Understanding IOL material properties and recognizing the vast differences between today’s available lenses is challenging for any surgeon. That is why I enlisted the help of an independent chemical scientist to clarify the basics of hydrophobic material design. This group of IOLs must not be categorized under a single overarching denomination, as variations in the chemical and physical characteristics of the polymers and the methods for manufacturing them can significantly affect performance outcomes.
MONOMER SELECTION
A number of acrylic monomers are used for IOL fabrication. Each monomer and its relative proportions affect the characteristics of the resulting polymer. A monomer is characterized by the presence of a polymerizable functional group—a reactive carbon-to-carbon double bond (ie, vinyl bond). Acrylate and methacrylate monomers (Figure 1) are typically preferred over simple vinyl monomers for IOL fabrication because of their higher reactivity.
Monomers possessing electron-rich aromatic moieties such as phenylethyl acrylate (Figure 2) exhibit greater refractive index values than nonaromatic monomer analogues; however, they are usually more expensive. These bulky aromatic groups also influence the mechanical characteristics of the material.
The presence of sterically significant aromatic groups within a polymer matrix reduces its chain density, with the consequence of enhancing individual chain flexibility and concomitantly reducing interchain entanglements. When used for IOL fabrication, these aromatic groups can result in water ingress; its subsequent accommodation may lead to the formation of glistening bodies (ie, vacuoles) within the body of the IOL.
MECHANICAL PROPERTIES
The combination of monomers with other constituents such as crosslinking entities determines the properties of the resultant polymer. Depending on the combination, mechanical characteristics of the resulting material can affect ease of folding and subsequent unfolding time, stability of the lens after implantation, and sensitivity to changes in temperature.
The mechanical parameters of hydrophobic IOL polymers are dependent on the glass transition temperature of the material (Tg; the temperature at which the polymer transforms from a rigid glassy polymer to a flexible compliant material). For polymethyl methacrylate (PMMA), this is room temperature, but the Tg values of polymers used for hydrophobic acrylic IOLs are typically below room temperature (22° C), thus ensuring easy manipulation such as flexing and rolling during the surgical procedure. At lower operating room temperatures, some hydrophobic acrylic materials with higher Tg values can behave as rigid materials, in which case folding and compressing are more challenging.
The structure of a particular monomer affects the magnitude of the polymer’s Tg, whether it is composed of a single monomer, to form a homopolymer, or of combinations of monomers. For a methacrylate monomer, the substituent group (R; Figure 1) has a chain structure, for which its flexibility is approximately proportional to its length. This controls the Tg of the resulting polymer. For example, PMMA (Figure 3) has a Tg of approximately 105°C, but polyethyl methacrylate (PEMA; Figure 4), with extra carbon in the substituent chain, has a Tg of approximately 65°C.
The use of monomers with different substituent R groups is a fundamental strategy employed by polymer formulation scientists to systematically control the Tg value when designing novel IOL materials. Acrylate monomers usually have lower Tg values than their methacrylate analogues, due to the smaller hydrogen substituent on the reactive vinyl double bond. The consequence of using an acrylate monomer is that a polymer chain formed exclusively from the coupling of acrylate vinyl groups is minimally hindered from rotation, thus imparting a greater degree of chain flexibility. Through combination of otherwise structurally analogous methacrylate and acrylate monomers, the Tg of the resulting copolymer can be manipulated by adjusting the relative proportions of each component. The Tg for a methacrylate copolymer typically lies between that measured for the homopolymers of each component.
The modulus, or stiffness, is also important in IOL materials design. The modulus and its components such as elasticity and viscosity can be manipulated by selecting monomers that produce ease of folding of the material. The properties of tensile strength and elongation affect the durability of the material, determining the degree of compression that can be achieved during injection. Despite the large role IOL design has on influencing incision size, it is ultimately the combination of the polymer’s mechanical parameters that establishes the boundaries in which the lens designer must work to minimize incision size.
IOL polymer composition also influences how temperature changes affect mechanical behavior. Generally, materials become less pliable as temperature decreases; such is the case with hydrophobic acrylic polymers. The magnitude of the change in mechanical properties with variation in temperature is a function of the monomer and crosslinker composition. Some polymers may show greater reduction in ease of folding and rate of unfolding than others at a particular temperature, and this must be considered for surgical conditions. For example, it would be advantageous for the IOL polymer to be freely foldable at the likely temperature of the anterior chamber during implantation—a condition depending on various factors including the temperature of the ophthalmic viscosurgical device.
POLYMERIZATION
Hydrophobic acrylic IOLs are manufactured by one of two methods: Either the lens optic is polymerized as a finished element (ie, cast molding), or the lens is lathe cut to the required geometries from a larger polymer article (ie, cryolathing). The latter process is performed at low temperatures to ensure that the material is kept below its Tg and therefore sufficiently rigid to accommodate precise machining. In either case, the polymer is formed through a free-radical polymerization reaction. This reaction can lead to additional variations among the types of hydrophobic materials.
