Clinical Overview
Hyaluronic Acid (HA) is a naturally occurring glycosaminoglycan found throughout the body's connective tissues. In the skin, it binds to water molecules to retain moisture, contributing to tissue mechanics, hydration, and cellular function.
The Molecular Weight Factor
The efficacy of HA is entirely dependent on its molecular weight (measured in kiloDaltons or kDa). The cosmetic industry typically uses Sodium Hyaluronate (the salt form of HA) because it is more stable and easier to formulate with, but it comes in vastly different sizes:
- High Molecular Weight (>1,000 kDa): Cannot penetrate the stratum corneum. Sits on the surface acting as a humectant and film-former. Prevents TEWL but can cause formulas to pill if used over 1%.
- Medium Molecular Weight (100-500 kDa): Penetrates the upper layers of the epidermis. Provides longer-lasting hydration.
- Low Molecular Weight (<100 kDa): Penetrates deeply. However, some clinical studies suggest that very low molecular weight HA (<50 kDa) can act as a pro-inflammatory signal by activating Toll-like receptors (TLRs) in the skin.
Formulation Best Practices
To achieve a cosmetically elegant serum without stickiness or pilling, formulators should use a blend of molecular weights at a total concentration not exceeding 1.5%. Always formulate with an occlusive agent (like squalane or dimethicone) in the routine, as HA in an arid environment can draw water out of the lower epidermis if the ambient humidity is low.