HA Filler Grades Explained: Monophasic vs. Biphasic and How to Choose

Why HA Concentration Tells You Almost Nothing

The number on the label — 18 mg/mL, 20 mg/mL, 24 mg/mL — describes how much hyaluronic acid is in the syringe. It does not describe how the HA is organised, how long it will last, how soft or firm the gel is, or where it should be placed.

A filler’s clinical behaviour is determined by three parameters that rarely appear on the label:

1. Cross-linking degree (how densely the HA chains are bonded to each other)
2. G-prime value (the elastic modulus — how much the gel resists deformation)
3. Cohesivity (how well the gel holds together rather than dispersing into tissue)

Understanding these parameters allows you to match product selection to indication — rather than relying on product names like “Deep,” “Volume,” or “Lyft” that differ between brands and reveal nothing about the underlying product architecture.

HA Cross-Linking: The Chemistry That Creates a Filler

Natural hyaluronic acid is water-soluble and biodegrades in skin tissue within days. To create a filler that lasts months, manufacturers cross-link the HA polymer chains into a three-dimensional gel network.

The BDDE Cross-Linker

The industry-standard cross-linker is BDDE (1,4-butanediol diglycidyl ether). BDDE forms covalent ether bonds between HA chains, creating a stable hydrogel scaffold. Almost all CE-marked HA dermal fillers use BDDE as their cross-linking agent.

The cross-linking process has two key variables:

Degree of Modification (DoM):
DoM measures what percentage of HA repeat units are bonded to BDDE molecules. A filler with 5% DoM has far fewer cross-links than one with 15% DoM. The practical difference: higher DoM creates a firmer, more durable gel; lower DoM creates a softer, more flexible gel that integrates more naturally with surrounding tissue.

Ratio of cross-linked to free HA:
Many fillers contain a blend of cross-linked HA (the structural scaffold) and free (uncrosslinked) HA (which contributes hydration and slide properties). The balance between these two fractions affects both the injection feel and the early post-injection appearance.

Monophasic vs. Biphasic Architecture

The terms monophasic and biphasic describe how the HA gel is structured at the manufacturing stage.

Monophasic Fillers

In monophasic production, all HA — both the fraction to be cross-linked and the free HA fraction — is combined before cross-linking. The result is a single, homogeneous gel phase. The HA chains are uniformly distributed throughout the gel without distinct particle structures.

Clinical characteristics:

• Soft, smooth injection feel
• Homogeneous distribution in tissue
• Natural-looking integration
• G-prime typically in the lower-to-medium range for surface and mid-depth indications
• Higher G-prime versions are achievable with elevated DoM for structural work

Typical indications: Fine lines, superficial lines, periorbital, skin hydration, mild contour correction.

Biphasic Fillers

In biphasic production, a highly cross-linked HA fraction is created first and then processed into discrete particles. These cross-linked particles are then suspended in a carrier gel of lower-concentration or uncrosslinked HA. The result is a two-phase system: dense gel particles dispersed in a softer carrier.

Clinical characteristics:

• More pronounced lift capacity
• Higher G-prime due to the dense particle fraction
• Firmer in-tissue feel initially
• May require more injection force than monophasic of equivalent HA concentration
• Supports the overlying tissue with a particulate scaffold

Typical indications: Mid-face volume, nasolabial folds, cheek augmentation, structural support, heavy correction.

Does Monophasic vs. Biphasic Determine Clinical Outcome?

The monophasic/biphasic distinction is an architectural characteristic, not a quality hierarchy. A well-formulated monophasic filler can achieve high G-prime values; a biphasic filler can be soft and tissue-integrative. Neither architecture is inherently superior — the final product performance depends on the complete formulation, including DoM, HA concentration, particle size (for biphasic), and gel cohesivity.

G-Prime: The Practical Selection Parameter

G-prime (G’) is the single most clinically relevant parameter for product selection. It quantifies the gel’s elastic stiffness — how much force is needed to compress the gel and how strongly it returns to shape when that force is removed.

G-prime is measured in Pascals (Pa) using oscillatory rheology.

G-Prime by Anatomical Zone

Note: G-prime ranges are approximate guidelines based on published literature and clinical consensus. Specific products vary, and injection technique significantly influences outcome regardless of G-prime value.

G-Prime vs. G-Double-Prime

G’ (elastic modulus) describes the elastic component of the gel — how much energy is stored and returned. G’’ (loss modulus, or viscous modulus) describes how much energy is dissipated as heat when the gel deforms. The ratio G’‘/G’ (called tan delta) indicates whether the gel behaves more like an elastic solid (low tan delta) or a viscous liquid (high tan delta).

For buyers, the practical implication: high G’ products hold their shape; low G’ / high tan delta products flow and integrate. Most high-G’ structural fillers also have low tan delta — they are designed to stay where they are placed.

Cohesivity: The Property No Number Captures Well

Cohesivity describes how well the gel holds together as a mass rather than dispersing into surrounding tissue. A highly cohesive gel behaves almost like a defined bolus — it can be placed precisely and maintains a shape. A less cohesive gel integrates diffusely into the tissue matrix.

Cohesivity is typically assessed in quality control by a “drop test” — dropping a sample of gel and observing whether it maintains a cohesive bead or disperses. It correlates with cross-linking architecture but is not directly captured by G-prime alone.

Clinical relevance:

• High cohesivity is desirable for structural work (chin, jaw, nose) where precise placement and shape retention are critical
• Lower cohesivity is desirable for superficial correction and skin boosters, where even tissue integration produces the best outcome
• Excessive cohesivity in the wrong plane can produce visible or palpable lumps — a risk in thin-skinned areas like the periorbital region

Reading a Technical Datasheet

When requesting product documentation from a manufacturer, the following parameters should be available in a technical specification sheet or product dossier:

A manufacturer who cannot provide these parameters on request is either unable or unwilling to be transparent about product specifications — which is itself a supplier evaluation signal.

Practical Summary

Choosing the right HA filler requires matching three parameters to the anatomical zone and desired clinical outcome:

1. G-prime — determines structural support level (low for soft, high for structural)
2. Architecture — monophasic for homogeneous integration, biphasic for particulate lift
3. Cohesivity — high for precise placement, lower for diffuse integration

HA concentration is a starting point, not a selection criterion. Two products at 20 mg/mL can have G-prime values differing by 300 Pa and clinical behaviours that are entirely different.

For a curated selection of HA dermal fillers across the full G-prime spectrum — from fine-line products to structural volumisers — see our product catalogue.