The ancient art of origami enables to create 3D shapes from a flat sheet of paper. This concept can be applied to a planar matrix composed of interconnected tiles, which can be self-folded and unfolded using actuated SMA bridges. Origami-inspired self-folding structures offer several advantages, including rapid in-plane fabrication, reduced assembly time, and lightweight designs with high rigidity. Previous research has primarily concentrated on macro-sized demonstrators that have proven advanced functionality in a variety of fields, including robotics and aerospace. In this work, we extend this concept to the microscale by creating a network of self-folding microactuators that can be programmed and controlled through local Joule heating. Our previous concepts faced limitations for reversible latching of tiles in their position after actuation. This paper presents the design, fabrication, and characterization of an origami-inspired bistable microdevices that utilize antagonistic SMA microactuators for bi-directional self- folding and heatable magnetic SMA films for reversible magnetic micro-latching.

The thermal induced martensitic phase transition in TiNiHf was exploited for bi-directional actuation with TiNiHf/SiO 2 /Si composites. When compared to free-standing films of similar thickness, films on a substrate exhibit a reduced fatigue effect upon thermal cycling and a smaller hysteresis width. Differential scanning calorimetry (DSC) and cantilever deflection measurements (CDM) results showed that the transition temperatures of fabricated TiNiHf films and TiNiHf/SiO 2 /Si bimorph composites decrease with thermal cycling. The change in transition temperatures after 40 thermal cycles is significantly reduced for TiNiHf films bound to a SiO 2 /Si substrate compared to the functional fatigue DSC results reported for freestanding films. The thermal hysteresis width is also reduced for TiNiHf films constrained by SiO 2 /Si and Si substrates compared to freestanding films of similar thicknesses. With proper composition selection and microstructural control, TiNiHf films can be promising SMA films for bistable actuators with PMMA/TiNiHf/Si composites.