Shape Memory Alloy
Shape memory alloys (SMA) are an actuator technique for shape-changing displays using metal alloys. Electrical actuation heats up the material which then undergoes temperature driven phase transformation between two different crystal forms (austenite = higher temperature phase / martensite = lower temperature phase). A change in crystal structure results in dimensional changes that are typically between 1% and 8%. The special property of SMAs is that, unlike most crystal transformations, the process is fully reversible. Besides a range of iron-based and copper-based alloys the two most prevalent SMAs are copper-aluminium-nickel (cheap) and nickel-titanium (NiTi) (more stable but superior thermo-mechanic performance). Both of them can be manufactured to almost any shape and size.
The challenges in designing SMA applications are to overcome their limitations, which include a relatively small usable strain, low actuation frequency, low controllability, low accuracy and low energy efficiency. SMA deactuation for example is typically slower than actuation, because it occurs by heat transfer to the ambient environment.