Broadly contact lenses can be divided into two types, Rigid and Soft.
Originally contact lenses were made of Perspex, an acrylic plastic which was impermeable to oxygen- this was a problem as the cornea, the clear part at the front of the eye, has no blood supply and receives its oxygen from the atmosphere via the tears. Clearly putting bits of plastic on the front of the eye will reduce the amount of oxygen the cornea receives. Corneal hard lenses were small in diameter, and employed various techniques in an attempt to get oxygen to the cornea- any oxygen that did get to the cornea was either flushed in behind the lens with fresh tears, or travelled through holes made in the lens.
Soft (hydrogel) contact lenses were invented to address the problems of the hard lenses, and were a major advance, as the water that was contained within the lens transmitted oxygen through it, allowing the cornea to breath better. A significant side effect of the soft lenses was that they were more comfortable.
Rigid gas permeable lenses came next- chemists found that by incorporating silicon into the perspex monomer, they were able to achieve significantly higher oxygen transmissibility than the soft lenses. This statement is an oversimplification- many polymer chemists were employed developing materials for gas permeable contact lenses .For many years, and arguably still today, Rigid Gas Permeable (RGP) lenses can transmit more oxygen that soft lenses, but soft lenses were more popular with the wearers due to their greater comfort.
It could be argued that if RGP lenses were invented before softs, that the soft lenses would not have been accepted by the professionals, as the rigids were a healthier long term option, but softs were much easier to adapt to- really there was no getting used to them, and the gradual increase in wear time was more to get the cornea gradually used to its new decreased oxygen supply than for comfort issues. Adaption to RGPs is generally a longer process, wearing them for an hour or two and then increasing by an hour per day. As a pervious RGP wearer, I was certainly watching the clock until I had achieved the wear time for the day, and was pleased to get them out- perhaps that is why I eventually “wimped out” and went for softs?
Soft contact lens manufacturers managed to increase the oxygen permeability of their lenses using higher water content plastics- more water meant more oxygen, and also by making the lenses thinner, which reduced the barrier to transmission. But still, RGPs were far more breathable- water is not a brilliant transmitter of oxygen, and a lens of 70% water was unlikely to be a 70% water content lens when it had been in the eye for a few hours, due to dehydration. Also, as water content increased, thickness had to increase to make the lens easy enough to handle, and mechanically strong enough, and this increasing thickness became a barrier to getting the air through. Nowadays it seems to make little difference if a soft lens is 55% or 70% water content, the transmissibility of both will be similar.
The massive advance in soft contact lenses was the invention of the silicone hydrogel. Scientists spent many years trying to develop a silicone hydrogel lens- silicone lenses were available, which provided high oxygen transmission, and they were soft, but silicone is a water repellent, and these lenses were not comfortable. Bausch and Lomb and CibaVision both independantly invented Silicone Hydrogel lenses at about the same time. These lenses allowed massive amounts of air through them, and were comfortable as they contained water. (The water now has only a secondary role of making the lens soft and therefore comfortable). Original first generation silicone hydrogels were fairly low water content, which meant they were great at transmitting the oxygen, but were a bit stiffer than the high water content soft that people were used to wearing, which lead to complaints of poorer comfort. These first generation lenses, Purevision and Night and Day, were approved for monthly wear. A side effect of Silicone Hydrogel CLs is that in layman’s terms the water is “glued” into the lens- silicon is a water repellent, which was why it took so long to develop them, but it means that the lenses are very dehydration resistant, which means end of day comfort can be excellent- no more drying out, or at least much reduced.
Second Generation SiH lenses have a higher water content than the first generation (so they have a lower oxygen transmissibility), but they are softer, more like the lenses soft wearers were used to. Not necessarily suitable for monthly wear, these lenses still usually allow more oxygen through than the cornea requires which means that they can be worn without time limits on a daily basis.
Most Daily Disposable contact lenses are traditional Hydrogel materials, and therefore have fairly poor oxygen performance- we would recommend these lenses for infrequent wearers- though they can be worn on a daily basis, they really should not be worn all waking hours, as the cornea does not get enough oxygen when the lenses are worn (and even less when you sleep).
Silicone Hydrogels would be the recommendation for every day wear, as there are no limits on how long they should be worn- most contact lens wearers would admit (if pushed) that they wear their lenses more than recommended, but with this type of lens they can wear then as often and as much as desired.
RGPs are still available and are still an option, but unless the prospective wear wants to commit to 6 or 7 days per week, most will have tolerance issues- softs can be worn on a more flexible schedule. RGP lenses can still better softs when it comes to particularly complicated prescriptions, such as very high degrees of astigmatism, but SiH lenses have meant that there are less and less reasons to recommend the Rigid lenses.