Magnetically Driven Micro and Nanorobots

Manipulation and navigation of micro and nanoswimmers in different fluid environments can be achieved by chemicals, external fields, or even motile cells. Many researchers have selected magnetic fields as the active external actuation source based on the advantageous features of this actuation strategy such as remote and spatiotemporal control, fuel-free, high degree of reconfigurability, programmability, recyclability, and versatility. This review introduces fundamental concepts and advantages of magnetic micro/nanorobots (termed here as “MagRobots”) as well as basic knowledge of magnetic fields and magnetic materials, setups for magnetic manipulation, magnetic field configurations, and symmetry-breaking strategies for effective movement. These concepts are discussed to describe the interactions between micro/nanorobots and magnetic fields. Actuation mechanisms of flagella-inspired MagRobots (i.e., corkscrew-like motion and traveling-wave locomotion/ciliary stroke motion) and surface walkers (i.e., surface-assisted motion), applications of magnetic fields in other propulsion approaches, and magnetic stimulation of micro/nanorobots beyond motion are provided followed by fabrication techniques for (quasi-)spherical, helical, flexible, wire-like, and biohybrid MagRobots. Applications of MagRobots in targeted drug/gene delivery, cell manipulation, minimally invasive surgery, biopsy, biofilm disruption/eradication, imaging-guided delivery/therapy/surgery, pollution removal for environmental remediation, and (bio)sensing are also reviewed. Finally, current challenges and future perspectives for the development of magnetically powered miniaturized motors are discussed.


INTRODUCTION
Many species in nature, such as magnetotactic bacteria, birds, bats, butterflies, lobsters, and salmon, can fly or swim over a long distance by perceiving navigation cues from geomagnetic fields. Some species (e.g., Amitermes meridionalis) even have the ability to (re)orient their bodies or nests according to geomagnetic information. Similarly, the locomotion of nanoscale and microscale objects in a predefined path by the navigation of magnetic fields, 1−4 which are mainly generated by moving charges (i.e., electric currents) and magnetic materials (such as permanent magnets), has drawn extensive attention owing to their tremendous potential for applications in biomedicine and environmental remediation. Such miniaturized objects are normally termed as "magnetically driven micro/nanorobots" (called "MagRobots" for short in this review), which is an important branch of micro and nanorobots. Micro/nanorobots are locomotive artificial machines with size in the micro or nanoscale and rationally designed to execute tasks on command via self-propulsion or an externally controlled propulsion mechanism. Ideally, micro/nanorobots should have the ability to undertake tasks via encapsulation/ functionalization with diagnostic or therapeutic agents, decoration with functional materials, or being fabricated into special micro/nano architectures; "delivery tasks" by moving toward targeted sites in a user-defined path or a theoretically and experimental optimized path; "execute tasks", for example, killing diseased cells/tissues, removing environmental pollutants as required; and "exit tasks" after the task accomplishment via recycling or in situ degradation. During task implementation, locomotion behavior is of great importance for micro and nanorobots. The migration of micro and nanorobots can be powered by multiple strategies including chemical catalysis (e.g., O 2 or H 2 generation) or chemical gradients, 5−11 external energy sources (e.g., magnetic field, 12−14 light, 15−21 acoustic wave, 22−25 or electrical field 26−28 ), and even motile cells (e.g., sperm cell, bacterial cell). 29−37 According to the power source, micro/nanorobots can be classified as chemically driven (or fuel-driven), magnetically driven, light-driven, ultrasound-driven, electrically driven. The word "driven" can be replaced by "powered", "actuated", or "propelled". According to their functionalities, micro/nanorobots can be named as micro/nanogrippers, 38−40 micro/nanodrillers, 41 micro/nanocleaners, 42,43 micro/nanoscavengers, 44 etc. Readers can refer to our latest review 45 to obtain a more detailed classification of micro/nanorobots based on geometric shapes, motion modes, and functionalities.
Chemically propelled micro/nanorobots are faster than those with other propulsion methods, but their locomotion lacks directionality. Moreover, they require toxic fuels such as H 2 O 2 , N 2 H 4 , HCl, urea, and NaBH 4 . 46,47 In comparison, those micro/nanorobots powered by external physical fields (such as magnetic, ultrasound, light, and electric fields) do not need toxic chemical fuels for propulsion, but their motion is relatively slow. 48−52 Light-propelled micro/nanorobots can move in water; however, depending on their composition, they need H 2 O 2 and a high-intensity light source, which could compromise their biocompatibility. On the other hand, micro/ nanomotors propelled by ultrasound are biocompatible but lack directionality control, making it difficult for them to perform specific tasks. Finally, micro/nanomotors propelled by electric field are very promising for fuel-free locomotion; however, its biological application is still limited and not yet fully demonstrated. Magnetically driven micro/nanomotors address most disadvantages presented by others propulsion principles and, until now, have been the more explored and used in many biomedical applications as well as for environmental control and remediation. Furthermore, magnetic medical microrobots can be driven by magnetic resonance imaging (MRI) systems, thus utilizing existing clinical MRI equipment for dual purposes, namely the imaging and tracking of microrobots, and their propulsion and motion control. 53,54 Likewise, clinical ultrasonography systems hold great potential to actuate ultrasonically driven microrobots. 45 In addition, among all the actuation strategies, the utilization of a magnetic field for manipulating miniaturized robots has unparalleled advantages, which are summarized as follows. (i) Remote maneuverability: magnetic fields provide a noninvasive way to manipulate matter owing to the inherent contactless characteristics of magnetic forces. Such a wireless actuation method allows for micro and nano agents to move in an untethered manner while keeping their local chemical environment intact. (ii) Fuel-Free: using a magnetic field for propulsion is a clean process that does not consume liquid fuel (unlike for chemically and photochemically propelled swimmers). This feature eliminates the harmful effects of toxic chemicals (e.g., hydrogen peroxide) on cells and tissues during their biological application processes. In addition, magnetic fields exhibit insignificant dependence on features and properties of surrounding environments and cause negligible damage to cells at low frequencies. (iii) Reconfigurability and programmability of magnetic materials: reconfigurability refers to the rearrangement of the swimmer's features such as the morphology, locomotion mode, or other motion parameters upon the application of magnetic fields or other external stimuli. Examples of reconfigurable structures are magnetically driven particulate swarms, 55−57 stimuli-responsive magnetic materials (i.e., ferromagnetic shape-memory alloys), or composite structures (i.e., smart magneto-polymer composites 58,59 or complex origami-like architectures 60 ). This type of structure can readily change its shape by changing the conditions of the applied magnetic fields (i.e., frequency or magnitude). Programmability refers to the ability to manipulate the components of the MagRobots in terms of their shape, magnetic shape, magnetic anisotropy, 61 and crystalline anisotropy to achieve a specific motion mode, position, or orientation when magnetic fields are applied. 62,63 For example, the orientation of a magnetic composite-based structure can be programmed by suitably aligning the particles within the composite matrix. 60 Specific shape-morphing small-scale systems can also be designed to exhibit both reconfigurability and programmability. 64 (iv) Recyclability of magnetic materials: after micro/nanorobots have completed their tasks, the separation and recycling of introduced foreign matter from water, biological fluids, or even tissues might be necessary in terms of biosafety and biocompatibility. Magnetic nano/ microrobots, as they are composed of magnetic building blocks (i.e., coating, segment, particulates), allow for a feasible and convenient magnetically assisted retrieval and recycling process. (v) Versatility: by combining a magnetic field with other actuation sources, the transport and delivery of functional cargos (e.g., drugs or a single cell at the nanosize level) can be achieved with high maneuverability and sensitivity. 65 Currently, various hybrid power sources, such as magneto-acoustic, 22,23,66 magneto-optical, 67 and magneto-chemotaxis, 68 have been reported, which provide dual propulsion modes in response to multiple stimuli.
Molecular machines are molecular components capable of implementing mechanical locomotion (as output) in response to particular external stimuli (as input). 69−72 Stimuli can be various energy inputs such as chemical energy, electric energy, light, photochemical, electrochemical energy, or pH gradient. 73−77 Although molecular machines can perform very complicated functions, most functions are limited to conformational movements. 78−82 In terms of practical uses, particularly for biomedical applications, the operator's real-time imaging and tracking of the tiny robots are required when they are carrying out specific tasks inside the human body. 10,83 This requirement may limit the applicability of molecular machines due to their nanoscale (<10 nm) size being too small to be readily visualized using traditional imaging techniques. By contrast, larger micro-and nanorobots can provide greater feasibility for bioimaging for the applications in medical fields. 53,84−86 To this end, swarms of micro/nanorobots can also be used for their imaging and positioning abilities. 87−89 Recent reviews about micro and nanorobots that focus on fabrication techniques, 51 geometric shapes (e.g., active particles, 90 Janus, 91 tubular, 92 hybrid actuators 81,93 ), actuation sources (e.g., light, 48,49 magnetic field 94 ), propulsion mechanisms, 82 and potential applications (e.g., cancer therapy 95 ) provide us with a basic understanding and up-to-date developments in this multidisciplinary and interdisciplinary area. A comprehensive understanding of how tiny machines behave under magnetic fields will inspire and trigger interdisciplinary and cross-disciplinary scientific and technological innovation for multiple applications. The goal of this review is to provide a general view of the locomotion behaviors of nano and microscale motors under the manipulation of a magnetic field and guidance for their rational design by describing the interaction of MagRobots and magnetic fields as well as actuation and movement mechanisms, and reporting state-of-the-art fabrication techniques. After demonstrating current applications in biological and environmental fields, a further outlook of this new and exciting field is presented.

