Not hard to imagine but very hard to do. There are several huge challenges:
- Power. Contact lenses are too small to hold batteries that will last for a reasonable period of time. You could possibly use inductive coils in conjunction with regular eyewear but that means a fair electromagnetic field aimed right at your eyes and some obvious health risks.
- Optics. This is actually 2 issues - First, you can't focus on anything that's literally right on your eye so you'd need to do something very clever here. Possibly direct laser projection from the lens to the retina. Second, all the electronics in the lens will be opaque and could occlude your regular vision. Let's suppose you can move most of it to the outside of the lens (as with coloured contacts now), the HUD part is still going to cover part of your pupil. So it needs to be really tiny and still produce a clear image. Not impossible maybe but very hard.
- Health. Less of a showstopper, but contact lenses need to breathe. Impeding the flow of oxygen to the surface of the eye can lead to corneal neovascularization - the growth of blood vessels over the cornea. So ideally these contact lenses need to be gas permeable as well.
Very hard they are. But that's what we have engineers for :).
See what you just did: you outlined some real challenges we face if we want to improve this technology. And now we have something that is in domain of science and engineering, so we can discuss ideas and solutions. We're in a much better state than if we just left it on "it's hard to imagine" or "it's impossible".
- Power. Contact lenses are too small to hold batteries that will last for a reasonable period of time. You could possibly use inductive coils in conjunction with regular eyewear but that means a fair electromagnetic field aimed right at your eyes and some obvious health risks.
- Optics. This is actually 2 issues - First, you can't focus on anything that's literally right on your eye so you'd need to do something very clever here. Possibly direct laser projection from the lens to the retina. Second, all the electronics in the lens will be opaque and could occlude your regular vision. Let's suppose you can move most of it to the outside of the lens (as with coloured contacts now), the HUD part is still going to cover part of your pupil. So it needs to be really tiny and still produce a clear image. Not impossible maybe but very hard.
- Health. Less of a showstopper, but contact lenses need to breathe. Impeding the flow of oxygen to the surface of the eye can lead to corneal neovascularization - the growth of blood vessels over the cornea. So ideally these contact lenses need to be gas permeable as well.