Polymer contact lens material

Contact Lens Material. Dr. Jay F. Kunzler and Dr. Joseph A. McGee, Department of Polymer Chemistry, Bausch Lomb, Chemistry Industry 21 (1995), 615. 

However, it should be imderstood that many recently developed contact lens materials contain polymers that block the transmission of light in the ultraviolet region. Therefore when an ultraviolet light source, such as a... 

Many other applications for plasma polymers in the Life Sciences have been dted, often in relation to implantable medical devices or materials, with the goal of concealing the device from the bodies defence mechanisms, or improving cell colonisation of the material, e.g. endothelial cell growth into vascular grafts. A number of excellent studies from the group of Hans Griesser (CSIRO, Australia) describe the use of plasma polymers as substrates to which biomolecules can be immobilised. These immobilisations have been demonstrated to enhance the medium-term acceptability of contact lens materials and may prove relevant to implantable devices. 

Recent experiments indicate that polymers that contain a balance of hydrophobic (nonpolar) and hydrophilic (polar) chemical groups show minimal protein adsorption and cell adhesion (6). With the intent of rationally designing a contact lens material that would minimize protein adsorption, the adsorption of lysozyme, albumin, and immunoglobulin G (IgG) to a series of hydrophobic and hydrophilic polymers and copolymers was measured. The polymers ranged from 100% poly(methyl methacrylate) (PMMA) to 100% poly(2-hydroxyethyl methacrylate) (PHEMA). Adsorption varied significantly for each protein, as did the elutability of the proteins from the surfaces. 

Contact lenses are the most common polymer product in ophthalmology. The basic requirements for this type of materials are (T)excellent optical properties with a refractive index similar to cornea good wettability and oxygen permeability ( ) biologically inert, degradation resistant and not chemically reactive to the transfer area ( ) with certain mechanical strength for intensive processing and stain and precipitation prevention. The common used contact lens material includes poly-P-hydroxy ethyl methacrylate, poly-P-hydroxy ethyl methacrylate-N-vinyl pyrrolidone, poly-P-hydroxy ethyl methacrylate, Poly-P-hydroxy ethyl methacrylate - methyl amyl acrylate and polymethyl methacrylate ester-N-vinyl pyrrolidone, etc. The artificial cornea can be prepared by silicon rubber, poly methyl... 

The design of new polymeric materials for contact lens application requires an extensive knowledge of polymer chemistry, polymer properties and the physiology of the eye (1-4). The properties that must be optimized in designing a new contact lens material are optical transparency, chemical and thermjd stability, wettability to tears, mechanical properties, dimensional stability, biol( cal compatibility and oxygen permeability. 

The solubility parameter method (31) can be used to calculate an estimate of oxygen solubility in the swollen polymer gel. Models for the difiusion of a gas through a swollen polymer gel are available but require large amounts of computer time (26). With both pieces of data for the permeability available, the capacity of different formulations to pass oxygen to the eye can be estimated. This provides a final numerical scrcerring for the possible formulations of contact lens material. Materials which pass all of the numerical criteria can then be synthesized in the lab and the correlation of predicted properties and measured properties determined. This capacity to estimate some of the properties of a proposed polymer composition (31,32) has sharply increased the capacity of research workers to develop new materials or fit formulations to new applications in a very short time. 

Hyon S-H, Cha W-I, Ikada Y, Kita M, Ogura Y, Honda Y (1994) Poly(vinyl alcohol) hydrogels as soft contact lens material. J Biomater Sci Polym Ed 5 397... 

Table 2 Representative monomers and polymers used as soft contact lens materials.

Not every polymer can be manufactured successfully into a contact lens. Several important properties for both ocular physiology and patient handling are required of a material for a contact lens appHcation (1,11). In addition, the type of lens appHcation, ie, rigid, flexible, or soft, will dictate the range and importance of the key properties. 

Water content indirectly affects other lens characteristics. Water evaporation from the lens can result in a dry eye sensation and subsequent desiccative erosion of the cornea. Clinical studies have shown the incidence of corneal erosion as a result of lens desiccation to be a material-dependent and water-content-dependent phenomenon (25,26). The nature of water and sodium ions in hydrogels has been studied primarily by nmr and thermal techniques (27,28). An empirical relationship between water mobility in contact lens polymers and desiccative staining has been proposed (29). 

Hard contact lenses are composed of a polymer that repels water because the constituent repeating units (the monomers that link together to form the polymer) are nonpolar, hydrophobic segments. The first hard contact lens was constructed in 1948 from the monomer known as methyl methacrylate (MMA), yielding the polymer poly(methyl methacrylate) or PMMA. This material offers durability, optical transparency, and acceptable wettability for optimal comfort. Today the rigid lens material of hard contact lenses is often constructed by combining MMA with one or more additional hydrophobic monomers to provide better gas permeability. 

Polysiloxanes (silicones) are one of the most studied classes of polymers. They exhibit a variety of useful properties not common to non-metal-contain-ing macromolecules. They are characterized by combinations of chemical, mechanical, electrical, and other properties that, when taken together, are not found in any other commercially available class of materials. The initial footprints on the moon were made by polysiloxanes. Polysiloxanes are currently sold as high-performance caulks, lubricants, antifoaming agents, window gaskets, O-rings, contact lens, and numerous and variable human biological implants and prosthetics, to mention just a few of their applications. 

The hydrophobicity of the surface prevents the wetting by tear and tends to expose dry surface of a contact lens. Therefore, rapid dehydration of the corneal tissues could occur, which could cause the damage of corneal epithelium. However, this explanation seems to be oversimplified in light of the adsorption of protein, which makes a hydrophobic surface wettable by tear fluid, as described in Chapter 26. Moreover, the highly hydrophobic surface characteristic of silicone rubber tends to encourage the deposition of protein and mucus of the tear on the surface of the lens. Lipids and lipid-soluble materials follow the same track and eventually penetrate into the bulk phase of the contact lens. Because of these undesirable factors, the use of silicone contact lenses of various chemical compositions and with surface treatments has not been successful but rather disastrous because of the interfacial characteristics of silicone contact lens on the cornea, which cannot be oflfset by these efforts. It indicates that more profound surface modification to cope with the problems rather than mere surface treatment is needed in capitalizing on the advantageous bulk properties of silicone polymers. 

In addition, many of the known hydrophilic monomers have been used to prepare synthetic hydrogel polymers and copolymers of a wide range of compositions, mostly for contact lens uses. In such materials, water insol-... 

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