Magnetic Fields and Magnetic Materials
Magnetic fields, as vector-valued functions of the position, originate from the movement of electric charge. Magnetic fields can be generated by two distinct sources: freely moving electric currents and magnetic materials. Typically, the former source is generated by the coil of an electromagnet that is externally controllable. The setups of a triaxial orthorhombic Helmholtz coil and eight electromagnetic coils (e.g., MiniMag, OctoMag) are representative and widely employed to generate magnetic fields for driving and steering MagRobots (see Section 2.2). The latter source is generated from the intrinsic magnetization of magnetic materials, specifically permanent ferromagnets, which can retain a large remnant magnetization.
To manipulate micro-and nanomachines by magnetic fields, a conventional strategy consists of incorporating magnetic components into nano/microstructures. Magnetic materials can be classified as a function of the magnetic susceptibility (x m ), a parameter that reflects how easy a magnetic material is magnetized. As such, magnetic materials are categorized as ferromagnetic (and ferrimagnetic) materials (x m ≫ 0), paramagnetic materials (x m > 0), and diamagnetic materials (x m < 0). Paramagnets and diamagnets are weakly attracted or repelled, respectively, to magnetic fields. Additionally, they cannot retain any magnetization once the magnetic field is removed. Ferro-and ferrimagnets are all strongly attracted to magnetic fields. Specifically, ferro-and ferrimagnets can retain magnetization, (i.e., exhibit remnant magnetization or remanence) after being subjected to a magnetic field. Usually, high remanence is a feature of hard-ferromagnetic materials, otherwise known as permanent magnets. Soft-ferromagnets, in contrast, exhibit low remanence. Both soft-and hard-magnets exhibit a hysteretic behavior, which means that to demagnetize these materials, a coercive magnetic field is necessary. This coercivity is large for hard-magnets and small for soft-magnets. When placing a magnetic small-scale robot with a volume v in an external magnetic field B, the device will display a magnetization M. If the device is subject to a magnetic field gradient ΔB, it will experience an attractive force (or repulsive if it is a diamagnet) as expressed in eq 1. If the device is subjected to a magnetic field, to minimize its energy, it will experience a torque as expressed in eq 2, which will cause the magnetic robot to orient in such a way that its easy magnetization axis is parallel to the direction of the applied magnetic field. The easy magnetization axis is usually governed by the shape (shape anisotropy) but can also be ruled by specific crystal orientations of the materials (crystalline anisotropy). Additionally, the easy magnetization axis can be programmed, for instance, by orienting magnetic nanostructures with a matrix of a composite component or by premagnetizing a material in a specific direction: Both magnetic forces generated in gradient fields and magnetic torque induced by spatially homogeneous or heterogeneous dynamic fields can function as "fuel" to actuate microscopic and nanoscopic motors in various environments. In terms of magnetic torque, weak homogeneous rotating or oscillating fields (see Section 2.3), which display higher efficiency in transforming magnetic energy into kinetic energy, are highly preferable. Magnetic fields offer a maximum of six degrees of freedom (DoFs) (i.e., three translational DoFs and three rotational DoFs) for absolute spatial manipulation of micro/nanorobots, depending on the setup of electromagnetic actuation systems (see Section 2.2). For instance, the widely used uniform rotating magnetic field with triaxial Helmholtz coil can supply three rotational DoFs, while MiniMag and OctoMag have five DoFs: two rotational and three translational DoFs.
high-resolution camera), a magnetic manipulation system, and a computer system with video capture and analysis ( Figure  1A). The magnetic manipulation system consists of a set of either permanent magnets or electromagnets 107−110 as the source of the magnetic field. Recent contributions 97,111,112 provide a systematic review of configurations of magnetic manipulation systems that can be applied to magnetic smallscale robots with sizes ranging from nanometers to millimeters. In this review, we will only focus on the commonly used magnetic systems employed for the manipulation of nanoscale and microscale robots.
One of the main differences between systems using permanent magnets and electromagnets is the fact that the magnetic field from a permanent magnet is persistent and its magnitude cannot be quickly changed. The distribution and strength of a magnet's field depend on its geometrical shape and size. For a magnetized object with a given geometry shape and magnetization, large magnets can project their field further into space. However, large magnets produce smaller magnetic forces as demonstrated in eq 1 because the change of field in space (i.e., spatial derivatives in the field) is less pronounced. By manually or automatically adjusting the position or orientation of a magnet, a translatory or rotational movement of MagRobots can be triggered. Direct utilization of portable magnet provides an easy-to-operate way to drive the motion of MagRobots by simply adjusting the position and orientation of a magnet ( Figure 1B). Although many researchers have reported the locomotion of magnetic micro/nanorobots by using single permanent magnets, the experimental reproducibility and accuracy are challenging aspects because the movement of magnets largely depends on their operator. Given the drawbacks of manual handling, many automatically operable magnet systems have been designed by integrating a magnet with a commercial robotic arm such as the LBR Med robotic arm from KUKA Robotics Corporation ( Figure 1C) and MH5 robotic arm from Yaskawa Motoman. Such an integrated system is more reliable and precise. Besides magnetic field gradients, magnetic torque can also be exerted on small-scale devices when the magnet rotates ( Figure 1D), which allows for rotational actuation mechanisms.
In magnetic actuation systems based on electromagnets, magnetic fields are generated from flowing currents through coils. A typical electromagnet is formed by wrapping insulated copper wires around a ferromagnetic core, which can concentrate and amplify the magnetic field and field gradient. An ideal soft magnetic material is often used as the core in order to avoid effects of hysteresis. On-demand setting of current in each coil can result in the required configuration of magnetic fields, such as rotating field, oscillating field, alternating fields, and conical fields, which will be discussed in Section 2.3. Different arrangements of coils constitute specialized electromagnet systems such as the Helmholtz coil, the Maxwell coil, the saddle coil, and the double-saddle Golay coil (detailed information can be found in ref 113). Helmholtz coil, containing two circular and coaxial coils with equal radius and same handedness of flowing current, is the first and most important arrangement. Because the field generated from the Helmholtz coil is near-uniform at the center of the coils, such a magnetic actuation system is appropriate for magnetic torque control. 114−116 Arbitrary uniform magnetic fields in a 2D plane or 3D space can be generated by two pairs of Helmholtz coils or triaxial Helmholtz coils, respectively. Triaxial circular Helmholtz coils are the most commonly used for actuating magnetic small-scale robots ( Figure 1E). The combination of Helmholtz coils with other types of coils can engender systems with multi-DOF capabilities. Maxwell coil is also composed of two circular coaxial coils with equal radius, but the current flowing through different coils coil has the opposite handedness. Maxwell coils can create uniform magnetic field gradients, saddle coils can generate a uniform field or a gradient field, and double-saddle Golay coils can produce a transverse gradient. A magnetic manipulation system with a stationary Helmholtz− Maxwell coil and a rotational Helmholtz−Maxwell coil has the capacity of 3D locomotion of a magnetic small-scale robot through the control of both magnetic forces and torques ( Figure 1F). 104 Its upgraded system using four different coil pairs (i.e., a Helmholtz coil, a Maxwell coil, a rotatory uniform saddle coil, and a rotatory gradient saddle coil) occupies a smaller volume and consumes less driving energy ( Figure  1G). 105 Given the practical clinical application of biomedical micro/nanorobots, saddle coil and Golay coil with tubular construction are preferable because they have high space efficiency and, hence, are capable of accommodating the human body. For example, a widely used magnetic resonance imaging (MRI) scanner in clinical practice incorporates a Maxwell coil and two orthogonal Golay coils. 117 A drawback of magnetic actuation systems consisting of paired coils lies in their restrictions on the shape and size of the workspace. In contrast, electromagnetic systems using several nonorthogonally distributed electromagnets, usually made of columnar coils with soft-iron cores, can break this limitation by arranging the electromagnets so that their generated dipoles keep their respective axes pointing to a common point in the given workspace. The first example of such configuration was the OctoMag, an electromagnet comprising a total of eight electromagnets. OctoMag is a system capable of generating magnetic forces and torques in three dimensions and allows for a 5-DOF magnetic control (3-DOF position and 2-DOF orientation). 118 OctMag is composed of four evenly distributed electromagnets in a plane with the orientation of 90°f rom a central axis and four evenly distributed electromagnets with the orientation of 45°from a central axis. MiniMag is the scaled-down compact version of the OctoMag ( Figure 1H). Utilization of OctMag and MiniMag has been reported to remotely manipulate micro-and nanorobots for targeted drug delivery, 119 minimally invasive ophthalmic surgery, 120 and stem cell transplantation in a rat brain. 121 Other configurations of electromagnets, such as square antiprism, cubic, open asymmetric, and so on, were summarized in a recent review. 113

