Dermal Filler Rheology Explained: G′, G″, Cohesivity & Product Selection

Evidence reviewed: September 2026. Dermal filler rheology is one of the most useful—and most misunderstood—parts of product selection. Terms such as G′, G″, G*, tan δ, viscosity, cohesivity and cross-linking describe different aspects of gel behaviour. None should be used alone to decide which filler belongs in a lip, cheek, chin or jawline.

Quick answer: G′ describes a gel's elastic response under defined test conditions; G″ describes its viscous response; G* combines both; tan δ describes the balance between viscous and elastic behaviour. Cohesivity describes how strongly a gel mass tends to remain together, while viscosity and extrusion characteristics influence flow and handling. These properties can help predict behaviour, but rheology values from different studies or manufacturers are not automatically comparable because testing conditions and methods materially affect the numbers.

Dermal filler rheology at a glance

What is G′ in dermal filler?

G′ (G prime), the storage or elastic modulus, measures the elastic component of a viscoelastic material during oscillatory testing. In simple terms, it reflects how strongly the gel resists deformation and stores recoverable energy under the conditions of the test.

A higher measured G′ is often associated commercially with firmer gels and structural applications. That can be useful shorthand, but it is incomplete. The clinical behaviour of a filler also depends on cohesivity, viscosity, normal force, HA concentration, cross-linking technology, particle/network structure, injection plane, volume and the mechanical environment of the tissue.

Does higher G′ mean better lifting?

No—not as a universal rule. Higher G′ can contribute to resistance against deformation, but “lifting capacity” is not a single rheological measurement. Published rheology research shows that fillers with apparently similar categories can have substantially different mechanical profiles, and clinical performance cannot be reduced to one number.

A very firm gel placed in a highly mobile or superficial area may be less appropriate than a softer, more integrating formulation. Conversely, a product designed for subtle superficial integration may not provide the mechanical behaviour sought for a structural projection objective.

What is G″?

G″ (G double prime), the loss modulus, represents the viscous component of the material's response—the portion of deformation energy dissipated rather than elastically recovered. HA fillers are viscoelastic, meaning they exhibit both elastic and viscous behaviour.

G″ receives less attention in marketing, but it is part of the overall mechanical picture and contributes to G* and tan δ.

What is G*?

G* (complex modulus) describes the total magnitude of the material's viscoelastic response and is mathematically derived from G′ and G″. It should not be confused with G′ alone.

Two fillers can differ in how their total mechanical response is divided between elastic and viscous components, which is why interpreting one headline value in isolation can be misleading.

What does tan δ mean?

Tan δ is the ratio G″/G′. It describes the relative contribution of viscous and elastic behaviour. A lower tan δ indicates a more predominantly elastic response under the particular measurement conditions; a higher value indicates a relatively greater viscous contribution.

Again, this is a material descriptor—not a treatment-area prescription.

What is filler viscosity?

Viscosity describes resistance to flow and changes with shear conditions. HA fillers are generally non-Newtonian, so their apparent viscosity is not necessarily constant. This matters because material experiences very different forces while being pushed through a needle or cannula compared with after placement in tissue.

Practitioners should therefore be cautious when a single viscosity value is presented as though it completely predicts injection feel or in-vivo behaviour.

What is cohesivity?

Cohesivity describes the tendency of a filler to remain together as a gel mass rather than disperse or fragment. It is distinct from elasticity. A filler can be highly cohesive without simply being “high G′”, and vice versa.

Several laboratory methods have been developed to assess cohesivity, including visual scales. Different methodologies make direct brand-to-brand comparisons difficult unless products were tested under the same conditions.

Cross-linking: why HA concentration alone does not tell the story

Native hyaluronic acid is rapidly degraded. Dermal fillers modify HA—commonly through cross-linking—to create a gel with greater persistence and altered mechanical properties. Manufacturers use different HA sources, concentrations, molecular weights, cross-linking processes and proprietary technologies.

More HA per millilitre does not automatically mean a stronger, better or longer-lasting filler. Network architecture and manufacturing process matter. Likewise, nominal cross-linking information cannot by itself predict clinical performance.

Why rheology numbers from different manufacturers may not be comparable

This is one of the most important points for practitioners comparing product brochures. Rheological measurements depend on the test protocol: temperature, frequency, strain, geometry, instrument, sample preparation and whether testing remains within the material's linear viscoelastic region can all affect the reported result.

Peer-reviewed comparative studies therefore provide the most useful comparisons when several fillers are tested on the same instrument using the same protocol. Comparing one manufacturer's G′ value from one method with another manufacturer's number obtained under different conditions can create false precision.

Rheology and filler selection by treatment area

The following is a clinical reasoning framework, not a universal product map. Product IFUs, patient anatomy and practitioner training take precedence.

Lip filler rheology: why “soft” is also too simplistic

Lips are mobile structures with multiple possible treatment objectives: contour, projection, body, hydration appearance and asymmetry correction. A useful lip product needs to behave appropriately during movement as well as at rest.

Choosing solely by a “soft filler” label ignores cohesivity, tissue integration, swelling, technique and patient anatomy. Excess volume and inappropriate placement can also contribute to an overfilled appearance or migration. See Lip Filler Migration: Causes, Prevention & When to Dissolve.

