Advancements in medicine, surgical techniques and materials have led to the now routine practice of placing devices in the human body on a permanent or semi-permanent basis. These biomedical implants must meet stringent requirements issued by the Food and Drug Administration (FDA), to ensure that components are safe.

Biomedical implants require surgery for implantation. Therefore, they should have a long life so that the patient does not have to undergo multiple surgeries to place and replace devices. To last a long time, biomedical implants must be resistant to corrosion, which is quite challenging in the human body, as it is basically a saline environment that will quickly corrode many materials. Corrosion is not just a threat to the device, but it is also a threat to the human body, as the corrosion products can be toxic or spall away to become embedded elsewhere.

Outside of biocompatibility, biomedical implant materials must be strong, yet lightweight. Consider a hip implant; it must be strong enough to support a large part of the weight of the patient, but should not be much heavier than the original hip, or else it may damage surrounding bone and tissue structures. A hip must also move around, and so it must have a hard surface with little texture to minimize wear. Just like corrosion products, wear particles can cause discomfort and may need to be surgically removed as well.

Comparison of alloys

While there are some polymeric and ceramic materials that are used in biomedical implants, this article will focus on a few common alloys. Overall, metal implants are more common for corrective and life saving surgeries. While plastics are used for cosmetic surgery, bone replacements, pacemaker leads, vascular ports, bone screws, plates and many other devices are more commonly metallic.

In order for a metal alloy to be used as a biomedical implant, it must be approved via the FDA process. Currently, the most common biomedical implant materials are several stainless steel alloys and titanium alloys. Cobalt-chromium alloys are growing in popularity, and some cases of precious metals (e.g., silver) occur as well.

Stainless steel

Perhaps the most common stainless steel alloy in the biomedical field is AISI 316L, though several others see occasional use. AISI 316L has the right combination of resistance to chemical attack and strength. While more dense than titanium alloys, it is often more cost effective. Stainless steel alloy parts can be manufactured in a number of ways, making them versatile in production and further reducing costs.

One disadvantage is that some people are allergic to stainless steel. In this case, another alloy must be chosen, either a titanium alloy, or in an extreme circumstance, a precious metal, such as silver.

Titanium

Commercially pure titanium is used in some biomedical implants. However, the preferred alloy in most cases is Ti-6Al-4V, which adds a little aluminum and vanadium as a grain refiner. In either case, titanium alloys have a high strength to weight ratio and high resistance to corrosion from both saline and acid attack. This, and other factors, make them biocompatible and suitable for long-term use in the human body. Titanium alloy implants can be investment cast or produced via powder metallurgy techniques.

A quick comparison of strength and density between AISI 316L and Ti-6Al-4V shows that the titanium alloy has the highest strength to density. Not shown is the higher cost of the titanium alloy.

Data source: MatWeb

Manufacturing techniques

There are many techniques for building biomedical implant devices. Currently, the two most popular are investment casting and powder metallurgy techniques, though additive manufacturing, such as 3D printing are becoming more popular.

Investment casting

One common technique for manufacturing biomedical implants is investment casting. In investment casting, a wax or foam positive is coated in a ceramic slurry and fired. This leaves a hollow shell behind. From there, molten metal can be poured into the shell to form the final shape. The ceramic is broken away, leaving behind the part.

Investment casting can be done reliably and economically, making it a good fit for moderate production numbers. It can be used for parts of various sizes, from relatively small components, such as ports to large components such as artificial hips. Furthermore, depending on the circumstance, custom parts can be manufactured in low production numbers and placed on the same “tree” as many other parts, driving down the cost slightly.

Powder metallurgy

In powder metallurgy, metal powder is either pressed into a shape and then sintered, or it is combined with a slurry and injected into a mold, then sintered. The former is considered traditional “powder metallurgy” and the latter is “metal injection molding” or MIM. In both cases, the starting feedstock is a metal powder mixed with a small fraction of a polymeric binder. High pressure forces the powder into contact and helps it retain its shape. Then, a heat treatment removes the binder and drives diffusion bonding between the particles, strengthening them.

MIM typically produces stronger bonds as compared to traditional powder metallurgy, and thus is often the preferred choice for biomedical devices. In either case, post processing of either hot isostatic pressing (HIPing), or grinding and polishing steps can be implemented to improve strength and surface finish.

Powder metallurgy techniques are often more expensive but can be used to create much smaller parts than casting. They can also be performed much more precisely, creating hermetic seals for electronic devices.

Summary

The market for biomedical implants will continue to grow. New medical advancements will drive the implant market from improving the final stages of life to extending life, meaning new implants will need to have longer lifetimes before replacement. Besides correcting defects, such as bone screws, pacemakers and other items, the advancements in sensor technology will continue to push the implant market. The ability to monitor glucose levels and other biomarkers in real time will lead to tougher demands on biocompatible alloys.