Keywords
aesthetics - hyaluronic acid fillers - elastic modulus - rheology
Hyaluronic acid (HA) filler injections for facial rejuvenation and soft-tissue augmentation
were the second most popular nonsurgical aesthetic procedures in 2019, with 4.3 million
procedures performed worldwide, an increase of 16% from the previous year.[1] Features that may contribute to the popularity of HA filler treatments include their
biocompatibility and degradability, overall safety and tolerability, high hydrophilicity,
ease of administration, minimal recovery time, immediate results, and low incidence
of immunologic reactions.[2]
[3]
[4]
[5]
[6] HA fillers used in aesthetic indications typically consist of chemically crosslinked
HA molecules, resulting in a hydrogel that is less susceptible to enzymatic degradation
(i.e., has longer duration) and has improved rheologic properties compared with uncrosslinked
HA.[7]
[8] Variations in manufacturing processes, such as degree of crosslinking, crosslinking
conditions (temperature, pH), molecular weight of the starting HA, and post-crosslinking
modifications (sieving/homogenization, addition of lidocaine, etc.), can impact filler
characteristics.[3]
[9]
[10]
[11]
[12]
Understanding the range of HA filler products from the standpoint of their rheologic
and physicochemical characteristics can provide an initial framework for predicting
treatment outcomes[13] and assist clinicians in selecting the appropriate attributes for each treated facial
area.[11]
[14] Rheologic and physicochemical properties of HA fillers impact performance characteristics
(i.e., lift capacity, resistance to deformation, and tissue integration), which, together
with injection technique (i.e., injection plane, location, volume) and the interaction
of the filler with the surrounding tissue, may affect clinical outcomes.[15]
There are different methodologies for measuring and characterizing the rheologic and
physicochemical properties of crosslinked HA fillers; the use of standardized in vitro
assays can provide the basis for understanding how different fillers may perform under
different situations.[7]
[16] This article presents data on rheologic and physicochemical characteristics of HA
fillers using consistent methodology to allow a scientifically based comparison. Guidance
on appropriately aligning HA rheologic and physicochemical characteristics to the
facial area being treated follows, with the goal of helping clinicians make informed
decisions about HA filler selection.
Overview of the Rheologic and Physicochemical Characteristics of HA Fillers
Overview of the Rheologic and Physicochemical Characteristics of HA Fillers
HA fillers are viscoelastic materials—i.e., they demonstrate both viscous and elastic
properties when subjected to shear deformation.[17] Once injected, HA fillers encounter various forces, such as relative movement (shear)
between tissue layers (skin, muscles, fat pads, bone), gravity, and/or compression
(by overlying tissues or external pressure).[15] Therefore, assessing the behavior of fillers in response to mechanical stress provides
clinically relevant information.[13] Rheology, the study of the way a material deforms and reacts under mechanical stress,
allows for this assessment.[18] Four rheologic parameters may be used as the primary measures of a gel's viscoelastic
properties: G* (a measure of the overall viscoelastic properties), G′ (a measure of
the elastic properties), G″ (a measure of the viscous properties), and tan δ (tan
delta, a measure of the ratio of the elastic to viscous properties).[7]
[17]
[19]
[20] Using a rheometer, a twisting force is applied to a gel between two plates to measure
these parameters[12] ([Fig. 1]). The tests are performed using a range of frequencies (e.g., 0.1 to 10 Hz) to simulate
variability in the degree of dynamic movement across the face.[12]
[21]
[22] As with any analytical technique, results will depend on not only the material (i.e.,
the filler) but also the instrument used for testing and on the experimental conditions
(frequency, amplitude, plate geometry, temperature, etc.). Comparing results from
rheologic studies that have used different methodologies is challenging because it
requires good scientific comprehension and understanding of limitations; thus, care
should be taken in making comparisons across different studies.[16]
[23]
Fig. 1 Schematic representation of a rheometer in oscillation mode. The gel is placed between
two plates of defined geometry to assess elasticity (solid behavior) quantified by
the elastic modulus, or G′, indicating how much the gel can recover its shape after
shear stress. The same experiment also measures G″, the viscous modulus. From these
measured parameters, G* and tan delta (δ) can be calculated.[11]
[17]
[20]
Definitions of the most common rheologic and physicochemical properties used to characterize
HA fillers are provided in [Table 1]
[7]
[11]
[14]
[17]
[20]
[24] and discussed in greater depth below.
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.
