# Biomaterials for Ophthalmic Applications
## 2 Biomaterials in Ophthalmology
### Contact Lenses
The selected hydrogel materials used for the manufacture of soft contact lenses are: allyl methacrylate (AMA); ethylene glycol dimethacrylate (EGDMA); 2-hydroxyethyl methacrylate (HEMA); methacrylate acid (MAA); 4-(2-methacryloyloxyethyl)-2-(2H-benzotriazol-2-yl)phenol (MAEBTP); 2-methacryloxymethyl phosphorylcholine (MPC); N-vinylpyrrolidone (NVP); poly(ethylene propylene glycol) dimethacrylate (PEPGDMA); poly(vinyl alcohol) (PVA); 4-tert-butyl-2-hydroxycyclohexyl methacrylate (TBHMA); sodium methacrylate (SMA); 1,1,1-tris(hydroxymethyl)propane; and trimethacrylate (TRIM). Recent studies aimed to improve the oxygen permeability and wearing comfort of soft contact lenses [39] and the development of new polymers with antifouling properties to reduce the adsorption of proteins and cells on the lens surface [40]. This may help to improve the biocompatibility of soft contact lenses, especially for long-term wear. Poly(dimethyl siloxane) (PDMS)-based lens materials combine the extremely high oxygen permeability of PDMS and the wearing comfort of conventional pHEMA hydrogels. Besides their application as corrective lenses, soft contact lenses can provide a drug delivery system for the anterior segment of the eye [41; 42]. The use of soft contact lenses as a drug delivery system is a challenging approach with no approved products on the market. The main challenge is to reach adequate levels of drug loading while ensuring controlled drug release at the same time. Although well-established polymers are used, the biocompatibility, transparency, and oxygen permeability of drug-loaded soft contact lenses have to be tested. Extensive in vitro and in vivo studies will be required to prove the safety and efficacy of the developed drug delivery systems. Soft contact lenses are manufactured mainly by lathe-cutting, spin-casting, cast-molding or a combination of these three [27; 32]. Lathe-cutting is the most expensive due to the number and variability of stages. It is usually reserved for the production of customized lenses with features that do not allow for mass production. The dry polymer is placed in a lathe where it rotates and a diamond tool generates the back surface of the lens. It is then mounted with adhesive wax, where the same process takes place to generate the front surface of the lens. It is then removed from the lathe and the edges are polished before inspection. The dry lens is hydrated in saline solution to obtain its final shape. It is inspected again before being placed in a sealed glass bottle, labeled and sterilized in an autoclave [27; 32]. Spin-casting fabrication is performed using a stainless-steel tool allowing for the manufacture of hundreds of thousands of female molds, produced by the pressure of a male tool in cast polypropylene which, once cooled, hardens. The female mold is placed on an axis which rotates about the lens axis, with the concavity upwards, and liquid monomers are introduced into the rotating mold. The final shape of the lens is determined by a combination of the temperature, gravity, centrifugal force, tension surface, the amount of liquid in the mold and rotation speed. The monomers are then irradiated with UV light to initiate polymerization. The dry lens is removed from the mold, its edges polished and the mold removed. The final steps are the same as for lathe-cutting, except that they are placed in a bag containing saline solution. The blister is sealed and sterilized in an autoclave [27; 32]. Cast-molding fabrication is the preferred way to make lenses. Both male and female casts are made of stainless steel, and the contour of the male tool head defines the shape of the front surface of the lens, while the contour of the head of the female tool defines the shape of the back surface of the lens. The mold's tongue and groove are made by pressing the tools in molten polypropylene which, once cooled, harden. The female mold is assembled, the concavity is made upwards and the liquid monomers are introduced into it. The male mold is then placed in the female mold.