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The photodiodes are made of a semiconducting polymer printed over a base of silver-containing ink

When he was an assistant professor at Princeton University, Michael McAlpine led the development of a 3D-printed bionic ear. Now an associate professor at the University of Minnesota, he has gone on to 3D-print a rudimentary bionic eye and it could eventually lead to versions capable of replacing the real thing.
A McAlpine-led team began with a hemispherical glass dome, similar in size and shape to the back of a human eye. Using a custom-built 3D printer, they then added stripes of an ink containing silver particles the ink successfully dried in place, as opposed to running down the inside of the dome and pooling at the bottom. Finally, over top of that ink base, a printed layer of a semiconducting polymer was added.
The result was an array of 3D-printed photodiodes, which are capable of converting light into an electrical current with an efficiency of 25 percent. McAlpine's team is now planning on boosting that efficiency, and incorporating many more of the photodiodes into a single dome. Ultimately, it is hoped that the technology could be used to create a fully-functioning bionic eye, which would restore a blind recipient's vision by stimulating their optic nerve in response to perceived light.
Additionally, the scientists are looking at ways of printing the photodiodes onto a soft hemispherical material that could be surgically implanted into the back of a patient's existing eye, where it would replace the retina.
Besides the earlier-mentioned bionic ear, incidentally, McAlpine has also brought us 3D-printed items such as skin-mounted electronics, guides for nerve regeneration, spinal-cord-injury "bridges," and model internal organs upon which surgeons could practice.
A paper on the research can be found in the journal Advanced Materials

Source: University of Minnesota