Complex Modulus (G*)
Complex modulus, or G*, measures the overall viscoelastic properties of a gel and
is commonly referred to as “hardness.”[17] G* describes the global response of the filler to deformation, takes into account
both the elastic component (G′) and the viscous component (G″), and is derived through
the equation .[17]
[20] This parameter represents the strength of the material (hardness) or the total energy
needed to deform it.[25]
Elastic Modulus (G′)
Elastic modulus (also known as storage modulus), or G′, measures the elastic properties
of the gel, specifically the ability of the gel to regain its original shape after
deformation.[25]
[26] G′ represents the energy stored in the material and recovered once the shearing
stress is removed.[17] Elastic modulus is the most common descriptor for HA fillers and represents a solid-like
behavior that reestablishes the shape of the filler once injected.[20]
[22] Fillers with low to medium elasticity (G′) are characterized as soft fillers.[27] Most HA fillers available are predominantly elastic, with nearly equal G′ and G*
values.[20]
Many manufacturers use the degree of crosslinking and gel concentration to influence
the softness or firmness of their fillers.[7] Increasing the degree of crosslinking will increase the elasticity of the gel, thus
elevating G′.[7] As the distance between crosslinks decreases, the overall matrix strengthens and
makes the gel stiffer or firmer (higher G′).[7] Decreasing the number of crosslinks lengthens the distance between the links of
the HA molecules, allowing for less force to deform the gel and leading to a softer
and less elastic filler (lower G′).[7]
[27] In HA fillers manufactured with the same technology and with the same degree of
crosslinking, increasing the HA concentration will lead to an increased G′, resulting
in a firmer filler.[7]
G′ is traditionally viewed as an indicator of the lift capacity of a filler.[13]
[17]
[23]
[27]
[28]
[29]
[30]
[31] However, there is not always a linear relationship between G′ and lift.[13] Among fillers with similar composition or crosslinking technology, G′ has a positive
correlation to overall lift capacity, but when comparing fillers with different compositions
or different crosslinking technologies, lift does not always correlate with increasing
G′ because many other parameters also influence performance.[13]
Viscous Modulus (G″)
Viscous modulus, or G″, measures the viscous properties of the gel and represents
the energy lost during deformation.[26] Hence, it is also known as the “loss modulus.”[26] G″ describes the inability of the filler to recover its shape after the sheer stress
is removed, and it is linked to the liquid behavior of the gel, allowing the gel to
deform and flow to some extent during injection.[11]
[17]
[20] HA fillers tend to have low G″.[14] For any HA filler to be effective, it needs to be viscoelastic, i.e., viscous enough
to be injected and initially molded, but elastic enough to resist shear deformation
forces and provide a durable correction once implanted into soft tissue.[11]
[17]
[20] It is important to note that G″ is distinct from viscosity, which relates to the
flow of the filler during injection and does not impact clinical performance.[8]
[17]
Tan Delta (tan δ)
Tan δ is the ratio between the viscous (G″) and elastic (G′) components of the HA
gel (i.e., tan δ = G″/G′) and evaluates the relative contributions of each property.[11]
[17]
[20] Tan δ >1 signifies a mostly viscous filler, whereas tan δ <1 indicates a mostly
elastic filler.[18] Most HA crosslinked fillers have tan δ <1 (i.e., G′ > G″).[17] While tan δ allows an understanding of whether the filler is more elastic or more
viscous, it is important to note that it does not provide information on the actual
magnitudes of G′ and G″.[32]
Gel Cohesion (cohesivity)
Gel cohesion (also called cohesivity) represents the adhesion forces within the gel
and characterizes how a filler behaves as a gel deposit once injected, which makes
cohesivity an important property to consider in the overall behavior of a filler.[17] At the time of injection, HA fillers with lower cohesivity tend to be easier to
mold and spread more easily.[17] However, when subjected to the compressive forces of the face, fillers with lower
cohesivity do not maintain their shape and projection as well as fillers with higher
cohesivity and similar G′.[17] When high compression is applied to a low-cohesivity gel, there is a risk of detachment/separation
of gel from the original deposit, which can result in filler migration.[17] When high compression is applied to a high-cohesivity gel, the gel deposit resists
this force more easily and retains its original shape.[17] Cohesivity is a function of both HA concentration and degree of crosslinking.[17] Under the same crosslinking technologies, increasing either the HA concentration
or the crosslinking degree increases cohesivity.[17]
Manufacturers use different methods for determining the cohesivity of HA filler products,
and no standardized assay exists.[16] Compression methods, such as the compression force test or the pull-away method,
use the rheometer to measure normal force (N) by subjecting the gel to vertical compression
(attempting to simulate the compression movements of the face) or positioning the
gel between two plates pulled apart at a constant speed.[9]
[14]
[17]
[30]
[33]
[34]
[35] Other cohesivity assays include the drop weight method, wherein an HA gel is pushed
through an opening at a constant speed and its weight is determined, and the visual
shear-stressed gel method, which is based on physical handling of the gel.[34]
[35]
Water Uptake
Water uptake (or swelling factor) is a measure of the ability of the HA gel to swell
from water uptake and is a function of both HA concentration and degree of crosslinking.[7]
[11] The crosslinking technology used in the manufacture of the filler highly impacts
the filler's in vitro swelling rate, and maximum swelling depends on the crosslinking
density of the network.[10] As the number of crosslinks increases, the chains are held more tightly together,
and their flexibility in moving apart (stretching to accommodate the water) becomes
more limited, thus reducing the swelling capacity of the gel.[10] Changes in water uptake mainly occur immediately post-injection and can contribute
to the initial volumization.[13]
There are different methods for assessing water uptake,[7]
[34]
[36] all intended to determine how much water the gel will absorb under optimal conditions.
