I'm looking for an NFC sticker that can be stuck onto someone's rib, that can be used to detect the motion of the rib. This would be used to work out breathing patterns and abnormalities to enable a device to send an emergency alert in case of breathing difficulties. It must be compatible with Android.
Sorry, this grew too big to be a comment.
NFC does not give any indication of distance. Weeeell, I suppose that theoretically you could try to measure signal strength - from expansive active RFID, not NFC, tags, not cheap passive tags - but that is absolutely not off the shelf & would require a great deal of expertise, eve if it is doable.
BLE 5.1+ has the option of Angle of Arrival/Departure which allow location to within 1cm (which might also mot be accurate enough), so that seems to be your best bet.
My question BLE 5.1 wearable dev board has received no answers, though :-(