Fifty years of Moore’s law scaling in microelectronics have brought remarkable opportunities for the rapidly evolving field of microscopic robotics1,2,3,4,5. Electronic, magnetic and optical systems now offer an unprecedented combination of complexity, small size and low cost6,7, and could be readily appropriated for robots that are smaller than the resolution limit of human vision (less than a hundred micrometres)8,9,10,11. However, a major roadblock exists: there is no micrometre-scale actuator system that seamlessly integrates with semiconductor processing and responds to standard electronic control signals. Here we overcome this barrier by developing a new class of voltage-controllable electrochemical actuators that operate at low voltages (200 microvolts), low power (10 nanowatts) and are completely compatible with silicon processing. To demonstrate their potential, we develop lithographic fabrication-and-release protocols to prototype sub-hundred-micrometre walking robots. Every step in this process is performed in parallel, allowing us to produce over one million robots per four-inch wafer. These results are an important advance towards mass-manufactured, silicon-based, functional robots that are too small to be resolved by the naked eye.
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We acknowledge the CNF staff, in particular T. Pennell, J. Clark, V. Genova, C. Alpha and G. Bordonaro, for guidance and support with the fabrication process; S. Norris and B. Bircan for discussions; funding from the Army Research Office (ARO W911NF-18-1-0032), the Air Force Office of Scientific Research (AFSOR) MURI Grant FA2386-13-1-4118, the Cornell Center for Materials Research DMR-1719875, NSF Grant DMR-1435829 and the Kavli Institute at Cornell for Nanoscale Science. This work was performed at the Cornell NanoScale Facility, an NNCI member supported by NSF Grant NNCI-1542081.
The authors declare no competing interests.
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Extended data figures and tables
a, X-ray reflectometry thickness measurements as a function of the number of growth cycles of ALD. The first 30 cycles are a nucleation phase where the film grows rapidly in thickness. Once the surface is covered in platinum, a bulk growth phase begins at a slower rate. b, Resistivity measurements (via four-point probe) as a function of ALD cycle number. We interpret the dramatic drop in resistivity after about 30 cycles as clear evidence that a continuous electrical film has formed.
a, Cross-sectional TEM image of a titanium–platinum SEA structure, with colours coded by electron energy loss spectroscopy. Silicon is shown in red, titanium in green, oxygen in blue and platinum in white. b, In-plane TEM image of an ALD platinum film showing crystal grain sizes of approximately 10 nm. Scale bars, 10 nm.
The curvature in a SEA is set by both adsorption of ions and pre-stresses built up during fabrication. Here we plot the curvature of a titanium–platinum SEA in the absence of any applied bias as a function of the titanium layer thickness. The curvature varies continuously from positive to negative as the titanium layer is increased. As the voltage is fixed to the open circuit potential, this effect is purely due to pre-stress. The sign of the curvature inverts at a titanium thickness of around 4 nm. Overall, the pre-stress provides an added level of control over the three-dimensional structure formed using SEAs and rigid panels.
a, Top: cyclic voltammogram of a titanium–platinum SEA over a large voltage range versus Ag/AgCl, swept at 1 V s−1. Standard features of platinum electrochemistry are observed: hydrogen adsorption and desorption peaks from about −0.5 V to −0.9 V and a broad peak at positive voltages that includes both oxygen-species adsorption and later oxidation of the platinum. When the sweep returns from the oxidation regime, there is an oxide reduction peak at about −0.3 V. Bottom: the curvature of a SEA over the same range, showing strong hysteresis in the platinum oxidation regime. b, Top: cyclic voltammogram for a titanium–platinum SEA, sweep rate 400 mV s−1, over a narrower sweep range, avoiding oxidation of the platinum. We find that the cyclic voltammogram is relatively reversible in both the hydrogen and oxygen adsorption/desorption regimes. Bottom: the curvature of a SEA over the same quasi-reversible range of voltages. We observe two branches of actuation, the hydrogen and oxygen-species adsorption regimes, each with a small amount of hysteresis. All measurements were performed in phosphate-buffered saline solution.
a, Cyclic voltammogram for a graphene–platinum SEA that is 70 μm long and 13 μm wide over the voltage range used here to actuate the robots: the oxygen-species adsorption/desorption regime. b, The SEA curvature (red dots) and the capacitive portion of the response (blue crosses). The latter is obtained by removing current that does not depend on the voltage sweep direction (Methods). A peak in charge transfer occurs in the same region where bending takes place. The integrated magnitude of charge transferred gives a surface charge density of 130 μC cm−2, consistent with typical values for oxygen-species adsorption43,44,45 The data are well represented by a standard charge transfer model with nearly ideal adsorption (dashed lines) (Methods).
a, A labelled scanning electron microscopy image of a silicon photovoltaic, 10 μm by 20 μm in size. b, A schematic cross-section of one of our photovoltaics. c, A current–voltage curve for a photovoltaic with (red curve) and without (blue curve) illumination. The devices were illuminated by a 785-nm wavelength laser with an intensity of 100 nW μm−2. The illuminated current–voltage curve shows an open circuit voltage (Voc) of approximately 700 mV.
a, Schematics of the three surfaces robots walked on: hexagonal arrays of knobs spaced 5 μm apart and 10 μm apart, and random arrays of knobs. b, We measure the maximum lateral velocity a robot takes at each step, then aggregate the results into a distribution of peak speeds for each robot body type and frictional surface type (robot types are depicted in the inset images). The upper and lower error bars represent the upper and lower quartiles. Substantial variability is expected for walking on rough surfaces: each step can provide a different contact geometry and force. The order of magnitude is constrained by an interplay between friction and drag: the robot speed is bounded by the maximum frictional force the feet can generate. The maximum frictional force is found to be 0.1–0.3 of the robot’s weight, consistent with friction. (See Methods for a detailed discussion).
Changing the voltage on a SEA relative to the surrounding electrolyte. Surface adsorption causes the actuator to curl to micron scale radii of curvature over small changes in voltage. (This video is in real time).
An array of electrically actuated SEAs. Actuators curl and uncurl together in response to the voltage signal on a common electrode. (This video is in real time).
Real-time video of the release of thousands of robots into solution. The robots self-fold upon release.
A four-photovoltaic robot walking across a microscope slide. A focused laser beam is directed to specific photovoltaics on the robot body, powering a corresponding set of actuators, and causing the robot to walk across a surface. (The video has been sped up by 8×).
A two-photovoltaic robot walking across a microscope slide. A focused laser beam is directed to specific photovoltaics on the robot body, powering a corresponding set of actuators, and causing the robot to walk across a surface. (The video has been sped up by 8×).
A microscopic robot walking in a circle. By changing the pattern of the rigid panels, we can change the robot’s locomotion. Here the back legs are rotated slightly relative to the robot body during fabrication. As a result, the robot walks in a circle. (This video has been sped up by 8×).
Microscopic robots drawn into a syringe. Due to their small size, microrobots are robust to handling, even when using macroscopic tools like syringes. (This video is in real time).
A robot actuating following injection through a pipette. In this video, a robot has been injected into an amoeba culture, demonstrating the robots are robust to handling. (This video is in real time).
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Miskin, M.Z., Cortese, A.J., Dorsey, K. et al. Electronically integrated, mass-manufactured, microscopic robots. Nature 584, 557–561 (2020). https://doi.org/10.1038/s41586-020-2626-9
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