For aesthetic healthcare professionals, choosing a dermal filler is about far more than knowing which product is labeled for a particular indication. The same facial region may respond very differently depending on the patient's anatomy, tissue characteristics, injection plane, treatment goal, and the physical properties of the filler.
One of the most useful concepts for understanding these differences is rheology - the study of how materials deform and flow when forces are applied.
Among the rheological properties discussed in filler science, G′ (G prime), the storage or elastic modulus, is particularly useful. But G′ should not be viewed in isolation. Cohesivity, viscosity, G″, tan δ, HA concentration, crosslinking technology, and the behavior of the product under clinical conditions all contribute to how a filler performs.
This guide breaks down G′ and explains how aesthetic practitioners can use rheological principles as one part of a broader, anatomy-informed approach to product selection.
What Is Filler Rheology?
Facial fillers are viscoelastic materials, meaning they demonstrate characteristics of both solids and fluids.
In the face, a filler is exposed to a constantly changing mechanical environment. Facial expression, muscle movement, tissue compression, gravity, and external forces can all deform the gel. Rheology helps us understand how a filler responds to those forces.
Several parameters are commonly used to describe filler behavior:
- G′ (storage/elastic modulus): describes the elastic component of a gel - how much energy it stores during deformation and can recover.
- G″ (loss/viscous modulus): describes the energy dissipated during deformation and reflects the viscous component.
- G* (complex modulus): represents the overall viscoelastic response.
- Tan δ: the relationship between G″ and G′ and an indicator of how predominantly viscous or elastic a material behaves.
- Cohesivity: describes the internal tendency of the gel to remain together rather than separate or disperse.
These properties are influenced by how a filler is manufactured, including its HA concentration, crosslinking technology, particle structure, and other formulation characteristics.
What Is G Prime?
G′ is a measure of elastic modulus. Put simply, it tells us how strongly a gel resists deformation and how readily it tends to recover its shape after a force is removed.
A useful mental model is a gummy bear versus honey.
Does a Higher G′ Always Mean a Better Filler?
No.
This is one of the most important concepts for clinicians to understand.
A high-G′ filler is not inherently "better" than a low-G′ filler. Instead, it may be better suited to a particular clinical objective.
Think of G′ as a tool-selection variable rather than a quality score.
If your goal is structural support or projection, resistance to deformation can be advantageous. If your goal is subtle blending in a mobile, superficial tissue plane, excessive stiffness may be counterproductive.
Published reviews generally describe higher-G′ fillers as being useful when greater resistance to deformation and projection are desirable, while lower-G′ fillers may be advantageous where softness, flexibility, and conformity are priorities.
Matching G′ to the Clinical Objective
Rather than asking, "Which filler has the highest G′?", consider asking:
"What mechanical behavior do I want from this filler in this tissue?"
Structural augmentation
When the goal is projection—for example, selected applications involving the chin, jawline, or cheek—a filler with greater elastic strength may provide useful resistance to compression and deformation.
These areas can experience substantial mechanical forces, so a product with appropriate structural characteristics may be advantageous.
Soft-tissue blending
In areas where the priority is smooth integration rather than projection, a softer, more flexible product may be preferable.
The lips are a classic example. The tissue is highly mobile, and the desired outcome is often softness and natural movement rather than rigid structural support.
Superficial correction
When treating superficial lines or transitioning between areas of different tissue thickness, a product with lower elastic strength may allow more conformability.
The objective is not simply to "fill the line." It is to create a smooth transition while minimizing visible or palpable product.
Dynamic facial areas
Areas exposed to frequent movement require a thoughtful balance between support and flexibility. A product that is too stiff for the tissue may behave differently from the desired clinical result, while a product with insufficient structural capacity may not provide the intended correction.
This is why filler selection should consider facial dynamics as well as anatomy.
G′ Is Only One Piece of the Puzzle
One of the limitations of focusing exclusively on G′ is that rheology is multidimensional.
Two fillers can have similar G′ values and still behave differently clinically because of differences in other physical properties.
Cohesivity
Cohesivity describes how strongly the gel components remain associated with one another.
In practical terms, cohesivity can influence how a filler holds together, spreads, and integrates with surrounding tissue. Higher cohesivity may be useful when maintaining a more defined implant or structural effect is desirable, while different levels of cohesivity may be advantageous in mobile tissues where moldability and integration are priorities.
