Do we really know the material our contact lenses are made of?

When you are about to buy a new pair of contact lenses, what influences your choice? The answer may be comfort, “I wear them but I do not feel them”…do you know how you can define the comfort of a pair of contact lenses? This is the point we should be more focused on, starting to discover a brand new world: the materials the lenses are made of.

But first it is important to make a premise: the materials are valued by their properties such as oxygen transmissibility, wettability, hardness, resistance to debris, thermal conductivity, specific gravity, thickness and biocompatibility. Oxygen transmissibility is the ability of the material of being flown through by gases; wettability as well as hydrophilicity is the ability of absorbing water; hardness stands for capacity of having strong or weak chemical links; debris are represented by proteins normally produced by our eyes which form a sort of deposit if the material does not resist; thermal conductivity is the ability of the material of spreading the heat produced by our cornea; specific gravity and thickness concern the geometry of the lens itself; biocompatibility is the ability of a foreign body to adapt to the human body, being recognized and accepted by it.

Now let’s try to make things clearer to understand among the wide world of materials used to produce contact lenses. Acronyms such as PMMA, CAB, HEMA,… a constantly evolving world that follows the scientific progress in the both medical and physical-chemical fields, a progress started since 1500 for the contact lenses.

The first was Leonardo da Vinci and then Descartes to study the fascinating world of contact lenses and their ability to change the eye sightedness. Contact lenses as we know them today (except for the materials) were produced in 1800 for the first time when the first glass prototype was created. It is easy to imagine that glass was not that comfortable material as it does not allow any oxygen transmissibility and wettability as well but it is absolutely hydrophobic, preventing the eye from being properly lubricated which could make the elimination of debris easier. Furthermore it is a heavy and hard material and not biocompatible at all. That is why studies went on focusing on plastic materials. The first classification we can do distinguishes rigid materials from soft ones. As concerns rigid materials, the first material used to experiment the creation of contact lenses was PMMA, a chemical formula which describes polymethylmethacrylate, a thermoplastic, transparent, stable, much lighter than glass and with excellent quality for the processing, a materiaI that hardly allows the onset of irritation or allergy. The PMMA had a rapid spread but its problem was being hydrophobic, that is to say poor water permeability and thus the passage of oxygen to the cornea, two extremely important elements for the definition of comfort and therefore of the lens portability. The following material introduced by physical and chemical studies was a polymer deriving from PMMA, a material that was considered as semi-rigid and for this reason named RGP, which stands for rigid gas permeable materials. The first new materials among this new category was the CAB, the cellulose acetate butyrate. It entered the market during the 70s improving the oxygen transmissibility of the material but it was still not enough because it was not that wettable and so it produced lipids deposits and irritations consequently.

To improve the oxygen transmissibility, attention went on composed of material that could put together different polymers (elements of material), each one could bring its own positive characteristics. As a result, silicone and fluorinated materials were mixed uo together. An interesting characteristic of fluoride component is its ability to alter the electrical balance and then to reject the proteins and lipids of the tear film from the lens, removing the risk of irritation and improving comfort for long times too. These copolymers correspond to MMA composed which are basically made of metilmethacrilate, that are based on silicone groups characterized by weaker links that enhance the oxygen transmissibility, giving resistance and low deformability to the material. On the other hand, silicone has a very low hydrophilicity.

This is the reason why monomers such as HEMA and NVP were added to silicone: a new generation of contact lenses was about to rise up, more commonly known as hydrogel contact lenses.

We are now able to say that RGP contact lenses are the most used as long-wear lenses and for a corrective goal as well. They were improved by the mix of copolymers that promoted the evolution of these contact lenses into a proper comfort. Other categories of lenses now have silicone components in order to improve their wettability, MMA to improve their duration through time and the optical elasticity, wettable agents and fluorine for wettability and debris resistance, coloring agents to obtain different colours.

As concerns the world of soft contact lenses, the most common materials are silicone hydrogel materials. Hydrogel was the adding component to silicone to improve its wettability. They have less volume that allows a greater oxygen transmissibility and more water content, up to 70%. Water still remains a significant factor to the thickness of the lens that requires a thicker layer which tends to reduce during the day, giving to the eye the exact quantity of necessary substances to the cornea (water, oxygen, heat) constantly.

Other high-interest compounds are GMMA glycerol methacrylate which has the characteristic of maintaining a good water balance, dehydrates very slowly as it is used and re-hydrates just as quickly when blinking, and phosphocholine, a biomimetic substance used to cover up the lenses enhancing a reduction of accumulation of debris deposits on the lens surface and a constant hydration: this is a component known as a biomimetic soft material that fits in the physiological contest perfectly.

REFERALE MARSHAL PRODUCT

all Product in list are GMMA material

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