Technological Challenges in Engineering a Retinal Implant

The artificial retina consists of an electrode-studded soft polymer array (shown) that is tacked to the retina inside the eye.

Researchers face numerous challenges in developing retinal prosthetic devices that are effective, safe, and durable enough to last for the lifetime of the individual.

The retinal device bypasses the eye's lost light-gathering function of the rods and cones with a video camera. The information captured by the camera is used to electrically stimulate the part of the retina not destroyed by disease. Stimulation is done with a thin, flexible metal electrode array that has been patterned on soft plastic material similar to that of a contact lens.

The device must be biocompatible with delicate eye tissue, yet tough enough to withstand the corrosive effect of the salty environment. It should remain stably tacked to the retinal macula and not overly compress or pull at the tissue--the resilience of which can be compared to that of a wet Kleenex®. The apparatus also needs to be powered at a high enough level to stimulate the electrodes, yet not generate enough heat to damage the remaining functional retinal cells.

Increasing Resolution. These images aproximate what patients with retinal devices ideally could see. It is hoped that increasing the number of electrodes will result in more visual perceptions and higher-resolution vision. Click on photo for larger image. [Credit: California Institute of Technology]

Moreover, just as the resolution of graphic images on a computer screen improves with greater pixel density, researchers assume that increased electrode densities will translate into higher-resolution images for patients. However, the area of the retina targeted for electrical stimulation is less than 5 mm by 5 mm. Consequently, as the number of electrodes increases, their size and spacing must decrease.

Furthermore, image processing needs to be performed in real time so there is no delay in interpreting an object in view. Development of effective surgical approaches is also critically important to ensure a successful implant (see sidebar, Ensuring Surgical Reproducibility, below).

In other words, engineering a retinal prosthesis is somewhat analogous to constructing a miniature iPod® on a foldable contact lens that works in seawater.

Promising Advances
Three devices of increasing resolution are now in testing or development. Several pioneering technological advances in fabrication and packaging by DOE national laboratories (see Artificial Retina Team Garners 2009 R&D 100 Award) have helped make the third-generation device a reality. For example, the significantly higher electrode density required new lithographic and etching techniques to pattern platinum films on soft polymer materials. Additionally, new stacking techniques were needed to layer metals and polymers on top of each other to achieve a narrower prosthesis in the section that delivers electrical charges to the electrodes. Other advances by DOE national laboratories include softening the rim of the array that is in contact with delicate retinal neurons and improving techniques to ensure none of the electrodes short-circuit.

Because the device’s advanced electronics operate in the eye’s saltwater environment, DOE’s national laboratories have spearheaded critical innovations in packaging. These innovations include higher interconnect densities of the individual contacts and attachment to the electronic chip while still preserving a leak-tight electrical connection. A novel, dual-sided integrated circuit also has been developed for stacking components and interconnecting them (see Lab Spotlight: Sandia National Laboratories). Research also is under way on various bioadhesives that could be used to attach the microelectrode array to the retinal surface.

Fabricated 200+ thin-film electrode array. The metal traces forming the electrodes in the artificial retina are less than a micrometer thick—less than 1% the thickness of a human hair.Click on photo for larger image. [Credit: California Institute of Technology]

Work in Progress
As the DOE Artificial Retina Project moves forward, prospects for additional progress include developing a device with 1000 or more electrodes that can be scaled up using the knowledge gained in creating the 200+ model. Various advances and spinoffs from this work already are beginning to pay off in other biomedical applications as well as in a wide range of hybrid surveillance systems, including environmental sensors, and for plant and bacteria studies (see story, Artificial Retina Project Spurring Spinoff Technologies).