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I discovered the purpose of what looked like an NFC tag in my toothbrush heads. More expensive smart versions of my toothbrush log how many times the head is used, and start nagging about replacement after a while. Unlike with printer ink, it at least doesn't disable old toothbrush heads.
https://hackaday.com/2023/05/27/hacking-a-smart-electric-toothbrush-to-reset-its-usage-counter/
It's just a rewritable NFC tag in the head. The nagging happens via a mobile app the brush connects to.
I took one head apart out of curiosity when I first got this thing, and it's not much. But here's the insides of my current head anyway. Not like my dumb toothbrush needs it.
The aluminium ring is just for weight, or support.