Is titanium the only metal alloy for making prostheses for the human body?

Titanium is a chemical element with symbol Ti and atomic number 22. It was discovered by the English clergyman William Gregor. Its name was coined by the German chemist, Martin Heinrich Klaproth, who created the name titanium from Titan, from ancient Greek mythology.

Titanium is a silver-colored metal of low density and high hardness. It is highly resistant to corrosion by fresh water, sea water, aqua regia (a highly corrosive solution formed by mixing concentrated nitric acid and concentrated hydrochloric acid, capable of dissolving gold, platinum and other metals) and chlorine.

In nature, it is always found together with other elements, usually in the interior of igneous rocks (or magmatic rocks, formed when magma cools and solidifies) and some sediments derived from them. One of the most exploited titanium-containing materials is rutile (a titanium dioxide, TiO2); an image of titanium dioxide powder obtained with a Scanning Electron Microscope (SEM) is presented in Figure 1a. Also, in Figure 1b, an Energy Dispersive X-Ray Spectroscopy Analysis is presented that allows the chemical characterization of the titanium dioxide sample.

Figure 1: (a) SEM image of titanium dioxide (TiO2) powder (b) Energy dispersive X-ray spectroscopy analysis of titanium dioxide powder sample.

Australia is the world's leading titanium producer with an estimated production of more than 1.5 million tons in 2021. South Africa and China are the next two producers of titanium with 1.16 and 1 million tons, respectively.

Titanium is among the ten most abundant chemical elements in nature, therefore it is not threatened at the moment, which will allow the development of new applications and innovations in the mechanical, aeronautical, energy, chemical, automotive, biochemical, electronic manufacturing and biomedical sectors with this chemical element.

Titanium is considered to be the most biocompatible metal that is neither harmful nor toxic to the body's tissues; so far, no allergic reactions of the body's defense system (immune system) have been reported.

One of its qualities is its resistance to corrosion by body fluids (blood, saliva, among others) due to the formation of a passive layer of titanium oxygen on the surface of the alloy, increasing its resistance to corrosion naturally in the presence of oxygen.

The titanium surface allows living tissues to grow (such as bone) and adhere practically anchoring themselves, showing a huge advantage over stainless steels (AISI 304 and AISI 316L) and cobalt-based alloys (CoCrMo and CoNiCrMo) that require the use of an adhesive to remain attached to the tissue. Titanium implants, such as jaw bone replacement parts, heel, bone screws, hip (see Figure 2a), dental implants or cranioplasty plates in surgery, last longer but are more expensive and difficult to manufacture.

Figure 2: (a) Titanium hip prosthesis (b) SEM image of the CoB-Co2B layer formed by thermochemical boriding treatment on the surface of a CoCrMo cobalt-based alloy.

In the last five years, the Surface Engineering Group of the Escuela Superior de Ciudad Sahagún, Universidad Autónoma del Estado de Hidalgo (UAEH) has been studying the surface hardening of cobalt base alloys (CoCrMo) whose nominal chemical composition is 0.14% C, 30% Cr, 7% Mo, 1% Ni, 0.75% Fe, 1% Si, 0.25% Mn, used in the biomedical and industrial sector through the application of various thermochemical treatments (boriding, nitriding, cementation, boron-nitriding, carboronitriding), which substantially increase surface hardness, contact fatigue behavior, wear resistance and corrosion resistance (tribocorrosion).

Figure 2b shows a cross section of the cobalt boride layer (CoB + Co2B) resulting from the application of the thermochemical boriding treatment, formed on the surface of a CoCrMo cobalt base alloy, allowing to increase the hardness values along the depth of the layer. For the CoB layer the hardness reaches 18 GPa, decreasing for the Co2B layer to 14 GPa and in the untreated substrate to 4 GPa.

The application of these processes in medical grade steels (AISI 304 and AISI 316L) and cobalt-based alloys (CoCrMo and CoNiCrMo) has aroused enormous interest, due to the fact that better properties are achieved with a low manufacturing cost compared to titanium alloys, This will allow in the future to manufacture replacement parts of a jaw bone, heel, shoulder, spinal rods, vascular stents, knee, hip, dental implants, fracture fixation devices or cranioplasty plates in low cost surgery and with an increased lifetime.

For example, in the case of hip replacement made of titanium, it has an average life time of 15-20 years, so it should be replaced with a new one; but in adults it is not very advisable. In this sense, the application of thermochemical treatments can help to cover this need to create prostheses with a longer useful life and a much lower cost.