Noninvasive, with any sort of accuracy that can be used for anything, is most likely impossible.
So back to "invasive"... what can we do... embedding a device under your skin, right around where your watch goes, that can be powered and communicate with the watch. While also making sure the device is not rejected by the body and stays in place.. AND has no other adverse side-effects. I think this is where it'll ultimately end up for the people who want it. sub 10 min procedure to put it in and should last for quite some time (maybe dissolves after 5 years for upgrades?)
My understanding is this isn't possible because of the size/invasiveness.
We need significantly better power sources that are either have lots of energy capacity, xor some energy capacity and able to charge while inside the body.
Either version they need to be small enough that the procedure is cheap and the body doesn't reject the device. Additionally, anything like this would need to go through regulatory bodies for medical devices, and would not be approved for "healthy people" (i.e. significantly reduced adoption).
But then youre still dealing with measuring it penetrating the skin and all the variables that have to with it - sensor position, changes in temperature and blood flow, etc.
We do it with pulse ox pretty well, but +\- 2-3% O2 sat and repositioning isnt a big deal.
Having the watch move and misreadibg blood sugar can have serious consequences.
Its a valid ideal but then you get into the fun world of toxicity, pharmacodynamics, dosing, inter-patient variability, compliance, drug-drug interactions.