As with other measures, absolute swelling factor values depend on experimental conditions,
but a range of values may be observed among fillers.[34] It is important to note that in vitro water uptake assessments represent the maximum
ability of the gel to absorb water (unconstrained water uptake), and once HA fillers
are injected in the face, many other constraints (e.g., composition and water content
of surrounding tissues, forces acting on tissues) will limit the fillers' ability
to fully expand.[11]
HA Concentration (mg/mL)
This measure is the total amount of HA found in the filler, comprising both insoluble
crosslinked HA and the soluble HA mostly derived from noncrosslinked HA added to facilitate
the passing of the gel through a needle.[7]
[11] The crosslinking technology determines variables such as HA concentration and degree
of crosslinking.[10] Fillers manufactured using the same technology and degree of crosslinking may have
increased elasticity (G′) with increased HA gel concentrations, which yield greater
molecular entanglements.[7] Assuming consistent degree of crosslinking, initial HA molecular weight, post-crosslinking
modifications, and other conditions such as increased HA concentration will result
in greater water uptake and longer duration of the filler.[37]
Rheologic and Physicochemical Measurements of HA Filler Products
Rheologic and Physicochemical Measurements of HA Filler Products
The previously described rheologic and physicochemical characteristics are important
for developing an understanding of HA filler characteristics that allows selection
of fillers that may be suited for each indication and facial area. However, for values
to be meaningful for direct comparison, studies of the rheologic and physicochemical
properties must be conducted using consistent methodology.
To obtain information on the rheologic and physicochemical properties of HA filler
products across manufacturers, different products were tested for G′, G″, tan δ, cohesivity,
and water uptake using the standardized methods described by Hee and colleagues.[13] Briefly, fillers were tested using a rheometer at 5 Hz with 0.8% strain; resistance
to compression to assess cohesivity was measured using maximum normal force at 0.8 mm/min
for 2 minutes; and water uptake was measured by dyeing any buffer that was not taken
up by the filler gel and calculating maximum absorption ratio as the percentage difference
between initial and final gel percentage.[13]
[Table 2] reports the rheologic and physicochemical values of HA filler products obtained
using this methodology.[13]
Table 2
Rheologic and physicochemical characteristics of HA fillers (data from Hee et al[13] and data on file, Allergan Aesthetics, an AbbVie company). All products were tested
under the same conditions using the same methodologies[13]
|
Filler product name[a]
|
HA (mg/mL)
|
G'5Hz (Pa)
|
G''5Hz (Pa)
|
Tan δ
|
Cohesivity/Fn (gmf)
|
Maximum water uptake, %
|
|
Belotero Soft+
|
20
|
40
|
42
|
1.050
|
16
|
<100
|
|
Belotero Balance+ / Lips Contour
|
22.5
|
128
|
82
|
0.641
|
69
|
664
|
|
Belotero Intense+ / Lips Shape
|
25.5
|
255
|
110
|
0.431
|
115
|
700
|
|
Belotero Volume+
|
26
|
438
|
103
|
0.235
|
97
|
370
|
|
Juvéderm Ultra
|
24
|
156
|
