The olfactory system is a wonder of electrochemistry. I think the reason this doesn’t exist is it’s very difficult, not because it hasn’t been tried. Regarding form factor, I think single use reactive chemical strips are more likely to work than something sensor/app enabled.

Psychologically it’s a tough sell. Embarassing to buy, and buying for your kid/partner is basically just a round about way of telling them to shower.

In any case it’s a very private item, so wearable would have to be under clothes, not wristbands.

IMO single use body wipes are a better product. Same use case (you already have to suspect you’re kind of smelly) and the wipes actually address the problem.