Active Micromotor Systems Built from Passive Particles with Biomimetic Predator-Prey Interactions

Shape-Tunable Janus Micromotors via Surfactant-Induced Dewetting

The ability to tune shapes of micromotors is challenging yet crucial for creating intelligent and functional micromachines with shape-dependent dynamics. Here, we demonstrate a facile strategy to synthesize Janus micromotors in large quantity whose shapes can be precisely tuned by a surfactant-induced dewetting strategy. The Janus micromotor is composed of a TiO₂ microparticle partially encapsulated within a polysiloxane microsphere. A range of particle shapes, from approximately spherical to snowman, is achieved, and the shape-tunable dynamics of the micromotors are quantified. Our strategy is versatile and can be applicable to other photoactive materials, such as ZnO and Fe₂O₃ nanoparticles, demonstrating a general approach to synthesize Janus micromotors with controllable shapes. Such shape-tunable micromotors provide colloidal model systems for fundamental research on active matter, as well as building blocks for the fabrication of micromachines.

Swarming behavior and in vivo monitoring of enzymatic nanomotors within the bladder

Enzyme-powered nanomotors are an exciting technology for biomedical applications due to their ability to navigate within biological environments using endogenous fuels. However, limited studies into their collective behavior and demonstrations of tracking enzyme nanomotors in vivo have hindered progress toward their clinical translation. Here, we report the swarming behavior of urease-powered nanomotors and its tracking using positron emission tomography (PET), both in vitro and in vivo. For that, mesoporous silica nanoparticles containing urease enzymes and gold nanoparticles were used as nanomotors. To image them, nanomotors were radiolabeled with either ¹²⁴I on gold nanoparticles or ¹⁸F-labeled prosthetic group to urease. In vitro experiments showed enhanced fluid mixing and collective migration of nanomotors, demonstrating higher capability to swim across complex paths inside microfabricated phantoms, compared with inactive nanomotors. In vivo intravenous administration in mice confirmed their biocompatibility at the administered dose and the suitability of PET to quantitatively track nanomotors in vivo. Furthermore, nanomotors were administered directly into the bladder of mice by intravesical injection. When injected with the fuel, urea, a homogeneous distribution was observed even after the entrance of fresh urine. By contrast, control experiments using nonmotile nanomotors (i.e., without fuel or without urease) resulted in sustained phase separation, indicating that the nanomotors' self-propulsion promotes convection and mixing in living reservoirs. Active collective dynamics, together with the medical imaging tracking, constitute a key milestone and a step forward in the field of biomedical nanorobotics, paving the way toward their use in theranostic applications.

External Power-Driven Microrobotic Swarm: From Fundamental Understanding to Imaging-Guided Delivery

Untethered micro/nanorobots have been widely investigated owing to their potential in performing various tasks in different environments. The significant progress in this emerging interdisciplinary field has benefited from the distinctive features of those tiny active agents, such as wireless actuation, navigation under feedback control, and targeted delivery of small-scale objects. In recent studies, collective behaviors of these tiny machines have received tremendous attention because swarming agents can enhance the delivery capability and adaptability in complex environments and the contrast of medical imaging, thus benefiting the imaging-guided navigation and delivery. In this review, we summarize the recent research efforts on investigating collective behaviors of external power-driven micro/nanorobots, including the fundamental understanding of swarm formation, navigation, and pattern transformation. The fundamental understanding of swarming tiny machines provides the foundation for targeted delivery. We also summarize the swarm localization using different imaging techniques, including the imaging-guided delivery in biological environments. By highlighting the critical steps from understanding the fundamental interactions during swarm control to swarm localization and imaging-guided delivery applications, we envision that the microrobotic swarm provides a promising tool for delivering agents in an active, controlled manner.

Ultrafast Growth and Locomotion of Dandelion-Like Microswarms with Tubular Micromotors

Dynamic assembly and cooperation represent future frontiers for next generations of advanced micro/nano robots, but the required local interaction and communication cannot be directly translated from macroscale robots through the minimization because of tremendous technological challenges. Here, an ultrafast growth and locomotion methodology is presented for dandelion-like microswarms assembled from catalytic tubular micromotors. With ultrasound oscillation of self-generated bubbles, such microswarms could overcome the tremendous and chaotic drag force from extensive and disordered bubble generation in single units. Tubular MnO₂ micromotor individuals headed by self-generated oxygen bubbles are ultrasonically driven to swim rapidly in surfactant-free H₂ O₂ solutions. A large bubble core fused from multiple microbubbles is excited to oscillate and the resultant local intensified acoustic field attracts the individual micromotors to school around it, leading to a simultaneous growth of dandelion-like microswarms. The bubble-carried micromotor groups driven by ultrasound could swarm at a zigzag pattern with an average speed of up to 50 mm s^-1 , which is validated in low H₂ O₂ concentrations. Additionally, such superfast locomotion could be ultrasonically modulated on demand. The ultrafast microswarm growth and locomotion strategy offers a new paradigm for constructing distinct dynamic assemblies and rapid transmission of artificial microrobots, paving the way to a myriad of promising applications.

Wafer-Scale Fabrication of Micro- to Nanoscale Bubble Swimmers and Their Fast Autonomous Propulsion by Ultrasound

Fuel-free, biocompatible swimmers with dimensions smaller than one micrometer have the potential to revolutionize the way we study and manipulate microscopic systems. Sub-micrometer, metallic Janus particles can be propelled rapidly and autonomously by acoustically induced fluid streaming, but their operation at acoustic pressure nodes limits their utility. In contrast, bubble-based microswimmers have an "on board" resonant cavity that enables them to operate far from the source of acoustic power. So far, they have been fabricated by direct writing techniques that limit both their minimum dimensions and the number that can be produced. Consequently, the size scaling of the properties of bubble swimmers has not been explored experimentally. Additionally, 3D autonomous motion has not yet been demonstrated for this type of swimmer. We describe here a method for fabricating bubble swimmers in large numbers (>10⁹) with sizes ranging from 5 μm to 500 nm without direct writing or photolithographic tools. These swimmers follow a previously proposed scaling theory and reveal useful phenomena that enable their propulsion in different modes in the same experiment: with magnetic steering, autonomously in 3D, and in frequency-specific autonomous modes. These interesting behaviors are relevant to possible applications of autonomously moving micro- and nanorobots.