Actuation Configurations for MagRobots
According to changes of the magnetic field vector with time, magnetic fields can be classified as static, dynamic (including a rotating magnetic field whose direction varies with time, an oscillating magnetic field whose strength varies with time), or on−off fields. Both static and dynamic magnetic fields can be homogeneous fields where the field vector modulus remains constant in space, or inhomogeneous magnetic fields where the field strength varies with position, that is, field gradient. 122 Rotating magnetic fields are widely adopted to induce rotational motion. For some micro and nanomachines with specific shapes (e.g., helical structure), such temporal−periodic rotational motion can be converted into translational corkscrew motion (see Sections 3.1 and 4.2), which leads to a net spatial displacement. In contrast, oscillating magnetic fields can be utilized to activate traveling undulatory locomotion for some MagRobots such as those with soft tails (see Section 3.2) and those consisting of solid segments linked with soft hinges (see Section 4.4). Rotational magnetic fields can also induce thermophoretic motion for ferromagnetic materials by generating heat energy 123 (see Section 3.4). Figure 2 summarizes different categories of magnetic fields and their corresponding field diagrams. 124

Effective Movements in MagRobots:
"Symmetry-Breaking Strategies" To begin this section, we would like to briefly introduce the hydrodynamic laws to understand how small-scale robots swim in a fluid. The Navier−Stokes equation, arising from Newton's second law, describes the motion of a Newtonian fluid as follows (eq 3): where vector ν and vector p (both of which are a function of position and time) are the flow velocity and pressure, respectively; ρ and η are the density and viscosity of the flow, respectively. The left-hand of the Navier−Stokes equation comprises the inertial forces, while the right-hand corresponds to the viscous forces. Here, we introduce an important dimensionless quantity called the Reynolds number (Re, expressed in eq 4), which is the ratio of inertial and viscous forces: L inertial forces viscous forces (4) where L is the characteristic length of an object moving in a fluid.
For small-scale devices and organisms (i.e., motile cells, bacteria), L is very small (Re ≈ 10 −4 ), which means that viscous forces rule their motion. A typical analogy of swimming at low Re is that a bacteria swimming in water is similar to a person swimming in honey. Considering that inertia forces are negligible in the low Re regimes, the Navier−Stokes equation can be simplified as an expression known as the Stokes equation: ν η∇ = ∇P (5) Note that this hydrodynamic equation is time-independent, meaning that no net displacement will occur after completing a cyclic process no matter if the speed of the swimmer is fast or slow. In other words, the resultant fluid flow exhibits instantaneous and time-reversible features. This is the socalled "Scallop Theorem," as introduced by the Nobel laureate Purcell ( Figure 3A). At low Reynolds number, a microscopic scallop can only perform back and forward movement (i.e., reciprocal motion). Once the actuation energy (such as a magnetic field) is removed, its motion is immediately halted due to the lack of inertial forces. Importantly, to generate a nonreciprocal translatory movement to execute tasks such as cargo delivery, Figure 3B summarizes some strategies employed to break Purcell's Scallop Theorem. The first method involves fabricating a small-scale robot with an asymmetric shape such as a tubular, 125 helical, 60,126,127 fishlike, 128 annelid-worm-like, 129 tadpole-like, 130 bullet-shaped, 22 star-shaped, 131 or even random-shaped 132,133 structure. In addition, an asymmetric shape (e.g., carpet, 134 ribbon 56 ) can also be formed by self-assembling colloid particles with a symmetric shape based on collective behavior. 90 A second approach consists of creating a micro-or nanostructure containing a flexible component, for example, a flexible tail, which can mimic the flagellum of a microorganism. 81,135 Velocity distribution (indexed by frame number of a video sequence) of a single beating flagellum or cilium from a cell or a microorganism during one cycle 136 indicated the generated traveling-wave motion (see Section 3.2) is nonreciprocal. Incorporating flexible components in between rigid structures to create multilink micro or nanoassemblies is also another possibility, which will be further discussed in Section 4.4. A recent strategy consists of integrating motile flagellated microorganisms and cells with magnetic micro and nanostructures to create biohybrid MagRobots (see Section 4.5). A third approach entails the use of a nonsymmetric actuation magnetic field. For example, a symmetric small structure can exhibit a translational motion by means of a traveling-wave 137 or a ciliabeating motion mechanism 138 under a nonsymmetric actuation field. The fourth approach is based on actuating magnetic small-scale devices in the proximity of a boundary (e.g., wall, interface) to break the spatial symmetry. The motion mechanism based on this method is called "surface-assisted propulsion", which will be discussed in Section 3.3. All these symmetry-breaking strategies evade the constraints of the famous Scallop Theorem. 100 Note that the Scallop Theorem only applies to Newtonian fluids. Time-reversible reciprocal locomotion can still generate an effective propulsion in non-Newtonian fluids (e.g., blood, saliva, mucus). 139

ACTUATION AND MECHANISMS OF MAGNETIC
ROBOTS Compared with macroscale motile robots, micro and nanoscale robots experience totally distinctive hydrodynamics. Hence, they exhibit distinctive assorted motion behaviors. A good understanding of various propulsion mechanisms is the basis for the design of propulsion microsystems including the shape and architecture of micro and nanorobots as well as the configuration of the magnetic field. The designed propulsion system must be able to overcome various resistive forces in the micro and nanodomains to realize the motion of small-scale robots effectively. The translational mechanisms of magnetic miniaturized machines could be broadly divided into three types: (a) corkscrew motion, (b) undulatory motion (i.e., traveling-wave motion), and (c) surface-assisted propulsion (i.e., surface walker).

