1. Advance Silicon hydrogel contact lens materials
Silicon hydrogels are the latest in a line of development of soft contact lens.
Due to its high level of hydration and comfort, it is most popular among new
and extended wearers. In silicon hydrogels materials, silicon is combined with
conventional hydrogel monomers. Silicon part (hydrophobic) provides high
oxygen permeability (dk) to the material and hydrogel part(hydrophilic)
provides flexibility, wettability to the material which therefore enhances the
clinical performance of contact lens.
All contact lens materials are polymers. The constituent monomers which
comprise the polymers determine the physical and chemical properties of the
materials. Repeating chains of monomers are arranged in patterns with cross-
linking between the polymer chains to afford strength. FDA has classified Si- Hy
materials into various generations and each generation have different
treatment to make material hydrophilic. The monomers commonly employed
in silicon hydrogel contact lens materials are mentioned below.
Balafilcon A is a homogeneous combination of a TRIS monomer derivative
that is copolymerized with the hydrophilic hydrogel monomer N- vinyl
pyrrolidone (NVP). Lotrafilcon A is the combination of unmodified TRIS
monomers with silicone elastomer interspersed with hydrophilic polyethylene
glycols (PEGs). Both balafilcon A and lotrafilcon A uses gas plasma technique as
a wetting strategy. Galyfilcon A and senofilcon A use a more hydrophilic
derivative of the TRIS monomer and polyvinyl pyrrolidone (PVP), which serves
as an internal wetting agent. Similarly, comfilcon A, asmofilcon A and enfilcon
A are the latest silicone hydrogel materials and uses a unique long-chain
siloxane macromer which is naturally wettable material.
These materials provide greatly improved Dk/t to the cornea and have
largely eliminated hypoxic signs, whether worn on a daily wear or overnight
basis. Thus silicon hydrogel contact lens are currently commercially available
with their major features being practised by optometrist .
