The current trend toward small incision cataract surgery has resulted in the use of elastomers as intraocular lens materials. Little has been published on appropriate methods of evaluating biomaterials intended for implantation in the eye. We present a methodology for such a study and report the results for two elastomeric silicone intraocular lens materials. The chemical, optical, and mechanical properties of the two materials were evaluated, as was their stability to hydrolytic and ultraviolet degradation. Qualitative correlations between these properties and clinical requirements are discussed. Both silicone materials possessed the necessary properties for use as small incision intraocular lenses.
This report describes the results of in vitro accelerated hydrolytic and ultraviolet aging studies performed on SI-18NB and SI-20NB silicone intraocular lenses. The hydrolytic aging study simulated the effects of 20 years in vivo. The ultraviolet aging study simulated the effects of 17 years in vivo. No significant changes in the focal length and resolution of the lenses were observed. Examination of the lens surfaces using scanning electron microscopy revealed no changes in surface morphology.
Using the internally placed elastic membrane and multi-chamber configuration, we designed a digitized mini optofluidic element for fast switching between refractive and diffractive states of preset optical powers. Relief surface was used in the diffractive state. We applied finite element analysis to establish membrane mechanical characteristics for switching at the force level produced by the ocular elements such as ciliary muscle or lower eyelid at eye downgaze. The prototypes were made to demonstrate proof-of-concept. Membrane conformance to the diffractive grooves and imaging quality were demonstrated. The analysis supported switching under the force level exerted by the ocular elements supporting the digitized optofluidic element potential for presbyopia correction by ophthalmic lenses.
A transparent poly (ether urethane) (PEU) was considered for use as a foldable intraocular lens material. The PEU was found to possess excellent mechanical, optical, and surface characteristics for this application. In vitro hydrolytic and ultraviolet aging studies suggested the PEU to be tolerant to conditions simulating 3-10 years of normal intraocular exposure. Different behavior was obtained, however, from intraocular and subcutaneous implantation of the PEU. After 6 months of intraocular exposure in the feline model, prototype PEU lenses had lost most or all of their optical resolving power. SEM analysis demonstrated scattered pitting and cracking on the lens surfaces. Degradation was found to be more extreme after as little as 30 days of subcutaneous exposure in rabbits. Severe pitting over the entire surface of implanted flat PEU specimens was observed by SEM. Macroscopic examination showed the samples to be frosty in appearance. It was postulated that the subcutaneous implant environment provides an accelerated in vivo model for materials intended for intraocular use. A minimum acceleration of 6-10x was estimated on a preliminary basis. The PEU studied here was found to be unsuitable for use as a foldable intraocular lens material.
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