About Cosmetic Formulation and the Skin/Part 3

Learn how to formulate more consciously and improve your formulation efficacy.

In the final Part 3, we will discuss cosmetic ingredients themselves.

Ingredients are able to penetrate through two main skin pathways: between and through the skin cells.

The intracellular pathway allows small hydrophobic molecules (below 500 Da) to permeate through tight lipid junctions between the cells. The transcellular route, in which the penetrants pass through the corneocytes themselves, enables the penetration of small hydrophilic or moderately lipophilic molecules (log p = 1-3).

So, what impact particular ingredients might have on skin penetration?

Phospholipids

Phospholipids such as lecithin, lysolecithin, and liposomes will interact with the SC lipid matrix and increase penetration of both lipophilic and hydrophilic molecules

Vegetable Oils

Plant oils such as olive, jojoba, and squalene have increased affinity with the skin and penetrate faster together with the actives dissolved in them.

Occlusive Emollients

Emollients such as mineral oil can cause an occlusive effect, under which the hydration of the horny layer takes place, resulting in increased penetration.

Emulsifiers

Emulsifiers are able to entrap lipophilic actives in o/w micelles, and their penetration can be improved. It works the same way for hydrophilic substances in w/o micelles.

Non-ionic Surfactants

Non-ionic surfactants enhance absorption by inducing fluidization of the SC lipids.

Anionic Surfactants

Anionic surfactants interact with both keratin (extension of filaments) and lipids (increase in fluidity) and alter the permeability of the skin.

Cationic Surfactants

Cationic surfactants interact with the keratin fibrils of the cornified cells and result in a disrupted cell-lipid matrix and enhanced absorption.

Fatty Acids & Essential Oils

Fatty acids (lauric, oleic) and essential oils terpenes (eucalyptus, rosemary, cinnamon, limonene) enhance skin penetration through disintegration of the intercellular lipid structure.

Humectants

Humectants such as glycerin, ethylhexylglycerin, propylene glycol, butylene glycol get deposited in the lipid layer, where they attract water and widen the pathway for transcellular skin penetration.

Moisturisers

Moisturisers such as urea or sodium pca have opposite effect to glycerin! They are likely to get deposited in the corneocytes, where they cause them to reversibly swell and reduce the path for transcellular penetrating agents.

Exfoliators

Peeling agents such as AHAs (lactic, glycolic, and malic acids), BHAs (salicylic acid), Nacetyl glucosoamine will reduce SC thickness and facilitate penetration.

Complexing Agents

Complexing agents such as cyclodextrins, have a molecular cavity and are able to encapsulate actives, improving their water solubility and bioavailability.

Solid Particles

Solid particles may increase dermal penetration of dissolved actives (particle-assisted penetration mentioned in Part 2). An optimal increase occurs with small particles and concentrations >20%.

Lastly, few words about ingredients polarity. Non-polar ingredients penetrate faster than polar. One way to increase polar ingredient penetration is to decrease water activity of the formula and use humectants!

I hope you enjoyed the series! Happy formulating!

Let's create beauty together.