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CONTACT LENSES
By Elif BAŞEL
1. INTRODUCTION
Contact lenses are thin, curved plastic disks designed to cover the cornea, the clear front covering of the eye. Contact cling to the film of tears over the cornea because of the surface tension, the same force that causes a drop of water to cling to the side of a glass. Contact lenses provide a safe and effective way to correct vision when used with care and proper supervision. They can offer a good alternative to eyeglasses, depending on your eyes and your lifestyle. Contact lenses are used to correct the same conditions that eyeglasses correct. (myopia, hyperopia, astigmatism, presbyopia).Special tinted contacts can be used to change the color of the eyes to various degrees. Contact lenses are sometimes used therapeutically in eye diseases where an uneven cornea blurs vision, such as keratoconus or scarring
PMMA lenses are more likely to scratch the cornea if the lens does not fit properly or if the lens is worn while sleeping. They are also more likely to slide off the cornea and become hidden under the lid. Rigid lenses traditionally had a reputation for "popping out" of the eye. New lens designs have minimized the chance of loosing a contact even during vigorous exercise. Rigid gas-permeable lenses may allow more protein build-up than rigid non-gas-permeable lenses. Protein build-up results in discomfort, blurring and intolerance to the lenses. You will need special cleaning solutions to dissolve the protein. Daily-wear lenses should never be worn as extended-wear lenses. Misuse can lead to temporary and even permanent damage to the cornea. People who wear any type of lens overnight have a greater chance of developing infections of the cornea. These infections are often due to poor cleaning and lens care. Improper overwearing of contact lenses can result in intolerance, leading to the inability to wear contact lenses. Most people who need vision correction can wear contact lenses, but there are some exceptions. Some of the conditions that might keep you from wearing contact lenses are: frequent eye infections, severe allergies, dry eye (improper tear film), a work environment that is very dusty or dirty and inability to handle and care for the lenses properly.
Contact lenses must be properly cleaned and disinfected when you remove them to kill germs and prevent infections. At the time you insert your contact lenses, you should thoroughly rinse the case with warm water and allow it to dry. All contact lens cases need frequent cleaning, including disposable lens cases. Soft extended-wear contacts are the most likely to have protein build-up and cause lens-related allergies. Soft daily-wear lenses are less likely to create problems. Rigid gas-permeable or disposable lenses may be good choices for someone with allergies. Homemade saline solutions have been linked to serious eye infections and should never be used. Any eye drops, even nonprescription ones, can interact with all types of contact lenses.
2. HISTORY OF CONTACT LENSES
In about 1508, Leonardo da Vinci sketched the forms of a new refracted surface on the cornea in his Codex on the Eye. He used the example of a very large glass bowl filled with water; immersion of the eye in the water theoretically corrected impaired vision. Descartes, in a treatise entitled Ways of Perfecting Vision, published in 1636, suggested applying a tube full of water directly to the eye to correct a refractive error. Even with these early suggestions, the real history of contact lenses did not begin until the 19th century. In 1801, Thomas Young described a neutralizing surface for the cornea that was a forerunner of the contact lens. Young took the biconvex lens from a small microscope and secured it at the end of a tube full of water. He placed the other end of the tube on his eye with the water (rather as Descartes had suggested). Sir John F. W. Herschel, the English astronomer and physicist, wrote and circulated widely the opinion that corneal contact lenses were optically feasible. In 1823, he proposed a lens with an anterior surface of the same refractive power as the eye and a posterior surface molded to fit exactly an irregular corneal surface. Herschel was also the first to suggest that an actual mold of cornea might be taken. His ideas lay dormant for 60 years. His suggestions were theoretically sound, but the problems of practical application were too difficult to overcome. With the introduction of anesthesia in 1884, contact lens technology advanced. Toward the end of 1880s, Fick and Kalt described contact lenses designed to correct refractive errors in eyes with normal contours. It was made in 1887 by F.A. Muller, a manufacturer of artificial eyes.
Glass scleral contact lenses were made from 1888 to 1938 and were fitted by a tedious method of trial and error. Glass lenses were heavy and were often attacked by the larimal fluid so that in about 6 months they become too rough to wear or see through. For these reasons, scleral lenses did not meet the needs of most people. In the 1930s, the availability of plastics that were lightweight, transparent, chemically stable, unbreakable, scratch-resistant, and easy to work with changed the course of contact lens technology. In 1938, Theo Obrig developed techniques for making plastic scleral lenses and suggested the use of fluorescein dyes to study fit. With the introduction of polymethylmethacrylate (PMMA), a flush-fitting scleral lens was made possible. Its therapeutic uses were developed by Ridley, in England, in 1954.
In 1937, Feinbloom fitted the first contact lenses that were a combination of soft plastic and hard glass. They were worn for 1 to 2 hours. A major development was the introduction in 1947 of hard plastic corneal contact lenses by Kevin Touhy in England. In 1950, George Butterfield introduced the concept of fitting the peripheral cornea of contouring the peripheral lens to be parallel to the cornea. The first corneal lenses to have any measure of widespread success were designed in the 1950s by Frank Dickinson, Wilhelm Sohnges, and John Neill. These lenses had a thickness of about 0.2 mm. Thinner lenses, of about 0.1 mm were introduced in the early 1960s. They had rounded edges and a smaller size that permitted greater tear circulation.
