Inspired by chasing–escaping behaviors of predator and swarming prey in nature, here we demonstrate a concept to create active micromotor systems from two species of passive microparticles with biomimetic predator–prey interactions. In this concept, the biomimetic predator–prey interactions are established in a binary particle system comprising the diffusiophoretic attractive microparticles (prey particles) and the diffusiophoretic repulsive ones (predator particles). In the absence of additional chemical fuels and external fields, the predator particles are attracted by and constantly chase the swarming prey particles, which, in response, escape from the former and show dynamic group reconfigurations because of the local repulsion. Based on this concept, various synthetic active micromotor systems have been demonstrated, including active ZnO–TiO2, Ag3PO4–TiO2, and ZnO–AgBr micromotor systems. As the predator and prey particles are powered by each other through the biomimetic predator–prey interactions, the concept proposed here provides an advanced method to develop not only a class of single micromotors powered by passive particles or “solid fuels” but also micromotor swarms capable of manipulating “moving cargo”. In addition, it also illustrates a proof-of-concept implementation of intelligent micro/nanomotor systems composed of heterogeneous individuals with complementary or cooperative functions.

Swarming micro/nanomotors can self-organize into cohesive groups to execute cooperative tasks. To date, research work has focused on the construction of egalitarian microswarms composed of similar individuals. The construction and collective behaviors of hierarchical leaderfollower-like microswarms are demonstrated. By inducing converging electrohydrodynamic flows under an AC electric field, dielectric microparticles with different sizes and dielectric properties can hierarchically self-organize into leader-follower-like microswarms under attractive electrohydrodynamic interactions, and show novel emergent collective behaviors. First, different from immobile single constituents or egalitarian clusters, the hierarchical microswarms autonomously move with tunable speed. Second, they exhibit multimode collective photoresponses emerging from different behaviors of the constituents in response to light signals. With a vertical UV signal, the photoresponsive followers tend to surround the leader and stop the microswarm. In response to sidewise UV signals, the constituents with stronger phototaxis would migrate to the position away from light stimuli, and thus the microswarms reorient parallel/antiparallel to the light direction and perform collective positive/negative phototaxis. Due to differential roles and huge design spaces of constituents, the hierarchical microswarms are envisioned to possess merits of high-efficiency, multiresponsiveness, and multifunctions, and may serve as intelligent micro/nanorobot systems for biomedicine and microengineering.

The key principle of designing a micro/nanomotor is to introduce asymmetry to a micro/nanoparticle. However, micro/nanomotors designed based on external asymmetry and inherent chemical and geometrical asymmetry usually suffer from tedious small-scale preparation, high cost, and/ or complexity of external power and control devices, making them face insurmountable hurdles in practical applications. Herein, considering the possible distinct properties of different polymorphs, we propose a novel design strategy of simple-structured micromotors by introducing inherent asymmetry in crystalline phases. The inherent phase asymmetry can be easily introduced in spherical TiO 2 particles by adjusting the calcination temperature to control the phase transition and growth of primary grains. The as-designed anatase/rutile TiO 2 micromotors not only show efficient autonomous motions controlled by light in liquid media stemming from the asymmetric surface photocatalytic redox reactions but also have a promising application in environmental remediation due to their high photocatalytic activity in "on-the-fly" degradation of organic pollutants, facile large-scale fabrication, and low cost. The proposed design strategy may pave the way for the large-scale productions and applications of micro/nanomotors.

Swarms and assemblies are ubiquitous in nature and they can perform complex collective behaviors and cooperative functions that they cannot accomplish individually. In response to light, some colloidal particles (CPs), including light active and passive CPs, can mimic their counterparts in nature and organize into complex structures that exhibit collective functions with remote controllability and high temporospatial precision. In this review, we firstly analyze the structural characteristics of swarms and assemblies of CPs and point out that light-controlled swarming and assembly of CPs are generally achieved by constructing light-responsive interactions between CPs. Then, we summarize in detail the recent advances in light-controlled swarming and assembly of CPs based on the interactions arisen from optical forces, photochemical reactions, photothermal effects, and photoisomerizations, as well as their potential applications. In the end, we also envision some challenges and future prospects of light-controlled swarming and assembly of CPs. With the increasing innovations in mechanisms and control strategies with easy operation, low cost, and arbitrary applicability, light-controlled swarming and assembly of CPs may be employed to manufacture programmable materials and reconfigurable robots for cooperative grasping, collective cargo transportation, and micro- and nanoengineering.

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Cargo delivery by micro/nanomotors provides enormous opportunities for micromanipulation, environmental cleaning, and drug delivery, etc. However, due to the limited driving force, single micro/nanomotors are usually difficult to transport cargoes...

Through experiments and simulations, we show that fuel-free photoactive TiO 2 microparticles can form mobile, coherent swarms in the presence of UV light, which track the subsequent movement of an irradiated spot in a fluid-filled microchamber. Multiple concurrent propulsion mechanisms (electrolyte diffusioosmotic swarming, photocatalytic expansion, and photothermal migration) control the rich collective behavior of the swarms, which provide a strategy to reversely manipulate cargo. The active swarms can autonomously pick up groups of inert particles, sort them by size, and sequentially release the sorted particles at particular locations in the microchamber. Hence, these swarms overcome three obstacles, limiting the utility of self-propelled particles. Namely, they can (1) undergo directed, long-range migration without the addition of a chemical fuel, (2) perform diverse collective behavior not possible with a single active particle, and (3) repeatedly and controllably isolate and deliver specific components of a multiparticle "cargo". Since light sources are easily fabricated, transported, and controlled, the results can facilitate the development of portable devices, providing broader access to the diagnostic and manufacturing advances enabled by microfluidics.

“Life-like” nonequilibrium assemblies are of increasing significance, but suffering from limited steerability as they are generally based on micro/nanomotors with inherent asymmetry in chemical composition or geometry, of which the vigorous random Brownian rotations disturb the local interactions. Here, we demonstrate that isotropic photocatalytic micromotors, due to the persistent phoretic flow from the illuminated to shadowed side irrespective of their Brownian rotations, experience light-programmable local interactions (reversibly from attraction to repulsion and/or alignment) depending on the direction of the incident lights. Thus, they can be organized into a variety of tunable nonequilibrium assemblies, such as apolar solids (i.e., immobile colloidal crystal), polar liquids (i.e., phototactic colloidal stream), and polar solids (i.e., phototactic colloidal crystal), which can further be “cut” into a predesigned pattern by utilizing the switching motor-motor interactions at superimposed-light edges. This work facilitates the development of active matters and motile functional microdevices.