity, swelling, or extrusion force
Method	Instrument, geometry, assay, or device setup
Conditions	Temperature, frequency, strain or stress, rate, medium, and time as applicable
Result	Value, unit, variability, and selected point from the test
Provenance	Independent, manufacturer-funded, or manufacturer-generated
Interpretation	Laboratory finding separated from label and clinical outcome

If compatible conditions cannot be established, describe the studies separately. Use Not reported under a comparable method rather than filling a table with visually comparable but methodologically incompatible numbers.

Return to Formulation and Gel Architecture before interpreting why two products may produce different measurements.