In 1964, Otto Wichterle introduced a soft hydrophilic plastic and with it made the first soft contact lenses. The soft contact lenses initially developed by Wichterle in Europe were introduced on a trial basis in the United States in 1965 by the National Patent Co. In 1966, Bausch and Lomb introduced the spin-cast soft lenses on an experimental basis. In 1967, the lathe-cut soft Griffith Lens was first seen in the United States; this later became the American Optical soft contact lens (Hales, 1982).
3. LENS TYPES AND MATERIALS
3.1 Hard Lenses
Hard lens materials are now given the suffix ‘-focon’ and are classified according to the Chemical Groups. Hard lenses are available in a wide range of materials and Dk values. Oxygen considerations, however, must take into account the barrier effect which reduces the Dk on the eye to approximately 55% of that measured in air together with centre and average lens thickness.
For physiological reasons lenses should be as thin as possible, but in practical terms making lenses too thin is counter-productive since they are very likely to distort throughout the power range and also become too brittle. In most cases, a realistic minimum centre thickness is 0.14mm even for high minus powers.
Although Dk is important, there are several other considerations which affect comfort, vision and life span. These include:
· Lens design.
· Fitting method.
· Manufacturing technique.
· Optical quality.
· Surface wetting properties.
3.1.1 Cellulose acetate butyrate
Cellulose acetate butyrate (CAB) was one of the first modern hard lens materials, introduced in 1977. By modern standards its Dk (between 4 and 8 x 10-11) is low and it is now less often fitted as the lens of first choice. Its main difficulty when manufactured by traditional lathing methods is dimensional instability. However, when manufactured by moulding, this problem has been largely overcome and lenses such as Conflex and Persecon E give very good clinical results.
Advantages
· Good wettability.
· Relatively inert.
· Does not attract protein.
· Low breakage rate.
· Very low incidence of PC.
· Relatively good for 3 and 9 o’clock staining.
Disadvantages
· Moulding necessary for dimensional stability.
· Limited range of lens design.
· Scratches easily.
· Attracts lipids from the tears.
· Corneal adhesion in some cases.
· Lens flexure and distortion on toric corneas with tight lids.
3.1.2 Silicone acrylates (siloxanes)
Silicone acrylates are copolymers in varying proportions of acrylate (PMMA) which provides lens rigidity and silicone which controls the degree of oxygen permeability. An excellent range of materials are now available with widely different properties and Dk values. They give superior oxygen and physiological performances compared with CAB, and most have stood the test of time in terms of dimensional stability and optical and mechanical results. They are routinely fitted for daily use and a limited degree for extended wear.
Advantages
· Wide range of materials available.
· Wide range of designs with practitioner control.
· Medium to high Dks available.
· Good dimensional stability.
· Good vision with limited lens flexure.
· Less easily starched.
Disadvantages
· Attract protein from the tears.
· Some materials are brittle with a breakage problem.
· High incidence of 3 and 9 o’clock staining.
· Some incidence of PC.
3.1.3 Fluorosilicone acrylates and fluoropolymers
This newer generation of materials is based mainly on fluorine.
Fluoropolymers incorporate fluorine into the polymer chain to improve oxygen permeability, wettability and deposit resistance. The 3M Fluorofocon A, for example, also includes N-vinylpyrrolidone (NVP) for its surface-wetting properties and methylmethacrylate for additional strength and stiffness.
Fluorosilicone acrylates are copolymers of silicone, fluorocarbon and methyl methacrylate.
Advantages
· Very high Dks possible.
· Suitable for flexible extended wear.
· Good wettability.
· Fewer deposit problems.
· Low incidence of PC.
· Easy to modify.
Disadvantages
· Brittle if too thin.
· Require careful manufacture.
· Dimensional stability depends on material and manufacture.
· Corneal adhesion in some cases.
3.2 Polymethyl methacrylate (PMMA)
Polymethyl methacrylate (PMMA) has been in use since the 1940s, first as a replacement for the earlier glass scleral lenses, and subsequently the material of choice with the development of corneal lenses. There are now large numbers of patients
Who have successfully worn PMMA for 25 years and longer. It is now failing into disuse with the advent of modern hard lenses, by comparison with which its permeability is negligible. However, its original merits of inertness and stability mean that it retains a place for a tiny minority of new patients who exhibit neither signs nor symptoms and are best left without refitting, although their corneas should be carefully monitored.
3.2.1 Modified PMMA
Although PMMA has relatively good surface-wetting properties by comparison with many hard lens materials, various versions with modified surface properties (e.g. BP-flex) have been introduced. The improved wettability can give moderately better comfort and the increased tear flow beneath the lens has been beneficial in reducing some of the edema problems inevitable with an almost impermeable material.
