Soft Lens Materials

Introduction

Soft contact lenses have had a massive impact on the global contact lens market since they became widely available in the early 1970s. Since their introduction, the number of soft contact lenses being prescribed around the world has steadily increased, and it is mainly the sale of soft contact lenses that is responsible for an industry that has been estimated to reach US$10 billion globally by 2025. A recent survey has indicated that soft lenses currently make up 87% of all contact lens refits worldwide ( ).

Saturation of the contact lens market with soft lenses has occurred primarily due to two reasons. First, soft lenses provide wearers with what they see as the two most important requirements for successful contact lens wear: good vision and comfort. The major obstacle as far as rigid lenses are concerned is generally accepted as being their lack of comfort and, in particular, their initial discomfort ( ). Second, advances in manufacturing technology have directed the industry towards disposable soft lenses.

Contact lens materials (both soft and rigid) are good examples of biomaterials. A biomaterial may be defined as a natural or synthetic material that is suitable for introduction into living tissues, especially as part of a medical device. The term encompasses a vast array of technologies, including tissue engineering, artificial organs, bioceramics, medical devices and implantable drug delivery systems. Contact lenses are classified as a medical device in most countries.

Very few of us are likely to get through life without having some kind of biomaterial introduced into our bodies. The most common examples include dental fillings, contact lenses, intraocular lenses, heart valves and stents. This list in itself highlights just how diverse biomaterials must be to satisfy their very specific end application; for example, a contact lens material has very different properties compared to a material used for dental fillings.

If a biomaterial is to be successful in its application, it follows that it must also be biocompatible. Biocompatibility refers to the ability of a material to perform with an appropriate host response in a specific application. An ‘appropriate host response’ includes not having toxic or injurious effects on biological systems. Biomaterials manufactured for use as contact lenses must not only satisfy all of these requirements for safe use within the eye but must also additionally have very specific characteristics, such as being transparent (and remain so on the eye), comfortable and relatively cheap to manufacture.

This chapter reviews the building blocks, properties and characteristics of the materials that are used to manufacture soft contact lenses and provides some of the history of development of these materials. Most clinicians are familiar with the United States Adopted Name (USAN) of a particular lens material (e.g. etafilcon A or lotrafilcon A), but any further understanding of the material is often lacking. This chapter aims to give meaning and background to these USAN names to help the reader understand and differentiate between different soft lens materials.

Polymers

All contact lens materials may be classified as polymers. The word ‘polymer’ is derived from ancient Greek, meaning: ‘many parts’. Polymers are solid materials (as opposed to gases or liquids) that are made up of high-molecular-weight chains (i.e. long chains), which in turn are made up from small repeating units. These repeating units are called monomers. Polymers are macromolecules (giant molecules) made from thousands of atoms. The term ‘polymer’ is therefore an umbrella term for materials that include plastics [e.g. polymethyl methacrylate (PMMA), used in the manufacture of ‘hard’ rigid lenses]; fibres (e.g. nylon); elastomers (i.e. rubbers such as silicone rubber); and the materials being discussed in this chapter, hydrogels.

The term ‘hydrogel’ is often used interchangeably with the term ‘soft’ when referring to contact lenses. Soft lenses are so named because they are made from water-swollen, cross-linked, hydrophilic polymers that are flexible and compliant. The term ‘hydrophilic’ is used to show that the networks from which these materials are made are ‘water-loving’.

The widespread use of polymers in many areas of our everyday lives has become a common and accepted phenomenon, so much so that they have been referred to as the ‘steel of the 21st century’. Polymers possess many properties that make them suitable for a wide range of applications, some of which are unique. These properties are in part due to the length of the molecules from which they are made. Additionally, polymers derive their unique characteristics from the ability of certain atoms to join together to form stable covalent bonds.

