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Director, Business Development
Technical Content Creator
One of the most common queries we get at TRI is “how can we support hair moisturization or hydration claims?” TRI can’t provide legal advice on claims, but our answer to this question is that many companies change the claim to moisturized feel in their marketing copy, and do not claim increased water content. Moisturized feel is related to consumer perceived hair surface smoothness, and ease of comb. Increased water content, paradoxically, has many negative effects on many hair types, and isn’t desirable. For very coily hair, however, the situation may be different. Research at TRI is starting to suggest that increased water content might be a good thing for coily hair and may help prevent hair breakage.
The art of translating consumer language into scientific measurements
Before starting to look at the science, it is perhaps worth taking a short moment to talk about the art of translation in cosmetic claims. It is often the job of the cosmetic scientist to convert what consumers say they want into hard, technical measures. This can be more challenging than it first looks, and hair moisturization is a classic case. What most consumers want is soft, smooth, and healthy-feeling hair, and they express this to us in terms of wanting ‘moisturized’ hair. It makes sense in other parts of the body; dry skin and dry lips, for example, are missing water, and, as a result, need moisturization. However, in most situations, the analogy doesn’t work for hair. Here is why.
More hydrated hair is rougher to the touch
Increasing the water content of the hair will make it swell. Hair swells much more in diameter than in length. Hydration causes swelling in both the cortical and cuticle cells. A study has shown that immersion of hair in water can increase cuticle step-height by over 50%. It is widely believed that the endocuticle layer in each cuticle cell, which has the lowest levels of disulphide bond cross-links, absorbs the most water. The important effect for the consumer is that the surface roughness of hair increases with hydration. Friction measurements show an increase in hair surface friction with increasing humidity. It is also well known that, for straighter hair types, combing forces are much higher on wet hair versus dry hair.
The take-home message is that, for most straighter hair types, the swelling of hair with hydration only has negative effects on hair smoothness and combability. We will come back to textured hair in a moment.
More hydrated hair is harder to keep in style
It is well known that driving water out from hair helps set it in a particular shape, whether that is making straight hair curly, or curly hair straight. Removing water converts hair from a soft, pliable material to a harder, more ‘set’ material. Some cosmetic scientists argue that heat styling works by taking hair down through a glass transition.
As a result, when water comes back into hair that has been set into an unnatural shape, then it is able to soften, and spring back into its original shape. For wavy/curly hair that has been straightened hair, this just means frizz. For straight hair that has been curled, it means a loss of shape and volume.
The take-home message is that, for all hair, hydration makes it harder to keep in shape.
Effects of water on textured hair
So far, we have shown that increasing water content has negative effects on most straighter hair types. However, the rules all change for highly coiled hair, where moisture can have positive effects. For example, textured hair is easier to comb when it is wet versus when it is dry because the water softens the curls and provides some weight to the hair. As a result, the hair becomes easier to detangle with the brush. Hair swelling and an increase in cuticle step height probably still occur, but the straightening effects of water clearly outweigh these effects.
Plasticization of the hair by water can also perhaps reduce hair breakage in highly coiled hair. Recent single fiber tensile tests performed at TRI show that occlusive treatments used on textured hair can maintain higher hair moisture levels and reduce premature fracturing of the hair when it is stretched, Figure 1. More work is being done in this area as part of the Lipids & Hair Breakage in textured hair Consortium project (2023-2024).
Figure 1. Increased moisture reduces premature fractures, i.e. hair breakage at <20% strain, in textured hair. (a) High premature breakage at 40% RH; (B) Low premature breakage at 80% RH. (Figures taken from Adlam et al, 2021).
The take-home message is that for textured hair, some hydration can help reduce breakage.
Instrumental tests for moisturized feel claim support
Ease-of-combing and dry smoothness tests can be used to measure moisturized feel.
Ease-of-combing tests at TRI use an Instron tensile tester, Figure 2. The instrument measures frictional forces as a hair tress is pulled through a comb at a controlled rate. Combing experiments are performed in the wet or dry state. The Instron is used to measure several parameters: peak combing force, also known as ‘maximum load’, combing energy and average combing force. Tresses are usually combed several times to obtain accurate measurements.
Figure 2: Ease-of-comb test at TRI.
The dry smoothness test at TRI measures the force required to pull a tress of hair through two rubber cylinders pressed together, simulating the ends of your fingers, Figure 3. Careful control of the speed at which the hair is pulled through, the pressure between the cylinders and the environmental conditions (room temperature and humidity), allow the determination of hair smoothing effects of formulations.
Figure 3: Dry hair smoothness test at TRI.
Instrumental tests for measuring hair moisture content
If you still want to measure the water content of hair fibers, there are several methods available that can reveal some interesting behaviors and trends. The most commonly encountered techniques for direct measurement of water content are dynamic vapor sorption (DVS) and thermogravimetric analysis (TGA) although the vibrational spectroscopies FT-IR and Raman can provide significant insight too.
DVS allows for measurement of moisture content (both through sorption and desorption) as a function of humidity or temperature and relies upon the mass of the sample changing, Figure 4. DVS can also be used to investigate the rate of water uptake and loss in the hair.
Figure 4: Hair sample prepared for a DVS experiment at TRI.
The ‘science bit’: Hair damage increases hair porosity and water content
Hair is designed to be showerproof and has a protective layer of lipids, including 18-methyl eicosanoic acid, or 18-MEA, present on the surface. For the most-part, this lipid plays an excellent role at repelling water, therefore ensuring that very little water can enter the fiber. However, over-exposure to UV light, or the use of chemical treatments can strip the hair of this guardian, allowing water to enter the fibers due to the increased porosity, Figure 5.
Figure 5: Oxidative treatments can strip hair of 18-MEA, allowing water ingress and hair damage.
In addition, some chemical treatments, for example coloring, perming or chemical straightening, alter the internal chemical structure of the hair fiber, and make it absorb more water. This alteration is often through oxidation of disulfide bridges within the protein backbone, which results in conversion of one S–S bond (a disulfide bridge) to two cysteic acid (SO3H) residues, Figure 6a. Fewer S–S bonds between α-helices in the keratin strands results in weakness and confirmational change. In addition, the polarity of the protein strands increases (i.e. the fibers become more hydrophilic) because there are more SO3H residues, Figure 6b. This means that any water molecules can easily form hydrogen-bonds to the newly installed O and H atoms, Figure 6c. The net effect is that chemically damaged hair absorbs more water.
Figure 6: (a) Chemical alteration of hair fibers can lead to oxidation of some disulfide bridges leading to cysteic acid (SO3H) residues; (b) comparison of polarity of an S–S disulfide bridge and an SO3H residue; (c) Ingress of water (pink) can decrease protein–protein hydrogen bonding (red dashes). NB for clarity, not all possible hydrogen-bonding interactions have been shown.
Conclusion
In summary, in most cases hair moisturization claims are usually supported using tests that provide evidence of an improvement in moisturized feel. These might be ease-of-comb experiments or dry smoothness measurements. Increasing hair moisture levels is not beneficial for most hair types, as it leads to increased hair friction and the loss of style. For highly coiled hair, however, moisture might be beneficial in terms of reduced breakage.
If you’re interested in the fascinating interactions between water and hair, Dr Trefor Evans’ talk is available in the TRI library and gives an excellent overview of the key tests undertaken at TRI and explains in further detail the key interactions between water and hair.
In addition, TRI Princeton provide a suite of analytical services that can measure the properties that customers desire when using hair-care products. Contact us to arrange a call with one of our experts to discuss your needs, or to gain further information about our services.