3.3 Soft lenses
Soft lenses are generally discussed according to the interrelated properties of water content, Dk and material type. Materials are now given the suffix ‘-filcon’ and are classified according to the Chemical Groups.
Water content and water uptake
Care must be exercised when interpreting brand names which include a numerical suffix, because these do not always accurately reflect the true water content.
Ionic and non-ionic polymers
Polymers can also be categorized into four groups by linking water content to ionic properties. It is a materials rather than a classical definition, but generally ionic polymers contain methacrylic acid and attract greater levels of deposit from the tears.
(1) Low water content, non-ionic polymers, e.g. Crofilcon (CSI), 38.5%.
(2) High water content, non-ionic polymers, e.g. Lidofilcon-A (B & L, 70%).
(3) Low water content, ionic polymers, e.g. Bufilcon-A (Hydrocurve II, 45%).
(4) High water content, ionic polymers, e.g. Etafilcon-A (Acuvue, 58%).
3.3.1 Clinical implifications of soft lens water content
Lenses have been produced with water contents from 3% to 85%. However, a great many of the lenses currently being used are still hydroxymethyl methacrylate (HEMA)-based, in the region of 38-45%.
Advantages of low water content lenses
· Greater tensile strength.
· Less breakage.
· Longer life span.
· Smaller swell factor.
· Better reproducibility.
· Easier to manufacture.
· Can be made thinner.
· Less dehydration on the eye.
· Less discoloration with age.
Disadvantages of low water content lenses
The disadvantages of low water content lenses relate mainly o the relatively low Dk values:
· A greater tendency to cause edema.
· A long-term tendency with thicker lenses to cause vascularization.
Advantages of high water content lenses
Most high water content materials have Dks between three and five times that of HEMA. Apart from their obvious application in edema cases, they have several other advantages:
· Better comfort because of material softness.
· Faster adaptation.
· Longer wearing time.
· Extended wear.
· Easier to handle because of greater thickness.
· Better vision because of greater thickness.
· Better for intermittent wear.
Disadvantages of high water content lenses
Despite these good features, there are nevertheless disadvantages with high water content lenses which preclude their use in some cases:
· Shorter life span.
· Greater fragility.
· More deposits, especially white spots.
· More discoloration.
· Reproducibility less reliable.
· More difficult to manufacture.
· Greater variation with environment.
· Fitting requires longer settling time.
· Greater variability with vision.
· More solutions problems.
· Corneal desiccation.
· Lens dehydration.
3.3.2 Clinical implications of soft lens thickness
The typical centre thickness for a ‘standard’ corneal diameter HEMA lens of power -3.00 D is in the region of 0.10-0.14 mm. Lenses below 0.10 mm may be regarded as ‘thin’; those below 0.07 mm as ‘ultra-thin’ and thinner than 0.05 mm as ‘hyper-thin’. They represent a very satisfactory way of increasing transmissibility (Dk/t) and improving physiological performance, as well as giving an inherent safety factor for patients who accidentally fall asleep. Low plus and aphakic lenses cannot be truly considered as ultra-thin because of their necessarily greater centre thickness. Nevertheless, ‘thin’ plus lenses give a more satisfactory result.
Oxygen performance for a lens cannot be judged solely in relation to its specified centre thickness, but must be considered for the entire lens. If an ‘average’ or ‘mean’ thickness is used, this itself requires definition to avoid error and give valid comparison.
Advantages of thin lenses
· Lower incidence of edema.
· Reduced lid sensation because of thinner edges.
· Reduced limbal irritation because of thinner edges and larger total diameter.
· Different fitting characteristic may provide better centration than standard lenses.
· Easier to fit because fewer fitting steps are necessary.
· Safer if patients accidentally fall asleep.
Disadvantages of thin lenses
· Handling is more difficult, especially in low minus powers below about -2.00 D.
· Higher breakage rate than standard thickness lenses.
· Life span is shorter, especially with heat disinfection.
· Visual acuity may be less good with toric corneas.
· Greater tendency to dehydrate on the eye and disturb precorneal tear film.
3.3.3 Dehydration of soft lenses
One of the major reasons for the clinical success or failure of a particular les on the eye relates to its dehydration characteristics.
Effects of lens dehydration
· Change in parameters and fitting.
· Reduced comfort.
· Reduced Dk.
· Disruption of tear film.
· Corneal desiccation and staining.
· Increased deposits.
· Reduced vision.
Factors influencing lens dehydration
Ocular factors
· Volume of tears.
· Quality and stability of tear film.
· Osmolarity of tears.
· Blinking habits.
· Size of palpebral aperture.
Other factors
· Lens material.
· Lens thickness.
· Temperature.
· Relative humidity.
· Draughts and wind.
· Systematic drugs.
· Alcohol.
The water content contained within the polymer matrix consists of ‘bound’ water directly attached to hydrophilic sites by ‘van der Waals’ forces and ‘free’ water which is readily lost by evaporation. The higher the bound water, the less any particular material will dehydrate on the eye.
