FULL PAPER www.ms-journal.de Some Considerations on the Composite Structure of the Human Eye Alexandra Maria Lazar and Mihaela Ioana Baritz* The construction of the human eyeball has characteristics of the composite structure considering the optically transparent media (cornea, crystalline) that participate in the formation of images on the surface of the retina. Both the cornea and the crystalline are formed by thin cellular layers that behave like structures composed of elastic curved plates. The first part of the article presents the theoretical aspects that are the basis of the biomechanical approach of this ensemble. In the second part, the aspects of the modeling of the optical behavior of the eyeball and elasticity of the lens are presented. The final part of the article presents the conclusions regarding the behavior of the composite structure of the ocular globe from a biomechanical and optical point of view. 1. Introduction The eyeball represents for the human body the second most im- portant structure after the brain because it is responsible for the perception, transformation, transmission, and assimilation of ≈85% of the information from the environment to the hu- man body. This aspect makes the whole visual system to be con- sidered a complex structure, evolutionary, and connected with other structures of the human body. The visual system as a whole contains several levels of biological composite structures that al- low the capture and transformation of light radiation into signals transmitted on the optic nerve, which, in turn or through feed- back, analyze and react to the information transported to the vi- sual cortex. In this area, the information from the two eyeballs is received, processed, and stored, and through the optical chiasm, the visual perception process is completed in its entirety. Looking from the point of view of the materials from which the optically transparent media of the eyeball are constituted, one can find a variety of properties that allow these tissues to exhibit character- istics of elasticity, deformability, and resistance to internal and external pressures. Of these structures, the lens of the eyeball is a tissue capable of changing, dynamically, its shape leading to the modification of dioptric power, a phenomenon known as the pro- cess of accommodation. This process is continuous, controlled by feedback through the visual cortex and gradually declines with A. M. Lazar, M. I. Baritz Product Design Mechatronics, and Environment Department Transylvania University of Brasov b-ul Eroilor nr.29, 500036 Brasov, Romania E-mail: mbaritz@unitbv.ro DOI: 10.1002/masy.201900103 age. A number of experimental studies [1] in- dicate that this capability is the response of the lens tissue to external forces, which, in turn, is modulated by the biomechanical characteristics of lens tissues. The knowl- edge of the mechanical properties of the lens is essential for understanding the phys- iological function of the accommodation process, of its functional reserve in the case of the elderly, and not least of the possi- ble surgical intervention to implant the fold- able lens. From the optical point of view, the lens provides one third of the dioptric power of focusing the eyeball and through the ac- commodation process, it adapts to any dis- tance of view to which the target objects are. What many studies have shown regarding the dependence of the level of the process of accommodation on the patients’ advanc- ing age are confirmed by the existence of presbyopia and the way in which the properties of the crystalline tissue modify these characteristics. [2] 2. Optical Modeling of the Eyeball The eyeball is composed of several optically transparent media that form a complex system, having the following characteris- tics: total volume = 6.5 cm 3 , total mass = 7–7.5 g, and average geometric dimensions as in Figure 1. The modeling of the opti- cal calculation of the eyeball is carried out under the conditions of accommodation of the lens and consists in determining its diop- tric power (Table 1). From the optical analysis of the basic element of the human eye, it appeared the necessity of using an optical system with ten spherical diopters, having a stratified structure, separating opti- cally transparent environments with different radii of curvature and refraction indices (Figure 2). 3. Modeling of the Elasticity of the Eye Lens From the point of view of the elastic behavior of such a construc- tive element, it is considered to be made of a material with elas- tic properties, stratified, axially symmetrical, uniform, and prac- tically incompressible. Approaching the study of the state of elas- ticity of a crystalline layer, by adopting the model of thin curved plate [3] came closest to the real behavior of this constructive ele- ment. For this each layer that forms the crystalline structure is considered as a thin curved plate (d i = 0.4 mm << D u = 8.5 mm) and is defined by a median curved surface. A surface with an Macromol. Symp. 2020, 389, 1900103 © 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1900103 (1 of 3)