The science of hair strength: the key to developing consumer relevant hair care formulations

Hair strength is commonly referred to as the tensile force required to break an individual hair fibre, where a higher force required suggests a stronger hair fibre. For a cosmetic scientist, there are a range of instrumental-based mechanical testing protocols that can be utilised to evaluate this parameter, each capable of examining different areas of the hair structure and quantifying how it can be altered, giving us an overall picture of how various factors have the potential to compromise the hair structure. The mechanical properties of hair are a direct reflection of its complex structure that is made up of three layers: the cuticle, cortex and medulla.

Cuticle

The outer cuticle layer is the hydrophobic sheath protecting the inner cortex. It is composed of flat, overlapping scales that run in a root to tip direction. The cuticle is extremely important in the cosmetics industry due to its influence upon the tactile properties of the hair.

Medulla

The medulla would be found in the centre of the hair fibre; however, a medulla is not present in all hair types. There is currently little known of the impact of the medulla with regards to mechanical or chemical properties within the hair, therefore is not an important area for consideration.

Cortex

The cortex makes up the majority of the hair fibre mass. It mainly contributes to the mechanical properties of the hair fibre and is the primary source of hair’s strength. It is primarily composed of cortical cells, which are spindle shaped, that run longitudinally along the hair fibre and are joined by a thin cortical cell membrane complex. Cortical cells are made of much smaller structures, of which macrofibrils are the main components and are composed of intermediate filaments that align along the length of each macrofibril. The intermediate filaments are embedded within a protein matrix, which contains amorphous protein structure, comprised of keratin associated protein, and possesses a high cystine content.

This is a simplified overview, but hopefully you can appreciate that the cortex is a complex structure, and the intertwining of all of these small structures is what provides the hair with its strength.

Because the cortex is responsible for hair strength, its treatment is essential. Even if it is protected by the cuticles, there are numerous factors that can have an impact on its structure and influence the mechanical properties of hair, here are a few examples:

Damage from external insults – heat from styling tools, chemical treatments such as perms or relaxers, and environmental aggressors such as UV rays are all capable of influencing the internal structure of the cortex, causing bonds to be broken and in some-instances re-formed. This repeated action on the hair fibre causes weakening of the internal structures, leading to hair that is more prone to breakage.

Natural hair morphology – Even virgin hair that has not been exposed to any external damage has the potential to be weakened. This is even more true for curly or coily hair for which kinks and twists occur naturally due to its morphology. These kinks and twists along the hair fibre represent areas of discontinuity in the cuticle layers, leaving the hair more susceptible to breakage during usual grooming practices. The cross-sectional diameter of the hair fibre also has a part to play, as thicker strands tend to be inherently stronger.

Hair aging – during the natural aging process, hair can lose its strength and pliability. Hormone production changes throughout our lifetime and imbalances can lead to unfavourable hair conditions. An example of this is the menopause, where studies have shown that the hair becomes more fragile and prone to breakage. The hair surface can also become rougher and more prone to frizz, with hair thinning also a common symptom. These effects are thought to be linked to a change in the hairs internal structure.

But how do these factors actually cause the hair to become weaker? The simple answer is that the bonds within the cortex that hold its complex structures together are disrupted and broken, which can lead to a loss in tensile strength, leaving the hair more susceptible to breakage.

There are three main types of bonds present within the cortex:

Hydrogen bonds
Salt bonds
Disulphide bonds

Each are responsible for bonding different areas of the cortical structure, and when combined, contribute to the overall strength of our hair.

Whilst contributing to its overall strength, hydrogen and salt bonds also have the ability to temporarily impact your hair's natural shape. This is because these bonds can readily be broken by the addition of water or heat, and can be easily reformed again. Think about when your hair is wet, your natural hair shape changes – this is where the hydrogen and salt bonds have been broken. Then we leave our hair to dry or cool down, our hair sets in its new shape – this is where the hydrogen and salt bonds have reformed.

Disulphide bonds are permanent bonds that are largely responsible for the strength of hair and its natural shape. Chemical treatments such as relaxers or permanent waving are designed to break disulphide bonds to permanently change the hairs natural shape. This change is irreversible and although steps are taken to reforms bonds within the cortex to maintain the hairs new shape, not all bonds will reform again leaving the hair weaker, more brittle and prone to breakage.

Prevention is often better than the cure, but this isn’t always possible with the myriad of styling practices we inflict on our tresses! There are a range of hair treatments on the market today which offer repairing benefits, to help restore strength to weakened hair. Here are some of our own solutions:

As the hair is made primarily of keratin and thanks to its good performance, there's a long-standing demand for keratin-based proteins in hair treatments. However, keratin is an animal-derived ingredient, often sourced from sheep’s wool, and due to the increasing vegan trend, the use of keratin is falling out of favour. KeraMatch V is an optimized blend of pea and potato protein, offering a plant-derived keratin alternative that replicates the performance and functional benefits of animal-derived keratin.

The performance testing of KeraMatch V showcased significant improvements of the overall quality of damaged and fragile hair, including European, Asian and Type VII textured hair, with increased strength performance while providing exceptional consumer perceivable improvements in the sensory attributes demonstrated in salon evaluations.

One of the instrumental tests we used to validate the strengthening benefits of KeraMatch V was single fibre tensile testing. This test is a widely used method to quantify hair strength and gives a fundamental indication of hair fibre integrity, measuring the amount of force that is required to break an individual fibre. The behaviour of a hair fibre undergoing this test is depicted by a typical stress-strain curve and allows us to compare the tensile properties of hair after exposure to different conditions and/or treatment with products or ingredients. In the video below, our hair care team in South Africa explain more about this test – give it a watch if you are interested to learn more!

Although instrumental analysis is important to validate the performance of our ingredients, we should not underestimate the importance of consumer perception of the efficacy of hair care products. As mentioned earlier, consumers identify “strong” hair to be healthy. There are many signals which tell consumers that their hair is weakened and not in great condition, such as hair breakage, visible split ends, frizz and hair that easily knots and tangles. Although instrumental analysis is important for hair care brands and ingredient suppliers to understand how our products are influencing the mechanical properties of the hair, we must remember that consumer assessment of their own hair strength probably does not involve pulling and manipulating individual fibres in a manner comparable to these instrumental methods.

At Croda, we value the consumer evaluation of our hair care ingredients and often utilise this method to support our instrumental testing. With salon facilities in our US, South Africa and Singapore laboratories, we are able to conduct salon evaluations with trained cosmetologists to determine the consumer perceivable benefits of our hair care ingredients. This was perfectly demonstrated in the data generated for KeraMatch V. We conducted several salon evaluations, comparing conditioner formulations containing KeraMatch V and an animal-derived keratin benchmark respectively. This study evaluated several wet and dry hair attributes and showed that KeraMatch V outperformed the animal-derived keratin, by significantly improving attributes such as wet and dry combing, feel and anti-frizz. Both the trained cosmetologists and panellists expressed an overall preference for hair treated with the conditioner containing KeraMatch V, commenting that hair felt smoother, was softer and overall felt healthier.

I hope you enjoyed the second blog in our hair care series, look out for my next blog where I will explore the booming scalp care market and why the maintenance of a healthy scalp is essential for healthy hair.