Generally, most water loss occurs within the first few minutes and high water content materials give greater dehydration. Tear film stability is better with thicker lenses and low water contents: it is worse with ultra-thin and high water content lenses.
3.4 Silicone lenses
3.4.1 Silicone rubber lenses
Silicone lenses differ from hard lenses in several ways. They can be flexed, stretched and turned inside out. They have excellent elastic properties, partly conform to the shape of the cornea in wear, and have excellently high Dks, in the region of 200 x 10-11. They are also unlike hydrophilic lenses because their natural state is dry and they are extremely tough. Since they do not absorb water to any significant extent, fluorescein can be used in their fitting and they do not need disinfecting in the same way as soft lenses.
Because of the amorphous nature of the silicone rubber raw materials, lenses are produced by a moulding and vulcanization technique, which also assists in maintaining good reproducibility. The main difficulty with silicone is that its natural surface is extremely hydrophobic, and it has been necessary to devise methods of rendering the surface permanently hydrophilic without interfering with any of its optical or physical properties. The final stage of manufacture is therefore surface treatment by ion bombardment.
Because of the following advantages and disadvantages, silicone has remained very much a minority lens with limited therapeutic applications.
Advantages of silicone lenses
· Very high Dk.
· Better and more stable vision than many soft lenses.
· Little variation in comfort or fitting with environmental factors.
· Low risk of loss or damage.
Disadvantages of silicone lenses
· Difficult to fit, requiring as much precision as hard lenses.
· A negative pressure effect, giving sticking, particularly if not correctly fitted.
· Breakdown in surface coating and difficulties with wetting.
· Build-up of deposits.
· Foreign bodies, especially with loose fittings.
3.4.2 Silicone resin lenses
Resin lenses differ from silicone elastomers because they are not flexible, having many of the physical properties of hard lenses. Dk values, however, are significantly lower than elastomers and they have so far found only limited application.
3.5 Other lenses
Soft-coated hard lenses
The Novalens (Ocutec) is a hard lens with 0.005 μm ‘soft’ coating of OH groups which gives the characteristics of a hydrophilic lens. The lens does not absorb water, but has a good surface wettability and improved comfort. The Dk is 55 x 10-11 and lenses are fitted according to hard lens criteria.
Styrene lenses
The Wesley-Jessen Airlens is produced from t-butyl styrene which contains no silicone. The material has a Dk of 24 at 35°C and a high refractive index of n = 1.525. Solutions containing chlorhexidine should be avoided, since they may interfere with the wetting angle of 54°C.
Collagen lenses
Collagen lenses are produced from biological polymers and one of the reasons for their development is biocompatibility. Their main applications have been therapeutic (Gasson and Morris, 1992).
4. PROPERTIES-RELATED REQUIREMENTS OF CANDIDATE POLYMERS FOR CONTACT LENSES
The necessity for fundamental design considerations of the macromolecular structure of polymers is becoming more prominent in view of the in ability of existing polymers to fulfill all the requirements of contact lens materials
Based on a careful analysis of desirable properties, polymer scientists are able to provide a list of requirements or criteria for contact lens development. This list, which is summarized in Table 4.1, may serve as a set of requirements to be achieved when a new polymer is designed or as criteria to be fulfilled when evaluating a candidate polymer.
Table 4.1 Structure-based requirements of polymers for contact lenses |
Requirements |
Phenomena and parameters to be investigated |
Chemical |
Chemical stability
Hydrolytic degradation
Enzymatic degradation
Thermal stability
Migration (release) of impurities |
Optical |
Refractive index
Discoloration at storage |
Surface |
Surface energy and contact angles
Chemical structure
Protein adsorption |
Mechanical |
Modulus of elasticity
Ultimate tensile strength
Strain at break
Tear propagation
Response to shear stress
Response to dynamic loading
Environmental aging |
Biologic |
Toxicity
Carcinogenicity
Sterility |
The properties presented here are some of the most important factors in the success of a new polymer for contact lenses. At the same time, the decision of whether or not to investigate all these properties of a new polymer depends on the sophistication of the laboratories available, the level of collaboration with polymer and physical chemists, and the level of effort. However, optimization of properties calls for investigation and achievement of at least the following requirements:
- Chemical inertness
- Optical transparency
- Wettability
- Dimensional and mechanical stability
- Adequate oxygen permeation
- Biological inertness
It is evident that design and evaluation of new or modified polymers for contact lenses is neither empirical nor scientifically easy.
4.1 Chemical Structure and Stability of Polymers
4.1.1 Chemical stability: backbone chains and functional groups
The term “chemical stability” encompasses characteristics of the backbone macromolecular chains as well as the type of the functional groups of the chain ends and substituents, which are related to the chemical inertness of a polymer under the physiologic conditions of the eye.
