Microparticles have demonstrated substantial application values in a large number of areas [1–4]. Commonly, the functions and properties of the particles are closely interrelated with their structures and morphologies [5–7]. Nonspherical particles possess unique features originating from their shape anisotropy such as optical properties and motion characteristics, which bring about fascinating potential applications including sensing, catalysis, and microactuators [8–13]. Currently, numerous engineering techniques are developed to prepare nonspherical microparticles such as stretching methods [14], lithography, micromolding, etc. [15–18]. The resultant particles have demonstrated their values as photonic materials, drug release vehicles, micromotors, and catalysts [19–23]. Although with many successes, a large portion of the above methods rely on complex equipment and sophisticated generation processes, and it is difficult to fabricate large quantities of anisotropic particles in a fast and efficient manner. Moreover, the morphological diversity of the resultant particles remains to be explored. Thus, novel preparation strategies for the continuous preparation of anisotropic particles are still anticipated.

Here, we presented novel jellyfish-inspired microparticles from microfluidics, as schemed in Fig. 1. Jellyfish are among the most representative of living organisms showing extremely irregular and exquisite shapes [24,25]. The unique umbrella-like shape of jellyfish facilitates their motion by radially expanding/contracting their body and, thus, pushing water behind [26,27]. This complex morphology is difficult to reproduce by conventional molding or templating methods. Alternatively, microfluidic is an emerging technique that masters precise control of fluid flow in constrained channels [28–30]. Benefiting from this, microfluidics has emerged as a promising method for the generation of various particles [31–36]. However, because the particles are mainly derived from fluid templates formed by oil–water emulsification, typically droplets, their shapes are generally spherical or slightly deviated from spherical because of the action of interfacial tension [37–42]. Thus, it is conceived that by introducing new fluid regimes through external forces, jellyfish-like particles could be prepared continuously in microfluidic systems.

In this paper, we employed a pulse-generating microfluidic system to generate complex flow configurations and fabricate jellyfish-like microparticles. By combining piezoelectric vibration with co-flow microfluidics, a photocurable fluid thread with periodic reentrance cavities was generated, which then evolved to jellyfish-like liquid templates and eventually solid particles through in situ ultraviolet (UV) polymerization. By adjusting the piezoelectric parameters, the morphology and size of the particles could be easily controlled. Furthermore, by incorporating graphene oxide/N-isopropylacrylamide (GO/NIPAM), the microparticles exhibited stimuli-responsive shrinkage behavior under near-infrared (NIR) irradiation. Owing to the anisotropic particle shape, such deformation could provide a propulsive thrust to propel the particle, in a way mimicking the jellyfish. Moreover, by further incorporating magnetic nanoparticles, the direction of movement of the particles could be controlled by a magnet. On the basis of that, we demonstrated the capability of the jellyfish-like microparticles in highly efficient adsorption of organic pollutants under external control. These results indicated the superiority of the pulse-generating microfluidic system in preparing highly anisotropic particles with implications for various applications.

In a typical experiment, we assembled a co-flow microfluidic device by coaxially inserting a tapered injection capillary into an outer collection capillary (Fig. S1). An inner aqueous pre-gel solution (polyethylene glycol-diacrylate, PEGDA) and the outer fluid of deionized water were simultaneously driven to the injection and collection capillary, respectively, and flowed in the same direction to form a steady jet formed downstream in the collection capillary. A piezoelectric stack (PSt150/7/20VS12, CoreMorrow) was employed to exert oscillation pulsations to the inner phase by a PTFE (polytetrafluoroethylene) film, and the inner phase fluid was modulated as desired (Fig. S2). When the amplitude of pulsation exceeded a certain value, the fluctuation caused the inner jet to periodically retract and go forward and then formed ligaments with a varicose head and thin-thread tail (Fig. 2A). Because of the large inertial force relative to the interfacial tension of the 2 aqueous phases [43], the ligaments continued to deform into a jellyfish-like shape in accordance with Poiseuille advection (Movie S1 and Note S1). Two key elements of preparing such unique particles are the low interfacial tension of the liquid–liquid system for the ease of deformation and the rapid solidification of the liquid for keeping the configuration. Then, the effects of the piezoelectric and the microfluidic parameters on the jellyfish-like ligament were systematically investigated in detail (Fig. 2B and C). It was found that the length l and head ratio h/l could be well controlled by changing the piezoelectric frequency and voltage, respectively, as plotted in Fig. 2D and E. Meanwhile, the inner flow rate would significantly influence the width of the ligament template (Fig. S3). These results demonstrated that, by simply programming the piezoelectric signals, the jellyfish-like ligament templates with controllable morphologies could be obtained on demand.

After rapid solidification by UV irradiation, the jellyfish-like ligament templates were polymerized into solid particles with the same shape. The resultant particles possessed an umbrella-like head with an internal cavity and a trailing tentacle and showed component homogeneity (Fig. 3A). The morphology was further confirmed by scanning electron microscopy (SEM), as depicted in Fig. 3B. Because the morphology of the ligament templates evolved from bullet to jellyfish at the microfluidic channel, by shinning UV at different positions of the collection tube, the resultant particles could preserve the corresponding morphologies (Fig. S4). Benefiting from the continuous fluid flow in microfluidics, this method enabled consecutive generation of jellyfish-like particles with uniform size and morphology, as shown in Fig. 3C and D. Moreover, the jellyfish particle length could be finely tuned by altering the piezoelectric frequency, as shown in Fig. 3E. The length distribution of the jellyfish particles demonstrated monodispersity, with a coefficient of variation of <5% (the SD divided by the mean value), as plotted in Fig. 3F.

