Nitinol: highly complex, yet super-elastic and enormously variable

Nitinol is perhaps the best-known shape memory alloy (SMA). Like most smart materials, the metal alloy of nickel and titanium is very versatile and can be used in a variety of applications. Gerd Siekmeyer at Acquandas GmbH knows all about the applications for Nitinol and the respective manufacturing processes.

Based in Kiel the company manufactures micro and thin-film components for the healthcare industry - especially as an OEM (original equipment manufacturer) - customized, medical components (active/passive implants, actuator and sensor components, instruments and systems)Five years ago, COMPAMED-tradefair.com had already asked the expert about shape memory alloys and their applications. This time around, we wanted an update on the latest smart material developments.

Mr. Siekmeyer, what were the trends and developments in the smart materials sector over the past few years, especially as it pertains to nitinol?

Gerd Siekmeyer: Materials manufacturers have made great strides in improving alloy melts. Nitinol smart materials require tight tolerances, homogeneity, and purity of the atomic alloying elements. The more precise this ratio and the purer the components, the greater the reproducibility of the mechanical properties of a semi-finished part or component will be.

Novel melting processes also promise fewer impurities in the melts (so-called non-metallic inclusions).

There is also a wider range of semi-finished parts (more suppliers, more products), resulting in smaller and larger strips, wires, tubes, and sheet metals for product engineering.

New manufacturing technologies such as laser lathe (high-speed precision turning and simultaneous ablation with an ultrashort pulse laser), ultrashort pulse laser or erosion processes likewise allow the fabrication of more complex, and smaller parts.

All this resulted in new smart materials applications in an increasing number of implants and instruments. In the past, smart materials were primarily used in vascular stenting, but are now also implemented in many modern medical devices: examples include applications pertaining to minimally invasive stroke treatment, bioelectronic neurotechnology or electrophysiology, catheter-based cardiac assist devices, in-body sensor systems, ophthalmological, neurosurgical, or robotic precision equipment or wearables.

Nickel titanium or nitinol is not a newcomer in the materials sector, but its applications seem to be endless. What are the current limits/obstacles in the production process?

Siekmeyer: Nitinol is a very complex material that only recently facilitates highly variable, safe, and excellent processing thanks to technical engineering. Its price per kilogram is more than ten times higher than plastic. Although you can "make everything imaginable" that someone creates on a modern CAD system, it doesn’t necessarily mean it is the best and most cost-effective way. Especially when it comes to smart materials, there are multiple specialized production processes that don’t correspond or compare exactly to conventional production methods or design solutions. To explore the full potential in this setting, designers, developers, and manufacturers must partner up and collaborate closely from an early stage to incorporate special manufacturing know-how into the product design. Ultimately, it is critical to strike the right balance and find the best solution.

What makes working with smart materials so exciting?

Siekmeyer: Unlike simple plastics or metals, smart materials will always be highly complex substances. However, the drawback of material complexity is also a huge advantage. The increased variability of the material class makes it possible to meet highly complex requirements on a more custom basis. Specific material properties allow us to "embed" comprehensive technical functions in the smallest spaces, which results in a considerably higher functional density. This makes smaller and more cost-effective product solutions feasible. The many facets of modern smart materials make them very exciting, but they also require a willingness to learn and explore new ways and possibilities.

What is the latest state-of-the-art of additive manufacturing and how do these processes work?

Siekmeyer: There are two types of additive manufacturing methods for Nitinol: the conventional production process is applied in 3D printing and uses metal powder. Here, you first use technology to pulverize a solid material to ultimately sinter and form it into a solid-state body. However, pulverization slightly modifies the chemistry of a material since it encourages the accumulation of more oxygen in an alloy, prompting porosity during the sintering process. A component made from this substance is subsequently best suited for medical devices such as orthodontic brackets or dental drills and less fitting for vascular implants.

The PVD sputtering method is an ultra-precision, additive manufacturing process, which we also implement at ACQUANDAS GmbH. Using the right alloy ratio to normative chemistry, the material is directly additively processed at the atomic level. In this high vacuum deposition technique, molecules are released from a nitinol solid-state body via high-energy ion bombardment – predominantly noble gas ions – and change into the gas phase. Sputtering is thus used for the molecular atomization of a material, which is then deposited via a plasma cloud on a substrate, where it forms a solid, non-porous film. This process facilitates instant high-precision and structured material thicknesses of up to 0.1 mm. The huge advantage of this process is the ability to form specific film systems with distinct properties and functions, even from different groups of materials.

ACQUANDAS is also engaged in some of the latest current research projects. What role do SMAs play in each setting?

Siekmeyer: The material properties of shape memory alloys (SMA) facilitate various applications of sensors. Strain gauges (sensors) utilize the specific electrical properties of SMAs combined with the super elasticity of the SMA material. This technology also supports high-temperature applications, which is especially beneficial when it comes to lightweight composites or textile products such as prosthetics, exoskeletons, or steerable catheters.

New SMA sensor components also make it possible to widen or improve the range of applications of standard sensors. For example, gas-tight, biocompatible enclosures or shape memory alloy thin membranes can be used for medical technology applications.

Applications of neurotechnology use components and parts for bioelectronic stimulation and nerve detection. Smart (SMA) material forms the mechanical medium that facilitates a targeted delivery via the vascular system. To treat neurological disorders, you could combine this with layered materials to enable an additional functionalization of these component.