History, nomenclature and terminology of contact lenses -ch. Anatomy and physiology of the eye -ch. Optics and contact lenses -ch. Indications & contraindications for contact lens wear -ch. Keratometry -ch. Biomicroscopy -ch.7. Fitting of rigid daily wear contact lenses -ch. Fitting of soft daily wear contact. JCAHPO Joint Commission on Allied Health Personnel in.
A technical overview of the development,manufacturing, and future of contact lenses
Justin Bergin
CE435
Introduction to Polymers
Department of Chemical Engineering
State University of New York at Buffalo
April 6, 2000
Abstract
Contact lenses are a modern technological breakthrough yet they have been in development for over 500 years.While the most important advances have come in the latter half of the 1900’s, this area of science is still undergoing tremendous growth.Chemical engineer’s are critical for the development and manufacturing of the contact lens of the future.This paper will attempt to give a brief overview of contact lenses.It will discuss the history, manufacturing, and science behind modern contact lenses.The paper will conclude by showing some of the tradeoffs of contact use, for no invention is without its drawbacks.
The History of Contact lenses
This first concepts of contact lenses were developed by Leonardo DaVinci in the year 1508.He drew sketches and depicted several forms of possible lenses.In 1632, Rene Descartes suggested the idea of the corneal contact lens, and in 1827 Sir John F. W. Herschel suggested that a glass lens could protect the eye from infection.1 These ideas were taken advantage of in the late 1800’s by Adolph Fick, Eugene Kale, and August Miller.Working independently, these scientists created the first corneoscleral shell contact lenses.1,2,3The lenses consisted of blown glass, and were molded from rabbit and cadaver eyes.These glass lenses were relatively successful at protecting the eye from infected eyelids, and they showed promise at correcting vision as well.Initial lens designs covered the whole eye, including the white portion.While visual correction was attained to a certain degree, they were hard to make and very uncomfortable.Often times it took patients several months to be fitted correctly.The lenses were also very thick and heavy.The thickness gave the eye and unnatural appearance, and the pressure from the weight of the lenses caused discomfort and damage to the eye.1It was reported that in some cases the lenses was so painful, that cocaine anesthetic was used.2
The next major development occurred in the year 1936 when William Feinbloom, an American Optometrist, fabricated the first plastic contact lens.In 1948, Kevin Tuohey then designed the first designed the first corneal contact lens from poly (methyl methacrylate) also known as PMMA.This went on to become what is known as the hard contact lens.Hard Contact lenses became available commercially in 1979.These lens designs worked well and improved models of them are still sold today. 2
During this time Otto Wichterle discovered poly(hydroxyethyl methacrylate), (HEMA), and in 1961 he created the worlds first soft lens. HEMA is a hydrogel that could be made into contacts using a simply and efficient spin cast method.Since different corrections could be made by simple varying the spin rate and mould optic, this became a very efficient method for manufacturing lenses.Bausch & Lomb eventually attained the patients, and the FDA gave approved for HEMA based contact lenses to be used for daily wear in 1971.While HEMA was discovered after PMMA, it was the first contact lens to be approved by the FDA. 1, 2
In the years since polymers introduction, contact lens technology has been increasing at a rapid rate.There have been improvements in manufacturing techniques, as well as an increase in the type of polymer used in the lens.Manufacturing techniques moved from the lenses being hand made and polished, to modern day lathes where the lenses are cut with lasers to computer precision.While all contact lenses consist of amorphous chain linked polymers, the type of monomer used can significantly change the property of the lens.This push in technology was fueled by the desire for extended wear lenses, increased permeability, comfort, and biocompatibility.
How Contact Lenses Work
To understand the technological advances in the contact lens industry it becomes necessary to have an understanding of how contact lenses work, and their desired.A soft contact lens molds to the shape of the eye covering both the cornea and limbus.This is possible because the lens is soft and flexible.The lens floats in the layer of tear that is present on top of the eye.The eyelid and attraction between the polymer and tear film hold the lens in place.4
Hard contact lenses are not flexible, and this means that they must be ground to precisely fit the eye. They only cover the cornea, and not the limbus.If hard contacts are not fit precisely to the eye they will move and not work correctly.They are held in place by precise fitting and their attraction to the tear film. 4
Figure 1:How Contact Lenses Work
Picture courtesy of Scientific American @ http://www.sciam.com/2000/1000issue/1000working.html
Hard and soft contacts work similarly in that as light is refracted as it passes through the polymer.By varying the thickness and the shape of the lens the amount of refraction can be varied to produce different amounts of visual correction.
Why Do People Wear Contact Lenses
Contact lenses can be worn for multiple reasons.The majority of contact lens users wear them for vision to correct myopia, and some research has shown that contacts slow of the progression of myopia.In addition, contact lenses can also be worn to change the color of the eye for aesthetic reasons.The information in table one lists only a few of the conditions that favor contact lens use. 5
Table 1:Typical Reasons For Contact Lens Use5
What Makes a Good Contact Lens, and Biocompatibility
Several things must be considered when designing contact lenses, but perhaps the most important is biocompatibility.While several factors can affect a lenses biocompatibility, perhaps the most important is the wetability of the lens.Contact lenses are constantly in contact with the fluid of the eye. When the contact is placed in the eye, a layer of tear separated it from the eye.This is commonly known as the tear film.The human tear consists not only of water, but also of protein, lipids, sodium, calcium, bicarbonate, and enzymes. 6
If a polymer is hydrophobic it will repel the water that makes up a majority of the tear surface.This disrupts the tear flow, and results in the deposition of an albumin film on the lens.This reduces the effectiveness of the contact, and can cause infection and/or irritation.14This is due to the contact lens hindering the tear film that covers the eye. Therefore, if a contact lens surface is highly hydrophobic it must treated to be made hydrophilic.Doping the polymer or treating the surface of the polymer can do this change in the morphology of the surface. 2
Wetability is not the only reason for deposits on contact lenses.Most contact lenses consist of monomers and cross-link materials that have charges on the monomers.This charge distribution results in the attraction of proteins.This is because proteins also have charge distributions, and they attract one another.Protein or lipid depositions create a biofilm in the lens.This can result in the lens losing its ocular properties, and the turns turning a yellow color. The yellow color is a result of lens spoilage, a result of the diffusion of proteins and lipid into the lens. The push for extended wear lenses is limited by the lenses biocompatibility. 2, 7, 8For extended wear, the contact lens must be highly hydrophilic and must resist the deposition of a biofilm on the lens.
