What do Sam Worthington, Peter Weller, and Will Smith have in common? They're all dreamy, and they all play characters in movies who have devices implanted into their bodies. In their films the only issues they seem to have with their artificial parts are emotional ones, but putting a foreign object into a human body without adverse physical effects is actually pretty hard to do.
Note the chiseled endoskeletal cheekbones.
Marcus (Worthington) in Terminator Salvation, is apparently the lone humanoid organism to have survived Skynet's experimentation to build a cyborg with a metallic endoskeleton. Why would this be so hard? (Other than the fact that a machine dominated society using humans as lab rats wouldn't really care that much if there were a lot of casualties.) Living tissue and many types of metals have problems getting along together.
YOU PUT WHAT, WHERE?
Metals for making cyborgs, or hip or knee replacements, have to be corrosion resistant and biocompatible. If you have an allergy to nickel, you sure don't want a nickel implant, or a nickel belt buckle, or a phone with nickel in it.
Metals shed ions. In order for the body to tolerate the metal well, it can't leach too much into the surrounding tissue. Although your body uses some metals for metabolism (copper, iron, magnesium, etc.), too much can cause toxicity. You don't want that metal plate in your head shedding too much of anything near your brain(1).
So you can imagine that corrosion resistant types of metals, plastics, or ceramics, would be a good idea. Using incompatible metals could interfere with normal tissue growth near the implant(2). Blood acidity and ph balance can have a corrosive effect on metals. If metals shed into the bloodstream, your whole body may have a (systemic) reaction. Organs that filter blood, like the liver and kidneys may accumulate unhealthy amounts of metal ions, which is bad. And don't forget cancer! You have to be careful that whatever you're implanting isn't carcinogenic 20 years after you've implanted it.
Cobalt-chromium alloys, stainless steel, and titanium have been reported to have minimal tissue reactions in rabbits, dogs, and people. If Terminator Salvation was going to be really accurate, we should have seen some rabbit cyborgs hopping around killing humans.
Although there are always many things to consider, many people enjoy their artificial joints (made of plastics and metals) or eye lenses (plastics), or boobs (silicone) and never have any problems. 300,000 women per year receiving breast implants can't be all wrong, can they(3)?
Once you find biocompatible materials to implant in the body, you also want them to last. Fatigue resistance is important. You don't want parts to break or bend like a lug nut on an airplane wing.
If we're talking about hip replacements, "... chrome is favoured if tensile and fatigue strength are required, titanium is favoured if load sharing with adjacent bone (uncemented prostheses) is required (titanium has a similar modulus to cortical bone)(4)." So it really depends on what you're going to do with that hip and whether you glue the prosthetic, or just jam it in there really tight.
Bo Jackson, cute enough to be a cyborg, and a super-human athlete.
The metal and the bone it's embedded in, making up the structure of an artificial joint, are able to withstand normal physical stresses. Professional athletes with joint replacements often end up with complications because they put super-human stress on their bodies. The metal may bend, the bone holding it may break, or the prosthetic starts to work its way lose. For this reason, RoboCop with his combination of exoskeleton and implanted sensory parts, Spooner and his entirely prosthetic limb, and Marcus with a full body endoskeleton are the creme de la creme of the Hollywood Cyborgs.
1. Which is why they use titanium or niobium in the skull.
2. This can lead to chronic inflammation and scarring in the area.
3. Women with breast implants do not qualify as cyborgs.
4. Page 394 from Imaging of the Hip and Bony Pelvis: techniques and applications, by Arthur Mark Davies, Karl J.Hohnson, Richard Wihehouse. Read it on Google books, otherwise it costs over $200!