Titanium alloy is often hailed as the "universal metal" in biomedical applications, covering almost every field of modern medicine from orthopedic implants to dental restorations, from cardiovascular stents to surgical instruments.
Why can seemingly ordinary metals "invisibly" shuttle through the human body and coexist peacefully with our tissues? What magical 'biological magic' does it possess? Where does this magic come from? Let's explore the truth in this article.
From fighter jets to human bodies, the magnificent transformation of titanium alloy
The story of titanium alloy began in the aviation industry. In the 1940s, scientists accidentally discovered that this metal used to manufacture fighter jets actually got along well with animal bones.
In the 1950s, titanium alloys officially entered the medical industry. The initial star product Ti-6Al-4V (containing 6% aluminum and 4% vanadium) performed well, but doctors found that the vanadium element in it may pose health risks. So material scientists began to "formulate new formulas" and replace vanadium with safer niobium, developing new alloys such as Ti-6Al-7Nb.
Scientists can customize titanium alloys suitable for different medical needs by adjusting the proportions of "ingredients" such as aluminum, vanadium, niobium, etc. Nowadays, the titanium alloy family has developed into three major sects:
Alpha type: high stability but moderate strength
Beta type: The most elastic and closest to real bones
α+β type: Balancing strength and toughness
In the world of medical materials, titanium alloy is like an all-around player - both sturdy and lightweight, and able to coexist harmoniously with the human body.
Perfect biocompatibility. What is the biggest fear if you put a piece of metal into your body? That's right, it's a rejection reaction. But titanium alloy can coexist peacefully with the human body, thanks to its special skill of automatically generating a protective film (titanium dioxide layer). It is almost insoluble in the human environment, can effectively block the release of metal ions, and will not be corroded by body fluids, nor is it easy to trigger immune system attacks.
The film layer is capable of adsorbing calcium and phosphate, enhancing the nucleation of hydroxyapatite that is a primary inorganic component of human bones and teeth, and enabling bone cells to directly adhere on titanium surface to form a "biological fusion". On the other hand, stainless steel and cobalt chromium alloys are known to release nickel and chromium ions at a slow pace which can potentially cause allergic or toxic reactions. They are usually encased in fibrous tissue and cannot be truly biocompatible.
Strong and lightweight. The second superpower of titanium alloy is its mechanical properties. Doctors have found that the ideal implant material should have two characteristics - strong enough to withstand daily activities, and elastic enough to be close to real bones. Titanium alloy perfectly meets these requirements.
Firstly, it is lightweight, with a density only half that of steel, yet its strength is similar; Secondly, its elasticity is moderate. The new titanium alloy has a elastic modulus of approximately 60GPa (gigapascal), which is very close to the human bone's 30GPa; Thirdly, it is fatigue resistant, it can be bent millions of times without fracturing.
Why does the human body not reject titanium alloy?
Titanium alloy has found a great application in contemporary medical field due to its outstanding overall properties, including dental restorations, cardiovascular stents, orthopedic implants and so on. So, why doesn't the human body reject titanium alloy? This begins with a developing interdisciplinary field at the interface of bone tissue and immune regulation – bone immunology.
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