American fashion designer Vera Wang said: “To me, eyewear goes way beyond being a prescription. It’s like makeup. It’s the most incredible accessory. The shape of a frame or the colour of lenses can change your whole appearance.” But the makeup’s colours and properties should suit the wearer, and, as we all have different physical facial features and eyes, so should our contact lenses.
Research shows that the global market for contact lenses is about $12-billion, and two-thirds of contact lens wearers are female. However, a 2009 research study conducted by Vanessa Moodley, an optometrist based in Durban, found that there is still a need to create lenses suited to the eyes of different races.
The secret to makeup is to find a colour that best suits one’s complexion and features. Similarly, Africans, Indians and Caucasians have been found to have varying cornea shapes. Moodley performed a contact lens fitting test on people of different races with over-the-counter contact lenses. In her study, published in the journal The South African Optometrist (now known as African Vision and Eye Health), she found that in a sample of 240 subjects, more than 80% of African subjects had a high number of incompatible lenses, while the over-the-counter contact lenses fitted the majority of Caucasian subjects. These differences were mainly attributed to a tight fit on the patient’s corneas and attests to the flat cornea in people of African descent. The cornea is a protective layer on the surface of the eyeball, covering the iris and pupil. It is also responsible for the refraction of light rays into the eye to form an image.
With age and use, minor defects form on the cornea and contact lenses are used to correct these defects and redirect the images appropriately for good vision. But contact lenses that fit properly need to be made, and out of materials better suited to the different climates across the world. For instance, some materials absorb water and could be better suited for use in moist and hot weather conditions.
Based on these needs, a team at the precision engineering laboratory at the Nelson Mandela Metropolitan University in 2013 decided to look into machining contact lenses and how to achieve high lens quality. For this, we used single-point diamond cutting, the most accurate optical fabrication technique available. In a way similar to cooking, our experimental research for making contact lenses takes a lot of practice to perfect. This process involved bringing together various ingredients in specific quantities in a particular sequence to create a single piece. To obtain our perfect contact lens recipe, we individually adjusted the tools’ cutting values (cutting speed, feed and depth of cut) to observe how each change influenced the accuracy of the contact lenses’ surface. From our tests, we generated various grades in quality of surface profiles. This was based on parameter choices and some combinations which negatively influenced the surface quality. For example, we found out that slower cutting speeds do not necessarily lead to better lenses.
To clearly understand the effects of cutting tools’ values while cutting contact lens plastics, we also analysed various combinations of cutting methods and the influence of environmental variables such as humidity and temperature. We discovered that a moderately humid environment provided better results and in combinations, some cutting factors improved the quality of lens surfaces.
Experimental tests also showed us that plastics’ properties changed during cutting. As cutting temperatures increased, the plastic lenses changed from being brittle like glass to being more rubbery at the point of cutting. This transition is critical in the manufacture of very accurate contact lens surfaces.
Another interesting discovery was that static electricity was generated when you cut contact lens plastic materials with diamonds, the same effect as rubbing socked feet on a carpet. This can dramatically affect the quality of the lenses. However, a slight increase in atmospheric humidity above 60% prevents static build-up.
To test our recipe and confirm if we were really making a more accurate contact lens, we undertook a molecular dynamics study. This allowed us to see each atom in the lens and what happened to it while we were cutting it. On this nanoscopic scale, we saw that specific combinations of cutting parameters created atomic shifts, which determined the smoothness of the lens surface. From this information, we were able to produce more accurate lenses suited to South Africa’s unique population.
My perfect recipe for making a very accurate contact lens comes from a combination of very complex parameters. Cutting speed, feed and depth of cut in combination with force and humidity are essential ingredients to achieve a highly accurate lens. However, improper combinations of these factors lead to high static charges and deformed surfaces.
As I have seen from our research, diamond machining is capable of making extremely fine optical surfaces and offers more flexibility in production than available methods. Current supplies of over-the-counter lenses are yet to meet the demands of the South African market – to create a contact lens that suits the African eye lies with an African solution. If makeup is made to suit its wearer, so should contact lenses.
Oluwole Olufayo attends Nelson Mandela Metropolitan University.