Corkscrew-like Motion
In nature, many microorganisms can coordinate their propulsion and orientation behaviors according to external stimuli with a motile appendage called a flagellum. Eukaryotic cells (e.g., spermatozoa) can produce a traveling-wave motion by making use of a flexible beating flagellum. In contrast, prokaryotic cells can perform a corkscrew-type motion by rotating their helical flagella. Bacteria (e.g., E. coli), as a representative of prokaryotic organisms, rely on the rotation of flagella for swimming. The flagellum, containing a basal body, a hook, and a filament, is the fundamental organelle for bacterial motion. There is a reversible motor inside the basal body controlling the rotation of the flagellum. The flagellum can not only trigger reorientation of the organism but also make them move forward and back. When the flagellum rotates in one direction with an action frequency ω 1 , the cell body counterrotates with the reaction frequency ω 2 (ω 2 and ω 1 are not equal) to balance the produced torque ( Figure 4A). Inspired by the bacterial flagellum for efficient movement, man-made helical micronanomachines, known as artificial bacterial flagella (ABF), 142−145 have been developed and investigated. Although there is no motor in the ABF system, external rotating magnetic fields provide a similar function for generating the rotation.
As discussed earlier, a MagRobot will align its easy magnetization axis parallel with the direction of a local homogeneous field upon experiencing a magnetic torque in that magnetic field. A continuously applied torque to a micro/ nanoobject under an external rotating field gives rise to the rotational movement of the body. For artificial magnetic micromachines containing chiral helices, a steady rotation around their helical axis can be effectively converted into nonreciprocal translational motion, with the direction parallel with the rotating axis of a two-dimensional planar rotating field. At the same time, the tail and head (sometimes it has no head) of ABF perform the same (clockwise or counterclockwise) orientation. This is distinct from bacteria, whose head and tail rotate in the opposite orientation. If the ABF consists of a single rigid body, then the head and tail will rotate with the same frequency (ω h = ω t ). Moreover, the progression direction (forward or backward) can be easily inverted by reversing the direction of rotation (i.e., clockwise or counterclockwise) of an applied magnetic field. In the magnetically actuated ABF system, similar to other magnetically controlled systems, magnetic materials are required in order to respond to the external field. Widely used ferromagnetic materials include Ni, Co, and Fe, while the frequently applied superparamagnetic materials include Fe 2 O 3 and Fe 3 O 4 . Up to now, various types of ABF systems have been investigated. 146,147 Some typical examples are shown in Figure 4B. 148 Many factors play a critical role in the movement of magnetic helical microswimmers such as solution properties (e.g., fluid viscosity, ion strength), geometrical parameters (e.g., helix pitch), surface characteristics (e.g., surface wettability, 149,150 roughness), magnetic field properties (e.g., frequency, intensity, rotating, or oscillating field), magnetization properties of magnetic materials, head/tail shapes, mechanical properties (e.g., rigid or flexible), and boundary condition (e.g., wall). The simulation demonstrates that helical swimmers exhibit the highest propulsion efficiency when the pitch angle is about 45°. 151 The optimal magnetization direction for helical microrobots is perpendicular to the helical axis in order to maximize the applicable magnetic torque around the axis. The motion mode and velocity of ABF are strongly associated with the applied field frequency. As shown in Figure 4C, at low frequency rotating magnetic fields (typically below several Hertz), a wobbling motion occurs when the axis of the helical MagRobot cannot align with the direction of the local field. 152,153 As the rotating field frequency is enlarged, the wobbling angle decreases from 90°to zero, where a wobbling angle of zero corresponds to the rotation along the long axis with a direct corkscrew-like thrust. (Ratio of viscous to magnetic torque (i.e., Mason number), helix angle, and helical size can also bring about shrinkage of the wobbling angle of helical MagRobots under temporal−periodic torques. 49 In the corkscrew-like motion region (also denoted as "synchronous" region), the translational velocity of helical MagRobots increases with the increased applied rotation frequency of an external magnetic field, performing a synchronous and linear relationship. Further increase with respect to a critical field frequency results in a decrease of the swimming velocity, which is attributed to the fact that the magnetic torque is not sufficient to maintain a synchronous relationship between the magnetic moment and the applied rotating magnetic field. The critical frequency is called the "step-out frequency". 154 Surface chemistry also influences the motion of helical MagRobots. Recently, it has been reported that magnetically driven helical microswimmers with hydrophobic surfaces possess larger step-out frequencies and higher maximum translatory velocities at low Reynolds numbers in comparison with those with hydrophilic surfaces. 155 The increase in hydrophobicity of the swimmer surface causes an increase in both the step-out frequency and the maximum forward velocity in a nonlinear mode due to the interfacial slippage. Importantly, the forward velocity of ABF is independent of their surface wettability when MagRobots are manipulated below their critical frequency. A 3D oscillating magnetic field, created by the combination of DC magnetic field B xy and oscillating B z field, can only cause the reciprocal back-and-forth motion of a helical microswimmer. When symmetry is broken by placing the microswimmer near a surface, the rocking motion results in a net displacement. Moreover, the asymmetric helix (with polystyrene head and helical Co/SiO 2 tail) exhibits much larger displacement than a nearly symmetric helix without a head under similar experimental conditions. 156 The viscosity disturbance in different solutions results in the difference of precession angle (i.e., wobbling angle) of helical MagRobots when the applied frequency of the rotating field is smaller than the step-out frequency. Taking advantage of this feature, the detection of instantaneous orientations (i.e., wobbling angle) of MagRobots provides an innovative approach to evaluate the viscosity of the local medium with high spatial and temporal accuracy, which makes ABF a novel prototype for mobile viscometers. 157

Traveling-Wave Locomotion/Ciliary Stroke Motion
Both traveling-wave propulsion and metachronal-wave propulsion, inspired by the flagella and cilia of eukaryotic cells, respectively, are capable of breaking temporal symmetry to overcome the Scallop Theorem and generate an effective net displacement. Because short and rigid nano/microrobots can only generate very limited net propulsion due to the reciprocal nature of an oscillating movement, the presence of an elastic component is crucial for achieving traveling-wave propulsion. However, net displacement can also be hampered if the motor is too long and flexible due to the increase of drag force. Hence, the size and elasticity must be taken into consideration in terms of design. Traveling-wave propellers have been created either by incorporating elastic tails (e.g., a chain of paramagnetic beads using DNA as the soft hinge 158 ) to a rigid head or by utilizing multilink nanowires connected by flexible segments (e.g., soft silver nanowire, 3 elastic polymeric nanocylinders composed of multiple bilayers of polyallylamine chloride and polystyrenesulfonate 97 ). The thrust from the backward-traveling wave generated by the undulatory motion of multilink artificial microswimmer, consisting of two magnetic nickel segments, two gold segments, and three soft silver, hinges upon the application of an oscillating magnetic field. Periodic mechanical deformation triggered fish-like locomotion at the microscopic level ( Figure 5A). 128 Other traveling-wave motion of wire-like MagRobots driven by an oscillating field can be found in Section 4.4.
Although the metachronal wave, which is produced by the oscillatory locomotion of ciliated protozoa through hydrodynamic interactions, can also drive an effective nonreciprocal movement. Because of the complexity of manufacturing these structures at micro-and nanoscale, only millimeter-scale (not nanoscale or microscale) robot systems that mimic the metachronal-wave movement of cilia have been reported ( Figure 5B). 159 To date, one artificial cilia-like magnetic microarchitecture, as the exclusive example with regard to the simple ciliary stroke motion, has been fabricated by means of a 3D laser lithography method. 138 The efficient movement of this microrobot in a fluid environment with a low Reynolds number was powered by the net propulsive force from the beating locomotion of cilia and its position and orientation can be precisely controlled by on−off fields with designated angle ( Figure 5C).

Surface-Assisted Motion
Apart from breaking the symmetry from the geometrical point of view, another strategy to overcome the Scallop Theorem and induce translational movement is to introduce a physical boundary to break the spatial symmetry. Such locomotion can be achieved by magnetically actuating a magnetic micro-or nanostructure when it lies in the proximity of a surface/ interface 160 or a wall in a liquid at low Reynolds number, or even a dry surface. 161 The micro and nanorobots based on this "surface-assisted locomotion" mechanism are called "surface walkers" or "surface rollers." Figure 6A exhibits a typical forward locomotion mode of a surface walker. Many magnetic micro and nanostructures have demonstrated such surfaceassisted propulsion including (but not limited to) nanorods, dimers, assembled colloids, microtubes, and Janus particles.
Simulations and experiments have confirmed that the dynamics and motion mechanism of surface walkers are governed by the boundary features (slip or nonslip), the degree of confinement (e.g., single or multiple confining boundaries, the distance of a MagRobot from the nearby boundary), fluid properties (e.g., finite inertia 162 ), magnetic fields (e.g., configurations, frequency, strength), and others. The presence of a boundary modifies the hydrodynamic stresses on self-propelled nano/ microrobots, resulting in a change in their orientation, velocity, trajectory, and even hydrodynamic bound states. 163 Stronger frictional forces near a nonslip confining boundary (wall or surface) can drive microdevices to move forward, resulting in a larger net displacement compared with those in proximity to a smooth boundary. Hydrodynamic interactions can create stable finite clusters ("critters") from an unstable front that is generated from the press of fingers. 164 Motion modes of surface walkers are frequency-and field type-dependent. CoPt semihard magnetic nanowires experience the motion transformation from tumbling to precession and then to almost rolling near a surface boundary by raising the frequency of the applied planar rotating field. In the tumbling region, the y-axial translational velocity of nanowires synchronously increases with the field frequency regardless of the applied magnetic moment. In the procession region, the velocity still slowly increases and then decreases after reaching the maximum. The decrease of speed is ascribed to a decline of the precession angle, resulting from the change of motion configuration. 165 Transformation of the motion mode can also occur in hematite peanut-shaped microrobots by using different magnetic fields, including a 1D oscillating magnetic fields (oscillating mode), yz-planar rotating magnetic field (rolling mode), xy-planar rotating magnetic field (spinning mode), and conical magnetic field (tumbling mode) corresponding to the collective configuration of liquid, chain, vortex, and ribbon, respectively ( Figure 6B). A 2D vortex can be self-assembled by rotating magnetic colloids in a plane parallel to the interface; however, such a vortex cannot produce net displacement. On the contrary, net displacement occurs in rolling mode and tumbling mode once a boundary is present.
Taking the chains with rolling mode as an example, net displacement along the x axis can be generated when the assembled magnetic chains are subjected to a yz-plane rotating field. In other words, the rotational motion of microrobots in a plane perpendicular to a nearby boundary can lead to nonreciprocal propulsion. Similar to the artificial bacterial flagella, the velocity of the individual peanut-shaped microrobots as well as that assembled chains (e.g., trimer and pentamer) linearly increases with applied frequency when the actuation frequency is below the step-out frequency. Above the step-out frequency, the increase of the rotating field's frequency causes a decrease of the microrobots' velocity owing to the considerable rise of liquid-induced viscous torque. In addition, the velocity of assembled chains is dependent on the number of microrobots composing the chains. Most importantly, collective formations and locomotion can be manipulated by a magnetic field in a programmable and reconfigurable fashion, providing versatile collective modes to meet multitasking requirements in complicated biological systems. 56,124,166 Magnetic microkayaks demonstrate processing motion in a double-cone rotating way, similar to the movement of a paddle, when placed in proximity to a solid surface under the rotating fields with kilohertz frequency ( Figure 6C). 167 In comparison with flat surfaces, research of magnetic nano/ microrobots on topographic surfaces is more challenging but more intriguing. Inspired by smooth-riding bicycles containing square-shaped wheels, utilization of a microroad with periodic bumps lead to 4-fold intensification in forward velocity of microwheels (μwheels) owing to the nonslip rotation of entire wheels. Because of the velocity difference between diamond μwheels and square μwheels on topographic surfaces, the separation of isomeric μwheels by symmetry can be fulfilled ( Figure 6D). 168 For surface walkers, climbing over a barrier is also possible by taking advantage of surface physics. A peanutshaped hematite micromotor with its magnetic moment vertical-aligning with the long axis can achieve rolling movement under a rotating magnetic field and wobbling movement under a conical rotating field. The magnetically actuated MagRobot can climb up and down a steep slope with a height of 8 μm through the wobbling motion mode. By combining rolling motion mode and wobbling motion mode, the MagRobots can be utilized to deliver and release cells to an appointed place and form complex cell patterns under the control of a magnetic field in a contactless fashion ( Figure  6E). 169 Except for these artificial barriers, magnetic actuation of MagRobots on the uneven surface of biological tissue (i.e., ex vivo swine bladder) was investigated by Zhang's group. 170 In addition to a rotating field, an oscillating magnetic field can also be adopted to actuate the translational movement of a surface walker. Under an oscillating field, microdimers consisting of Ni-SiO 2 magnetic Janus microspheres are able to roll on the solid surface after sedimentation treatment. In contrast, no net displacement can be produced when Janus microspheres are returned to the bulk of the liquid by acoustic levitation ( Figure 6F). 171

