a biomaterial
A biomaterial is a material designed to work inside or alongside the living body — a hip implant, an arterial stent, a contact lens, a dental filling. Above all it has to get along with living tissue.
The key requirement is biocompatibility: it must not be toxic, must not be rejected, and should be mechanically well-matched to the tissue it works with. Common examples are titanium (it integrates with bone and resists corrosion thanks to a stable oxide film), 316L stainless steel, cobalt-chromium alloys, alumina and zirconia ceramics for wear surfaces, ultra-high-molecular-weight polyethylene for joint cups, hydrogels, and bioresorbable polymers such as PLA that dissolve away once their job is done.
The honest caveat is that a stiffness mismatch causes real trouble: a titanium implant is far stiffer than bone, so it carries the load the bone used to carry, and the unloaded bone slowly weakens and resorbs — a phenomenon called stress shielding. And there is no universally 'bioinert' material; compatibility depends on the site, the loads, and how long the material stays in the body.
A titanium hip stem is press-fit into the thigh bone. Titanium works as a biomaterial because a thin, stable oxide film keeps it from corroding in salty body fluid, and bone grows right up to it (osseointegration). But titanium is far stiffer than bone, so it carries load the bone used to; the unloaded bone slowly resorbs — stress shielding — a reminder that matching stiffness matters as much as being non-toxic.
A hip implant must be non-toxic, corrosion-resistant, AND mechanically matched to bone.
There is no universally 'bioinert' material — compatibility depends on the site, the loads, and the time in the body; a material safe as a bone plate can fail as a blood-contacting surface.