Transparency refers to the clearness (clarity) of a material. Among other factors, it is a function of the chemistry, purity, and hydration of the material. No material is completely transparent, as some light will always be reflected, absorbed, and scattered. Transparency is often expressed as a percentage of incident light of a certain wavelength that passes through a sample of the material. Values for most clear (nontinted) contact lens materials range from 92% to 98%.

The hardness of a lens material is an important quality affecting its ability to be used for the manufacture of contact lenses and its durability. Generally, hardness is an attribute that is more relevant to rigid lens materials than soft materials. Stiffness is the degree of flexibility of a material, and this can be an important factor when a lens material is selected for a patient. More flexible materials usually result in better initial comfort but do not mask or correct corneal astigmatism, as they tend to drape over the cornea and conform to its shape. Stiffer materials retain their shape during handling and will make insertion and removal of the lens easier.

The tensile strength of a material is a value that expresses how much stretching force can be applied before it breaks. Materials with a high tensile strength tend to be more durable, as they are better able to withstand the forces applied during lens handling procedures (i.e., cleaning, inserting) without tearing.

The modulus of elasticity is a constant value that expresses a material’s ability to keep its shape when subjected to stress and to resist deformation. Materials with a high modulus are stiffer, resist deformation, hold their shape better, are easier to handle, and may provide better visual acuity.
Many silicone hydrogel materials have a lens modulus much greater than hydrogel materials. The stiffer lens may have the benefits mentioned previously, or it may adversely affect the lens performance by causing edge lift or fluting, superior epithelial arcuate lesions (SEALs), mucin balls, or giant papillary conjunctivitis [GPC, also called contact lens papillary conjunctivitis (CLPC)].⁷,⁸,⁹ Materials with a low modulus of elasticity are less resistant to stress. Most hydrogel materials fall in the low-modulus category. Modulus values can be found in Table 10.2.¹,⁷,⁹,¹⁰

Refractive index of a lens material is the ratio of the speed of light in air to the speed of light in the material. Materials with higher refractive indices cause more refraction of incident light. For soft lens materials, the index of refraction is related to water content. Generally, increasing the water content lowers the refractive index. A hydrogel lens material with a water content of 80% has a refractive index of about 1.37, and one with a water content of 42% has a refractive index of about 1.44, compared to silicone hydrogel materials (lotrafilcon A and balafilcon A), which have a refractive index of 1.43.¹¹,¹²

The surface wettability of a soft lens is an important property. The wettability aids in the closure of the lid over the lens, thereby improving comfort and preventing changes to the papillary surface on the internal surface of the lid.¹³ The very wettable surface creates a stable, even tear film. This assists with optimizing comfort, visual acuity, and deposit resistance. As silicone is naturally hydrophobic, its use in soft lenses created a dilemma until the ability to increase the wettability of silicone was discovered.

Early silicone hydrogel materials used a surface treatment to cover up the hydrophobic properties of silicone. Ciba Vision uses a gas plasma technique to apply a uniform plasma coating, approximately 25 nm thick with a high refractive index, on the surface of its silicone hydrogel lenses (lotrafilcon A and B) (Fig. 10.1). The gas plasma technique is also used by Bausch & Lomb to apply a plasma oxidation surface treatment to its silicone hydrogel lens (balafilcon A). This surface treatment results in glassy silicate islands on the surface of the lens. The islands leave small areas of exposed hydrophobic regions; however, the wettability of the silicate appears to create a bridge over these areas to produce a net hydrophilic surface.⁶ Menicon combines the benefits of plasma coating and plasma oxidation for a plasma surface treatment on its lens (asmofilcon A) called Nanogloss surface modification. The manufacturer reports that this creates a smooth surface with a low contact angle.¹⁰ Silicone hydrogel materials have also used an internal wetting agent, polyvinyl pyrrolidone (PVP), to create wettability (Vistakon, galyfilcon A, and senofilcon A).¹ CooperVision reports that its silicone hydrogel lens (comfilcon A) has no surface treatment or wetting agent. Instead, the lens material contains two silicone-based macromers (a large monomer preassembled to transfer advantageous properties to the final polymer).⁶ These macromers, when incorporated into the material with hydrophilic monomers, result in a naturally wettable lens (Fig. 10.2).

Another factor that has changed the wettability of soft contact lenses is the incorporation of PVA or PVP, moisturizing agents, into daily disposable hydrogel lens materials. Potentially, adding these agents to the lens material will result in increased wettability, increased comfort, and enhanced tear film stability. Early results indicate that this may be beneficial to dry-eye patients and provide increased comfort throughout the day, including end-of-the-day comfort.¹⁴,¹⁵

Contact lens materials may possess an electric charge, or they may be electrically neutral. This attribute is especially important in soft lens materials, as it affects factors such as solution compatibility and deposit formation. Materials that have an electric charge are said to be ionic. The charge results from the presence of electrically charged groups in their chemical formulation. In most cases, this is an overall negative charge. The presence of a negative ionic charge causes the material to be more reactive, especially in solutions that are acidic. This, in turn, can cause dimensional changes and even material degradation.

An ionic charge may also cause a material to be more prone to deposit formation. Most deposits are positively charged substances from tears that are attracted to the negative ionic charge of the lens material. Materials that are electrically neutral are said to be nonionic. These materials tend to be more inert and less reactive with tear constituents, so they also tend to be more deposit resistant.

Most contact lens materials, both GP and soft, absorb some water. The amount absorbed is usually expressed as a percentage of the total weight. When a material absorbs water, it swells, a fact that must be considered during the manufacturing process to achieve precise specifications. Materials that absorb <4% of water by weight are referred to as hydrophobic materials; those that absorb ≥4% water are termed hydrophilic polymers. With hydrophilic polymers (hydrogels),
increasing the water content generally increases Dk. However, this often increases lens fragility and may make the material more prone to deposit formation. Even if it were possible to create a 100% water content lens material, the Dk of water is 80; therefore, this lens would still be unable to meet the Holden-Mertz criterion for extended wear (87 × 10⁻⁹).¹ Silicone hydrogel materials have low water contents, because the material is dependent on silicone, not water, to transmit oxygen.