Engineering Micromotors with Droplet Microfluidics - ACS Publications

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Engineering Micromotors with Droplet Microfluidics Chunmei Zhou, Pingan Zhu, Ye Tian, Min Xu, and Liqiu Wang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b00731 • Publication Date (Web): 15 May 2019 Downloaded from http://pubs.acs.org on May 16, 2019

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Engineering Micromotors with Droplet Microfluidics Chunmei Zhou1,2,‡, Pingan Zhu1,2,‡, Ye Tian1,2,3, Min Xu4, and Liqiu Wang1,2,* 1

Department of Mechanical Engineering, the University of Hong Kong, Hong Kong, China

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HKU-Zhejiang Institute of Research and Innovation (HKU-ZIRI), 311300, Hangzhou, Zhejiang,

China 3

Sino-Dutch Biomedical and Information Engineering School, Northeastern University, Shenyang

110016, China 4

Center for Transport Phenomena, Energy Research Institute, Qilu University of Technology

(Shandong Academy of Sciences), Jinan 250014, China ‡ These authors contributed equally to this work. * Corresponding author. E-mail: [email protected]

Abstract Micromotors have promising potential in applications ranging from environmental remediation to targeted drug delivery and noninvasive microsurgery. However, there are inadequacies in the fabrication of artificial micromotors to improve the design of structure and composition for motion performance and multifunctionality. Here, we present a microfluidic fiber-confined approach to creating droplettemplated micromotors with precisely-engineered anisotropies in 3D structures and material compositions. The shape anisotropy comes from controllable deformation in droplet templates and material anisotropy originates from versatile emulsion templates. Containing Pt and magnetic nanoparticles (NPs), micromotors are endowed with both catalytic propulsion and magnetic guidance which are capable of performing tasks of precise catching, skillful delivering, and on-demand releasing of cargos. Droplet microfluidics allows us to systematically and independently vary the shape and size

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of micromotors and the distribution and content of NPs for the study of their influences on motors’ mobility and improve the design. Our results are useful for fabricating micromotors with well-controlled morphology and composition that is beneficial to designing sophisticated microrobotic systems for realworld applications. Keywords: micromotors, anisotropic microparticles, droplet microfluidics, microfiber-confined fabrication, self-assembly

Micromotors are microscopic machines that transform various energies into effective mechanical motion. According to energy sources, including magnetic,

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electric,

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micromotors can be propelled by chemical fuels,

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acoustic,

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and light

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physical fields

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such as motile

stimuli, and biological agents

cells. Owing to the micrometer-scale size, micromotors are treated as promising components to power miniaturized robotic systems for the performance of specific tasks such as environmental remediation, 9, 10

targeted drug delivery, 10-12 noninvasive surgery, 13-15 and organized assembly. 16, 17 Although much progress has been made,

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the study of artificial micromotors is still in its infancy.

Inspired by the motile microorganisms and molecular motors, artificial micromotors are committed to mimicking the functionality and swarm intelligence of biological motors. However, there are challenges in achieving efficient motion in complex environments and functionalizing artificial micromotors.

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Operated at low Reynolds number, micro/nanomotors need to overcome viscous drag and even Brownian motion to maximize the propulsion speed; complexity in environments imposes further hurdles on improving motion efficiency of micromotors, for example, in channels with high confinement, 19, 20 in the human body with variable geometries, and in biofluids with the non-Newtonian fluid property. To overcome these challenges, optimization in the 3D structure of micromotors is highly desired. In addition, functionalizing micromotors is imperative to fulfill multiple tasks such as sensing,

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navigation, communication, cargo loading and delivery,

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which requires the incorporation of multiple

functional components and variable materials. Given that most micro/nanomotors are particle based,

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developing methods for the fabrication of anisotropic microparticles with precisely-engineered structure and composition is of primary importance. It is challenging to fabricate micromotors with well-designed 3D structure and precisely-controlled distribution of materials at microscale by conventional fabrication techniques.

