**4. Structure-function relationship of conventional and novel CL materials**

Newer, soft lenses (hydrogel) are made from polymers that are inherently flexible or may become flexible through absorption of fluid into the polymer matrix. In general, when speaking of rigid CLs, most commonly used polymers are poly(methyl methacrylate) or PMMA, polyacrylamide (PAA), cellulose acetate butyrate (CAB), and in order to make them gas-permeable various mixtures based on PMMA doped with silicone or fluorine are used. Also, many modification of PMMA based materials are also present in the market (such as poly (2-hydroxyethyl methacrylate (PHEMA), poly (2-hydroxypropyl methacrylate) (PHPMA) etc.).

Fig. 2. Structure of poly methyl methacrylate.

The functions that contemporary contact lense materials are most often required to fulfill, encompass:


4 Will-be-set-by-IN-TECH

(*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

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 quality of vision.

(PHPMA) etc.).

encompass:

Fig. 2. Structure of poly methyl methacrylate.

**3.2 Light perception and visual signal transmission**

the activity of the modulation cells present in different retinal layers.

**4. Structure-function relationship of conventional and novel CL materials**

Newer, soft lenses (hydrogel) are made from polymers that are inherently flexible or may become flexible through absorption of fluid into the polymer matrix. In general, when speaking of rigid CLs, most commonly used polymers are poly(methyl methacrylate) or PMMA, polyacrylamide (PAA), cellulose acetate butyrate (CAB), and in order to make them gas-permeable various mixtures based on PMMA doped with silicone or fluorine are used. Also, many modification of PMMA based materials are also present in the market (such as poly (2-hydroxyethyl methacrylate (PHEMA), poly (2-hydroxypropyl methacrylate)

The functions that contemporary contact lense materials are most often required to fulfill,

• **Dimensional stability** rendered through sufficient strenght and stiffnes – these properties are obtained by matrix of methylmethacrylate, which provides hardness and strength, and ethylene glycol dimethacrylate (EGDMA) which acts as a cross-linking agent adding to

• **Normoxia** rendered through material flexibility and gas permeability – these propeties are obtained by addition of several components, such as silicone (increases flexibility and gas

dimensional stability and stiffness but reducing water content.

permeability through the material's silicon-oxygen bonds (*Si* − *O* − *Si*); however, it also brings in the disadvantage of poor wettability), fluorine (improves gas-permeability (less than *Si*) and improves wettability and deposit resistance in *Si*-containing lenses).

• **Proper adhesion** through controlled wettability – adequate level of adhesion is controlled by inclusion of hydroxyethyl-methacrylate – the basic water-absorbing monomer of most hydrogel-based soft lenses; methacrylic acid and n-vinyl pyrolidone (NVP) monomers are also added, both of which absorb high amounts of water and are usually adjuncts to hydroxyethyl methacrylate to increase lens water content.

**Rigid contact lenses** made of PMMA are recognized by excellent mechanical properties of dimensional stability (flexure resistance, stiffness and resistance to breakage) but practically have no oxygen permeability. Inclusion of *Si* and *F* has introduced gas-permeability properties but have compromised surface characteristics and stiffness. By chemically balancing silicone acrylate (SA) with stiff crosslinking monomers (esters) manufacturers have been able to achieve sufficieng gas-permeability without significantly compromising stiffness. Most contemporary gas-permeable lenses are composed of fluorosilicone acrylate that, due to oxygen's preference to dissolve into fluorinated materials, practically draws in the oxygen from the atmosphere and transports it towards the corneal surface, utilizing its *Si* − *O* − *Si* component.

**Hydrogel based lenses** draw their advantage from the fact that hydrogels are materials that absorb and hold water inside their polymer matrices causing the spaces between the polymer chains to expand. Anything dissolved in the water can potentially enter the hydrogel matrix, depending on the molecular size and the matrix pore size. The pore size ranges from 0.5*μm* to 3.5*μm* for low and high-water content lenses, respectively. The oxygen permeability of hydrogel based contact lenses originates from their water content. Because various hydrogel polymers can greatly alter its chemical and physical properties, they may react differently to changes in pH, osmolarity, temperature and the components of the various lens care products that are used.

**Silicone hydrogel based lenses** use *Si*-doping which simultaneously decreases water content and increases permeability. Having in mind that the solubility of oxygen is very high in *Si* when compared with water, silicone doped hydrogel lenses permeability dramatically increases with lowering water content – which is a logical step towards improving functionality of CL materials. However optimal transport conditions for water and ion through a silicone lens, require an adequate amount of water. Synthesizing *Si*-hydrogel lenses is challenging since involves mixing non-polar (oxygen-rich silicone) and polar component (water) to produce lenses that renders high oxygen permeability, good wettability, flexibility, good optics, reasonable lens movement on the eye and general biocompatibility.

### **4.1 Conventional materials used in our investigation**

We used two types of CL materials: gas-permeable CLs that are manufactured from fluorosilicone acrylate based material (Soleko *SP*40*TM*) and standard non-doped PMMA CLs. The aim was to test the response of this material's surface roughness quality on the nano-level using standard nanotechnological methods and new nano-photonic method.
