Nitinol is a smart material that requires smart manufacturing. Nobody understands this better than Dr Bernd Vogel, who’s company, Endosmart, has over the past 17 years become one of the main suppliers of disposable nitinol instruments for the medical market. Being asked to describe the shape-memory effect of nitinol, Vogel takes out a wire in the form of a heart, completely straightens the wire, and then slowly dips it into the coffee in front of him. The hot coffee appears to completely reverse the change in the wire, which again shows the perfect form of a heart.
When was the shape-memory effect discovered?
Dr Bernd Vogel: The first observation of such an effect goes back to the 1930s, when Arne Olander, a Swedish chemist, reported ‘rubber-like’ behaviour in a gold-cadmium alloy. But it was not until 1962 that researcher William J Buehler discovered these properties in a family of nickel-titanium alloys at the Naval Ordnance Laboratory – NOL. As such, he named the family of this alloy nitinol.
Since then, nitinol has been used worldwide because of its unique shapememory properties. The majority of commercial applications for the material take advantage of its extreme elasticity, also known as super-elasticity. In this state, the substance can tolerate a considerable amount of bending without any permanent damage taking place. These extreme elastic properties revolutionised medicine.
In the medical device industry, many procedures involve the implantation of a wide range of metallic constructions. Contrarily, traditional materials, such as stainless steel and titanium, lack the flexibility and elasticity inherent in living tissues. The absence of biomechanical compatibility has a negative effect on adjoined tissue, resulting in unfortunate scenarios, such as the loosening of bone implants.
However, the properties of nitinol are far closer to living tissue than any other material. With this in mind, nitinol has been continuously used as an implant material for bone fractures. By having access to super-elastic nitinol tubes, the way was paved for vascular implants, like selfexpanding stents, filters and grafts.
Do standards exist for designers who do not have a deeper knowledge of the nitinol material?
The American Standard of Testing Material (ASTM) committee has developed standards for nitinol wrought materials applied as medical devices and implants. In these standards, testing methods and specification of nitinol in the form of wire, tube and sheet are defined. But without deeper knowledge, these standards are too broad to source readily available material off the shelf. The lack of standardisation allows the industry to develop standards of its own. This results in variation within the material from one supplier to another. It has to be kept in mind that the material properties are so sensitive that when a slight change of nickel content or cold work takes place, it can have significant effects on the behaviour of the material.
Which processing methods can be applied to nitinol?
The fabrication of stand-alone nitinol components can be scaled into three groups. The first is structuring the wrought material. If the product is fabricated out of tubes or sheets, then the following methods are suitable: laser cutting, wire EDM, waterjet cutting and chemical etching. On the other hand, if a wire, strip or ribbon is needed, the profiling would mostly likely be an abrasive technique, such as grinding. In some cases, eroding or chemically etching can also be successful.
The second is shape setting. The process that is required to set the shape is similar whether beginning with wire, sheet or tube material. The structured component from step one must be constrained on a fixture, followed by a heat treatment. The heat treatment may be a salt bath, sand bath, an air or vacuum furnace, or various heating methods, such as inductive heating.
The third is surface finish. Depending on the heat treatment, the material will have a different surface condition or oxide layer. The resulting surface finish will depend on the performance of the instrument. For implants, it will have an effect on durability, corrosion and other properties.
If the nitinol component is only one part of the assembly, the designers have to look carefully into the joining methods. Nitinol can be easily welded to itself by using a laser or ebeam. However, one must consider that welds do not have superelastic properties and, therefore, should be placed in a section of the assembly where only slight deformation occurs. When joining nitinol to other materials, such as titanium or stainless steel, it can be quite a difficult process. Research has shown successful joining by using soft soldering with aggressive fluxes, while resistance or diffusion welding can also be applied. But generally, alternative mechanical options, like crimping or shrink fitting, are preferable.
Is there the possibility of combining this smart material with smart manufacturing, and can it be used as part of industry 4.0?
First of all, we have to clarify the meaning of smart manufacturing and why the phrase ‘manufacturing 4.0’ has a good chance to become the non-word of 2018. The 4.0 stands for the four evolutions of industry. First came the mechanisation with water and steam power, followed by mass production with assembly lines and electricity. The third evolution was automation with calculations over the past few decades.
Manufacturing 4.0 can be described as a cyberphysical system using AI and IoT, which means the connection of technologies at a global structure. When looking into a factory of medical device manufacturers, the quality of the product is the first thing we have to think about. Following this is the productivity and, finally, the profitability. These parameters do not depend on whether you have a complete hands-on operation or an automated process.
What impact does that have on processing nitinol?
