I am a postgraduate researcher at the University of Leeds. I have completed my master's degree in the Erasmus Tribos program at the University of Leeds, University of Ljubljana, and University of Coimbra and my bachelor's degree in Mechanical Engineering from VTU in NMIT, India. I am an editor and social networking manager at TriboNet. I have a YouTube channel called Tribo Geek where I upload videos on travel, research life, and topics for master's and PhD students.
Metallic Alloys in Biomedical Implants
Table of Contents
Introduction
Metallic alloys are more commonly used in medical implants than any other pure metals because they offer improved mechanical and tribological properties along with biocompatibility. In the 1920s, stainless steel alloy (18-8 SS) was introduced as a more corrosion-resistant option. Since then, materials like 316L stainless steel, cobalt alloys, and titanium alloys have been developed. This offers high mechanical strength and good biocompatibility (which are non-toxic and do not release harmful elements, and do not cause allergic reactions). In general, no metal is completely inert or immune to corrosion within the human body. Therefore, metals and their alloying elements are assessed based on their toxicity levels and durability when used in the body. It is also important to note that certain metals naturally exist in the human body and play specific functional roles.
Figure-1 Different materials used for biomedical applications [2]
Common metallic alloys used for implants
Stainless steel
Stainless steel is an iron-based alloy containing elements like chromium (Cr), nickel (Ni), molybdenum (Mo), manganese (Mn), silicon (Si), copper (Cu), and carbon. Its high corrosion resistance was first discovered in the early 20th century due to the formation of a stable chromium oxide film that prevents further oxidation. One of its types, 316L stainless steel has reasonable biocompatibility and has been widely used in medical implants due to its low cost, acceptable biocompatibility, and good mechanical properties. However, studies have shown a risk of nickel ion release due to bio-corrosion when implanted in the human body. To improve the biocompatibility of 316L stainless steel, researchers have explored replacing nickel with high concentrations of nitrogen. Nitrogen serves as a stabilizer for the austenitic phase, which helps to avoid the issues associated with nickel. The new alloy ASTM F2229 is free of nickel and has high nitrogen content and exhibits superior resistance to pitting corrosion wear, and general corrosion compared to traditional 316L stainless steel.
Stainless steel alloys offer several advantages, such as being relatively inexpensive and demonstrating suitable biocompatibility and strong mechanical properties. They are widely used in medical applications and are approved by the FDA for temporary biomedical implants. The newly developed ASTM F2229 stainless steel has shown improved performance over traditional 316L stainless steel, with better fatigue strength, wear resistance, and corrosion resistance, and without the risk of nickel ion release. However, stainless steel alloys also have notable disadvantages. They exhibit poor resistance to bio-corrosion and pitting corrosion, which can be problematic in the human body. Their fatigue performance can be reduced when exposed to corrosive environments. Additionally, stainless steel alloys lack bioactivity, meaning they do not promote bone growth or integrate well with bone tissue. Even though ASTM F2229 offers enhanced properties compared to 316L stainless steel, it still faces challenges with bone bonding, which may lead to issues such as implant loosening.
Figure-2 Corrosion of stainless-steel implants [3]
Cobalt Alloys
In the early 20th century, the CoCrMo alloy was introduced for use in aircraft and later found applications in medical implants. This alloy, composed of cobalt (Co) as the base metal and alloyed with chromium (Cr), molybdenum (Mo), tungsten (W), carbon (C), and nickel (Ni), demonstrated exceptional corrosion and wear resistance along with excellent mechanical properties such as ultimate tensile strength, fatigue strength, and Young’s modulus, even at high temperatures. The alloying elements Cr, Mo, and Ni contribute significantly to its superior performance in these areas. Despite the high toxicity of Co, Cr, and Ni, CoCrMo alloys are highly biocompatible due to their resistance to corrosion, which limits the release of these toxic elements. These alloys have successfully replaced stainless steel in joint replacements because of their excellent wear resistance. However, over time this metal-on-metal contact in implants can generate debris and release cobalt and chromium ions into the body, which can lead to severe adverse effects.
Figure-3 Image of cobalt chromium dental implants [3]
Titanium alloys
Titanium alloys were initially used in aerospace applications and were first introduced for dental implants in the 1950s and later gained popularity for bone implants. Titanium is non-toxic even at high doses and has a low density (4.8 gm/cm³) which offers superior specific strength compared to other common alloys. Although pure titanium has limited use due to its relatively low mechanical and fatigue strength, it performs well in corrosive environments, thus it is suitable for biomedical applications. To improve its mechanical properties, titanium is often alloyed with elements like aluminum (Al), vanadium (V), niobium (Nb), zirconium (Zr), and molybdenum (Mo). Ti6Al4V is a particularly common titanium alloy used in biomedical applications due to its excellent mechanical properties.
Titanium alloys are known for their high specific strength, low density, excellent corrosion resistance, and superior biocompatibility and osseointegration compared to stainless steel and cobalt-based alloys. However, they have several drawbacks: they are relatively expensive, difficult to forge, sensitive to notches which affects fatigue strength, and have limited bending strength, hardness, and wear resistance. These issues restrict their use in long-term load-bearing implants and joint applications. Despite these limitations, titanium alloys remain highly promising for other biomedical applications.
Figure-4 Titanium based ortho implants [4]
Reference
[3] https://www.beautyzir.com/blogs/news/advantages-of-cobalt-chromium-alloy-in-dental-applications
[4] https://www.alltraumaimplants.com/blog/titanium-orthopedic-implants.php
