Key facts
- What it is
- A glycosaminoglycan found naturally in skin, where it binds large amounts of water
- In dermal fillers
- Cross-linked gel that occupies space and holds water; firmness and spread vary between products
- In skin boosters
- Lightly or non-cross-linked HA placed in the dermis to hydrate and support, not to add contour
- Topically
- A humectant that draws water into the outer layer; it does not cross the barrier into the dermis
- Reversibility
- Injected HA is gradually broken down by the body and can be dissolved with hyaluronidase
What hyaluronic acid is, and how it works
Hyaluronic acid is a long chain of sugar units, a glycosaminoglycan, that the body makes itself. In skin it sits in the dermis between collagen and elastin fibres, where each molecule binds many times its own weight in water; that water content is much of what makes young skin look plump. The amount in skin declines with age and ultraviolet exposure, which is one reason skin becomes drier and thinner.
Because the natural molecule is cleared within days, HA used in fillers is cross-linked: the chains are chemically bonded into a gel that holds its shape and resists breakdown for months. The degree of cross-linking sets how firm the gel is and how far it spreads, so one product suits lips and fine lines and another supports the cheek or chin. Whatever its form, injected HA can be broken down with hyaluronidase, which is why it is described as reversible.
How hyaluronic acid is used in aesthetic medicine
As a dermal filler, cross-linked HA occupies space: it is placed in a chosen plane to restore volume or support in the lips, tear troughs, cheeks, chin or jawline, as described on the volume restoration page. Dr Sin Yong uses HA where reversibility matters most, such as the tear trough and lips, and biostimulatory products for deep structural volume.
As a skin booster, lightly or non-cross-linked HA is placed in fine droplets or defined points into the dermis to hydrate the skin itself rather than change its contour; Profhilo is a thermally stabilised hybrid complex of high and low molecular weight HA designed to spread through tissue. The skin booster page sets out how the classes are chosen. Topically, HA is a humectant in serums and moisturisers: it draws water into the outer layer so skin feels smoother for a time, and the topical hyaluronic acid entry in the Skin A–Z explains what the evidence does and does not support.
“A moisturiser sits on your skin. A skin booster is a different conversation, at a different depth.”
Dr Sin YongOn what the category actually means
What hyaluronic acid is often confused with
The same name on a serum and on a filler leads people to expect the same thing from both. They are not the same intervention. The intact skin barrier exists to keep large molecules out, so hyaluronic acid applied to the surface hydrates the surface it sits on and does not become dermal hyaluronic acid. A booster or filler bypasses the barrier with a needle; a cream cannot, and no serum provides the structure of injected product.
HA filler is also confused with the collagen biostimulators. Radiesse, Ellansé and Sculptra are not hyaluronic acid; they work by prompting the body's own collagen and cannot be dissolved. And an HA skin booster is not a small filler: it is formulated to spread and hydrate, and will not lift a cheek or project a chin.
When hyaluronic acid is not the answer
Hyaluronic acid adds water and volume. It does not tighten lax skin, reposition descended tissue, relax a muscle or clear pigment, so a complaint of sagging, dynamic lines or brown patches belongs to a different mechanism. A face that already carries too much filler needs less product, not a different brand of the same molecule, and that is the work of filler correction.
Some HA products draw in water and can read as puffiness in areas prone to swelling, such as under the eyes, and product placed too superficially under thin skin can show as a bluish tint. Injected HA is deferred where there is active infection, inflammation or an acne flare at the injection sites, in pregnancy and while breastfeeding, and in anyone allergic to a component of the product. Topical HA is well tolerated by most skin types, and its limit is depth rather than tolerance.