The –C-C- and –Si-O- bonds of most presently available contact lenses are relatively stable, and they do not degrade under the mild chemical conditions of the tear fluid. Backbone chains with double bonds (for example –C=C-) should be avoided, since they tend to react relatively easily in the presence of oxygen and ultraviolet light to form unstable peroxidic bonds. These in turn can decompose to form carbonyl-containing chains of smaller molecular weight, leading to polymers of inferior mechanical strength and stability. Etheric (-CO-) and esteric (-COO-) bonds in the main macromolecular chain are relatively stable, although these structures are known to promote crystallinity and therefore considerable change of refractive index of many polymers. Some types of backbone structures are known to break down under hydrolytic or enzymatic conditions. For example polyamino acids, polylactic acids, and related structures are known to biodegrade under various physiologic conditions, and they should be avoided in contact lens materials.
4.1.2 Aging and environmental effects
Regardless of how stable the chemical structure of a polymer for contact lenses is, aging and environmental effects are to be expected for long-term applications. Some of these effects may be observed as increase in turbidity (for soft contact lenses), warping, and discoloration.
Environmental aging may be result of slow oxidation or hydrolysis, influence of ultraviolet light, temperature changes, and chemical reactions. It frequently leads to polymer degradation and considerable change of the mechanical strength of polymers.
Aging of hydrogels (soft contact lenses) is more complicated as a result of the formation of microgels, heterogeneous structures, microcrystallinity, paracrystalline phases, and so on. As a result of these chemical reactions and physical rearrangements, Hydrogels become considerably ore turbid, sometimes loosing their dimensional stability because of syneresis of the structure. These phenomena have been analyzed for PHEMA, PVA and polyacrylamide gels.
4.2 Physical Properties of Contact Lenses
4.2.1 Oxygen permeability, oxygen transmissibility and equivalent oxygen percentage
According to clinical investigations, the cornea metabolism requires a constant delivery of oxygen. The cornea uses oxygen to maintain its clarity, structure, function and obtains its oxygen from air as illustrated in Fig. 4.1. When the eye is open, the cornea receives oxygen from atmospheric air (21% oxygen) that at sea level corresponds to a partial pressure of oxygen of about 159 mmHg. On the other hand, the cornea in the closed eye receives oxygen mainly from the vasculature of the palpebral conjuctiva at a partial pressure of about 55 mmHg. Corneal swelling up to 2-4%, accompanied by endothelial changes, has been found after overnight sleep. In this case, the swelling is mainly attributed to the decrease in the Osmolarity of the tear film due to tear evaporation decrease. It is assumed that the closed-eye condition (55 mmHg) provides sufficient oxygen to the corneal surface (Compañ et al., 2003). It is found that hydrogel contact lens induced, closed eye corneal swelling occurs chiefly in the posterior direction (Erickson et al., 1999).
Figure 4.1 Diagram of the eye and the sources of oxygen for the cornea (Nicolson and Vogt, 2001)
Oxygen permeability
The oxygen permeability of a material is generally referred to as Dk. The units of 10-11 cm2/s ml O2/ml x mmHg (sometimes referred to as Fatt units) are often omitted for convenience. In this nomenclature, D is the diffusion coefficient – a measure of how fast dissolved molecules of oxygen move within the material – and k is a constant representing the solubility coefficient or the number of oxygen molecules dissolved in the material.
The Dk value is a physical property of a contact lens material and describes its intrinsic ability to transport oxygen. It is defined as ‘the rate of oxygen flow under specified conditions through unit area of contact lens material of unit thickness when subjected to unit pressure differences’. It is not a function of the shape or thickness of the material sample, but varies with the temperature. The higher the temperature the greater the Dk.
Oxygen transmissibility
Oxygen transmissibility is referred as Dk/t, with units of 10-9 cm/s ml O2/ml x mmHg. Here, t is the thickness of the lens or sample of material, and D and k are as defined above.
The Dk/t for a particular lens under specified conditions defines the ability of the lens to allow oxygen to move from anterior to posterior surface. The value of t is generally an average lens thickness for powers between ±3.00 dioptres (D). Outside of this range it is necessary to apply a nomogram. Oxygen transmissibility is not a physical property of a contact lens material, but it is a special characteristic related to the sample thickness.
Equivalent oxygen percentage (EOP)
The equivalent oxygen percentage (EOP) refers to the level of oxygen at the surface of the cornea under a contact lens. For the uncovered cornea exposed to the atmosphere, the amount of oxygen available is 20.9%. With the eye closed the cornea receives 8%, whereas to avoid edema the EOP should be over 10% (Dk/t = 24.1), and for no overnight swelling it needs to be as high as 18% (Dk/t = 87). An EOP profile (Figure 4.2) for a lens of known material and thickness shows whether it can provide enough oxygen to avoid corneal edema.
Figure 4.2 Equivalent oxygen percentage profile
4.2.2 Water content and water uptake
The water content is the amount of fluid taken up by a lens material as a percentage of the whole under specified conditions:
|
|
Wt of fully hydrated lens – Wt of fully dehydrated lens
Wt of fully hydrated lens
|
|
Water content (%) = x 100
|
|
Wt of fully hydrated lens – Wt of fully dehydrated lens
Wt of fully dehydrated lens
|
|
Water uptake (%) = x 100
Water is lost by evaporation when a hydrogel lens is worn on the eye. This is in part caused by a rise in temperature and is accompanied by a tightening of the fit.