While the surface must resist the formation of a biofilm it must also be semi-permeable.The human eye does not receive adequate blood flow to supply the eye with enough oxygen, or to remove enough carbon dioxide.The eye relies on its exposure to the air for aid.If the contact lens does not provide adequate permeability, the eye suffers serious health effects. 2, 9,10, 11,12, 13This permeability, DK, is typically measured in units of Barrers (10-10 cm3 O2 (STP) cm/cm2s cmHg).24Where D is the diffusion coefficient, and K is the solubility coefficient.14 For a contact lens to be acceptable for extended wear they must have a DK of 100 Barrers. 2, 24The actual amount of oxygen reaching the cornea is called the oxygen transmissibility.This is in terns of DK/L, where L is the thickness of the lens.It can be seem that the amount of oxygen reaching the eye is inversely proportional to the lens thickness, L.14
While the polymer that makes up the lens is important it is also important, for it to be produced in a way so that there is no contamination to the eye.This means not only that that polymer itself is capable of being in contact with the eye, but also that it can be produced in a way so that any residual monomer or solution does not pose a health risk.It is important to test the monomers for biocompatibility, and to assure that the monomers used in the lens are highly pure. Adobe reader 8 mac os x download.
In addition to the biochemistry of the lens, it must also be physically acceptable.A big requirement of the lens is that it must be lightweight.This allows it to be placed on the eye comfortably for extended periods of time without causing ocular strain.This is difficult because the lens must also be strong to avoid tearing and/or scratching, have a reasonably high modulus of elasticity for ease of handling, and yet still soft and flexible enough to feel comfortable on the eye.
The size and specific gravity of the lens also become important factors for eye comfort.If the lens is too thick it will interfere with the eyelid and cause discomfort.If the specific weight of the lens is significantly different from that of tears the lens will have a tendency to move up or down.This makes the lens unstable on the cornea, and makes fitting problematic. 14
Finally a good contact lens must be affordable.It has to be able to be made cheaply and efficiently so that it can be purchased at a reasonable cost.
Manufacturing of Contact Lenses
There are three primary ways in which contact lenses are manufactured.The first method of manufacturing is referred to at lathe cutting.In this method, the monomer is bulk polymerized into rods.The rods are then cut into buttons, which are placed in the lathe.The lathe is then guided by computer to cut the button into a lens.This method has a high yield, and moderate quality. 4
Figure 2: Schematic of Lathes
Picture courtesy of Scientific American @ http://www.sciam.com/2000/1000issue/1000working.html
Figure 3: Modern precision Lathes, and polishing machines for hard contacts
Pictures courtesy of C & H Contact Lens, Inc.@ Http://www.chcontacts.com/smrtmach.htm
The second method of forming contacts is referred to as spin casting.In this method, the liquid monomer is placed in a mold, the mold is then rotated. The monomer is then polymerized inside the rotating mold.This method produces a low yield, but high quality lenses.Spin Casting is good because the lens optics can be varied by simply varying the speed of rotation, and/or the shape of the mold. 14, 2
Figure 4: Spin Cast Mold
The final method for producing contacts is by cast molding.In this method the monomer is placed into a mold, the mold is then cast, and the monomer is polymerized.The lens parameters are varied by changing the shape of the anterior and posterior molds.This method produces high yield, and high quality lenses.It is the most popular form of manufacting.4
Figure 5: Cast Mold
Picture courtesy of Scientific American @ http://www.sciam.com/2000/1000issue/1000working.html
Hard Contact Lenses
Currently fifteen percent of the thirty million contact lens users wear what is known as hard contact lenses. 4 There are several kinds of hard contact lenses, the most historic being impermeable hard contact lens, and now the most common are the rigid gas permeable (RGP) lens, and silicone acrylate based lenses.All of these lenses consist of an amorphous three dimensional polymer matrix (typically a MMA derivative) that is below its glass transition temperature.The lenses are typically very stiff and have a high modulus of elasticity.This gives them a high tear strength, and makes them very easy to handle.4
The impermeable contact lens was the first type of hard contact to be developed.It consists of PMMA only.The MMA monomer is polymerized via ultra-violet or infrared radiation in the presence of cross-linkers and initiators.The lenses were then made by the lath cutting manufacturing process. 2
PMMA is an ideal polymer to be used for hard contact lenses because it is cheap and easy to make.It is moderately hydrophobic, which also contributes to it repelling proteins effectively.It has a typical oxygen permeability of 0.5 DK, which makes it effectively an impermeable membrane to oxygen and carbon dioxide.This impermeability is what restricts PMMA lenses from being used more then about 8 hours at a time.2, 23
This restriction has caused tremendous research in the area in hard contact lens permeability.It is possible to make theoretical calculations with respect to contact lenses by applying a modified version of Henry’s law, and Fick’s Law.The modified version of Henry’s law for polymers below their glass transition temperature can be simplified as followed:
C = KDp + CH(bp/1+ bp)
C = KDp + CHbp(bp << 1)
C = (KD + CHb)p
C = K’Dp