Rheology
G′, G″, tan δ — why the same millilitre behaves differently in different planes
Two syringes. Same volume. Same molecule. One builds a chin that holds for eighteen months; the other turns a tear trough into a lumpy blue shelf. The difference is not skill. It is rheology.
Rheology is the physics of how a gel deforms under load. Facial tissue is a mechanical environment — it is compressed by chewing, stretched by smiling, and pulled by gravity all day. A filler is a material implanted into that environment. Choosing one without understanding its mechanics is like specifying concrete by colour.
This chapter is the technical partner to deep fat compartments and the papillary dermis: those chapters tell you which plane; this one tells you what to put in it.
I once watched a very good injector blame their technique for a lumpy tear trough for a full year. The technique was fine. The gel had a G′ built for a chin.
G′, G″ AND TAN Δ DEFINED
Three numbers that tell you where a gel belongs.
G′ — the elastic (storage) modulus. How much the gel resists deformation and springs back. High G′ means firm, structural, resistant to compression. This is your projection and lift parameter. Measured in pascals (Pa).
G″ — the viscous (loss) modulus. How much the gel flows and dissipates energy rather than storing it. High G″ means the material behaves more like a thick liquid than a solid.
G* — the complex modulus. The vector sum of G′ and G″; total resistance to deformation. Useful as a single "firmness" figure.
tan δ — the ratio G″ / G′. The single most useful number nobody quotes. A tan δ below about 0.3 means the gel is predominantly elastic — solid-like, structural, stays where you put it. Above about 0.5 it is predominantly viscous — soft, spreading, integrating, moving with tissue.
"G′ tells you what a filler resists. tan δ tells you what a filler is."
COHESIVITY, SWELLING AND CONCENTRATION
The properties everyone confuses with G′.
Three further properties matter clinically and are frequently confused with G′.
- Cohesivity is how strongly the gel holds itself together as a single mass. A high-cohesivity gel stays as a defined bolus and projects; a low-cohesivity gel fragments and spreads. Two products can share a G′ and behave completely differently because of cohesivity. Cohesivity, not G′, is what makes a filler suitable for a periosteal bolus at the chin.
- Swelling capacity (hydrophilicity) is how much water the gel draws in after placement. High-swelling products keep growing for one to two weeks — which is desirable in lips and disastrous in a tear trough. The classic "it looked perfect on the day" complaint is usually a swelling-capacity error, not a volume error.
- HA concentration (mg/ml) correlates loosely with duration but not with firmness. A 24 mg/ml lightly cross-linked gel is soft; a 20 mg/ml heavily cross-linked gel is firm. Cross-linking density and technology matter more than concentration.
MATCHING RHEOLOGY TO PLANE
Firm gels go deep. Soft gels go shallow. That's the whole religion.
| Plane | Mechanical demand | Target G′ | Target tan δ | Swelling |
|---|---|---|---|---|
| Intradermal / papillary | Hydration, light scatter, no projection | Very low (< 50 Pa) | High (> 0.5) | Low preferred |
| Deep dermis | Fine line correction, blending | Low (50–150 Pa) | Moderate–high | Low |
| Superficial subcutaneous | Soft volume, glide, must move with expression | Low–medium (100–250 Pa) | Moderate | Low–moderate |
| Lip submucosa | Constant deformation, must not fracture | Low (80–200 Pa) | Moderate, high cohesivity | Consider carefully |
| Deep fat / supraperiosteal | Projection under load, resists compression | High (300–800 Pa) | Low (< 0.3) | Low, high cohesivity |
| Tear trough | Thin skin, unforgiving, static | Low (< 150 Pa) | Moderate | Lowest available — non-negotiable |
| Nose (dorsum, cannula) | Precision, must not spread laterally | Medium–high | Low, very high cohesivity | Minimal |
Read the table backwards and it becomes a diagnostic tool. If a result looks lumpy and visible, you probably used too high a G′ too superficially. If it looks flat and disappeared in four months, you used too low a G′ too deep.
PRODUCT COMPARISON TABLE
Relative positioning, not gospel specification.
