Why is modulus important?

Karen French , PhD BSc (hons) MCOptom Karen French is an optometrist working in independent and hospital practice in Cambridgeshire, UK.  She graduated from City University with an honours degree in optometry.  She has also completed a PhD at Aston University, under the supervision of Professor Brian Tighe, where her research interests included synthesis and characterisation of novel hydrogels for use as biomedical materials including contact lenses.

The widespread use of silicone hydrogels has refocused attention on the mechanical properties of contact lens materials and the ocular complications that can arise as a result of stiffer, less flexible lenses.  A contact lens in situ on the eye is subject to external forces from the eyelids.  It is also subject to external forces during handling and during the manufacturing process.  The success of a contact lens material and the impact of these external forces are governed by the materials’ mechanical properties. 

From the perspective of the contact lens wearer, the two main qualities to be achieved with their contact lenses are comfort and good vision.  Comfort can best be achieved from a flexible contact lens that drapes easily over the cornea and has minimal interaction with the eyelids during blinking.  But a high degree of flexibility can be a disadvantage when trying to achieve optimum vision.  The optical performance of a lens will be significantly improved if it can mask corneal astigmatism.  An increase in stiffness or rigidity will achieve this but at the expense of initial comfort.  The ease of manufacture of a contact lens, along with the reliability of the quoted parameters and dimensional stability will also be influenced by the mechanical characteristics of the contact lens material.

In order to fully understand the mechanical properties of contact lens materials it is necessary to define and discuss some of the terminology used.  Several terms can be used to describe the mechanical behaviour of any material.  Polymers are often described as strong or maybe tough.  They maybe considered elastic or stiff and inflexible or even brittle.  These terms all describe different kinds of mechanical behaviour and the response of a material to a specified external force.

Stress is the force per unit area required to change the shape of a solid.  It is calculated from the following equation:

Stress = F/A

where F is the force applied to the sample and A is the cross-sectional area of the sample.

Strain is a term used to describe the deformation that a sample undergoes in the direction of the force applied during testing.  It can be measured as the percent change in length of the sample relative to the sample’s original length at any given point, i.e.   

where L is the length of the sample and L0 is the sample’s original length.

The modulus of a material describes how well it resists deformation.  A material with a higher modulus is stiffer and has better resistance to deformation.  The modulus is defined as the force per unit area required to produce a deformation or in other words the ratio of stress to strain. 

There are several different types of modulus that can be measured.  The ‘bulk modulus’ relates to the change in the volume of an object when subjected to pressure changes.  The ‘shear’ or ‘rigidity modulus’ is a value relating to how a material behaves when a horizontal force is applied in a direction tangential to its surface or when a material is twisted during torsional testing. 

The elastic or Young’s modulus (E) is the value most often quoted in the context of contact lenses.  It is a measure of how well the contact lens material resists deformation by pulling or stretching.  For a truly elastic material - one that will return to its original shape after deformation, Young’s modulus is a constant value and the stress is proportional to the strain applied.  However, few polymeric materials are truly elastic and are actually classified as viscoelastic (having both viscous and elastic properties).  For viscoelastic materials, which include contact lens materials, Young’s modulus varies with the amount of stress applied.  The value quoted for Young’s modulus for these materials is usually the initial value at very low strains where the proportionality of stress to strain is maximum.

What features govern modulus?
It is important to appreciate that modulus is a property of the material rather than the contact lens.  The lens thickness and geometry will also have a bearing on its mechanical behaviour.  A thick lens made from a low modulus material may still be considered relatively inflexible or stiff.  A thinner lens made from a low modulus material will drape over the cornea, distributing itself evenly on the ocular surface with minimum lid interaction.  This allows the fitting of the lens to be less dependent on the lens parameters.  There is greater comfort initially for the patient.  There is a lower incidence of mechanically induced ocular complications.  A low modulus, however, also means that the lens material has poor handling characteristics and reduced durability. 

