True Story of Poly(2-Hydroxyethyl Methacrylate)-Based Contact Lenses: How Did It Really Happen
Miroslava Duskova-Smrckova, Institute of Macromolecular Chemistry, Czech Academy of Sciences
Jiří Podešva, Institute of Macromolecular Chemistry, Czech Academy of Sciences
Jiří Michálek, Institute of Macromolecular Chemistry, Czech Academy of Sciences
Modern hydrogels are usually tailor-made for the given purpose and application, be they synthesized by radical-initiated or stepwise process-es, performed in a standard way, or by 3D printing. Since the times of the invention of the first hydrophilic plastic “swellable Perspex”, prepared by O. Wichterle’s group in the 1960s using the radical polymerization of 2-hydroxy-ethyl methacrylate (HEMA),1 much effort has been devoted to a detailed study of this polymer. This was due both to its use for pioneering hydrogel contact lenses (the so-called “swelling plastic”) and to its interesting properties. Poly(2-hydroxyethyl methacrylate) (PHEMA) is distinguished by a good swellability (primarily in hydrophilic and partially also in hydrophobic media) and by a very good compatibility with living tissues. Even after swelling in aqueous media it keeps its mechanical strength and flexibility and is stable in time. That is why this material has found so many applications. Besides the medicinal use in the fields of ophthalmology, implants, or systems for drug transport and releasing, there are less known but no less successful uses for sorbents with a large intrinsic surface or separation monoliths in chromatography.
Thus, PHEMA remains a subject of lively scientific interest, as indicated by the number of papers with this keyword, published every year. At the same time, it represents an important model polymer both for the scientific research of synthetic hydrogels and for biomedicinal applications, including testing experiments of tissue engineering. This paper brings information on the history of the research and applications of this unique monomer and its polymers, with special regard to hydrophilic contact lenses. It is the authors’ ambition to put some erroneous historical data straight. Moreover, we consider it useful to briefly outline also the classification and history of the whole phenomenon of contact lenses.
2. EXCITING HISTORY OF CONTACT LENSES IN GENERAL
What is the contact lens? The basic definition reads: Contact lens is a small optical system placed directly on the cornea. All the issues and problems related to the contact lenses follow therefrom.Contact lenses can be categorized in various ways. However, according to M. F. Refojo, the fundamental division is based on the nature of the material. Most simply, contact lenses could be distinguished into rig-id ones and soft ones, the latter then into hydrophobic and hydrophilic. Further categorization, necessary in connection with the development of new materials for contact lenses, is given in more detail in the Appendix. In current sources, this division is, regrettably, often oversimplified.The idea of contact lenses is very old, reaching back as far as the 16th century and Leonardo da Vinci concepts, and its implementation is closely connect-ed with the development of material science. Various inventors tried to use a broad spectrum of materials for contact lenses.For example, when poly(methyl methacrylate)(PMMA) was introduced into the market (1933) and its relatively good biocompatibility was discovered, a way was opened for new medicinal applications of this plastic.
Thanks to its optical properties, PMMA found its main use in ophthalmology (as a material for contact lenses, later for intraocular lenses, spectacles, etc.). This was the beginning of the era of polymers or covalent polymer networks in contactology, a brief history of which is presented in a tabulated form in the Appendix. After PMMA had been tested and finally abandoned, the following development of contact lenses was carried out to improve the properties of the lenses, namely, their permeability for gases (primarily oxygen) and also for water-soluble substances and ions. Although both of these requirements were met excellently by hydrogels studied by Wichterle and Lím, another branch of the research continued towards the silicone elastomers (1965) which offered a high permeability for gases and showed good softness but were hydrophobic.
These properties were then responsible for problems met when removing these lenses from the eye, namely, mechanical damage to a testing person’s cornea. As a consequence of this, contact lenses based purely on silicone hydrophobic elastomers are no more accessible in the common market. Still another route of the development resulted in rigid gas-permeable (RGP) materials (1974), usually copolymers of alkyl methacrylates and siloxane meth-acrylates (possibly also fluoroalkyl methacrylates) which guarantee a high permeability for oxygen11 but are hydrophobic and do not allow the transport of watersoluble substances.
Diverse variants of high-swelling hydrogels for contact lenses have continuously been being developed which had, in dependence on the equilibrium water content, a higher permeability for both water-soluble substances and gases. In addition to the basic sparsely crosslinked PHEMA, other glycol methacrylates were used, such as diethylene glycol methacrylate, triethylene glycol methacrylate, dihydroxyalkyl methacrylates (e.g., glycerol methacrylate), acrylamide, and, for ionogenic materials, also methacrylic acid sodium salt. Besides the acrylic acid derivatives, also 1-vinyl-2-pyrrolidone and polyvinylalcohol found their use as materials for high-swelling hydrogel contact lenses.12Thus, in the sixties and seventies, the development headed toward soft contact lenses based on PHEMA or similar hydrophilic methacrylates, as will be discussed below. Later, however, silicone hydrogel lenses of the first generation were developed and introduced (1998–1999, according to the territory) and became an important milestone. Based on the first experience, the second generation arrived in 2004 and soon after (2006) even the third one. Interestingly, the first relevant patent dates back to 1979.
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