From Material to Application
In the development of applications based on shape memory alloys (SMA), material and application development have always gone hand in hand. Both improvements in material properties and a better understanding of key aspects have led to new applications. This article provides a historical overview of the systems developed so far and the associated problems, pitfalls, and special features.
We owe the discovery to accident
The discovery of the shape memory effect in NiTi alloys dates back to the late 1950s. The story of this discovery is particularly fascinating because the “greatest inventor of all time – accident” (Mark Twain) lent a hand not just once, but twice. A young researcher named William J. Buehler was tasked with producing a tip for a military missile that could withstand special challenges such as heat, fatigue, and impact force. The first accidental discovery occurred after Buehler found that a 1:1 alloy of nickel and titanium was suitable for this task. A bundle of cast NiTi rods fell to the floor, and he noticed that the still-warm rods produced a distinctly higher-pitched, less-damped sound than the cold rods. This revealed for the first time that temperature, and the associated phase transformation, appeared to strongly influence the stiffness and damping of the material.
Chance lent a hand once again in 1961, when Buehler presented a NiTi sample at a meeting of the laboratory’s management. The sheet, folded like an accordion, was passed around, examined by the participants for its mechanical properties, and bent in the process. One of the attendees, a pipe smoker, heated the sample with his lighter, and to everyone’s surprise, the accordion-shaped strip contracted and returned to its original shape. In 1963, Frederick E. Wang joined Buehler’s team, and he subsequently succeeded in scientifically describing the materials science fundamentals and, ultimately, the crystal structure of Nitinol. In 1967, the first scientific conference on NiTi shape memory alloys was held.
Once the materials science fundamentals had been established, the first products based on SMAs were developed. Initial commercial applications focused on the military sector. In 1969, the Raychem Corporation developed a hydraulic coupling (the so-called Cryofit Connector), which was first used in F-14 fighter jets.
The coupling is initially expanded at temperatures well below 0 °C to allow the insertion of a hydraulic line and then stored at around –70 °C. For installation, the coupling is slid over the two pipes to be connected. The phase transformation triggered by the ambient temperature causes the coupling to shrink, creating a permanent connection capable of withstanding pressures of up to 350 bar. Similar approaches were used in the following years in the development of various electrical and mechanical connectors.
The 1980s bring cyclic applications
All first-generation SMA applications in the 1970s had one thing in common: the shape memory effect was used only once, as SMA semi-finished products were not yet suitable for cyclic operation. This changed in the early 1980s. A better understanding of manufacturing processes—both in terms of melting and semi-finished product production as well as post-processing—enabled new applications.
In the field of medical technology in particular, the development of superelastic tubular semi-finished products for producing stents significantly advanced shape memory technology. In household goods—for example in coffee machines or scald-protection fittings—and also in the automotive sector as bypass valves in oil circuits, expertise in SMA spring production led to new products. The second generation of SMA applications in the actuator sector is essentially characterized by the cyclic operation of the systems.
The temperature rises with the ’90s
SMA developments in the 1990s were marked by major advances in materials engineering. For example, the first NiTiCu semi-finished products were created, as well as SMA wires with very high transformation temperatures and, above all, high cyclic stability. In addition, the development of the first SMA models using the finite element method laid the foundation for designing components subjected to multi-axial loads. As a result, these advances in materials engineering led in particular to the availability of high-quality small-diameter SMA wire semi-finished products and, in the early 2000s, to the development of the first products with electrically activated SMA wires. A milestone here is the Omnipod insulin pump from Insulet. In this device, two opposing SMA wires are alternately activated and used to dose insulin as needed.
The 2000s – Make yourself comfortable
Another, rather unusual SMA application in the toy sector was introduced in 2003. In the “Baby Bright Eyes” doll, eye movement was achieved using electrically activated SMA wires. One advantage of SMA wire actuators—the ability to produce very slow and natural movement—turned into a disadvantage here. It was only after the market launch that it became apparent that the high degree of realism frightened children. As a result, production of the doll was discontinued after just a few months.