Monomers play a crucial role in determining the structure of the polymer formed through any polymerization process. All polymerizable components, including monomers and crosslinkers, have reactivity characteristics dependent on structural considerations. This reactivity is further influenced by the polymerization conditions and the presence of other components. Because more than one polymerizable component is always employed, the monomer constituents can be assembled into polymer chains exhibiting great architectural diversity. In an ideal situation, the polymer units alternate consistently to create a regularly alternating copolymer. However, due to differences in the relative reactivity of the monomers at a set of polymerization conditions, distribution of the monomer units tends to result in the formation of copolymer blocks that contain alternating segments composed of only one constituent monomer. When there is considerable disparity in the reactivity of the monomer components, these blocks may be large, affecting the properties of the polymer network.
The precise conditions of the polymerization process, such as the amount of initiator and the delivery of activation energy (thermal or radiative: eg, ultraviolet light), contribute to the architecture, possibly leading to differences in the physical properties of the resultant polymer. Additionally, other differences in the nature of the monomers, such as polarity and solubility, can prevent the polymerized unimonomeric blocks from forming homogeneous blends. If blocks are formed from other monomer components, discrete immiscible phases may result, potentially affecting material properties, most notably optical clarity.
The above considerations can also affect the degree of conversion of monomer units into the resulting polymer. The amount of residual monomer determines the viability of the resulting IOL. Polymer formulation scientists can then evaluate whether subsequent processing steps (eg, extraction of leachable components) are required to enable safe lens implantation.
MANUFACTURING PROCESSES
Following polymerization, the material used to fabricate the IOL is subjected to several processes that can affect IOL performance. With lathe-manufactured IOLs, the material is cooled below room temperature to ensure that acceptable optical surface quality is produced. The rate at which the material is cooled, the working temperatures, and the subsequent rate of heating to ambient conditions contribute to the thermal history of the material. The lens can then be subjected to additional thermal conditioning; however, the thermal history of the lens during transportation, storage, and immediately prior to the surgical procedure equally contributes to the material properties. Studies1 examining the tendency of an IOL to form glistenings show that thermal history and exposure to significant temperature gradients can influence the severity of glistening formation.
The ways that manufacturing processes influence the properties of an IOL can vary from manufacturer to manufacturer; these factors can include polishing, packaging, and sterilization. At each stage, the polymer can be subjected to physical conditions that could affect its morphology.
Exposure to chemicals may also play a role in the behavior of an IOL. Such exposure can result in ingress of undesirable contaminants into the polymer matrix or surface deposition processes that may alter the nature of the lens’ interaction with the ocular environment (eg, as contributing factors to formation of posterior capsular opacification).
Polymerization reactions result in a certain amount of unpolymerized monomers, some of which can act as sensitizing agents in the intraocular environment. Therefore, IOL manufacturers typically employ an extraction process, achieved through the use of organic solvents, to swell the polymer and solubilize the unreacted monomers. This process affects the morphology of the polymer, and the degree of swelling and amount of residual monomer will alter the amount of free space in the polymer. Furthermore, the solvent itself can be difficult to remove completely from the polymer matrix. Any residue will act as a plasticizing agent, reducing the elastic modulus of the polymer.
OCULAR BIOCOMPATIBILITY
The nature of the components used and the manufacturing method influence how the lens material behaves in the ocular environment. Steele McIntyre, MD; Liliana Werner, MD, PhD; and Nick Mamalis, MD, provide an overview of ocular biocompatibility associated with hydrophobic acrylic IOLs in their article, Hydrophobic Acrylic Lenses: A Primer, on page 39.
CONCLUSION
The wide range of commercially available hydrophobic acrylic IOLs offers diverse properties that cannot be categorized simply. The monomer components used to produce the IOL polymer, along with precise manufacturing conditions, affect the ultimate viability of the resulting lenses; this is far more complex than can be described by differences in classic modulus, Tg, or refractive index parameters in isolation. These design factors, manipulated by polymer formulation scientists and lens designers, affect the mechanical properties of the lens, the susceptibility of the material to the formation of glistening bodies, and other biocompatibility issues that determine the material’s eventual effectiveness during implantation and when residing in the ocular environment.
Such performance differences can be further complicated by various manufacturing processes, most notably the thermal history of the lens polymer. The rapidly growing global markets for hydrophobic acrylic IOL polymers are indicative of the success of this IOL technology. The above design and performance considerations, as outlined to me by an independent chemical scientist, should be carefully considered when selecting the most suitable lens for implantation. Additional research and longer follow-up is needed to evaluate the qualities of this most important product used in modern cataract surgery.
Khiun F. Tjia, MD, is an Anterior Segment Specialist at the Isala Clinics, Zwolle, Netherlands. Dr. Tjia states that he is a consultant to Alcon Laboratories, Inc., and Hoya Corp. He is the Co- Chief Medical Editor of CRST Europe. Dr. Tjia may be reached at e-mail: kftjia@planet.nl.
TAKE-HOME MESSAGE
• The mechanical parameters of hydrophobic IOL polymers are dependent on the glass transition temperature of the material.
• Hydrophobic acrylic IOLs are manufactured by one of two methods, cast molding or cryolathing.
• IOL performance differences can be complicated by various manufacturing processes including the thermal history of the lens polymer.
- Kato K,Nishida M,Yamane H,et al.Glistening formation in an AcrySof lens initiated by spinodal decomposition of the polymer network by temperature change.J Cataract Refract Surg.2001;27:1493-1498.
