Stretchable Auxetic SMA Actuators

Recently, sputtered thin-film superelastic alloys patterned in serpentine and auxetic geometries were shown to have several advantageous mechanical and electrical properties over traditional stretchable conductors such as thin-film copper. This means that structured thin-film shape memory alloys (SMAs) can be used as actuators, wires, electrodes, and substrates for applications such as soft robotics and wearable electronics. Two-dimensional thin-film SMAs structured into auxetic geometries can also be shaped to remember three-dimensional shapes with complex curvatures, which has several uses in actuators, medical implants and surgical tools.

Typically, SMA actuators are configured into so-called “mono-stable” configurations to switch between two different shapes. Upon removal of the heating source, the mono-stable SMA actuator will return to the low-temperature shape due to the reverse phase transformation. A drawback to this approach is that the SMA requires a constant heating source in order to hold the actuator in the high-temperature shape. Bi-stable SMA actuators based on trimorph SMA composites (i.e. polymer/SMA/metal) can overcome this limitation because they can hold two different shapes at room temperature.

The fabrication and characterization methods presented in this talk will show how stretchable electronics and auxetic metamaterials can be combined with functional SMA properties to create a new class of stretchable actuators and hybrid electronics. Flat-wire and flat sheet SMAs are structured primarily through laser processing techniques. Sputtered superelastic and SMA thin-films are fabricated via layer-by-layer deposition and photolithography. These methods allow other components, such as polymers, metals, and microelectronics, to be seamlessly integrated into the fabrication process.