Kathrin E. Peyer

How Should Microrobots Swim

Microrobots have the potential to dramatically change many aspects of medicine by navigating through bodily fluids to perform targeted diagnosis and therapy. Researchers have proposed numerous micro-robotic swimming methods, with the vast majority utilizing magnetic fields to wirelessly power and control the microrobot. In this paper, we compare three promising methods of microrobot swimming (using magnetic fields to rotate helical propellers that mimic bacterial flagella, using magnetic fields to oscillate a magnetic head with a rigidly attached elastic tail, and pulling directly with magnetic field gradients) considering practical hardware limitations in the generation of magnetic fields. We find that helical propellers and elastic tails have very comparable performance, and they generally become more desirable than gradient pulling as size decreases and as distance from the magnetic-field-generation source increases. We provide a discussion of why helical propellers are likely the best overall choice for in vivo applications.

Characterizing the Swimming Properties of Artificial Bacterial Flagella

Artificial bacterial flagella (ABFs) consist of helical tails resembling natural flagella fabricated by the self-scrolling of helical nanobelts and soft-magnetic heads composed of Cr/Ni/Au stacked thin films. ABFs are controlled wirelessly using a low-strength rotating magnetic field. Self-propelled devices such as these are of interest for in vitro and in vivo biomedical applications. Swimming tests of ABFs show a linear relationship between the frequency of the applied field and the translational velocity when the frequency is lower than the step-out frequency of the ABF. Moreover, the influences of head size on swimming velocity and the lateral drift of an ABF near a solid boundary are investigated. An experimental method to estimate the propulsion matrix of a helical swimmer under a light microscope is developed. Finally, swarm-like behavior of multiple ABFs controlled as a single entity is demonstrated.

Artificial bacterial flagella for micromanipulation

This article presents an overview of recent developments in artificial bacterial flagella (ABFs) and discusses challenges and opportunities in pursuing applications. These helical swimmers possess several advantageous characteristics, such as high swimming velocity and precise motion control indicating their potential for diverse applications. One application is the manipulation of small objects within liquid, which is the focus of this review. Preliminary results have shown that ABFs are capable of performing microobject manipulation either directly by mechanical contact or indirectly by generating a localized fluid flow. The latter approach can be used for batch manipulation without direct contact, also implying possibilities for flow control in lab-on-a-chip systems. Miniaturized helical swimmers are also promising for biomedical applications, such as targeted drug delivery and implantation or removal of tissues and other objects.

Controlled Propulsion and Cargo Transport of Rotating Nickel Nanowires near a Patterned Solid Surface

We show that rotating Ni nanowires are capable of propulsion and transport of colloidal cargo near a complex surface. When dissimilar boundary conditions exist at the two ends of a nanowire, such as when a nanowire is near a wall, tumbling motion can be generated that leads to propulsion of the nanowire. The motion of the nanowire can be precisely controlled using a uniform rotating magnetic field. We investigate the propulsion mechanism and the trajectory of the nanowire during the tumbling motion and demonstrate cargo transport of a polystyrene microbead by the nanowire over a flat surface or across an open microchannel. The results imply that functionalized, ferromagnetic one-dimensional, tumbling nanostructures can be used for cell manipulation and targeted drug delivery in a low Reynolds number aqueous environment.

Magnetic Helical Micromachines

Helical microrobots have the potential to be used in a variety of application areas, such as in medical procedures, cell biology, or lab-on-a-chip. They are powered and steered wirelessly using low-strength rotating magnetic fields. The helical shape of the device allows propulsion through numerous types of materials and fluids, from tissue to different types of bodily fluids. Helical propulsion is suitable for pipe flow conditions or for 3D swimming in open fluidic environments.

Selective trapping and manipulation of microscale objects using mobile microvortices.

Controlled manipulation of individual micro- and nanoscale objects requires the use of trapping forces that can be focused and translated with high spatial and time resolution. We report a new strategy that uses the flow of mobile microvortices to trap and manipulate single objects in fluid with essentially no restrictions on their material properties. Fluidic trapping forces are generated toward the center of microvortices formed by magnetic microactuators, that is, rotating nanowire or self-assembled microbeads, actuated by a weak rotating magnetic field (|B|< 5 mT). We demonstrate precise manipulation of single microspheres and microorganisms near a solid surface in water.

Targeted cargo delivery using a rotating nickel nanowire

UNLABELLED This paper reports an approach to perform basic noncontact and contact manipulation tasks using rotating nickel nanowires driven by a rotating magnetic field. A rotating nanowire is capable of propulsion and steering near a solid surface by a tumbling motion. The FEM simulation shows that fluid flow is induced around the rotating nanowire, which was applied to manipulate micro-objects in a noncontact fashion. Pushing, pulling, and rotation tests of individual polystyrene microbeads are conducted on a solid surface. In addition, targeted delivery tasks of biological samples, e.g., individual flagellated microorganisms and human blood cells, are demonstrated. The results imply that rotating magnetic nanowires are good tools for handling cellular and subcellular objects in an aqueous low-Reynolds-number environment and have potential for single-cell analysis. FROM THE CLINICAL EDITOR In this study, the authors report the ability to push, pull, and rotate individual polystyrene microbeads on a solid surface. Furthermore, they demonstrate targeted delivery of biological samples, implying that rotating magnetic nanowires are good tools for handling cellular and subcellular objects.

Behavior of rotating magnetic microrobots above the step-out frequency with application to control of multi-microrobot systems

This paper studies the behavior of rotating magnetic microrobots, constructed with a permanent magnet or a soft ferromagnet, when the applied magnetic field rotates faster than a microrobots step-out frequency (the frequency requiring the entire available magnetic torque to maintain synchronous rotation). A microrobots velocity dramatically declines when operated above the step-out frequency. As a result, it has generally been assumed that microrobots should be operated beneath their step-out frequency. In this paper, we report and demonstrate properties of a microrobots behavior above the step-out frequency that will be useful for the design and control of multi-microrobot systems.

Hybrid Helical Magnetic Microrobots Obtained by 3D Template‐Assisted Electrodeposition

Hybrid helical magnetic microrobots are achieved by sequential electrodeposition of a CoNi alloy and PPy inside a photoresist template patterned by 3D laser lithography. A controlled actuation of the microrobots by a rotating magnetic field is demonstrated in a fluidic environment.

Assembly, Disassembly, and Anomalous Propulsion of Microscopic Helices

Controlling the motion of small objects in suspensions wirelessly is of fundamental interest and has potential applications in biomedicine for drug delivery and micromanipulation of small structures. Here we show that magnetic helical microstructures that propel themselves in the presence of rotating weak magnetic fields assemble into various configurations that exhibit locomotion and a change in swimming direction. The configuration is tuned dynamically, that is, assembly and disassembly occur, by the field input. We investigate a system that consists of two identical right-handed helices assembled at their center in order to model the motion of assembled swimmers. The swimming properties are dependent on both the component design and the assembly configuration. For particular designs and configurations, a reversal in swimming direction emerges, yet with other designs, a reversal in motion never appears. Understanding the locomotion of clustered chiral structures enables uni- and multidirectional navigation of this class of active suspensions.