Contact lenses are made either out of glass (hard lenses) or silicone hydrogel (soft lenses). In both cases, they feature some level of oxygen permeability, which is an indispensable requirement for the wearer’s comfort. There are also hybrid contact lenses made out of rigid gas-permeable plastic and featuring a peripheral fitting zone that is softer than the optical part, but these are generally rare. The hydrogel monomer resin is the most popular choice of the three, as the spectrum of applications for softer contact lenses is a lot wider.
Manufacturing Contact Lenses
The most widely used method of manufacturing contact lenses is molding. The material is added in rotating molds and then shaped by centripetal forces or added and revolved at high speed to take its shape inside the rotating mold. For precision cutting needed to manufacture highly customized solutions, diamond turning on a CNC lathe can also be used. In this case, the concave shape of the contact lenses can only be achieved via the removal of material, and this is usually done through high-precision ball polishing.
After the lens gets its shape, the silicone hydrogel resin needs to be heat-cured in order to be polymerized. For this purpose, lab ovens of ultimate precision need to be brought into play. The thermal curing cycles for the hydrogel may last for up to 34 hours, depending on the size and thickness of the contact lenses. Throughout this whole time period, the heating must remain uniform and within the specified stringent range. As this process takes place, sterilization is also happening as a side effect, which is crucial for contact lenses.
Contact lenses are very delicate products, and at the same time, they are supposed to attain very specific physical properties and optical characteristics. This calls for high levels of manufacturing precision, accuracy, and caution. As expected, for all these prerequisites, there’s an extensive list of special requirements that need to be met otherwise the lenses will have a defect. Here are the most important of these requirements:
- When using rotating molds and injection molding, the lens will have to remain moist throughout the process. If they dry up, they are discarded, as re-hydrating them won’t fix the minor yet key defects that appear on the lens surface.
- The temperature range during the curing cycles inside the lab oven must not deviate from +/-3 degrees Celsius, and the heating uniformity must reach the utmost level. For example, the Despatch LNB oven that is suitable for contact lenses curing features a temperature uniformity of +/- 0.8 OC.
- The atmosphere inside the oven chamber must feature low concentrations of oxygen (down to 100 ppm), otherwise, there’s a risk of distortions and discolorations on the lenses’ surface. The LNB series uses a nitrogen blower for the air heater, ticking this box too.
- Depending on the lens and its features, the curing temperature requirement may be as high as 150 OC (302 OF).
As contact lenses are medical devices, they are rigorously inspected for any faults that may render them unusable. The quality control is done via intense magnification to ensure that there are no irregularities at the microscopic level, while shadow graphing is also used to measure the curvature and diameter of the lens and make sure that it matches the manufacturing specifications. If all is found to be perfect, or at least within the range of the acceptable deviation tolerance, the lens is packaged in a glass or plastic vial and is filled with a saline solution which helps maintain the required moisture in the cured silicone material.