4.2.3 Wettability
Wettability is the ability of a drop of liquid to adhere to a solid surface. The lower the cohesive forces within a liquid, the greater the attraction between the liquid and the surface. Thus, superior wettability enhances the spread of liquid over a surface.
Contact angle is a measure of the hydrophilicity of a surface. The contact angle may be measured in a variety of ways:
· Sessile drop method: measures the tangent to a drop of liquid placed on a sample surface (Figure 4.3).
Figure 4.3 Sessile drop methods (A, advancing angle; B, receding angle)
· Captive bubble method: measures the tangent to an air bubble formed on the surface of an immersed sample.
· Wilhelmy balance method. A sample is immersed or withdrawn vertically from a liquid.
· Direct meniscus method.
Figure 4.4 Captile bubble method
Both the advancing and receding angles are measured. These are formed when liquid s added or removed from the controlled liquid drop used for measurement (Figure 4.2). The higher the contact angle, the more wettable the surface. Typical values are given in Table 4.2, which demonstrates the great inconsistency between different methods. Comparisons can therefore only be made when the same method has been employed (Hartstein, 1982).
Table 4.2 Wetting angles
In a study it is shown that the wettability of a silicone can be improved if a polar plastic mold derived from a polar polymer is used to create the anterior surface of the lens (Lai and Friends, 1997).
4.2.4 Protein adsorption
Although hydrophilic surfaces are desirable for contact lenses since that promote water wettability, hydrophilicity promotes another phenomenon, namely the tendency of the surface to adsorb proteins and to induce deposition of proteinaceous layers on contact lenses. It is known that the protein adsorbing on the surface of polymers in contact with artificial tear solutions can drastically change the surface wettability of the surface. By working with the copolymers of PHEMA and PMMA and by controlling the molar fraction of PHEMA (that is, the amount of hydroxyl groups), they found that the minimum level of adsorption of total protein, albumin and lysozyme was in the range of 30% to 45% PHEMA in the copolymer (Harstein, 1982). Soltys-Robitaille et al. 2001, investigated the adsorption of lysozyme and human serum albumin (HSA) onto hydrogel contact lenses as a function of lens surface charge by using two techniques, the colorimetric BCA (bicinchoninic acid) assay and MALDI-ToE (matrix assisted laser desorption ionization mass spectrometry). Lysozyme, a positively charged protein at physiological pH, was only detected on the anionic surface charged contact lenses. Neither the cationic nor the non-ionic lenses deposited lysozyme, possibly due to charge repulsion. As expected, the neutral polymacon and the cationic HEMA lenses did not deposit lysozyme. In the case of HSA, this negatively charged protein deposited onto the surface of the cationic HEMA lenses. This study demonstrated that the impact surface charge had on in-vitro and subsequently in-vivo deposition levels, the knowledge of which could be helpful in the development of new material and surface chemistries.
4.3 Optical Properties
In general numerous polymers, both diluent free and swollen, have a refractive index close to that of the cornea, that is around n = 1.37 at 34°C. Commercial polymers with a refractive index in the range of 1.32 to 1.45 can be used as contact lenses, since they transmit most of the light in the range of the visible spectrum, that is, 4000 to 7500 Å. Polymers with a desirable refractive index include various polyvinyl ethers, a range of polyacrylates and polymethacrylates, polyvinylacetate, and most Hydrogels.
Structural parameters that unfavorably affect the refractive index of polymers are the degree of cross-linking and the degree of crystallinity. Increase the values of both parameters leads to relatively turbid or translucent materials. Mechanical or environmental degradation of carelessly prepared polymers with chemically unstable bonds is the cause of contact lens discoloration. Plasticizers and additives must be avoided for the same reasons.
The refractive index of contact lenses can be determined rather easily with simple or sophisticated refractometers at constant temperature (Harstein, 1982).
4.4 Mechanical Properties
4.4.1 Influence of structure
The physical properties of polymers at 34°C (eye temperature) is the determining factor in the classification of contact lenses as “soft” or “hard”. From a mechanical point of view, rubbery, swollen or unswollen, semicrystalline films of low degree of crystallinity are suitable for soft contact lenses. Glassy, purely amorphous or semicrystalline polymers and rubbery, semicrystalline polymers with a high degree of crystallinity are suitable for hard lenses.
Important mechanical properties of polymers (Figure 4.4) for contact lenses are the elastic modulus, defined as the ratio of stress (load per area) over strain (dimensional change per initial length) and usually expressed as modulus at 0% strain (initial modulus) as shown in Fig.4.4 ; the ultimate tensile strength, defined as the stress at the breakpoint; the strain at break, defined as the maximum attainable strain before the material breaks; and the tear propagation energy, defined as the energy required to tear a film with a determined cut.
