Hyaluronic acid (HA) is a polysaccharide belonging to the glycosaminoglycans, made up of disaccharide units constituted of N-acetylglucosamine and D-glucuronic acid.
1. Introduction
Hyaluronic acid (HA) is a polysaccharide belonging to the glycosaminoglycans, made up of disaccharide units constituted of N-acetylglucosamine and D-glucuronic acid (
Figure 1). It is a component of the connective, epithelial, and neural tissues and it represents a substantial constituent of the extracellular matrix (ECM)
[1][2][3][4][5][6]. HA was discovered for the first time in the vitreous humour of the eye in 1934, and in 1964 it was synthesized in vitro
[7][8][9]. HA has a wide range of molecular weights ranging from 2 × 10
5 to 10
7 Da
[10][11][12][13]. The HA average molecular weight can influence its physico-chemical properties
[3][14].
Figure 1. Chemical structure of hyaluronic acid (HA).
Among the many biological effects, HA is involved in cell differentiation, embryological development, inflammation, wound healing, viscoelasticity, etc.
[15]. As it has been observed, the molecular mass and the mode of its synthesis or degradation define the HA biological effects
[3][16][17]. By a passive mechanism, high molecular weight HA (HMW-HA) permits the tissue hydration, contributes to the osmotic balance, and stabilizes the ECM structure. On the other hand, HA interacts with different receptor binding proteins, and its molecular weight can influence the receptor affinity or its uptake by the cells, leading to opposite effects. For example, HMW-HA inhibits the cell growth (angiogenetic activity) and protects the articular cartilage due to its lubrication properties. Low molecular weight HA (LMW-HA) has angiogenetic activity and can induce tumor progression or presents pro-inflammatory activity
[15][16]. Thus, the biological activity of HA is due to its binding to different receptors. For example, the binding HA-CD44 transmembrane receptor mediates cell adhesion and migration in many physiological or pathophysiological processes: (a) angiogenesis; (b) ECM structure (linking the HA with cytoskeleton); (c) inflammation (upregulation of the receptors overexpresses the interleukin-1); (d) wound healing; (e) malignant tumors (e.g., pancreatic, breast, lung, etc.). The CD-168 receptor (Receptor for Hyaluronan-Mediated Motility, RHAMM) localized on the cell surface has an important relevance in cell migration. When the receptor is situated intracellularly, it affects the activity of the mitotic spindle. As a result, the HA-RHAMM links can influence the inflammation and tissue repair processes. The HARE (Hyaluronan Receptor for Endocytosis) receptors modulate the glycosaminoglycans clearance. The lymph absorption of HA, implicitly the HA turnover, is controlled by LYVE1 (Lymphatic Vessel Endothelial Hyaluronan receptor-1). As a result, HA-LYVE1 interaction influences the tissue biomechanical properties, including its hydration. Referring to the HA interaction with TLRs (Toll-Like Receptor), it is noticed that LMW-HA has an inflammatory effect, because of its agonist activity on TLR-2 and TLR-4. On the other hand, a high mass of HA decreases the binding capacity to the receptors, forming a dense coat around the cell and covering the receptor surface
[15][16][18][19].
Nowadays, there are a lot of studies conducted in order to elucidate the mechanism of action and the biosynthetic pathways of HA, or to optimize its biotechnological production, in order to synthesize derivatives with superior properties and to improve its therapeutic utilization
[16].
The list of substances that are restricted or prohibited in the EU for use in cosmetic products does not include hyaluronic acid and sodium hyaluronate (NaHA). As an example, using hyaluronic acid, sodium hyaluronate, or potassium hyaluronate (KHA) in cosmetics is not restricted in Japan
[20].
