CeFlowBot: A Biomimetic Flow-Driven Microrobot that Navigates under Magneto-Acoustic Fields

Acoustofluidic Actuation of Living Cells

Acoutofluidics is an increasingly developing and maturing technical discipline. With the advantages of being label-free, non-contact, bio-friendly, high-resolution, and remote-controllable, it is very suitable for the operation of living cells. After decades of fundamental laboratory research, its technical principles have become increasingly clear, and its manufacturing technology has gradually become popularized. Presently, various imaginative applications continue to emerge and are constantly being improved. Here, we introduce the development of acoustofluidic actuation technology from the perspective of related manipulation applications on living cells. Among them, we focus on the main development directions such as acoustofluidic sorting, acoustofluidic tissue engineering, acoustofluidic microscopy, and acoustofluidic biophysical therapy. This review aims to provide a concise summary of the current state of research and bridge past developments with future directions, offering researchers a comprehensive overview and sparking innovation in the field.

Ultrasound robotics for precision therapy

In recent years, the application of microrobots in precision therapy has gained significant attention. The small size and maneuverability of these micromachines enable them to potentially access regions that are difficult to reach using traditional methods; thus, reducing off-target toxicities and maximizing treatment effectiveness. Specifically, acoustic actuation has emerged as a promising method to exert control. By harnessing the power of acoustic energy, these small machines potentially navigate the body, assemble at the desired sites, and deliver therapies with enhanced precision and effectiveness. Amidst the enthusiasm surrounding these miniature agents, their translation to clinical environments has proven difficult. The primary objectives of this review are threefold: firstly, to offer an overview of the fundamental acoustic principles employed in the field of microrobots; secondly, to assess their current applications in medical therapies, encompassing tissue targeting, drug delivery or even cell infiltration; and lastly, to delve into the continuous efforts aimed at integrating acoustic microrobots into in vivo applications.

Steerable acoustically powered starfish-inspired microrobot

Soft polymeric microrobots that can be loaded with nanocargoes and driven via external field stimuli can provide innovative solutions in various fields, including precise microscale assembly, targeted therapeutics, microsurgery, and the capture and degradation of unwanted wastewater fragments. However, in aquatic environments, it remains challenging to operate with microrobotic devices due to the predominant viscous resistances and the robots' limited actuation and sensing capabilities attributed to their miniaturization. The miniature size prevents the incorporation of onboard batteries that can provide sufficient power for propulsion and navigation, necessitating a wireless power supply. Current research examines untethered microrobot manipulation using external magnetic, electric, thermodynamic, or acoustic field-guided technologies: all strategies capable of wireless energy transmission towards sensitive and hard-to-reach locations. Nonetheless, developing a manipulation strategy that harnesses simple-to-induce strong propulsive forces in a stable manner over extended periods of time remains a significant endeavor. This study presents the fabrication and manipulation of a microrobot consisting of a magnetized soft polymeric composite material that enables a combination of stable acoustic propulsion through starfish-inspired artificial cilia and magnetic field-guided navigation. The acousto-magnetic manipulation strategy leverages the unique benefits of each applied field in the viscous-dominated microscale, namely precise magnetic orientation and strong acoustic thrust.

Acoustic Streaming-Induced Multimodal Locomotion of Bubble-Based Microrobots

Acoustically-driven bubbles at the micron scale can generate strong microstreaming flows in its surrounding fluidic medium. The tunable acoustic streaming strength of oscillating microbubbles and the diversity of the generated flow patterns enable the design of fast-moving microrobots with multimodal locomotion suitable for biomedical applications. The acoustic microrobots holding two coupled microbubbles inside a rigid body are presented; trapped bubbles inside the L-shaped structure with different orifices generate various streaming flows, thus allowing multiple degrees of freedom in locomotion. The streaming pattern and mean streaming speed depend on the intensity and frequency of the acoustic wave, which can trigger four dominant locomotion modes in the microrobot, denoted as translational and rotational, spinning, rotational, and translational modes. Next, the effect of various geometrical and actuation parameters on the control and navigation of the microrobot is investigated. Furthermore, the surface-slipping multimodal locomotion, flow mixing, particle manipulation capabilities, the effective interaction of high flow rates with cells, and subsequent cancerous cell lysing abilities of the proposed microrobot are demonstrated. Overall, these results introduce a design toolbox for the next generation of acoustic microrobots with higher degrees of freedom with multimodal locomotion in biomedical applications.

Two-photon microscopy for microrobotics: Visualization of micro-agents below fixed tissue

Optical microscopy is frequently used to visualize microrobotic agents (i.e., micro-agents) and physical surroundings with a relatively high spatio-temporal resolution. However, the limited penetration depth of optical microscopy techniques used in microrobotics (in the order of 100 μm) reduces the capability of visualizing micro-agents below biological tissue. Two-photon microscopy is a technique that exploits the principle of two-photon absorption, permitting live tissue imaging with sub-micron resolution and optical penetration depths (over 500 μm). The two-photon absorption principle has been widely applied to fabricate sub-millimeter scale components via direct laser writing (DLW). Yet, its use as an imaging tool for microrobotics remains unexplored in the state-of-the-art. This study introduces and reports on two-photon microscopy as an alternative technique for visualizing micro-agents below biological tissue. In order to validate two-photon image acquisition for microrobotics, two-type micro-agents are fabricated and employed: (1) electrospun fibers stained with an exogenous fluorophore and (2) bio-inspired structure printed with autofluorescent resin via DLW. The experiments are devised and conducted to obtain three-dimensional reconstructions of both micro-agents, perform a qualitative study of laser-tissue interaction, and visualize micro-agents along with tissue using second-harmonic generation. We experimentally demonstrate two-photon microscopy of micro-agents below formalin-fixed tissue with a maximum penetration depth of 800 μm and continuous imaging of magnetic electrospun fibers with one frame per second acquisition rate (in a field of view of 135 × 135 μm2). Our results show that two-photon microscopy can be an alternative imaging technique for microrobotics by enabling visualization of micro-agents under in vitro and ex ovo conditions. Furthermore, bridging the gap between two-photon microscopy and the microrobotics field has the potential to facilitate in vivo visualization of micro-agents.

