The most practical medical purpose I’ve seen is as a prosthetic implant for people with brain/spinal cord damage. Battelle in Ohio developed a very successful implant and has since received DARPA funding: battelle.org/…/battelle-led-team-wins-darpa-award…. I think that article over-sells the product a little bit.

The biggest obstacle to invasive brain-computer implants like this one is their longevity. Inevitably, any metal electrode implanted in the brain gets rejected by the immune system of the brain. It’s a well-studied process where a glial scar forms, neurons move away from the implant, and the overall signal of the device decreases. We need advances in biocompatibility before this really becomes revolutionary.