Developing affordable titanium implants in Africa
Michael Oluwatosin Bodunrin is an early career researcher and AESA-RISE Postdoctoral Fellow based in South Africa. The AESA-RISE Postdoctoral Fellowship is an Alliance for Accelerating Excellence in Science in Africa (AESA) Programme that builds on the foundation of the Regional Initiative in Science and Education (RISE), which has for a decade prepared PhD- and masters-level scientists and engineers in sub-Saharan Africa through competitively selected, university-based research and teaching networks, to respond to an urgent need to increase the number of researchers in Africa. It is a partnership between the AAS and the Carnegie Corporation of New York (CCNY).
AESA-RISE is implemented through the AESA platform - a funding, agenda-setting, programme management initiative of the African Academy of Sciences (AAS) in partnership with the African Union Development Agency (AUDA-NEPAD) and with the support of Wellcome, Bill and Melinda Gates Foundation and the United Kingdom Department for International Development (DFID).
Titanium alloys are coveted in a wide range of land-based applications especially in the biomedical industry where they are used for making implants and prostheses. Around 80 million people in Africa are considered disabled and this is likely to increase with an increasingly aging population. Accidents and their resultant injuries have contributed to a high prevalence of disability and significant impacts on quality of life in Africa. Producing implants and prostheses locally would help in tackling this disability burden.
Titanium is one of the three most popular light metals including aluminium and magnesium. Titanium alloys have similar strength as steel but are about 40% lighter than steel. They don’t rust easily when exposed to harsh environment and are quite biocompatible. However, titanium-based products are 50 times more expensive than steel and 10 times more expensive than aluminium.
Despite being the ninth most abundant element in the Earth’s crust, it took more than a century from discovery, before it could attract any commercial interest. It’s beneficiation, that is, adding economic value to titanium by separating it from other elements in its mineral is challenging due to the difficulty in extracting titanium from its naturally occurring state. Its strong bond with other associated elements made beneficiation almost impossible.
Like many other pure metals, the use of pure titanium is limited, so other elements are added to it in well-controlled amount to form alloys. In the 1950s a few alloys were developed including Ti-6Al-4V (90% titanium, 6% aluminium and 4% vanadium), that became the gold standard in making aero engines and military parts.
As the science of titanium advanced, over 100 alloys developed but less than 25% of these alloys achieved commercial status due to the costs of producing them. Even those that fell within the commercial category were only used if there were no alternative materials that could perform better.
Researchers have continued to search for ways to reduce the cost of processing titanium alloys in order to make titanium-based products as affordable and accessible as steel and aluminium. The factors contributing to their high cost include the use of expensive alloying elements like vanadium, niobium and molybdenum and multi-stage and complicated shaping and finishing processes such as forging, rolling or machining. The combination of these processes account for almost 70% of the total cost of titanium alloys. Elements like vanadium, molybdenum and niobium do not form unwanted features in the internal structure of the alloy while cheaper elements; iron, chromium and nickel may do when added in large quantities. Using these cheaper elements requires more control if the formation of unwanted features is to be avoided.
Several strategies have been adopted to lower costs in low-and-middle income countries. These strategies include replacement of expensive alloying elements with cheaper ones that perform similar functions; optimization of shaping and finishing processes; or use of powder metallurgy processing routes instead of the conventional melting approach. The powder metallurgy routes involve consolidating powders of titanium and other alloying elements under pressure and temperature to produce a near-net shaped finished product. These routes either minimize or eliminate the need for forging, rolling or machining during processing. However, it is still very challenging to produce large parts with complex and intricate shapes using powder metallurgy route. In sub-Saharan Africa, the serious infrastructural deficiency and poor funding are the major barriers faced by scientists who are interested in titanium research.
Description of study
Bodunrin’s AESA-RISE research project focuses on developing low-cost titanium alloys by using a combination of different approaches. The project was inspired by the abundant titanium resource in South Africa, one of the world’s largest reserves of titanium-bearing minerals, the effort of the government to set up a robust titanium industry and the growing need for affordable titanium implants and prosthesis for management of bone injuries in Africa.
The methods adopted involved using both computational modeling and experimental techniques to establish critical parameters that would help reduce cost and waste associated with processing of titanium alloys. In Bodunrin’s research group, what they have done differently is to partially substitute expensive alloying elements with a cheaper one rather than total replacement as done previously by other researchers. For example, in the case of Ti-6Al-4V alloy we partially replaced vanadium with 3% iron by mass to make Ti-6Al-1V-3Fe alloy. Ti-6Al-4V is the most investigated and most used titanium alloy; it accounts for over 50% of the entire titanium alloys used worldwide. Most of the new experimental titanium alloys are compared with Ti-6Al-4V alloy. We also produced another variant where we partially replaced vanadium with iron and reduced the aluminium content from 6% to 4.5% by mass to make Ti-4.5Al-1V-3Fe alloy. Using this approach, our cost analysis showed that we could earn up to 10% savings from the total cost of procuring raw materials alone.
Since forging, rolling and machining contribute significantly to the cost of titanium alloys and lots of waste is generated during these processes, we optimized different processing parameters to determine the best way to process the newly designed alloys in a reproducible manner and with minimal waste generation.
We have produced the alloys in different scales. First, we produced about 10g and 50g buttons of the different alloys using vacuum electric arc furnace, the ideal furnace for melting titanium alloys. But due to size limitation we improvised on the vacuum induction furnace to scale up the production of the alloys and about 2kg of each alloy was melted and casted. At each stage we examined the internal structure using different microscopes and we searched for formation of unwanted features in the internal structure of the alloys. We also tested for hardness, strength and ductility and compared the results with Ti-6Al-4V alloy.
So far, newly designed titanium alloys that are potentially cheaper than Ti-6Al-4V alloy have been developed. The critical parameters that would ensure reproducibility and minimal waste generation during shaping of the alloys have been established. We have published process maps that would guide manufacturers that may be interested in the development and processing of the new alloys. We are now using computational techniques to test the critical parameters that we obtained from our study to see the performance of the alloys under different industrial conditions.
For the first time in South Africa, bulk experimental titanium alloys (about 2kg each) were produced by improvising on the vacuum induction melting furnace. The strength of the new alloys is similar to that of Ti-6Al-4V but we are working on improving the ductility of the alloys.
This research seeks to draw the attention of South African government and other policy organs to the potential opportunities that are available in developing unique titanium alloys in Africa. It also seeks to inform the wider public that the development of affordable titanium alloys is an important step towards achieving affordable titanium implants in Africa.
To sustain the local development and production of titanium alloys using existing infrastructures like the vacuum induction melting furnace, one must incorporate multidisciplinary measures. For example, the crucible (ceramic container) used in melting titanium alloys is not readily available in Africa, hence there is need for experts in ceramics engineering to come up with ideas on how to develop crucibles that can be used to successfully melt titanium.
It is not enough to develop low-cost alloys without developing a finished product that can be tested and recommended to consumers. Consequently, future research project will concentrate on the development and performance evaluation of finished biomedical products such as dental implants, ancillary nails or plates and screws for management of bones fractures. It implies that future research would involve experts from other disciplines like ceramic engineers, mechanical engineers, orthopedic surgeons and dentists to mention a few.