December 19, 2017
Continued advancements in medical technologies are prolonging lives all over the world. The development of new uses of metals and alloys in internal and external medical applications has stimulated the progress being made. In 2016, more than 50 million surgical procedures were performed in the US alone. As that number grows each year, the demand for minimally invasive surgeries is also increasing as it offers many attractive benefits including shorter recovery times and reduced overall impact on the patient during the procedure.
Stainless steel is still the most commonly used material for medical instruments due to its versatility, biocompatibility and relatively low cost. Other materials, such as titanium and cobalt-chrome have also contributed to advancements. Nitinol (UNS N01555), a nickel-titanium shape memory alloy, is a star performer among medical metals and increasingly used in many medical applications.
Nitinol guides the way
Nitinol is an extraordinary material with super-elasticity and shape memory properties. It is 55% nickel-45% titanium and an attractive material for medical implants. Nitinol is biocompatible and because of the alloy’s outstanding properties, many applications where stainless steel was previously used have been replaced with Nitinol over the past ten years.
The top three applications for Nitinol medical wire are vascular guidewire, diagnostic guidewire and dental arch wire. Every year, several hundred tonnes of nickel go into the manufacturing of Nitinol and stainless medical wire, which are used both in life-changing solutions and in life-saving situations. According to a 2014 research report published by Grand View Research, “Global guidewires market is expected to reach US $2.19 billion by 2020. Growing prevalence of target diseases coupled with growing geriatric population base is expected to drive guidewire demand over the next six years.”
A guidewire is a thin, flexible, medical wire inserted into the body to guide a larger instrument, such as a catheter, central venous line, or feeding tube. While historically used in coronary procedures, the guidewire has become an integral part of a growing number of medical procedures with its use steadily increasing and expanding into more medical specialties.
“Nitinol wire has 16 times better super-elasticity and can withstand 8% strain compared to stainless wire, which can withstand around 0.5% strain before being deformed from its initial shape. For example, take a paper clip made of Nitinol. You could bend it to a 90 degree angle and it would spring back to its original shape”, says David Plumely, Nitinol Product Manager at Fort Wayne Metals, a leading supplier of Nitinol, stainless steel and specialty medical wires, based in Fort Wayne, Indiana, in the US. “Nitinol wire has great “push ability”, good kink resistance and is able to maintain its straightness. The properties are very valuable to guidewire. We have seen beyond 10% annual growth in our production of Nitinol in the last ten years and we see continued growth opportunities for Nitinol in the future, both in medical and non-medical areas.” Another important application for Nitinol is in reinforced catheter polymer tubes, where braided Nitinol acts as reinforcement between the inner and outer polymer layers.
Stents that accommodate large strain
Nitinol’s extraordinary ability to accommodate large strain coupled with its physiological and chemical compatibility with the human body makes it an attractive material in medical device engineering and design. A key area of application for Nitinol is stents. Nitinol stents can be fabricated at one temperature, folded smaller at another temperature, then inserted into an artery where the body heats the material above its transformation temperature and it returns to its original size.
Nitinol vascular stents are made from a gun drilled bar. Once processed into exact dimensional requirements, they are laser cut into shapes. Vascular stent tubes used to treat aneurysms are typically 25.4 mm (1”) in diameter and “crushed down” into only 6-7 mm (1/4”) diameter, before being inserted into a delivery device tube and into the patient’s aorta. The stent will expand on its own and take its original shape, enabling doctors to repair the aneurysm.
High strength stainless steel applications
“There are many applications for Types 302 (UNS S30200), 304 (UNS S30400) and 316L (UNS S31603) austenitic stainless medical wire”, says Austin Lucas of Fort Wayne Metals. These fine medical wires have diameters ranging between 2.54 mm and 0.1016 mm, and can be drawn into even finer diameters. “Stainless alloys are attractive materials because of the high strengths (400-500+ ksi) that can be achieved in cold work condition, for example 0.127 mm wire with 450 ksi. This is important in the manufacturing of very small and thin parts, such as is typical in many medical applications”. Uses will be found in a wide range of applications within vascular therapy, neuro-stimulation therapy, endoscopy, orthopaedic medicine, orthodontics and implant dentistry.
During open heart surgery the rib cage must be separated by breaking the sternum so the surgeon can reach the heart. Upon completion of the surgery, the ribs will be pulled back together and closed with the help of sternum closure wire. This is typically made from Type 316L stainless medical wire. Type 304 stainless medical wire is commonly used as guidewire for initial entry into veins and arteries in vascular procedures. Some stent designs use Type 304 stainless steel. Other common applications for Types 304 and 302 stainless steels are stylets, catheters, springs, needles and mandrels. In many surgical interventions, staples are commonly used for closing wounds. These are made from Types 316L and 304 stainless wires. The latter grade is also used in dental arch wire.
Wires, pins and needles
Kirschners Wires (K-wires) and Steinmann Pins are made from implant grade 316L and used by orthopaedic surgeons as implantable devices for fixation of bone fractures, for bone reconstruction, and as guide pins for insertion of another implant. They may also be implanted through the skin so that a pulling force may be applied to the skeletal system. Once the bone has healed four to six weeks later, the pins will be removed. Other uses for Type 316L stainless include orthopaedic cables.
Stainless medical wires used in medical implant applications are produced using Vacuum Arc Remelt (VAR), a secondary melting process for materials with elevated chemical and mechanical homogeneity for critical applications. Such controlled chemistry and high purity material is needed to ensure consistency in fatigue strength, which is of utmost importance in many medical applications.
Stainless medical wire is also used for the manufacturing of needles. Suture needles, for example must exhibit exceptionally high strength and ductility to resist bending and breaking, as well as provide stiffness and tissue penetration performance to enable good control in the hands of the surgeon. In order to meet these material requirements, suture needles are commonly made from high performance age-hardened martensitic stainless alloys such as Custom 455® (UNS S45500) and Custom 470®.
The continued pursuit of innovative applications
Various other nickel-containing specialty wires also find medical uses, such as, FWM 1058®/Elgiloy®/Phynox®/Conichrome® (UNS R 30003), which is a cobalt-chrome-nickel-iron alloy. Applications include wire based stents, filters, pacemaker leads and orthopaedic implants. In addition, cobalt-nickel-chrome-molybdenum alloy 35NLT® (or other versions of the alloy (UNS R 30035)), with its high modulus of elasticity, finds uses in pacing leads, stylets, catheters and orthopaedic cables and permanent implantation applications.
Medical technological advances go hand in hand with the availability of high performance materials with outstanding properties. Biocompatible materials such as Nitinol and nickel-containing alloys must not only match the present needs of medical device designers, but provide them with parameters for new ideas and possibilities. With the continued pursuit of new applications, these are certainly extraordinary and inspiring materials for future innovation.