https://newatlas.com/soundbrenner-core-wearable-metronome/56599/

Even freeform jazz musicians need to keep time. You could opt for a stomp-based rhythm machine like the excellent BeatBuddy to help keep your playing tight when no-one else is around, or even go old school and plump for the click-click of a metronome. Soundbrenner has updated the latter with the Core, a wristwatch-like wearable that vibrates to mark time.