Can hair really be hydrated?

As a reminder, each hair fibre comprises three layers: the medulla, the innermost layer; the cortex, representing 80% of the fibre; and the cuticle, the outer protective layer. The cortex is made of keratin and contains melanin pigments that give hair its colour. It also serves as the main zone for water storage and absorption. The cuticle comprises flat cells that overlap like tiles, and their cohesion is essential to limit water loss. When these cells become disorganised due to alteration of the lipid intercellular cement, the hair becomes more porous: it retains less water and absorption increases.

Hair cannot be hydrated in the strict sense as it is biologically inert, but its water content can be maintained.

Hydrating hair does not mean wetting it in the shower. Water does not enter the hair fibre. Evaporation follows. The real challenge of hydration is to support water retention in the cortex while protecting the cuticle to prevent loss. For this, use conditioners or masks that contain humectants, able to attract and retain water within the hair fibres. Among the most common humectants are glycerine, hyaluronic acid, and urea. These hygroscopic molecules bind water around them and ease its uptake by the cortex when the cuticle is more permeable for a time after shampooing.

To keep water in the hair, the cuticle’s lipid barrier must be restored. This is where emollients and occlusive agents act. Plant oils such as coconut oil or apricot oil, rich in fatty acids, can form a film on the fibre surface, reducing water evaporation. Some studies show that lauric acid, a main component of coconut oil, penetrates the hair and reduces cuticle swelling and stress on hair fibres caused by water.

Other studies have examined the effect of nanoemulsions on the hydration of textured hair samples from a female donor. Some hair fibres underwent a bleaching process, with or without subsequent recolouring. For reference, a nanoemulsion is a mixture of two or more immiscible liquids in which one is finely dispersed in the other, forming droplets at the nanoscale. Specifically, three nanoemulsions were studied: a control nanoemulsion, a version enriched with 7.5% coconut oil, and a third containing 7.5% coconut oil and 0.1% wine lees extract. These were applied to the three hair types and hydration was observed.