Some studies realized by the Cosmetic Ingredient Review (CIR) experts panel, based on the application of cosmetic HA in various concentrations, showed acute, short-term, or chronic toxicity
[20]. Additionally, some side tolerable effects (scaling, erythema, and pruritus) were observed, after the use of a topical product with hyaluronic acid (0.01%), hydroquinone (4%), and glycolic acid (10%) for melasma treatment
[4]. Subsequently, HA and NaHA can be nebulized and used in cosmetic products which can be applied as aerosols (e.g., hair spray)
[20]. The nebulized particles cand be stored at different levels of the respiratory system, depending on their size and concentration. Because of this fact, safety assessment of cosmetic aerosols is an important issue. The protective effect of HA on the respiratory system was noticed in some studies
[21], however the propellant gas, vapors, and other soluble compounds (e.g., alkanes, alcohols, stabilization polymers, bentonite, aluminium chlorhydrate, perfume oils, cosmetic colorings, complexation agents, lanolin derivates, plant extracts, etc.) associated with hyaluronan in cosmetic aerosols could induce respiratory sensitization effects such as: rhinitis, conjunctivitis, wheeze, dyspnea, or asthma. Moreover, the insoluble particles from aerosols could be responsible for pulmonary overload, leading to chronic toxicity (e.g., chronic inflammation, fibrosis, including lung tumor). These effects are related to their concentration, exposure duration, or particle size. For example, the assessment of the inhalation toxicity of products with insoluble particles with a size below 10 μm is recommended. Regarding these aerosols, an exposure duration of 5 min is indicated, and also it is necessary to avoid the exposure to fine droplets of lipophilic substances, which could produce “acute respiratory syndrome”
[22].
Due to various biological activities, HA products are increasingly in demand. Thus, in 2016, the total market of HA (pharmaceuticals, beauty, and personal care) exceeded 141 tones and it is expected to grow more than 30% in 2021 (
Figure 2). The most significant increasing of HA market is estimated to be in Europe and Asia
[23].
Figure 2. Hyaluronic acid market-regional comparison.
Some formulations containing HA are already available on the market, with a large experience in their use. At the same time, for other products it is necessary to perform subsequent investigations to confirm their efficacy. HA is a special moisturizing active ingredient, used in cosmetics, particularly formulated as emulsions or serums, claiming hydration and skin elasticity effect. These skin biophysical parameters are closely related to anti-wrinkle effect, but no rigorous scientific evidence does justify this statement completely. Additionally, it should be taken into consideration that the efficacy of hyaluronic acid depends largely on the molecular weight
[10].
Hyaluronic acid is one of the most efficient and safe ingredients used frequently in cosmetics. HA properties can be improved by other bioactive ingredients (e.g., plant extracts, vitamins, amino acids, peptides, proteins, minerals, saccharides, probiotics, etc.). Nowadays, there are a multitude of cosmetics containing HA, marketed by different manufacturers. The previously published papers present separately these advantages of HA or bioactive ingredients. In our paper, we present firstly the biological effect of HA on skin level, after which the portfolio of some popular manufacturers was analyzed, commercially cosmetic brands and products containing HA were identified, and their declared qualitative composition was evaluated. Subsequently, the additional biologic effects and the toxicological potential of the other active ingredients were presented.
2. Use of Hyaluronic Acid in Cosmetology
Nowadays, HA is one of the most widely used active ingredients in cosmetic formulations. General perception about skin regeneration is of constant interest for both industry professionals and consumers. It is evident that the skin is an indicator of individuals’ health and HA is one of the main factors for healthy skin
[24]. As shown above, hyaluronic acid is a biopolymer considered of primary interest from a scientific point of view, due to its multitude of applications in cosmetic and biomedical fields. Such being the case, exploration on this ingredient is increasing in many interdisciplinary domains targeting, on the one hand, the improvement of production processes in terms of biotechnology and on the other hand the development of new formulations incorporating hyaluronan or HA-based innovative ingredients. Scientific efforts are moving nowadays towards the production of appropriate molecular weight biopolymers. This specific aspect relies precisely to the biological function, as indicated by bibliographic studies. Although HA was synthesized a very long time ago, it is still needed to investigate this active ingredient in terms of physico-chemical and biological properties
[16].
HA has a multitude of applications based on specific properties such as: (1) high hygroscopicity; (2) viscoelastic nature; (3) biocompatibility; (4) non-immunogenicity. Nevertheless, the HA skin penetration mechanism is still barely understood. A multitude of factors are studied, including the existence of HA receptors for an active transport and a particular structure of the hydrated HA. The general hydration effect of the skin may also optimize dermal absorption of active ingredients and can assist their retention within the moisturized epidermal layers. HA is appropriate for biomacromolecules because it ensures protein stabilizing properties. However, the precise mechanism for the transdermal transport of HA remains to be elucidated
[10][25][26].
