Why Did Contact Lenses Move From Hydrogel to Silicone Hydrogel?

On the side of a lens box, some products say hydrogel and others say silicone hydrogel. On the eye, both are soft, flexible lenses. Soft lenses had already appeared in the 1960s, so why did a material called silicone come along decades later?

There was a limit, hard to get past, to leaving the job of carrying oxygen to the cornea to the water in the lens alone, and silicone was added as another way around that limit. In return, a new problem had to be solved: a surface that repels water.

This article does not rank materials. It follows only the history and principles of why the material changed. How to read the numbers on a box is covered separately in our article on comparing lens specifications.

The cornea takes its oxygen from the air

A 1991 symposium publication from the US National Academies Press explains that the cornea, which has no blood vessels, must get oxygen from the atmosphere and swells if it cannot.

A contact lens is the very object that covers that surface. According to the same publication, less than 2% of the tears under a soft lens are exchanged with each blink. Since tears can hardly carry the oxygen in, the oxygen has to pass through the lens. How do you cover the eye and still let oxygen through? That question has followed the history of lens materials throughout.

The arrival of plastic that holds water

The usual starting point given for soft lenses is a paper on water-compatible gels published in Nature in 1960 by Czechoslovak chemists Otto Wichterle and Drahoslav Lím.

The 2013 report from the tear film society TFOS presents the work of Wichterle's team as the basis of hydrogels for soft lenses. The representative material, polyHEMA, was a polymer holding about 38% water. Materials that raised the proportion of water by blending in other components followed.

Oxygen travelled through the water in the lens

In a hydrogel, the passage for oxygen is water. A 2023 materials review explains that because oxygen permeates through the water portion of a lens, oxygen permeability depends on total water content. Water content is the proportion of water in the lens.

This structure has a ceiling in principle. The 1991 publication wrote that the Dk value, which expresses a material's oxygen permeability, was tied to water content in the hydrogels of the time, with a possible upper limit of about 50. It adds that when a lens dehydrates, oxygen transmission falls accordingly. Less water means a narrower passage.

The end of the road for adding water

Would adding a lot of water solve it? The TFOS report writes that there were hopes higher water content would mean better wetting, more comfort and more oxygen. But it soon emerged that dehydration was more pronounced in high water content lenses, and in some cases this led to drying of the corneal surface and discomfort at the end of the day.

The problem of sleep came on top of this. According to the 1984 Holden and Mertz study cited in the 1991 publication, keeping corneal swelling within a certain level required a supply equivalent to 9.9% oxygen for daytime wear and more than 17% for overnight wear.

The publication concluded that the oxygen the hydrogels of the time could supply was limited, and predicted that the arrival of new hydrogels not relying on water alone could open the way to continuous wear. This was an outlook from before silicone hydrogel lenses reached the market.

Silicone: a different route

There were other materials too. The 1991 publication wrote that soft silicone lenses had the highest oxygen permeability of all materials, and the 2023 review explains that oxygen can dissolve into the silicone portion and pass through it.

The problem was that silicone does not get along with water. According to the review, silicone surfaces have a low affinity for water and do not wet well, and the material is also on the stiff side, which can cause discomfort. The TFOS report wrote that there were many attempts to use the oxygen permeability of silicone rubber in lenses, but that they bore fruit only in the late 1990s.

The two materials that emerged then had water contents of 24% and 36%, lower even than the 38% of early polyHEMA. What set them apart was that oxygen passed through well despite the small amount of water. Water-rich parts and silicone-rich parts sit together in the material, and oxygen mainly takes the silicone route. A tendency also appeared for more water to actually reduce oxygen transmission.

The surface: a new problem to solve

The TFOS report explains that siloxane, a silicone component, gives high oxygen permeability but is also the cause of poor wetting, so various methods were brought in to make the surface compatible with water.

The methods did not settle on just one. The report's 2013 table of materials lists, side by side, methods including plasma coating that puts a 25-nanometre film on the surface, plasma oxidation that alters the surface, and internal wetting agents that put water-compatible components into the material with no surface treatment.

The fact that lenses became stiffer was also seen as a problem early on. The TFOS report concluded that, as of 2013, high-level evidence could not answer whether oxygen supply affects comfort. That oxygen was the reason the material changed is one matter; which is better for comfort is another.

Made for overnight wear, ended up used by day

The TFOS report says the original intention of silicone hydrogels was continuous wear. A 2001 approval document from the US Food and Drug Administration states that a lens in the 24% water material mentioned above could be prescribed for up to 30 nights of continuous wear. This is what was approved at the time for a specific lens. Whether overnight wear is possible varies from lens to lens, so a professional's prescription should be followed.

But the trend went in another direction. According to TFOS's 2023 contact lens report, silicone hydrogels came to be used mainly for daily wear and accounted for 74% of soft lens prescriptions worldwide in 2021. Hydrogels have not disappeared. It is more accurate to say that the centre of prescribing moved than that one material replaced the other.

The change in materials was a process of hitting the limit of water as a passage, finding another passage and then solving the surface problem that passage created.

Next time you look at a lens box, look for the material name and the water percentage together. A low number is not bad and a high number is not good. It is enough to remember that even the same number means a different route for oxygen, depending on whether the lens is hydrogel or silicone hydrogel.