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For example, most

micromotors are in simple geometric shapes of tubes, rods, helixes, and spheres;12,

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functional

materials, such as catalyst, 3 are often homogenously distributed in micromotors. Although the emerging 3D printing technology is able to control the shape complexity and material composition for motor fabrication,

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low throughput is a non-negligible concern. Without the ability to systematically and

independently vary the size, structure, and composition of micromotors for large-scale fabrication, it is a formidable challenge to improve the motor design for practical applications. In contrast, droplet-based microfluidics provides a versatile platform 25 for engineering nano- and microscale droplets and particles with high precision,

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and is a good candidate for us to effectively synthesize a myriad of

micromotors with high throughput. In particular, droplet-based microfluidics is effective in generating a range of droplet templates with the typical generation frequency ranging from O(102) to O(104) Hz per channel potentially useful for the production of uniform microparticles with well-engineered shape (spherical, rod, disk, crescent), compartmentalization (Janus, core-shell, multi-layered), and microstructures (surface or internal structure), 27, 29 which can use a variety of materials including photocurable, thermo-curable and ionic cross-linkable polymers, inorganic compounds, metals, metal oxides, and stimuli-sensitive compounds.

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This technique can fabricate microparticles by design through the

precise placement of materials in different regions, such as occupying a part, encapsulated inside, and covering the surface of the microparticle, and thus represents a highly promising approach for multi-

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functional micromotors. Geometrical confinement can also be applied to precisely deform droplet templates to form nonspherical shapes.

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Selective solidification of the droplet templates produces

various polyhedral microparticles. 33 Therefore, droplet microfluidics offers precision and controllability to enable the fabrication of micromotors with systematically-varied size, structure, and composition. We develop a microfluidic-based microfiber-confined method to create anisotropic microparticles with precisely-engineered 3D structures and material compositions for artificial micromotors. We started with the spinning of alginate microfibers 34 that encapsulate curable oil droplet templates, 35, 36 followed by drying microfibers to compress the droplets into different shapes. After droplet solidification and removal of microfibers, anisotropic microparticles with various 3D shapes were obtained. The structure diversity includes curved surfaces, flat surfaces, sharp edges, and pointed ends. Taking advantage of various emulsion templates gives rise to anisotropy in compartmentalization and chemical composition of micromotors. We then doped Pt and magnetic (Fe3O4 and Fe) NPs into microparticles for the fabrication of micromotors with controllable mobility where Pt catalyzes the decomposition of H2O2 for propulsion and Fe3O4/Fe allows controlled motion under external magnetic fields. We investigated the organized self-assembly behavior and demonstrated the ability of micromotor assemblies to precisely catch, skillfully deliver, and on-demand release cargos. We also studied the influences of the shape and size of micromotors and the distribution and content of Pt NPs on motors’ mobility. This work advances the fabrication of anisotropic microparticles in optimizing the structure and functionality to improve the motion efficiency and design of sophisticated micromotors.

Results and Discussion Geometry-Confined Anisotropic Microparticles

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The central idea of fabricating anisotropic microparticles with 3D morphologies lies in geometrically shaping the soft droplets using volume-shrinking microfibers. The general fabrication process is shown in Figure 1a, involving four steps: microfiber fabrication, microfiber dehydration, droplet solidification, and microfiber dissolution. We made a capillary microfluidic device (see Figure 1b) to mimic the spinning process of spider silks 34 for the fabrication of oil-encapsulating microfibers, in which the inner phase of oil droplets spontaneously assembled into ordered arrays after their production and the outer phase of sodium alginate was ionically crosslinked when flowing into the calcium chloride solution. We then dried the hydrogel fibers in the air to decrease their radial dimensions by which oil droplets were compressed into 3D nonspherical shapes, followed by curing droplets into solid microparticles. After dissolving alginate microfibers in ethylenediaminetetraacetic acid disodium salt dihydrate (EDTA) solution, we obtained 3D microparticles with controllable anisotropic shapes (Figure 1c). We employed this method to fabricating thermo-curable polydimethylsiloxane (PDMS) and photo-curable trimethylolpropane ethoxylate triacrylate (ETPTA) microparticles with different 3D shapes, as shown in Figure 1d-1g.

Controlling the Shape of Microparticles Confined in microfibers, oil droplets assemble into well-organized packings. A range of microparticles are templated from deformed droplets in different packing assemblies: spindle, drum, zig-zag shape, and 3D polyhedral particles from multilayer packings (Figure 1c-1e). During water evaporation, the volumeshrinking fibers compress droplets in the radial direction. When droplets are sparsely packed, the compressed droplets freely elongate in the axial dimension and evolve into two pointed ends in a spindle shape; when the distance between adjacent droplets decreases, they would squeeze against each other to form two parallel flat sides in a drum shape; further decrease in the droplet spacing produces zig-zag

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packings that lose the axial symmetry along the fiber; finally when the density of droplets are large enough, they organize into two or multiple layers (see Figure S1) that produce polyhedral microparticles. As such, different degrees of droplet deformation yield various topological features of engineered microparticles, including curved and flat surfaces, sharp edges, pointed ends, and their combinations (see Figure 1f-1g). To discern different microparticles, we develop a model to depict the packing of droplets within microfibers.

