Microparticle Manipulation using Multiple Untethered Magnetic Micro-Robots on an Electrostatic Surface Steven Floyd*, Chytra Pawashe*, and Metin Sitti Abstract— This work presents the control of multiple un- tethered rectilinear magnetic micro-robots (Mag-μBots) with dimensions 250 × 130 × 100 μm 3 actuated by pulsed external magnetic fields, which translate by induced stick-slip motion at speeds of up to 4 mm/s immersed in silicone oil. Multiple Mag-μBot control is enabled by employing an array of individually addressable electrostatic surfaces to selectively anchor individual Mag-μBots. Coupled parallel and uncoupled serial motion of multiple robots is demonstrated, and they can combine to form an assembly that is also capable of motion. Manipulation of 230 μm diameter microspheres is also demonstrated cooperatively by two Mag-μBots in a fluid environment, and is enhanced when the two Mag-μBots are combined. An analysis of the electrostatic anchoring forces and the forces relevant to manipulation is discussed. I. I NTRODUCTION The recent emergence of sub-millimeter sized mobile micro-robots has introduced new approaches to power de- livery and control at the micron-scale. The current designs in literature, including electrostatic [1], [2], electromagnetic [3]–[7], laser driven thermal impact [8], and bacteria pro- pelled systems [9], [10], have resulted in successful wire- less control of individual micro-robots. These micro-robots are also capable of manipulating micron-scale objects in their respective workspaces. For example, Zhang et al. [4] demonstrate 6 µm spheres being manipulated by a micro- swimming flagellar device, and Frutiger et al. [5] show that 150 µm gold discs can be moved by a mobile micro-magnetic actuator. These approaches offer a valuable alternative to conventional micro-grippers controlled by a multi-degree- of-freedom macro-scale positioning system, which can be complex, difficult to control, and expensive [11]. In addition, such a system does not share the advantages of untethered micro-robots where the micron-scale end-effector is entirely contained within the workspace. For any contact-based micro-manipulation method, stic- tion between the end-effector and micro-object becomes relevant at the micron-scale, making the release of grasped micro-objects difficult. Methods to combat this problem can include using ice to form and break connections between end-effectors and micro-objects, vibrating the end-effector to release a grasped object, employing vacuum to selectively * Equally contributing co-first authors S. Floyd and C. Pawashe are with the Department of Mechanical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, USA [srfloyd, csp]@andrew.cmu.edu M. Sitti is with the Department of Mechanical Engineering and Robotics Institute, Carnegie Mellon University, Pittsburgh, PA 15213, USA sitti@cmu.edu capture and release micro-objects, and using electrostatic at- traction and repulsion to manipulate micro-objects [12], [13]. Alternatively, the micro-object can be immersed in fluid, where micro-manipulation can be easier because stiction effects dramatically reduce. However, this limits applications to situations where the micro-object can be immersed, which is not always desired. A new challenge in micro-robotics is the control of multi- ple untethered agents, where the power delivery and control mechanisms may not be conducive to this task. Donald et al. [1] demonstrate the control of four electrostatically actuated MEMS micro-robots, which are all designed to be physically different to respond differently to the global driving electric field. Motion among these micro-robots is coupled, and requires sophisticated algorithms to create paths for each micro-robot. Using magnetic-resonant micro-robots, Kratochvil et al. [6] demonstrate that decoupled motion is possible. Like in [1], these individual micro-robots must be physically different, so that their response to the global driving fields are unique. To enable the control of multiple magnetic micro-robots (Mag-µBots) in our electromagnetically actuated system, we introduce a structured surface where electrostatic forces can be applied in order to selectively anchor Mag-µBots to the surface, which we demonstrate in [14]. This allows for any unanchored Mag-µBot to be driven by the encompassing magnetic fields, while keeping anchored Mag-µBots immo- bile. This approach allows for both the uncoupled serial actuation and coupled parallel actuation of multiple Mag- µBots. In this scheme, Mag-µBots do not need to be specially designed, reducing complexity of micro-robot fabrication. Using Mag-µBots, the manipulation of microparticles can be performed within a fluid environment, which reduces the effects of stiction. In addition to forces exerted by the Mag-µBot by contact manipulation, fluid forces can also affect microparticles, caused by the displacement of fluid by the Mag-µBot while moving. This effect is explored in [15], where a single Mag-µBot can manipulate microparticles as small as 50 µm, limited by the imaging resolution of the system. The combination of multiple Mag-µBots and microparticle manipulation can lead to the vision of teams of micro-robots playing soccer, which is a goal for the RoboCup Nanogram league [16]. II. TOOLS AND CONCEPT A rectilinear Mag-µBot with dimensions 250 × 130 × 100 µm 3 is actuated by six independent electromagnetic coils, aligned to the faces of a cube approximately 11 cm on a