**1. Contact lense industry**

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The contact lens (CL) industry and market have displayed a high level of dynamism in the past few decades, and have evolved into a rapidly changing field in which science and everyday practice constantly interact, not only through broadening of material and product portfolio, but through innovative therapeutic and diagnostic solutions as well.

Stable market growth with numerous rearrangements in different product segments is constantly taking place, mainly stirred by innovative material and optical design. The standardly used hydrogel materials are being rapidly replaced by silicone doped hydrogel materials. The analyses of customer CL usage and satisfaction indicate continued market growth in future, however with many changes in product profile and significant increase in multifocal and daily disposable lenses market share.

The main impulse behind the dynamism of CL industry stems from results of scientific and technological improvements, which are enhancing medical field and reminding us that the focal point of sustainable development lies in scientific investigations.

### **2. Contact lenses in present, past and future**

The technology of materials used in CL production has improved vastly in the past decades starting from glass and moving to polymer based materials (PMMA) with, eventually, major steps being taken in including hydrogel and doped-hydrogel based materials, shifting the functionality of CLs from rigid gas-impermeable (RGP) to soft gas-permeable materials represented by silicone hydrogel materials that are now in use.

This chapter will focus on multimodal applied research of rigid gas-permeable contact lenses (CL) that are manufactured from fluorosilicone acrylate based material. Our multimodal research comprises measurement of intermolecular interactions on the basis of optical, mechanical, morphological and magnetic properties of CL material.

The role of our research in such a complex system of CL industry was introduction of new diagnostic modalities through improved material characterization.

In the course of last decade, scientists have developed different possibilities of "on eye"CL application that are not related to its optical capabilities for which they were invented in the first place – correcting eye's refractive error. Furthermore, improvement in CL material manufacturing, both soft and rigid gas permeable, are mostly directed towards increasing oxygen permeability and wearing comfort. Rarely today, CL producers are dedicated to improving CL material properties for the purpose of enhancing the quality of vision, on the contrary, by doping them with silicone, for example, the optical properties become even worse.

Apart from it's properties to correct eye's refractive anomaly (dioptric power), the most frequent factors influencing quality of vision while wearing RGP CL are those related to the fact that visible light, on it's way to the eye's "perception area"– the macula, must pass through CL material itself, and all it's characteristics can seriously modify it.

Geometrical optics and related functions of the eye (vision acuity etc.) should not be considered as the only one mechanism of light interaction with human organism. We also consider physical inputs that influence the functioning of the central nervous system (CNS) on the basis of optical-neuronal interactions, and point to perspectives of investigating the role of CLs in modifying the influence of light for therapeutic purposes, or using the CLs as a potential diagnostic tool in monitoring the state of other systemic parameters (such as serum glucose level etc). As a rule, when comparing "naked eye"vision with the vision aided by CL it is inevitable to conclude that vision with CLs is of lower quality, due to reduced contrast sensitivity, sub-normal color's perception, spherical and chromatic light aberrations. All these are considered mostly consequence of CL material imperfectness. The aim of our research is to organize a setup for development of a novel material for RGP CL production which should, after adequately lathe cut and polished, improve it's optical properties in transmitting visible and "near visible"light, while increasing contrast sensitivity for "on eye"usage and improving color perception with simulaneous reduction in both higher and lower order light aberrations. New, advanced CL materials, are still needed because, regarding biocompatibility and oxygen permeability, the advantages offered by high oxygen delivery have solved many hypoxia-related clinical problems, but the complications related to inflammation, infection and mechanical insult to the cornea still occur. The good news is that the industry continues to work on the next innovative materials and designs while our patients, and us, enjoy the silicone hydrogel lenses and the big step they represent toward safer and more effective extended (toward continuous) contact lens wear.

### **3. Physiological considerations related to contact lenses wearing**

### **3.1 Tear layer considerations**

Tear layer is constantly renewed tripartite film that covers conjunctival and corneal surface. All layers are separated:


The tear layers have many different functions:


Fig. 1. Structure of sublayers that constitute tear layer.

Presence of any contact lens on the front surface of the eye influences physical and chemical properties of the tears and disturbs tear film stability. These changes in the healthy tear film are most likely caused by the following effects on the eye:

• Contact lens surface

2 Will-be-set-by-IN-TECH

improving CL material properties for the purpose of enhancing the quality of vision, on the contrary, by doping them with silicone, for example, the optical properties become even

Apart from it's properties to correct eye's refractive anomaly (dioptric power), the most frequent factors influencing quality of vision while wearing RGP CL are those related to the fact that visible light, on it's way to the eye's "perception area"– the macula, must pass

