Engineering a Light-Attenuating Artificial Iris.

The human iris provides eye color and facial symmetry while serving a central role in maintaining visual input to the retina. Using the sphincter and dilator muscles, the natural iris quickly and dynamically alters the size of the pupil, allowing for a 30-fold change in light transmission to the retina.1,2 Excessive light exposure associated with diminished pupil constriction can lead to damage of retinal photoreceptor cells causing a decline in vision over time.3 Congenital iris defects, such as aniridia or iris coloboma, as well as trauma-induced iris defects, are the two main contributors to reduced functionality of the iris.4,5 Albinism, in which patients lack the melanin pigment that provides opacity to the natural iris, can also create a larger area through which excessive light can enter the eye.6 The reduced adaptive response of the iris to light may result in photophobia, glare and haloes, double vision, and decreased visual acuity.7 Treatment options to alleviate the adverse effects of iris damage are limited to wearing sunglasses, colored contact lenses, or artificial iris intraocular implants. Although not approved by the Food and Drug Administration (FDA), Morcher GmbH (Stuttgart, Baden-Wurttemberg, Germany), Ophtec BV (Groningen, The Netherlands), and HumanOptics AG (Erlangen, Bavaria, Germany) fixed-diameter artificial iris implants have been used sparingly in the United States under the FDA's compassionate care clause.8 Morcher's and Ophtec's implants for partial or complete aniridia use a fixed colored disc composed of polymethylmethacrylate (PMMA), a mechanically stiff polymer, which solely blocks peripheral light entering the eye.9–11 In contrast to the hard polymers used by Morcher and Ophtec, HumanOptics' artificial iris lens is made of a flexible silicone polymer that allows for a smaller incision site.12 Including an image of a natural iris allows the HumanOptics implant to be used as a cosmetic option to restore or change iris color. However, its fixed pupil size and thus invariable light transmittance fails to dynamically reduce bright light entering the eye.13,14 Morcher, Ophtec, and HumanOptics' artificial iris implants have been shown to decrease glare and photophobia as well as increase visual acuity, but postsurgical complications include decentralization due to migration of the implant, inflammation caused by repeated contact with peripheral tissues, and anterior uveitis.15–17 Additional undesirable effects include secondary glaucoma, myopic shift, and a ghosting phenomenon causing double vision.18,19 Furthermore, despite their success in reconstructing the shape and color of the natural iris, all three implants have a set pupil diameter and amount of light transmitted, thus lacking the ability to adapt to changes in incident light. One important criterion for improving the current treatment options would be to provide the flexibility to attenuate the incident light dynamically. The development of photo-responsive artificial iris implant should mimic the functionality of the natural iris while restoring iris color and shape. A well-designed artificial iris lens would continuously monitor and reversibly decrease the transmission of light based on the incident light intensity. For example, an increased intensity of ambient light would result in increased activation of the photosensitive component, which should absorb and block transmission of a higher percentage of the bright light, thereby protecting the sensitive photoreceptor cells of the retina. To overcome a major limitation of current iris defect treatment options, rapid activation and reversal times of less than 1 minute would be preferred as well as the synthetic and biocompatible lens materials. Photochromic or photo-responsive materials are activated by light to undergo a change in molecular conformation. This change switches between a colorless more transparent state and colored opaque state associated with decreased light transmission.20 Photochromic materials have traditionally been used for nonmedical applications, including memory storage systems, semiconductors, and light-based electric switches. In addition, these materials have been proven safe and effective as external ophthalmic devices such as Transition lenses. Incorporating a photo-responsive material encased within a biocompatible inert polymer without leakage provides an improved means of fulfilling the design criteria. In this study, we present a novel artificial iris that combines photochromic and polymer components to create an ophthalmic device that partially mimics the natural iris and can have long-term implant stability in the eye, which is a major advance over current commercial artificial irises.