it isn’t weak at all. where in your environment are you going to accidentally bump into a 100-1k Hz 20mT magnetic oscillation? any electronic device you encounter about is going to have less than uT fields even in contact. an induction cooktop might reach 1mT in contact, but 10s of kHz, and of course quadratic falloff.

what makes you think the coating is depletable? my read of the paper is that these nanoparticles are extremely sturdy. when they recovered them from the sacrificed mice there was no degradation observed.

of course, that doesn’t address your question of removal. the particles are present in cells that are injected into the target tissue. presumably the cells could be engineered to flag themselves to the immune system for destruction and clearance, then you just need to see how many are left over after you centrifuge your study mice.