Hair is a universal feature that defines much of our physical appearance, and its texture, colour, and health can play a key role in our self-esteem and personal identity. Whether curly, straight, frizzy, or fine, every strand of hair is a product of intricate chemistry that shapes its structure and properties. To truly understand how hair behaves, we must explore the molecular makeup of hair, its growth cycle, and the effects of various chemical treatments on it. This article will dive deep into the chemistry of hair structure, revealing how the tiniest molecules have such a profound impact on the way hair looks and feels.

The basic structure of hair

Hair is primarily made up of a protein called keratin, which belongs to the family of fibrous structural proteins. Keratin is responsible for providing strength, elasticity, and resilience to hair, making it resistant to physical and chemical wear. Every strand of hair has a complex three-layered structure:

The Cuticle: This is the outermost layer, composed of overlapping dead cells arranged like scales. These cells serve as a protective shield, safeguarding the inner parts of the hair shaft from environmental damage. The condition of the cuticle largely determines how shiny and smooth hair looks. A well-maintained cuticle layer reflects light and provides a silky appearance, while a damaged cuticle can result in frizzy, dull hair.

The Cortex: Beneath the cuticle lies the cortex, which makes up the majority of the hair's mass. The cortex consists of long chains of keratin proteins bundled together, giving hair its strength, elasticity, and colour. The chemical bonds between these proteins are what control many of the hair's properties, such as curliness or straightness. Within the cortex, pigments like melanin give hair its natural colour.

The Medulla: The innermost part of the hair, the medulla, is often not present in fine or light-coloured hair. It serves no significant structural role but may provide some insulation in thicker hair types.

The molecular chemistry of keratin

Keratin is a key player in hair structure, consisting of long chains of amino acids. These chains form through a process called polymerisation, in which smaller molecules called monomers (amino acids) join to form a polymer (keratin). The primary amino acid in keratin is cysteine, which contains sulphur atoms. The sulphur atoms from cysteine molecules form strong disulphide bonds, which give hair much of its resilience and shape. The more disulphide bonds between keratin chains, the curlier the hair will be.

Additionally, keratin proteins are held together by other types of chemical bonds:

Hydrogen bonds: These are weak bonds that form between the keratin chains due to attraction between hydrogen atoms and other atoms like oxygen or nitrogen. Hydrogen bonds are easily broken by water, which is why hair becomes more pliable and easier to style when wet.

Salt bonds: These ionic bonds form between the negatively charged and positively charged parts of amino acid chains. Salt bonds are sensitive to changes in pH and can be broken when exposed to acidic or alkaline substances.

While hydrogen and salt bonds are easily disrupted and reformed, disulphide bonds are much stronger and more permanent. This explains why heat and water temporarily change the shape of hair, while chemical treatments like perms can alter hair structure more permanently.

Hair growth and structure formation

Hair grows from hair follicles embedded in the scalp, with each follicle undergoing a cycle of growth, rest, and shedding. The growth phase, or anagen phase, can last several years, during which the hair follicle produces new hair cells that are gradually pushed out, creating the hair shaft. As these cells move up and away from the follicle, they die and harden in a process called keratinisation. 

During keratinisation, the hair shaft is formed as keratin proteins in the cells align into a rigid structure. Once formed, hair is essentially composed of dead cells filled with keratin, which is why it can be cut, styled, and chemically treated without causing pain.

The role of hair bonds in texture and curl

The texture of hair, whether it is straight, wavy, or curly, is largely determined by the shape of the hair follicle and the chemical bonding within the hair shaft. The more disulphide bonds present, the curlier the hair will be. This happens because disulfide bonds create a tighter link between keratin proteins, which causes the hair to twist or curl. Conversely, straight hair has fewer disulphide bonds, which allows the keratin chains to lie flat.

In addition to disulphide bonds, the physical shape of the hair follicle also contributes to hair texture. Round follicles tend to produce straight hair, while oval or asymmetrical follicles result in wavy or curly hair.

The chemistry of hair treatments: From dyeing to perming

Hair treatments often rely on manipulating the chemical bonds in hair. One of the most common treatments is hair dyeing, which involves the use of chemicals to alter the hair's colour. Hair dye molecules must penetrate the cuticle and reach the cortex, where the natural pigments reside. Permanent dyes contain ammonia to lift the cuticle and peroxide to break down melanin, allowing the new dye to be deposited. However, this process can weaken the hair structure by damaging the protective cuticle and breaking down keratin proteins.

Perming and relaxing are other treatments that permanently alter hair texture. Both involve breaking the disulphide bonds in keratin proteins and then reforming them in a new configuration. A perm typically adds curls or waves to hair, while relaxing straightens it. The process begins with the application of a reducing agent, which breaks the disulphide bonds in the hair. The hair is then reshaped (by wrapping it around rods for curls or straightening it) and a neutralizing agent is applied to reform the disulphide bonds in their new shape.

While effective, chemical treatments can be harsh on hair, excessive use of dyes, perms, or relaxers can damage the cuticle and weaken the keratin structure, leading to brittle, frizzy, or dry hair. That's why many people now turn to "keratin treatments", which are designed to repair damaged hair by applying a keratin-based product that coats the hair shaft, temporarily smoothing and strengthening it.

The science of hair damage and protection

Hair is subject to numerous environmental and chemical stressors, from heat styling to UV radiation, that can damage its structure. When the cuticle layer is worn down, the inner cortex is exposed to further damage, which can weaken the hair and cause split ends. Over time, this leads to a rough, brittle texture, and hair becomes more prone to breakage.

One way to protect hair is through the use of conditioners and serums that smooth the cuticle and create a protective barrier around the hair shaft. These products often contain silicones, oils, or proteins that help to restore shine and reduce friction. For more intense repair, products containing hydrolysed proteins can penetrate the hair shaft and reinforce the keratin structure from within.

Heat styling tools, like curling irons and straighteners, can also cause damage by breaking hydrogen bonds and temporarily altering the structure of hair. Over time, repeated heat exposure can weaken the cuticle and degrade the hair's natural proteins. Heat protectant sprays, which form a barrier between the hair and heat, can help minimise this damage.

The future of hair chemistry: Innovations and sustainability

The beauty industry is constantly evolving, and new developments in hair chemistry aim to improve both the effectiveness and safety of hair care products. Recent innovations include bond-building treatments, which claim to repair damaged hair by re-linking broken bonds within the hair shaft. These products often contain ingredients like bis-aminopropyl diglycol dimaleate, which helps to rebuild the disulphide bonds in keratin, offering a more permanent repair solution than traditional conditioners.

Sustainability is also becoming a key focus, as consumers demand hair care products that are both effective and environmentally-friendly. Companies are exploring natural, biodegradable alternatives to synthetic chemicals, such as plant-based keratin and eco-friendly packaging.

Conclusion

The chemistry of hair structure is a fascinating interplay of proteins, chemical bonds, and molecular interactions that define everything from texture to strength. Understanding the science behind hair not only helps us appreciate its complexity but also informs better choices in hair care and treatments. Whether you're dying, perming, or just protecting your locks, the secrets to healthy, beautiful hair lie in the chemistry of the molecules that make it up.

Renald Blundell is a biochemist and biotechnologist with a special interest in Natural and Alternative Medicine. He is a professor at the Faculty of Medicine and Surgery, University of Malta.