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Contact Lens Materials
Introduction
An overview of factors that influenced materials development
To appreciate the current spectrum of commercial contact lens materials and their structural variations, it is useful to summarise briefly the way in which materials have developed over time. Accounts of early attempts to improve vision by use of a lens contacting the eye are limited to a few isolated observations ( ). Practical success was not realised until techniques for fabrication of lenses from glass were sufficiently developed ( ).
Poly(methyl methacrylate) (PMMA) replaced glass in the late 1930s; the material was more durable, more readily fabricated and claimed by some to show better ocular compatibility ( ). During the same broad period, there was also a change in emphasis from scleral to corneal contact lenses, which placed different demands on material design and development. The only measurable physical property universally considered to be of practical importance for contact lens manufacture at this time was refractive index ( ).
For more information on the early soft and rigid gas permeable (RGP) materials developments, see Chapter 1 and also Section 8 , History, available at: 
https://expertconsult.inkling.com/ .
The Nature and Behaviour of Contact Lens Materials
Silicon and glass
Although more than 100 elements are known and more than 80 exist in a stable form in the world around us, just four of these (carbon, nitrogen, oxygen and hydrogen) make up 95% (by weight) of all living matter on Earth. They are not, however, the four most abundant elements on Earth (which are oxygen, iron, silicon and magnesium). The structure and function of the human body has provided an inspiration for biomaterial development over the last 50 years. In this respect silicon is an anomaly because it is virtually non-existent in the human body but has become important in the field of ophthalmology. The unique clarity and rigidity of glass gave it a significant position in civilisation. These properties made it an obvious choice for ophthalmic applications, examples of which stretch back more than 2000 years. For example, ancient civilisations used glass to represent the eye in statues of birds and animals, many of which still exist. Pellier de Quency, an 18th century French ophthalmologist, suggested the use of a glass shell, surrounded by a silver ring to accommodate sutures, as an artificial cornea. It was not until the 20th century, however, that successful glass tissue-contacting devices were made in the form of scleral contact lenses.
The word glass is used quite broadly in materials science to describe a solid material which is hardened and becomes rigid without crystallising. Glass as we know it is more accurately described as silicate glass. It is formed by solidifying individual silicon dioxide (silica) clusters into a solid but noncrystalline (i.e. amorphous) framework. Commercial glasses are based on this silica framework, together with additives, to break down any crystalline structure and reduce the melting point. The older forms of glass had only three basic ingredients: sand (silica), soda ash (Na 2 O) and lime (CaO). Types of glass were then developed for more demanding applications which required chemical resistance or thermal stability, by adding small amounts of other components. In all of these, however, silica is the predominant component, usually comprising 70%–80% by weight of the total composition.
More recently, silicon has grown in importance in ophthalmic biomaterials. Its use is substantially unrelated to either its ability to form silicate glasses or its natural abundance, but it has useful properties in the field of organic materials (as distinct from inorganic or mineral-based materials). The following provides a picture of the nature and structure of the class of materials known as polymers.
The nature of polymers
The unique properties that polymers possess arise from the ability of certain atoms to link together to form stable bonds. This is quite different from the nature of glasses, in which the solid is held together by electrostatic forces. Foremost among the atoms that can form linking bonds with each other is carbon (C), which can link together with four other atoms either of its own kind or, alternatively, atoms of, for example, hydrogen (H), oxygen (O), nitrogen (N), sulphur (S) or chlorine (Cl). It is this property of carbon that forms the basis of organic chemistry or the chemistry of carbon compounds. Most of the polymers that we encounter fall within the realm of organic chemistry, defined in this way. These polymers may be purely natural (such as cellulose), modified natural polymers (such as cellulose acetate) or completely synthetic (such as PMMA). To some extent, silicon (Si) resembles carbon in this way, especially in its ability to link to carbon, hydrogen and oxygen. Although the number of silicon-based polymers is relatively small and their structural versatility is limited, they have some properties that are not achievable in carbon-based compounds. In the context of contact lenses, one of these properties (oxygen permeability) is advantageous, whereas another (water repellency) is a disadvantage.
