To expand on one aspect of hostile, some white blood cells greet foreign bodies with micromolar concentrations of a variety of oxidants and radicals: hypohalous acids (not just the familiar household bleach, hypochlorous acid, but also the nastier hypobromous acid), peroxides, superoxide, nitroxides, maybe even hydroxyl radical.
Realizing that, it's easy to see why autoimmune diseases are so devastating.
Of course, it's chemical warfare down there. Using strong acids and the like to destroy bacteria and virus structures is something I'd heard about, at the microscopic level, but I'd absolutely never thought about its implications on a macro scale before.
Presumably they sheath these devices in inert plastics or something in an attempt to counteract all this?
We do. Silicone, gold, porcelain, and titanium are actually pretty good for being bio-inert as well as many many other compounds. The metals obviously come with many issues too (heav metal poisoning, weight, different elasticity, etc) and the plastics are not as durable or tough as the metals. Also, these things tend not to be 'squishy' so impacts and wear and tear are tough on them.
Fun fact: Gold teeth caps are really good for you, though maybe not your love life. Gold has very very similar mechanical parameters to your dentin.
Overall, bioengineers are making great strides into these compounds. We are developing new heart stents that coil and expand in response to heat or can just be injected and will solidify and expand wherever they are needed. A cool development sector is in 'smart' hip replacements. They have nano-pores in the titanium to induce bone growth into the implant and create a better and more healthy bond.
Realizing that, it's easy to see why autoimmune diseases are so devastating.