Contact lens

 

As simple as it may sound, there is complex chemistry behind contact lenses that many of us wear every day. Here we take a look at some of the different chemicals that have been used over the years and how contact lenses have gradually evolved. 

More and more people are turning to contact lenses to correct vision in their daily lives. The choice can be based on lifestyle or cosmetic considerations. So switching to Contacts can give you the comfort of a life without glasses. Soft contact lenses were first introduced in the 1970s and the number of wearers continues to grow every decade.

History:

Contact lenses were first invented in the late 1800s. The first contacts were made of blown glass and were very painful to wear. The next development occurred in the 1930s and 1940s, which brought with it hard plastic contacts. These were much more comfortable, but still did not absorb water or allow enough oxygen to pass through the lens to the eye.

 

In 1971, soft contact lenses were officially launched in the United States. Soft plastic lenses are more flexible and easier to use. They also absorb water, which helps increase the flow of oxygen to the eyes. In 1982, a man from Denmark invented disposable lenses. The next breakthrough didn't come until 1995 when disposable contact lenses hit the market.

Eye problems:

In normal vision, light penetrates the cornea and is focused on a single point on the retina. This is in the back of the eye. 

 Visual disturbances are common when the cornea does not focus properly, usually due to its shape. Or there may be trouble adjusting the curvature of the natural lens. You may be familiar with the more common nearsighted or farsighted vision disorders. However, eye problems present themselves in a variety of different problems. There are also many different solutions.

Myopia is also known as nearsightedness. This means that you can see objects up close, but your distance vision is blurry. This occurs when the eyeball is too long, causing light to focus in front of the retina. The contact lens used for this condition is a concave or divergent lens that increases the focal length of light entering the eye. The concave lens has a thinner center and thicker edges that deflect extracted light from objects that are far away. The deflected light is then combined by the cornea and lens of the eye to produce a clear image on the retinal layer.

Hyperopia is also known as farsightedness. This means that your distance vision is fine, but nearby objects will be blurry. This occurs when the eyeball is too short for light to focus behind the retina. The contact lens, which is naturally convex or converging, is used to correct this condition. The converging lens reduces the focal length of the light, proportionally reducing the image distance. Combines light rays from an object before they reach the eyes. The converging light is then deflected by the lens of the eye. The combination of two lenses allows the image of close objects to be focused on the retina.

Astigmatism Irregularly shaped corneas cause astigmatism. This distortion causes light to focus on several different points along the fundus. It is corrected with lenses that have different thicknesses and curvatures across the lens. It all depends on the specific visual needs of the person. A special lens known as a toric contact lens is used to correct the curvature of the cornea of ​​the eyes that causes astigmatism. 

It is designed in such a way that the lens can develop different focusing potentials in the vertical and horizontal directions. The toric lens has different focal lengths and refractive power, which are formed at a 90 ° angle to each other. When light rays from an object pass through the toric lens at right angles, the multiple focal points achieved on the retina are corrected.

Presbyopia occurs when a person has difficulty concentrating on nearby objects. This is because the lens loses its elasticity. So this problem is more common in the elderly. It is usually corrected with bifocal and multifocal lenses. Bifocal contact lenses have a concentric bifocal pattern; in other words, it is concave in the middle, but convex around for a clear view of the near and far regions. Wearing monovision contact lenses is another way to correct presbyopia, with the lens of one eye focusing on nearby objects while the other eye focuses on objects that are far away.

Rigid contact lenses:

Rigid poly (methyl methacrylate) (PMMA) contact lenses were the first available. Before that, glass lenses had to be worn. PMMA lenses were not very permeable to oxygen, so improved rigid materials were developed. These newer rigid lenses contain compounds that contain silicone and/or fluorine. Also known as gas permeable (GP) contact lenses, they maintain the shape of the cornea and allow the cornea to oxygenate.

 

The GP lens obscures the liquid lens in the center of the cornea and lens and completely eliminates the abnormality in the anterior area of ​​the eye. It also contracts the cornea to a smoother level so that it can take the correct shape of the posterior lens to allow clearer vision. People with astigmatism can get the most out of GP contact lenses. Rigid lenses are chemically inert, so they can be used in harsher environments than soft lenses.

Soft contact lenses:

Soft contact lenses are the most commonly used. Soft lenses are more flexible than rigid lenses and can be rolled or bent smoothly without damaging the lens. While rigid lenses require a period of adjustment before achieving comfort, new soft lens wearers tend to report lens awareness rather than pain or discomfort. 

These are based on hydrogels, the first of which was poly (hydroxyethyl methacrylate) (PHEMA). Hydrogels are networks of cross-linked polymer chains that are highly absorbent of water. More recently, siloxane-containing hydrogels have been developed that are more permeable to oxygen. 

Typically, soft contact lenses are mass-produced, while rigid contact lenses are manufactured to precise specifications for the individual patient. Spincast lenses: a soft lens made by spinning liquid silicone in a rotating mold at high speed. 

Molding is used to make some brands of soft contact lenses. Rotating molds are used and the molten material is added and shaped by centripetal forces. Injection molding and computer control are also used to make near-perfect lenses. The lens stays wet throughout the molding process and never dries out or rehydrates.

Contact lens solution:

Wearing contact lenses can be more comfortable if you clean them regularly. Visual acuity is also better. Cleaning solutions help remove dirt and protein build-up from the lens surface. There are many different multipurpose solutions on the market that you can use to clean, disinfect, and store contact lenses. Cleaning solutions are peroxide or multipurpose solutions. The peroxide solution uses peroxide for disinfection; Multipurpose solutions use polymeric detergents such as biguanide or polyquaternium. Both contain other cleaning and hydrating chemicals. 

Boric acid is the active ingredient found in some brands of multipurpose contact lens cleaning solutions. Boric acid, like hydrogen peroxide, is a disinfecting and antifungal agent used in contact lens solutions to prevent the growth of microorganisms in the eye. These sterile solutions are often preserved with ascorbic acid or edetate disodium. Ascorbic acid is an effective preservative to protect against bacterial and fungal growth on contact lenses.

Reference:

1) https://en.wikipedia.org/wiki/Contact_lens

2) https://www.allaboutvision.com/en-in/contacts/contact-lenses/

3) https://www.uofmhealth.org/health-library/ut1799

4) https://healthfully.com/the-ingredients-in-contact-lens-cleaning-solution-5905862.html

5) https://coopervision.com/blog/how-our-contacts-are-made