Although it has been around for close to 50 years, nitinol continues to challenge the world's leading medical device designers. While the alloy helps device manufacturers achieve things they could only have dreamed of, its use must be carefully considered before being implemented.
How is it being used today, how has that changed, and what might the future hold as the understanding of the material and its integration with medical devices evolves?
Shape and elasticity
The unique property of nitinol is its ability to undergo phase transformation when subjected to temperature changes, external loadings or a combination of both. The two most important phases are known by the terms 'martensite' and 'austenite'. Transformation between these two phases due to heat or mechanical loading, making nitinol a dynamic composite, exhibits shape memory and superelasticity.
In its martensitic, or low-temperature phase, nitinol is ductile and can be deformed easily. When heated, the material transforms to its austenitic phase, which allows it to return to its pre-deformed state. For this reason, this temperature-related shape recovery is known as shape memory.
Austenitic, or the high temperature phase, can switch into martensite under mechanical loading such as external force or displacement. On release of the external force or displacement, Martensite returns to austenite, or the parent shape.
This shape recovery upon release of the external loading is called superelasticity, which allows nitinol to recover at much larger amounts of deformation than what could be recovered through standard elasticity. However, the discovery of shape memory and superelasticity in nitinol did not immediately lead to the massive commercial products seen today.
In contrast, it took decades to develop the first commercial use of nitinol, which focused on the shape memory effect on products such as pipe couplings, industrial actuators. While technically successful, these applications did not lead to any recognisable commercial use of nitinol.
Understanding and commerciality
One of many reasons holding nitinol back from massive commercial use was the lack of knowledge of the material's processes and superelasticity. Much of the alloy's early technical development focused on refining the melting processes and understanding the basic metallurgical mechanisms.
Nitinol melting is particularly challenging because it must be melted under a vacuum to minimise impurities. In addition, the alloy is sensitive to the composition of the nickel and titanium that changes slightly will lead to different mechanical properties and therefore different end products.
Knowledge about the processing of the devices using superelastic nitinol was incremental until the 1980s: cellular telephone antennas, eyeglass frames and orthodontic archwires made from nitinol were the first to see large-scale commercial success. These products indicated that the commercial use of nitinol had changed from using shape memory to superelasticity, although many nitinol medical devices today originated from ideas using shape memory.
The medical commercial use of nitinol is credited to two landmark medical devices developed during the late 1980s: Mitek's Homer Mammalok for breast tumour localisation, and Nitinol Medical Technologies's Simon Nitinol Filter, which traps potentially deadly blood clots in the venous system, developed. These successes opened the door for Class III medical devices. The applications of nitinol in medicine have since multiplied.
Another significant change after the late 1980s was the sharing of knowledge and understanding of nitinol among medical device engineers, from a processing technology standpoint and a device development standpoint, thanks to the persistent efforts from nitinol producers to educate the industry on how to design, test and manufacture nitinol medical devices. A key element was the formation of the Shape Memory and Superelastic Technologies Society, and the launching of its international conferences starting in 1994.
With significant improvement in the understanding of manufacturing process technologies for producing nitinol devices, particularly advances in laser-cutting technology, surface finishing and the introduction of the thin-walled micro tubing at the end of last century, the nitinol self-expanding stent market took off with the 1998 market release of the Cordis SMART Stent.
This marked the domination of peripheral vascular products such as stents and guidewires.
Followed by the success of the nitinol stents, endovascular aneurysm repair devices, inferior vena cava filters and embolic protection devices have been developed into significant commercially successful products.
The development of ASTM standards that apply to the nitinol raw material itself and various test methods originally characterised the material's use. The F2063 wrought material standard gives medical device engineers assurance that the starting material meets the basic requirements for use in a medical device.
Meanwhile, nitinol's final properties are mostly determined by the processing it undergoes during device manufacturing. Testing standards, such as the F2129 corrosion testing standard, describe the acceptable test methods for verifying many of the critical outputs of the nitinol medical device manufacturing process, including the effects of heat treatment and surface finishing.
Equipped with more knowledge about the material and processes, nitinol manufacturing has advanced beyond blacksmithing. Today, most nitinol-based medical device manufacturing methods have implemented advanced engineering quality control systems, sophisticated manufacturing and test method validations, statistical process control and lean manufacturing tools. A few even go beyond the traditional manufacturing that they offer design, analysis and testing.