As discussed, nitinol, with its superelastic and shape-memory properties, comes into play when all other materials fail, because the alloy’s properties and the needs of the medical industry overlap. On the other hand, conventional machining, like milling and turning, cannot be applied using titanium or stainless steel. Within every process, one has to be aware of what effect it has on the properties of the material. The amount of cold work, the heat treatment, the rate of strain, and the number of cycles all have an impact on material properties and therefore change the stress-strain behaviour.
Because of this, the industry is having a hard time converting nitinol fabrication into an industrial automation. Today, medical products made out of nitinol are mainly processed within manual operations – handcrafted with a low degree of automation. This makes the outcome dependent on the worker, and the process is difficult to validate. So, with regards to the four evolutions of nitinol processing, we are quite a way from nitinol 4.0.
What, possibly, is the next big thing for nitinol in the medical field?
To investigate automation, there must be a need for serial production. In the past few decades, the alloy became popular in the treatment of peripheral vascular diseases. Stents were the first medical products with a high demand. Unfortunately, the variety of versions needed for these implants is so high that the profitability to install automated processes is questionable. Therefore, these implants are mainly processed with a low-level degree of automation.
Today, as well as in the future, more surgical procedures will be operated using flexible endoscopes and interventional radiology. Using these methods, the surgeon is able to reach a point of interest in the body in a minimally or non-invasive way. The first mass-produced article in this field was the super-elastic, kink-resistant nitinol guide wire. Due to its elasticity, the ability of the wire to follow a tortuous path in the body – and still rotate smoothly – lead to improved performance. In material science, these wires fall into the category of deformation-resistant applications.
Moreover, the largest family in medical instruments is the group of shaperestoring applications. These products are temporarily deformed in order to be introduced into the body and, when removed, spring back to their original shape without the need for increasing the temperature. They are used for various functions, such as loops, snares, retractors and baskets – as well as for removing foreign bodies and blood clots.
In the urological field, there is already a worldwide, yearly need for stone-retrieval baskets. The same instrument can also be used to catch various stones that are 1–12mm in size. Therefore, the diversity of versions needed in this field is low. This enables the possibility to install a certain degree of automation in the manufacturing process. The most common version of stone-retrieval baskets are manufactured from wires, which form a basket-like structure.
Additionally, the elliptical-basket form is more suitable for retrieving stones in tube-like structures, such as the ureter. When going into the kidneys, where stones have to be picked mostly from the bottom of hollow organs, ball-shaped basket structures are more suitable. But this is where the drawback of these baskets becomes apparent. In order to form a basket, the wires have to meet again in the distal end of the basket, where there should be an open space in order to more easily pick stones from the bottom wall of the organ. The optimal retrieval device would be like the mouth of a fish, assisted by suction, in which all types and sizes of stones can be easily caught.
Due to the special arrangement of nitinol wires and a tube-in-tube solution, the device closes from the very distal end, being able to also pick stones that have already grown into the tissue wall. Having this in mind, the joining of nitinol is difficult and should be avoided; the wires are profiled and then set into the needed geometry. Since the shaft of the device should be strong and kink resistant, it should be built out of one strong nitinol wire, which allows more flexibility when going towards the distal end of the device. Therefore, the challenge is to integrate a fish-like catching structure to the thin wire construct.
One has to be aware that every processing step has an effect on the nitinol properties, which is why most devices are being processed within manual operation and handcrafted with a low degree of automation. The drawback, again, is that the outcome is dependent on the worker, and the process is especially difficult to validate. However, a degree of automation is moving into these products.
The profiling can be done with a centreless grinding method, taking the needed wire directly from the coil without the need of a machine operator. Directly integrated into this production line is the control of the profile with a 3D laser sensor. In order to function properly, as well as increasing durability, this retrieving device has to have a particular wire section with elliptical cross-sections, rather than standard round crosssections. This can be directly integrated into the measuring detection.
This process is difficult to automate, with the perfect parameters for the heat treatment being directly adjusted to the measured material parameters of the raw material on the coil.
Here, a self-learning process can be installed in order to have the desired superelastic property in the needed temperature window for the finished device. This is crucial because nitinol properties of the raw material differ a lot so the processing parameters have to be adapted, with additional ageing processes needed.
For the family of disposable products, a heat treatment can be done in air. This is because a slight build-up of oxide is not an issue, and in this case, may even be desired. Therefore, an additional surface treatment can be avoided.
What’s the future of nitinol?
20 years ago, physicians had rarely heard about this material. Today, the term nitinol is used in almost every vascular conference, as well as in the field of cardiology, orthopedics and endoscopy. For the future, the field of nitinol offers a high potential for the field of orthopedic implants. In addition to this, new developments in the sputtering process of thin films will open the field for nitinol for use in small stents, valves and actuators. Along with the extrusion of complex cross sections and laser sintering processes, the casting of nitinol and deep-drawn articles are not yet commonly available. Nonetheless, processes are being developed within specialised companies and thus will open a completely new field of application. And remember, for some things, nitinol is the only possible solution.