Application of Magnetic Fields in Other Propulsion Approaches
Approaches such as chemically or photochemically induced propulsion lack the level of control of magnetically driven micro and nanoswimmers, especially in terms of directionality, control over the speed, and ON/OFF motion features. However, chemically and photochemically driven swimmers are very useful for chemistry-on-the-fly applications such as water remediation applications. To provide better controllability on the motion aspects of these chemical and photochemical swimmers, the integration of magnetic components has been widely adopted. For example, a single TiO 2 −PtPd−Ni nanotube 173 performed autonomous motion through the bubbles generated from the decomposition of hydrogen peroxide ( Figure 7A). To control the directionality of bubble-propelled small-scale machines along any predetermined paths, the assistance of other power sources is necessary. After the application of a static magnetic field, the motion direction of those self-propelled nanodevices is controllable. A similar function of orientation control was found in fuel-free light-driven small-size robot systems, 174 urease-powered nano/micromotors, 175 cell-powered nanomicromachines, 176 and acoustically actuated micronanoscale vehicles. 97,177 Furthermore, the combination strategy can amplify the propulsive thrust by harvesting energies from different sources, 178 resulting in more efficient task processing capabilities. A Janus microrobot, using three types of nanomaterials as engines, was capable of swimming by bubble propulsion, light-powered propulsion, and magnetic-actuated motion ( Figure 7B). 179 Compared with only bubblepropulsion, the bubble−magnetic dual propulsion mode boosted the velocity of microrobots up to 3 times, while the bubble−light dual mode could increase it up to 1.5 times. Because of the synergetic effect of the three energy sources (i.e., chemical energy, light, and magnetic field), the ternary bubble−light−magnetic mode exhibited a much higher speed than binary bubble-light mode. 179 By switching on and off a magnetic field, the on-demand control of nanoand microscale robotic systems via braking or accelerating the propulsion process was demonstrated. Obvious growth of velocity was observed in an ultrasound-powered Janus micromotor when a static magnetic field switched from "OFF state" to "ON state" as shown in Figures 7C. 180 Moreover, the use of external magnetic fields allowed for controlling the directionality to the acoustically driven microrobots.

Magnetic Stimulation of Micro/Nanorobots beyond Motion
In addition to direct motion control, magnetic fields can be used as the energy source for triggering hyperthermia, 181 thermophoresis, and magnetoelectricity. Magnetic hyperthermia refers to the heating of cells, tissues, tumors, or systems to temperatures up to 42°C by converting magnetic energy into heat radiation. 182,183 Such function is preferable for treating cancer cells while minimizing damage to surrounding healthy tissues as nanoscale and microscale robots can be externally delivered to the infection site with the assistance of real-time image guidance (e.g., clinical MRI scanner, magnetic particle imaging scanner 184 ) and subsequent hyperthermia treatment is localized by only focusing on the tumor tissue. Recently, an approach that combined hyperthermia features with the propulsion force of nanoswimmers has been utilized to clear away plaques in a clogged blood artery. The nanorobots consisted of cellulose nanocrystals, Fe 2 O 3 NPs, and Pd NPs. 185 As demonstrated in Figure 8A, the flow of the bloodstream went back to its normal state after the blockage site from animal fat was fully melted and removed. Magnetically induced thermophoresis refers to a self-diffusive motion generated by the local temperature gradient induced by the nano/microrobot itself under an external field. An alternating (AC) magnetic field has been used to heat the spherical Janus robot half-capped with magnetic material (i.e., Fe 19 Ni 81 alloy), giving rise to self-thermophoretic motion 123 as shown in Figure  8B. Besides, the high heating power generated by the magnetic field was also reported to trigger a Fischer−Tropsch synthesis. 186 In this process, the magnetic nanoparticles acted as magnetically induced heterogeneous catalysts.
Magnetic fields can also be used to trigger electric polarization if magnetoelectric materials are incorporated in small-scale motile devices. 187,188 Magnetoelectric materials are single-phase or composite materials, which become electrically polarized when subjected to an external magnetic field. 187,189 To operate at room temperature, magnetoelectric materials are usually made by intimately coupling magnetostrictive and piezoelectric components, although certain single compounds, such as bismuth ferrite (BiFeO 3 ), exhibit magnetoelectric features at room temperature. When a magnetic field is applied to these materials, the magnetostrictive part changes its dimensions. In turn, the magnetostrictive part stresses the piezoelectric part, which subsequently becomes electrically polarized. Magnetoelectric composites can be processed as bilayered or multilayered composite structures, core−shell architectures, or as particulate matrix composite films. 190 Because of their ability to generate electric fields in a wireless fashion (i.e., external magnetic fields), magnetoelectric materials integrated into small-scale robots can serve at least two purposes: (a) magnetic navigation due to the responsiveness of the magnetostrictive component to magnetic fields and (b) application of an electric field to the surrounding environment (i.e., electrolytes, cells, tissues) due to the piezoelectric block. Switching between these two capabilities is managed by changing the conditions in which the magnetic fields are applied, for example, by changing the frequency of an oscillating magnetic field or by swapping between gradients (for motion) and oscillating magnetic fields (for triggering the magnetoelectric effect). The delivery of electric fields is interesting for a wealth of applications, especially in the biomedical domain such as cell electrostimulation and differentiation, 191 electroendocytosis-mediated drug delivery, 192 irreversible electroporation for cancer treatment, 193 cell fusion, 194 or even cell destruction. 195,196 Magnetoelectric nanorobots or microrobots, despite being less investigated, have been utilized for targeted cell manipulation, 197 neuronallike cell differentiation, 13 and targeted drug delivery. 198 For instance, a helical microswimmer, incorporating core−shell magnetoelectric nanoparticles (i.e., CoFe 2 O 4 as the core and BiFeO 3 as the shell) into a hydrogel matrix was able to induce the differentiation of neuronal cells due to the generation of charges upon magnetic stimulation. 13 On-demand drug release for killing cancer cells was demonstrated by FeGa@P(VDF-TrFE) core−shell nanowires upon the application of an AC magnetic field because of the magnetoelectric coupling effect. 198 It is believed that magnetoelectrically induced drug release is caused by the rupture of drug−carrier bonds when the dipole moment triggered by a magnetic field goes beyond the threshold value (i.e., drug−carrier bond strength) and breaks the intrinsic charge distribution on atoms 199,200 as suggested by Khizroev's group ( Figure 8C).

