Looking at Rockley stuff in particular makes me hopeful this could be solved, if it's solvable with spectrophotometry at all. I'd even be OK with something larg

Looking at Rockley stuff in particular makes me hopeful this could be solved, if it's solvable with spectrophotometry at all. I'd even be OK with something larger, perhaps ankle-worn, as long as I don't have to puncture my skin at all.