What are Contact Lenses Made of?
Hundreds of millions of 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.
First, 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.
Soft Contact Lenses
Starting first with soft contact lenses, materials are used that cling well to water molecules. Soft lenses, as their name implies, are very flexible and delicate, and remaining that way means staying very well hydrated. Anyone that's accidentally left out a soft contact lens over night knows that they can dry out very quickly, and shrink and turn brittle when they do so.
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.
This high water content helps keep eyes feeling moist and fresh, and also allows quite a lot of oxygen to pass through the lens as well. 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 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. Some lenses will hold approximately 40% water, by weight. These are considered low water content lenses. Medium water content lenses vary between 40% and 60% water content, and high water content lenses are those that have more than 60% water content.
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.Another feature of hydrogel materials used for soft contact lenses is their surface charge, which can affect how quickly protein deposits form on the lenses during wear.
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.
Rigid Gas Permeable Contact Lenses
Often referred to as RGPs, or even just hard lenses, rigid gas permeable lenses have been on the market since the 1970s. 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.
But these lenses 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.
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.
The lens's oxygen transmissibility level is measured in Dk units, where the lower the value, the less oxygen that's passed through the lens.Gas permeable contact lens materials generally are classified according to their "Dk" value, which is a measure of their oxygen permeability. 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 lenses have made many leaps in progress. 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 GP lenses to become more fragile and caused them to dry out and accumulate lens deposits more easily.
Eventually, fluorine was added to GP lens polymers to solve these problems. Today's fluoro-silicone/acrylate GP 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.
However, despite advances in GP lens technology, some people cannot adapt to wearing rigid GP lenses and choose soft lenses for comfort reasons.
Hybrid Contact Lenses
Soft lenses are popular because of their comfort, and RGP lenses due to their superior vision correcting ability and oxygen permeability. But the demand for a lens that does both was high enough that hybrid lenses were developed. These lenses are the best of both worlds, and use the same materials found in the other lenses detailed above.
The center 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.
But since the edges of RGP lenses can be bothersome for many people, a ring of soft silicone hydrogel was added around the edges. This provides comfort and easy of application normally only found in soft lenses.
However, they have not gained widespread use, 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 often happened on removal, as removal is tricky - 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.
Author: John Dreyer Optometrist Bsc(Hons), MCOPTOM, DipCLP
Created: 24 Apr 2015, Last modified: 4 Mar 2020