. 2022 Feb 3;38(2):116–123. doi: 10.1055/s-0041-1741560

Table 1. Rheologic and physicochemical characteristics of HA fillers measured in vitro.

Parameters	Definitions	Relevance
Complex modulus ¹¹ ¹⁷ ²⁰	G*, or hardness, measures overall viscoelastic properties of a gel.	For most HA fillers, G* and G' are similar. The value of G* is derived from the formula .
Elastic modulus ¹¹ ¹⁷ ²⁰	G′, or elasticity, measures the elastic properties of the gel and its ability to recover its shape after shearing stress is removed.	The most common descriptor for HA fillers, G′ is a measure of the strength (firmness). G′ is influenced by the degree of crosslinking and total HA concentration.
Viscous modulus ¹¹ ¹⁴ ¹⁷ ²⁰	G″, or loss modulus, measures the viscous properties of the gel and its inability to recover its shape.	HA fillers tend to have low G″.
Tan delta (δ) ¹¹ ¹⁷ ²⁰ ²⁴	Tan δ is the ratio between the viscous and the elastic components of the HA gel (G″/G′).	Tan δ characterizes whether the gel is more viscous or more elastic (proportion of G″ to G′). Tan δ is usually low in crosslinked HA fillers, meaning that the elastic behavior under low shear stress is dominant over the viscous behavior.
Gel cohesion (cohesivity) ¹¹ ¹⁷	Cohesivity measures the resistance to vertical compression/stretching.	This property characterizes how a filler behaves as a gel deposit once it is injected and subjected to forces. Gel cohesion is influenced by HA concentration and the crosslinking and sizing/homogenization of the gel.
Water uptake ⁷ ¹¹	Water uptake, or swelling factor, measures the ability of the gel to swell from water uptake.	Water uptake/swelling factor helps anticipate the initial volumization of an implanted gel. It is influenced by the degree of crosslinking and HA concentration.
HA concentration ⁷ ¹¹	This parameter is the total amount of HA found in the filler, expressed as mg/mL, and includes insoluble and soluble HA.	Insoluble HA is the crosslinked HA and the foundation for the effectiveness and durability of the filler. Soluble HA is the noncross-linked and rapidly degradable form of HA (from HA fragments, or usually added for facilitating extrusion). HA concentration impacts all the parameters.

Abbreviation: HA, hyaluronic acid.