What are Contact Lenses Made of?
Nearly all modern soft contact lenses are made of hydrogel, a material that loves water, and clings to and absorbs it readily. In fact, they love water so much that some contact lenses are actually only 25% hydrogel, and 75% water by weight.
Most people know what contact lenses do, but very few know what they are made out of. The answer to that is slightly more complicated than one might assume, and will depend on the type of contact lens in question.
Let's consider the three main contact lens varieties available today:
- Soft Contacts Lenses
- Rigid Gas Permeable Lenses
- Hybrid Lenses
Each of these three are designed with different goals in mind, and will achieve slightly different results in vision correction that the others. But more important than their individual capabilities, each is made from very different materials, in fact, even within each group there are different lens materials used.
What Are Soft Contact Lenses Made Of?
Soft contact lenses use of hydrogel, a material that loves water, and clings to and absorbs water readily. Some soft contact lenses are actually only 25% hydrogel, and 75% water by weight as their name implies soft contact lenses, are very soft, flexible and delicate, and the addition of hydrogel helps them remain that way.
Anyone that has accidentally left out a soft contact lens over night will know that they can dry out very quickly, and shrink and turn brittle when they do so.
The high water content helps keep eyes feeling moist and fresh, and also allows quite a lot of oxygen to pass through the lens. Most plastics are barriers for oxygen, but by adding water to the material, oxygen atoms are able to dissolve into the lens and then get absorbed by the eye. A newer material being used, called silicone hydrogel, is particularly good at oxygen transmission. In fact, silicone hydrogel contact lenses can pass up to six times more oxygen than some standard hydrogel lenses. Examples of contact lenses that contain silicone are Dailies Total 1 and 1 Day Acuvue Trueye.
Not all hydrogel lenses will hold the same amount of water s are graded on their water content
- Low water content lenses - will hold approximately 40% water
- Medium water content lenses - will hold between 40% and 60% water
- High water content lenses - Will hold more than 60% water
Different brands of soft contact lenses have different water content and lens thickness. Generally, hydrogel lenses that have a low water content are thinner than soft lenses that have a high water content.
There is significant variation in thickness and water content of hydrogel contact lenses because people will respond differently to the materials. Some contact lens wearers are more comfortable wearing thin, low water content lenses; others are more comfortable wearing thicker, moderate and high water content lenses.
Just like there are advantages of having high water content lenses, there are also advantages to low water content lenses, too. The less water content in a lens, the thinner it will be. It may dry out more quickly, but it will also be far less noticeable on the eye, especially when properly hydrated.
Hydrogels are classified as either ionic or non-ionic. Ionic materials have a negatively charged surface and therefore may attract positively charged proteins in the tear film. Non-ionic hydrogels are treated to reduce this negative surface charge and therefore may be less prone to attract protein deposits.
The FDA uses four categories to classify soft lens materials:
- Category 1 = low water, non-ionic
- Category 2 = high water, non-ionic
- Category 3 = low water, ionic
- Category 4 = high water, ionic
Which option is best will depend on each individual person's own physiology and needs. Those with dry eyes may want thicker, wetter lenses, while those with more sensitive eyes need thinner, less noticeable lenses.
To find out the water content of your contact lenses check the box, all contact lenses display their contents along with water content, normally on the back of the box.
What Are Rigid Gas Permeable Contact Lenses Made Of?
Often referred to as RGPs, or even just hard lenses, rigid gas permeable contact lenses have been on the market since the 1970's. They differ from soft lenses in that they are not flexible, and are a bit smaller in diameter too. Something they share with hydrogel soft lenses is that they allow a lot of oxygen to pass through them, as their name implies.
Another similarity between soft lenses and RGPs is that they are rated and grouped into sub-categories. Where soft lenses are categorized based on their water content, RGP lenses are divided into sections based on how much oxygen they allow to pass through them.
Unlike soft contact lenses, these RGP's don't have very much water content at all, and pass oxygen in a different manner. Rather than dissolving it in water, RGP lenses are very porous on a microscopic level, and these tiny holes allow the oxygen molecules to permeate the lens.
The lens's oxygen transmissibility level is measured in Dk units, which is a measurement of their oxygen permeability.Gas permeable contact lens materials generally are classified according to their "Dk" value, . Materials with a high Dk transmit more oxygen to the eye than those with a low Dk value:
- Low Dk is < 12
- Medium Dk is 15-30
- High Dk is 31-60
- Super Dk is 61-100
- Hyper Dk is > 100
Hard contacts had existed for many years prior, but the first ones to allow any oxygen to pass through them at all were introduced in the 1970s. These early RGPs were made of a material called silicone acrylate, and only had a Dk value of 12. They weren't very good by today's standards, but they were the best option available at the time.
Over time RGP contact lenses have made many leaps in progress and their Dk levels. The early models were classified as Low Dk, and were followed by Medium Dk (15-30), and High Dk (31-60). But then improvements kept coming, and better category names were needed. Super Dk includes value of 61-100, and Hyper Dk is anything above that.
Since the level of oxygen passed through the lenses has risen so much, there are now RGP lenses available that can be worn for extended periods of time, even over night.
Initially, increased oxygen permeability was achieved by adding more silicone to the lens materials. However, this ultimately caused RGP lenses to become more fragile and caused them to dry out and accumulate lens deposits more easily.
Eventually, fluorine was added to RGP lens polymers to solve these problems. Today's fluoro-silicone/acrylate RGP lenses are optimized for oxygen permeability, lens stability and surface wetting characteristics.
Because of their hardness and because they do not fluctuate significantly in their water content, gas permeable contacts generally have superior optical characteristics and provide sharper vision than soft lenses.
Despite advances in RGP lens technology, soft lenses continue to be more for comfortable and for this reason are the favourite choice of Optometrists and contact lens wearers alike.
What Are Hybrid Contact Lenses?
The centre of the lens is rigid, made of the same material as RGP lenses. They offer fantastic visual clarity, and keep the eye feeling fresh by providing it with plenty of oxygen, a ring of soft silicone hydrogel was added around the edges to provide comfort and easy application normally only found in soft lenses.
Soft contact lenses are popular because of their comfort, and RGP lenses due to their superior vision correcting ability and oxygen permeability, not surprisingly there was demand for a lens that does both, hence the introduction of hybrid contact lenses. Hybrid lenses are the best of both worlds, and use the same materials found in the other lenses detailed above.
Hybrid contact lenses have not gained the widespread use that was anticipated, mainly because it is hard to attach the soft skirt to the rigid silicone lens and so their lifespan was limited as the soft skirt often became detached. This would often happened on removal, as you cannot 'pinch' them to get them out, like a soft lens, and you cannot use your lids and finger pressure to blink them out, like a RGP.
Hybrid contact lenses are a great choice for people with low to high levels of astigmatism and for people with mild to moderate keratoconus.
Author: John Dreyer Optometrist Bsc(Hons), MCOPTOM, DipCLP
Created: 24 Apr 2015, Last modified: 11 Sep 2020