Nanosized shape memory actuators that fold themselves into 3D configurations could enable locomotion, novel metamaterial design, and highly-fidelity sensors. CREDIT: Cornell University.
Micron-sized shape memory actuators could allow atomically thin two-dimensional materials to fold themselves into 3D configurations with just a quick voltage burst. Photo shows what could be the world’s smallest self-folding microscale origami duck transformed from a flat sheet at a low voltage of ~1?V and holding its 3D shape without the voltage. CREDIT: Cornell University.A new type of shape-memory actuator based on nanometer-thick platinum films capped with titanium dioxide could enable low-power, electrically programmable microrobots, according to scientists from Cornell and the University of Pennsylvania [Liu et al., Science Robotics 6 (2021) eabe6663, https://doi.org/10.1126/scirobotics.abe6663]. Shape-memory materials take up a temporary configuration and return to their original form when stimulated by temperature, light, or an electric or magnetic field. Typically shape-memory actuators are made from polymers, alloys or ceramics, so a metallic device driven by standard electronics could be hugely useful.
“We’ve learned how to build complex systems and machines at human scales, but we haven’t learned how to build machines at tiny scales,” says Paul L. McEuen, who led the study with Itai Cohen. “[Now] we have invented the world’s smallest electrically programmable shape memory actuator that enables a broad spectrum of micromachines.”
The devices consist of a 7-nm-thick layer of platinum grown by atomic layer deposition capped on one side with a 2-nm layer of titanium or titanium dioxide. Voltage-induced electrochemical oxidation and reduction reactions on the platinum surface create strains within the oxide layer that bend the layers. During oxidation, oxygen and platinum atoms switch positions and an oxide layer builds up, forming a barrier that prevents further growth, so that once the actuator is bent it stays in position even after the voltage is removed.
“We made a CMOS-compatible, voltage-driven, and nanometer-thin shape-memory actuator. The tighter the bends, the smaller the folds, and the tinier the footprint of each machine. It’s also important that these bends can be held without voltage because that minimizes the power consumption of microrobots,” explains first author Qingkun Liu.
This new class of actuator, which operates in aqueous solution, is capable of a high degree of curvature and fast response, in less than 100 ms. The devices only require a small voltage, of less than 1 V, to change shape and can stay in position for hours without any driving voltage at all. Moreover, the devices can flex back and forth between configurations thousands of times without failing.
To demonstrate the capabilities of the approach, the researchers created the world’s smallest origami bird that can fold and unfold its wings on the application of a small voltage. The team also fabricated actuating micro-positioners and self-folding Miura-origami metamaterial sheets. The actuators could be used to create electrically responsive microscale robotic elements including pop-up origami-inspired three-dimensional structures, morphing metamaterials, and mechanical memory elements.
“Think of all the things robots do for us at the macroscale,” says Cohen. “Could we have micro-factories that transform the way we make things or develop surgical robotic instruments able to perform more dedicate surgeries on length scales smaller by a factor of ten than we can currently?”
In the future, adds Liu, the researchers hope to develop microactuators with a footprint of 100 µm that can operate in air and be integrated with control circuits, sensors, and memory devices. This could enable the creation of microrobots able to sense their environment and make decisions autonomously, carry out complicated tasks at the microscale and interface with biological materials.
“These microrobotic systems could be used to synthesize nanomaterials, manipulate biological materials, and deliver drugs to specific locations,” suggests Liu.
Julia R. Greer, Ruben F. and Donna Mettler Professor of Materials Science, Medical Engineering, and Mechanics and Director of the Kavli Nanoscience Institute at Caltech believes the work represents a breakthrough in terms of reconfigurable and responsive robotics.
“The actuators with dimensions below a millimeter are very responsive to low applied voltages and can curl up into rolls with diameters of 500 nm or less, which is an unprecedented change in curvature from an initially flat thin plate/film,” she points out. “The key material innovation here is the incorporation of a shape memory effect, induced by electrochemical oxidation of the nanocrystalline Pt film.”
The ability to switch between different configurations enables devices and robots fabricated from these shape memory actuators to perform as excellent swimmers in aqueous environments, she adds.
This article was first published in Nano Today 38 (2021) 101167.