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

140 JOURNAL OF COSMETIC SCIENCE will most likely cause less edema, but a larger volume of gel needs to be injected to achieve the same results as a less saturated gel.5,27,28 CONCLUSIONS This review intended to gather the most relevant information on the properties of HA present in available literature. Besides being the most abundant polysaccharide in the human skin, HA as a biomaterial shows a promising role in different areas of medicine (most predominantly from an aesthetic point of view), with HA fillers being used to perform tissue lifting, but also being used as a healing agent, thanks to its effects on fibroblasts and other cells. Therefore, it is crucial to have an intricate understanding of HA’s rheology and physicochemical properties, which will influence its clinical outcomes. There is not one universal dermal filler adequate for all patients and anatomical areas, rather there are multiple products with slight differences in said properties. Viscoelasticity (and especially the elastic modulus—G’), cross-linking, HA concentration, cohesivity, particle size, and hydrophilic expansion are some of the most important and influential factors to consider, especially since they all impact one another. All these factors have a role in defining HA’s longevity, expansion and lifting capacity, biocompatibility, and behavior in the patient’s tissues. Despite this, there is little research that presents a truly detailed and integral description of all of HA’s characteristics. Due to this and the growing concern for aesthetics and effective tissue regeneration methods, it is important for future research to further explore this area and establish better defined protocols. REFERENCES (1) Brandt FS, Cazzaniga A. Hyaluronic acid gel fillers in the management of facial aging. Clin Interv Aging. 2008 3(1):153–159. (2) Zhao X. Synthesis and characterization of a novel hyaluronic acid hydrogel. J Biomater Sci Polym Ed. 2006 17(4):419–433. (3) Lee DY, Cheon C, Song S, et al. Influence of molecular weight on swelling and elastic modulus of hyaluronic acid dermal fillers. Polym Korea. 2015 39(6):80–976. (4) Wu GT, Kam J, Bloom JD. Hyaluronic acid basics and rheology. Facial Plast Surg Clin North Am. 2022 30(3):301–308. (5) Clark CP, III. Animal-based hyaluronic acid fillers: scientific and technical considerations. Plast Reconstr Surg. 2007 120(6)(suppl):27S–32S. (6) Wang HM, Chou YT, Wen ZH, Wang CZ, Chen CH, Ho ML. Novel biodegradable porous scaffold applied to skin regeneration. PLOS ONE. 2013 8(6):e56330. (7) Hashemi SS, Rajabi SS, Mahmoudi R, Ghanbari A, Zibara K, Barmak MJ. Polyurethane/chitosan/ hyaluronic acid scaffolds: providing an optimum environment for fibroblast growth. J Wound Care. 2020 29(10):586–596. (8) Noh I, Kim GW, Choi YJ, et al. Effects of cross-linking molecular weights in a hyaluronic acid- poly(ethylene oxide) hydrogel network on its properties. Biomed Mater. 2006 1(3):116–123. (9) Hu M, Yang J, Xu J. Structural and biological investigation of chitosan/hyaluronic acid with silanized- hydroxypropyl methylcellulose as an injectable reinforced interpenetrating network hydrogel for cartilage tissue engineering. Drug Deliv. 2021 28(1):607–619. (10) Toole BP. Hyaluronan: from extracellular glue to pericellular cue. Nat Rev Cancer. 2004 4(7):528–539.