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I will be coating it with polymethylmethacrylate if I get them
Where are you going to be putting it is what will determine if it is too big. If you are going to do finger I would say it is too big at almost 10mm.
I've placed larger but I certainly don't advise doing so. The thing is, as the size of the magnet increases, the likelihood of tissue damage occurring increases. Because it's so large anything that you hold will crush the tissue in between. It's not instantaneous or anything at that size. It's just something to consider.
Another consideration is that while Au has a long record of safety, many many grinders have tried a gold coated magnet and had it fail. Some hypothesize it's because the gold is so soft. It's always interesting to get more data though. Has anyone here had success with a gold only coating? As far as the PMMA goes, what crosslinking agent are you going to use?
Given that the average persons finger in only about 16-20 mm wide I think that 10 mm after the coating process is very large if the intent is to put it in a finger for sensing for my liking at least. If you are looking at using it is a lifting magnet in the palm it might not be too big. For comparison the Sense52 is 3.5 mm