You’re probably concerned with knowing what ingredients go into your foods and medications, but have you ever considered what goes into your contact lenses? There are a few basic types of contact lens materials, making up three different classes of lens: soft, rigid gas permeable, and hybrid. There are also a few new materials being used in the latest contact lens technology. Let’s take a look at what you’re putting in your eyes.
Soft contact lenses are the most popular lens type. Flexible, comfortable and inclined to stay in place, they’re often the best contact lens choice for active people. Soft lenses are made of unique materials called hydrogels. Invented by Czech chemist Otto Wichterle in the early 1960s, hydrogels are water-loving (hydrophilic) plastics. Soft lenses made of hydrogels maintain their suppleness and flexibility by absorbing water from the surrounding environment. Unfortunately, the environment that they draw moisture from is your eyes.
This means that when worn for too long, hydrogel lenses can sometimes dehydrate your eyes, leaving them feeling dry and uncomfortable. However, many new soft lenses have surface treatments that preserve the water content and prevent drying.
Various brands and types of soft contact lenses offer different water contents and lens thicknesses. Hydrogel lenses are available in three general categories: low, medium and high water content. The water content of hydrogel lenses allows oxygen to pass through which is crucial. Unlike other organs in the body, which receive oxygen from the bloodstream, your eyes get oxygen directly from contact with the air. Contacts which don’t allow oxygen to reach the corneas can cause serious eye damage.
Soft, oxygen-permeable lenses allow air to reach the eyes, making them serviceable for daily, monthly or continuous wear. Generally, hydrogel lenses with low water content are thinner than soft lenses with high water content. However, new contact lens technology has introduced silicone hydrogels, which has higher oxygen permeability than standard hydrogels. Because the silicone lenses allow more oxygen to pass through, these lenses do not need to have as high of a water content to ensure the same comfort. This means lighter, thinner lenses that can be safely worn for longer periods of time.
A final feature of hydrogel lenses is their surface charge, which can be either ionic or non-ionic. Ionic hydrogels have a negatively charged surface and can attract positively charged proteins, which can lead to build-up under the lenses. Non-ionic hydrogels are treated to reduce the surface charge and are less prone to attracting deposits.
Also called “hard” or “GP” contact lenses, RGP lenses have been around for a lot longer than soft lenses, although their composition has changed drastically throughout their history. Originally made of glass and later polymethyl methacrylate (PMMA), RPG lenses are now made of fluoro-silicone/acrylate. This oxygen-permeable material is a silicone acrylate with added fluorine. Unlike soft lenses, rigid gas permeable lenses don’t rely on their water content to transmit oxygen to the eyes. Instead, the microscopically porous design of RPG lenses allows oxygen to reach the cornea. Gas permeable contact lens materials are classified by their oxygen permeability, referred to as their “Dk” value. The higher the Dk value of a set of rigid lenses, the more oxygen they allow to reach the eye. A low Dk value would be less than 12; a high Dk value is over 30.
Typically less than a centimeter across, RGP contact lenses are smaller than soft lenses. They’re designed to cover the pupil of the eye, where vision is processed. The edges of RGP lenses extend only to the outer edge of the iris, unlike soft lenses which are designed to fit under the eyelids. Because of this size difference, GP lenses can feel uncomfortable for a short time, until your eyes adjust to the feel of the lens when you blink.
Although they take some getting used to, RPG lenses have some advantages over soft lenses. A GP lens with a high Dk rating is more oxygen-permeable than the best soft contacts, making them a healthier option for your eye. They are also more durable and easier to clean than soft contacts. Because of their rigidity and because their water content doesn’t change, RGP lenses also generally provide somewhat crisper vision than soft lenses. However, some people have difficulty adapting to rigid GP lenses and prefer the comfort of soft lenses.
Some of the latest contact lens technology, hybrid contact lenses are a combination of soft and rigid GP contact lenses. They have a rigid GP center covering the optical zone with a soft outer ring of hydrogel contact lens material. This melding of the two combines the visual clarity of rigid contacts with the comfort of soft contacts.
These hybrid contact lenses correct a wide variety of vision problems, including astigmatism, keratoconus, presbyopia, myopia (nearsightedness), and hyperopia (farsightedness). Unlike RGP lenses, hybrids can often be worn by people with irregularly shaped corneas.
Of the various types of contact lenses, they all have a few things in common. All modern contact lens materials are safe, non-toxic, oxygen-permeable (to varying degrees), can treat most vision problems, and are available in tints and bifocals. They also share some of the same disadvantages: most contact lenses need to be cleaned regularly with a special solution. All contacts must be handled carefully, and all require a prescription. Aside from these similarities, though, there are some distinct advantages and disadvantages to the different materials.
The most obvious advantage of soft hydrogel lenses is their comfort. They require almost no adjustment period. They also conform to your eye, so they stay in place better than RGP lenses. Soft contacts offer several wearing options, including disposable, daily wear, and extended wear types. Their primary disadvantage is cost, as they must be replaced frequently. Soft lenses are also more difficult to clean, more likely to harbor bacteria, and more likely to collect protein deposits.
The clear advantage of RGP lenses is enhanced vision. The shape and design of RGP contacts allow for crisper, clearer eyesight than soft lenses can offer. They also cost less; a set can last for several years with proper care. They allow more oxygen to reach the cornea and are less prone to bacteria and protein build-up. The downside to RGP lenses is that they require daily cleaning, they can slip around on your eyes, and they can cause some slight discomfort.
Hybrid contact lenses have both the advantages and disadvantages of the other two types. They provide better comfort than RGP lenses and clearer vision then soft; however, they’re costlier than both types and, since they’re a relatively new contact lens technology, they offer fewer brands and types to choose from.
|Soft Contact Lenses||Comfortable, conform to the eye, stay in place, multiple wear options.||Expense, difficult to clean, collect build-up and bacteria|
|Rigid Gas Permeable Lenses||Excellent vision, long lasting, inexpensive, healthier for the eyes.||Less comfortable, require daily maintenance, can slide out of place.|
|Hybrid Lenses||Comfortable, excellent vision, stay in place.||Expensive, few options.|
Whether soft, rigid or hybrid, contact lens technology offers a wealth of advantages over eyeglasses. Contact lenses offer the full range of vision, including peripheral. They never fog up, get blurred by rain, or fall off your face. They don’t put pressure on sensitive areas like your nose and ears, and they don’t need constant readjusting. When choosing the best contact lens material for your needs, discuss your options with your eye doctor.
Updated May 8th, 2014
Todd Childs, O.D. has been practicing optometry for the past twelve years. He currently practices at South Valley Optical in Draper, Utah. Dr. Childs earned his Doctor of Optometry at Southern California College of Optometry.