Rheology Decoded: The Happy Injector’s Guide to Choosing the Right Dermal Filler for the Right Job

Felicity McGuire
Jul 16
4 min read

Have you ever wondered why one dermal filler lifts beautifully while another spreads too easily?

Or why some fillers glide smoothly through the syringe while others seem to fight back?

That is the magic (and science) of rheology—the study of how materials flow and deform under stress. While it might sound like something best left to scientists in lab coats, rheology is something you consider every day in the clinic, whether you realise it or not.

This guide is here to change the way you think about fillers forever. We’ll break down the core rheological concepts into bite-sized, relatable chunks, using jelly babies, honey pots, and pillow faces to help you visualise exactly what’s happening beneath your patients’ skin. By the end, you’ll not only understand the ‘why’ behind your filler choices, you’ll also be equipped to make even better ones.

What Is Rheology (and Why Should You Care)?

Rheology refers to how a substance responds to forces like pressure, stretching, or movement, and dermal fillers deal with all these forces once they’re injected.

In fact, fillers are expected to handle an impressive list of demands:

Flow easily through a fine needle (this is shear stress).
Stay exactly where you put them (this involves cohesion and yield stress).
Spread smoothly into the tissue to achieve natural contours (spreadability).
Resist continuous pressures from muscle movement, gravity, and external forces like sleeping on one’s side (this involves viscoelasticity and compression resistance).

In other words, every time you pick up a syringe, you’re already making rheology-based decisions.

The Forces at Play: Shear, Compression, and Stretching

Let’s start with what your filler experiences after injection:

1. Shear Forces

These occur when skin layers slide over each other, often in highly mobile or superficial areas like the perioral region, jawline, or cheeks.

2. Compression Forces

Think of the pressure from muscles during talking, chewing, or even sleeping with your face on a pillow. Areas like the chin and nasolabial folds are under constant compression.

3. Stretching Forces

Every smile, frown, or raised eyebrow stretches the skin—and the filler within it.

These forces aren’t occasional—they’re constant. That’s why understanding how fillers behave under stress is the key to achieving long-lasting, natural-looking results.

The Core Rheology Players: What Every Injector Should Know

1. Viscoelasticity: The Bounce-Back Factor

Viscoelasticity refers to the balance between two opposing properties:

Elasticity (the ability to bounce back to shape).
Viscosity (the tendency to flow or spread).

Fillers display both traits to varying degrees, depending on their design. To measure viscoelasticity, scientists use oscillatory testing, which gently wobbles the filler to simulate the forces it will face under the skin.

This testing produces key values:

G’ (Elastic Modulus): How well the filler resists deformation. High G’ fillers hold their shape like a gummy bear.
G” (Viscous Modulus): How easily the filler flows under pressure, like thick honey.
G* (Complex Modulus):** The combined measure of overall resistance to movement—imagine a jelly baby with both bounce and wobble.
Tan δ: The ratio of G” to G’. It shows whether the filler behaves more like an elastic solid or a viscous liquid.

Why It Matters:

High G’ fillers provide structure and lift—perfect for cheeks, jawline, or chin.

Low G’ fillers are soft and spreadable—ideal for lips or fine lines.

Tan δ < 1 means the filler holds its shape (more elastic).

Tan δ > 1 means it spreads and blends easily (more viscous).

2. Compression Resistance: Holding Strong Under Pressure

Compression resistance tells you whether a filler can withstand the daily vertical pressures it faces—especially in high-load areas.

These forces are most significant in the:

Chin
Jawline
Nasolabial folds
Cheeks (especially in side-sleepers!)

Fillers designed for these areas undergo compression testing using advanced lab equipment that simulates these downward forces over time.

One landmark study in 2016 (Molliard et al.) highlighted the importance of compression properties for understanding how fillers behave in vivo. In simpler terms, if a filler can’t handle compression, it won’t last long in the face.

3. Cohesion & Crosslinking: Will It Stay or Spread?

Cohesion describes how well a filler sticks together within itself—think of it like the inner glue of the gel.

In Practice:

High Cohesion: The filler holds together firmly—great for building projection in areas like the chin or cheeks.
Low Cohesion: The filler spreads more easily—ideal for superficial smoothing or delicate blending in areas like the tear trough or forehead.
Cohesion is closely related to the crosslinking process—a chemical technique that creates bonds within the filler’s hyaluronic acid structure. The more tightly crosslinked the filler, the more cohesive (and typically firmer) it becomes.

4. Yield Stress & Shear Thinning: The Needle Test

Here’s where injectors really feel rheology in action—at the syringe tip.

Yield Stress:

This refers to the force required to get the filler moving through the needle.
Low yield stress
means the filler flows easily but may spread too much afterward.
High yield stress makes the filler harder to inject but helps it hold shape better in the tissue.

Shear Thinning:

Many fillers exhibit shear thinning—they become less viscous as more force is applied:

High shear force during injection → smoother flow.
Resting in the tissue (low shear) → thicker, more stable.
The best fillers strike a careful balance between these properties, giving injectors control during placement while maintaining a natural result post-injection.

How to Match Filler Properties to Facial Zones

Here’s a practical reference for selecting fillers based on their rheological behaviour and the forces at play in different facial areas:

In Summary – Rheology Is the Secret to Consistent, Confident Results

You don’t need a PhD to understand filler rheology, but knowing how your products behave under pressure, movement, and time can transform your injecting.

Here’s what to remember:

Every face and facial zone presents unique mechanical challenges.
Rheology helps you personalise your treatments beyond brand names.
Focus on forces first, then pick the product to match.
Don’t be swayed by marketing; be guided by behaviour.

Before Your Next Treatment, Ask Yourself:

What forces will this filler face here—shear, compression, or stretching?
Am I aiming for structure, lift, spread, or subtle blending?
Does this filler have the right combination of elasticity, cohesion, and flow?

By approaching every treatment with these questions, you’ll make smarter filler choices, leading to more predictable, long-lasting, and natural-looking outcomes for your patients.

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