Contact lenses represent one of the single largest categories of medical devices, with more than 31 million contact lens wearers in the United States alone, according to the Association of Contact Lens Manufacturers. Many of these lenses are disposable, which translates into a domestic volume of more than 1 billion lenses per year. Economically sustaining this volume requires highly efficient manufacturing and testing methods, with an emphasis on low cost and high reproducibility. At the same time, these lenses incorporate increasingly complex shapes and nanostructured surfaces to support improved vision correction, including astigmatism and bifocal function. One limiting factor in producing these shapes and surfaces has been the performance of the traditional metrology instruments used to characterize the lenses themselves, as well as the tools and processes used to create them.

This article demonstrates how this need is now increasingly met by the 3-D optical microscope. The noncontact, gauge-capable tool enables surface metrology with higher throughput sampling density than previously possible. This tool can characterize all three critical parameters—form, local shape such as steps or edge heights, and roughness—with high accuracy. It can also be used during R&D and process development as well as in manufacturing metrology of intraocular lens (IOL) implants. Like contact lenses, these implants are also small, polymer-based lenses with increasing shape complexity. But since they represent a permanently implanted device, comprehensive testing of form and function is even more critical than for contacts. 

Contact Lens Materials

Contact lens materials and manufacturing methods have changed considerably since the first experimental glass lenses were developed more than 100 years ago. These developments have been driven by the need to produce lenses with characteristics that improve function and wearability. These characteristics include optical clarity, chemical stability, mechanical durability, oxygen permeability, and wettability.  

Around 70 years ago, the development of contact lenses based on a hard plastic, polymethylmethacrylate (PMMA) was a key step forward towards mass public acceptance of contact lenses. But the major advances that drove widespread adoption of the technology were the development of hydrogel materials about 30 years ago, soon followed by silicon-containing gas permeable (GP) lenses. And in the past decade, the majority of contact lenses have switched to silicone hydrogel materials. The term hydrogel refers to a soft and flexible hydrophilic material that is hydrated during use (i.e., contains up to 70% by weight of water).

Although these are the basic classes of commonly used materials, each manufacturer uses a proprietary blend of monomer additives to optimize different lens properties. For example, disposable lenses sacrifice durability to maximize the other characteristics, while long-wear lenses prioritize oxygen permeability.

Contact Lens Shape Evolution

In recent years, there has been an evolution in the shapes and surface complexity of contact lenses, enabling more comprehensive and subtle vision correction. This evolution has been driven in part by the changing age demographics of contact lens wearers. Specifically, first generation of contact lens wearers needed the lenses to correct near-sightedness as this was the  only conditionthat spherical contact lenses could correct at that time based on the spherically curved lens surfaces created by conventional lens fabrication methods. Today, the fastest growing segment of the contact lens market is in the 40+ age category. Many of these users are already long-time contact lens wearers (due to near-sightedness) who are now also affected by presbyopia (a gradual hardening of the eye’s lens, which limits the ability to accommodate—focus at short distances, for example). Since presbyopia is an age-related condition and we have a naturally graying U.S. population, the demand for bifocal contact lenses that correct for both near-sightedness and presbyopia is growing in parallel with increasing age of the baby boom generation.

Bifocal lenses can be designed in several different ways. The predominant methodology is to create lenses with alternating optical powers structured within a concentric circular pattern (see Image 1). Each concentric lens ring is structured for either distant or near vision, and the eye looks through both powers at the same time. The visual system automatically selects the correct power choice based on the distance to the object on which the eye is focusing, thereby providing continuous near and far vision.

To further complicate matters, many older patients also need correction for astigmatism, where the ocular system needs different powers of correction in different axes. Toroid-shaped lenses have long been used in eyeglasses for this purpose, in situations where the lens has two different focusing powers orthogonal to each other. But with contact lenses, preventing the lens from rotating is not as easy as with traditional eyeglasses. Two widely used methods for maintaining angular orientation in contact lenses are the double slab-off design, which uses gravity, and the prism ballast, which uses a “watermelon seed” shape. In both cases, blinking forces of the eye hold the lens in place.