As knowledge increases, the applications of superelasticity are dominant in the medical device field with wide use of nitinol-based stents, guidewires, surgical and diagnostic instruments, scoliosis rods and embolic protection filters. Nitinol has become the material for novel medical devices, particularly in the minimally invasive implantable or diagnostic devices and instruments – some even say that nitinol is a solution looking for problems.
Failure is the mother of success, and the great successes of nitinol's medical use are accompanied by mistakes and lessons. At the turn of the century, many in-vivo fractures of nitinol medical implantable devices were reported. Even though some of these fractures do not result in immediate clinical consequences, the effects of these in-vivo fractures are substantial and have misled regulatory and medical care professionals in the belief that nitinol is bad in fatigue resistance.
While in-vivo fractures of nitinol occurred partly because some optimistically thought fatigue was not an issue for superelasticity since the material can recover from large strains with negligible residual, the most critical issues are that these medical devices went to clinical use without the necessary knowledge about material fatigue and the medical device use environment.
The combination of a lack of material fatigue and a lack of design requirements, not nitinol itself, are to blame for many in-vivo fractures.
It is better understood that although nitinol is a superelastic material, it has its limitations. While superelasticity gives rise to superb fatigue performance at high amplitude and high mean strains, the alloy's fatigue crack growth threshold is lower than most metals used in medicine.
The indication of lower fatigue crack growth threshold and faster crack growth rate is that nitinol's fatigue is difficult to detect because prior to final fracture there is no sign of material degradations such as unrecoverable plastic deformation or reduction of the forces or both. Knowledge of nitinol fatigue has advanced tremendously in the past 20 years, yet fatigue is one of the highest unmet needs in nitinol medical use.
There remains a lack of understanding about fatigue mechanisms, multiaxial material behaviours, stabilised material properties, descriptive material constitutive law, effective computational methods, and multiaxial fatigue tests and predictive methods. Nitinol fatigue remains in an infant state and is a bottleneck in device design.
The success of nitinol in the peripheral vascular space has spurred development in areas such as general surgery, structural heart disease and orthopaedics. As use of nitinol becomes a reality, the more critical issue is the need for a better understanding of the nitinol fatigue.
For example, a nitinol frame in a transcatheter heart valve, a combination of the stent and bioprosthetic heart valve technology must demonstrate a longer fatigue life than a stent through a more rigorously fatigue safety analysis with a higher fatigue safety factor than a peripheral stent, mainly because of the difference in the device's clinical criticality due to the consequences caused by a fracture of nitinol-based medical devices.
If the transcatheter heart valve frame fractures, it may lead to valve malfunction and a complete heart failure, and the quick death of the patient. However, fractures of a peripheral stent, such as those observed in superior femoral artery, may not lead to any immediate clinical consequences.
As such, nitinol fatigue in structured heart disease applications must be addressed more rigorously than in peripheral stent applications.
Factors such as longer fatigue cycles, higher confidence levels on fatigue limit and more specific device fatigue tests and accelerated fatigue failure modes must be evaluated, so nitinol fatigue should remain an active research area. Increasing criticality about the use of nitinol's medical use also drives the studies on material corrosion.
The past decade has shown improved understanding of nitinol corrosion behaviour. Because nitinol contains 50% nickel, its corrosion performance has been scrutinised. Like stainless steel, nitinol is protected from corrosion by a protective surface layer composed of titanium dioxide.
When properly processed, the biocompatibility and corrosion resistance of nitinol is excellent and provides flexibility and the necessary strength for many applications. Lately, overlapped medical devices are being used where the corrosion of nitinol needs to be addressed in more depth, and its interaction with other metals, different environments, wear and other biological reactions studied.
Less-invasive and active future
Nitinol will continue to be an important material for the medical devices market as procedures become less invasive. As the understanding of extreme biomechanical environments in the human body improves, more sophisticated devices that take full advantage of nitinol's properties will be developed. Peripheral stents that can tolerate severe in-vivo loads such as bending, axial compression, crushing and twisting, are on the horizon.
New applications will require the advances of materials technology in emerging areas such as thin-film nitinol, porous nitinol and new alloys with improved mechanical and physical properties.
A combination of the micro-electrical-mechanical systems that use the chip fabrication technology and nitinol will be another exciting and successful area for active medical implantable devices.