Many polymers are composed of hydrocarbons [i.e. carbon (C) and hydrogen (H) alone] such as polyethylene and polystyrene. However, even though the basic makeup of many polymers is carbon and hydrogen, other elements can also be present. Oxygen (O), nitrogen (N), chlorine (Cl), fluorine (F) and silicon (Si) are commonly found in the molecular makeup of polymers. Many polymers have carbon backbones (these are considered organic polymers), but some can incorporate silicon or phosphorous backbones (these are considered inorganic polymers).

The atoms making up a polymer and its geometric arrangement give each polymer its chemical distinctiveness and, thus, its particular use and function. Polymers themselves may be completely natural (e.g. cellulose), partly natural (e.g. cellulose acetate) or completely synthetic (e.g. PMMA). Most of the polymers used in the manufacture of soft contact lenses fall into this last category, i.e. they are man-made.

The Structure of Polymers

A polymer chain can be described by specifying the kind of repeating units present and their spatial arrangement. In this way, several broad categories of polymer can be described.

A homopolymer is one in which only one type of monomer is used, i.e. the units are chemically and stereochemically identical, with the exception of the end units. If the chain units are arranged in a linear sequence, the polymer is referred to as a linear homopolymer. This is shown schematically in Fig. 4.1 . Departures from this simple array lead to structures of increasing geometric complexity. A nonlinear or branched structure is shown in Fig. 4.2 .

The chemical differences between linear and branched polymers may be quite small; yet, because of the structural differences, the two polymers can have quite markedly different properties. A good example of these differences is found between low-density polyethylene (branched) and high-density polyethylene (linear). Low-density polyethylene is commonly used as a packaging film (e.g. cling film and for carrier bags), whereas high-density polyethylene is used for making pipes and durable plastic bottles because of its higher impact strength.

Nonlinear and network structures can also be prepared from a collection of linear chains by covalently linking together chain units selected from different molecules. Such a system is said to be cross-linked. This is shown schematically in Fig. 4.3 . Here, X represents the chemical species (the cross-linker) that covalently links together the A units from different molecular chains. When a sufficient number of units are intermolecularly cross-linked, an infinite network is formed. A cross-linker is an important ingredient in a soft contact lens monomer mix, which will be discussed later.

A copolymer is one in which more than one type of monomer is used. The properties of a copolymer not only depend on the chemical nature and amounts of the counits but also vary markedly on how the units are distributed along the chain. For linear copolymers, three ‘ideal’ arrangements can be described. The first is an alternating copolymer, which is shown in Fig. 4.4 . In this scenario, each monomer prefers to interact with a fellow monomer rather than itself.

At the opposite extreme is the ordered or block copolymer, where there is an overwhelming tendency for a unit to be succeeded by another of the same kind. Here, long sequences of one type of unit alternate with long sequences of another kind ( Fig. 4.5 ).

The third major classification is the random copolymer. Here, different units are randomly distributed along the chain ( Fig. 4.6 ).

Departing from the restrictions of a linear array, branched copolymers, known as ‘graft polymers’, can also be prepared. The backbone of the molecule is composed of one type of unit, and the long side chains, or grafts, are made up of another. More sophisticated types of graft polymers have backbones made up of different repeating units and several distinctly chemically different side groups. This type of polymer is represented schematically in Fig. 4.7 .

One final important classification of polymers is into either amorphous or crystalline polymers (i.e. their macromolecular order) ( Fig. 4.8 ). Crystalline polymers have a geometrically regular structure and are generally stiff, resistant to chemicals, and tough. They have limited use as materials for contact lenses owing mainly to their poor optical qualities (i.e. they tend to be translucent or opaque). A good example of a semicrystalline polymer is polypropylene, which is often used to make casts in the cast-moulded manufacturing process of contact lenses.

Amorphous polymers, on the other hand, do not have a regular structure. The polymer chains intermingle and are in random positions (imagine a pile of spaghetti on a plate), which often allow for these polymers to be transparent. Depending on their chain mobility, amorphous polymers can be classified as either ‘plastic’ or ‘glassy’ ( ).