Is it really possible to hydrate your hair?

As a reminder, each hair fiber is composed of three overlapping layers: the medulla, the innermost layer; the cortex, accounting for nearly 80% of the hair; and the cuticle, the external protective layer. The cortex is made of keratin and also contains melanin pigments, which give hair its color. It is also the primary water-storage region. The cuticle, in turn, consists of flat cells that overlap like shingles, and their cohesion is essential for limiting water loss as well as preventing its ingress. When these cells become disorganized, often due to degradation of the lipid intercellular cement, the hair becomes more porous: it holds water less effectively and absorbs it more readily.

This is why the verb "hydrate" is actually misused when it comes to hair. The goal of hair care isn’t to force water into the fiber. When water seeps into a porous strand, it causes a cuticle swelling, further disrupting the scales that make it up. This swelling-and-shrinking cycle, which repeats with each wash and dry, weakens the fiber, raises the risk of breakage, and promotes split ends. It is also for this reason that it’s not recommended to let your hair air-dry, since moisture lingers longer. The danger water poses to hair fibers is illustrated by the images below, taken with an atomic force microscope. They clearly show the cuticle lifting when the hair is immersed. Without applying a conditioner to smooth and reseal it, the cuticle remains raised and vulnerable to external aggressions.

It is preferable to speak of “conditioning” one’s hair rather than “hydrating” it.

Hydrating, or more precisely conditioning one’s hair, involves preventing water from entering and exiting it. To achieve this, the cuticle scales must be thoroughly smoothed. This notably involves the systematic application of a conditioner after each wash. Indeed, using a conditioner flattens the cuticle scales along the hair shaft thanks to the cationic surfactants it contains. This treatment also increases the adherence of the cuticular scales to the hair shaft, enhancing the hair’s light-reflecting properties and reducing friction between hair fibers.

It is also important to maintain the cuticle’s lipid barrier. This is where emollients and occlusive agents come into play. Plant oils, such as coconut oil or apricot oil, rich in fatty acids, can form a film on the surface of the fibers, thereby reducing water evaporation. Some studies have shown that lauric acid, one of the main components of coconut oil, can penetrate the hair shaft and mitigate cuticle swelling from water as well as the stress it imposes on the hair fibers.

Other studies have investigated the effect of nanoemulsions on the hydration of textured hair donated by a woman. Some hair fibers underwent a bleaching process, with or without subsequent recoloring. 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 their hydration was then observed.