The Science of Youthful Skin and the role of Hyaluronic acid injections

Dr Kalli Dodou

1Founder of Kallistia Aesthetics Clinic

2 Associate Professor at the University of Sunderland, UK


Why is young skin free from wrinkles? The short answer is: because it is resilient to permanent deformation induced from stress.

Throughout my career, I have been researching the effect of mechanical stress on the properties of materials. Indeed, if we consider our skin to be a “material” too, we can explain, prevent up to an extent, and manage the appearance of wrinkles.

Young skin behaves like an elastic solid. The key property of an elastic solid is resilience, ie. its ability to bounce back to its original shape when stress (ie a mechanical force over a specific surface area) is applied to it. Our facial skin is exposed to such forces daily because of our facial expressions such as frowning, smiling, raising our eyebrows. When we are young, our skin is resilient to the forces from facial expressions; it bounces back every time. If these facial expressions are frequent eg we constantly raise our eyebrows when we speak, our skin is in a constant effort to reverse the deformation caused.

The elastic behaviour of our skin is because of the presence of collagen and elastin proteins. Their role is enabled by hyaluronic acid which is a humectant and retains water into the skin; a hydrated skin is plump and flexible. Imagine it as a “hydrated elastic solid” or hydrogel that can sustain deformations and bounce back every time.

Collagen, elastin, and hyaluronic acid are abundant in our skin when we are young. As we age, their levels drop, and our skin becomes less elastic and less plump; therefore less resilient to the effect of facial expressions.

One approach to avoid wrinkles, would be to avoid facial expressions….to make a conscious effort to not frown or raise our eyebrows. However, this cannot be a sustainable or realistic measure because as the collagen levels drop and our skin becomes less elastic, then the force that can cause a permanent deformation (ie a static wrinkle) is much lower! This means that the same facial expressions can result in wrinkles (permanent deformations) as our skin ages.

To recap the above:

  • Young skin à elastic material à resists deformation and bounces back à yield stress is much higher than applied stress by facial expressions à no wrinkles!
  • Mature skin à plastic/ ductile material à deforms easily without bouncing back à yield stress is low and within the range of facial expressions à static wrinkles appear.

How do Phormae and Sisthaema skin boosters restore skin’s youthfulness?

Both Phormae and Sisthaema skin boosters contain hyaluronic acid gel, ie hyaluronic acid polymer dissolved in water (20mg/ml and 25mg/ml respectively)

  • The hyaluronic acid in Phormae is uncrosslinked (linear polymer chains) and has molecular weight of around 1300 kDa (1,300,000 Da) according to its manufacturer.
  • The hyaluronic acid in Sisthaema is also uncrosslinked but has a wide molecular weight range; from 200 kDa to 1500 kDa. Sisthaema also contains Trehalose.

You might now think…is there a difference between uncrosslinked and crosslinked hyaluronic acid? What is the significance of the molecular weight of hyaluronic acid?

Crosslinked hyaluronic acid is used in the manufacture of fillers. It absorbs moisture within the skin and swells to a predetermined maximum volume, which can then fill-up wrinkles. It behaves like a structured elastic solid and can confer shape and volume when it’s injected into the skin. The higher the degree of crosslinking, the stiffer the filler gel, and the more difficult to flow in the needle; that’s why the hyaluronic acid in fillers is always partially crosslinked and not completely crosslinked. The partial crosslinking enables a degree of liquid-like (viscous) behaviour to allow flow. A completely crosslinked gel would have no flow and would behave like an elastic implant. In addition, the crosslinks resist the natural enzymatic degradation of hyaluronic acid by skin enzymes and the volumizing effect can last for months. Accidental injection of excess filler must be dealt quickly by the administration of medicinal hyaluronidase enzyme due to the very slow degradation of the crosslinked hyaluronic acid within the skin.

Skin boosters, eg Phormae & Sisthaema, contain uncrosslinked hyaluronic acid gel which is not an elastic (solid-like) material and therefore they don’t have volumizing or contouring effect. In fact, the uncrosslinked hyaluronic acid gel is a viscous liquid (its viscosity depends on the concentration of hyaluronic acid in water) and flows easily via the needle into the dermal tissue where it provides an instant moisturizing and hydrating effect.  Because it’s uncrosslinked, it can undergo gradual enzymatic degradation to lower molecular weight hyaluronic acid fragments into the skin, which are useful to stimulate collagen synthesis. That’s why the regenerating effect starts a couple of weeks after injection. A gradual increase in collagen synthesis in combination with the hyaluronic acid gel in the extracellular matrix, can diminish the depth of existing wrinkles and prevent the formation of new wrinkles by increasing skin elasticity and resilience against deformation.

There is a fine balance between the requirement for the fragmented hyaluronic acid that can induce collagen production, and the complete enzymatic degradation of the injected hyaluronic acid gel which would undermine the plumpness of the extracellular matrix. The presence of trehalose in the Sisthaema formula is intended to keep this balance by protecting the hyaluronic acid from complete enzymatic degradation in the skin; this synergy between hyaluronic acid and trehalose is therefore claimed to provide a prolonged dual effect of hydration and collagen synthesis.

The above scientific insight is my own interpretation on the mode of action of the Phormae and Sisthaemic skin boosters. I have observed the effects of these formulas on my clients’ skin, especially on aged skin, and I look forward to continuing providing skin regenerating transformations using these products.


“Dr Kalli assumes no responsibility or liability for any errors or omissions in the content of this site. The information contained in this site is provided on an “as is” basis with no guarantees of completeness, accuracy, usefulness or timeliness. This blog has been purposely written for Sisthaema Hevo-T website, therefore please do not copy this blog post”
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