The raw material of the hydrogel is called HEMA. HEMA is hydrophilic and has a water content of 38% by itself. By cross-linking with other polymers, the water content can be up to 80%. The world's first soft cloaking material (OttoWichterle, 1961) was made from HEMA, called PHEMA. Nowadays, most conventional hydrogels are made up of the copolymer of PHEMA and other monomers.
It is the earliest material used in the production of contact lens hydrophilic material. The main advantage is water absorption, with a water content of about 38%. The material is soft, but its disadvantage is only partial oxygen permeability.
HEME Mixed Materials
Using HEMA as the matrix, other monomer components are added to improve the hydrophilicity, oxygen permeability, anti-precipitation, and other functions of the lens.
The non-HEMA materials are hydrophilic soft lens material, which does not contain HEMA components. They are stronger and more oxygen permeable than most HEMA polymers.
The water content of regular hydrogel lenses varies from 30% to 58%. When oxygen passes through the hydrogel, it needs to be carried by water molecules in the material. Therefore, the higher the water content of the lens, the higher the oxygen permeability value. The softer the lens, the more comfortable it is to wear, which is conducive to the metabolism of the cornea. However, at the same time, the higher the water content of the lens, the faster the water evaporation, easy to dehydration, and easy to cause dry eyes. A lens with high water content has low material density, poor durability, and easy breakage, so they are suitable for making lenses that are frequently changed or discarded. On the contrary, the lens with low water content has good formability, easy operation, durability, and is not easy to dehydrate and cause dry eyes.
Nesofilcon A and Omafilcon A
Nesofilcon A is super water glue. This is the proprietary material of Bausch & Lomb and is now used in only one product, the Bausch & Lomb Pure Suncast (launched in the US in 2012). Water content is 78%, and the oxygen permeability is DK/T =42.
Through PC Technology, the Omafilcon A material contains substances that mimic the composition of cell membranes--- including human eyeball cells, and locks water, and prevents dryness.