At first glance, eyelashes seem very simple – a row of fine hairs arranged in a curve above and below the eye. Beneath their delicate appearance lies an intricate structure that has evolved to balance protection with resilience. With this article I want to so that each eyelash is a micro-engineered miracle, which can endure all sorts of insults while helping to shield the eyes.
Every eyelash is a complex layered structure and although it appears simple to the naked eye, its internal structure is a hierarchy, extending from the atomic level of bonds between individual amino acid, up to a wrapped up bundle of fibres that makes the visible hair fibre itself.
Keratin provides the majority of the hair structure and it is composed, at the lowest level, of amino acids. These basic building blocks assemble into keratin proteins which are long, microscopic coiled chains. Several keratin molecules then twist together to form coiled-coil tetramers, and groups of tetramers combine to form intermediate filaments. These filaments bundle into microfibrils, which assemble further into macrofibrils
At this point, everything is still invisible to the naked eye, but the fibres are now visible under a microscope as the cortex of the hair. This dense and deep hierarchy which extends from amino acids through keratin proteins, the various filaments, up to the entire hair body results in a material that is has many useful properties. It is incredibly strong despite its light weight. Its resilience allows it to bend without breaking, then return back to its original form. It can resist being crushed and will resist being torn – with the hair more easily plucked out the follicle than the shaft torn apart.
Around the outside of the hair are overlapping cuticle cells. They overlap like the petals of a rose, often with 5 to 7 overlapping layers protecting the hair, and the cuticle is the first line of defence for a hair. They shield the interior of the hair from damage while also allowing the hair to rub against other hairs or surfaces with minimal friction. Eyelash cuticles are typically more tightly packed than those of scalp hair, giving them greater rigidity and helping them maintain their natural curvature. Conditioners are designed to smooth and protect this outer cuticle layer. Un-conditioned hair is ragged as the cuticles are lifted, which makes the surface look similar to a Christmas tree. Conditioners work by depositing positively charged conditioning agents onto the negatively charged hair surface. This reduces static and forces the cuticles to flatten, so the hair becomes smooth again on a microscopic level. This decreases friction between hairs and the smoother surface improves light reflection.
The main structural material of the eyelash is α-keratin, a fibrous protein rich in the amino acid cystine. This amino acid is linked by sulphur bonds to other molecules of cystine to give the resulting keratin strands a tight, spring-like structure that resists bending and breakage.
Surrounding the cuticles on the surface of the hair is a lipid envelope known as the F-layer. This outer layer contains a unique fatty acid called 18-methyl eicosanoic acid (18-MEA), which makes hair water repellent. In eyelashes this layer is particularly important due to the exposure of the hairs to tears or water from external sources like rain. The waterproof nature of eyelashes, combined with blinking, help to quickly move the water away from the eye and help keep it clear.
All hair colour is determined by the amount and type of melanin stored within the cortex. It is inserted into the growing hair by cells called melanocytes that are found in the tissue surrounding each follicle. There are two main forms of melanin: eumelanin which produces a dark pigment, and pheomelanin which produces a red or yellow tone. Darker hairs are richer in eumelanin, lighter hairs have less eumelanin, and red hair has a high proportion of pheomelanin. The proportion varies immensely between people and also between individual hairs.
Melanin acts as a natural sunscreen by absorbing ultraviolet (UV) radiation before they can damage proteins found within the hair. Studies suggest that eumelanin-rich hairs demonstrate far greater photostability than pheomelanin-dominant ones, which are more prone to structural damage from sunlight.
Cumulative damage from sunlight leads to degradation of the hair, by breaking down the keratin bonds which cannot be repaired. This results in the hair becoming weaker and ragged, which gives a poorer appearance as well as increasing the likelihood of the hair breaking. This knowledge led us to the creation of the world’s first sun protect eyelash serum that shields the lash from UV damage.
As we age the cumulative oxidative stress causes a decline in melanocyte activity and a gradual reduction in melanin synthesis, leading to greying or colour dilution. Interestingly, eyelashes tend to turn grey at a later age than the scalp hair and this is thought to be related to an increased expression of a certain gene in the eyelash follicle compared to the scalp. Melanin also acts within the hair as an anti-oxidant, so a reduction in melanin also makes hair more susceptible to oxidative damage from the sun and pollution.
All human eyelashes share the same layered keratin structure but there are differences in the curvature, thickness and pigmentation due to subtle genetic and anatomical differences. The shape of the follicle, and the aperture from which it exits the skin, determines how straight or curled of any hair on our body. All eyelash follicles tend to have a comma shape when viewed on the horizontal axis and a curve when viewed in full profile, shown in the centre of the following image.
A paper published in the British Journal of Dermatology (Na et al., 2006) reported that eyelash curvature and diameter differ by ethnicity. They found that when compared to Europeans, East Asian lashes are generally straighter, slightly thicker, with more overlapping layers of cuticles. There were fewer lashes in the Asian population but no difference in length. These distinctions are believed to arise from differences in follicle shape and the orientation of the dermal papilla within the eyelid margin. A 2010 paper in the same journal, commissioned by L’Oreal, found that the curl of the eyelash is likely achieved by the greater activation of certain genes on one side of the follicle compared to the other which leads to an asymmetrical growth pattern that curves away from the eye.
Ultraviolet light, pollution and friction from mascara removal can all degrade the eyelashes to varying levels. Waterproof mascaras and the perming solutions used for lash lifts are particularly problematic because they strip away the protective 18-methyl eicosanoic acid (18-MEA) lipid layer on the cuticle surface. Once this barrier is lost, the overlapping cuticle cells begin to lift, exposing the cortex and reducing the lash’s flexibility and gloss.
