Contact lens oxygen permeability

Copolymers of diallyl itaconate [2767-99-9] with AJ-vinylpyrrolidinone and styrene have been proposed as oxygen-permeable contact lenses (qv) (77). Reactivity ratios have been studied ia the copolymerization of diallyl tartrate (78). A lens of a high refractive iadex n- = 1.63) and a heat distortion above 280°C has been reported for diallyl 2,6-naphthalene dicarboxylate [51223-57-5] (79). Diallyl chlorendate [3232-62-0] polymerized ia the presence of di-/-butyl peroxide gives a lens with a refractive iadex of n = 1.57 (80). Hardness as high as Rockwell 150 is obtained by polymerization of triaHyl trimeUitate [2694-54-4] initiated by benzoyl peroxide (81). 

The number of contact lens wearers has grown to an estimated 24 million in the United States and 50 million worldwide. Concurrendy, there has been a proliferation of contact lens manufacturers and products. The 1980s saw the widespread introduction of lens products made of more oxygen-permeable materials, ie, rigid gas-permeable (RGP) materials that made PMMA lenses virtually obsolete and high water content hydrogels that competed with HEMA-based lenses. 

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°. 

While the silicone polymer phase provides a high enough oxygen permeability, it also dominates the surface characteristics, i.e., the surface of silicone hydrogel is too hydrophobic to be used as a contact lens. The silicone phase also readily absorbs... 

To be suitable for long-term wear, a contact lens material has to satisfy several criteria. It must be hydrophilic enough to maintain a stable, continuous tear film on its surface, resist fouling by tear components, not irritate the eye, and be comfortable to wear. It should also be realized that the metabolism of the cornea is highly dependent on dissolution and adsorption of atmospheric oxygen and therefore any lens material must have sufficient oxygen permeability to maintain this, else corneal anoxia will set in. 

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. 

Iliis paper descnbes a novel contact lens materim based on a methacrylate end capped poly (trifluoropropylmethylsiloxane)(TET), octafluoropentyl-methacrylate (OFPMA) and a wettmg agent 2-vinyl-4,4-dimethyl-2-oxazolin-5-one (VDMO). The trifluoropropylmethyl siloxane, fluorinated methacrylate methacrylate and VDMO in the correct concentration provide a lipid resistant, oxygen permeable, hydrophilic contact lens material. 

Fig. 3. Equivalent oxygen permeability comparison of contact lens materials. For reference air = 21 % Oj. (from Hill)...

Fig. 4. (Photo)-Rigid oxygen permeable contact lenses and a new generation extended wear hydrogel lens incorporate a variety of organosilicon substituted monomers (courtesy L. Seidner, Permeable Technologies).

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. 

Many materials are employed in the production of soft contact lenses. The material with the widest application in the industry is HEM A [13]. HEM A became a popular soft contact lens because of its clarity, comfort, good vision, and excellent wetting characteristics. HEMA is a hydrogel, and hydrogels are cross-linked polymer networks that are swollen in water. Hydrogels are typically classified by whether they are neutral or ionic. Anyone who has had a piece of dirt in the eye understands the need for good wettability when it comes to a contact lens. The wettability of a soft contact lens in the ocular environment is a function of the surface chemistry while it is hydrated, which determines the biocompatibility of the lens [6]. A lens surface that does not wet will cau.se discomfort and potentially increased deposits. These deposits can affect vision and the physical properties of the lens [3,4]. The major drawback of HEMA is the permeability of the material to oxygen [12]. Extended wear can result in increased vascularization of the cornea... 

Silicone Hydrogels. A new class of hydrogels based on silicone has been developed (14). These materials were developed in an attempt to combine the high oxygen permeability of polydimethylsiloxane and the excellent water absorption characteristics of conventional non-silicone hydrogels. This new class of hydrogels was developed primarily for contact lens application. These silicone hydrogels are also prepared by free-radical poljnnerization techniques. 

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