Hierarchical Integration of Thin-Film NiTi Actuators Using Additive Manufacturing for Microrobotics

Shape memory alloy (SMA) actuators can provide significant advantages for small-scale robotics given their robustness, energy density, and low voltage actuation. However, NiTi thin films typically found in SMA microactuators do not often provide useful forces and displacements for microrobotic applications. This work presents a fabrication process in which NiTi thin film actuators are integrated with two-photon polymerization (TPP) 3D printing to scale these actuators up for use in mesoscale systems. Individual unimorph actuators are characterized with respect to uniformity across many actuators so that actuators can be arrayed together for even larger forces or combined toward the operation of complex mechanisms. The resulting actuators are fast to prototype, reliable and stable (up to 5000 cycles), and can utilize complex geometries that are otherwise challenging to achieve with conventional MEMS microfabrication techniques. A 2D positioner is demonstrated by combining six individually controlled actuators with conventional mm-scale fabrication techniques (3D stereolithography printing, wire bonding and PCB assembly). The actuators are controlled by a commercial microcontroller and powered using a standard Lithium polymer battery. [2020-0208]

I. Introduction

To enable functional microrobots that can interact with the world, it is necessary to integrate robust, reliable, high work density actuators with electronics and batteries in small volumes [1]–[3]. Shape memory alloy (SMA) provides an attractive option to address these actuator requirements given its low impedance (for low voltage actuation) and its inherently high work density (~ 1 J g⁻¹) [4]. The low voltage operation of SMA has made it particularly compelling for microactuators found in applications such as microswitches [5]–[7], microsensors [8], microrelays [9], micropumps [10], [11], microvalves [12], and microgrippers [13], [14]. SMA is also reliable; if strains are below 2%, it can survive millions of cycles [15]. In addition, SMA based actuators can potentially be powered by unconventional power sources including laser irradiation [7], [16] and fuels [17].