Figure 4.4 Stress-strain behavior of typical polymeric material under stress relaxation Eo, Slope of stress-strain curve at є = 0 (initial modulus); єb, strain at break; τb,ultimate tensile strength, τy, yield stress.
Increased degrees of crystallinity (Fig.4.4) lead to mechanical strength as observed by increased values of the modulus, ultimate tensile strength, and strain at break. Extensive cross-linking leads to increased modulus and tensile strength but decreased strain at break. Therefore mechanically weak, soft contact lenses may be reinforced by cross-linking and crystallization processes.
Transition from the glassy to the rubbery state (for example, by adsorption and swelling agent) leads to two or three orders magnitude of lower modulus.
In general, polymers with room temperature elastic moduli in the range of 109 to 1010 Pa are candidates for hard contact lenses. Room temperature elastic moduli in the range of 106 to 107 Pa are characteristic of materials suitable for contact lenses. Polymers with moduli lower than 5 x 105 Pa are mechanically weak and they tear on application in the eye (Harstein, 1982).
In a study, changes in the surface viscoelastic and adhesive properties of bulk hydrated and dehydrated pHEMA based contact lenses were monitored as a function of humidity by atomic force microscopy (AFM). Stiffness, elastic modulus, viscous deformation, and adhesion properties were extracted from AFM force versus distance interaction curves and indicate that surfaces of bulk-dehydrated lenses are dry at humidities up to 85% ad that there is very little net diffusion of water into the bulk. For the bulk-hydrated lens, the surfaces are rigid and dry under ambient humidity, and soften dramatically at ~60% relative humidity indicating an increased presence of water, which plasticizes the surface layer. This method can be used to compare surface water content of various classes of hydrogel and to test the effectiveness of various surface treatments hydrogel material in enhancing the water content of the surface region (Opdahl et al., 2003).
4.4.2 Evaluation of mechanical properties
Mechanical properties can be determined by simple or sophisticated techniques under static or dynamic loading. Static loading measures basic properties of contact lenses such as moduli. Dynamic loading determinations may be necessary to simulate the blinking action of the eyelid.
An important molecular parameter of polymers, the relaxation time, somewhat complicates the determination of their mechanical behavior. The relaxation time is the time necessary for the relaxation process of long macromolecular chains to occur. This parameter is necessary because macromolecular chains have a thermodynamic tendency to rearrange and slowly return to their equilibrium state on application of external load. To some extent warping and other undesirable materials problems observed when using certain gas-permeable lenses are related to this relaxation process.
Mechanical tests can be carried out by stress relaxation, creep (constant stress), or constant rate of extension experiments. The temperature must be controlled at 34°C during experimentation.
The importance of dynamic loading and repetitive loading experiments is that they simulate the eyelid motion over the contact lens.
Incorporation of a different monomer in the structure of a desirable polymer imparts improved mechanical and physical properties. This can be done either by mechanical mixing (polymer blends) or by chemical reaction (copolymers) (Harstein, 1982). The combination of two hydrophilic monomers, N,N-dimethylacrylamide (DMA) and N-vinyl pyrrolidone (NVP), in formulations containing polysiloxanes produced silicone hydrogels with properties which can be manipulated by varying the ratios of these two monomers. It was shown that high levels of NVP can be used to maintain excellent wettability when used with DMA while maintaining the levels of water content, oxygen permeability, and low modulus (Lai and Valint, 1996).
4.5 Biological Properties
Since lenses come in contact with the eye, they should be considered as biomaterials, which must fulfill most of the general biological requirements of all other biomedical polymers. Specific toxicity, carcinogenicity, and sterility tests for the evaluation of biomaterials in general have been recommended. The structure plays a very important role in toxicity and carcinogenicity of biomaterials in general. It is believed that toxic reactions with the tissue are the result of functional groups of the polymer of residual impurities or products of partial degradation, which can be leached to the surrounding fluid. Some of the most common tests of toxicity of contact lenses are cornea irritation and agar overlay tests on the polymer and its water-soluble extracts. Sterilization procedures depend on the surface properties and degree of swelling of the contact lens. Most of the heat treatment techniques are t be avoided because of the potential change of the mechanical properties and the considerable extent of the thermal degradation of most biomaterials at temperatures above 100°C. Of the chemical sterilization techniques, very few can work with soft contact lens materials because of the potential permanent adsorption of sterilizing agents on the hydrogel surface. A preferred sterilization technique is irradiation at low-dose levels, although it is well understood that this method may cause some change in the degree of cross-linking of the polymer (Harstein, 1982).
5. CARE SYSTEMS FOR SOFT LENSES
A wide variety of products are presently available for contact lens care. One must keep in mind the efficiency of the product as to both cleaning the lens and its ability to disinfect. Thus there must be some guidelines for a systematic method of selecting an appropriate effective hydrogel care system for a given person or type of material. Available care products fall into four categories (Stein and Slatt, 1984).