In wound regeneration, HA has mainly cosmetic applications. In skin care formulations, it can be used as a moisturizing component, because of its hydrophilic nature. Using cosmetic products such as creams or lotions that contain HA helps to moisturize the skin and to improve elasticity, thereby decreasing the depth of wrinkles. It is assumed that, when applied onto the surface of the skin, HA solutions form an occlusive layer, absorb moisture, thereby hydrating the skin, and default wrinkles filling occurs. HA is assumed to stimulate the migration of epidermal cells. Additionally, the occlusive properties given by HA may allow the biologically active substances incorporated in cosmetics to persist in the skin layers and possibly make it easier for them to penetrate the epidermis. According to previous studies, some cosmetic HA products have been proven efficient in protecting the skin from UV irradiation. At the same time, sunscreen products containing hyaluronic acid help to maintain a firmer skin, protecting it from the injurious impact of UV radiation, due to the potential antioxidant effect of HA
[27].
In cosmetic formulations, hyaluronic acid has the function of a viscosity modifier and/or a skin conditioning agent. HA is mainly used in anti-ageing cosmetic products. LMW-HA has the ability to enhance the level of moisture of the skin and expedite regeneration. HMW-HA forms a viscoelastic film when applied onto the skin and has a moisturizing effect. The main action of the HMW-HA polymer is film forming and it reduces evaporation of water from the skin and thus possessesan occlusive effect. Additionally, HMW-HA, Medium molecular weight (MMW-HA), and LMW-HA hygroscopic properties justify the ability to maintain skin hydration
[27][28].
HA is also of particular importance as a delivery system of active ingredients. Currently, there are some commercially available formulations incorporating actives in different concentrations. These products are designated for the topical treatment of actinic keratosis and skin inflammatory diseases. In fact, it has been proven that HA enhances the penetration of the active ingredient through the stratum corneum (SC), which behaves as a barrier to the entry of the molecule into the deeper layers of the skin, and the holding and locating the active ingredient in the epidermis. Topical preparations containing HA in formulation are used for their healing properties, decreasing the skin irritation. A topical preparation that contains HA (0.2%
w/
w sodium hyaluronate (NaHA)) as a main component is currently available for the amelioration of acute and chronic wounds (areas of grafted skin, post-surgical incisions, etc.)
[13][29][28].
A significant number of in vitro and in vivo studies have shown the effectiveness of HA treatment as: anti-inflammatory, skin regeneration and chondro-protective effect, anti-ageing and immunosuppressive effects, etc.
Although hyaluronan has various applications, subsequent research and technological development are needed, because there are currently certain issues to be elucidated. Firstly, further consideration of aspects regarding HA metabolism and receptor clustering analysis is necessary in order to explain the various biological actions and to foresee the effects that canvary with the molecular weight of HA. Some pharmaceuticals and/or cosmetics can incorporate HA with different molecular weight. Thus, studies are necessary for assessing the implications of molecular weight in the HA effects. Next-generation products with derivatives of crosslinked HA-conjugated polymer-delivery systems and drug substances should be developed, granting a high level of biocompatibility, prolonged half-life, and permanent in situ performance. Therefore, clinical exploration is imperative to fully characterize the safety and efficacy profile of these substances. So far, recent in vitro studies have shown promising results regarding the safety and efficacy of these promising and novel compounds: for example, HA-CL (urea-crosslinked hyaluronic acid) showed a significant biocompatibility with human corneal epithelial cell, having antioxidant, anti-inflammatory, and skin regeneration properties
[16][30].
HA is used in cosmetic formulations in concentrations ranging from 0.2 to 1%. The maximum concentration of NaHA in a body lotion is 2%. When a rate of 1 mg/cm
2 of a product is applied, the contribution of hyaluronic acid is 0.02 mg/cm
2 of skin
[20].
