Source: Bausch & Lomb.
Oxygen transmissability to the open eye
Development of Contact Lenses
Development of contact lens materials has accelerated in the last century since the advent of the original glass shell lenses. Starting with the use of PMMA in 1937, followed by the development of the corneal lens in 1947, the pace of contact lens development has increased as the biocompatible materials and manufacturing sectors matured. Otto Wichterle, a Czech chemist, developed the modern soft lens in 1961, Bausch & Lomb brought the first contact lenses made of poly-HEMA (Poly(2-hydroxyethyl methacrylate)—a polymer that forms a hydrogel in water—to the market in 1971, mass-produced, disposable lens were popularised by Johnson & Johnson Vision Care in 1988 and Ciba Vision were the first company to develop and commercialise silicone hydrogel (SiHy) contact lenses.
Data from Robert W Baird (Jeff Johnson, OD, CFA, director, senior research analyst) indicates that the value of the worldwide contact lens market is approximately US$7.1 bn, with the US market valued at approximately US$2.4 bn at the manufacturer level in 2012. Premium lenses such as torics (20%) and multifocals (5%) making up significant portions of the market and spherical lenses accounting for the majority of the remainder.
Daily disposable lenses continued to grow in popularity, accounting for around 39.5% of 2012 worldwide sales. Daily disposables are the ideal lenses for patients who want an irregular wearing schedule for sports, work or social events. They eliminate altogether the need for cleaning and solutions—ideal for the atopic and indolent patient alike.
During the development of such a lens there are several key parameters to optimise, including lens design, water content, Dk—oxygen permeability, which allows the cornea to breathe, modulus, mechanical property, resistance to deformation, tear strength—if low, lens is fragile and breaks easily—surface properties, coefficient of friction, wettability (contact angle) and surface chemistry/charge. During lens development, each property must be optimised for performance based on the required modality of the lens. Modulus of the material is important for numerous reasons and a balance must be struck in order to attain the best performance from a material formulation. Oxygen transmissibility to the open eye for some of the leading brands are shown in the figure.
HyperGel: An evolution in silicones and hydrogels
One of the most exciting recent developments in daily disposable contact lenses came in May 2012 with the release of the HyperGel material (nesofilcon A) by Bausch & Lomb. The material is used in the Biotrue ONEday contact lens family and is the first daily disposable lens inspired by the biology of the human eye. The bio-inspired lenses contain 78% water, the same water content as the cornea, while delivering virtually the same oxygen level as the open eye. The true breakthrough of the HyperGel material is that it has an outer surface that is designed to mimic the lipid layer to prevent the lens from dehydrating and therefore maintain consistent optics. This is important because current contact lenses can dehydrate throughout the day, leading to discomfort and blurred vision. The lipid layer of the tear film contains naturally occurring surfactants that provide a natural barrier against dehydration. The HyperGel formulation contains a specially developed surfactant that is permanently enriched at the surface of the contact lens during the manufacturing process.
Recent studies of the new daily disposable contact lens have shown that, even after 16 hours of wear, the lens was losing, on average, only 1.5% water, compared with other lenses, which lose 6% to 8% water under normal conditions. To assess dehydration in the lens, clinicians also conducted studies in humidity-controlled rooms, with a relative humidity of around 5%, comparable to desert conditions. Even under such arid conditions, water loss with the new daily disposable contact lens was only 1.5%. HyperGel is an exciting new class of material not a silicone hydrogel and not a conventional hydrogel but an evolution of contact lens material.
Medical Plastics News would like to thank Austin Coffey, chair of the European Medical Polymers Division of the Society of Plastics Engineers, for providing this article.