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what are the problems and the health risks with partially exposed implants?
your body usually rejects things that are partially exposed, if it can. this is a common problem in the body mod community with dermal punches and many transdermal piercings. basically anything that uses a surface barbell will reject in most people given enough time, and should be considered a temporary mod. this isn't painful or dangerous, the body will just slowly grow new tissue under the offending object until it falls out completely, but it might leave a nasty scar.
that's really the least bad thing that can happen. you also run the constant risk of infection, as you are giving bacteria a convenient way to bypass one of your body's best defenses.
things that are implanted more deeply, beneath all the layers of skin, can't be rejected, and also grant even more lucrative and dangerous access to the inside of your body from the outside world. you want to die of blood poisoning? this is how you die of blood poisoning.
The skin does not fuse to the implant so there is always an opening into your body. People have been trying to get around this but looks like its still a work in progress.
The keyword to search for is transdermal.
HERE is a thread where they tried a coating experiment to have the skin grow onto the implant and make a seal.
Unfortunately there is no conclusion in that thread, but its a good start.