Figure 1. Microfiber-confined fabrication of anisotropic microparticles. (a) Schematic of the four-step fabrication process: microfiber fabrication, microfiber dehydration, droplet solidification, and microfiber

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dissolution. (b) Schematic of the single emulsion microfluidic device. Inset: micrograph of oil droplet generation. (c) Phase diagram of particle shapes regarding the dimensionless D = Ddroplet/Dfiber and F = fDfiber/u. Four types of microparticle are created with predicted boundaries of F = 1/(1.385D), F = 1/D, and F = 2/D. (d-e) Four kinds of PDMS (d) and ETPTA (e) microparticles: spindle, drum, zig-zag shape, and polyhedron. PDMS and ETPTA are dyed by Oil Red O. (f-g) Scanning electron microscopy (SEM) images of the spindle, drum, zig-zag shape, and polyhedral PDMS (f) and ETPTA (g) microparticles, for images from top left to bottom right. Scale bars, 500μm in the inset of (b) and 200μm in (d-g).

The packing structure is controlled by the volume fraction of droplets, depending on the two independent parameters: droplet diameter Ddroplet and frequency of droplet generation f. We determine their dimensionless forms to be D = Ddroplet/Dfiber and F = fDfiber/u, respectively, denoting Dfiber as the diameter of microfibers and u as the fiber-spinning velocity where u = 4(Qi + Qo)/(πD2fiber). The transition between different microparticles is determined by the competition between the droplet spacing Ls and the fiber length Lf ejected during the time period t = 1/f for the generation of one droplet, where Lf = u/f. We experimentally find that when Ls < 1.385Ddroplet (Figure S2) microparticles change the shape from spindle to drum. Therefore, the transition boundary of Lf = Ls yields f/u = 1/(1.385Ddroplet), leading to F = 1/(1.385D). The transition between the drum and zig-zag type and between zig-zag and multilayer (more than one layer) occurs at Ls = Ddroplet and Ls = 0.5Ddroplet, respectively, leading to the boundaries of F = 1/D and F = 2/D, respectively. Figure 1c verifies our model, in which the experimental results agree well with predictions. We can further control the particle shape by varying the condition of fiber drying. Hanging the microfibers in the air enables a uniform compression force in the radial direction to fabricate axisymmetric microparticles, as shown in Figure 1d-1g. In contrast, placing the drying microfibers on a rigid substrate renders the bottom of microparticles flat, by which half-spindle and half-drum particles are fabricated (Figure S3). We envision more possibilities in engineering particle shapes by

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manipulating the fiber-confining conditions, such as using topologically patterned substrates and altering the number and spatial organization of substrates.

Figure 2. Anisotropic microparticles from double emulsion templates. (a) Schematic of the microfluidic device. Inset: micrograph showing the generation of double emulsion droplet. (b-d) Non-spherical coreshell microparticles in spindle, drum, zig-zag, and bilayer shapes (b), with increasing core size (c), and with increasing core numbers (d) from left to right. (e-f) Micrograph (left) and SEM (right) images of anisotropic liquid/solid hybrid ETPTA microparticles (e) and hollow PDMS microparticles (f). (g-h) Micrograph (left three) and SEM (rightmost) images of the crescent spindle (g) and half-spindle (h) ETPTA microparticles at different fabrication stages. Scale bars, 500μm in the inset of (a) and 200μm in (b-h).

Anisotropy from Double Emulsion Templates We apply the fabrication strategy to double emulsion templates to realize anisotropy in material compositions in addition to particle topologies. To produce double emulsion droplets, we introduced an innermost phase liquid into the microfluidic device, as shown in Figure 2a. We realize a variety of coreshell material compositions for microparticles using different liquids, such as by choosing PDMS and ETPTA (Figure 2b-2d), fluorinated oil (HFE 7100) and ETPTA (Figure 2e), air and PDMS (Figure 2f),

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and air and ETPTA (Figure 2g-2h) as the inner and middle phase liquid, respectively. The core-shell particles from PDMS and ETPTA are highly tunable in their shell shape (Figure 2b), the size ratio of the core to the shell (Figure 2c), and the number of cores (Figure 2d) as a result of the precisely engineered double emulsion templates and drying microfibers. In addition to hard cores, we can create liquid cores inside solid shells for the fabrication of liquid/solid hybrid microparticles (Figure 2e) and cavities inside micro-shells for the fabrication of hollow microparticles (Figure 2f). If air bubbles are removed during the fabrication, a different family of anisotropic microparticles is engineered to have crescent shapes (Figure 2g-2h). Dry of microfibers without substrate confinement produces axisymmetric particles (Figure 2g) while drying fibers on rigid substrates renders the bottom of the crescent particles flat (Figure 2h).

