Med-Tech Innovation Smart materials
prosthetic is used in practice. One problem is that the pupil is inevitably fi xed both in position and appearance. In varying light conditions a real eye will vary because of 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.
A liquid crystal display was proposed for
light-reactive ocular prosthetic eyes by Leuschner and Lapointe et al.3,4
in which a liquid crystal display
or light valve (a device for varying the quantity of light) 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 surrounding iris, which can affect the realism of the resulting prosthetic. In attempting to create a user customised solution, the requirement to provide a complex array of addressable patterns to reproduce a pupil of appropriate size and shape also needs to be addressed. Other forms of light reactive ocular prostheses have been proposed. For example, LaFuente5
utilises a magnetically actuated
Figure 4: Responsive prosthesis – graphical representation (Worrell 2013)
the bulk of the product cost, and a disposable bougie that can be manufactured for less than £20. The ultimate aim for this device is for the bougie section to be used routinely in clinical practice, with the additional option to attach the controller element if the airway proves too diffi cult to manage with the bougie alone. Initial prototype testing has shown that the device has the potential to deliver a signifi cant increase in diffi cult airway management success rate, whilst achieving a low per use cost for the NHS. The production of fully working prototypes presents a number of technical and engineering challenges that need to be overcome. These include determining the optimum balance of proximal rigidity and distal fl exibility in the body of the device, and working out the most effi cient manufacturing method to incorporate all the necessary elements within the core of the bougie.
Because this device fi lls a unique and sizeable gap in the existing market, the product is an ideal candidate for development through to full market release.
Prosthetic eyes
In the second case study smart materials are being used to create life-like prosthetic eyes. Realism is an important consideration in the design of ocular prosthetics, which are generally handmade and bespoke, a necessity brought about by the variation in form as a result of the wearer’s needs. They can range in style from typical manufactured pieces that can 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
14 ¦ September/October 2014
pupil; Schliepman et al. and Budman et al. describe the use of a liquid crystal display;6,7
and Friel incorporates a
photochromic pigment to simulate a dilating pupil.8 Smart materials are currently being investigated as
a result of a joint research project being undertaken by Nottingham Trent University and Refl ex Systems. The research project is based on developing a Light Reactive Ocular Prosthesis that employs an electroactive polymer technology (EAP) and utilises the EAP to the create the image of a dilating and contracting pupil. Figure 4 provides a simple graphic representation of the proposed device.
EAP technology has possible applications in various
fi elds, for this research application the visual impression of dilation can be achieved. When applying EAP technology to the visual representation of a dilating or contracting pupil, the technology is integrated by stretching a transparent elastomer that is encapsulated under tension. The elastomer is encapsulated between two rings of the same size and thickness. Once encapsulated, a circular fi lm of a conductive paste can be painted onto the transparent stretched elastomer; this will represent the artifi cial pupil. When a voltage is applied to the conductive paste, the “black dot” that represents the pupil can be made to appear to dilate. The functionality to visually represent dilation and contraction of an artificial pupil is provided by an elastomer with a dielectric strength stretched between these two electrodes and acted upon by Maxwell pressure9
as a result of the presence of a high voltage
electric fi eld provided by the electric fi eld. Maximum input voltage to the prosthetic eye
microcontroller is 5V, which allows the system to be battery powered. Output voltage from the controller is applied to a subminiature transformer supplying a voltage of up to 1.5 KV to the EAP. This voltage is varied dependent on ambient light conditions; increasing the input voltage to the EAP provides the impression of dilation and decreasing the input voltage provides the impression of contraction.
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