MAGNETIC ROBOTS IN THE MAKING:
FABRICATION APPROACHES

(Quasi-)Spherical MagRobots
Colloidal magnetic particles have attracted scientists' attention not only because of their individual properties but also due to an emergently investigated phenomenon called "swarm" or "collective behavior", 57,201−208 which is a term inspired by many phenomena in nature such as flocking of birds or teamwork behaviors of insects. How to manipulate and actuate a large number of tiny robots with collective behaviors for Chemical Reviews pubs.acs.org/CR Review potential in vivo applications, particularly in complex biological media and in a precisely controllable and programmable fashion, is the ultimate objective of scientists. The selfassembled MagRobots not only are capable of loading or unloading defined cargos on command but also transport them to a defined site (e.g., microfluidic system or biological environment), providing great potential for localized therapy and targeted drug delivery 209 owing to their easy synthesis and versatile multifunctionalities by material design, structure optimization, and surface modification. The collective behavior via colloidal self-assembly presents a rapid, reversible, and programmable bottom-up approach to fabricate MagRobots by employing simple colloidal particles as building blocks. In the presence of a magnetic field, both commercially purchased paramagnetic materials (e.g., μm-sized Dynabeads 134,210 ) and experimentally synthesized magnetic colloidal particles can be self-assembled into desired sizes and shapes (such as carpet, 134 wire, 211 lasso 210 ). Yang et al. 210 recently reported on superparamagnetic PVA-linked colloidal chains by applying a one-dimensional DC magnetic field with a strength of around 20 mT in the vertical direction to a diluted epoxyfunctionalized Dynabeads solution. After the formation of linear chains, a circularly planar rotating magnetic field was operated to transform the chains into a lasso shape. By steering the magnetic field strength and phase lag, lassos can capture cargo through curling behavior and precisely transport it on the ground of a wheel-type mechanism at high velocities. Inspired by ants' cooperative behavior to create a bridge with their bodies when encountering a vanished or nonexistent road ( Figure 9A), Zhang's group used a self-organized magnetic swarm robotic system as building blocks to form a microswitch to repair broken microcircuits. Each component of the system was made of a conductive gold-coated superparamagnetic Fe 3 O 4 nanoparticle. Under a programmed oscillating field, these magnetic nanoparticles can self-reconfigure into a ribbon-like microswarm to act as a conductive bridge between two disconnected electrodes. The patterns and behaviors of the swarming MagRobots depend on the amplitude ratio and input oscillating frequency. Moreover, the elongation of the microswarm is reversible by altering the amplitude ratio. 211,212 By applying an xy-plane rotating magnetic field with a few milli-Tesla (mT), microwheels of superparamagnetic beads can be self-assembled ( Figure 9B). 213 For microwheels lying on a surface, magnetic torque generated by a 2D rotating field can only induce a spinning movement of the micromachines without net displacement. After inputting a 3D oscillating field by adding a varied component vertical to the plane of the rotating field, that is, the microwheels were reoriented until they tilted to a surface, they began to translate with a velocity of around 100 μm s −1 . 213 Inspired by the rolling motion of neutrophiles on the vasculature walls, superparamagnetic beads can accumulate and roll on the surface of confined boundaries using a combination of magnetic and acoustic fields. 66 3D laser lithography is among the most popular techniques used to fabricate small-scale robots with desired architecture. Burr-like spherical porous MagRobots were prepared by using a direct laser writing system followed by depositing Ni thin films for magnetic actuation and Ti thin films for biocompatibility via a sputtering system ( Figure 9C). 214 The fabricated microrobots can carry and deliver targeted cells to a predetermined location in vitro and in vivo under the control of a field gradient. In vitro experiments conducted in a microfluidic chip showed that cell-loaded microbots could be transferred along the blood vessel-like microchannel to a predefined area to release cells (i.e., MC3T3-E1 preosteoblasts). These free cells moved toward the tissue chamber through migration channels. In vivo experiments conducted on nude mice also confirmed that burr-like magnetic microrobots exhibited excellent cell loading, carrying, and release capabilities. In a similar fashion, Jeon et al. used 3D laser lithography and sputtering to fabricate cylindrical, hexahedral, helical, and spherical MagRobots. 121 The use of a magnetic field gradient induced the pulling motion of cylindrical and hexahedral MagRobots, while the rotating field caused corkscrew motion for helical MagRobots and rolling motion for spherical microrobots. 121 Spherical microrobots with Janus structure were fabricated by Martin Pumera's group ( Figure 9D). 209 The Janus structure, formed by half-covering superparamagnetic polymer particles with catalytic Pt layer, can self-propel due to the catalytic decomposition of hydrogen peroxide and can be steered by an external magnetic field. Polymer particles with a tosyl group-rich surface provided the chance to bind anticancer drugs. In addition to drug loading and delivery, the microrobots could also manipulate cells when they assembled into a chain under magnetic guidance.

Helical MagRobots
Helical architectures, inspired by the flagella of bacteria, enable micronanomachines to convert rotational motion to a translational corkscrew motion by using a low-strength magnetic field in low Reynolds number liquids. Various micro-and nanofabrication techniques have been used to prepare helical micro/nanostructures, including templateassisted electrochemical deposition (TAED), 215 laser ablation, 216 direct laser writing and 3D printing, 127,155,217−220 glancing angle deposition, 126,221 coiled flow template, 222,223 biotemplate, 224,225 and origami-based self-scrolling technique. 60,226 Laser micromachining allows the creation of arbitrary 3D structures. Piezoelectric soft MagRobots, which can deliver PC12 cells by employing a rotating magnetic field to induce neuronal differentiation under the stimulus of acoustic waves, were fabricated by Salvador Pane's group. 216 Helical MagRobots consisting of piezoelectric polymer matrix and CoFe 2 O 4 magnetic component were formed by laser ablation of composite film coated on the surface of copper wire by dipcoating method, followed by etching copper wire with acidic ferric nitrate solution ( Figure 10A). Steering of helical parameters such as pitch, pitch angle, and the ratio can be achieved by altering the laser spot size, laser motion speed, and rotating speed of copper wire. The helix microstructure can move in a corkscrew manner along its long axis by a rotating field.
3D/4D printing provides a feasible approach to fabricate soft micro/nanorobots with predesigned shapes. 227−235 Recent reviews give a summary of functional soft robots created by 3D printing 45 and 4D printing 236 technique. 3D-printed enzymatically biodegradable soft helical microswimmers have been designed by Panéand co-workers. 237 Two-photon polymerization (a type of 3D printing technique) was adopted to print photo-cross-linkable gelatin methacryloyl (GelMA) helical microswimmer. To decorate GelMA architecture with Fe 3 O 4 nanoparticles for magnetic actuation, GelMA microstructures were immersed in a water suspension of PVP-coated Fe 3 O 4 nanoparticles ( Figure 10B). Another work about hydrogel-based biodegradable helical microswimmers with length of 20 μm and diameter of 6 μm was reported by Metin Sitti's group. 127 3D printing of double-helical architecture was realized by two-photon polymerization technique from a precursor mixture of GelMA, photoinitiator, and biofunctionalized superparamagnetic Fe 3 O 4 nanoparticles. Such double-helical architecture allows these micromachines to host high therapeutic cargo loading and swimming abilities under a rotating magnetic field.
Although template-assisted electrochemical deposition (TAED) has been widely used to fabricate tubular micromotors, this method can also be employed to generate helical architectures. 238−240 A representative example was demonstrated by fabricating platelet−membrane-cloaked magnetic helical nanomotors in Joseph Wang's group. 215 Pd helical microstructures with a length of 3−5 μm were synthesized by coelectrodepositing a Pd/Cu bilayer on an electrochemical platform using a polycarbonate template and followed by selectively etching the Cu with nitric acid. Afterward, Ni/Au thin films were deposited on the surface of the helical nanostructure via the electron beam evaporation method. To make the gold surface negatively charged, surface modification of the magnetic helical microstructures was carried out by overnight incubation of the microrobots with 3-mercaptopropionic acid. Then, platelet-membrane-derived vesicles were adsorbed, bound, and fused onto the negatively charged gold surface by ultrasonic mixing ( Figure 10C).
Helical MagRobots can also be produced by glancing angle deposition (GLAD). 241−243 In this approach, a seed layer, normally created by spreading a monolayer of silica beads on the substrate, is required to function as the nucleation site. Prior to deposition, the seed layer is fixed at a glancing angle with respect to the input vapor flux of a specific material. During the deposition process, a helical silica structure grows starting from an individual seed particle by continuously rotating the substrate. The pitch and chirality of asymmetric helical structures are changeable by adjusting the speed and direction of rotation. Finally, a layer of magnetic material is deposited in the resulting silica helical tail. While this method can batch-produce uniform helical nanostructures, this process is still limited in terms of material selection and shape. To make the magnetic section (i.e., Ni) of helical microstructure stable in acidic solution, helices were covered with an 8 nm Al 2 O 3 thin film by atomic layer deposition. The stabilized helical micropropellers can be further functionalized with urease ( Figure 10D). 221 Inspired by origami designs, Huang et al. 244 exploited thermoresponsive gel composites reinforced with magnetic nanoparticles to fabricate microswimmers with various 3D architectures by using a one-step photolithography technique and capitalizing on the self-folding of the hydrogel upon hydration ( Figure 10E). During the gel polymerization process, a static uniform field was used to align the encapsulated magnetic nanoparticles. The folding axis direction of the MagRobots was consistent with the alignment direction of the magnetic particles as the swelling was constrained along the reinforcement direction. The produced microswimmers could change their shapes to adapt to local environmental variations in mechanical constraints and osmotic pressure. 244 Hollow helical microstructures can be obtained by first synthesizing magnetic helical microfibers composed of calcium alginate hydrogel and Fe 3 O 4 nanoparticles from coiled flow templates in glass-capillary microfluidic devices, followed by biosilicification and dicing process ( Figure 10F). The produced microswimmer containing inflexible alginate/protamine/silica shell exhibited good mechanical performance for cargo transport. 222 Utilization of bevel-tip capillary and syringe Chemical Reviews pubs.acs.org/CR Review pump, heterogeneous core−shell hydrogel microsprings with calcium alginate hydrogel as shell components and functional materials (e.g., magnetic particles, agarose, cell-suspended collagen) as core components were produced. 245 Because nature provides us with plenty of helical micro-and nanoarchitectures, preliminary attempts to extract the helical xylem vasculature of plants 224 and Spirulina cyanobacterial green−blue microalgae 246−248 as templates to fabricate biohybrid helical micro-and nanomachines open a new insight into strategic designs. The advantage of biohybrid small-scale robots is in the biocompatibility and biodegradability characteristics of the biotemplates. Cell-based helical microswimmers can be acquired from multicellular Spirulina via a single cost-effective dip-coating process in superparamagnetic Fe 3 O 4 solution. 248 Because of the intrinsic properties of microalgae, the prepared microswimmers allowed for in vivo fluorescence imaging without additional fluorescent markers. Moreover, large swarms of microswimmers can be accom-plished inside the rat stomach by an external rotating magnetic field with the assistance of imaging. 248 Model small molecules, as well as biomacromolecules, can be loaded into Spirulina cells by controlling their dehydration and rehydration. 246 The micromachine loaded with molecular cargo can be magnetically driven in an intestinal tract phantom, thus providing the possibility of targeted molecular delivery for gastrointestinal diseases. By modifying their surface with polydopamine via dopamine self-polymerization ( Figure 10G), Spirulina-based magnetic helical microswimmers exhibit an enhanced photoacoustic signal and photothermal effect. 225 In addition to the above-mentioned helical MagRobots, many other helical architectures have been created. 144

