The basis of biocompatibility of titanium alloys

(1) The formation and protection of surface oxide films
In air, the surface of titanium alloy rapidly reacts with oxygen to form a dense oxide film, mainly composed of titanium dioxide (TiO₂). This oxide film is extremely thin, typically ranging from a few nanometers to several tens of nanometers, yet it offers remarkable protective effects. It acts like a sturdy "armor", isolating the titanium alloy substrate from human tissues and preventing the release of metal ions from the titanium alloy into human tissues, thus avoiding immune responses and inflammation caused by the toxicity of metal ions. Additionally, this oxide film is chemically stable and is unlikely to react with various chemical substances in the human body, ensuring the long-term stability of titanium alloy within the human body. For instance, in artificial hip joint implantation surgeries, the oxide film on the surface of titanium alloy implants can effectively prevent direct contact between the alloy and human tissue fluid, reducing the risk of infection and ensuring the safety of the implants.

(2) Low elastic modulus property
Human bones have a certain elastic modulus, with the elastic modulus of normal cortical bone being approximately 10-40 GPa. Traditional medical metallic materials such as stainless steel and cobalt-chromium alloys have a relatively high elastic modulus, generally around 150-200 GPa, which is significantly different from that of human bones. When these materials are implanted into the human body as implants, due to the mismatch in elastic modulus, the stress borne by the bones is reduced, leading to a "stress shielding" phenomenon, which in turn causes bone atrophy and bone mass loss. In contrast, titanium alloys have a relatively lower elastic modulus. For instance, the commonly used Ti-6Al-4V alloy has an elastic modulus of about 110 GPa, which is closer to that of human bones. This enables titanium alloy implants to deform in concert with human bones under force, resulting in a more uniform stress distribution, effectively reducing the "stress shielding" effect, promoting a tighter bond between the bone and the implant, and maintaining the normal physiological function of the bone.

(3) Non-toxic and non-allergenic
Titanium alloy itself does not contain elements harmful to the human body, and it is chemically stable in the human body, not releasing any toxic or harmful substances. Meanwhile, titanium alloy has a relatively small impact on the human immune system and rarely causes allergic reactions. In contrast, the nickel element in materials such as nickel-based alloys may cause allergic reactions in some people, limiting their application in the biomedical field. The non-toxic and non-allergenic nature of titanium alloy enables it to coexist peacefully with human tissues, providing a safe and reliable guarantee for long-term implantation in the human body. It plays a significant role in high-safety-demand applications such as dental implants and cardiovascular stents.