My team and I built this. We tried to make it work on smartwatches, but necessary Hz, battery life, and UX limitations (ie: being in a "fitness session" to access necessary sensors) all proved to be an inadequate user experience. So we just created dedicated hardware and firmware.
Check us out here: https://immutouch.com/
Here's TechCrunch's write up on us: https://techcrunch.com/2020/03/09/dont-immutouch/