*Invited* SMA Microactuators: From Research to Applications

SMA microactuators make use of planar structures that are fabricated e.g. from magnetron sputtered SMA films, rolled SMA foils or melt-spun ribbons. Initially, SMA microactuators have been fabricated through costly piece-by-piece processing and pick-and-place assembly as parallel fabrication is hampered by material and process incompatibilities. In recent years, considerable progress has been made to enable wafer-scale processing of SMA planar structures and, thus, to reduce the barriers for market introduction. This review gives an overview of the achievements in SMA microtechnologies for the example of SMA microvalves. These achievements include novel transfer bonding technologies to enable integration and processing of SMA films on different substrates, wafer-scale micromachining as well as technologies for selective release from a substrate. In addition, novel methods for electrical and fluidic interconnections have been developed to enable modular designs as well as flexible plug-in interfaces, which greatly facilitate industrial solutions.

The design, fabrication, and performance of SMA microvalves for fluidic control systems are presented in detail. The design consists of SMA microbridges of 20 µm in thickness that generate a homogenous stress profile when deflected in out-of-plane direction. By direct heating the microbridges with an electrical current, the initial flat shape is recovered. The corresponding force is used to perform either a normally open, normally closed or bistable microvalve function depending on the valve layout. An array of microvalves is integrated in a common layer to reduce the number of components. For selective electrical control, an electrical backplane is stacked onto the microvalve layer. Closed-loop flow control is realized by combining SMA microvalves and microflow sensors.

Recent developments include freely configurable fluidic microsystems, e.g. for lab-on-a-chip applications. It is expected that the new SMA microtechnologies enable the introduction of SMAs in other microsystems applications including microoptical devices in information technology or implantable devices in the medical field.