Shape-memory actuators allow machines ranging from robots to medical implants to hold their form without continuous power, a feature especially advantageous for situations where these devices are untethered and power is limited. Although previous work has demonstrated the creation of shape-memory actuators using polymers, alloys, and ceramics, the need for micrometer-scale electro–shape-memory actuators remains largely unmet, especially ones that can be driven by standard electronics (~1 volt).
Here, we report on a new class of fast, high-curvature, low-voltage, reconfigurable, micrometer-scale shape-memory actuators. They function by the electrochemical oxidation/reduction of a platinum surface, creating strain in the oxidized layer that causes bending. They bend to the smallest radius of curvature of any electrically controlled microactuator (~500 nanometers), are fast (<100-millisecond operation), and operate inside the electrochemical window of water.
The device consists of a nanometer-thick platinum thin film capped on one side by a passive layer. The platinum is grown via atomic layer deposition (ALD), and the ones reported here are 7 nm thick and capped on one side by a 2 nm thick layer of either sputtered titanium or ALD titanium dioxide. By patterning 1 mm thick rigid polymeric panels on top of the actuators, we localize bending to the unpatterned region and produce what are effectively folds.
The microactuator has stable states with and without the platinum oxide layer even if the voltage is removed, i.e., shape memory. The shape-memory “read,” “write,” and “erase” operations of a simple gripper are shown below. The microgripper consists of hinges linking 10 mm–by–10 mm rigid panels. In the oxidized state, the hinges are flat and the gripper is open. To erase the state, a voltage of −0.5 V is applied to the microactuator relative to the Ag/AgCl reference electrode that causes the gripper to change into the closed position. To read the state, we lift the voltage probe and observe the bending state of the device at its open-circuit potential (OCP) in the microscope. To re-write the original state, a voltage of 1.1 V is used to return the gripper to the open position.
The electrochemical mechanism behind the shape-memory operation is illustrated in the figure below: Low voltages cause reversible electrochemical adsorption of O2−/OH− ions decomposed from the water onto the Pt (states I and II). At higher positive writing voltages, the initial O2− adatoms undergo an interfacial place-exchange process with the Pt surface atoms, leading to a quasi-three-dimensional surface lattice, and finally exchanging with the deeper Pt ions to form a nanometer-thick nonstoichiometric oxide PtOx (states III and IV). Unlike the adsorption of ions onto the surface at low voltages, this oxidation process is irreversible within a finite electrochemical window over the time scales of hours, well beyond what is needed for many applications.
The writing/erasing operations, switching speed, and cycling stability of a SEA are shown below.
The actuator can be rapidly switched between the oxidized and reduced states using brief electrical pulses of 1.1 and −0.5 V, respectively, a process reminiscent of writing/erasing electronic states in a nonvolatile information storage device. Switching between these states is very fast. The actuator returns to the completely reduced state within 20 ms. The oxidation process is slower, but with the most rapid change still occurring within 100 ms. The oxidation/reduction is highly repeatable, with only a 5% variation in the oxidized/reduced states after ~100 cycles of operation.
The new physics we explore is how to predict the amount of oxidation time at a set voltage to form a platinum oxide layer of a given thickness and achieve the desired bending curvature. The oxide thickness is linear in voltage above 0.7 V. With the calibration of the strain between the oxide layer and the substrate, we can determine the curvature for any combination of oxidation voltage and time.
This predictable behavior allows for the design of microrobotic elements with controlled multistable configurations. These structures use origami- and kirigami-based designs with actuators and rigid panels. Mountain and valley folds are dictated by the growth order of the Pt and inert layers and location of the rigid panels. Bidirectional folding creates combinations of mountain (downward) and valley (upward) folds, greatly expanding the space of the shapes compared with unidirectional folding. We demonstrate that these shape-memory actuators can be used to create basic electrically reconfigurable microscale robot elements including actuating surfaces, origami-based three-dimensional shapes, morphing metamaterials, and mechanical memory elements. Our shape-memory actuators have the potential to enable the realization of adaptive microscale structures, bio-implantable devices, and microscopic robots.
Read the full article published in Science Robotics here: https://robotics-sciencemag-org.proxy.library.cornell.edu/content/6/52/eabe6663
Read the focus article of our work here: https://robotics.sciencemag.org/content/6/52/eabh1560
Our work is also featured on the cover: https://robotics.sciencemag.org/content/6/52
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