And now, thanks to this light-sensitive tissue, it can do something powerful—make heart cells beat on command. Most importantly, this technique doesn’t require any genetic changes to the cells.

What’s more, the device’s transparency offers even more advantages, say the researchers, who used light to monitor and control heart rhythm (optocardiography) in their rat subjects.

First, the heart is surrounded by a layer of fat. Will implantation involve penetrating this fat and anchoring the device to the muscle? Or will the device and its light source be delivered throug ...