The single characteristic that unites both silicon-based and carbon-based polymers is the fact that, as the name (poly-mer) suggests, they are composed of many units linked together in long chains. Thus if we can imagine a molecule of oxygen and a molecule of water enlarged to the size of a tennis ball (the molecular size of water is very similar to that of oxygen), a molecule of polyethylene or poly(methyl methacrylate) on the same scale would be of similar cross-sectional diameter but something like 200 feet in length. It is the gigantic length of polymers (sometimes called macromolecules) in relation to their cross-sectional diameter that gives them their unique properties, such as toughness and elasticity. The links between individual atoms are inclined to each other at an angle (the bond angle), which means that chains are not straight and rodlike but ‘kinked’. One major difference between the siloxane (Si—O—Si) backbone and the carbon (C—C) backbone is in the ease of rotation (rather like a crankshaft in a car engine) resulting from differences in size and bond angles of the individual atoms.
The individual building blocks from which polymers are formed are termed ‘monomers’. To indicate that a polymer contains more than one type of repeating monomer unit, for example when two different monomers are polymerised together, the description ‘copolymer’ is used. Copolymer is a general term and can be used to describe polymers obtained from mixtures of more than two monomers. Most contact lens polymers are formed from monomers that are characterised by the presence of a carbon-to-carbon double bond that opens to form a linked chain. It is the way in which the structural and functional groups interact with each other and with their surrounding environment that governs the interaction of polymer chains and the resultant properties of the polymer itself. The siloxane polymers are structurally different and, as a consequence, the methods of polymer synthesis are not interchangeable with those used for carbon backbone polymers. It is this simple fact that has made the preparation of hybrids of silicone-based and carbon-based polymers very difficult to achieve.
For further information on the nature and development of polymers, see the Section 9 , Addendum, available at: 
https://expertconsult.inkling.com/ and Chapter 3 in the 5th edition of Contact Lenses .
Lens Properties
Clinical evaluation of lens performance is a topic of detailed study involving effects of material structure ( ), production techniques ( , ) and assessment of the biological response (Bergmanson 2001, , ). Although the relevance of mechanical properties is implicit in many evaluations of contact lens comfort and performance, little attempt has been made to assess the relative trade-off in importance of, for example, mechanical properties, wettability and oxygen permeability. Despite this, the concept of ‘the ideal contact lens’ has been regularly discussed from the time that the issue was first raised by Kamath in the late 1960s ( ).
The process of material development over time has been characterised by intermittent changes rather than regular stepwise increments. As interest in new lens materials grew, design began to involve the amalgamation of increased understanding of lens characteristics, clinical studies and, ultimately, practitioner feedback. Development was initially limited and typically carried out in small companies, but as larger companies realised the commercial opportunities of the contact lens marketplace, both the development and the scale of commercial production increased.
For information on the search for materials to replace PMMA, see Chapter 1 and Section 8 , History, available at: https://expertconsult.inkling.com/ website.
Since the 1980s, the spectrum of commercially produced rigid materials increased and reached the stable platform of properties now available to the clinician ( , , ). This has changed relatively little year on year.
A dramatic step-change in contact lens properties was brought about by the arrival of soft hydrogel lenses that had their genesis in the work of Otto Wichterle in Czechoslovakia. A reduction in lens stiffness, brought about by the inclusion of water to a polymer network, meant that soft lenses were relatively fragile compared with rigid materials. This period was marked by a huge growth in the understanding of corneal hypoxia ( ) and the complementary importance of surface properties (tear wettability), mechanical properties (comfort and handling) and transport properties (oxygen permeability) in the design of contact lens materials ( ). Experience of hydrogel chemistry grew steadily, driven mainly by the desire to achieve higher water contents, with the consequent increase in oxygen permeability that this brought. To increase oxygen transmissibility, lens thickness was reduced, but thin–high water content lenses inevitably compromised lens durability. The current generation of conventional hydrogels reflects a more complete understanding of hydrogel network structures and their effect on mechanical durability.