68
|
0.436
|
96
|
580
|
|
Juvéderm Ultra XC
|
24
|
207
|
80
|
0.386
|
96
|
622
|
|
Juvéderm Ultra Plus
|
24
|
214
|
74
|
0.346
|
116
|
515
|
|
Juvéderm Ultra Plus XC
|
24
|
263
|
79
|
0.300
|
112
|
454
|
|
Juvéderm Ultra 2
|
24
|
188
|
75
|
0.399
|
95
|
574
|
|
Juvéderm Ultra 3/Smile
|
24
|
238
|
71
|
0.298
|
104
|
426
|
|
Juvéderm Ultra 4
|
24
|
164
|
66
|
0.402
|
105
|
614
|
|
Juvéderm Volite
|
12
|
166
|
30
|
0.181
|
12
|
<100
|
|
Juvéderm Volbella with lidocaine
|
15
|
271
|
39
|
0.144
|
19
|
133
|
|
Juvéderm Volift with lidocaine
|
17.5
|
340
|
46
|
0.135
|
30
|
184
|
|
Juvéderm Voluma with lidocaine
|
20
|
398
|
41
|
0.103
|
40
|
227
|
|
Juvéderm Volux
|
25
|
665
|
49
|
0.074
|
93
|
253
|
|
Restylane Fynesse
|
20
|
134
|
58
|
0.433
|
30
|
677
|
|
Restylane Refyne
|
20
|
116
|
50
|
0.431
|
49
|
516
|
|
Restylane Kysse
|
20
|
236
|
50
|
0.212
|
85
|
373
|
|
Restylane Defyne
|
20
|
342
|
47
|
0.137
|
60
|
318
|
|
Restylane Volyme
|
20
|
239
|
50
|
0.209
|
91
|
354
|
|
Restylane Vital Light
|
12
|
84
|
49
|
0.583
|
12
|
<100
|
|
Restylane Vital
|
20
|
667
|
172
|
0.258
|
27
|
<100
|
|
Restylane
|
20
|
864
|
185
|
0.214
|
29
|
<100
|
|
Restylane Lyps
|
20
|
976
|
166
|
0.170
|
31
|
<100
|
|
Restylane Lyft
|
20
|
977
|
198
|
0.203
|
32
|
<100
|
|
Restylane SubQ
|
20
|
1055
|
123
|
0.117
|
42
|
<100
|
|
Teosyal Puresense Redensity II
|
15
|
114
|
43
|
0.372
|
16
|
239
|
|
Teosyal Puresense First Lines
|
20
|
105
|
44
|
0.419
|
18
|
250
|
|
Teosyal Puresense Kiss
|
25
|
314
|
66
|
0.209
|
74
|
380
|
|
Teosyal Puresense Deep Lines
|
25
|
301
|
64
|
0.214
|
82
|
300
|
|
Teosyal Puresense Ultra Deep
|
25
|
348
|
54
|
0.155
|
87
|
250
|
|
Teosyal RHA1
|
15
|
133
|
54
|
0.406
|
22
|
260
|
|
Teosyal RHA2
|
23
|
319
|
99
|
0.310
|
77
|
420
|
|
Teosyal RHA3
|
23
|
264
|
67
|
0.254
|
109
|
427
|
|
Teosyal RHA4
|
23
|
346
|
62
|
0.179
|
115
|
366
|
Abbreviation: HA, hyaluronic acid.
a All product trade names are the property of the respective owners (Belotero products,
Merz Aesthetics; Juvéderm products, Allergan Aesthetics, an AbbVie company; Restylane
products, Galderma Laboratories, LP; Teosyal products, Teoxane Laboratories). All
products tested, except Juvéderm Ultra and Juvéderm Ultra Plus, contained lidocaine.
Selecting the Appropriate HA Filler Based on Its Rheologic and Physicochemical Characteristics
Selecting the Appropriate HA Filler Based on Its Rheologic and Physicochemical Characteristics
Considerations Pertaining to HA Filler Characteristics and Specific Facial Regions
As described, rheologic and physicochemical properties have implications for the clinical
performance of HA fillers, and their alignment to the facial area being treated can
help optimize clinical outcomes. HA fillers are expected to function not only as volumizers
in areas that have volume deficit and wrinkles or deep folds, but also to look and
feel natural, whether in static or more dynamic areas of the face.[13]
Below are guiding principles for using HA fillers in facial aesthetic correction based
on pertinent rheologic and physicochemical properties. [Fig. 2] summarizes the recommended filler characteristics for each facial region.
Fig. 2 HA filler characteristics recommended in facial aesthetics. HA, hyaluronic acid.
