Did you know that each chemical bond in hair Keratin is responsible for its behavior towards hair products used?
The hair shaft is divided into three main regions: Cuticle, cortex, and when mature and thick enough, the medulla ; “baby hair” vs adult hair. Human hair fibers are covered by multiple layers of cuticle cells (Figure 1). The cuticle is a chemically resistant region which provides the inner structure of the hair protection from damages caused by environmental agents, cosmetic treatments, and industrial processes. The cuticle is of flap overlapping scales (keratinocytes), each cuticle cell contains a thin proteinaceous membrane, the epicuticle, covered with a lipid layer that includes the 18-methyl eicosanoic acid (18-MEA) and free lipids responsible for the hydrophobicity of the hair. This inner structure of hair consists mainly of the cortex which provides hair strength, moisture, and color, whereas texture is related to the properties of the hair surface. The cortex contains several intermediate keratin (hair protein) filaments, cross-linked with keratin-associated proteins via extensive disulfide bonds. The keratin and bonds in your hair determine its texture and shape and give each hair around a third of its overall strength.
Fig. 1 Hair Structure Diagram.
The cells within the hair follicle are produced and mature in an upward moving process through the follicle, known as keratinization. During keratinization cells absorb keratin. As the cells continue to move upward they lose their nucleus and die off, producing the non-living keratinized cells that emerge from the scalp; The hair. In other words, hair is dead material. Hair is composed primarily of these keratin proteins (~88%), that are of hard fibrous type. Keratin is composed of polypeptide chains, that when broken down, you obtain individual amino acids. Each polypeptide of keratin is held together by four different types of bonds: The Hydrogen bonds, the Salt bonds, the Cysteine bonds (disulfide bonds) and the sugar bonds. Each one of these bonds serve a purpose to the hair’s strength, elasticity and moisture (Fig 2).
Fig 2. The Hair Chemical Bonds and Their Purpose
Hydrogen bonds, which contribute to the elasticity and strength of hair, allowing the hair to be stretched and returned back to its original shape. The hydrogen bonds allow us to temporarily change the shape of our hair with the aid of water or heat. These bonds are electrically controlled and account for about one-third of hair’s strength. The Salt bonds which are ionic bonds and are affected by change in pH, where they are broken down by acidic or alkaline solutions. Like hydrogen bonds, salt bonds also account for approximately one-third of hair’s strength. Additionally you have the sugar bonds which also hold the peptide chain together by connecting amino acids. The Sugar bond gives hair toughness but little strength and some moisture is contributed to the hair as a by-product of this bonding.
Lastly the disulfide bonds, also known as the cystine bonds or S-bonds differ from hydrogen and salt bonds as they are chemical side bonds held together by two sulfur atoms attached to cysteine amino acids within the polypeptide chains. Disulfide bonds which can not be broken by water or heat, are affected by extreme pH. However, unlike hydrogen and salt bonds, once the disulfide bonds are broken down and reshaped, the effect can not be reversed.
How Are Your Hair Products Affecting These Hair Bonds
pH – Products with extreme pH such as hair relaxers and permanent waves can permanently alter or change the non-living keratinized cells that emerge from the scalp (hair) via the S-bonds. Alkaline chemical procedures treatments can cause the hair to stretch beyond the normal 30% of its original length, and irreversible changes occur when hair is stretched between 30% and 70% however stretching beyond that, can cause major fracture and damage to hair. These hair treatments also have the tendency of removing the 18-MEA (mentioned above) that is responsible for the hydrophobicity of the hair by increasing its hydrophila. Water absorption causes hair shaft swelling, making it highly porous: a common feature of damaged and dry hair. The use of the proper hair products can restore hair cuticle damage and prevent hair breakage by reducing friction and water pick up, however the changes in the disulfide bonds are permanent, at least for the non-living keratinized hair.
pH goes hand in hand with the hair’s ability to hold moisture and maintain its sebum. The natural pH of your hair strands and scalp is on the acidic side, between 3.8 & 5.5. When the hair is at the proper pH level, the cuticles are tightly ‘closed’, giving hair a healthy, smooth feel and a shiny appearance. Therefore, it is best to use hair care products that maintain and balance your scalp & hair pH.
