138 JOURNAL OF COSMETIC SCIENCE NASHA was one of the first technologies used in the dermal filler market, having been used to cross-link Restylane fillers. NASHA fillers contain a HA concentration of 20 mg/ mL and consist of low-molecular-weight HA. Restylane fillers are more appropriate for mid-to-deep dermal injections, such as nasolabial folds.38 In this technology, a small amount of BDDE is added to the filler, meaning the degree of cross-linking ends up being a minute percentage, usually from 1% to 2% in the final product (and 10–15% in the original matrix). After the sizing process, the HA “pearls” (microspheres) obtained are suspended in either a phosphate-buffered solution or noncross-linked HA gel. According to a 2016 study by Micheels et al., NASHA fillers are noncohesive and biphasic.34 HA Concentration. Total HA concentration corresponds to the amount of HA per mL of final product, and it is usually expressed in mg/mL.14,18 Other than the cross-linking degree and molecular weight, the manufacturing process and total HA concentration (consisting of the insoluble portion of cross-linked HA and the soluble portion of free HA) also influence the gel’s viscoelastic and rheological properties (namely its hardness) with lower concentrations creating softer fillers.14,19,26,29 Fillers with higher HA concentrations also show higher cross- linking degrees and elastic modulus, meaning they are better able to resist deformation and support and volumize the tissues, usually needing to be placed in a deeper plane within them.24 It is important to keep in mind that even though there are listed concentrations for each HA filler, sometimes there are variations in the concentration of cross-linking agent and the percentage of cross-linked HA versus uncross-linked HA, which doesn’t contribute to the clinical outcome. The same can happen with different batches of the same filler brand. Because of this, the listed concentrations may not always be completely indicative of a product’s true performance. Still, there are some brands that share the cross-linking percentage for distinct batches, making it easier for clinicians to correctly choose the most appropriate one for their patients.32 Particle Size. After a HA filler is cross-linked, it presents itself as a large gel mass. However, it must be able to deform itself to pass through fine-bore needles into the skin. For this to happen, the filler must go through a sizing process, which includes passing it through several sieves. Only after this process is completed, do the HA fillers contain gel particles of a defined average size making it possible for them to flow as easily as possible through a needle. Once the filler is injected, it must regain at least part of its original structure so that it can sustain the tissues (possible thanks to the G’).14,18,22 Fine-bore needles represent an advantage in reducing negative side effects such as edema, pain, tissue trauma or bleeding.18 Depending on the sieving method utilized, different dermal fillers can have different particle sizes if they never surpass a stipulated maximum size, beyond which the gel particles could clog the needle. Another way of sizing a gel is through a homogenization process. This results in a smoother and softer gel, thanks to its broader distribution of particle sizes, and lower G’ values. Because these gels flow easier, there is no maximum particle size that needs to be respected, as softer particles are easily deformed to pass through the needle. Thus, fillers obtained though the sieving process tend to show higher viscosity and to need a higher extrusion force, that is unless they have uncross-linked HA in their composition.14,18,22 Still, even when the average particle size is smaller (which would make for lower extrusion forces), there may be sporadic flow of the product through the needle if there is still a considerable number of bigger particles. Because of this, whether a gel is firmer or softer, it is better for the particles’ sizes to be uniform.18 Particle size is also a determinant property for gel hardness as bigger particles also result in a harder gel.16

139 HYALURONIC ACID AESTHETIC FILLERS Another important thing to keep in mind is that, as previously mentioned, the particle size will influence the filler’s durability. Usually, a filler made up of smaller particles degrades at a faster rate inside the body as smaller particles have a bigger total surface area available for which enzymes to attach themselves. Additionally, they also show lower volumizing abilities. Despite this, most HA fillers currently available in the market show similar particle sizes, so this may not have as big of an impact in clinical differences.14 Regardless, particle size remains one of the most important elements responsible for defining a filler’s characteristics.17 Water Absorption and Hydration Properties. Another essential property of HA to consider is its hydrophilia, with it being able to retain 1,000 times its volume in water.1,25 The presence of amine and hydroxyl groups, which form hydrogen bonds with water and negatively charge the HA, is one of the main reasons for HA’s high solubility, as it creates a viscous clear liquid when in contact with water.9,14 HA’s hydrophilic properties are what determine the gel’s capacity to absorb water and expand, which inherently links these properties to the filler’s lifting capacity.4,25,39 These properties are defined by the insoluble portion of HA.16 The lifting capacity itself depends on HA’s cohesivity, which was previously described as an important property that maintains a HA filler’s integrity and keeps it together. There is also a known connection between the elastic modulus (G’) and the filler’s water absorption capacity and, consequentially, its lifting capacity.25 It is then easy to understand that the filler’s water absorption also shows an association with HA concentration and the degree of cross-linking, since usually a higher cross-linking density/stronger gel means lower chain flexibility and a lower capacity to absorb water.2,3,14,15,18,26,29 However, studies also claim that stronger fillers usually show higher lifting capacities,3 and HA swelling caused by water uptake also depends on surrounding tissues characteristics, such as its pH.27 The swelling ratio (or gel fluid uptake) is a measurement that translates the gel’s ability to absorb water and expand its volume by binding water while remaining in one single in vitro phase (since gels can only absorb a limited amount of water, restricted by the polymer network, before becoming a two-phase system in which there would be HA gel particles suspended in excess water). This ratio is thus used to determine a gel’s hydration/saturation level.15,26,29,40 If the swelling ratio is 1, the gel is at equilibrium, and as it gets higher, the gel gets further away from equilibrium and becomes more cohesive.15,16,31 HA fillers achieve equilibrium hydration (full saturation) when a balance is struck between the elastic forces of the swollen HA and its osmotic forces. So, when a filler with a regular cross-linking degree (and around 5.5 mg of HA for every mL of water) is injected, we can consider it to be near equilibrium, which means it will not swell any further. Unlike dermal fillers with higher concentrations of free HA, which are below equilibrium and will swell more postinjection, they have the capacity to absorb water from the surrounding tissues, granting the filler its volumizing effect. This means that the higher the HA concentration, the more a filler will absorb water and swell.4,14,18,26,40 In the research, the majority of swelling ratio and water absorption capacity studies are related to different biomaterials and not always specifically related to HA fillers.2-9,13,15,20,26,29,39-43 In fact, there is little evidence comparing water absorption and expansion capacity of different HA fillers. However, it is still important to keep in mind that there is a possibility that excessive water absorption may lead to tissue trauma, overcorrection, and greater edema. This is especially important when working on dark circles as using an incorrect HA filler may lead to increased eyelid edema. In short, if a filler is more saturated (nearer to equilibrium), it