Published rheology figures vary with test frequency, temperature and laboratory method, so treat the values below as relative positioning rather than absolute specification. Always cross-check against the current manufacturer data for your market.
| Product family | Technology | Relative G′ | Character | Typical plane |
|---|---|---|---|---|
| Juvéderm Voluma / Volux | Vycross | High to very high | Highly cohesive, projecting, notable water uptake | Deep fat, supraperiosteal, jaw and chin |
| Juvéderm Volift / Volbella | Vycross | Low to medium | Smooth, integrating; Volbella is the low-swelling fine gel | Subcutaneous, lips, perioral |
| Restylane / Lyft (classic) | NASHA | High | Firm, particulate, minimal spread, strong projection | Deep structural, nose, chin |
| Restylane Refyne / Defyne / Volyme | OBT / XpresHAn | Low to medium | Flexible, stretches with expression, low swelling | Dynamic areas — nasolabial, marionette, lips |
| Belotero Balance / Soft | CPM | Very low | Exceptional dermal integration, very low Tyndall risk | Dermis, tear trough, fine lines |
| Belotero Volume / Intense | CPM | Medium to high | Cohesive with good tissue integration | Deep fat, midface |
| Teosyal RHA 1–4 | Resilient HA | Low (RHA1) to high (RHA4) | Designed for stretch and recovery under movement | Graded by number, dermis through deep fat |
| Skin boosters (Profhilo, Restylane Skinbooster) | Non- or lightly cross-linked | Negligible | No projection; biostimulatory and hydrating | Dermal and immediate subdermal |
| Radiesse (CaHA) | Microspheres in gel carrier | Very high undiluted | Rheology changes entirely on dilution; not reversible | Subdermal and supraperiosteal only |
HOW TO READ A MANUFACTURER DATASHEET
Read the test frequency before you read the number.
- Find the test frequency. G′ is usually quoted at 1 Hz or 5 Hz. Numbers taken at different frequencies are not comparable.
- Find the temperature. Body temperature (37 °C) values differ from room temperature values.
- Ignore "cross-linking percentage" as a marketing claim. It correlates poorly with clinical behaviour.
- Look for cohesivity data. Most manufacturers omit it, which tells you something in itself.
- Ask about water uptake. If the representative cannot tell you the swelling factor, do not use it in a tear trough.
- Check extrusion force. High extrusion force through a 27G cannula means you will push harder — and pressure is a safety variable, not just a comfort one. See complications and vascular occlusion.
THE FOUR CLASSIC RHEOLOGY ERRORS
Four ways to make an expensive gel look cheap.
1. High G′ in thin skin. Visible ridges, palpable edges, Tyndall. The tear trough and the perioral skin punish this within a fortnight.
2. Low G′ deep. A soft gel placed supraperiosteally is compressed flat by the tissue above it. The patient sees nothing, you conclude they "need more", and you have created a volume habit.
3. High swelling in a static area. Under-eye and pyriform regions have nowhere to expand into. Choose the lowest-hydrophilicity product you have access to.
4. Rigid gel in a mobile area. A high-G′, low-tan-δ gel in the nasolabial fold or the lip fractures under repeated deformation, producing lumps and sometimes accelerating migration. Dynamic zones want flexibility, not firmness.
Final Word
You do not need to memorise pascals. You need to be able to look at a syringe and answer three questions: does it resist compression, does it move with the tissue, and how much water will it pull in? Those three answers place the product in a plane, and the plane is the treatment.
Depth chooses the product. The product does not choose the depth.
Questions Injectors Actually Ask
Short answers. The long ones are above.
- What is G prime in dermal filler?
- G′ (the elastic or storage modulus) measures how strongly a gel resists deformation and returns to its original shape. High G′ gels hold projection against tissue pressure and belong deep, on bone; low G′ gels spread and integrate and belong superficially.
- What is tan delta and why does it matter clinically?
- Tan δ is G″ divided by G′ — the ratio of viscous to elastic behaviour. A low tan δ gel behaves like a solid and stays where you place it; a high tan δ gel flows and drapes. It predicts whether a product will project or blend.
- Which filler should be used in the deep versus superficial plane?
- Match rheology to plane: high G′, low tan δ, high cohesivity products supraperiosteally for structural support; mid-range gels in the deep subcutaneous plane for volume; low G′, high tan δ gels in the superficial subcutaneous and dermal planes for fine lines and skin quality.
- Is a higher HA concentration always a stronger filler?
- No. Concentration, cross-linking, particle size and cohesivity are separate variables. A high-concentration gel can be soft and a low-concentration gel can be firm; only the rheology figures on the datasheet describe how a product will behave in tissue.