While metal injection molding (MIM) techniques are widely used in the medical device industry, there are distinct advantages when using medical-grade titanium alloys. The titanium metal injection molding (TiMIM) process produces pore-free parts that provide better corrosion resistance and biocompatibility than stainless steel alloys. Titanium is also significantly more efficient when used in ultrasonic applications.
In this article, we’ll take a look at the characteristics of medical-grade titanium and how our titanium-only manufacturing facility ensures Praxis’ TiMIM parts are biocompatible.
Characteristics of Medical-Grade Titanium Alloys
Titanium alloys are known for their resistance to corrosion and biocompatibility, making them the ideal choice for medical devices. The most common titanium alloy for medical-grade applications is Ti-6Al-4V, which stands for ~6% aluminum, ~4% vanadium, and the balance titanium. As with all titanium alloys, there are strict limits for elements such as oxygen, nitrogen, hydrogen, and carbon. Any other residual or tramp elements are limited as well.
When the average person thinks of “oxidation,” they think of rust: rusty cars, rusty nails, and so on. With rusty pieces of steel, the oxidation damages the material, penetrating deep into it, and eventually destroying it entirely.
Rust is only one example of oxidation. Some materials, such as aluminum and titanium alloys, oxidize quickly. In the case of titanium, however, instead of the oxidation acting as a destructive layer, the oxide on the surface forms a protective barrier, preventing the oxygen from penetrating deeper into the bulk of the material. The titanium oxide that forms as a thin film on the surface of titanium alloys is a hard, chemically resistant material, which acts as a thin protective layer around the metal.
What’s the difference between rust and the protective oxide layer? Rust tends to not form a cohesive layer on the surface. When steel rusts, the oxide layer expands and is not bonded well with the layer beneath and flakes outward, exposing fresh steel for an oxide attack. This process continues until the steel has been completely reacted with the oxygen and corroded away.
With titanium alloys, the oxide maintains the existing cohesive bonding to the layers under the oxide layer, meaning it does not flake off and oxygen can not reach the un-oxidized metal beneath it. Once the surface is completely coated with oxide, the reaction has been run to completion, and no further oxidation can occur.
In the harsh environment of the human body, any implant is subject to corrosion. The human body is practically a saltwater bath of varying pH, which wreaks havoc on most materials. Plain carbon steel would quickly rust away, destroying the device and contaminating the body with rust particles. In contrast, titanium alloy parts resist the constant chemical attack of the human body; the device does not deteriorate or generate rust particles that can cause health problems.
Should the thin oxide layer get damaged, the speed at which titanium alloys oxidize means that the oxide layer will quickly regrow, even while immersed in the human body. While most implants will never get physically damaged in this manner, some wear surfaces, such as the contacts on replacement joints, could under extreme conditions.
Besides corrosion, implant designers must also consider biocompatibility; otherwise, the body might not accept the implant and could damage itself trying to fight the foreign object. The body will attack itself over ingrown hair, yet many medical devices are made from metal. Part of the challenge of developing medical devices is to have the human body not identify the device as an intruder to be attacked.
Biocompatibility is one of the major advantages of medical-grade titanium alloys. The commonly-used Ti-6Al-4V (and other titanium alloys) are inert in the body, meaning the body will generally accept components made from these alloys.
The reason for the high biocompatibility is titanium’s passive oxidation layer. The thin oxide layer that prevents the titanium alloy from corroding also prevents chemical reactions in the body. Chemically, the material is inactive, and will not react with the surrounding tissues, bodily fluids, or bone. This chemical inertness dramatically decreases the likelihood of adverse reactions.
ASTM International (formerly the American Society for Testing and Materials) developed a standard that describes the requirements of titanium metal injection molded (TiMIM) parts for medical implant technologies. The standard, ASFM F2885, contains specifications on density, chemistry, mechanical properties, and other concerns related to the TiMIM process. In particular, this standard covers Ti-6Al-4V used in surgical implants. For more information, ASTM F2885 can be purchased here.
The TiMIM Process
Titanium metal injection molding (TiMIM) is a technique for forming high volumes of small parts with complex geometries. Titanium alloy powder and thermoplastic binder are forced through a heated barrel to a nozzle, and then forced into a mold. Subsequent debinding and sintering removes all of the binder and densifies the part. Post-sintering processing removes the residual porosity, creating a uniform and pore-free product. Final parts are hermetic and can be used in application that require high hermeticity.
The TiMIM process produces parts with incredible tolerances (±0.3%) and consistent surface finishes. Surgical applications are generally very demanding from a surface finish perspective, and this can be driven by aesthetic and functional considerations. Praxis has many different polishing techniques to satisfy our customers’ product requirements. Surface finish is a tremendously important consideration, and sometimes a single part will have different surface finish requirements, depending on the part’s functionality.
The Praxis Advantage
Praxis only works with titanium-based alloys, and this gives us a distinct advantage over conventional MIM houses that offer an array of different materials. Because we have been specializing in titanium for over 15 years, we offer our customers unique and focused expertise, and act as a collaborative partner to make their concepts a reality. All of our emphasis is on producing the highest quality medical-grade titanium parts on the market.
In fact, we have developed an infrastructure to specifically service implant markets, including:
- A team of titanium powder metallurgy experts
- Designated titanium-only facility
- FDA-registered, ISO-13485 certified manufacturing facility
- Electronic document control
- AMS-2750 compliance
- Qualified MIM process/certified product
- Demonstrated long-term process capability
- Internal feedstock production
Furthermore, because we ONLY work with titanium alloys, there is no risk of contamination from other metals. Titanium is especially sensitive to contamination, which negatively affects its mechanical performance. In powder metallurgy, a small contaminant in a commonly-shared molder, mixer, or other processing tool can lead to costly issues and delays for medical device manufacturers.
Praxis’ medical device parts are free from contamination from other materials because only titanium-based alloys are used in our facility. As a result, Praxis is the only company on the market to produce medical device parts that conform to ASTM F2885.
For more information on our medical-grade titanium and TiMIM capabilities for medical implants, contact our team today to discuss your product’s unique needs.