Intraocular Lenses

IOLs are the other common type of ophthalmic “device.”  They have revolutionized the treatment for opacification, or clouding of the native lens, commonly referred to as cataracts. Unlike the contact lens, the IOL is implanted in the eye on a permanent basis. Pioneered by the world-renowned ophthalmic surgeon, Sir Harold Ridley, many people now consider the IOL to be the single most successful implant, especially given the low rate of postoperative complications (POCs). This success is, in part, a result of extensive process and quality control by IOL manufacturers. Surface roughness measurements are an important component of this testing since some complications are thought to arise from the interaction between calcium ions found in eye fluids and the material of the IOL.

The IOL is a clear plastic lens with two protrusions on opposite edges, called haptics (see Image 2). These integral parts of the device commonly take the form of S-shaped hook extensions or rectangular lobes, depending on where and how the lens is to be secured in the eye. Like contact lenses, IOLs have gone through several material evolutions and revolutions. In addition, the initial IOLs were hard objects fabricated from PMMA (as were contact lenses).

The development of soft IOLs was a huge leap forward, although for different reasons than contact lenses. Specifically, a soft IOL made of hydrophilic acrylic, hydrophobic acrylic, or silicone-based material enables the lens to be folded during insertion into the eyeball. The advantages of this less disruptive surgery are obvious. Additionally, flexible lenses can be attached to the capsule, which enables limited natural focusing, just like the original native lens.

Manufacturing Today’s Soft Lenses

Cast molding is the primary manufacturing method for producing the complex geometries and detailed shapes of today’s contacts and IOLs. Here, monomeric material is thermally cured while trapped between two molds that define the front and rear surfaces of the lenses. In the case of contacts, these are referred to as the central posterior curve and the central anterior curve.

To maximize product consistency while minimizing fabrication costs, the “soft” mold is molded from a reference tool commonly referred to as a master pin. This tool is typically a machined stainless steel part, created by turning with a diamond blade. Using CNC fabrication, this pin is therefore a positive model of the final lens, including all the correct shape features and nanometer-level structures required in the lens.

To enable volume fabrication, multiple front- and back-curve molds are then assembled in mold inserts. A polymer is injected in the mold and thermally cured. The lenses are then hydrated, where they absorb 20% to 70% water by mass. Of course, this means that the final lenses are larger than the machined dehydrated form. This factor has to be very carefully allowed for when designing and machining the master pins.

Using secondary molds for volume fabrication means that the master pins are used sparingly. However, they must eventually be replaced. This process involves making secondary molds and prototype lenses that must be tested. The lenses rarely meet the target prescription the first time. Instead, the results of these tests are used to create or modify new pins, and another batch of prototype lenses. The limitations of traditional testing methods, and to a lesser extent the hydration expansion effects, means that up to six iterations are required to create a new set of conformal master pins. In addition, during regular production, all batches of lenses must be sample tested for overall shape, nanoscale features, and surface roughness. The latter is important to avoid irritation and increased risk of infection with contacts and to eliminate their potential as initiation sites for POCs (in the case of IOLs). Lastly, batch data is often embossed on the lenses via the molding process. It is imperative that these characters are deep enough to be seen without being too deep or sharp-edged to cause irritation.

There are two traditional methods for testing the shape and surface details of the final lenses. Both have limitations for this application. One is a noncontact, optical method for measuring overall surfaces called Fizeau interferometry. Because this is a laser method, it cannot be used to quantify step heights greater than one quarter of the laser wavelength, which translates into step heights of 160 nm. However, many of today’s lens designs incorporate nanofeatures with greater step heights.

A method that can measure the aspheric profile and nanometer features of modern lenses is the stylus profilometer. This instrument provides for the motion of a fine needle under low pressure over the surface of the lens, thereby obtaining an accurate, two-dimensional profile. By measuring at two azimuthal angles, the shape of the test component is approximately understood. However, the typical soft hydrated gel lens has to be frozen prior to test with this contact method, which adds to test preparation time.

The 3-D Optical Microscope

The 3-D optical microscope is a well-proven instrument in many other applications, including quality control of medical devices such as dental implants and orthopedic surgical prosthetics. It is now being increasingly adopted (or evaluated) for the metrology needs of soft lens manufacturing. Specifically, this instrument is a high-speed, noncontact 3-D tool that can quantify the overall surface contours of a lens surface, the height and width of all surface features, as well as surface roughness, in seconds.