Flexible MagRobots
Flexible or soft small-sized robots refer to a nanoscale and microscale robotic system completely or partially comprising soft components or architectures that function as carriers, templates, hinges, joints, actuators, sensors, or reser-   262,263 Third, flexible and soft small-scale robots are more desirable for biomedical applications as these devices are more adaptive in complex biological scenarios, especially in confined, hard-toreach tissues and vessels of the body when compared with swimmers made from rigid and hard parts. Soft robots can be constructed with stimuli-responsive polymer materials that enable shape transformations and the realization of other tasks depending on environmental changes (i.e., pH, 264,265 temperature). For example, PPF/pNIPAM-AAc magnetic microgrippers with pNIPAM-AAc serving as a thermoresponsive swelling hydrogel segment, polypropylene fumarate (PPF) as a nonswellable stiff segment, and Fe 3 O 4 nanoparticles for the magnetic actuation were prepared by serial photolithographic method ( Figure 11A). The thermoresponsive soft self-folding microgrippers could be directed or retrieved to the desired location under the magnetic field to execute their tasks (e.g., to load or release therapeutics) in response to temperature stimulus at around physiological temperature without the need of wires, batteries, or other sources. 39 Similarly, another thermoresponsive soft microrobot was manufactured and employed for pick-up/release applications due to the temperature-sensitive P(OEGMA-DSDMA) layer. 266 Because of the pH-responsive property of 2hydroxyethyl methacrylate (PHEMA), the PHEMA/PEGDA-Fe 3 O 4 bilayer soft microrobot formed via photolithography ( Figure 11B) performed the trapping of drug microbeads at about pH 9.58 by full folding motion and the release of drugs by unfolding motion at about pH 2.6. 265 Biocompatible magnetic "hairbots," derived from functionalized hair ( Figure 11C), can display heightened osteogenic differentiation capacities of mesenchymal stem cells under magnetic actuation compared with nonmagnetic hairbots. Moreover, a magnetic field with repulsion mode endowed stem cells with higher osteogenic activity compared with the attraction equilibrium or nonequilibrium mode. 267 Liquid metals (LM) have also been recently used to create shapemorphing flexible microrobots. An ice-assisted transfer printing method was used to fabricate Fe 3 O 4 NPs-incorporated EGaIn LM micromotors ( Figure 11D). Because ice can be easily removed, this method provides great convenience for transferring LM-based micromotors to arbitrary desired substrates. Irradiation from an alternating magnetic field could cause the dramatic morphological transformation of LM-based micromotors in an aqueous environment. Moreover, the resulting LM-based microswimmer exhibited high propulsion velocity (over 60 μm s −1 ) under an elliptically polarized magnetic field as compared with its rigid counterparts. 268 The utilization of DNA as a flexible component is another method to create soft micro/nanorobots is shown in Figure  11E. Artificial flagella with a length of several micrometers were generated using a self-assembled DNA bundle. 269 After attaching the soft DNA flagella to a magnetic microbead via biotin−streptavidin coupling interaction, a hybrid microrobot was constructed. The fabricated magnetic microrobots can be propelled like peritrichous bacteria under a homogeneous rotating magnetic field. Similarly, Reḿi Dreyfus and coworkers 158 used biotinylated double-stranded DNA as "soft" hinges to link red blood cells decorated with streptavidinmodified superparamagnetic particles. In this way, another type of flexible artificial flagella was prepared via the specific biotin− streptavidin interaction.
Origami as a self-folding process provides a top−down approach to fabricate soft robots with transformable morphologies. A complete origami robotic system normally comprises power, sensing, actuation, and computation subcomponents. 270−273 Readers are suggested to read the review article written by Daniela Rus and Michael T. Tolley to obtain more information about the design, fabrication, and control of origami robots. 274 Self-folding origami MagRobots with various body designs (i.e., tubular body and helical tail, tubular body and spiral tail, helical body and planar tail, etc.) were created by Nelson's group. 60,63 The micro-origami swimmers were endowed with reconfigurable morphologies, controllable mobility, and even programmable magnetic anisotropy by embedding magnetic nanoparticles into selffolding hydrogel bilayers (i.e., one supporting layer and one thermally responsive layer). Because of the programmable shape-morphing feature of the origami-based microrobots, an artificial microsized "bird" was created to mimic the different flying modes of a real bird, including "flapping," "hovering", "turning", and "side-slipping" (Figure 11F). 64

