Medical Device Link.

MATERIALS

Image: iStockphoto

Obtaining new properties

Grain refinement and microstructural size are critical in the design of nearly all metallic structures. In aircraft jet turbines, large crystals are grown along the load axis to create strength in a preferred direction. In fine medical wire, grain sizes are reduced to promote resistance to fatigue and to increase strength properties. Proposals by Hall and by Petch quantified the strengthening mechanisms of grain refinement in the familiar Hall–Petch relationship.¹ In recent years, the microstructural size scale has generally been divided into three ranges:

• macroscale comprises the range from hundreds of microns to millimeters
• microscale comprises tens of microns
• nanoscale has arbitrarily been defined as 100 nanometres or less.

Today, microstructural process output can be monitored using advanced high resolution scanning electron micro-scopy (SEM) and transmission electron microscopy (TEM) with resolutions approaching several angstroms. These techniques have been used directly in the production of carbon nano-structures and in nanodeposition of metallic materials.² To better understand the nanoscale regime, many researchers today are heavily involved in the study of microstructure and accompanying property relations at the nanoscale.³ The study described in this article provides an insight into the beneficial properties that can be imparted to wire for medical devices through microstructural control at the nano level.

Grain size and fatigue behaviour

Figure 1: Polyslip surface features and 200 nm PSB protrusion near crack front in System B alloy after 2.05 x 105fd cycles at 690 MPa.

Grain boundaries exist as a result of crystal misorientations that occur in most crystalline solids.⁴ Many authors have demonstrated the formation of persistent slip bands (PSBs), or microscopic regions, in which repeated irreversible plastic deformation occurs in surface grains.^4,5,6,7 These PSBs possess a preferred orientation and/or size relative to the applied fatigue loading axis. Although the detailed mechanisms of the effect of grain size are still debated,⁸ it is generally accepted that irreversible plastic slip (micro-scale plastic deformation) occurs more readily in larger grains. It has also been shown that the stress field acting at the tip of a PSB behaves in a similar way to the stress field found at the tip of a crack and that these PSBs are often incipient sites of crack growth. ^4,6,7 An example of PSB formation as a result of fatigue is shown in Figure 1. Here, the alloy system is ASTM F562 chemistry 35 cobalt (Co)-35 nitinol (Ni) -20 chromium (Cr) -10 Molybdenum (Mo) (hereinafter referred to as
System B (SPS Technologies, Jenkintown, Pennsylvania, USA).

Recent papers have given significant attention to the behaviour of nanoscale microstructures in fatigue.^3,8,9 Some studies have reached phenomenological conclusions, and others have achieved physically quantifiable results. Studies that have achieved quantifiable results have been limited to pure metals such as copper, aluminium, titanium and binary intermetallics such as iron aluminides.^8,9 Recent research has primarily focused on theoretical discussion of what should occur and secondly on a limited amount of experimental data. Hoppel et al. showed that ultra fine grain structures achieved through equal channel angular pressing (ECAP) in brass, copper and aluminium resulted in a significant reduction in fatigue life in strain-controlled low-cycle fatigue testing.⁹ This result may be due to the high dislocation or internal lattice defect density expected from the ECAP process. Deleterious effects on crack growth were found by Hanlon et al. in nanocrystalline (NC) nickel. In damage tolerant design in which fatigue lifetime is gauged by the remaining life in a cracked structure, this behaviour would result in reduced product life and warrant the use of a conventional microcrystalline (MC) structure. However, the results were positive in the high cycle fatigue testing where they showed an increase in the fatigue threshold.³ For the purpose of the study discussed below, low cycle fatigue is considered to be less than 10⁵ cycles; high cycle fatigue is defined as greater than 10⁶ cycles.

Figure 2: Typical wire diameter to grain size ratio in MC System B medical wire.

Taking tight control

To be useful and predictable in engineering applications, structural metals and alloys require strict microstructural control. The level of control required is dependent on the application as well as the size of the finished component. As demonstrated in several recent studies,^10,11,12 the importance of metallurgical consistency is critical to fatigue behaviour in medical devices that incorporate fine wire elements such as microcoils or cables used in cardiac rhythm management systems. Intrinsic inhomogeneities in the alloy matrix such as constituent particles (inclusions) and large grains as well as extrinsic surface defects can result in premature wire failure during fatigue loading.¹² New thermo-mechanical processing technology (patent pending) has recently been developed that allows control of grain size in medical wire in the sub 1 micron range.¹²

Microcrystalline medical wire

Figure 3: Typical MC grain structure in System A medical wire.

