Hexoskin
Wrist trackers, while comfortable, are notoriously inaccurate when it comes to heart-rate tracking. That's fine if you're just trying to get a general idea of how many calories you burned in a single workout, but not good for serious training.
Heart rate monitors that strap around your chest are far more accurate, but are less comfortable particularly for long training sessions or vigorous sports.
A comfy workout shirt with the sensors built in? Brilliant!