Our eyelashes protect our eyes from foreign bodies. Now a study shows that the shape and structure of the small hairs are perfectly designed to quickly drain water and thus preserve our clear vision. The microstructure within each individual eyelash as well as the slightly curved shape and the arrangement of the eyelashes next to each other contribute to this. The results can help improve artificial eyelashes – be it for cosmetic purposes or for technical applications.
Over the course of evolution, the amount of hair on our bodies and faces has decreased significantly. However, our eyelashes have been preserved. Many other mammals also have these sturdy, slightly curved hairs on the upper and lower eyelids. But what function do they fulfill? Previous studies have suggested that eyelashes protect our eyes from small particles and other foreign bodies. However, the exact function is still controversial.
Microstructure resembles a staircase
A team led by Shan Zhou from the Chinese Academy of Sciences in Beijing has now taken a closer look at human eyelashes. To do this, the researchers examined the microstructure of each individual hair under an electron microscope. On the other hand, they tested the properties of the eyelashes in their natural arrangement and with mascara on three-dimensional models and on human volunteers.
“Our results show that human eyelashes are able to allow liquids to slide off quickly and in a targeted manner through a combination of different structural features,” reports the team. This begins at the microscopic level: “Electron microscopic images of naturally shed eyelashes revealed that the hairs have a step-like microstructure.” Several layers of overlapping cells form a kind of staircase that leads down from the base to the tip of the eyelash. For example, if a drop of water hits the eyelash, it tends to move down the stairs, i.e. towards the tip of the eyelash.
Optimized for liquid removal
In addition, the macroscopic shape of the eyelash helps to quickly remove water. As Zhou and his team report, the curved shape of the eyelash corresponds to a so-called brachistochrone – i.e. the path between two points on which a ball travels most quickly from the starting point to the end point due to the influence of gravity. Arranging your eyelashes next to each other also helps get rid of liquids quickly. “Eyelashes are densely arranged at the base and quasi-parallel,” describes the team. This means they form an effective and flexible barrier. The eyelashes ensure that we hardly get any sweat in our eyes even during strenuous exertion and that our eyes stay largely dry even in the shower or in the downpour.
“To investigate the underlying mechanisms, we dropped a drop of water onto the eyelashes of a test subject looking straight ahead,” explain the researchers. They recorded the experiment with a high-resolution camera. The result confirms the assumptions based on the structural observations: “The drop slides off the curved eyelashes without causing any backflow or visible residue,” report Zhou and his team. Using calculations, they also showed that the drop slides off 20 percent faster than on a rigid, straight slope.
Practical applications
From the researchers’ perspective, these results could have numerous practical implications. The team showed that the use of cosmetic products such as mascara reduces the hydrophobic properties of the eyelashes because the layer of make-up covers the sophisticated microstructure of the eyelashes. Even if the eyelashes are curled upwards with the help of pliers and similar beauty tools, they have a harder time draining water. “A child’s naturally curved eyelashes exhibit a design optimized for both protection and aesthetic appearance,” Zhou and colleagues write. “For people with sparse eyelashes, artificial eyelashes that follow the natural shape could offer a solution that allows the appearance to be improved without compromising the protective function.”
But according to the researchers, artificial eyelashes could also be important beyond cosmetics. “Understanding eyelash structure has broader implications and can inspire the development of innovative water protection solutions for various applications,” they explain. It would be conceivable, for example, to equip the lenses of photo drones with eyelashes, or to provide electronics with filter screens made of eyelashes that deflect water and at the same time ensure good ventilation.
Source: Shan Zhou (Chinese Academy of Sciences, Beijing) et al., Science Advances, doi: 10.1126/sciadv.adr2135