Interest in using hyaluronic acid as a cosmetic ingredient in skin care products occurred with the discovery that the amount of HA found in natural skin diminishes with age, and when reintroduced into the skin care products, it keeps skin hydrated, attenuates the appearance of wrinkles, and smooths the skin. HA has many qualities that make it superior to other substances used in skin regeneration, with pronounced moisturizing and anti-ageing effects
[27][28]. Biological activity and HA penetration into the skin depends on the molecular weight of this substance showing different effects on the skin, as presented in
Figure 4.
Figure 4. Hyaluronic acid activity, molecular weight dependence and claimed effect of HA. (TEWL—Transepidermal Water Loss; DEJ—Dermoepidermal junction).
It has been demonstrated by some researchers, that HA has extraordinary cosmetic and nutricosmetic efficacy in improving diverse skin imperfections such as wrinkles, periorbital and nasolabial folds, and skin ageing. These types of effects of HA have been correlated with their capacity to induce the augmentation of soft tissue, to hydrate the skin, stimulate collagen, and rejuvenate the faceas summarized in
Figure 5 [31].
Figure 5. Cosmetic and nutricosmetic effects of HA.
2.1. Hydratation Effect of HA in Cosmetic Formulations
The amount of hyaluronic acid synthesized is more substantial in the epidermis than in the dermis. Since the dermis is much thicker than the epidermis, it comprises four to nine times more HA, but it was demonstrated that for equivalent tissue quantities, the epidermis synthesizes four times more hyaluronic acid than the dermis. In the epidermis, HA is located in the intercellular matrix of the basal and spinous layers. Similarly as in the dermis, the hygroscopic properties of the hyaluronic acid are of substantial relevance in hydrating the deep layers of the epidermis, but its contribution goes further than conventional hydration
[32][33][34].
HA, which has the ability to bind water up to 1000 times its volume, has a relevant contribution to cellular growth, adhesion, and membrane receptor function. The major biologic role of HA in the intercellular matrix is to reinforce the intercellular structures and to produce the elastoviscous fluid matrix that firmly envelops collagen and elastin fibers. HA holds moisture, and provides firmness and radiance to the skin as well
[35][36]. HA can be used topically to regenerate the skin and support hydration, although its very high molecular weight prevents its penetration through the SC
[37][38].
2.2. Anti-Ageing Effect of HA in Cosmetic Formulations
HA also has an important role regarding skin ageing. Cells lose their ability to produce HA with ageing. The skin becomes drier, thinner, and looser, leading to wrinkling, among other significant changes
[39]. Skin ageing is also associated with a decrease of skin moisture. Hyaluronic acid (hyaluronan) has a unique capacity to link and retain water molecules
[40]. As it was shown, hyaluronic acid is a natural component that is present in the whole body. In a 70 kg individual there are 15 g of hyaluronic acid, 5 g of which are replaced daily. HA is naturally and constantly renewed because of its rapid degradation, but its renewal tends to slow with age and external aggressions. Therefore it is necessary to act very early, sustaining an optimal hyaluronic acid turnover, similar to that of young skin, in order to prevent the signs of ageing
[41][42][43][44].
In relation to its biological effects at skin level, it is known that hyaluronic acid is actively involved in skin cell signaling (by binding the CD44 and LYVE-1 receptors) and thus influences the ECM stability. It has been noticed that HA has an impact on the growth of keratinocytes which protect the epidermis from ageing
[10][16][45][35][46]. Hyaluronic acid is used in cosmetic preparations for its elasticity effect and for giving shape to the periorbital area after HA cosmetic treatment
[47]. Additionally, the chemical double binding structure of the D-glucuronic acid unit confers antioxidant properties to hyaluronic acid. Furthermore, HA restrains the proliferation of the skin cells via the CD44 receptor and HA also has anti-inflammatory properties on the skin
[41][48].
Hyaluronic acid is applied in a multitude of anti-ageing products. For example,
Figure 6 presents the effect of an anti-ageing cream incorporating 0.5% (
w/
w) LMW-HA (20–50 KDa) and 3% (
w/
w) encapsulated HMW-HA (1–1.4 MDa) on periorbital wrinkles before treatment and after 28 days of treatment (Protocol Report No. 300924/19/JSHR/Agreement No. 331/30 August 2019 JS. Hamilton Romania S.R.L)
[49].
