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Rewriting Life

A Sharper Future for Retinal Implants

Researchers grow neurons on a photosensitive polymer hybrid.

Retinal implants can already restore sight to people who have lost it owing to degenerative eye diseases like macular degeneration or retinitis pigmentosa. Now new research suggests a way to make higher-quality, more biocompatible retinal implants by integrating living neural cells with a soft organic polymer semiconductor.

Green light: Hippocampal neural cells (stained green with a fluorescent dye) are grown on a light-sensitive base.

A retinal implant restores vision by sending a signal from a video camera attached to a pair of glasses to electrodes implanted on the back of a person’s retina. But the silicon or platinum components typically used to for the electrodes tend to produce images of limited quality, and can leave the retina scarred.

Organic semiconducting polymers are softer and more flexible than silicon, and they have useful mechanical and electrical properties, making them ideal for biomedical applications. In fact, they are already used in some medical devices such as glucose sensors and the electrodes that record neural activity in the brain. Researchers at the Italian Institute of Technology have now shown how organic polymer could be used to make better electrodes for retinal implants.

One of the first possible applications for a polymer-neuron interface could be in optogenics, says Guglielmo Lanzani, a professor at IIT who led the research, which is published in the January 18 issue of Nature Communications. “Our short-term goal was to establish communication between an [organic] semiconductor and a neuron,” says Lanzani. 

The most advanced existing retinal implant, called the Argus II, is made by a company called Second Sight, and is currently undergoing clinical trials. The signal from the camera activates 60 electrodes on a chip, and these in turn stimulate neurons in the retina, causing an image to form in the brain. But the images that the Argus II produces are blurry, and users can only make out rough shapes and read large letters. “[The platinum electrodes are] very thin, they’re very flexible, but they’re not this lovely, soft substrate,” says Gerald Chader of the Doheny Vision Research Center and a collaborator at the Bioelectronic Research Lab at the University of Southern California, who was not involved with the new study.

The Lanzani lab grew neural cells in a petri dish directly on top of the polymer. Light shined on the polymer activates the photodiodes, which stimulate individual neurons much the way light-sensitive photoreceptor cells in a healthy eye cause neurons to fire. (In contrast, the Argus II stimulates up to hundreds of thousands of neurons at one go.) Developed further, the Lanzani approach could lead to a retinal implant that produces much clearer vision.

The researchers grew rat embryonic hippocampal neurons on a substrate consisting of photodiodes made of indium tin oxide coated with a layer of photoconducting organic polymer, and another organic layer that functioned as an adhesive. The rat neurons and the diodes were immersed in an ionic solution. The diodes, when activated by light, triggered a charge imbalance in the ionic solution, which caused the neurons to fire.

Next, Lanzani plans to use printing techniques to position the photodiodes on the device precisely, more closely mimicking the geometry of the photoreceptor cells in the human retina. He says the photodiodes could be color-sensitized, to produce images in color rather than just black and white.

James Weiland, another expert on retinal prostheses at the University of Southern California, calls the device a “good first start, one that is worth pursuing further.” He notes that the flexibility of the substrate means the material would fit better on the retina.

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