Self-Assembly of Anisotropic Microparticles Controlling the self-assembly process is vital to creating complex architectures for performing various tasks. For example, intelligent robotic swarms of animal behaviors observed in nature.

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have been designed to faithfully mimic the complexity

We show in Figure 3 that anisotropic microparticles exhibit

distinct self-assembly behavior to spherical particles. For microparticles made from PDMS and ETPTA, they assemble together attributed to the attractive hydrophobic surface interactions.

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Assembly of

spindle-shaped microparticles could build flower-like symmetric structures in which the long axis of each particle pointing radially to the center (Figure 3a-3c). Figure 3a displays the transition of an asymmetric chain-like assembly to a flower-like pattern with a six-fold symmetry (supporting video S1). The flower-like assemblies have strong mechanical stability to resist disturbance from fluid flow for keeping the structural integrity, in which particles are anchored and supported to each other through their longer ends. This character could explain the symmetry found in higher order assemblies when the

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particle number exceeds four; by contrast, the lower order structures provide more rooms for particles to hovering, thereby with a lower degree of symmetry (Figure 3b-3c). Imparting material anisotropy to microparticles allows further manipulation of self-assembly. We explore the self-assembly of cavityPDMS hollow microparticles with anisotropic density at the two ends, which results in the observed two states of assembly: microparticles can lay either parallel to (Figure 3d) or vertical to (Figure 3e) the water-air interface.

Figure 3. Self-assembly of spindle microparticles. (a) Snapshots showing the transition of six halfspindle PDMS microparticles from chain-like to flower-like assemblies. (b-c) Organized self-assembly of two to five spindle PDMS (b) and ETPTA (c) microparticles, from left to right. (d-e) Two distinct

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self-assembly states of spindle hollow PDMS microparticles that lay either parallel to (d) or vertical to (e) the water-air interface. Scale bars, 200μm.

Micromotors with Magnetic Guidance We fabricate magnetic microparticles responsive to the external magnetic field for the design of artificial micromotors. For this purpose, we add Fe3O4 nanoparticles into PDMS microdroplets during microparticle fabrication, in which magnetic nanoparticles can be forced to distribute unevenly at either the flat or the curved surfaces of microparticles (see the dark region in Figure. S3b). The anisotropic shape of microparticles and uneven distribution of magnetic nanoparticles give rise to more possibilities in the locomotion of microparticles in addition to translational motion. Figure 4a shows two cycles of half-spindle microparticles rotating along their central axes in response to the magnetic field, in which the microparticles switch between two basic states of postures (supporting video S2). Similar reversible rotation is also observed using half-drum microparticles with magnetic nanoparticles located near the curved surface (see Figure S4 and supporting video S3). This robust control suggests high sophistication in the swimming strategy, which could be a useful motion control to guiding micromotors passing through complex environments, such as blood vessels with variable geometrical confinement. We also demonstrate the cargo delivery ability of microparticle assemblies that perform collective tasks, mimicking the swarm intelligence in animal groups.

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Magnetic microparticles assemble into an

integrated structure and perform tasks of “on-the-fly” catch and transport of cargos to targeted site guided by an external magnetic field, as shown in Figure 4b-4c (also see supporting video S4-S5). The cargo (spherical particle) can be released from microparticle assemblies by gently disturbing the fluid flow, while the assembled architectures still keep the integrity owing to the strong mechanical stability and hydrophobic attractions (Figure 4b-4c). As such, such micromotor assemblies may be used as robotic systems in drug delivery for therapeutics

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and in the construction of artificial structures.

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To facilitate these applications, the cargo-delivery vehicles must have sophisticated cargo-towing approaches to overcoming complexities in environments. Figure 4d-4e shows the translational and rotational movements of cargo-loaded robotic systems composed of two-micromotor and threemicromotor assembly, respectively; three types of translational motions are demonstrated: drifting, retreating, and advancing (see supporting video S6-S7). Therefore, the magnetic robotic systems are capable of precise capture, skilled transport, and targeted release of cargos.

