What are contact lenses made of? Lens materials and how each is made

The main lens materials at a glance

Soft lenses: hydrogel and silicone hydrogel

Soft contact lenses are made of hydrogels: polymers that absorb water and become soft and flexible. The classic soft lens materials are based on HEMA (hydroxyethyl methacrylate). In a hydrogel, oxygen reaches the eye mainly by dissolving in the water inside the lens, so the oxygen supply depends on the water content and the thickness of the lens.

Silicone hydrogels combine a hydrogel with silicone, which lets far more oxygen pass through the material itself. In many markets they are now the most common material for everyday soft lenses.

Most soft lenses are cast-molded in large numbers. Custom soft lenses, for unusual prescriptions or eye shapes, are lathe-cut instead. The material is cut while it is still dry and hard (a "xerogel"), and only becomes soft when the finished lens is hydrated. Because the lens swells as it takes up water, the lathe cuts it smaller than the final size, by a known factor for each material.

Rigid gas permeable lenses: silicone and fluorosilicone acrylates

Rigid gas permeable (RGP or GP) lenses keep their own shape on the eye. That is why scleral lenses, ortho-k (night) lenses and lenses for keratoconus are rigid: their shape does the optical work. Their materials are mostly silicone acrylates and fluorosilicone acrylates. The silicone lets oxygen pass through the material; the fluorine helps the surface wet and resist deposits.

Lens makers compare these materials by their oxygen permeability, written as Dk, together with how they wet, how stable they are and how well they machine. GP lenses are almost always lathe-cut, one by one, from small buttons of the material.

PMMA: the first plastic lens material

PMMA (polymethyl methacrylate, the same family as acrylic sheet) was the first plastic used for contact lenses. It is very clear, stable and easy to machine, but it lets almost no oxygen through, so the eye depends on tear exchange under the lens. Gas permeable materials have replaced it for contact lenses. PMMA is still used for some rigid intraocular lenses.

Intraocular lenses: acrylics, PMMA and silicone

Intraocular lenses (IOLs) replace the natural lens inside the eye, for example in cataract surgery. Most modern IOLs are foldable, so they can be inserted through a small incision. They are made of hydrophobic acrylic, hydrophilic acrylic or silicone; rigid IOLs are made of PMMA. Many acrylic and PMMA IOLs are lathe-cut and their haptics milled; silicone IOLs and some acrylic designs are molded. Our article how intraocular lenses are lathed explains the process.

How the material decides the way a lens is made

• Volume: high-volume soft lenses are molded; specialty and custom lenses are lathe-cut, often one lens for one eye.
• Rigid materials are lathe-cut directly to their optical surface.
• Hydrogels are lathe-cut dry and hydrated afterwards, so the design allows for the swell.
• Soft-at-room-temperature materials, such as some hydrophobic acrylics, are molded or lathed with the material cooled.

All lathe-cut lens materials are cut with a single crystal diamond tool, because the surface it leaves is the optical surface of the lens. See how contact lenses are made.

What Bulunmaz does

Bulunmaz makes single crystal diamond tools in natural diamond and MCD for lathing contact lenses and intraocular lenses: roughing and finishing tools, inserts and haptic milling cutters, made to fit your lathe, and resharpens them. We do not make lenses or lens materials. This article explains materials and manufacturing; it is not advice on which lens suits an eye. See our diamond tools for lathe-cut contact lenses and IOLs.