The Chemistry of Contact Lenses
For objects that essentially look like thin plastic films, there's a surprising amount of chemistry behind contact lenses. This chemistry is designed to maximise comfort whilst they are being worn, and as such it's also been constantly evolving and improving. Today's post takes a look at some of the chemicals they have been composed of over the years, as well as what's in contact lens cleaning solution.
Contact lenses have been around since the late 1800s, although in a far more uncomfortable guise. They were originally made of glass, folded from the eyes of rabbits or corpses, and could only be worn for short durations, no more than a few hours. These glass lenses were subsequently made slightly thinner, but didn't really lend themselves to comfort, and as such never really became particularly popular.
This all changed with the advent of plastics around the 1930s and 40s. Polymers, long molecules made up of multiple identical smaller units called monomers, were flexible and could be made into a much thinner lens to sit on the surface of the eye. The first polymer to be utilised as a direct corneal lens was poly(methyl methacrylate) (PMMA), which had a number of advantages over glass. It had better clarity, and of course was more lightweight and comfortable. However, these hard lenses still had problems; they did not allow oxygen to pass through, which can cause adverse effects on the eye, though this was not known at the time. More importantly, they were still rather uncomfortable.
The real breakthrough that led to the contact lenses most of us lens wearers wear today came in the 1950s. Czech scientists used a different polymer, poly(hydroxyethyl methacrylate) to create soft, flexible hydrogel lenses, which had the added advantage of being permeable to oxygen. These lenses were more comfortable, and as such could be worn for longer.
Hydrogel lenses contain networks of cross-linked polymers that are hydrophilic (water-loving) but also insoluble. They attract and absorb water; this is due to the presence of highly electronegative atoms such as oxygen in the polymer, which can form hydrogen bonds with water. By definition, a hydrogel must contain at least 10% water by weight, but many of them can hold much more – some up to a thousand times their original dry weight. You can see this hydrogel in action when you remove contact lenses. When removed from moisture, the lenses gradually shrivel up and become hard and brittle as the water evaporates. However, if placed back in water, they swell up and become flexible once again.
Though these soft lenses were much better at allowing oxygen through, there was still room for improvement. This was initially accomplished by adding other polymers, co-polymers, to the hydrogel mix, in order to modify the permeability of the lens. Even with these advances, however, it was still difficult to develop a lens that could be worn for extended periods without causing oxygen deprivation in the eye. Making the lenses thinner helped, but there was a limit to both this, and the degree of oxygen-permeability of the polymers in use. Subsequently, different types of polymers were required to push the oxygen-permeability of the lenses further.
Enter the polysiloxanes, or silicones. These are silicon and oxygen containing polymers, whose applications are wide and varied. They're also even more oxygen-permeable than water, so the oxygen permeability of the lens hydrogel could be increased still further. This allows the possibility of continuous-wear contact lenses, which can be worn overnight and for an extended period of time without depriving the cornea of the eye of oxygen.
The only issue is that these silicone is hydrophobic (water-repelling). As such, it's prone to problems such as sticking to the eyes if used on its own. To solve this problem, the first generation of silicone hydrogel lenses used surface treatments in order to make them hydrophilic. The second generation switched to the use of wetting agents such as polyvinylpyrrolidone (PVP), and more recently the third generation simply contained a modified polymer which was itself designed to be hydrophillic.
It's not just contact lenses themselves which require an array of chemical compounds to function – contact lens cleaning solution also requires a number of different components. There are two types of cleaning solution: peroxide solutions, or multi-purpose solutions.
Peroxide solutions use peroxide as a disinfection agent, typically at around a 3% concentration. A neutralisation catalyst is also present in the contact lens case (usually platinum, palladium, or silver) to help eventually break the peroxide down into water and oxygen, as otherwise it could cause damage to the eyes when the lenses are put back in.
Multipurpose solutions, on the other hand, commonly contain polyalkylene biguanides or polyquaternium chemicals. These are both polymers with antimicrobial activity. They're polymers that are actually formed from more effective antimicrobial monomers, but these would be too harsh to put into your eye when you put your lenses back in!
Both types of cleaning solution will also contain a range of other compounds to help maintain the lenses. Biphosphonates break down the proteins that can end up stuck to the lens after a days' wear, and moisturising and conditioning chemicals ensure that the lenses remain in good condition whilst stored, so they're fully functioning when you pop them back in your eye.
So, if you're a contact lens wearer, it's clear you've a lot of complex polymer chemistry to thank for your clarity of vision. If you're interested in digging deeper into the chemistry, a number of free-to-access studies are linked below.
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Hydrogel lenses: materials and manufacture – C Maldonado-Codina
Contact lens solutions – J Kemsley, C&EN
Contact lenses – The Organic Solution
Contact lens materials – C McDonnell
Soft contact lens polymers: an evolution – P C Nicolson (£)
Pedro on October 28, 2015 at 2:57 pm
Wow! A lot of interesting facts related to a common item of daily usage! Thanks for the informations!
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