How were silicone hydrogel contacts developed?

The difficulty of combining silicone rubber with a typical hydrogel-forming monomer was the first major obstacle in the development of the silicone hydrogel contact lenses. The first answer that springs to mind is to combine HEMA with TRIS, the monomer used in the preparation of RGP lens materials. However, combining hydrophobic TRIS with hydrophilic HEMA and then hydrating the product presents the same difficulty as trying to combine oil and water to form an optically clear product.

The earliest patents were granted to the Toyo Contact Lens Company in 1979 with Kyoichi Tanaka and four others as the named inventors. However, it was not until the mid-1990s that the patents explicitly addressed the question of lens movement.

The first commercially available silicone hydrogel contacts adopted two different approaches. The first approach, by Bausch& Lomb,i s a logical extension of its development of silicone monomers with enhanced compatibility in hydrophilic hydrogel-forming monomers. The second, by Ciba Vision, was the development of siloxy macromers containing hydrophilic polyethylene oxide segments and oxygen permeable polysiloxane units. Polyethylene oxides, better known as polyethylene glycols or PEGs, are very hydrophilic materials and are widely used as components of surfactants, foodstuffs and various biomaterials.

Until the mid-1990s, three measurable properties of a contact lens material were based on wetting properties, mechanical properties, and oxygen permeability in order to predict the minimum acceptable baseline performance. The publication of the Ciba Vision patent in 1996, entitled ‘Extended Wear Ophthalmic Lens’,proposed a fourth, of property measurement linked to lens movement on the eye – hydraulic and ionic permeability – and demonstrates that biphasic materials present a successful way of balancing these properties.