Origami-inspired Bistable SMA Microdevice

We present the design, fabrication, and characterization of an origami-inspired microdevice consisting of antagonistic shape memory alloy (SMA) microactuators for bi-directional self-folding and heatable magnetic SMA films for reversible magnetic latching. The present research aims at microactuation in 3D space following the concept of origami, which allows folding of any 3D structure starting from a planar sheet of paper. One can transfer this idea to a planar matrix, which consists of many so-called tiles that are interconnected by actuated SMA bridges enabling active folding and unfolding. Previous work mainly focused on macro-sized demonstrators that have proven their functionality in various fields such as robotics and aerospace. Here, we transfer this concept to the microscale by designing a scalable network of self-folding microactuators that can be programmed and controlled through local Joule heating. The microfabrication includes sputter deposition of TiNiCu and NiMnGa films, micromachining of the films and polymer tiles by optical lithography, and hybrid integration of miniature permanent-magnetic NdFeB structures. A major challenge is the setting of the different memory shapes. We design pad-like structures at the end of the microactuators, which serve as electrical interconnections for short-term DC Joule heat treatments. Single SMA bridge microactuators reveal bidirectional folding angles of ±85°, which are enhanced by the additional magnetic latching forces. Our demonstrator device consists of four triangular tiles that can be folded on demand to either a pyramid or a table. Current research aims at understanding the limitations in further miniaturization and in performance due to thermal cross-coupling.