Field-Controlled Microrobots Fabricated by Photopolymerization

Field-controlled microrobots have attracted extensive research in the biological and medical fields due to the prominent characteristics including high flexibility, small size, strong controllability, remote manipulation, and minimal damage to living organisms. However, the fabrication of these field-controlled microrobots with complex and high-precision 2- or 3-dimensional structures remains challenging. The photopolymerization technology is often chosen to fabricate field-controlled microrobots due to its fast-printing velocity, high accuracy, and high surface quality. This review categorizes the photopolymerization technologies utilized in the fabrication of field-controlled microrobots into stereolithography, digital light processing, and 2-photon polymerization. Furthermore, the photopolymerized microrobots actuated by different field forces and their functions are introduced. Finally, we conclude the future development and potential applications of photopolymerization for the fabrication of field-controlled microrobots.

Microfluidic systems for particle capture and release: A review

Microfluidic technology has emerged as a promising tool in various applications, including biosensing, disease diagnosis, and environmental monitoring. One of the notable features of microfluidic devices is their ability to selectively capture and release specific cells, biomolecules, bacteria, and particles. Compared to traditional bulk analysis instruments, microfluidic capture-and-release platforms offer several advantages, such as contactless operation, label-free detection, high accuracy, good sensitivity, and minimal reagent requirements. However, despite significant efforts dedicated to developing innovative capture mechanisms in the past, the release and recovery efficiency of trapped particles have often been overlooked. Many previous studies have focused primarily on particle capture techniques and their efficiency, disregarding the crucial role of successful particle release for subsequent analysis. In reality, the ability to effectively release trapped particles is particularly essential to ensure ongoing, high-throughput analysis. To address this gap, this review aims to highlight the importance of both capture and release mechanisms in microfluidic systems and assess their effectiveness. The methods are classified into two categories: those based on physical principles and those using biochemical approaches. Furthermore, the review offers a comprehensive summary of recent applications of microfluidic platforms specifically designed for particle capture and release. It outlines the designs and performance of these devices, highlighting their advantages and limitations in various target applications and purposes. Finally, the review concludes with discussions on the current challenges faced in the field and presents potential future directions.

Multistimuli-responsive microrobots: A comprehensive review

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.

Recent Process in Microrobots: From Propulsion to Swarming for Biomedical Applications

Recently, robots have assisted and contributed to the biomedical field. Scaling down the size of robots to micro/nanoscale can increase the accuracy of targeted medications and decrease the danger of invasive operations in human surgery. Inspired by the motion pattern and collective behaviors of the tiny biological motors in nature, various kinds of sophisticated and programmable microrobots are fabricated with the ability for cargo delivery, bio-imaging, precise operation, etc. In this review, four types of propulsion-magnetically, acoustically, chemically/optically and hybrid driven-and their corresponding features have been outlined and categorized. In particular, the locomotion of these micro/nanorobots, as well as the requirement of biocompatibility, transportation efficiency, and controllable motion for applications in the complex human body environment should be considered. We discuss applications of different propulsion mechanisms in the biomedical field, list their individual benefits, and suggest their potential growth paths.

Review of Bubble Applications in Microrobotics: Propulsion, Manipulation, and Assembly

In recent years, microbubbles have been widely used in the field of microrobots due to their unique properties. Microbubbles can be easily produced and used as power sources or tools of microrobots, and the bubbles can even serve as microrobots themselves. As a power source, bubbles can propel microrobots to swim in liquid under low-Reynolds-number conditions. As a manipulation tool, microbubbles can act as the micromanipulators of microrobots, allowing them to operate upon particles, cells, and organisms. As a microrobot, microbubbles can operate and assemble complex microparts in two- or three-dimensional spaces. This review provides a comprehensive overview of bubble applications in microrobotics including propulsion, micromanipulation, and microassembly. First, we introduce the diverse bubble generation and control methods. Then, we review and discuss how bubbles can play a role in microrobotics via three functions: propulsion, manipulation, and assembly. Finally, by highlighting the advantages and current challenges of this progress, we discuss the prospects of microbubbles in microrobotics.

Delivery and actuation of aerosolized microbots

For disease of the lung, the physical key to effective inhalation-based therapy is size; too large (10's of μm) and the particles or droplets do not remain suspended in air to reach deep within the lungs, too small (subμm) and they are simply exhaled without deposition. μBots within this ideal low-μm size range however are challenging to fabricate and would lead to devices that lack the speed and power necessary for performing work throughout the pulmonary network. To uncouple size from structure and function, here we demonstrate an approach where individual building blocks are aerosolized and subsequently assembled in situ into μbots capable of translation, drug delivery, and mechanical work deep within lung mimics. With this strategy, a variety of pulmonary diseases previously difficult to treat may now be receptive to μbot-based therapies.