Cheek, chin and jawline: structure is more than G′

Structural facial treatments often require resistance to deformation, but projection depends on the interaction between the gel, injected volume, tissue plane, surrounding anatomy and placement pattern. G′ can inform product selection without becoming the entire decision.

This becomes especially important in whole-face planning. Read Profile Balancing with Dermal Fillers for the broader assessment framework.

Does rheology affect needle and cannula choice?

Product flow and extrusion characteristics interact with needle/cannula dimensions. Narrower lumens can increase the force required to deliver a viscous gel, but device choice must also account for anatomy, intended plane and manufacturer instructions.

Do not change to a non-recommended device simply to overcome injection resistance. See Cannula vs Needle for Dermal Fillers.

Extrusion force: why injection feel is not the same as G′

Extrusion force describes the force required to expel a filler through the delivery system. It is affected by the gel, syringe, needle/cannula dimensions and injection rate. A filler that feels harder to push is not necessarily one with the highest G′.

This distinction matters because tactile injection experience is sometimes incorrectly used as a proxy for rheological firmness.

Does rheology determine vascular-occlusion risk?

No rheology value makes a filler vascular-safe. Vascular complications are influenced by anatomy, tip position, injection behaviour, volume, pressure and other procedural factors. Material properties may affect how a filler behaves after intravascular entry, but selecting a lower or higher G′ product is not a substitute for vascular-risk management.

Practitioners should maintain an established emergency pathway. See Managing Vascular Occlusion After HA Filler.

Seven rheology mistakes practitioners should avoid

1. “Higher G′ means a better filler.” It does not.
2. Comparing numbers from unrelated laboratory methods. Test conditions matter.
3. Using HA concentration as a quality ranking. Manufacturing architecture matters too.
4. Treating cohesivity and G′ as the same property. They are not.
5. Assuming injection force reveals firmness. Extrusion is influenced by the whole delivery system.
6. Choosing a filler by treatment-area marketing alone. Define the anatomical objective first.
7. Ignoring the IFU. Rheological reasoning does not override manufacturer indications and instructions.

A practitioner buying framework

When comparing dermal fillers for clinic stock, ask:

1. What clinical role is missing from the current portfolio?
2. Is the objective structural projection, contour, dynamic integration or superficial refinement?
3. What peer-reviewed rheological data exist, and were competing products tested under comparable conditions?
4. What does the manufacturer IFU specify for indication, plane and delivery device?
5. How does the product's rheology fit the anatomy rather than merely its marketing category?
6. Does the range allow the clinic to cover distinct objectives without unnecessary duplicate stock?
7. Is the practitioner trained and comfortable with the product and its complication profile?

How should a clinic build a dermal filler portfolio?

A strong clinic formulary does not need every available filler. It needs meaningfully different tools. A rational range may include products suited to dynamic lip treatment, structural projection and contouring, and softer tissue integration—selected from product-specific evidence and manufacturer guidance.

Explore the Longeva professional dermal filler range →. For brand-level decision support, read Juvéderm vs Restylane and HA vs Non-HA Dermal Fillers.

Frequently asked questions

What does G′ mean in dermal fillers?

G′ is the elastic or storage modulus measured during rheological testing. It describes the elastic component of a gel's response to deformation under defined test conditions.

Is a high G′ filler better?

No. A higher G′ may be useful for some structural objectives but inappropriate for others. Product selection depends on the complete material profile, anatomy, plane and treatment endpoint.

What is the difference between G′ and cohesivity?

G′ describes elastic response; cohesivity describes the tendency of a gel mass to remain together. They are separate characteristics.

Which filler rheology is best for lips?

There is no universal numerical specification. Lip treatment requires consideration of dynamic movement, integration, cohesivity, swelling profile, desired endpoint and the product IFU.

Which rheology is best for chin and jawline?

Structural objectives often favour products designed to resist deformation and maintain projection, but no single G′ threshold defines an appropriate chin or jawline filler.

Can I compare G′ numbers between brands?

Only cautiously. Results are most comparable when products are tested using the same methodology and conditions.

Does more cross-linking mean longer-lasting filler?

Not as a simple rule. Degradation and clinical persistence are influenced by multiple features of the HA network, manufacturing technology, placement and patient factors.

Practitioner takeaway

Rheology turns product selection from “which brand do I usually use?” into a more sophisticated question: what material behaviour does this anatomical objective require?

G′ matters—but so do G″, G*, tan δ, viscosity, cohesivity, cross-linking, extrusion behaviour, tissue plane and product-specific evidence. The strongest purchasing decisions use these properties together, recognise the limitations of cross-study comparisons and still place patient anatomy and manufacturer guidance first.

Browse professional dermal fillers at Longeva Pharma →

References and further reading

• Fundamentals of hyaluronic acid filler rheology and physicochemical properties.
• Rheologic and physicochemical characteristics of hyaluronic acid fillers.
• Rheologic and physicochemical properties of hyaluronic acid dermal fillers: links to clinical performance.
• Cohesivity of hyaluronic acid fillers and implications for product characterisation.
• Review of rheological properties and clinical applications of hyaluronic acid dermal fillers.

Professional information only. Intended for appropriately qualified healthcare and aesthetic professionals. Rheological properties do not replace manufacturer IFUs, formal injection training, individual patient assessment or complication-management protocols.