So anyone who follows me on twitter might know that I have had some eye issues in the past. I have had blocked eye lid glands which were exacerbated by my repeated wearing of contact lenses and this got me to thinking…what the heck are my contact lenses made from and why were they making my eyes dry?
In 1936, polymethylmethacrylate (PMMA) was introduced and this led to the first commercial example of contact lenses. A big improvement in the industry came in the 1970s when Wichterle1 introduced the soft hydrogel, polymacon or polyHEMA (poly-2-hydroxyethyl methacrylate). HEMA is synthesised from methacrylic acid and ethylene or propylene oxide and then polymerised to give polyHEMA.
Polymacon (or Soflens by Bausch & Lomb) was quickly followed by a range of soft contact lenses made from hydrogels. It was swiftly understood that the cornea uses oxygen to maintain its function and obtains this oxygen from the air and with this knowledge in hand contact lenses rapidly improved. 2
Soft contact lenses worn by most people now are still made from hydrogels. A hydrogel is a network of hydrophilic polymer chains that is insoluble in water. The main advantage of hydrogels is that they are very highly absorbent and may absorb up to thousands of times their dry weight in water. To be a hydrogel, water must constitute at least 10% of the total weight (or volume) of the polymer. When the content of water exceeds 95% of the total weight (or volume), the hydrogel is said to be super-absorbent. Have you ever noticed that when you leave your contact lens out of the solution it shrivels up and hardens but when you put it back in solution it becomes soft and flexible again? This is because it is a hydrogel.
Simply put, hydrogels work by highly electronegative atoms (from polar groups) in the polymer causing a charge asymmetry which favours hydrogen bonding with water. There are two types of hydrogels:3
– Covalently cross-linked
– Absorb water until they reach equilibrium swelling
– High stability in harsh environments (high temp, high/low pH)
– Non-covalently cross-linked (e.g. electrostatic interactions)
– Weaker and more reversible form of interaction
– Respond to changes in temperature and pH
Contact lenses are required to do a lot: maintain a tear film for clear vision, sustain normal hydration, allow oxygen to permeate and be non-irritating and comfortable. The lens must have excellent surface characteristics being neither hydrophobic nor lipophilic (and there are many publications on the surface properties of contact lenses should you want to find out more). It is pretty amazing that they do all this work whilst allowing us to see properly. Interestingly, the reason we are supposed to only wear our contact lenses for only ~12 hours a day is because keeping them in too long can cause continuous corneal hypoxia. Hydrogel-based contact lenses are thought to reduce this as oxygen can diffuse through the lens. 5 Contact lenses can be a made from a range of polymer hydrogels, many of which now contain silicone as these are thought to allow more oxygen to the eye and therefore lead to healthier eyes.
As the name suggests, hydrogels form sticky gel like materials which can be used in a wide variety of applications from drug delivery to tissue engineering. There are some great reviews on hydrogels in biology and medicine by Pappas and Langer4 and Hoffman3 and I highly recommend reading more because hydrogels really are interesting things!
I haven’t gone into the polymer chemistry involved in the synthesis of hydrogels but I hope you have learnt something about the gooey things we put in our eyes on a daily basis.
1 O. Wichterle, D. Lim, Nature, 1960, 185, 117-118.
2 P. C. Nicolson, J. Vogt, Biomaterials, 2001, 22, 3273-3283.
3 A. Hoffman, Adv. Drug Deliver. Rev., 2001, 944, 62-73
4 N. Peppas, J. Z. Hilt, A. Khademhosseini, R. Langer, J. Adv. Mater., 2006, 18, 1345.
5 I. Fatt I, R. St Helen, Am. J. Optom. Physiol. Opt., 1971, 48, 545