As overnight wear became more popular, the relationship between available oxygen and corneal health changed. Clinical complications associated with chronic corneal hypoxia were increasingly reported. Instrumentation for measurement of corneal oedema improved, and techniques for measurement of oxygen permeability were standardised. Values quoted in manufacturers' literature became more conservative, and in 1984, Holden and Mertz published data to support their view that to prevent overnight hypoxia-induced oedema, continuous-wear contact lenses should have an oxygen transmissibility corresponding to an oxygen permeability of 87 Barrers for a lens of 0.1 mm centre thickness ( ). There has been an ongoing division of opinion about the merits of increasing this value, but it remains the most widely quoted baseline requirement for continuous-wear lenses.
To understand the permeability of polymers and the relevance of this information, we have to look more closely at the nature of permeability. The coefficient P, for a given species (the permeant) through a polymer is a product of two terms:
These two terms are the diffusion coefficient, D, and a solubility term, S. It has become conventional in the contact lens field to replace the term S by k, and to refer to the permeability coefficient as Dk. Whereas the diffusion term is related to the mobility of the polymer chains and the ease with which the permeant (e.g. oxygen) molecule can meander through them, the solubility term is governed by the amount of oxygen that the material can dissolve at a given ambient partial pressure. Thus k is a partition coefficient. Values of Dk are conveniently quoted in Barrers, where 1 Barrer = 10 −11 cm 3 O 2 (standard temperature and pressure [STP]) cm/s cm 2 mm Hg.
Thus Dk (or P) is the permeability coefficient for a given material, and the transmissibility of a sample of a given thickness t (such as a contact lens) of that material becomes Dk/t. Since lenses are around 0.1 mm thick, the values of contact lens transmissibility are usually quoted with units of 10 −9 (cm × mLO 2 )/(s mL mm Hg), which keeps them numerically similar to Dk values.
The precise way in which the oxygen permeability varies with water content at a given temperature was established in the mid-1970s. The relationship is an empirical one in which the logarithm of permeability (Dk) is seen to increase linearly with the percentage equilibrium water content (EWC, here referred to as W). That is:
where A and B are experimentally determined constants for a given temperature.
This means that if the water content and the constants A and B are known at a given temperature (say 34°C), a reasonably exact value of the oxygen permeability can be calculated. There is a clear pitfall here, however, because of the ways in which the water content varies with temperature, it is not possible to make comparative predictions of the permeabilities at 34°C for different materials from their water contents at room temperature. Wichterle's first hydrogel, poly(2-hydroxyethyl methacrylate) (polyHEMA), is atypically well behaved as a hydrogel in respect of the stability of its water content with temperature. Therefore it cannot be taken as a model for the behaviour of other hydrogel polymers, especially those containing ionic groups ( ). If the water content of hydrogels were to remain unchanged between 20°C and 34°C, the oxygen permeability would almost double over that temperature range. Since water contents usually fall with this temperature rise, however, the gain in oxygen permeability between room temperature and eye temperature is significantly less for most contact lens materials. Unfortunately, although the methodology for measurement of Dk values is well established, the factors identified above give rise to some variability in quoted permeabilities, even of ostensibly identical materials. These points are illustrated in Fig. 2.1 , which collects in the form of a scatter graph the quoted Dk values for a series of commercial lenses as a function of water content. The figure contains reference data for hydrogel membranes at both 25°C and 34°C, for which both Dk and water contents have been measured at each temperature. The clinical caveat is that quoted water contents and Dk values, taken together, should be used as a guide – but no more than that – to on-eye transmissibility.
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