Polyhedral Oligomeric SilSesquioxane (POSS) Fillers as Refractive Index Enhancers for Silicone Intraocular Lenses, 18-9333Printer Friendly Version
Inclusive Dates: 06/28/02 - 10/30/02
Background - Silicone chemistry is an important industry in various technology areas. Electronic packaging, sealants, medical tubing, and implantable medical devices are a few examples of the industries that rely on silicone technology. In the ophthalmic device market, a number of companies manufacture silicone intraocular lenses (IOL) for replacement of the natural lens after cataract surgery, with silicone lenses accounting for approximately 40 percent of the current IOL market.
A major advantage of silicone lens materials for ophthalmic use is the flexibility and elasticity of the material, along with biological inertness. Flexibility allows the lens to be folded and implanted through a much smaller incision than is possible with a standard hard lens, and elasticity allows the material to rebound to its unperturbed state when the folding pressure is released with perfect retention of the optical properties of the lens. Changing the composition of the lens material from a pure dimethylsiloxane to a dimethyldiphenyl siloxane increases the refractive index (n) of the resulting lens, which allows a thinner lens to be manufactured. The thinner lens then can be inserted through a smaller incision, resulting in more predictable surgical outcomes. There are limitations to increasing the refractive index of silicone materials by changing the composition of the siloxane backbone. Decreasing dimethyl composition of these silicone copolymers renders the materials more crystalline and with concomitant loss of elastomeric properties. Additionally, these silicone compositions require a filler to increase the mechanical strength of the material.
Unfilled silicone materials have very poor tensile and tear strength without the addition of reinforcing fillers to prevent crack formation and crack propagation. This component is critical in foldable silicone IOLs, as the material is stressed significantly in the folding process during surgical insertion. The interaction between filler particles and the siloxane (silicone) matrix significantly increases the physical properties of silicones such as tear strength, tensile strength, and elongation before failure. The most commonly used fillers for silicones are fumed silicas with a large surface area that have been surface treated to increase compatibility. The surface treatment enhances the interaction between the silicone polymer and the filler, and assists in matching the refractive index between filler and polymer components.
This project was directed at a high-risk development project using nanoscale filler materials to increase the refractive index of standard dimethyl and dimethyldiphenyl siloxane materials. This project's intent was an effort in the promotion of the development of silicone materials with a novel approach to reinforcing fillers.
Approach - In this project the team has investigated methods of enhancing the physical properties of silicone polymers by using Polyhedral Oligomeric SilSesquioxane (POSS) nanoscale fillers. These novel POSS fillers were envisioned to function both as siloxane-reinforcing agents to add strength to the cured polymer matrix and as refractive index enhancers. Refractive index matching is typically required because the particle size of the filler (fumed silica, for example) has a macroscopic particle size and will cause refraction of light if the refractive index of the filler is different from that of the siloxane. The use of nanoscale POSS fillers was expected to retain optical clarity because these are molecular scale materials and allow the development of materials with tailored refractive index properties.
Accomplishments - Two paths were taken in investigating the properties of POSS materials with silicone polymers. First, on a fundamental level, the team attempted to estimate the refractive index of the bulk POSS filler materials, since these values are largely unknown for POSS compounds. With this information, the calculation of the contribution that the POSS material would make to the volume refractive index of the composite silicone can be estimated. Values for the contributions from silicon atoms, in the silsesquioxane oxidation state [RSiO1.5], to the molar refractive index were not readily available. However, those for the organic corner capping groups can be accurately calculated within approximately one percent of the measurable value. Using these standard methods for the calculation of the molar refractive index, the team estimated the contribution from silicon in the silsesquioxane oxidation state from compounds with saturated, unsaturated (vinyl), and phenyl organic groups on the silicon atom. With this information, the team was then able to estimate the molar refractivity of the POSS compounds to within one percent for those compounds that are liquids. The measurement of the value for POSS solids is more difficult and will be pursued at a later date.
The team further investigated the behavior of three POSS compounds (dodecaphenyl, octaisobutyl, and octamethyl POSS) with the potential for compatibility in dimethyldiphenylsiloxanes by direct mixing with heating, and also by co-precipitation from a common solvent. In each case, the team found that the POSS materials have limited solubility in relatively low molecular weight siloxanes and did result in homogeneous and transparent mixtures as was anticipated. Thus these materials are not useful for optical materials purposes.