Multistimuli-responsive microrobots: A comprehensive review

Abstract

Untethered robots of the size of a few microns have attracted increasing attention for the potential to transform many aspects of manufacturing, medicine, health care, and bioengineering. Previously impenetrable environments have become available for high-resolution in situ and in vivo manipulations as the size of the untethered robots goes down to the microscale. Nevertheless, the independent navigation of several robots at the microscale is challenging as they cannot have onboard transducers, batteries, and control like other multi-agent systems, due to the size limitations. Therefore, various unconventional propulsion mechanisms have been explored to power motion at the nanoscale. Moreover, a variety of combinations of actuation methods has also been extensively studied to tackle different issues. In this survey, we present a thorough review of the recent developments of various dedicated ways to actuate and control multistimuli-enabled microrobots. We have also discussed existing challenges and evolving concepts associated with each technique.

Keywords: acoustic; catalytic; magnetic actuation; microrobots; multistimuli responsive; optical.

FIGURE 1

Representative examples of multistimuli-responsive microbots for different applications.

FIGURE 2

Mechanisms of magnetic propulsion (A) torque-dependent actuation under time-varying magnetic fields and (B) force-dependent actuation under non-uniform magnetic fields. Reproduced with permission (Yang and Zhang, 2020). Copyright 2020, Wiley-VCH.

FIGURE 3

Chemical propulsion mechanisms (A,B) diffusiophoretic, (C) electrophoretic, and (D) bubble induced propulsion. Reproduced with permission (Safdar et al., 2018). Copyright (2018) Wiley-VCH.

FIGURE 4

Examples of acoustically powered microrobots. (A) Asymmetric metallic rods propelled by self-acoustophoresis. Reprinted with permission (Wang et al., 2012). Copyright 2012 American Chemical Society. (B) Bimetallic rods propelled by acoustically activated flagella. Reproduced with permission (Ahmed et al., 2016a). Copyright 2016 American Chemical Society. (C) SEM image and schematic of cup-shaped swimmers along with different propulsion behavior at different acoustic frequencies. Reproduced with permission (McNeill et al., 2020). Copyright 2020 American Chemical Society. (D) Metallic microtubes propelled by acoustically vaporized bubbles of perfluorocarbon. Reproduced with permission (Kagan et al., 2012). Copyright 2012 Wiley-VCH.

FIGURE 5

Mechanisms of light-actuated propulsion. (A) Self-diffusiophoresis, (B) Self-electrophoresis. Reproduced with permission (Zhang et al., 2017c). Copyright 2017 American Chemical Society. (C) Self-thermophoresis. Reproduced with permission (Xuan et al., 2016). Copyright 2016 American Chemical Society.

FIGURE 6

Examples of different tasks performed by magnetically guided multistimuli responsive microrobots. (A) Self-assembly of nanorods into geometrically regular multimers. Reprinted with permission (Ahmed et al., 2014). Copyright 2014 American Chemical Society. (B) Experimental results of the reversible spreading and regathering of nanoparticles in a swarm. The applied fields are shown schematically. (Wang et al., 2021). Copyright 2021 American Association for the Advancement of Science (C) Cell manipulation by a chain of microrobots. Reproduced with permission (Villa et al., 2018a). Copyright 2018 Wiley-VCH. (D) 3D tornado-like swarming pattern formation. Reproduced with permission (Ji et al., 2020). Copyright 2020 American Chemical Society.

FIGURE 7

Examples of microrobots with optical brakes. (A) A balance of catalytic (Pt side) and optical (TiO₂ side) reactions to halt the motion. Reproduced with permission (Chen et al., 2018b) Copyright 2018 Wiley-VCH. (B) Motion and on-the-fly optical brakes are achieved by shape engineering of urchin-like TiO₂ microrobots. Reproduced with permission (Oral et al., 2022). Copyright 2022 Wiley-VCH.