Highly Acid‐Resistant, Magnetically Steerable Acoustic Micromotors Prepared by Coating Gold Microrods with Fe₃O₄ Nanoparticles via pH Adjustment

Abstract

There is mounting interest in designing magnetically steerable nano‐ and micromotors for next generation medical nanorobotics, which requires biocompatibility for each individual component. Although various magnetic materials (e.g., Ni, Co, and Fe3O4) have been incorporated into micromotors, their acid resistance remains largely unexplored. In this article, a simple one‐step method to prepare magnetic microrods via electrostatic attraction between paramagnetic magnetite nanoparticles (Fe3O4 NPs) and gold microrods at appropriate pH values is reported. The as‐prepared Fe3O4‐coated micromotors can be powered by MHz ultrasound and easily steered by external magnetic fields, and perform well in harsh working conditions such as high acidity, high viscosity, and high ionic strength. In particular, extended exposure to solution of pH as low as 0.9 has a minimal effect on the speed, steerability, or cargo‐transporting capability of micromotors coated with Fe3O4 NPs, in stark contrast with those containing Ni segments. Considering the many challenges of biomedical applications, acid‐resistant, magnetically steerable Fe3O4‐coated micromotors powered by MHz ultrasound can be a promising prototype for the future development of medical nano‐ and microrobotics.

From Passive Inorganic Oxides to Active Matters of Micro/Nanomotors

“…Furthermore, dual operation of ultrasonic and magnetic stimulus could be applied to propel Au@Fe 3 O 4 NPs rods. [ 74 ]…”

“…Furthermore, Fe 3 O 4 NPs are incorporated into Au rods via electrostatic attraction and the microrods are able to be driven by both ultrasonic and magnetic fields. [ 74 ] Such motors present good motion performance under harsh conditions such as high acidity, high viscosity, and high ionic strength.…”

Density Asymmetry Driven Propulsion of Ultrasound‐Powered Janus Micromotors

“…[ 15–17 ] In particular, ultrasound offers a unique opportunity to power such devices towards biological applications due to its non‐invasiveness, tunability, and prolonged operation time. [ 18 ] Micro/nanomotors powered by kHz [ 19 ] to MHz ultrasonic standing waves [ 20,21 ] are capable of biocompatible propulsion with spatial maneuverability, [ 22–24 ] transport of therapeutic payloads, [ 25 ] and pre‐concentrate biological targets [ 26–28 ] (for example, bacteria and cancer cells).…”

Trends in Micro‐/Nanorobotics: Materials Development, Actuation, Localization, and System Integration for Biomedical Applications

“…In addition to the self‐propelled and external field‐steered hybrid m‐bots, the motion and direction of the m‐bots can be controlled with the external fields, avoiding the use of chemical fuels. [ 255 ] Wang et al. [ 249 ] developed a magnetic/acoustic hybrid fuel‐free nanorobot composed of a segment of Au nanorod and Ni‐coated Pd helical structure (Figure 8e), which can be actuated by either a magnetic or an ultrasonic field in an on‐demand manner (Figure 8f,g).…”