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Figure 4. Remote control of magnetic micromotor assemblies. (a) Two reversible states switched by turning on and off a magnetic field using half-spindle PDMS microparticles in which the magnetic nanoparticles selectively gather at the flat surface. (b-c) Microscopic images showing the catch, delivery, and release of a sphere cargo by micromotor assemblies composed of two (b) and three (c) magnetic PDMS microparticles. (d-e) Versatile swimming strategies of micromotor assemblies composed of two (d) and three (e) magnetic PDMS microparticles for cargo transport. Scale bars, 500 m.

Mobility of Hybrid Micromotors We further incorporate Pt and/or Fe NPs into microparticles for the fabrication of hybrid micromotors with chemical propulsion and magnetic guidance. To propel micromotors, Pt NPs are located asymmetrically at one side of the ETPTA micromotors to catalytically decompose H2O2 for the propulsion of motor. The final speed of a micromotor is determined by the balance between the driving force and viscous drag. The distribution and content of Pt NPs influence the driving force, while the shape and size of micromotors affect the viscous drag. Therefore, controlling/improving the speed of micromotors requires precise control of the above-mentioned four parameters for a deep understanding of their influences on mobility. With the ability to independently tailor the morphology of micromotors and doping of Pt NPs using droplet microfluidics, we successfully fabricated six types of ETPTA micromotors with spherical, spindle and half-spindle shapes and Pt NPs located aside and at the tip of micromotors (Figure 5a-5c). The mobility of micromotors is measured in a 30% H2O2 aqueous solution, where all micromotors differentiate from each other (Figure 5d). Compared to the spherical micromotor, only the half-spindle micromotor with side-located Pt NPs displays a larger propulsion velocity, while the other four types move no faster than the spherical one. Meanwhile, different motion speeds are observed when Pt NPs are located at different positions of micromotors with the same shape, such as by using the spindle and half-spindle micromotors. Therefore, both the shape of micromotors and NPs distribution affect mobility.

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Figure 5. Mobility of engineered micromotors. (a) Schematic of ETPTA micromotors with different shapes and Pt distributions: spherical shape with Pt located aside (i); spindle shape with Pt at the tip (ii) and on the side (iii); half-spindle shape with Pt at the bottom (iv), at the tip (v) and on the curved side (vi). (b-c) SEM images of the spherical (b) and the spindle (c) micromotors showing that Pt NPs (~ 100 nm in diameter) are selectively embedded at the side of the motors (iii-iv). (d) Velocities of ETPTA micromotors with different shapes and Pt distributions. All micromotors have the same size (~170 µm in

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diameter) and the same content of Pt NPs (1 wt%). (e) Velocities of spherical ETPTA micromotors with different sizes and contents of Pt NPs. (f-h) Guidance of the motion of ETPTA micromotors by magnetic fields. The micromotors display direction change (f), orbital revolution (g), and rotation (h) under magnetic fields. Scale bars, 100 m in (i) of (b-c), 20 m in (ii, iii) of (b-c), 1 m in (iv) of (b), 500 nm in (iv) of (c), and 200 m in (f-h).

We also characterize the influence of motor size and content of Pt NPs on the moving speed using spherical micromotors, as shown in Figure 5e. According to Stokes’ law, the viscous drag is Fd =6π Rum

, where η is the viscosity of H2O2 solution, R is the radius of micromotor, and um is the

speed of micromotor. Increasing motor size R leads to a large drag force, thereby rendering the moving speed smaller, consistent with our experimental observations (Figure 5e). Moreover, increasing (decreasing) the concentration of Pt NPs increases (decreases) the decomposition rate of H2O2 for a larger (smaller) propulsion force, making the micromotor move faster (slower), as shown in Figure 5e. Adding Fe NPs into Pt-doped micromotors renders the micromotors responsive to external magnetic fields further. Such hybrid micromotors are chemically propelled by Pt-catalyzed decomposition of H2O2 and magnetically guided with Fe NPs. We demonstrate the direction change, orbital revolution and rotation of micromotors when the magnetic field is varied correspondingly, as shown in Figure 5f-5h, respectively (see supporting video S8-S10). The magnetic guidance imposes an additional degree of control for more flexible mobility of micromotors. Directing micromotors in complex environments is of practical importance. We further demonstrate the guidance of micromotors by geometrical variations without magnetic field. The micromotors are prone to attaching to interfaces, so as to lower the free energy. Leveraging this principle, we present that the micromotors can revolve around a bubble with a liquid-air interface (Figure 6a, supporting video S11) and be trapped by a cylinder with a solid-liquid interface (Figure 6b-6c, supporting video S12-S13). Using multiple cylinders, we are able to direct the transport of micromotor between adjacent cylinders

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(Figure 6d, supporting video S14) and capture multiple micromotors separately (Figure 6e, supporting video S15). Varying the spacing between cylinders allows more flexible control over the mobility of motors. By placing three cylinders in an isosceles triangle array, we found that the motor hovers selectively between cylinders with the closer spacing (Figure 6f-6g, supporting video S16). We expect that such geometrical variations would be useful in applications involving selective capture of motors and directional cargo transport.