Water – Water, whether it be in our hair products or the atmosphere, contributes majorly to the hair’s behavior. Moisture (water or water base products) aids in the hair’s stretch by helping the hair swell and contract. As aforementioned, hydrogen bonds allow us to temporarily change the shape of our hair with the aid of water or heat. High humidity breaks the hydrogen bonds and your hair can’t retain its styled shape. Additionally changing from one humidity to another (high to low or low to high) can also change the shape of your hair. For example If your hair was styled at high humidity, it would be better to stay at the same humidity than to go to a less humid location and vice versa. It is ultimately a matter of equilibrium of your hair’s moisture and that of the atmosphere. There is generally more water in your hair than in the air, but if the air around the hair gets drier, water will start seeping out of hair into the air. Inversely, if the air around your hair gets more humid, water will start to enter your hair and will move until the ratio of water in the air and in your hair is at an equilibrium. For example; If your hair was styled in a damp environment, such as a humid bathroom, you then walk outside into a less damp environment (high to low humidity), water will move out of your hair. Conversely, if your hair was styled when it was dry and sunny, then a storm happens later (low to high humidity) water will move into your hair. A similar occurrence can take place when we use heat on our hair as well, such as blow drying the hair using heat can reduce the hair’s moisture compared to when it is dried without heat.
Aside from humidity, static change can cause individual hair strands to repel each other resulting in “fly-aways”, frizz. Many hair care products are formulated to decrease the build-up of charge by increasing the hair’s moisture content. Hair with a rough cuticle can also be frizzy as can be the case with chemically treated hair without the proper maintenance. Rough hair cuticles can also happen if the hair is damaged by other factors, or if the wrong products have been used, like a poorly formulated hair product, or lack of conditioner. These occurrences can also contribute to the hair being under-moisturized, feeling dry, brittle, with very little stretch or pliability.
Oils – Oils which are lubricants that can play an important role in protecting hair from damage when they are properly used. Some oils can penetrate the hair and reduce the amount of water absorbed in the hair, leading to a lowering of swelling. This can result in lower hygral fatigue (constant wetting of the hair; repeated swelling and drying), a factor that can damage hair. The oil can fill the gap between the cuticle cells and prevent the penetration of the aggressive substances such as surfactants into the follicle. However not all oils are equally effective, and there is a ‘good’ way and a ‘bad’ way of using oils on the hair but that is a subject for another article. Oils can give hair shine but not necessarily sheen, a factor that is due to hair’s essential fatty acids which are found in the cuticle and cortical cell membrane complex and make up 85% of the total hair lipid content but only make up 2-6% of the hair’s overall weight. When essential fatty acids are depleted, the hair can feel rough, dry and lack elasticity.
Conditioning the hair often and using products with moisture rich ingredients aid in maintaining the hair’s moisture balance. Conditioners function to improve combability, restore hydrophobicity, seal the cuticle, eliminate or minimize frizz and friction, enhance shine, smoothness and manageability of the hair. This in turn, will also contribute to less hair breakage and hair loss. Conditioners contain anti-static and lubricating substances that can be divided into 5 main groups: Polymers, oils, waxes, hydrolyzed amino acids and cationic molecules. The ideal conditioner will have a pH that is closer to that of your hair and scalp. It would be formulated to help restore hair’s hydrophobicity and neutralizing static electricity with the capacity of entering the fiber, reaching the cuticle surface or the inner part of the cortex.
References For This Article:
- Robbins CR, Chemical and Physical Behavior of Human Hair (5th ed), 2012. (cross ref)
- Hair Shaft Damage from Heat and Drying Time of Hair Dryer. Ann Dermatol. 2011 Nov;23(4):455-462. English. Published online Nov 03, 2011. (cross ref)
- Gavazzoni Dias, M.F. Hair cosmetics: An overview. Int. J. Trichol. 2015, 7, 2–15. (cross ref)
- Sinclair, R.D. Healthy hair: What is it? J. Investig. Derm. Symp. Proc. 2007, 12, 2–5. (cross ref)
- Role of Internal Lipids in Hair Health. J Cosmet Sci. Sep/Oct 2018;69(5):347-356. (cross ref)