Well, there's a useful way to recycle them, offered by Tufts University professor Lisa Gulatieri. As noted in this article:
Gualtieri started RecycleHealth in April with the goal of giving unused activity trackers — mostly Fitbits so far, but RecycleHealth accepts all devices — a second life. The company has collected about 20 devices so far and has plans to donate them to the Montachusett YMCA in Fitchburg, Massachusetts, where they will be used to help older and lower income individuals have access to devices, as well as to learn about how those populations interact with activity trackers.
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