How do magnets fail over time?
So I got a TiN coated finger magnet implanted at the same time as a friend (~6m ago). They didn't reject and are working well. As my friend learned about the potential for failure, she has become very concerned about health complications. I want to organize more details about the exact series of events that occur when a magnet breaks down to reassure her that a tiny sliver isn't going to break off, travel to her heart and kill her.
I'm familiar with the coating structure of this type of magnet (Nd - Ni - Cu - Ni - TiN). As I've been searching through the forum I have been trying to piece together what exactly happens to the magnet mechanically when the liquid in your body degrades the coating and makes it to the sintered neodymium core. I'm also interested in warning signs that indicate it's time to get the magnet removed.
Can anyone furnish me with a timeline of events?
I'm familiar with the coating structure of this type of magnet (Nd - Ni - Cu - Ni - TiN). As I've been searching through the forum I have been trying to piece together what exactly happens to the magnet mechanically when the liquid in your body degrades the coating and makes it to the sintered neodymium core. I'm also interested in warning signs that indicate it's time to get the magnet removed.
Can anyone furnish me with a timeline of events?
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So far my understanding is:
1) Tiny surface imperfections in the coating allow your salty bodily fluids to penetrate the magnet and gradually degrade it's structural integrity.
2) A potential energy difference can form between the outermost TiN layer and the adjacent Ni layer speeding up the degradation process.
3) If there's pain, redness, or swelling remove the magnet.
4) If you notice a gradual reduction in the strength of the magnetic field, remove the magnet because it's starting to break down.
5) Rejection =/= Failure.
6) Time periods for the lifespan of a magnet vary
Please let me know areas where I'm incorrect or there are gaps in my understanding.
As much as I would like to assure you it's impossible for it to migrate to her heart, I can't. It's incredibly unlikely, and will almost definitely not kill her, but subdermal migration is certainly a real concern with any kind of implant. The likelihood of it making to the heart is like winning the lottery, and such a small object likely won't restrict any biological processes.
Migration is not your primary concern however, the biotoxicity of the NdFeB (Neodymium Iron Boron) when dissolved by your immune system during a rejection. Typically it's obvious enough that it's been rejected that the implant can be removed, and I haven't found any evidence showing it's life threatening to have it dissolve. To be fair I haven't seen any evidence on the contrary.
Look into a few academic databases and see what studies come up, i'll do some more research later.
You are more or less correct. However i'd like to expand on the "Potential energy difference." This is called galvanic corrosion (Among other names, but this'll work) and depends on the nobility of the metals. One highly noble metal in contact with a low noble metal will increase the galvanic corrosion, and the ratio of cathode to anode contact also matters. Your body is filled with electrolytic compounds, and when dissociated within a solution (i.e. blood) it creates ions and anions (+ and -) and can carry a current. This current can cause a small amount of energy to go through the magnet, and depending on the composition, can cause galvanic corrosion to varying degrees.
Galvanic corrosion is certainly a concern regarding the long term viability of a magnet, but it is generally a gradual process. It's not something you should lose sleep over, but it is something that should be kept in mind particularly when creating your own magnets. The thing you should concern yourself with are the tiny imperfections within your coating as you mentioned.
One last thing, the implant requires an immunologically inert or biologically inert/biocompatible (There's a lot of terms, effectively synonyms) coating because of your immune system attacking it. Your immune system sends T cells to fight the foreign object just like an infection, and does a great job of tearing down materials and/or pushing them out of the skin (rejection.) The goal is to prevent an immune response in the first place, not counteract it.
My friend has now seen this post and is much more comfortable with the prospects. There were too many unknown unknowns. We'll both keep an eye out for signs of failure and I'll try to keep up with the development of coatings as we asymptotically approach 0% chance of failure. Maybe TiN+parylene will be the one, if we get the procedure down.
You had mentioned doing more research about magnets dissolving, and I'm also curious. Even with lifting magnets, it's such a small amount of NdFeB that it likely doesn't matter. We mine as well know for sure, though.
All the relevant studies I found were >15 years old and of limited value (our use case is pretty obscure). I did find this recent article that catalogues a host of studies relating to medical uses of neodymium magnets, if anyone is interested:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6323575/
Based on the limited data I have seen, I suspect toxicity isn't a major concern. The more immediate problem would be the localized damage a fragmented hunk of metal will do to your fingertip, which isn't a big deal if handled promptly.
While I am definitely not suggesting we go out and kill a bunch of mice it would be interesting to know the ld50 of NdFeB and its effects at different levels in the body.
I had a coating fail and wound up with bits of magnet I couldn't get out sitting around in my finger for half a year. No symptoms of metal poisoning, it didn't appear to spread out much (I'm very pale and the particles were visible through my skin), the dissolved magnet formed a small lump of tissue around it, and the doctor who eventually took it out wasn't worried about any kind of systemic issues.
@dr4gon
I don't think toxicity is something we have to worry too much about. With the sheer quantity of NdFeB we'd have to pump those mice full of to get data on organ failures and such, they'd probably have other issues first.