Intriguingly, by taking advantage of the flexibility in fluidic manipulation of microfluidics, we prepared dual-layered jellyfish-like microparticles. Specifically, a capillary microfluidic device was constructed, comprising a 2-stage nested channel design (Fig. S5), and the piezoelectric vibration was induced to the middle phase (Fig. 4A). The 2 inner fluids were pumped to the microfluidic channel and pulsed periodically, forming a dual-layered stream, which eventually resulted in the formation of dual-layered jellyfish-like microparticles (Fig. 4B). We demonstrated the dual-layered structure of the particles in both the head and the tail through a confocal laser scanning microscope (CLSM) (Fig. 4C and Movie S2). Furthermore, by tuning the microfluidic parameters (the innermost phase flow rate Q_i and the middle phase flow rate Q_m), the proportion of the 2 components could be controlled by φ = Q_i/(Q_i + Q_m), as shown in Fig. S6. To keep the morphology and size of the dual-layered particles consistent, the dispersed flow rate Q_i + Q_m was fixed, and it was found that the increase of φ would increase the inner layer thickness, as plotted in Fig. 4D. Similar to that of the simple jellyfish microparticles, the dual-layered jellyfish-like particles could also be continuously fabricated with uniform size and morphology, as shown in Fig. 4E. Such dual-layered structure of the particles could facilitate the addition of functional ingredients as needed, thus supporting various functionalities.

To impart the particles with stimuli-triggered motion ability, functional GO/NIPAM composite hydrogels were employed as the component of the dual-layered particles so that they could respond to NIR. Benefiting from the photothermal effect of the GO/NIPAM hydrogel, the particles could undergo contraction under NIR irradiation. It was found that when the particle was exposed to the NIR light, the bioinpired jellyfish appeared to shrink within about 2 s, and it would return to the swelling state within about 1 s. As a result, the particles could be propelled to move forward. We validated this process by tracking the displacement of a single particle under NIR irradiation, as shown in Fig. 5A and B and Movie S3. Moreover, by incorporating Fe₃O₄ nanoparticles into the innermost layer, the particles could be endowed with additional magneto-responsiveness. As the Fe₃O₄ nanoparticles were dispersed in the inner layer of the particles and occupied a large proportion in the head, the response of the particles to the magnetic field should be asymmetric, thus facilitating controllable movement. When the particles were placed in a relatively weak field (the magnet was set 4 cm from the jellyfish), they reorientated and turned their heads toward the magnet (Fig. 5C and D and Movie S4). When the magnetic field was further enhanced (moving toward the particles to 2 cm), the particles would move directionally to the magnet side, as shown in Fig. 5E and F and Movie S5. In general, with the increase of the concentration of Fe₃O₄ nanoparticles, the average velocity of the jellyfish increased accordingly (Fig. S7). These features demonstrated the prominent controllability of the bioinspired jellyfish-like microparticles under multiple external stimuli.

Because of the versatile motion controllability features, the multi-stimuli-responsive jellyfish particles were expected to be applied in the adsorption of water impurities. Furthermore, because GO owns extraordinarily large surface areas and large numbers of polar groups, it has a natural advantage in impurity adsorption [44,45]. It was found that with the absence of external stimuli, the jellyfish particles showed faster adsorption than spherical particles with the same volume. This could be attributed to the larger surface area of the former (Fig. S8). We then tested the adsorption process using these microparticles with movement under combinational control of magnetic field and NIR (Fig. 6). The particles could be drawn from one side of the cuvette to the other by alternating the direction of the magnet (Fig. S9 online). Moreover, NIR irradiation could be exerted to cause a circulation of the swarm of particles, which was probably due to the cooperation of the particle movement and Rayleigh–Benard effect. This thus enhanced the contact of the particles with the dyes in the solution (Fig. 6B). Benefiting from these effects, the particles under dual stimuli achieved the best adsorption performance, as manifested by the color change (Fig. 6A). It was also found that with sufficient adsorption, the color of the particles changed from brown to blue, indicating that the pollutants were adsorbed by these particles, as shown in Fig. 6C and D. Then, we studied the adsorption kinetics of these particles with different stimulation modes using a model pollutant methylene blue (MB). We measured the real-time concentration of MB (C) relative to its initial concentration (C₀) at different time points for each group, as plotted in Fig. 6E. It was found that the magnetic stimuli group showed faster and more effective adsorption capability than the nonstimuli group due to efficient movement. In addition, the magnet/NIR stimuli group showed superior adsorption capability even to the magnetic stimuli group, which suggested that the particles under combinational control could have a larger chance of contact with the pollutants in the solution.

In summary, we have developed novel bioinspired jellyfish microparticles from piezoelectric microfluidics. The jellyfish microparticles were derived from specific flow configurations through the piezoelectric pulsation of a fluid jet. The size and morphology of the particles were readily overmastered by altering the piezoelectric and microfluidic parameters. Furthermore, multi-compartment jellyfish microparticles with dual-layer structures were facilely achieved by tailoring the channel geometry of the microfluidics. Moreover, by employing the GO/NIPAM hydrogel precursor and magnetic nanoparticles into the flow templates, the resultant jellyfish microparticles were imparted with stimuli-responsive motion ability. Such distinct characteristics of the bioinspired particles have demonstrated their superiority in water purification and have been promising materials for other related areas including biomimetics, smart robotics, etc. Hence, future endeavors can be made in the systematic investigation of the locomotion of these jellyfish particles with variable morphologies and stimuli conditions to achieve more efficient adsorption efficiency. In addition, other materials and control methods can also be developed to realize intelligent drug delivery in, e.g., constrained blood vessels. The potential applications of this strategy can be expanded by developing alternative stimuli-responsive materials (e.g., liquid crystal) and designing more complex network topologies. We believe that this study would also shed light on the microfluidic fabrication of anisotropic microparticles that would open an avenue to a wide range of applications.