A blind man sees his fiancée’s smile for the first time. Another walks around at night, navigating via streetlamps and headlights. Yet another reads his own name (and spots a typo). All three had lost their sight years before, as an inherited disorder destroyed the light-sensing cells of their retinas. But they had since been fitted with retinal implants that took over from the broken cells, sensing incoming light, and converting it into electrical impulses delivered to the brain. The devices are a long way from 20/20 vision, but they have nonetheless restored sight to those who had lived without it for years.

These retinal implants seem miraculous, but they have a major drawback: they rely upon a working optic