However, the knowledge gained in the design and electrical control of such systems subsequently led to the most successful SMA application in the automotive sector to date. Starting in 2005, the company Actuator Solutions successfully marketed an SMA-driven pneumatic valve for seat comfort systems. Such valves are used to inflate or deflate air bladders in vehicle seats. This allows the seat geometry to be adjusted statically, for example in the lumbar area, or dynamically, as in massage systems. The market success of this application was due in particular to the combination of various SMA advantages—such as noiseless operation, low weight, and a simpler, more cost-effective design—in a single product. To date, more than 120 million of these valves have been manufactured, and production continues. With this expertise, a first application in the field of optics—autofocus systems for drone cameras—was also developed.
SMA 4.0 – Moving into the smartphone
While third-generation SMA applications relied heavily on high-quality and well-controllable SMA wire semi-finished products, developments in the fourth generation show that further potential could be unlocked in the areas of cyclic stability of SMA actuators, miniaturization, and control. In addition to the development of new processes for producing ultra-pure melts, this is also due to advances in semi-finished product manufacturing, with the availability of very thin wires or foils of reproducible quality.
A groundbreaking fourth-generation application is the autofocus (AF) and optical image stabilization (OIS) systems developed by Cambridge Mechatronics (CML). Using a total of eight SMA wires, each only 25 µm thick and a few millimeters long, arranged in a parallel-kinematic configuration, the lens of smartphone cameras can be finely positioned in all degrees of freedom at frequencies of up to 20 Hz. This makes it possible to actively compensate for hand vibrations and capture extremely sharp images or videos even in low-light conditions.
The market launch of such systems took place as early as 2015, but at that time their market share remained relatively small due to high costs. This changed with CML’s introduction of the 8-Wire SMA AF+OIS system in 2023. A publication from October 2024 reported a total production volume of 80 million units. Compared to the figures published in February, this corresponds to a production rate of almost 4 million units per month. In addition, this application achieved significant progress in control technology: for the first time, the sensory properties of SMAs were consistently used for position control, since in such small installation spaces there is simply no room for sensors. An automation-friendly design and the development of a dedicated driver circuit specifically for this application also reduced system costs to the point where market success became possible.
The first commercial applications based on SMA thin films became established towards the end of the 2020s. Of particular note here are the activities of the company memetis in the field of microfluidics. Using structures made from SMA ribbons, it was possible for the first time to produce very small actuators for valves.
What the 5th generation will bring
In summary, the eventful history of SMA applications shows that each milestone achieved in materials engineering has enabled the development and marketing of new products. Current advances in high-temperature alloys and in the AI-assisted development of ternary or quaternary alloys suggest that this trend will continue. In the automotive sector in particular, it can be expected that future 5th-generation SMA actuators will make operating temperature ranges possible that currently still seem unrealistic. Additional developments in automated process chains, quality control, miniaturization and MEMS integration, as well as textile and plastic integration, will provide the manufacturing foundation for the fifth generation of SMA applications.
Pictorial sources:
Figure 1:
KAUFFMAN, G.B., MAYO, I. The Story of Nitinol: The Serendipitous Discovery of the Memory Metal and Its Applications. Chem. Educator 2, 1–21 (1997). https://doi.org/10.1007/s00897970111a
Figure 2:
https://www.intrinsicdevices.com/history.htm
Retrieved April 11, 2025
Figure 3:
Michael Weissflog, smarthoch3
Figure 4:
https://www.forbes.com/2003/02/12/cx_ah_0212tentech.html
Retrieved April 11, 2025
Figure 5:
https://www.actuatorsolutions.de/products/pneumatics-and-fluidics/
Retrieved April 11, 2025
Figure 6:
https://www.cambridgemechatronics.com/en/news/resources-blogs/sma-lens-shift-afois-how-cml-integrates-af-and-ois/
Retrieved April 11, 2025
Figure 7:
memetis GmbH, Image crecit: Sascha Linke
Dr.-Ing. Kenny Pagel
- Head of Shape Memory Technology Department, Fraunhofer IWU Dresden
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