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It seems some very specific alloys like implant surgical steel and titanium don't cause inflammation in the human body, but when I asked my doctor about it, they simply said "it just has some chemical property that doesn't cause it."

So...what exactly is special about these metals compositions as to not cause inflammation like most other things would?

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    $\begingroup$ Surgical steel can cause some inflammation (basically everything can) - it's just far less inflammatory than other metals and many other materials. You may want to edit your question to avoid an attack from the pedants. I'd also suggest removing the x-files references, they're mostly just distracting and not important for your actual question if you want it to be about biology and not science fiction. $\endgroup$
    – Bryan Krause ♦
    Jan 31, 2018 at 16:54
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    $\begingroup$ Usually people irrationally complain if there's no context or apparent reason for a question. $\endgroup$
    – John Joe
    Jan 31, 2018 at 16:56

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Some can

First the terms you are looking for is biocompatibility and bioactivity. The study of them is called biomaterial science. Both fields are relatively new so complete answers do not exist, there has been a certain amount of trial and error in find suitable materials.

There are a dozen factors that go into these things, toxicity, whether they corrode in the body, whether they are soluble, chemical reactivity, whether they trigger an immune response, whether they can bond with bone or other tissues, mechanical resilience, steric and electrical properties, wetting, even how smooth it can be made can have an effect. The other important thing is many materials have very different biocompatibility depending how and where they are used, different parts of the body can have very different conditions, for instance material used in a bone screw might be useless in a dental implant (different PH and chemical exposure) or a heart valve (different mechanical and electrical requirements).

Titanium has remarkable biocompatibility with internal bone implant, its already low reactivity actually gets better as its surface oxidizes as it becomes slightly polarized and will incorporate many cellular surface molecules(passivation). Many of the factors that make titanium useful in this way are still being studied.

Surgical steel is a broad term that covers several types of steel used for multiple medicalpurposes, it includes steel implant and steel only used for instruments. Some forms of surgical steel work great for instruments but are rarely for internal implants, often because the same nickel content that makes them corrosion resistant is also bioreactive, triggering an immune response, but its superior mechanical properties means it still is sometimes used as implants, especially temporary implants were corrosion is less of an issue. (paper, sorry it is only available in German). Nickel free surgical steels used in cutting implements is stronger and harder (what you want in a medical blade) but without nickel it will quickly corrode in the body.

Part of the reason your doctor did not know was some aspects are still unknown in general and the study of these properties is rather specialized so a general practitioner may have never studied the parts that are known. Consider an pilot still know very little detailed metallurgy of their engine components, and still be an excellent pilot becasue they don't really need to know. Heck most of reason I know anything about it is I used to hang out with a guy who made titanium implant screws for a living and it just sounded kinda interesting so I read up on it.

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  • $\begingroup$ Coincidence ? Vitallium has been used in gas turbines since the 30's ( the first engines, and it still has a place in these engines.) $\endgroup$
    – blacksmith37
    Jan 31, 2018 at 22:33
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As far as I know "surgical steel" for cutting tools is never used for implants . And, reactions to metals are mostly allergys. Surgical steels are martensitic ( hard) chrome stainless , eg 440 A, B, C, and 420. Some surgical tools are 316 stainless which is also used for implants. Better implants are made of titanium usually with a titanium oxide surface ; bone will attach to the oxide surface but generally not to 316 SS. There is Ni in 316 stainless and some people are allergic to Ni. Anything with Cu would be a terrible choice as it has strong toxic properties; eg , it is used to clad ships to prevent barnacles . There are some other minor use of ,eg. Vitallium ( alloy of chrome and cobalt) and others.

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  • $\begingroup$ After looking at Johns' reference above, I see Vitallium ( aka. Stellite 21, aka cobalt/chrome) is becoming more common again .It is likely replacing some 316 SS because of nickel allergies. And is cheaper than Ti after you put on the plasma spray oxide . Ti is still the better choice , good bone bonding, lower density , lower stiffness - more like bone. $\endgroup$
    – blacksmith37
    Jan 31, 2018 at 22:29
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Surgical steel doesn't cause inflammation because (i) it has very little solubility, and (ii) what little iron ions do get dissolved into bodily fluids are chemically too simple to be recognized by the immune system and trigger an immune response. Moreover, the human body naturally contains small amounts of iron, e.g. bound to myoglobin in cardiac and muscle tissue.

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  • $\begingroup$ I would think a practicing graduate physician for 20+ years would know an answer like that off the top of their head, are you sure it's that simple? Copper metal itself is insoluble, and copper is also found in the human body, and copper metal has an extremely simple chemical formula Cu, and yet something tells me a copper implant is still going to cause inflammation if insert it into someone. Could it possibly be more related to the ligands it forms? $\endgroup$
    – John Joe
    Jan 31, 2018 at 17:23
  • $\begingroup$ Copper and iron have slightly different physical and chemical properties, but basically that's it. Steel is used because additives like silicon or carbon permit fine tuning of different physical properties like tensile strength and expansion coefficient + it's cheap and abundant. Most things you can achieve with steel you can achieve with titanium as well, but titanium is a lot more expensive, so you use it only when you really need to. $\endgroup$
    – Stefan
    Jan 31, 2018 at 17:29
  • $\begingroup$ But see what are those "slight differences" that define whether something causes inflammation or not? $\endgroup$
    – John Joe
    Jan 31, 2018 at 17:32
  • $\begingroup$ Inflammation may be caused by complex macromolecules such as proteins and nucleic acids, which are recognized by the immune system or (a lot more rarely) simple molecules, which become bound to larger molecules, like proteins. In the latter case, the immune system is able to recognize the small molecule-protein complex, which initiates inflammation; such a potentially inflammatory molecule is called a hapten. Copper may be slightly more prone to binding/adsorbing to proteins because of factors such as electronic structure, the solubility of the specific material/alloy, and others. $\endgroup$
    – Stefan
    Jan 31, 2018 at 17:45

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