On the correlation between the curvature of the human eyelash and its geometrical features

New Insights Into the Anatomy of the Eyelashes With Regard to Ethnic Differences Against a Clinical Background

BACKGROUND

To investigate the root depth, root angle, and light and scanning electron microscopical anatomy of human eyelashes relevant to eyelash ablation.

METHODS

Eyelash root depth, the angle between eyelash root and skin epithelium, spatial relationship, and scanning electron microscopical features of the eyelashes were studied on 4 upper and 4 lower eyelids of Caucasian (n = 4) and Indian (n = 4) cadaver heads according to a set protocol.

RESULTS

There were significant differences in the mean eyelash root depth between Indians (2.3 ± 0.38 mm) and Caucasians (1.9 ± 0.26 mm; p = 0.007), as well as between upper eyelids and lower eyelids (1.9 ± 0.2 mm vs. 1.8 ± 0.1 mm). The mean angle between the lash follicle root and the skin epithelium was 75 ± 11 degrees and similar in both ethnic groups. The eyelash bulb was located close to the tarsal plate and meibomian glands and formed an angle of less than 15 degrees with the eyelash root. Scanning electron microscopy studies revealed that the eyelash bulb was 202 ± 12 μm wide in Indians and 170.6 ± 16.8 μm wide in Caucasian eyelids ( p = 0.08). The eyelashes were placed more closely in Indian eyelids than in Caucasian eyelids ( p = 0.03). The width of the cuticle layer varied between the hair shaft and the inner eyelid segment.

CONCLUSIONS

There are differences in eyelash root depth, inter-eyelash distance, and cuticle thickness between Indian and Caucasian eyelids. The oblique orientation of the eyelash root and close proximity of the eyelash bulb to the tarsal plate should be kept in mind while doing the electroepilation procedure.

Nanoindentation study of human fingernail for determining its structural elasticity

BACKGROUND

Human nails play an important role in transmitting force to the fingertips, and their mechanical properties are important indices. The nail has a three-layered structure consisting of top dorsal, middle intermediate, and under ventral plates, and its internal structure is believed to affect its mechanical properties. However, this has not been investigated in previous studies.

METHODS

The Young's moduli of the top, middle, and under plates were measured using nanoindentation, and a theoretical model was developed to estimate the structural elasticity for the bending deformation of human nails, which is an index describing the deformability of the nail without depending on its external dimensions. The structural elasticity of human nails was compared with that of human hair collected from the same person. The effect of the softening treatment on the nails was also evaluated.

RESULTS

The Young's moduli of the top, middle, and under plates measured using nanoindentation were 2.9, 3.1, and 2.8 GPa, respectively. The structural elasticity of the nail was estimated to be 2.9 GPa, approximately 75% that of hair. Moreover, softening treatment with a urea cream reduced the structural elasticity of the nail to 70%.

CONCLUSION

This paper proposed a method for estimating the structural elasticity of a human nail with a three-layered structure. This index is a mechanical property with "Pa" as a unit, and is useful for comparing deformability with the Young's modulus of other homogeneous materials or for investigating the effect of various treatments quantitatively.

Division of force among layers constituting human hair during bending and tension

Human hair is a three-layered structure comprising the inner medulla, middle cortex, and outer cuticle layer. When a hair is subjected to bending or tensile load, each of these layers absorbs a certain amount of the force applied. However, the magnitude of the force absorbed by each layer is not easy to estimate. This is because, in addition to Young's modulus of each layer, the absorption depends on the area of each layer as seen in the cross-section. This study used a strategic way of combining experiment and theory and found that Young's modulus of the cuticle layer changes in the face of bending and tension. Considering this, the ratio of force sharing inside a human hair was estimated. Bending and tensile tests were conducted on single human hairs to determine the structural elasticity for both deformations which expresses the deformability of the hair independent of its external dimensions. Moreover, Young's modulus of each layer was determined by nanoindentation of hair cross-section. By comparing the structural elasticity determined experimentally with that determined theoretically, Young's modulus of the cuticle layer against tension was found to be 25% of that against bending. Based on this finding, it was found that the cuticle layer bears 35% of the force endured during bending, and the cortex bears the rest; when subjected to tension, the cortex bears more than 90% of the force.

Modeling the protective role of human eyelashes against ultraviolet light exposure

The role of eyelashes in ocular radiation protection has been hypothesized for some time. There is however no quantitative knowledge of the shading they provide. The ocular protection provided by eyelashes is investigated in this study. A numerical model able to simulate an arbitrary source of light to illuminate a 3-dimensional head model with realistic details was used for this purpose. The eyelashes' filtering effect was studied for various light incidence angles, diameter and density of cilia. Using average values provided by literature to define their characteristics, we found that eyelashes reduce ultraviolet light received by the cornea by about 12-14%, with maximum values of 24%. These results suggest that the eyelashes can be an important element of the human eye protection system and their role should be further investigated.

Structural elasticity for tensile deformation of a single human hair and the comparison with it for the bending deformation

Human hair is a multi-layered structure, which consists of the inner medulla, middle cortex, and outer cuticle. Therefore, the mechanical properties of the hair are related not only to the Young's modulus of each layer but also to the internal structures. Although the tensile test of a human hair has been performed elsewhere, the deformability of the hair for the tensile deformation is determined as the Young's modulus of the hair structure, which is similar to that of metals. In this paper, the structural elasticity of a single human hair for the tensile deformation, which expresses the deformability of a hair by tension without being dependent on external dimensions and shape, is defined based on the theoretical model, and is measured by performing the tensile test under the digital microscope observation. The values of the structural elasticity for the tensile deformation of the hair samples collected from healthy persons are compared with the values obtained for bending deformation. The structural elasticity for the tensile deformation of the hair sample is found to be lesser than that of the bending deformation, and this is verified to be always valid provided the Young's modulus of the outer cuticle is greater than that of the middle cortex.

How eyelashes can protect the eye through inhibiting ocular water evaporation: a chemical engineering perspective

Bionics is a fascinating subject that has inspired many inventions through learning from biological structures and functions. In this work, a coupled multi-physics model has been developed to characterize ocular water evaporation with realistic eyelash structures taken into account. From a chemical engineering perspective, the protective function of human eyelashes in terms of evaporation inhibition has been rationally revealed. Systematic investigations were carried out to elucidate the effects of different eyelash lengths, orientations and inlet air directions on water evaporation on the ocular surface. The results clearly demonstrate that regardless of inlet air directions and eyelash orientations, increasing eyelash length from zero to an optimal length can effectively reduce water evaporation. However, further increase in the eyelash length can lead to enhanced evaporation. For the normal and parallel inlet air directions, the optimal eyelash length is around 15-30% of the eye width and can offer approximately 10-30% evaporation reduction when compared with the cases without eyelashes. These values are independent of the eyelash orientation. This investigation provides valuable data for in-depth understanding of the protective function of the eyelashes, which can be used in the future to improve and optimize bionic designs inspired by human eyelashes.