Ultraviolet radiation is one of the most significant contributors to hair degradation. It breaks the disulphide bonds that hold keratin strands together, and UV oxidises both proteins and lipids within the hair shaft. In a 2015 paper in the Journal of Photochemistry and Photobiology, researcher scientists demonstrated that UV rays from normal sunlight damaged both the structure of the hair and the colour. An older paper by Bhushan (2008) looked at hairs under scanning electron microscopy (SEM) and found that hairs exposed to sunlight display increased surface roughness, missing cuticles, and reduced adhesion between scales. This surface damage weakens the hair and alters how it reflects light, leading to loss of sheen.
Air pollution is increasingly recognised as a significant factor in hair and scalp damage. Particulate matter (PM₂.₅ and PM₁₀), polycyclic aromatic hydrocarbons, heavy metals, and airborne ozone can all interact with the hair surface to cause damage by oxidation. The result is a measurable decline in hair tensile strength, increased surface roughness, and fading of natural pigmentation.
A 2018 study in the Cosmetics Journal reported that exposure to airborne pollutants alters both the cuticle morphology and the lipid composition of hair. The 18-methyl eicosanoic acid (18-MEA) layer that coats the hair surface is particularly sensitive to oxidative attack, and once it is disrupted, cuticle lifting and protein loss follow. In addition to these surface effects, chronic exposure has been linked to scalp inflammation and premature follicle ageing, highlighting the need for protective formulations that include antioxidants and barrier-enhancing ingredients.
At Opti Laboratories, we have observed similar effects in our own testing. In a study that is pending publication we found that measurable damage to the hair after just eight hours of UV exposure. We then repeated the experiment using our sun protect serum and demonstrated that after the same 8 hours in the sun, these protected hairs maintained smooth and continuous cuticle alignment. Even without UV exposure, the treated samples displayed lower average cuticle edge angles, suggesting that protective coatings can reinforce the hair’s natural lipid barrier, act as an anti-oxidant, and improve the overall resilience of the hair.
Eyelashes are more than decorative. They are adaptive biological fibres designed for protection and endurance. Their compact cuticle structure, keratin strength, and dense pigmentation make them a miniature model of biomaterial engineering.
Protecting this structure matters as the hair has no in-built mechanism to repair itself. This means that any damage builds up over time, leading to duller hairs that are more likely to break off and shed.
Every coat of mascara and every hour of sunlight influences how the lash fibre weathers over time. Maintaining the integrity of the cuticle and lipid layer is essential for preventing brittleness and ensuring lashes continue to perform their dual role: safeguarding the eye and defining its beauty.
For more information on how to protect your lashes, contact us or browse our scientifically-proven range of products.
Reference:
Bhushan, B. (2008) ‘Nanoscale characterization of human hair and hair conditioners’, Progress in Materials Science, 53(4), pp. 585–710.
Available at: https://www.sciencedirect.com/science/article/abs/pii/S0079642508000029
Kim, K.E., Cho, D. and Park, H.J. (2018) ‘Air pollution and skin diseases: Adverse effects of airborne particulate matter on various skin diseases’, Cosmetics, 5(1), 17.
Available at: https://www.mdpi.com/2079-9284/5/1/17
Lademann, J., Patzelt, A. and Darvin, M.E. (2020) ‘The effects of environmental pollutants and oxidative stress on hair and scalp’, Skin Appendage Disorders, 6(4), pp. 262–272.
Available at: https://karger.com/sad/article/10/4/262/902859/The-Effects-of-Environmental-Pollutants-and
Ryu, H.S., Kim, D. and Shin, H. (2021) ‘The impact of air pollution on hair biology: A review of mechanisms and protective strategies’, International Journal of Trichology, 13(5), pp. 193–199.
Available at: https://pmc.ncbi.nlm.nih.gov/articles/PMC8534476/
Nascimento, A., Almeida, F. and Cestari, T. (2015) ‘Effects of solar radiation on hair and photoprotection’, Journal of Photochemistry and Photobiology B: Biology, 153, pp. 240–246.
Available at: https://www.sciencedirect.com/science/article/pii/S1011134415003206
Na, J.I., Kwon, O.S. and Kim, B.J. (2006) ‘Ethnic characteristics of eyelashes: a comparative analysis in Asian and Caucasian females’, British Journal of Dermatology, 155(6), pp. 1170–1176.
Available at: https://doi.org/10.1111/j.1365-2133.2006.07495.x
Aumond, S., de Lacharrière, O., Saint-Léger, D. and Loussouarn, G. (2018) ‘The eyelash follicle features and anomalies: A review’, Clinical, Cosmetic and Investigational Dermatology, 11, pp. 191–201.
Available at: https://pmc.ncbi.nlm.nih.gov/articles/PMC6147748/
Zhang, Q., Li, X. and Zhou, J. (2014) ‘Human eyelash characterization’, Journal of Cosmetic Science, 65(5), pp. 297–304.
Available at: https://www.researchgate.net/publication/26870800_Human_eyelash_characterization
Tohmyoh, H., Takahashi, M. and Nitta, K. (2018) ‘On the correlation between the curvature of the human eyelash and its geometrical features’, Proceedings of the Institution of Mechanical Engineers, Part H: Journal of Engineering in Medicine, 232(10), pp. 1013–1019.
Available at: https://www.researchgate.net/publication/326184736_On_the_correlation_between_the_curvature_of_the_human_eyelash_and_its_geometrical_features