Microrobots span dimensions from nanometers to sub-millimeters, can navigate biological fluids/tissues and localize to specific targets, and—owing to their miniaturization, untethered actuation, and multimodal locomotion—can access deep, narrow, and complex regions (e.g., vasculature and brain tissue) with minimal invasiveness, enabling broad prospects in targeted drug delivery, minimally invasive surgery, cell manipulation, and imaging. Yet propulsion and motion control at low Reynolds number remain fundamental challenges, motivating diverse external-field actuation schemes; while electric and optical approaches are constrained by potential cellular damage and limited tissue penetration, magnetic and acoustic actuation have attracted particular attention due to favorable biocompatibility and tissue penetration for in vivo use. Importantly, single-field paradigms exhibit intrinsic trade-offs: magnetic actuation offers precise controllability but suffers from limited propulsive force due to spatial field decay and increased system/fabrication complexity, whereas acoustic actuation provides strong propulsion and functional effects (e.g., cavitation and sonochemistry) but still struggles with accurate directional and 3D control and with the resolution–penetration trade-off. Accordingly, hybrid magneto-acoustic actuation—combining magnetic steering with acoustic propulsion/activation—has emerged as a systematic solution and a growing research focus. "In this article, we provide a comprehensive overview of hybrid magneto-acoustic microrobots, covering their actuation mechanisms, representative structural designs, biomedical applications, and key challenges and future directions." said the author Xiaoming Liu, a researcher at Beijing Institute of Technology.
The working mechanisms of hybrid magneto-acoustic microrobots can be broadly divided into two categories: (i) magnetic steering with acoustic propulsion, where magnetic responsiveness (e.g., via embedded magnetic nanoparticles) enables programmable orientation/trajectory control through alignment of induced dipoles or intrinsic magnetic moments, while ultrasound-induced acoustic streaming supplies efficient thrust—often generated by microbubbles, asymmetric Janus geometries, or cilia arrays—thereby combining precise navigation with powerful locomotion in complex environments. From a field-property perspective, magnetic fields are typically anisotropic and well suited for accurate directional control but less effective for uniform velocity regulation, whereas acoustic fields elicit more isotropic responses that provide strong energy input yet limited directionality; accordingly, this scheme assigns the main propulsion energy cost to the acoustic field and uses the magnetic field primarily for directional adjustment to improve energy efficiency and mitigate the force limitation caused by rapid magnetic-field decay. (ii) magnetic propulsion with acoustic manipulation, in which magnetic actuation ensures targeted locomotion and precise positioning, and ultrasound is applied on demand to trigger functional operations (e.g., mixing or drug release) via acoustic streaming, cavitation, and sonochemical effects, effectively decoupling locomotion from manipulation and reducing mutual interference.
Magneto-acoustic microrobots have advanced rapidly thanks to robust propulsion, precise positioning, multifunctionality, biocompatibility, and remote controllability. Authors highlights three application domains: targeted drug delivery, minimally invasive surgery, and medical imaging. Targeted delivery improves efficacy and reduces systemic side effects, yet passive transport is hindered by drug instability, poor localization, and biological barriers; magneto-acoustic microrobots can navigate complex environments and enable precise guidance/control using external magnetic and acoustic fields. Typical work involves using ultrasound induced local effects for targeted release/enhanced therapy, such as using magnetic controlled sonodynamic nanorobots to precisely deliver sonosensitizers to tumors and enhance ROS generation, thereby improving tumor cell killing. Alternatively, magnetic microbubbles can be used to load drugs, with magnetic targeting first to enrich the lesion, and then focused ultrasound to rupture the microbubbles and trigger drug release. In terms of minimally invasive surgery, these microrobots can act as microsurgical tools for tissue puncture, biofilm degradation, and thrombus removal; their size and maneuverability enable access beyond conventional instruments, while acoustic stimulation at the target enhances local penetration/effects—potentially reducing incision size, tissue damage, infection risk, and recovery time. In a study on thrombolysis, magnetic microbubbles undergo cavitation and rotation under ultrasound to form microfluidics, mechanically disrupting the fibrin network and forming microchannels, promoting the infiltration of drug loaded nanodroplets, ultimately leading to thrombus rupture due to cavitation within the fibrous network. In the field of medical imaging, clinical use requires real-time imaging to track individual or collective microrobots for accurate navigation; designs using gas-filled microbubbles or MNP coatings enhance acoustic contrast for deep-tissue ultrasound guidance, enabling high spatiotemporal tracking of motion and localization. Microrobot accumulation (e.g., magnetic microbubbles) at lesions under magnetic targeting amplifies local contrast; they can be monitored by ultrasound and collapsed under focused ultrasound for on-demand release; the authors note potential integration with fluorescence or X-ray for multimodal imaging.
Overall, microrobots driven by magnetic and acoustic fields are well suited for in vivo operation due to favorable biocompatibility and tissue penetration; however, single-field actuation is constrained by limited magnetic propulsion and the difficulty of precise steering under acoustic actuation. Consequently, hybrid magneto-acoustic systems exploit complementary coupling—magnetic precision control combined with acoustically enabled strong propulsion/functional activation—to improve locomotion efficiency and functional capability without sacrificing directionality. Recent magneto-acoustic microrobots have demonstrated advantages in high thrust, accurate positioning, low energy consumption, and decoupled locomotion–manipulation control, supporting targeted drug transport, tissue puncture/biofilm degradation, and multimodal imaging, with additional potential in microgravity and space life science contexts. Nevertheless, clinical translation is still limited by major safety and control challenges, including long-term risks from residual magnetic nanoparticles after degradation, insufficient adaptability of control strategies in dynamic physiological environments, and the lack of selective individual control in swarms. "Future research should focus on three priorities: (i) developing low-toxicity, controllably biodegradable magnetic materials together with feasible in vivo clearance strategies for residual particles; (ii) tightly integrating magneto-acoustic control with real-time, high-resolution medical imaging to enable image-feedback intelligent control (e.g., MPC and reinforcement learning) under dynamic disturbances such as blood flow; and (iii) designing swarms with differentiated physical properties to flexibly switch between collective control and selective individual control." said Xiaoming Liu.
Authors of the paper include Tingting Wang, Zhuo Chen, Qiang Huang, Tatsuo Arai, and Xiaoming Liu.
This research was supported in part by the National Natural Science Foundation of China (grant 62273052 to X.L. and grant W2431050 to T.A.), the Beijing Natural Science Foundation (grant L248102 to X.L. and grant IS23062 to T.A.), and the Grant-in-Aid for Scientific Research (23K22712 to T.A.) from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
The paper, "Advanced Microrobots Driven by Acoustic and Magnetic Fields for Biomedical Applications" was published in the journal Cyborg and Bionic Systems on Nov. 10, 2025, at DOI: 10.34133/cbsystems.0386.
