Life-like prosthetic eyes: the call for smart materials

Ocular prosthetics, or artificial eyes, are designed to aesthetically replace missing, damaged or underdeveloped eyes. Realism is a key consideration in the design of ocular prosthetics, as this benefits the confidence and comfort of the wearer. Ocular prosthetics are generally handmade and bespoke, a necessity brought about by the variation in form as a result of the varying needs of the wearer. Ocular prosthetics can, however, range in style from standard manufactured pieces that may be ordered remotely to pieces designed and handmade over the course of a number of consultations between the maker and the wearer. The resulting prosthetic can be extremely realistic, but problems remain in relation to how the prosthetic is used in practice. One such problem is that the pupil is inevitably fixed both in position and appearance. In varying light conditions, a real eye will vary by its pupil varying in size. This can result in a user’s real eye having a differently sized pupil to that of the prosthetic, which reduces the realism of the prosthesis. Light-reactive ocular prosthetic eyes have been proposed, for example, by Leuschner and Lapointe et al. [1,2], in which a liquid-crystal display or light valve is used to simulate a varying pupil. These proposals may, however, result in additional problems such as a lack of contrast between the pupil and the surrounding iris, which can affect the realism of the resulting prosthetic, as well as the need to provide a complex array of addressable patterns to reproduce a pupil of appropriate size and shape, which may conflict with the need for a customized solution in each individual case. Other forms of light-reactive ocular prostheses have been proposed, such as in US4332039 [101] disclosing a magnetically actuated pupil, in US6139577 and US6576013 [102,103], both of which disclose the use of a liquid-crystal display and in US5061279 [104], disclosing the use of a photochromic pigment for simulating a dilating pupil. The use of smart materials for medical products and applications is of course not a new idea; opinion varies, but it’s useful to perhaps provide a general definition for a smart material: ‘a smart material is one that reacts to a stimulus in a reproducible way, the reaction, in most cases, also tends to be reversible’. Interestingly, it is considered that a material cannot be smart by itself; the material must be part of a system. A smart system is a device or object that integrates smart materials, sensors and/or actuators, which will sense and then respond to any changes in the appropriate environment. A good example is provided by materials that are created to change their color at a particular temperature. Thermochromic behavior provides a useful visual response as a result of a thermal stimulus; a good but simplistic example is given where a bath plug changes color when the bath water is too hot. This can be described as a smart behavior where there is a material reaction as a response to a change in environmental conditions. As with all smart materials, no material can be smart in isolation – it must be a part of a structure or system. Smart materials are currently being investigated as a result of a joint research project Life-like prosthetic eyes: the call for smart materials