Upper Face
In areas of the upper face where filling and volume restoration are required, as in
the temporal fossa,[14]
[38]
[39]
[40]
[41] the HA filler should have high elasticity or resistance to deformation (G′) and
medium to high cohesivity. To address forehead contour, the filler chosen should have
a medium to high G′ and a low to medium cohesivity,[42] which would allow for molding and some degree of spread upon injection.
Midface
The infraorbital area is characterized by very thin tissue overlying bone with skin
that is only a few millimeters thick. Therefore, a filler for this area should have
low to medium elasticity or resistance to deformation (G′) and a low cohesivity for
ease of spreadability and to prevent overcorrection, lumps, and bumps.[12]
[17]
[20] Because the aesthetics of the periorbital area are highly sensitive to minimal volume
changes, a filler with low water uptake should be used to minimize the risk of swelling
and puffiness under the eyes.[40]
[43]
The zygomatic and submalar areas are subject to dynamic contraction forces of the
lip and cheek elevators. Therefore, the fillers used in these areas need to have a
medium to high elastic modulus (G′) to resist shearing and medium to high cohesivity
to withstand compression forces of the overlying tissue and maintain projection.[44]
[45]
This degree of cohesivity is essential to ensure minimal separation and avoid product
displacement that may occur after repetitive contraction of the overlying musculature.[46] To provide projection, the fillers to be used in the midface should have a high
lift capacity. Several HA filler products with the described rheologic and physicochemical
properties have demonstrated effectiveness for the treatment of the midface.[14]
Lower Face
The lower face is an area characterized by a high degree of dynamic movement; loss
of volume and structural support in this area, resulting in marionette lines, nasolabial
folds, or accordion lines, requires consideration of distinct rheologic characteristics,
such as medium elasticity (G′) and low to medium cohesivity,[17]
[47]
[48] with a moderate lift capacity. The ideal filler for this region would need to be
easily moldable, have low projection, be nonpalpable, and integrate well with facial
movement, as it will be subjected mostly to shearing and mild compression forces.
However, to correct severe folds, a filler with higher cohesivity is recommended,[14]
[17]
[47]
[48] although it could be harder to mold after injection.[17]
Lips
To enhance the lips, fillers are usually described as soft, i.e., having low to medium
elasticity (G′) and low to medium cohesivity, since the challenge in this area is
to avoid edges and bumps. Also, a low swelling factor is usually recommended to avoid
unnatural-looking results.[11]
[49] For a smoothing effect, lip fillers require lower lift capacity and easy moldability.[50]
[51] Increasing the cohesivity from low to medium or even to high will contribute to
projection and volumization.[9]
[50]
[52] There are several HA fillers with the appropriate combination of elasticity, cohesivity,
softness, and water uptake that have been shown to be effective for treating the lips.[14]
[50]
[51]
Nose, Jawline, and Chin
The chin, jaw, and nasal dorsum are areas of low shear stress but are characterized
by high compression, with taut skin and muscle over bony structures. Thus, the filler
of choice to enhance contouring and provide structure should have high elasticity
(G′) and medium to high cohesivity[42] and provide high lift capacity and resistance to deformation. Such a filler would
minimize lateral spreading and maintain a sharp vertical projection over time. Different
products with the appropriate balance of these rheologic properties have demonstrated
effectiveness for these regions in clinical trials.[14]
[42]
Fine Lines and Improvement of Skin Quality Attributes
HA filler products can improve superficial wrinkles by filling in shallow lines, thus
smoothing the skin and leading to an appearance of improved skin quality. Fillers
with low HA concentration that exhibit low to medium elasticity (G′) combined with
low cohesivity are best suited to treat superficial fine lines, such as those in the
periorbital and perioral areas.[14]
[53]
[54] As mentioned earlier, HA fillers with low cohesivity are generally easier to mold
and have increased spread in tissues. As these fillers are usually injected superficially,
they require low lift capacity, low resistance to deformation, and good tissue integration.
This type of filler will integrate well with the surrounding tissue, will perform
well with dynamic movement, and will be less likely to result in visible edges and
bumps or palpabality.[14]
Conclusion
The face is a dynamic and complex structure, and therefore the requirements for each
area of the face should be taken into consideration when choosing a filler. This overview
of the rheologic and physicochemical properties of HA fillers, together with a summary
of rheologic and physicochemical values for multiple products measured using the same
methodologies, will provide a valuable resource for clinicians. Aligning the rheologic
and physicochemical properties of HA fillers to the facial area being treated, along
with using the appropriate injection technique, can help clinicians select the right
product to achieve optimal aesthetic results.
