Materials for electronically controllable microactuators

Abstract

Abstract: Electronically controllable actuators have shrunk to remarkably small dimensions, thanks to recent advances in materials science. Currently, multiple classes of actuators can operate at the micron scale, be patterned using lithographic techniques, and be driven by complementary metal oxide semiconductor (CMOS)-compatible voltages, enabling new technologies, including digitally controlled micro-cilia, cell-sized origami structures, and autonomous microrobots controlled by onboard semiconductor electronics. This field is poised to grow, as many of these actuator technologies are the firsts of their kind and much of the underlying design space remains unexplored. To help map the current state of the art and set goals for the future, here, we overview existing work and examine how key figures of merit for actuation at the microscale, including force output, response time, power consumption, efficiency, and durability are fundamentally intertwined. In doing so, we find performance limits and tradeoffs for different classes of microactuators based on the coupling mechanism between electrical energy, chemical energy, and mechanical work. These limits both point to future goals for actuator development and signal promising applications for these actuators in sophisticated electronically integrated microrobotic systems.

Keywords: Actuation; Microelectromechanical systems (MEMS); Nanoelectromechanical systems (NEMS); Robotics.

Figure 1

Actuators that respond to electronic control signals yet operate at dimensions under a millimeter have enabled a variety of remarkable applications. Such tiny devices can be used to make microscopic robots, turn 2D lithographic patterns into controllable 3D origami structures, pump liquids under user command with electronically controlled cilia, and manipulate cells and microorganisms with microgrippers. These applications are evolving rapidly, thanks to electronic control: circuits can be used to generate behaviors that respond to external stimuli or are reprogrammed on-demand.

Figure 2

Examples of electronically controlled microactuators. (a) Actuators that operate at low voltage (sub 10 V) and high curvature (>1 mm⁻¹) can readily integrate with circuits at the microscale. In recent years, several classes of actuators have emerged that meet these demands, spanning operating voltages from ~100 mV to 3 V and curvatures up to 1 µm⁻¹. Broadly, actuators can be classified as electrochemical (bulk^,,, and surface^,), thermal,^, and piezoelectric. (b) Examples of each class of actuators have been demonstrated at the microscale, including surface electrochemical actuators (top panel), bulk electrochemical actuators using the lithiation of silicon to buckle microscale beams or the charging of polymer layers to control microgrippers^,, (second from top), thermal actuators for microscale grippers and origami (second from bottom), and nanometer-thick aluminum nitride piezoelectric actuators (bottom panel).

Figure 3

Examples of microrobots with bending microactuators. (a) Microrobots with thermal microactuators made with nitinol shape-memory alloys (SMAs). Direct laser actuation heats the hinges, causing them to bend and the robots to walk. These microrobots walk on land at speeds close to a body length per second and can be tracked with onboard retroreflectors. (b) Microrobots with surface electrochemical actuators and onboard digital control electronics. Both the legs and the circuit on these robots are powered by light. The onboard microelectronic circuit generates clock signals to drive the legs and set the gait of the robots. These microrobots operate in aqueous environments, move at close to 0.1 body lengths per second, and can change behavior in response to optically delivered commands. PVs, photovoltaics; IC, integrated circuit; SEA, surface electrochemical actuator.

Figure 4

(a) Efficiency versus power consumption per square millimeter for actuators operating at 1 Hz. The dotted line shows the approximate power per millimeter square for a silicon photovoltaic (PV) in bright sunlight (given by 1 mW/mm incident light intensity and assuming PV efficiency is about 10%). Even for microactuators with comparable efficiencies, power consumption can vary over almost six orders of magnitude. (b) Efficiency versus strain for microactuators shows a general relationship between the two: more efficient actuators operate at higher values of strain. In the case of electrochemical actuators, this result can be rationalized by looking at the dominant scaling behavior for electrical and mechanical energy. Moreover, if efficiency and strain scale together, then there are fundamental limits on actuator performance set by the elastic limits of the constituent materials. Indeed, vertical lines show where constituents for electrochemical actuators would begin to fail, indicating that within this class, further improvements in efficiency could be impossible without material innovation. Data are drawn from the following references: electrochemical bulk,^,, electrochemical surface,^, battery, thermal,^, and piezoelectric. SEAs, surface electrochemical actuators; EAPs, electroactive polymers.

Figure 5

Force per square against actuator response time shows that microactuators can operate over a wide range, but evidently face a performance tradeoff. An engineer can currently choose between a fast, weak actuator or a slow strong one, but no actuator achieves both high force and fast response. For thermal and electrochemical actuators, these limits arise from transport constraints: an actuator needs to reach thermal (chemical) equilibrium to impart force. Future work could engineer these transport properties, thereby improving response time. Data are drawn from the following references: electrochemical bulk,^,, electrochemical surface,^, battery, thermal,^, and piezoelectric.^, SEAs, surface electrochemical actuators; EAPs, electroactive polymers.