Magnetic micro-/nanoscale swimmers: Current status and potential applications

Abstract

Because of the small size, controlled mobility, and versatile functionalities, micro-/nanoscale swimmers have drawn great attention and shown various potential applications, ranging from biomedicine to environmental remediation, such as minimal invasive surgery, targeted therapy, cell manipulation, heavy metal ions adsorption in waste water, and so on. Development of micro-/nanoscale swimmers meets different challenges compared with traditional large-size robots, because the Renyolds number is very small (10 - 5–10 - 2) in their movement environment, that is, inertia is negligible compared to viscous forces. Such a low Renyolds number leads to that several difficulties must be overcome to actuate and steer the motion of micro-/nanoscale swimmers, including critical design of micro-/nanoscale swimmers whose movement must be non-reciprocal, and appropriate energy supply which can be applied to micro-/nanoscale swimmers constantly. Great efforts have been made to propel micro-/nanoscale swimmers at low Renyolds number, and at present, the propulsion mechanisms can be divided into two aspects: Self-propulsion and propulsion using external fields. The former means that micro-/nanoscale swimmers can obtain and convert energy from their surrounding environment to mechanical energy by themselves, including self-electrophoresis, self-diffusiophoresis, bubble propulsion, and so on. While the latter refers to that the motion of micro-/nanoscale swimmers is propelled by an external field such as electric field, acoustic field, magnetic field and light. Among the propulsion methods, magnetic field has aroused high research interest as one of the most promising means to actuate and steer magnetic micro-/nanoscale swimmers wirelessly, because low-strength magnetic field with low frequency is easy to be obtained and tuned, which is also considered harmless to biological cells and tissues, and decays slowly in living body. In this review article, magnetic micro-/nanoscale swimmers are divided into two aspects and introduced separately: Magnetic field actuated swimmers which are powered and steered by magnetic field, and magnetic-field-steered swimmers whose motion is powered by other means but steered by the magnetic field. On one hand, the power source of micro-/nanoscale swimmers actuated by magnetic field comes from the induced magnetic torque or field gradient, and the magnetic field that has been applied to actuate micro/nanoscale swimmers includes rotating magnetic field, magnetic field with field vector oscillating up and down in a plane, magnetic field with field gradient along or perpendicular to the direction of the field, and so on. Herein, the motion mechanisms and fabrication methods of helical swimmers and flexible swimmers actuated by rotating magnetic field and oscillating magnetic field, respectively, which are inspired by microorganisms in nature, are introduced in detail. Moreover, other kinds of magnetic swimmers, such as U-shaped microrobot actuated by magnetic field gradient, magnetic device pushed along a surface by a rotating dipole field, nickel nanowire propelled near a patterned solid surface by rotating magnetic field, and so on, are also discussed. On the other hand, we describe several kinds of magnetic micro-/nanoscale swimmers whose motion are powered by self-electrophoresis, bubble or metabolism, and steered by magnetic field, such as multi-segment nanorod based nanomotors, microtubular jet engines, Janus microparticles and magnetotactic bacteria. Herein, the magnetic field is only used to control the motion direction of swimmers and has no effect on the swimming speed. In summary, we have overviewed the recent research progress on the application of magnetic field in micro/ nanoswimmers, and introduced relevant mechanisms in this work, which could pave the way for the research and development of swimming machines and robots at small scales.