[nradov]

Theoretically what could work is to flip things around. Instead of drawing blood from the patient and analyzing it in a lab, miniaturize the lab and implant it in the patient. That way the sensors have continuous direct access to blood or other fluids.

Obviously there are huge challenges to designing such an implanted medical device, and it could only ever work for a limited set of assays, but I expect something like that will eventually be possible. We already see diabetics wearing continuous glucose monitors, which gives a preview of how such implanted sensors might operate.