https://www.theverge.com/2018/5/30/17411268/cornea-transplants-3d-printing-human-cells-eyes-science

Scientists have 3D printed the thin protective film over the eye, called the cornea, using human cells — and it’s the most advanced version of an artificial cornea yet. Should the technology improve, it could help millions of people see again.

It was tricky to find the right recipe for an ink that’s thin enough to squirt through a 3D printer’s nozzle, says Che Connon, a tissue engineer at Newcastle University who was one of the creators of the artificial cornea. This bio-ink didn’t just have to be thin — it also had to be stiff enough that it could hold its shape as a 3D structure. To get the right consistency, the researchers added a jelly-like goo called alginate and stem cells extracted from donor corneas, along with some ropy proteins called collagen.