Externally, the 3-D optical microscope looks similar to a conventional optical microscope, and the lens (or pin) being tested is placed on the sample stage. A broad wavelength band high-brightness LED (HB-LED) source sends light into the objective and is split into two paths by a partially reflective mirror. One of the paths is focused on the sample surface and the other is deflected to an extremely flat internal reference surface. When these two reflections are recombined, the image contains a series of dark and light bands that are a direct function of the profile of the test surface; they are analogous to elevation contours on a topographic map. Moreover, they only appear as sharp stripes in the image where the microscope is perfectly focused on that part of the sample.

In operation, the instrument’s computer steps the microscope through a full range of focus positions and captures the band contours on a digital camera, while also noting the precise focal depth at which each part of the striped image is sharpest. Onboard software then uses this data to calculate a 3-D surface map over the entire field of view (i.e., tens of thousands of surface points, or pixels) simultaneously. It is this multiplex aspect that makes the 3-D optical microscope such a versatile tool, enabling complete characterization of the entire lens or pin surface

The 3-D optical microscope does not have step height measurement limitations, so it can measure all types of lenses. And in the latest generation of instruments (see Image 3) which use 64-bit software and multicore processing to provide a faster and more intuitive software workflow, a high rate of data collection is accomplished. A patented dual HBLED illumination source provides increased light throughput, and hence higher speed and superior data in comparison to previous generation systems which used a single lamp source or a single LED. Images 4 and 5 show false color surface data obtained using a ContourGT.

3-D Optical Microscope in Action

The benefits of white light interferometry have been demonstrated in a recent application at a major lens manufacturing facility. The plant produces 100+ master pins per year to produce 48 different designs ranging from +6.0 to +30.0 diopters in 0.5 diopter increments. Manufacturing costs are plant specific depending upon the manufacturing region. However, an average cost per iteration is about $2,500, which includes fully loaded engineering and small batch manufacturing costs. In the past, using a 2-D stylus instrument, master pins typically required four iterations of machining and prototyping, for a total average cost of $10,000. In spite of all these iterations, in the year prior to acquiring a 3-D optical microscope, 67 of the plant’s 100+ pins required some level of rework even after being initially accepted for production. The result: 667 iterations on average were required to produce the 100+ pins per year, for a total cost close to $1.7 million dollars per year.

Using the 3-D optical microscope, this same plant has drastically reduced the number of iterations to a much lower, but proprietary, level. This covers a reduction in both iterations for creating new pins and rework after initial acceptance. Without revealing exact cost savings, plant management indicates that the benefits and savings associated with using the 3-D optical microscope pays for the initial capital outlay for the tool in a matter of months.

In another application, a leading high-volume manufacturer of contact lenses recently adopted the 3-D optical microscope to enable metrology of identification markings on its lenses for consistency of height and shape. The details of this metrology are proprietary, but the broad benefits of its use can be shared to illustrate the advantage this type of metrology can bring to a production process in the ophthalmic industry.

Highly polished metal tools, or mandrels, can be used to create a master mold (or wafer) for contact lenses across a broad range of optical powers for many different consumers. The lenses have identification marks imprinted onto them by the tools, which if not well controlled, can detract from wearer comfort. By monitoring the height of these identification marks using a 3-D microscope, the contact lens manufacturer improved wearability and ultimately increased its share of the contact lens market.

An estimate by Bruker Nano finds that if all runs of contact lens production tools were controlled using this technique, a significant   improvement to contact lens wearability and comfort could be made. This would result in ultimately improved product quality delivered into the market and reduced waste materials for manufacturing.

Today, using the 3-D optical microscope, a major manufacturer is already  successfully controlling the height and protrusion of identifying markings on its products, resulting in consistent wearer satisfaction and ultimately increased market share per publicly available data.  Although a precise increase in market share is hard to estimate, given the high-volume nature of this business, the benefits associated with using the 3-D optical microscope are clear.  The capital equipment investment in metrology can substantially pay for itself in saved material cost and repeat customer business for comfortable contact lens use.

Conclusion

Contact lenses and IOLs are volume-produced medical devices whose successful function critically depends on their surface characteristics. Today’s lenses correct vision problems with increasing levels of finesse, so these surface characteristics must now encompass overall contour, and surface roughness, plus the width, height, and location of nanometer-scale features. As a result, the noncontact, gauge-capable tool, the 3-D optical microscope, is fast gaining acceptance as a superior, high-speed method to measure these characteristics, because it streamlines lens production and significantly reduces manufacturing costs.