Figure 6. Manipulating micromotors by geometrical variations. (a) The micromotor revolving around a bubble. (b-c) Trapping one (b) and multiple (c) micromotors by a cylinder. (d-e) Directing the trajectory of one motor (d) and trapping three motors (e) by three cylinders in an equilateral triangle array. (f-g) Schematic (f) and image (g) showing the selective hovering of a micromotor between adjacent cylinders

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with the closer spacing. Arrows indicate the direction of motor trajectories. Scale bars, 200 µm in (a) and 500 µm in (b-e) and (g).

The reaction pathway of H2O2 decomposition on Pt is still unclear.

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Therefore, the propulsion

mechanism of motors based on Pt-catalyzed decomposition of H2O2 is under debate. Three possible hypotheses have been proposed: i) self-diffusiophoresis with local concentration gradients of neutral and ionic solutes,

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ii) asymmetric release of O2 bubbles,

oxidation and reduction of H2O2 at the motor surface. propulsion speed by salts

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3

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and iii) self-electrophoresis with separate Recent experiments on the reduction of

support the hysteresis of self-electrophoresis, as the motor speed should

be insensitive to salt concentration if the propulsion is attributed to neutral self-diffusiophoresis and bubble release. The propulsion of micromotors in our experiments is due to a combined mechanism of oxygen-bubble release and self-electrophoresis. We found that the propulsion speed sharply decreases when NaCl is added into H2O2 (Figure 7a), as salt increases the electrolyte conductivity, which reduces the decomposition-induced electric field that drives the electrokinetic flows around micromotors. Figure 7b contrasts the displacement of micromotors for different concentrations of salt, where the displacement increases with time almost linearly for propulsion without NaCl but displays the stair-step increase in the presence of NaCl (also see Figure 7c). The sharp increase in the displacement arises from the asymmetric release of O2 bubble, whereas the slow movement of micromotors is attributed to the weakened electrokinetic flows (Figure 7c). We observed that the release of O2 bubble barely affects the displacement of micromotors in H2O2 without NaCl (Figure 7d, supporting video S17) but contributes significantly to the displacement when NaCl is added into H2O2 (Figure 7e, supporting video S18), which suggests that the propulsion is self-electrophoresis dominant without salt and bubble-release dominant with NaCl. Therefore, the speed of micromotors is determined by the frequency of bubble

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release in the presence of NaCl, which gradually decreases as the bubble release time increases with the concentration of NaCl (Figure 7f). Moreover, NaCl slows down the decomposition of H2O2, as evidenced by the higher bubble release time (Figure 7f), decreased growth rate of O2 bubble volume (Figure 7g), and smaller bubble size at release (Figure 7h). Therefore, reduction in the speed of micromotors with NaCl is attributed to two effects: i) reduced self-electrophoresis by the increased electrolyte conductivity and ii) slowdown of O2 bubble release by the decreased H2O2 decomposition.

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Figure 7. The propulsion mechanism of micromotors based on Pt-catalyzed decomposition of H2O2. (a) Propulsion speed versus the concentration of NaCl for the spherical ETPTA micromotors embedded with 1.5 wt% Pt NPs. (b) Displacement of micromotors with time at different concentrations of NaCl. The three arrows indicate the abrupt increase of displacement due to bubble release for each of the three cases with the addition of salt. (c) The plot of displacement versus time for the abrupt increase indicated by the three arrows in (b), respectively. (d-e) Snapshots displaying the motion of a motor during the growth and release of O2 bubbles in H2O2 without (d) and with 10 wt% NaCl (e). (f) Bubble detachment time increases with the concentration of NaCl. (g) The growth rate of the O2 bubble volume without (39.2 nL s-1) and with 10 wt% NaCl (13.7 nL s-1). The constant rate of H2O2 decomposition gives rise to the linear increase of bubble volume with time. (h) Critical bubble diameter at release decreases as the concentration of NaCl increases. Scale bars, 100 µm.

