December 19, 2017
Those who are old enough to remember may recall the hit American TV show “The Six Million Dollar Man” that ran for five seasons in the mid-1970s. The show featured an astronaut—Colonel Steve Austin—who had been badly injured in a crash. Doctors decided to rebuild him “better…stronger…faster.” It was science fiction then, but now many of those original viewers share something with Colonel Austin: they also benefit from artificial body parts that improve quality of life, ease pain, and improve mobility.
How additive manufacturing improves hip and joint fit
Artificial joints stand at the top of the list. Globally, millions of individuals have joint replacements. In fact, just in the US, more than a million hip and knee joints are replaced annually. These prosthetic joints are either made entirely of metal, or contain significant quantities of metal. Many metals are used, including stainless steel, nickel alloys, and titanium, in large part owing to their durability and ability to withstand corrosion in the human body.
Until now, those parts have been off-the-shelf in standard sizes, but that may soon change. Additive manufacturing (AM—the process of designing finished products on computers and printing in 3D) holds the potential for each joint to be created so that it exactly fits each patient’s needs. A great deal of research is currently focused on this area, to improve the design, functionality, and cost-effectiveness of these implants.
The key single advantage in this new AM technology is the complexity involved in simulating bone. It is difficult to create a complex structure with a level of porosity and texture that allows the actual surrounding bone to embrace the foreign implant. Various techniques have been tried, but nothing comes close to the promise of additive manufacturing in achieving this goal.
A bright future for 3D printed stents
At the same time, an entirely new potential field for nickel-based implants is about to take off: vascular medicine and the treatment of blood vessels. Global numbers are hard to come by, but in America alone, over 500,000 stents are implanted annually. These are small mesh tubes constructed of metal whose job is to keep coronary arteries open in order for blood to pass.
In this area, the 50-year old nickel-titanium alloy Nitinol (UNS N01555) has shown itself to be superior. Highly flexible and durable, with the ability to maintain a specific geometry (referred to as shape-memory), and a high strength-to-weight ratio, Nitinol has proved to be an ideal candidate for multiple vascular medical applications including stents, valves, and other devices. The shape memory property is particularly valuable, since Nitinol can self-expand and adapt to the shape of blood vessels.
Nitinol can be difficult to work with and few companies have been able to successfully meet the manufacturing challenge of making finished products with conventional manufacturing technologies. However, that all may be about to change, with the likely eventual application of additive manufacturing. Some companies and research institutions are focusing specifically on Nitinol in this regard.
Currently, some AM researchers have been able to develop personalized stents out of polymers. Compared with off-the-shelf medical stents, these are less likely to move in the blood vessel and cause resulting complications.
Some companies also use Nitinol stents cut with lasers, but to date, they have not yet been able to apply additive manufacturing technologies to the effort. Nonetheless, it will probably be just a matter of time before the industry routinely fabricates personalised stents from Nitinol. The Bionic Man would be jealous…