Geometrical optics and related functions of the eye (vision acuity etc.) should not be considered as the only one mechanism of light interaction with human organism. We also consider physical inputs that influence the functioning of the central nervous system (CNS) on the basis of optical-neuronal interactions, and point to perspectives of investigating the role of CLs in modifying the influence of light for therapeutic purposes, or using the CLs as a potential diagnostic tool in monitoring the state of other systemic parameters (such as serum glucose level etc). As a rule, when comparing "naked eye"vision with the vision aided by CL it is inevitable to conclude that vision with CLs is of lower quality, due to reduced contrast sensitivity, sub-normal color's perception, spherical and chromatic light aberrations. All these are considered mostly consequence of CL material imperfectness. The aim of our research is to organize a setup for development of a novel material for RGP CL production which should, after adequately lathe cut and polished, improve it's optical properties in transmitting visible and "near visible"light, while increasing contrast sensitivity for "on eye"usage and improving color perception with simulaneous reduction in both higher and lower order light aberrations. New, advanced CL materials, are still needed because, regarding biocompatibility and oxygen permeability, the advantages offered by high oxygen delivery have solved many hypoxia-related clinical problems, but the complications related to inflammation, infection and mechanical insult to the cornea still occur. The good news is that the industry continues to work on the next innovative materials and designs while our patients, and us, enjoy the silicone hydrogel lenses and the big step they represent toward safer and more effective

through CL material itself, and all it's characteristics can seriously modify it.

extended (toward continuous) contact lens wear.

specific pre-albumin, lactoferin and other.

The tear layers have many different functions:

• Many of its constituents prevents ocular infection.

**3.1 Tear layer considerations**

glycocalix of the microplicae.

All layers are separated:

of the tears.

**3. Physiological considerations related to contact lenses wearing**

Tear layer is constantly renewed tripartite film that covers conjunctival and corneal surface.

• **Mucin layer** is the innermost and it is anchored to the corneal and conjunctival epithelial

• **Aqueous layer** is the middle one and represents 90% of the tear film thickness. It consists of water and salts dissolved in it, together with dissolved glucose, lysosime, for tear's

• **Lipid layer** is the outermost, secreted by the Meibomian glands and it retards evaporation

• Keeping the surface of cornea smooth and that way making it optically clear.

• During blinking tear film lubricates the friction area between lids and ocular surface.

worse.


In comparison to the normal pre-ocular tear film, conventional (hydrogel) soft contact lens wearers have pre-lens tear film with all three layers of the tear film reduced. Tear film quickly deteriorates and renders poor surface wettability capability (weak attachment of ocular surface epithelium to the mucus layer). Pre-rigid gas permeable lens tear film is even thinner and more unstable then that of soft lens. Its lipid layer is often absent so the aqueous layer quickly dehydrates. In silicone-hydrogels, pre-lens tear film is something like combination of the previous two tear films although lipid layer is always present. As in the case of pre-ocular tear layer, good quality pre-lens tear film layer is *conditio sine qua non* for adequate optical properties of the eye.

Each eye consists of light receptor layer in the retina, optically active tissues that focuses the light on the receptor layer and nerve fiber system (retinal ganglion cell's fibers and optic nerve) which conducts electrical impulses created during electro-chemical reaction in the receptor layer (provoked by light absorption) all the way to the visual cortex and other parts of the brain.

Optically, eye functions very similar to photo-camera, inside the eye it is totally dark and this is provided by its outermost protection layer – the *sclera*, and pigmented parts of the middle layer – the *choroid*, which also provides most of the eye's blood supply. Light transmitting into the eye is absolutely controlled by the very agile diaphragm – the *pupil*. In order for light to be focused at the center of the eyes' reception layer it has to be refracted by *cornea*: absolutely transparent convex-concave lens and further refracted by another agile tissue in the eye - *the crystalline lens*. In order to prevent light reflection from behind the receptor layer to interfere with and disturb the primary light stimulation of the receptor cells, a pigmented cell layer is situated just behind the receptor layer and absorbs all the residual light.

*Emmetropisation* of the eye is a very sensitive process of dosing the eye's optical activity in relation to its axial growth. Any disturbances in this results in refractive errors of the eye (*myopia*, *hyperopia* and *astigmatism*). Eye, just as any other optical device, is also prone to higher order light aberrations that influence the stimulation of the receptor cells therefore influencing the quality of vision.

### **3.2 Light perception and visual signal transmission**

Eye's light receptor cells (rods and cones) have large amounts of photo-sensitive pigments (*opsin* and *retinen*). Once stimulated by visible light (397 – 723 nm) these photo-sensitive pigments are changing their structure which leads to the chain of events that are ending in nerve activity. Rods are much more sensitive to the smallest amounts of light stimulation and they are most active in the dark environment (*scotopic* conditions) when pupil (the diaphragm) is dilated in order to receive as much light as possible. On the other hand, cones are much less light-sensitive and are active only in *photopic* conditions when the pupil shrinks in order to prevent too much light entering the eye which can disturb highly sophisticated visual functioning like color vision and small detail discrimination. Light stimulation blocks *Na*+/*K*<sup>+</sup> channels in the receptor cell and disturbs the balance of ions in the sense of hyper-polarization of the cell. This results in reduction of the amount of the synaptic neuro-transmitter (normally released in certain amounts without light stimulation) that triggers electrical activity in the retinal ganglion cell which is conducted to the brain. Before it reaches the optical nerve, electrical activity created in the receptor cells is changed by the activity of the modulation cells present in different retinal layers.