Conclusion We demonstrated a microfluidic-based fiber-confined method for highly controllable fabrication of micromotors with anisotropies in 3D shapes and material compositions. The microfluidic platform provides a wide variety of emulsion templates for engineering the structure and composition of microparticles, advancing the capacity to integrate various functional units in the design of sophisticated micromotors and/or microbots. Assembled in volume-shrinking hydrogel fibers, microdroplets are compressed by the fiber and their neighbors in a highly tunable manner. Solidified microparticles are then featured by curved surfaces, flat surfaces, sharp edges, and pointed ends, where the anisotropic shape is flexibly controlled and precisely predicted by altering the volume fraction of microdroplets. The resultant anisotropic microparticles self-assemble into symmetric structures with strong mechanical stability to keep the structural integrity. Endowed with magnetic controllability, the stable assemblies can be used as miniaturized robotic systems to precisely catch, skillfully deliver, and on-demand release of cargos, mimicking swarm behaviors of animal groups. Therefore, the micromotor assemblies may be applied to targeted drug delivery and construction of artificial architectures. The independent control over the morphology and composition of microparticles allows us to fabricate different types of micromotors for the comparison of their mobility under certain conditions, thereby finding the right way 19


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to the improvement of moving speed. The microfluidic method also enables the fabrication of hybrid micromotors for more flexible control over their movements. These results suggest that the microfluidicbased fiber-confined method would be a robust and versatile platform for engineering micromotors.

Methods Fabrication of Microfluidic Devices The microfluidic device that produces single emulsions was fabricated by coaxially aligning a tapered cylindrical glass capillary (C2) in an outer one (C1). The outer capillary C1 was tapered with a narrowing neck to focus the fluid flow for the production of smaller droplets. An additional innermost capillary C3 was inserted into the capillary C2 for the production of double emulsions. The outlet of microfluidic device was inserted into a plastic petri dish that contains 10 wt% aqueous solution of calcium chloride (Sinopharm Chemical Reagent Co., Ltd).

Fabrication of Anisotropic Micromotors The standard process of microparticle fabrication involves four steps: microfiber fabrication, microfiber dehydration, droplet solidification, and microfiber dissolution. In producing oil-encapsulating microfibers, liquids were pumped into the microfluidic device using high precision syringe pumps (Longer Pump, LSP01-1A). The outer liquid was a mixture of 4 wt% sodium alginate (Sigma) and 2 wt% poly (vinyl alcohol) (PVA, 87-89%, Mw = 13000-23000, 87-89% hydrolyzed, Sigma-Aldrich), which was crosslinked by calcium chloride to produce hydrogel microfibers, mimicking the spinning process of spider silk. Microfibers were then dried in the air to compress the encapsulated microdroplets. When deformed, droplets were solidified to freeze the 3D shapes, after which alginate fibers were

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dissolved by EDTA (Mw = 292.24 g mol-1, Sinopharm Chemical Reagent Co., Ltd) solution.

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Microparticles were then washed, dried, collected, and stored for usage. We used a mixture of PDMS and silicone oil (containing 9 wt% curing agent) as the inner phase liquid to produce PDMS microparticles. Mixing PDMS with a certain amount of silicone oil is to decrease liquid viscosity (see Table S1) for the easier formation of the droplet. We call the mixture as PDMS oil for short in the context. Microfibers were cured at 80 °C for 30 minutes to solidify PDMS microdroplets. We used ETPTA oil (Mw = 428 g mol-1, Aldrich) containing 2 wt% photo-initiator 2-hydroxy-2methylpropiophenone (Aldrich) as the inner oil phase to produce ETPTA microparticles. Microfibers were exposed to UV light (Power 400 w, wavelength λ = 365 nm) for 20 minutes to photo-polymerize ETPTA microdroplets. PDMS/ETPTA core-shell microparticles were fabricated by using PDMS oil and ETPTA oil as the inner and middle phase liquid, respectively. Microfibers were firstly photo-cured and then thermo-cured to solidify ETPTA and PDMS successively. Liquid-solid hybrid microparticles were fabricated by using fluorinated oil and ETPTA oil as the inner and middle phase liquid, respectively. Microfibers were photo-cured to solidify ETPTA shell. PDMS hollow microparticles were fabricated by using air and PDMS oil as the inner and middle phase fluid, respectively. Microfibers were thermo-cured to solidify PDMS shell. ETPTA crescent microparticles were fabricated by using air and ETPTA oil as the inner and middle phase liquid, respectively. Driven by buoyancy, air bubble spontaneously occupied one side of the ETPTA oil droplet and was encompassed by an ultrathin layer of the oil film. After photopolymerization, the fragile thin film of ETPTA shell collapsed, leaving behind crescent ETPTA microparticles.