G″ and viscosity
G″ reflects the viscous component of the gel.
Viscous behavior influences how the filler responds to deformation and contributes to properties such as flow and injectability. A product's behavior during injection is therefore not adequately described by G′ alone.
Tan δ
Tan δ provides information about the relative contribution of viscous versus elastic behavior.
Rather than memorizing isolated numbers, clinicians can use these concepts to develop a more complete picture of how a product may behave under different mechanical conditions.
Why You Should Be Careful Comparing G′ Between Brands
This is where filler rheology can become particularly confusing.
A published G′ number may appear to give clinicians an objective way to rank products. But G′ measurements are highly dependent on the testing methodology and formulation.
Differences in rheometer settings, frequency, temperature, strain, and other testing conditions can affect reported values. Furthermore, relationships between rheological measurements performed outside the body and actual clinical behavior are not always straightforward.
So rather than creating a universal list of fillers from "lowest G′" to "highest G′," clinicians should interpret manufacturer data and published studies in context.
A G′ value is most useful when you understand how it was measured and what clinical behavior it is intended to describe.
A Practical Framework for Choosing a Filler
When selecting a product, consider the following sequence:
1. Start with the anatomy
Identify the tissue layer, thickness, mobility, vascular anatomy, and structural characteristics of the treatment area.
2. Define the objective
Are you trying to:
3. Consider the mechanical demands
Ask how much compression, shear, stretching, and movement the product will experience in the intended location.
4. Evaluate the rheological profile
Look beyond G′ and consider G″, cohesivity, tan δ, and other available physicochemical data.
5. Consider injection depth and technique
The same filler may behave differently depending on where and how it is placed. Product characteristics should therefore be considered alongside injection plane, volume, instrument choice, and technique.
6. Match the product to the patient
Patient-specific anatomy and tissue quality should always take precedence over a rigid product-selection algorithm.
7. Stay within the product's approved use
In the United States, FDA-approved fillers have specific indications, and those indications vary by product. Practitioners should review the current labeling for the specific product being used.
G′ and Safety: An Important Distinction
Rheology can help us understand product behavior, but it does not make an injection safe by itself.
Dermal filler complications can include common effects such as bruising, swelling, redness, pain, and tenderness. Rare but potentially devastating complications can occur when filler is unintentionally injected into a blood vessel, including tissue necrosis, visual impairment or blindness, and stroke.
That means product selection should always be integrated with:
- Detailed facial anatomy
- Appropriate patient selection
- Knowledge of vascular anatomy
- Appropriate injection technique
- Product-specific training
- Recognition and management of complications
- Appropriate emergency protocols
Rheology is one component of clinical decision-making—not a substitute for anatomical knowledge or injection expertise.
The Bottom Line
G′ is useful because it gives aesthetic clinicians a way to think about a filler's elastic behavior and resistance to deformation.
But the best filler is not necessarily the one with the highest G′.
The more useful question is:
What combination of material properties is appropriate for this patient's anatomy, this tissue plane, and this treatment goal?
A higher-G′ product may be useful when projection and structural resistance are desired. A lower-G′ product may be more appropriate when flexibility, softness, and conformity are priorities. Between those extremes, clinicians can use the broader rheological profile—including cohesivity and viscous behavior—to make a more nuanced decision.
Ultimately, understanding filler rheology allows aesthetic healthcare professionals to move beyond memorizing product names and toward anatomy-informed, goal-directed product selection.
A Note for Aesthetic Healthcare Professionals
Filler rheology is a useful framework for understanding product behavior, but published rheological values should not be treated as direct predictors of clinical performance. Product labeling, peer-reviewed evidence, manufacturer-specific data, anatomy, technique, and individual patient factors should all inform treatment decisions.
References
- Sundaram H, et al. Basics of dermal filler rheology.
- Lee W, et al. Basic rheology of dermal filler.
- Edsman K, et al. Rheologic and Physicochemical Properties Used to Differentiate Injectable Hyaluronic Acid Filler Products.
- The Rheology and Physicochemical Characteristics of Hyaluronic Acid Fillers: Their Clinical Implications.
- U.S. Food & Drug Administration. Dermal Fillers (Soft Tissue Fillers).
