# Biomaterials for Ophthalmic Applications
## 2 Biomaterials in Ophthalmology
### Contact Lenses  
Obtaining good visual acuity may require the use of an optical correction, which can be achieved by converging or diverging contact lenses, whose power is expressed in diopters. There are four common refractive disorders, namely myopia; hypermetropia; astigmatism and presbyopia. The first glass contact lenses appeared in the 19 th century. In the 1880 sm the first scleral glass lenses appeared with [PERSON]. The process of making contact lenses was described in 1888 [2; 27]. [PERSON] introduced PMMA, a material considered biologically inert, light, easy to manufacture and resistant to breakage, in 1936. Hard lenses apparently appeared due to an error during the process of manufacturing PMMA scleral lenses and led to the popularization of contact lenses [2; 27]. Silicone elastomer lenses were introduced in the 1960s as a silicone soft contact lens that does not retain water, is very permeable to gases, and has a hydrophobic surface [27]. Hydrophilic lenses date back to 1972 with the introduction of the soft lens on the market, which was an immediate success thanks to its comfort and its superior biocompatibility; however, an improvement in their gas permeability was required, after which they were made thinner and had a higher aqueous content [28]. In 1974, the first rigid gas permeable lenses were invented. One of the first materials was cellulose acetate butyrate (CAB), which provided a gas permeability higher than that of PMMA, but was prone to deformation. [PERSON] successfully incorporated silicone into the basic structure of PMMA to introduce a new family of polymer contact lenses, silicone acrylate. In 1994, a technique drastically reduced the production costs and helped to conceive of the first daily contact lenses. More than one decade of intensive research and development was needed to develop silicone hydrogel contact lenses which improved hypoxia problems related to contact lenses [27; 29; 30]. Soft contact lenses are the most popular type of contact lens, accounting for 88% of worn lenses [28; 31; 32]. Hydrogel contact lenses are made with a component of stable polymer, which can absorb or bind water. Polymer pores allow the liquid to penetrate the material, making it hydrated. Due to the particular environment of the eye, the lens must be safe, inert, non-toxic, biocompatible, easy to produce, maintain a stable and continuous tear film, be permeable to oxygen, maintain normal corneal metabolism, be ion permeable to maintain the movement of eyes, be comfortable and provide a stable and clear view [27; 28; 31; 33; 34]. Nowadays, soft contact lenses are able to provide high oxygen transmission to the cornea, good tear film moisture for comfort and superior vision, good material strength and adequate water permeability to maintain lens movement [28]. Silicone hydrogel is the result of years research to combine silicone with conventional hydrogel monomers, leading to a superior capacity of oxygen transmission and lower likelihood of dehydration in the eyes [28; 33; 34]. There are several polymers and polymerization conditions suitable for contact lens manufacturing. The variety of materials explains why there is a wide range of contact lenses available. In Figure 1, the most important factors when considering materials for contact lenses are presented. This choice of material influences comfort, wear time and cost, and in turn influences patient/practitioner choice. In Table 1, the advantages and disadvantages of the most commonly used materials to produce contact lenses are shown. PVA is a synthetic polymer that contains many hydroxy (-OH) groups, one in each repeating monomer unit. This leads to PVA's excellent hydrophilic and biocompatible properties [35]. PVA contact lens hydrogels have low protein absorption rates. PVA has been used as a tool for producing more comfortable lenses [6] or to facilitate the loading of a colored pigment into the lens. Silicone-based hydrogels include silicone, siloxanes, fluorosiloxanes, and derivative materials.