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The spatial distribution of Fe3O4 nanoparticles was controlled by an external magnetic field during the fiber-drying process. In this case, magnetic nanoparticles settled in microdroplets along the direction of the magnetic field thereby assembling at one side of microparticles after droplet solidification. To fabricate hybrid ETPTA micromotors, Pt and Fe NPs were added into the ETPTA phase in the generation of ETPTA-droplet-encapsulated microfibers. Afterward, microfibers were dehydrated using different methods to deform microdroplets into different shapes and create different NPs distributions: immersing microfibers in water for more than one day produced spherical ETPTA microdroplets with Pt NPs located aside; vertically or horizontally hanging the microfiber in air fabricated spindle-shaped microdroplets with Pt NPs at the tip or on the side, respectively; drying microfibers on a rigid glass substrate deformed microdroplets into a half-spindle shape with a bottom flat surface and an upcurved surface and placing the glass slide (with microfibers) vertically, horizontally with the curved surface upward and horizontally with the flat surface upward made Pt NPs settle at the tip, flat and curved side of microdroplets, respectively. After dehydration, microfibers were exposed to UV light for 20 minutes to photopolymerize the ETPTA droplets. Then microfibers were dissolved in an EDTA solution to get different hybrid ETPTA micromotors. The size of micromotors was well-controlled by the size of ETPTA microdroplets.

Magnetic Control of Artificial Micromotors A permanent magnet was moved manually to control the motion of artificial micromotors. The single and assembled micromotors moved in the same way as the magnet did: moving the magnetic translationally and rotationally generated translational and rotational motion of artificial micromotors, respectively. In addition, Pt and Fe NPs doped hybrid ETPTA micromotors were guided by the magnet to perform the motion of direction change, orbital revolution, and rotation.

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Characterization Optical microscope images and videos were captured by an inverted microscope (Nikon TS100) equipped with a high-speed camera (Phantom M110). SEM images were captured by a Zeiss scanning electron microscope (EVO MA10). Viscosity was measured by a rotary rheometer (Kinexus pro+).

Associated Content Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: . Multi-layer microparticles, dimensionless length L/Ddroplet of microfibers, half-spindle and half-drum micro-particles, reversible switch between two different states, and material properties of

PDMS/silicone-oil mixtures (PDF) Video S1: The self-assembly of six half-spindle PDMS microparticles (AVI) Video S2: The reversible switch between two states of half-spindle PDMS magnetic microparticles (AVI) Video S3: The reversible switch between two states of half-drum PDMS magnetic microparticles (AVI) Video S4: Delivery of a sphere cargo by the two-motor assembly (AVI) Video S5: Delivery of a sphere cargo by the three-motor assembly (AVI) Video S6: Diverse motion skills of the two-motor assembly in cargo transport (AVI) Video S7: Diverse motion skills of the three-motor assembly in cargo transport (AVI) Video S8: Direction change of the ETPTA hybrid micromotor under magnetic field (AVI) Video S9: Orbital revolution of the ETPTA hybrid micromotor under magnetic field (AVI)

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Video S10: Rotation of the ETPTA hybrid micromotor under magnetic field (AVI) Video S11: Revolution of the micromotor around a bubble with a liquid-air interface (AVI) Video S12: Trapping one micromotor by a cylinder (AVI) Video S13: Trapping multiple micromotors by a cylinder (AVI) Video S14: Directing the trajectory of one micromotor by three cylinders (AVI) Video S15: Trapping micromotors by multiple cylinders (AVI) Video S16: Hovering of a micromotor between two selective cylinders (AVI) Video S17: Displacement of the micromotor during bubble growth and release without NaCl (AVI) Video S18: Displacement of the micromotor during bubble growth and release with 10 wt% NaCl (AVI)

Author Information Corresponding Author *E-mail: [email protected] ORCID Chunmei Zhou: 0000-0003-4888-1962 Pingan Zhu: 0000-0001-5847-1831 Ye Tian: 0000-0003-2134-1536 Liqiu Wang: 0000-0002-6514-4211

Acknowledgments The financial support from the Research Grants Council of Hong Kong (CRF C1018-17G, GRF 17210319, 17204718, 17237316, 17211115 and 17207914) and the University of Hong Kong (URC

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201511159108, 201411159074, and 201311159187) is gratefully acknowledged. This work was also supported in part by the Zhejiang Provincial, Hangzhou Municipal, and Lin’an County Governments.

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Engineering Micromotors with Droplet Microfluidics - ACS Publications

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