Hyaluronic acid (HA) plays an important role in the viscoelasticity and hydration of the skin (click here to read our previous blog). It happens to be a very large molecule that has been described as the most hydrophilic (i.e. ‘water-loving’) substance in nature since it can retain up to 1,000 times its own weight of water. As we age, the skin’s ability to produce HA decreases along with a reduction in the actual size of the HA molecules, which means that the skin’s elasticity and capacity to retain moisture are in turn reduced. Due to its well-established link to skin health, HA is used in numerous cosmetic applications in the form of high-end over-the-counter lotions, dermal fillers and advanced injectable treatments such as Profhilo®. The question is, can a topical lotion be as effective as an injectable formulation? Purely in terms of the science, let’s look at the ability of HA to penetrate the skin.
The epidermis is the outermost layer of skin and is typically made up of four distinct sublayers (see diagram below). The stratum lucidium is an extra layer that is only found in the skin of the palms and soles of the feet.
The HA used in cosmetics formulations comes in different molecular weights. But what is ‘molecular weight’? If you can remember your chemistry, molecular weight is used to describe the mass of molecules and is measured by the unit ‘Daltons’ (Da) with particularly large molecules measured in ‘kilodaltons’ (1 kDa = 1,000 Da). Here’s some molecular weights as examples; water = 18 Da, ethanol = 46 Da, aspirin = 180 Da, insulin = 5,808 Da (or 5.8 kDa), haemoglobin = 64 kDa, albumin (egg white protein) = 65 – 70 kDa. With large biological molecules, molecular weight is often considered a direct measurement of their physical size. With respect to the molecular weight of HA… how long is a piece of string? Because HA is a branched, chain-like molecule made up of any number of repeating units, its molecular weight is theoretically unlimited. The HA naturally found in the body ranges from 5 kDa to 20,000 kDa, but for the purposes of this blog we can consider low molecular weight HA to be of the order of tens of kDa whilst high molecular weight HA can be thousands of kDa (i.e. millions of Daltons).
One scientific paper from the journal Skin Research and Technology specifically looked at the penetration of hyaluronic acid of different molecular weights through human skin (Essendoubli, 2016). Their experiments showed that regardless of the size of the HA, be it ‘small’ (20-50 kDa), ‘medium’ (100-300 kDa) or ‘large’ (1,000 – 1,400 kDa) most of it was found in the very outermost layer of the epidermis, the Stratum corneum which is made up of dead cells. Any good moisturising product will hydrate this layer of skin. However, using state-of-the-art techniques, the research showed that some of the ‘small’ HA had penetrated through to the bottom of the epidermis (about 0.1 mm). The ‘medium’ HA only penetrated half of this distance (0.05 mm) whilst the ‘large’ HA was unable to penetrate beyond the Stratum corneum at all. These findings agree with previous results which showed that low molecular weight HA improves skin hydration whereas high molecular weight HA stays on the skin’s surface forming a protective film. In conclusion, topical applications cannot get HA beyond the epidermis into deeper dermis. High molecular weight HA formulations cannot penetrate beyond the outer dead layer at all; but it is very good at protecting and moisturising the skin. But there really is no way that the large HA molecules desirable for skin hydration and elasticity can reach beyond the very outermost layer of skin by topical application alone.
If you want to get HA into the dermis, you’re going to have to inject it. Profhilo® is an injectable HA formulation that has been specifically developed to hydrate the dermis directly with a concentrated mixture of high molecular weight HA (1,100 – 1,400 kDa) and low molecular weight (80 – 100 kDa) molecules. Here at Cosmetic Perfection, Emma offers this exciting product along with dermal fillers and wrinkle-relaxing injections.
The HA used in cosmetics formulations comes in different molecular weights. But what is ‘molecular weight’? If you can remember your chemistry, molecular weight is used to describe the mass of molecules and is measured by the unit ‘Daltons’ (Da) with particularly large molecules measured in ‘kilodaltons’ (1 kDa = 1,000 Da). Here’s some molecular weights as examples; water = 18 Da, ethanol = 46 Da, aspirin = 180 Da, insulin = 5,808 Da (or 5.8 kDa), haemoglobin = 64 kDa, albumin (egg white protein) = 65 – 70 kDa. With large biological molecules, molecular weight is often considered a direct measurement of their physical size. With respect to the molecular weight of HA… how long is a piece of string? Because HA is a branched, chain-like molecule made up of any number of repeating units, its molecular weight is theoretically unlimited. The HA naturally found in the body ranges from 5 kDa to 20,000 kDa, but for the purposes of this blog we can consider low molecular weight HA to be of the order of tens of kDa whilst high molecular weight HA can be thousands of kDa (i.e. millions of Daltons).
One scientific paper from the journal Skin Research and Technology specifically looked at the penetration of hyaluronic acid of different molecular weights through human skin (Essendoubli, 2016). Their experiments showed that regardless of the size of the HA, be it ‘small’ (20-50 kDa), ‘medium’ (100-300 kDa) or ‘large’ (1,000 – 1,400 kDa) most of it was found in the very outermost layer of the epidermis, the Stratum corneum which is made up of dead cells. Any good moisturising product will hydrate this layer of skin. However, using state-of-the-art techniques, the research showed that some of the ‘small’ HA had penetrated through to the bottom of the epidermis (about 0.1 mm). The ‘medium’ HA only penetrated half of this distance (0.05 mm) whilst the ‘large’ HA was unable to penetrate beyond the Stratum corneum at all. These findings agree with previous results which showed that low molecular weight HA improves skin hydration whereas high molecular weight HA stays on the skin’s surface forming a protective film. In conclusion, topical applications cannot get HA beyond the epidermis into deeper dermis. High molecular weight HA formulations cannot penetrate beyond the outer dead layer at all; but it is very good at protecting and moisturising the skin. But there really is no way that the large HA molecules desirable for skin hydration and elasticity can reach beyond the very outermost layer of skin by topical application alone.
If you want to get HA into the dermis, you’re going to have to inject it. Profhilo® is an injectable HA formulation that has been specifically developed to hydrate the dermis directly with a concentrated mixture of high molecular weight HA (1,100 – 1,400 kDa) and low molecular weight (80 – 100 kDa) molecules. Here at Cosmetic Perfection, Emma offers this exciting product along with dermal fillers and wrinkle-relaxing injections.
Links:
Essendoubli, 2016. https://www.ncbi.nlm.nih.gov/pubmed/25877232