1. Cleaning agents.
2. Disinfecting solutions-chemical and thermal.
3. Rinsing solutions.
4. Lubricating-rewetting-cleaning eye drops.
Cleaning agents
Soft lenses worn over a period of time become gradually coated and cloudy. This cloudiness not only interferes with the clarity of the vision but act as an irritant, making the patient uncomfortable. The coating may also cause adverse tissue responses. The cloudiness is usually related to surface debris consisting of protein, inorganic films, mucin, lipids, and minerals.
6. PROBLEMS WITH CONTACT LENSES AND SOLUTIONS
Sometimes adverse response to contact lens wear occur and reports have noted an increased incidence of contact lens-related ulcers, mostly due to Pseudomonas aeruginosa. For daily wear, regular cleansing and disinfection procedures must be used to minimize the risk of biofilm growth. However, organisms able to survive in ophthalmic solutions may adhere to lens surfaces and thus transported to the eye. Wear, handling, cleansing of contact lenses change the physico-chemical properties of the contact lens surfaces, like hydrophobicity, electrostatic charge and surface roughness and therefore affect the adhesion of bacteria. It is shown that the physico-chemical properties of rigid gas permeable lens surfaces changed slightly during the first 10-50 days of wear, but end-stage lenses all had increased surface roughness, concurrent with increased bacterial adhesion (Bruinsma et al., 2003).Extended wear contact lenses also pose a risk for infectious keratitis, contact lens associated red eye (CLARE) and corneal ulceration due to bacterial adhesion to the lens followed by adhesion to and invasion into the cornea. Corneal infections occur in one out of 2800 users of daily wear contact lenses, but the incidence of corneal infection increases to one in 500 users of extended wear contact lenses. It has been estimated that the risk of bacterial infection increases with each additional night wear of extended wear contact lenses. Bacterial keratitis due to Pseudomonas aeruginosa (an opportunistic gram negative pathogen) is a potentially serious complication of extended-wear contact lens use. Rediska et al., 2002 describes the use of exogenous soluble pooled human polyclonal antibodies to reduce P. aeruginosa adhesion to contact lens surfaces and corneal endothelial cells in vitro.
Many contact lens wearers are also allergy sufferers who may experience allergic ocular symptoms such as ocular itching, tearing, redness, as well as seasonal, perennial or contact lens papillary conjunctivitis (CPLC). It was shown that the use of 1-day disposable lenses (Etafilcon A) was an effective strategy for managing allergy-suffering contact lens wearers (Hayes et al., 2003).
It is well known that patients presenting a tear film deficiency are poor candidates for fitting contact lenses. Rigid gas permeable lenses are considered to be the option of choice for a poor tear film. However, sometimes RGP lenses are not the best solution for optical reasons or for the unwillingness of the patient to change wearing habits or to bear the mild discomfort that occurs during the fist few days of the wearing schedule. In these cases, hydrogel contact lenses may be the only choice. In a study the relationship between the central thickness of hydrogel contact lenses and the overall comfort of patients with tear deficiency were investigated. The results indicated that thicker lenses were preferred in terms of comfort, dryness sensation, photophobia and handling whereas thinner lenses were considered to give a better quality of vision. As a whole, thicker lenses were tolerated by the patient and central thickness was found to be of significant when choosing a contact lens, especially when the tear film is deficient (Gaskets et al., 2002).
7. CONCLUSION
Contact lens materials are a small microcosm in the world of biomaterials. They offer an attractive, effective option for non-invasive sight correction. Although the modern fitter and wearer have many choices of types of contact lenses, the ideal lens has not been devised. The ideal lens would have the following characteristics: It would fully correct the refractive error with good optics. It would not produce physiological or pathological changes in the eye. In other words, there would be no edema, no staining, no vascularization, and no corneal warping. It would be gas-permeable to permit free transmission of oxygen and carbon dioxide. It would be comfortable from the moment it was first placed on the eye, and it would continue to be comfortable. The lens could be capable of continuous wear if desired. It would be easy to insert, remove, center, and handle. Its care including storage, wetting, cleaning, and sterilization, would be simple. Its manufacture would be simple, with its specifications accurately reproducible. It would be inexpensive, cosmetically acceptable, and durable. It would have the therapeutic capacity. Contact lens evolution has been continues to be an interesting series of developments to catch the properties of an ideal lens. Today, daily disposable wear, extended wear, flexible wear (daily and overnight wear at the patients discretion) or conventional wear (insertion each morning and removal with cleaning and disinfecting in the evening) lenses are the main wearing alternatives offered by the major manufacturers. The ocular environment places high demands on the performance of contact lenses as biomaterials. Permeability to oxygen is a key performance to characteristic for contact lenses. The expression Dk is an intrinsic property of a material to transport oxygen through its bulk; it is thickness dependent. The term Dk/t refers to the oxygen transport properties of a contact lens of thickness t, in nm and is called oxygen transmissibility. Dk/t is generally viewed as a representation of the ability of a specific lens to deliver oxygen to the cornea. Sometimes adverse responses to contact lens wear occur and reports have noted an incidence of contact lens-related corneal ulcers, mostly due to Pseudomonas aeruginosa.
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