| Storage/Elastic modulus (G′) |
It measures the energy stored by the gel during deformation and is used to recover its original shape. It represents the elastic behaviour of a gel or how much it can recover its shape after shear deformation. Unit of measurement: pascal (Pa).
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| Loss/Viscous modulus (G″) |
It measures the energy lost on shear deformation through internal friction. It represents the inability of the gel to recover its shape completely after shear deformation. Unit of measurement: pascal (Pa)
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| Complex modulus (G*) |
It measures the total energy needed to deform material using shear stress. It represents how difficult it is to alter the shape of an individual crosslinked unit of filler. Unit of measurement: pascal (Pa)
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| Tangential delta (tan δ) |
It is a measure of the ratio of viscous to elastic components of G*, defined as tan δ = G″/G′ It refers to the elasticity of a filler. Measures whether a filler is more elastic (gel-like) or more viscous (liquid-like).
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| Cohesivity |
It describes the internal adhesion forces holding together individual crosslinked HA units. It is an indicator of the filler’s resistance to vertical compression or to stretching. Three different methods and units of measurement are available: (a) linear compression test, (b) average drop-weight (c) Gavard-Sundaram Cohesivity Scale. Each method has a specific unit of measurement.
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| Complex viscosity (η*) |
It is a measure of the resistance to deformation when shear stress is applied [9]. For fillers, it corresponds to the concept of “thickness” or of “resistance to flow” during the injection. Unit of measurement: pascal-second (Pa·s)
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Physicochemical characteristics
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| Swelling ratio (mL/g) or hydration capacity (mL/mL0) |
It describes the HA filler water uptake capability expressed by the volume that a determined quantity of the biopolymer contained in the filler or a determined volume of gel could reach when incubated with phosphate-buffered saline [10,11,12,13]. It is correlated with the thickness of the polymer network, with crosslinking grade, with HA concentration. Unit of measurement: mL/g or mL/mL0 (mL is volume of the fully swollen gel and mL0 is initial gel volume)
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| HA concentration (mg/mL) |
It indicates the mg of HA in 1 mL of gel, including both nonextractable (insoluble) and extractable (soluble) HA. The insoluble HA is the crosslinked component that provides long-lasting filler’s presence in the soft tissue. The soluble HA is the not crosslinked part that is easily metabolized and does not contribute to the extended duration.
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| Degree of modification and degree of crosslinking |
The degree of modification is the stoichiometric ratio between the sum of mono- and double-linked BDPE residues and HA disaccharide units. It provides the total amount of linked BDDE in comparison to total amount of HA. It describes the total change in the polymer after modification. It is indicated in percentage (%). The degree of crosslinking is the stoichiometric ratio between BDDE residues that are double-linked and HA disaccharide units. It is indicated in percentage (%). The crosslinking ratio is the ratio of crosslinked BDDE to the total number of BDDE molecules bound to HA.
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