04. Medical applications of stainless alloys
Owing to the unique combination of properties of stainless alloys they are indispensable in modern medicine. The required materials properties include good corrosions resistance in body fluids, biocompatibility, good strength to-weight ratio, fatigue resistance and a low magnetic susceptibility. The term biocompatibility is not well-defined but can be described in simple terms as the ability to perform adequately in a specific situation in the human body.
The requirements are often so extreme that the properties of traditional stainless steels are insufficient. For instance, resistance to wear in joints often requires implants of Co-Cr alloys. In other applications titanium alloys are to be preferred owing to a phenomenon called osseointegration, which leads to excellent adherence between the implant and the bone. One of the first to implement osseointegration was the Swedish scientist Brånemark, who started to use dental implants of titanium in dental medicine in the mid-60s.
A rather special application is cardiovascular stents where shape memory alloys based on Ni and Ti have been used very successfully. The field is vast, the number of applications enormous and, therefore, impossible to cover here. Instead I will confine myself to a selection of a few applications. Medical applications of special alloys are found mainly within the following three areas: Dental and Surgical Instrumentation, Cardiovascular, and Reconstruction. Traditional stainless steels are found essentially in instrumentation and trauma.
Dental and surgical
The wide variety of dental and surgical instruments includes reamers, hooks, clamps, retractors, screwdrivers, surgical blades, scalpels and surgical needles. For instrument applications, traditional stainless steels (mainly austenitic and martensitic) most frequently perform satisfactorily. However, the example given here is related to the use of the precipitation hardened stainless steel Sandvik Nanoflex with its unique properties. It was originally tailor-made to suit the requirements for surgical needles, for which a combination of high strength and ductility is essential.
Owing to the additional strength provided by precipitation in an already hard martensitic matrix a tensile strength exceeding 3000 MPa is attainable. Because of the combination of high strength, ductility and good edge retention it is suitable not only for suture needles but also for drills, screwdrivers, taps, reamers and blood lancets. An example of suture needles is shown in Fig 1a together with an ocular surgery (Figure 1b). The eye is very fragile and extreme care must be exercised. Needless to say, the needles need a combination of high strength and ductility to perform adequately.
Cardiovascular applications include pacemaker and ICD (Implantable Cardioverter-Defibrillator) leads, cardiac stents, guidewire systems, sensor wires, artificial heart valves, braided catheters and cardiovascular retractors. In some of these applications, such as guidewires and sensor wires, type 304 and 316 austenitic stainless steels perform satisfactorily. However, cardiovascular applications often require rather special alloys such as F562 (a 35% Co, 35% Ni, 20% Cr, 10% Mo alloy), ASTM B684 (a Pt-Ir alloy with 10% or 20% Ir) or a composite material with a core of a noble metal. F562 is employed as stents and in pacemaker and ICD leads owing to the combination of corrosion resistance, high strength and resistance to fatigue.
ASTM B684 is an alloy serving this purpose. When electrical conductivity is of prime importance, such as in pacemaker and defibrillator leads composite wires with a core of a noble metal such as Ag, Au or Pt are often used (Figure 2). A stent is an expandable device inserted into the human body to keep the passageway of a vessel open. The stent is usually brought into its final position using a guidewire, which is often made of an austenitic stainless steel (e.g. vacuum remelted 304 or 316). However, to produce the final expansion of the stent the shape memory effect provided by a Ni-Ti alloy such as Nitinol is often employed. The transformation temperature, defined as the temperature at which the stent assumes its original shape, can be tuned very precisely by altering the composition slightly. Prior to the surgery, the stent is compressed after which it is inserted into position. The final expansion takes place when it assumes the temperature of the body (Figure 3).
The longer life expectancy in combination with the growth in world population emphasizes the need for replacement implants in the human body. This involves complete or partial replacement of hips, knees, shoulders, wrists and elbows. Suitable materials are austenitic stainless steels (ASTM F138, nitrogen-rich ASTM F1586), titanium alloys (mainly Ti-6Al-4V) or cobalt-based alloys (e. g. ASTM F75). Owing to the softer elasticity of Ti and its alloys they are closer to the elasticity of bone and are, therefore, less likely to fail by fatigue. Spikes are often made of Ti-6Al-4V. Factors influencing the choice of material are the cost and the age of the patient. Implants play an important role in trauma situations i.e. repairing and supporting bones after accidents. Such implants are frequently made of titanium or stainless steels.
A joint is subjected to repeated loading. Therefore, wear resistance and fatigue resistance are crucial in determining the longevity of an implant.For instance, insufficient wear resistance leading to debris (wear particles) from the implant may cause so called osteolysis. If this is the case implant loosening may occur and a new surgery ensues. Surface coating of the exposed surfaces, for example a ceramic coating, is often employed to improve the wear resistance and the adherence. The example chosen here is a knee implant where two metallic components, a femoral component in the upper position and tibial component in the lower position are used. A polymeric layer is inserted in between these two components to avoid direct contact between the metals. Cobalt Chromium alloys and ceramics in combination or together with a polymer are so far the only material combinations that can survive in an artificial articulating joint. The wear exposed components are often mounted on a material with better ingrowth capability such as titanium or stainless steel.
The topic of medical applications is immense and impossible to cover in a short article of this kind. My aim was merely to show some selected examples from the medical field to emphasize that stainless alloys play a key role in improving the quality of life. Let me elucidate my last statement with one single example taken from real life. The pacemaker was co-invented in Sweden by Rune Elmehed and Åke Senning in 1958. Their first client was Arne Larsson, who was 43 years old at the time, fighting for his life. Thanks to the pacemaker technology he was granted another 43 years and died in 2001 at the age of 86 surviving both his rescuers.
This article was first published in Stainless Steel World Magazine in December 2017.