Typically, medical wire processing involves microstructural refinement through repeated cold reduction and progressively shorter dwell recrystallisation anneals that yield increasingly fine distributions of grain.¹¹ Despite continual refinement of grain size, as wire diameters become smaller, the average number of grains through a transverse section is typically reduced. Thus, in a relative sense, the microstructure becomes increasingly coarse as wire diameters become smaller. Grain structure of a typical medical wire of MC low inclusion alloy 35 Co-35 Ni-20 Cr-10 Mo, referred to here as System A is shown in Figure 3.

The study

Table I: Comparison of ASTM F562 chemistry.

The ASTM F562 alloy system was originally engineered for use as a high strength and fatigue resistant macroscopic alloy for aerospace bolt fasteners, not as a material to serve the medical industry.¹⁴ As a large scale fastener, the presence of 5–20 micron nonmetallic constituent particles (inclusions) in the alloy is not necessarily detrimental to performance. In the past two decades, this alloy system has been designed into medical devices typically as a fine wire of 12.5–500 microns in diameter. Many of these devices are intended for permanent or semi-permanent implantation in the body, thus increasing the need for microcleanliness. A variant of the ASTM F562 System A was commercially introduced in 2003 in response to this need. This system is processed using carefully controlled melt techniques that eliminate titanium nitride particle forming elements and reduce the distribution of inclusions.¹⁰ The nominal chemistry of System B and this improved System A alloy, are shown in Table I. This novel material has recently been approved for use in devices ranging from implantable pacemakers to neurological sensing and stimulation devices.

Figure 4: Grain size distribution in NC System A medical wire.

A novel microstructure was then created using System A alloy: a wire of 178 microns that comprised nanoscale grain diameters. This material was analysed using SEM and found to possess a nominal grain size of approximately 100 nanometres. The observed grain size distribution is shown in Figure 4. ECAP methods incorporate severe repetitive shear deformation that result a high dislocation density and a lower saturation limit of the dislocation cell size.¹⁵ This defect state should possess a reduced overall ductility and diminished resistance to crack propagation. Unlike ECAP grain refinement methods, the novel process technology used in this trial should result in significant dislocation recovery and a resultant low initial dislocation density.

Results

Figure 5: Surface microstructure in a System A failed specimen tested at an alternating stress of 1103 MPa. Section failed in 1 M cycles at this test level.

MC medical wire, which is typically used in cardiac pacemaker leads, of Systems A and B possesses fatigue strengths of approximately 800 MPa and 650 MPa, respectively, at greater than 100 million cycles.¹² In addition, both of these materials are cold drawn to promote these strength levels, which results in a relatively low axial ductility of 2–3.5% strain to rupture. The results of this NC study using System A included an average grain size of approximately 100 nm, axial ductility of greater than 10% to rupture in a monotonic tensile test, and a fatigue strength exceeding 1000 MPa at greater than 100 million cycles. An SEM micrograph of the surface of the NC microstructure is shown in Figure 5. These results were achieved in medical wire with a diameter of 178 microns, which is suitable for coil and/or medical cable production.

Figure 6: Stress-life (S-N) representation of ASTM F562 medical wire fatigue data showing realised increase in fatigue strength over the past decade.

In 1997, work completed by Altman et al. demonstrated fatigue strength of less than 400 MPa at the 100 million cycle life.¹⁶ The current results indicate a stress at 100 million cycles that is more than 150% above Altman’s 1997 benchmark. This dramatic improvement in fatigue was realised while achieving at least 300% better material ductility. In Figure 6, the increase in fatigue strength is presented relative to historical data. Further work, including TEM studies, is in progress to better understand fatigue damage evolution mechanisms in this novel material system. Continued research should allow realisation of even greater damage safety thresholds and better medical devices.

Future work

Low dislocation density in the NC study with System A has not yet been confirmed with TEM examination, although this is in progress. Evidence of the low damage state and the effect of grain size on fatigue were confirmed through high cycle fatigue testing and uniaxial tension testing.

In the last decade, there has been significant growth in research on ultra fine microstructures and their physical properties. Ni is one of the more complex materials that has recently been examined as a candidate for nanoscale microstructural control to achieve high strength.¹⁷ Despite these advances, at the time of writing, little has been done to explore property relations in nanostructured materials of complex alloy systems such as high strength super alloys. The true study of nanostructure has only been possible since just before the turn of this century. Much more research is needed to fully understand and utilise the unique property capabilities of these highly engineered microstructures.

---

Jeremy E. Schaffer M.Sc. is an Engineer at Fort Wayne Metals Research Products Corporation, 9307 Avionics Drive, Fort Wayne, Indiana 46809, USA, tel. +1 260 918 3772, e-mail: jeremy_schaffer@fwmetals.com

Some of the research for this work was performed at Purdue University, West Lafayette, Indiana, USA.