In general the mechanical properties of conventional hydrogel materials are governed by the equilibrium water content, with lower water content materials generally showing greater strength and higher modulus than those with a high water content.  The water content of conventional hydrogel materials also governs the oxygen permeability.  Hence the desire to find hydrogels with the right balance of oxygen permeability and mechanical properties.  The chemical structure of the hydrogel polymer can also play a part in governing the mechanical properties of the final material.  This is illustrated by the fact that articular cartilage, a naturally occurring hydrogel composite, has a tensile strength more than ten times greater than pHEMA but actually has a much higher water content of 80%.  Development of hydrogel materials with a view to obtaining the right balance of oxygen permeability and mechanical properties concentrated on investigating different monomer combinations, and the effective cross-link density of the final material – including not just covalent cross-linking but also ionic, polar cross-linking and steric interaction between side groups on the polymer back bone.  This allowed the production of high water content, low modulus materials with acceptable handling characteristics.

The advent of silicone hydrogel materials and their current widespread use in contact lens practice has resulted in a renewed interest in the mechanical properties of soft contact lens materials.  The first generation silicone hydrogel materials: balafilcon A (PureVision, Bausch and Lomb) and lotrafilcon A (Focus Night & Day, CIBAVision) have a much higher modulus than conventional hydrogel materials (Table 1).  This gives rise to lenses which behave differently from their conventional hydrogel counterparts.  Materials with higher relative silicone contents have higher oxygen permeability.  The higher silicone content, however, also results in a lower water content which generally means a material with a higher modulus.  As with conventional hydrogel materials, the polymer architecture also governs the modulus of the material.  The use of ‘bifunctional’ macromers in the polymerisation process results in increased cross-link density of the final polymer.  Materials with higher cross-link densities are more likely to have a higher modulus.  Similarly the presence of bulky siloxanyl groups leads to an increase in material modulus. 

The introduction of the second generation silicone hydrogel materials, galyfilcon A (Acuvue Advance, Johnson & Johnson), senofilcon A (Acuvue Oasys, Johnson & Johnson) and lotrafilcon B (AirOptix, CibaVision) has seen the arrival of silicone hydrogel materials with lower  moduli.  Galyfilcon A and senofilcon A have moduli somewhat lower than the first generation materials.  This has been achieved with a lower silicone content and also by the incorporation of an internal wetting agent, Hydraclear, based on polyvinylpyrrolidone.  This long chain high molecular weight molecule is extremely flexible when fully hydrated and along with the increased water content allows both materials to have a lower modulus.

More recently CooperVision have launched their comfilcon A material (Biofinity) which is interesting because it maintains a high level of oxygen permeability despite its relatively high water content.  The advantage of the high water content is that the material has a lower modulus than the first generation silicone hydrogels but without as much compromise on the oxygen permeability.

Current trends in the materials science have seen the use of novel macromer architecture to reduce the modulus of silicone hydrogel materials to levels closer to that of conventional hydrogel material.  Monofunctional macromers reducing cross-link densities and unique pendent or terminal groups within the polymer chain are all possible material modifications.

Clinical consequences of increased material modulus
The increased modulus makes these materials easier to handle and more durable.  But the initial comfort of the lens may be reduced and some patients notice greater lens awareness. 

The fitting of the lens is more critical than with conventional soft lens materials.  A lens with a higher modulus is less likely to conform to the eye curvature and as such needs a back optic zone radius that more closely matches the corneal curvature.  A flat fitting higher modulus lens will not drape as well over the cornea and can result in the lens edge lifting away from the cornea or ‘fluting’.  Fluting results in patient discomfort and can lead to ocular surface damage. 

Another significant problem with these materials is that, particularly when worn in an extended wear modality, there are several ocular complications that can arise as a result of mechanical irritation, such as superior epithelial arcuate lesions, contact lens related papillary conjunctivitis, and mucin ball production. 

Table 1 Properties of currently available silicone hydrogel lenses in comparison with some more traditional hydrogel materials