Shape memory alloy (SMA) wires, such as nitinol, with diameters less than 8µm (~.0003”) can be produced by the accumulative drawing/rolling and bonding technique, which has been used to make other microscale metallics [1-2]. Further, Nitinol wire or fiber forms have already been explored for use in complex textile production for advanced function . In this working example, as shown in Figure 1 (a,b), over 2,000 NiTi wires with a diameter of 2.5 µm (approx. 0.0001 in) were produced by drawing inside a specially tuned and sacrificial, deformable matrix.
With the recent approval of two magnesium-based implants in Europe, we may be at the beginning of a paradigm shift in medical intervention. In the near future, devices whose structural support is only needed temporarily, such as stents, staples, and screws, may be predominantly made of absorbable metals which dissolve harmlessly and even beneficially over time. Of the three nutrient metal classes (Mg, Fe, Zn), magnesium has, of late, received the most attention.
At Fort Wayne Metals, we are applying our knowledge of Nitinol alloys and processing to decrease the stress-temperature sensitivity of Nitinol and increase the temperature range where superelasticity is possible. The superelastic (SE) properties of Nitinol are generally used in moderate temperature environments. Room temperature and body temperature are the most common.
Fort Wayne Metals is engaged in alloy design, process development, and thermomechanical conditioning and test development of low through high temperature nitinol and NiTi ternary alloys for actuator applications. Custom product forms range from ultrafine filament (e.g. 50 µm) through larger wire (e.g. 2-5 mm), cables, strip and other custom product forms. The present work on low temperature actuation using superelastic grade NiTi is adapted from a talk given by the authors at SMST 2015 .
Unique microstructures and properties of a ternary Ni46.7Ti42.8Nb10.5 alloy reported in one of our recent studies  shows great potential of this alloy system in applications that require high stiffness and large mechanical energy dissipation.
In August of 2015 progress towards making wire with microgrooves in the guidewire size range was summarized in A New Take on Wire Geometry – Functional Grooves. Now the focus has shifted to comparing the mechanical performance to that of solid round wire.
In our July 2015 installment , we discussed Fort Wayne Metals’ recent development of a beta titanium, nickel-free superelastic alloy. At the time, as shown in Figure 1 below, we were able to design excellent shape setting response in linear wire segments, e.g. for applications such as kink-resistant guidewire and stylets. Straight shapeset geometries were achieved through conventional stress-annealing of a suspended wire segment as well as continuous reel-to-reel wire lengths.
Medical device design with absorbable metals has the potential to revolutionize patient care by providing effective short-term therapy and then harmlessly dissolving away. One of the primary hurdles to overcome is that of premature material fracture which could potentially lead to improper device function. One potential avenue to solve this problem is to harness and use a natural property of metals to our advantage, namely galvanic activity. When two differing metals are in proximity to one another in a conductive solution, one material will be electrochemically dissolved and the other protected from dissolving. This is the same principle by which a potato battery or galvanized steel works
You may know that Fort Wayne Metals has been in the business of providing effective DFT® composite wire solutions for more than 30 years into industries such as cardiac pacing and neurovascular stenting. DFT wires are commonly ordered with a specific area fraction (%) of another metal, like silver for conductivity (e.g. 35N LT-DFT-28%Ag), or platinum for x-ray-opacity (e.g. NiTi-DFT-10%Pt) – these DFT wires with less than 50% area core fractions are known as thick wall composites.
Magnesium is known mainly for its light weight, but in recent years it has received considerable attention for a much different reason: the ability to be absorbed by the human body. Being a necessary nutrient, the body has natural ways of breaking down metallic magnesium over time. This means that in many medical devices, the inert stainless steel or titanium currently used could be replaced by magnesium. Doing so would have the potential to reduce long-term complications and eliminate secondary procedures. It may also enable new applications that broaden the scope of medical treatment.
Recent processing advancements have enabled Fort Wayne Metals to form fine U-shaped features into continuous lengths of 0.010” to 0.020” wire in most of our common alloys. We are still in the R&D phase, but several combinations of groove width, depth, and diameter are possible for the same overall wire diameter. How could this technology enable your next generation of products? For more information, please contact RDTeam1@fwmetals.com.
While Nitinol is becoming more and more popular in the world of medical devices due to its superelastic and shape memory properties, its nickel content is cause for concern with some applications. The Fort Wayne Metals R &D team has therefore begun investigating nickel-free alternatives.
Fort Wayne Metals is a designer’s toolbox, for high performance wire composites. You may know that Fort Wayne Metals has been in the business of providing effective DFT® composite wire solutions for more than 30 years. These solutions end up in devices that require a combination of properties not possible in mono-metals, like high fatigue with electrical conductivity (35N LT-DFT-Ag) or superelasticity with high relative x-ray-opacity (NiTi-DFT-Pt).