Shedding Light on a New High-Volume Contact Lens Manufacturing Process

BY TONY HOUGH, MBA, BA
MARCH 1998

State-of-the-art technology allows Focus Dailies to bring a new level of affordability to daily disposable lens wear. Here's a glimpse into how it works.

Until now, daily disposable contact lenses have not been easily affordable for most patients in North America, resulting in limited acceptance. While patients have shown a high initial interest in them, they often reject it when presented with the yearly cost. The daily disposable market will advance only if manufacturers can produce a high quality product that is affordable for full-time wear. Anticipating that daily disposable soft lenses will take a significant share of the contact lens market, major manufacturers have carried out vast reviews of their production technologies. Clearly, molding is the only manufacturing technology capable of delivering the high volumes of lenses necessary to cope with the demand generated by the daily disposable modality. For about the past seven years, the development of high-volume soft lens manufacturing technologies could be described as one of incremental automation, with relatively modest evolutionary changes being added to established, successful techniques.

The Key Innovations of Lightstream

Occasionally, a product emerges which demands the attention of the technology watchers. When this happens in the contact lens industry, the interest is in the number of novel steps in the process of getting the new lens to the marketplace. The production technology behind CIBA Vision's new daily disposable soft lens, Focus Dailies, represents one of the most significant developments in soft lens manufacturing in the 1990s. There are numerous innovations in the manufacturing process used for Focus Dailies. This process, using techniques known as Lightstream Technology, advances the art of molding by introducing several features that have been long regarded as highly desirable.

The innovations in the Lightstream manufacturing process include:

Reusable molds -- the molds are made from high-quality quartz and glass and are used repeatedly to ensure master-mold quality in each lens.

Fully hydrated molding -- a water-soluble polymer is used to cast fully hydrated lenses which have dimensions matching those of the mold exactly and which require no subsequent extraction steps.

Edges formed by a unique photolithographic process -- the lenses are cross-linked in the molds by controlled exposure to ultraviolet light. The Lightstream manufacturing process derives its name from this stream of ultraviolet light. An opaque mask is used to prevent cross-linking at the periphery, thus forming the lens edge.

Any one of the features above would cause interest in manufacturing circles, but when taken together, they identify the Lightstream process as being highly innovative.

The Dailies Technology Think Tank

The contact lens design process should ensure that the product can be efficiently manufactured in terms of quality, capital requirement and unit cost. Particularly in the case of a very high-volume product such as daily disposables, production capability considerations are integral to the development process. The development team behind the new Focus Dailies lens is CIBA Vision's New Lens Technology Group (NELTEG), based in Grossostheim, Germany. This is a multidisciplinary team encompassing skills in material chemistry, lens design, manufacturing process, clinical practice and marketing. Throughout the development process, every relevant aspect of the product was engineered by the NELTEG team.

Material Chemistry

The innovation of the Focus Dailies lens goes beyond its manufacturing process. Change to current manufacturing practices began with a review of material chemistry. Using a patented process, polyvinyl alcohol (PVA), a highly biocompatible material widely proven in biomedical applications, is converted to nelfilcon A, the polymer of which Focus Dailies are made (Table 1). Using this water-soluble, pre-purified polymer, the Lightstream process eliminates the need for the time-consuming extraction of impurities, solvents and residual monomers. It is a goal of high-volume, low-cost lens production to minimize the number of manufacturing steps, as every additional step adds to the unit cost.

TABLE 1:  Focus Dailies Material Properties

Material name Nelfilcon A Composition Modified polyvinyl alcohol Water content 69% FDA classification Group 2, Non-ionic Oxygen permeability 26 x 10-11 barrers

Lens Design

The Focus Dailies lens geometry is a tri-curve back and front surface with the overall lens profile controlled for thickness to maximize oxygen delivery. The back surface is more complex than a standard co-axial tri-curve. The outermost peripheral zone is a narrow band of reverse bevel with a common tangent at the point of transition. It is generally accepted that this type of peripheral construction improves lens comfort by enhancing on-eye dynamics. A typical front optic zone diameter is about 8.0mm, with two surrounding front peripheral zones used to control lens thickness. The designed edge thickness is 75 microns. The manufacturing industry is familiar with very complex designs produced by lathe cutting, but so far the geometries produced by molding have all been relatively simple. The Focus Dailies geometry represents a new level of sophistication for a spherical soft lens manufactured by molding.

The Lightstream Process

The high-precision molds are a key element of the Lightstream process (Fig. 1). The lens geometry is incorporated in the mold surfaces -- a significant engineering feat. The manufacturing process is divided into modules, with each module enclosed in a laminar flow cabinet. The complete manufacturing process is fully automated, from the nelfilcon A polymer being metered into the molds to the lenses emerging in strips of five blister packs ready for autoclaving. In-process inspection of finished lenses is computer-controlled. Batches are sampled in the traditional method for volume production.

FIG. 1. The reusable quartz and glass molds. The high-precision mask which defines the edge is incorporated into one-half of the mold.

The water soluble nelfilcon A polymer is cross-linked in the mold by exposure to ultraviolet light which is restricted to the area within the mask. The surrounding polymer which is not cross-linked is then washed away with purified water, forming the edge. A thin edge profile, designed to minimize lid awareness, is defined by the three-dimensional shape delineated by the shadow of the ultraviolet lightstream at the edge of the mask and the cavity between the two mold surfaces. Figure 2 illustrates a typical edge profile. After cross-linking, the mold carrier is opened and the lenses are removed from the molds by vacuum grippers. Every lens is inspected for defects by an automated, computer-controlled process. The molds on their carriers are cycled continuously around the production module. When the lenses are removed prior to packaging, the molds are washed with water and go around again. The production modules are enclosed in environmentally controlled cabinets. The lenses are deposited in blister packs which are filled with a metered amount of saline and sealed, all within an environmentally controlled enclosure. The strips of five blister packs are fed from the production module to the sterilization area, packed in 90-lens packages and labeled.

FIG. 2. A typical Focus Dailies edge profile

THE LIGHTSTREAM PROCESS

Step 1: Nelfilcon A polymer solution is metered into a 'female' mold.

Step 2: One-half of the mold is lowered to form the lens, but the two never touch.

Step 3: A beam of UV light passes through the mold to cross-link the polymer.

Step 4: The polymer shielded by the mask is washed away to leave the final lens.

Step 5: Using a vacuum gripper, the final lens is placed in a blister pack.

A Technology For the Future

Strong performance depends on a fast response to rapidly developing customer demands. In developing the Lightstream Technology to produce the Focus Dailies lens, CIBA Vision has demonstrated what can be achieved with an integrated product and manufacturing strategy. The quality and unit cost of the Focus Dailies lens should enable CIBA Vision to grow the daily disposable category and make this a lens of choice for many eyecare practitioners and their patients.

This article was reprinted with permission from the November 7, 1997 issue of The Optician.

Tony Hough is a technical expert in the field of contact lenses with 19 years of experience in contact lens manufacturing technology. He is a consultant based in Cambridgeshire, UK.