Plastic contact lenses

In addition, highly water-absorbing and oil-absorbing resins are of interest. These have developed rapidly in recent years by unique grafting and cross-linking of hydrophilic polymers. Transparent polymeric materials with optical functions are also noteworthy. Some are biocompatible, such as poly(2-hydroxyethyl methacrylate), which serves as a material for soft contact lenses. Plastic optical fibers are also widely used as substitutes for glass and quartz devices in various fields of technology, especially the biomedical and communication sciences. 

Surface modification of a contact lens can be grouped into physical and chemical types of treatment. Physical treatments include plasma treatments with water vapor (siUcone lens) and oxygen (176) and plasma polymerization for which the material surface is exposed to the plasma in the presence of a reactive monomer (177). Surfaces are also altered with exposure to uv radiation (178) or bombardment with oxides of nitrogen (179). Ion implantation (qv) of RGP plastics (180) can greatiy increase the surface hardness and hence the scratch resistance without seriously affecting the transmission of light. 

In humans, a contact lens lies in the conjunctival sac of the eye. In a closed eye, this sac is a slitlike space between the conjunctiva-covered eyeball and the eyelids. Contact lenses are small polymer bowls that float on tears superficial to the corneal eye layer and correct existing visual deficiencies similarly to glasses. While the idea of the contact lens was formulated as early as 1508, it was not until the 1800s that contact lenses became a reality. Hard plastic contact lenses were invented around 1936, and although soft lenses were invented in 1960, they were not available on the commercial market until 1971. 

Chlorhexidine is also cationic like BKC and exhibits similar incompatibilities. It is not as stable as BKC to autoclaving and may irritate the eyes. It tends to be more favoured in Europe than in the United States, and is particularly used in contact lens products. Chlorobutanol and phenylethyl aclohol are also widely used in ophthalmic products. However, chlorobutanol will hydrolyse in solution, and autoclaving is not usually possible without loss of preservative activity. It is also volatile and may be lost through the walls of plastic containers. 

Gamma irradiation of 25 kGy or above and accelerated electrons tend to cause similar changes to the product/pack. These processes are normally used as part of an aseptic process to sterilise the pack components before use, since many products tend to degrade if terminally sterilised. Some contact lens solutions and normal saline products can be satisfactorily gamma irradiated in a suitable plastic or metal pack (aerosols). 

Uses Crosslinking agent for actylic/vinyl resins PVC plastisol comonomer (reduces initial vise, and oil extractability. and improves ultimate hardness, heat distort., hot tear strength, and stain resistance) adhesion promoter hardener used in cast acrylic sheet and rod, contact lens, elastomers, ion exchange resins, dental polymers, adhesives, coatings (paper, plastic), cosmetics, paints, sealants, photopolymers, electronics (photoresists, solder masks)... 

Successful application. First plastic hernia mesh. "First plastic contact lens. PVC, poly(vinyl chloride). 

Paragon Grows As Contact Lens Button Holder. Plastics News, pp. 1, 20, Jan. 1990. 

The first plastic ( hard ) comeal contact lens was developed in 1936, fabricated from poly(methyl methacrylate), PMMA (VI).However, these lenses were not commercially available until the mid 1960s. In comparison, soft contact lenses were first comprised of poly (hydroxy ethyl methacrylate), PHEMA (VII), and became commercially available shortly thereafter. Both types of lenses are comprised of a 3-D amorphous network of cross-linked polymer chains. However, the soft hard character of a lens material is directly related to the ambient operating temperature relative to its glass-transition temperature (Tg). Whereas PMMA has a Tg of 100-120°C, the Tg of PHEMA ca. 10-15°C) lies below room temperature. ... 

So-called soft contact lenses are actually made from a polymer hydrogel with a composition of up to about 50% water (the older hard lenses were a solid piece of plastic). If you are a contact lens wearer, you will know what happens when you leave a soft lens to dry out it shrinks and becomes hard like plastic. Dried up lenses will regain their former shape if soaked in water as the polymer network swells and refills with water. Some of the most current contact lens technologies, such as overnight or continuous wear lenses, are based on silicone hydrogels (polymers with a silicon backbone). 

Personnel involved in the handling of methanol require eye and skin protection from the irritating properties of methanol in the event of a spill. Contact lenses should not be worn, since plastic lens materials may absorb and concentrate methanol against the eye. Additional respiratory protection is not required with adequate local explosion-proof ventilation. 

Hard lenses can be defined as plastic lenses that contain no water, have moduli in excess of 5 MPa (500 g/mm ), and have T well above the temperature of the ocular environment. Poly(methyl methacrylate) (PMMA) has excellent optical and mechanical properties and scratch resistance and was the first and only plastic used as a hard lens material before higher oxygen-permeable materials were developed. PMMA lenses also show excellent wetting in the ocular environment even though they are hydrophobic, eg, the contact angle is 66°. 

Cast molding is an increasingly used manufacturing process for both rigid gas-permeable and hydrogel contact lenses. In this process, two molds, made from a variety of plastics, are used. A female mold forms the lens front surface (convex) and a male mold forms the lens back surface (concave). The plastic molds are made from metal tools or dies that are usually stainless steel, precision lathed, and polished to the specified lens design. A variety of mold materials are used. The polymerized, hardened lens is released from the mated molds and is processed in much the same way as the spin-cast lenses described above. 

Comeal shields are medical devices that are used as a bandage for protection of the cornea and to allow healing following surgery. Initially, hydrophilic soft contact lenses used for vision correction were employed as comeal shields. Collagen was then introduced as a substitute for the plastic noneroding bandage lens. They are widely used... 

Small droplets can be used to fabricate lenses and lens arrays suitable for use in fiber optics, laser diodes, imaging arrays, displays and optical switches. Cured optical plastics are used instead of glass to produce lenses where the curvature of the lens can be adjusted by control of surface tension and contact angle of droplets deposited on flat substrates. 

---