I've had one for around 8 years now, but it has lost its charge. I will need to re-magnetize it at some point. Mine used to be sensitive as well but I haven't had an issue with that for several years. Smashing that finger though can be pretty rough! I've never heard of one of these magnets rejecting (I am assuming it is in a silicone sheath). I do currently have a transdermal horn that has been rejecting pretty hard for the past 5 years that I really need to do something about though.

Yea, hers is just a gnarly color right now. The silicone might have broken or her body is just not having it.

Rather than a solid magnet, I wonder if a ferrofluid tattooed into the skin would have a similar effect.

It would most likely poison you. These magnets need to be coated in a bio-compatible sheath in order to protect yourself. Your body will break down the magnet into a powder, which is never good.