Figure 6. The images of skin texture before product application (D0) and after 28 days (D28) of regular application of an anti-ageing cream incorporating 0.5% (
w/
w) LMW-HA (20–50 KDa) and 3% (
w/
w) encapsulated HMW-HA (1–1.4 MDa)
[49].
3. Cosmetic Products with HA and Its Derivates Available on the Market
The cosmetic industry has been using HA for over 20 years for its great skin moisturizing properties. In 2016, over 5900 end products launched on the market contained either HA or hydrolyzed HA, with more than 70% of these products now dedicated to the mass market and masstige market
[50]. According to the price, cosmetic companies can position their products on the market as: (1) premium products; (2) mass-premium products; (3) mass-market products
[51]. For marketing, it is known that the price plays a psychological role in a collective imagination, for a product with a high price, consumers attribute a high value. Premium cosmetics include products with a higher purchasing price, while mass-market products have, in general, a lower price.
HA and its sodium and potassium salts are important cosmetic ingredients that are incorporated in moisturizing and anti-ageing products. Additionally, products that contain HA represent only 5%, while more than 95% of the total products contain sodium hyaluronate
[50]. Hyaluronic acid and its derivates are incorporated in a multitude of cosmetic products for eye contour, lips, facial, and neck care, anti-cellulite body care, or cosmetic color conditioning in different cosmetic categories: creams, lotions, serums, masks
[35][44]. A significant number of cosmetics based on hyaluronan have been launched on the market in the last years. Some examples of last products launched in the period 2015–2020 are listed in
Table 1, depending on product category or proposed use, trade name and producer, and the incorporated HA forms in the cosmetic formulation. Additionally, the market segment of HA cosmetic products is indicated.
Table 1. Commercially available cosmetics incorporating HA and HA derivatives.
Some manufacturers launched cosmetic products on the market, containing HA or hyaluronates in combination with other active ingredients, like botanical extracts, vitamins, probiotics, amino acids, peptides, proteins, etc. These compounds improve the cosmetic formulation qualities and benefits, awarding additional claims.
3.1. Bioactive Compounds in Cosmetics with HA and HA Derivates
Different vegetal extracts incorporated in HA available cosmetics can claim different additional effects such as antioxidant, anti-inflammatory, skin conditioning, hydrating, anti-wrinkle, skin whitening, or photoprotective properties. From the cosmetovigilance point of view, vegetal extracts are mostly considered as safe for cosmetic use, but some minor adverse effects (e.g., irritation, sensitization, allergic contact reactions) have been reported.
Literature data contains a wealth of information describing the aspects regarding composition, effects, and also adverse reactions of diverse bioactive ingredients incorporated into commercially available HA or HA derivates cosmetics. Some examples are mentioned below, describing their bioactive components, cosmetic claims, and benefits, as well as reported adverse effects as indicated in Table 2.
Table 2. The benefits and potential adverse effects of botanical extracts used in cosmetic products associated with hyaluronic acid and HA derivates.
3.2. Other Active Ingredients in Commercialy Available HA and HA Derivates Cosmetics
Besides plant extracts, commercially available cosmetics containing HA or NaHA incorporate different categories of active ingredients (e.g., probiotics, amino acids, peptides, proteins, vitamins, saccharides, or other active compounds like allantoin, lactic acid, lecithin, urea, Superoxide Dismutase (SOD), gold, malachite extract) claiming additional effects, such as moisturizing, anti-ageing, antioxidant, keratolytic, skin lightening, depigmenting, etc. These active ingredients are considered safe when used in cosmetic products and show good skin compatibility. Minor adverse reactions like contact dermatitis were reported.
More data indicating the category of other active ingredients, the cosmetic claims, skin benefits, and reported adverse effects in commercially available hyaluronan cosmetics are presented in Table 3.
Table 3. Benefits, cosmetic claim, and toxicity of some active ingredients used in cosmetics together with hyaluronic acid and HA derivates.