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Types of Lenses

1.1 History of Contact Lenses

A history of contact lenses spanning over almost 500 years has been detailed (1, 2). It is based on historical works, scientific papers and journal articles and looks at both the modern disposable lens as well as the hard and soft lenses that came before. Some important events are collected in Table 1.1.

Table 1.1 History of contact lenses (2).

Year Inventor Issue 1508 Leonardo da Vinci Corneal neutralization 1637 René Descartes Fluid-filled tube 1685 Philippe de La Hire Neutralization of cornea 1801 Thomas Young Three color theory of perception 1827 George Biddell Theory of astigmatism 1845 Sir John F. W. Herschel Convex lenses 1846 Carl Zeiss Optical instruments 1851 Johann Nepomuk Czermak Water-filled goggle 1887 Adolf Eugen Fick First successful contact lens 1961 Otto Wichterle Soft contact lenses 1979 Kyoichi Tanaka Silicone hydrogel materials

In 1508, Leonardo da Vinci first had the idea of placing a corrective lens directly onto the surface of the eye (3-5). In 1637, René Descartes proposed another idea in which a glass tube filled with liquid is placed in direct contact with the cornea.

In 1887, Adolf Eugen Fick, a German physiologist, created the first successful contact lens (6). Glass-blown scleral lenses remained the only form of contact lens until 1938, when poly(methyl methacrylate) (PMMA) was developed, and Mullen and Obring used the plastic to manufacture scleral lenses. Obring developed the Plexiglass series in New York in 1940 (4).

In 1961, the Czech chemist Otto Wichterle invented soft contact lenses (7, 8). In 1970, rigid gas-permeable contact lenses were developed, and widely accepted for the advantages of small diameter (about 9 mm) and gas permeability. Silicone hydrogel materials were developed in 1979 (9). In 1999, an important development was the launch of the first silicone hydrogels onto the market. These new materials showed an extremely high oxygen permeability with comfort performance (4).

The factors that influenced the development of special materials have been reported (10). Accounts of early attempts to improve vision by use of a lens contacting the eye are limited to a few isolated observations (11). Practical success was not realized until techniques for fabrication of lenses from glass were sufficiently developed (12). PMMA replaced glass in the late 1930s. This material is more durable, more readily fabricated and was claimed by some authors to show a better ocular compatibility (13). During the same broad period of time, there was also a change in emphasis from scleral to corneal contact lenses, which placed different demands on material design and development.

More is demanded from ophthalmic treatments using contact lenses, which are currently used by over 125 million people around the world (14). Improving the material of contact lenses is currently a rapidly evolving discipline (10).

A search has been performed of the titles of papers in the Scopus database to identify contact lens-related articles published this century (15). The ten most highly cited papers were determined from the total list of 4,164 papers found. Rank-order lists by count were assembled for the top 25 in each of four categories: authors, institutions, countries and journals. A 20-year subject-specific contact lens h-index was derived for each author, institution, country and journal to serve as a measure of impact in the field. The top 10 constituents (of the top 25) of each category were ranked and tabulated (15).

1.2 Materials

Contact lens materials (10) are typically based on polymer- or silicone-hydrogel, with additional manufacturing technologies employed to produce the final lens. These processes are simply not enough to meet the increasing demands for contact lenses and the ever-increasing number of contact lens users (14).

An advanced perspective on contact lens materials has been presented, with an emphasis on materials science employed in developing new contact lenses (14, 16). The future trends for contact lens materials are to graft, incapsulate, or modify the classic contact lens material structure to provide new or improved functionality. Also, some of the fundamental material properties are discussed, and the outlook for related emerging biomaterials is presented.

Contact lens materials and lens types, treatment for contact lens and tear film complications, and myopia correction and contact lenses for abnormal ocular conditions have been detailed (17). Current topics in this field are miniscleral lenses, keratoconus, corneal crosslinking, and pediatric, cosmetic and prosthetic contact lenses. Furthermore, simulation programs for scleral lens fitting, sagittal values, soft toric mislocation, front vertex power, orthokeratology and rigid lens design are discussed.

1.3 Monomers

The monomers that can be used for contact lenses, which are described in the following sections and in both tables and references, are collected in Table 1.2.

These issues will be detailed in the following sections of this chapter.

1.3.1 Monomers for Block Copolymers

A block copolymer that contains both hydrophobic and hydrophilic blocks with amino acid groups has been described (18).

The principal monomers for such block copolymers are a combination of two monomers capable of forming a hydrogel; such monomers are collected in Table 1.3.

Table 1.2 Monomers for contact lenses.

Monomers and monomer types Usage References 2-Hydroxyethyl methyacrylate N-Vinyl-2-pyrrolidone Methyl methacrylate Isobornyl methacrylate tert-Butylcyclohexyl methacrylate Soft lenses (19) Hydrophobic monomers Strengthening agents Table 1.8 Hydrophilic monomers   Table 1.8 Hydrophilic monomers Hydrogels Table 1.10 Azlactones Surface treatment Table 1.12 Acrylamide N-Hydroxyethyl acrylamide N-Isopropyl acrylamide 2-Acrylamido-2-methylpropane-sulfonic acid 2-Hydroxyethyl methacrylate 2-Hydroxyethyl acrylate Macromers (20) Acryl monomers Water absorbable Table 1.16 Poly(siloxane) Water absorbable Table 1.17 4-(Phenyldiazenyl) phenyl methacrylate Blue-light blocking (21) Acrylates UV-blocking Table 1.22 Silicone hydrogel Multifocal lenses Table 1.23 Acrylates Non-silicone hydrogel Table 1.25 Crosslinking agents Non-silicone hydrogel Table Table 1.26 Oxyperm Oxygen permeable Table 1.32 Ionoperm Oxygen permeable Table 1.32

Table 1.3 Monomers (18).

Monomer Monomer 2-Ethylphenoxy acrylate 2-Ethylphenoxy methacrylate 2-Ethylthiophenyl acrylate 2-Ethylthiophenyl methacrylate 2-Ethylaminophenyl acrylate 2-Ethylaminophenyl methacrylate Phenyl acrylate Phenyl methacrylate Benzyl acrylate Benzyl methacrylate 2-Phenylethyl acrylate 2-Phenylethyl methacrylate 3-Phenylpropyl acrylate 3-Phenylpropyl methacrylate 3-Propylphenoxy acrylate 3-Propylphenoxy methacrylate 4-Butylphenoxy acrylate 4-Butylphenoxy methacrylate 4-Phenylbutyl acrylate 4-Phenylbutyl methacrylate

Side-chain-linked amino acids are collected in Table 1.4. Some of these compounds are shown in Figure 1.1.

Table 1.4 Side-chain-linked amino acids...