Wire-like MagRobots
Most rod-like MagRobots are fabricated by template-assisted electrochemical deposition (TAED). 275−279 In general, anodic aluminum oxide (AAO) or polycarbonate porous membranes are employed as templates. These membranes are commercially available and are usually composed of cylindrical pores, although sophisticated designs and complicated fabrication of porous membranes with different pore geometries or with variable pore diameter can be realized. 280,281 Because of the nonconductive nature of these templates, prior to the electrodeposition of material, a layer of a conductive thin film (usually gold) is deposited on one side of the membranes by electron beam evaporation or other physical vapor deposition methods. The length of the nanostructures (i.e., nanorods, nanowires) is adjustable by regulating the electrodeposition time. After deposition, metal-based nanowires are released by dissolving the membrane template. Usually, ferroand ferrimagnetic nanowires and nanorods align with their long axis parallel with the direction of the applied magnetic fields. Two main strategies exist to align cylindrical magnetic nanostructures perpendicular to their long axis: (a) by placing segments of magnetic material sufficiently separated along a nonmagnetic structure (in order to minimize dipolar interactions) and (b) premagnetizing the nanowires/nanorods along their short axis. The first case can be achieved by synthesizing multisegmented nanowires/nanorods using pulsed plating electrodeposition or sequential deposition by alternating different electrolytes. 282,283 In the second approach, a nanowire/nanorod has to be made from hard-magnetic materials so that it can preserve a sufficiently large remanence after being premagnetizing in a specific direction. Figure 12A shows the fabrication of electrodeposited hard-magnetic CoPt nanowires and the procedure for their premagnetization along their short axis. 165 Figure 12B shows a comparison between a soft-magnetic CoNi and a hard-magnetic CoPt nanowire and their alignment upon the application of a magnetic field. While the premagnetized hard-magnetic nanowire aligns with its short axis to the applied field, the soft-magnetic is aligned along its long axis. In a rotational magnetic field, a nanowire/ nanorod that aligns with its long axis with the applied magnetic field can only exhibit a tumbling motion. 284 However, a nanowire-like MagRobot that is premagnetized along its short axis can display a richer variety of motion mechanisms such as tumbling, rolling, precession, or wobbling locomotion as a function of the magnetic field frequency. Another strategy to possess multiple motion modes is to integrate premagnetized nanowires into nonmagnetic structures. For instance, a single Ni nanowire only shows a sole tumbling motion. 284 After assembling two polystyrene beads into a Ni nanowire to construct a dumbbell-like MagRobot, the fabricated microstructure possesses three motion modes (i.e., rolling, wobbling, and tumbling) ( Figure 12C). 285 When adding flexible segments such as hinges or tails to nanowires, the assembled MagRobots display traveling-wave motion under the steering of an oscillating magnetic field. A multiple section microstructure of Au−Ag−Ni−Ag−Ni−Ag− Au, using three elastic Ag nanowires as hinges and fabricated by sequential electrochemical deposition, can mimic the swimming of a fish with a speed as high as 30 μm s −1 ( Figure  12D). 128 In a similar fashion, the two arms of a Ni−Ag−Au− Ag−Ni MagRobot are capable of executing an out-of-phase wobbling motion by a planar 2D oscillating field and propel the movement of the body with a velocity of around 30 μm s −1 ( Figure 12E). 3 A Ni-hinge-Ni-hinge-Ppy nanorobot involving a flexible polypyrrole (Ppy) tail has the ability to break the reciprocal motion at the temporal dimension, exhibiting an Slike motion mode by making use of its eukaryote-like tail with the assistance of an oscillating field, leading to maximum propulsion speed of 0.93 body-lengths s −1 ( Figure 12F). 97 Chemical Reviews pubs.acs.org/CR Review Inspired by the electric field, a knifefish, which can produce electricity through its electrocytes, was developed as a multifunctional Ni-Ppy-PVDF MagRobot containing a soft polyvinylidene fluoride (PVDF) tail. Taking advantage of the intrinsic piezoelectric performance of the PVDF tail, the surface of the fabricated MagRobots exhibits an enhanced release of cargo owing to the electrostatic repulsion generated by the magnetically induced piezoelectric effect. By changing the magnitude and rotational frequencies of the applied rotating magnetic field, three different locomotion modes (i.e., tumbling, wobbling, and corkscrew-like motion) with different translation speeds and drug release behaviors were observed ( Figure 12G). Interestingly, the application of an on−off magnetic field can actuate the release of drugs in a pulsatile approach. 286

Biohybrid MagRobots
Because of their excellent biocompatibility and extremely low toxicity, biohybrid mineralized motors, which often integrate synthetic nanostructures/nanoparticles with natural nonmobile cells (e.g., pollen, spores) or motile cells (e.g., bacteria, sperm), are currently of great interest. 135,287 Four methods are commonly used to produce biohybrid micro/nanorobots. The first method consists of directly using nonmotile cells as templates and then integrating magnetic nanomaterials and other functional building blocks such as inorganic nanostructures or molecules. Capitalizing on this approach, several pollen-based, 288−290 spore-based, 291 microalgae-based, 292,293 sperm-based 294 magnetic micromotors have been fabricated. In general, pollen and spores have the merits of excellent biocompatibility characteristics and structural uniformity. Some even have unique architecture (e.g., hollow cavity), which can facilitate specific applications. For instance, researchers have loaded drugs into two hollow air sacs of pine pollen grains via vacuum loading technique ( Figure 13A). The experiments demonstrated that pollen-based biohybrid MagRobots not only exhibit efficient drug-encapsulation ability but also can release them on demand. 288 By altering the Chemical Reviews pubs.acs.org/CR Review vectors of programmatically controllable magnetic fields, individual pollen-based micromotors with encapsulated magnetic Fe 3 O 4 inside present three distinct modes of locomotion (i.e., rolling, tumbling, and spinning) and these individuals were able to form a dynamic collective phenomenon under the steering of an external magnetic field. 288 Spore-based microrobots composed of G. lucidum spores, Fe 3 O 4 nanoparticles, and functionalized carbon nanodots have been synthesized via rapid, direct, and low-cost methods ( Figure 13B). The prepared spore@Fe 3 O 4 @CDs microrobots can detect bacterial toxins. 295 As mentioned above, Spirulina, with the innate spiral morphology, has been utilized as a biological template to create helical microswimmers 248,292 ( Figure 13C). Sperm-based soft MagRobots were fabricated by decorating Fe 2 O 3 nanoparticles on the surface of immobile sperm cells via the electrostatic selfassembly ( Figure 13D). The highest swimming speed of sperm-templated micromotors can reach 6.8 ± 4.1 μm s −1 (0.2 body length/s). 294 The second method of preparing biohybrid micromotors is to cloak functionalized synthetic nanomaterials with cell membranes. This method can enhance the biocompatibility of micromotors to the largest extent and avoids recognition by the immune system. Recently, cell membranes/vesicles from red blood cells (RBCs) 296 (Figure 13E), platelets 215 ( Figure  13F), and even dual cells (e.g., RBCs and platelets 297 ) were utilized as camouflage to cover the surface of functionalized synthetic nanomaterials. The magnetic nanoparticles embedded into these biohybrid nanomachines play a role in magnetic guidance. The locomotion of these cell-based biohybrids can be powered by a magnetic field or other driving forces. For example, the random movement pattern of a Janus RBC-Mg motor can be driven by hydrogen bubbles generated by the reaction of Mg and water. The addition of Fe 3 O 4 nanoparticles to the Janus micromotors can make the miniaturized machines move precisely along a predetermined path. 296 The third method to fabricate hybrid small-scale swimmers consists of combining active locomotive cells that are born with flagella, among which sperm and bacteria are widely used. 31,68,298−301 In this method, the motile cell either adheres to the surface of a synthetic particle (normally in the micrometer scale) or another cell or be trapped into a special microstructure. For example, bacteria-driven microswimmers were fabricated by attaching a single E. coli. bacterium to a drug-loaded polyelectrolyte microparticle via viscoelastic connection of the bacteria−particle interface ( Figure 13G). The E. coli-powered motor exhibited the chemotaxis behavior under a chemical concentration gradient. Fe 3 O 4 nanoparticles embedded within the polyelectrolyte microparticles functioned as a steering wheel, thus providing the biohybrid motors with directional control over the directionality and enabling guidance of the drug-loaded swimmers to target breast cancer cells in vitro. 68 Similarly, the magnetic guidance was also employed in bacterium-RBC micromotors, which were fabricated through the strong conjugation chemistry between the erythrocyte and E. coli bacterium ( Figure 13H). In addition, negatively charged microalgae with ellipsoidal morphologies (i.e., Chlamydomonas reinhardtii algal) were integrated with positively charged polyelectrolyte-functionalized magnetic microsphere via electrostatic interactions ( Figure 13I). The motile microalgae function as an actuator while the microparticle can be used for cargo encapsulation and magnetic steering. 293 In addition, various customized magnetic microstructures (such as tetrapod, 31 microtube, 298 and helix 247 ) have been prepared to capture the task-carrying spermatozoa to form sperm-hybrid microrobots (known as "spermbots"). Sperm cells with high vitality serve as a motile component of hybrid microrobots to complete specific tasks, for example, targeted drug delivery, 31 as shown in Figure 13J. However, they can also act as carriers when they have motility deficiencies. In such cases, the remotely controlled assisted fertilization relies on the synthetic magnetic microstructures of spermbots under the guidance of external magnetic fields. 302 The fourth approach consists of adopting a live immune cell to engulf the whole magnetic passive functional materials by taking advantage of the phagocytosis processes of immune cells. 303 As a consequence, biohybrid "immunobots", 304 as termed by Metin Sitti's group, can be formed. After a magnetic double-helical microswimmer was completely internalized by a macrophage, the biohybrid macrophage-based MagRobots were able to perform magnetically driven rolling locomotion along predetermined trajectories by steering the magnetic helical component. The robots were able to swim uninterruptedly even with the presence of cells blocking their pathway. In the absence of a magnetic field, the immunobots could autonomously move by crawling and actuated by the self-propelled movement of the macrophages in a biological environment ( Figure 13K). 304

Targeted Drug/Gene Delivery
The precise and efficient transportation of therapeutic payloads to target sites, especially to those confined and hard-to-reach locations of the body, is challenging for passive drug delivery systems. The past decade has witnessed a boom in the development of active smart drug delivery systems using external field-driven miniaturized micro-and nanomotors. Particularly, magnetically driven micro and nanorobots offer several advantages as small agents for targeted cargo delivery including but not limited to remote, precise, and minimally invasive maneuverability, and potential recyclability of residual administered drug-carriers, which often results in serious side effects to healthy organs and tissues. 306−309 In most cases, very low field strength (in the mT range) is sufficient for the actuation of MagRobots without causing damage to healthy cells.
Before the steerable delivery of cargos (e.g., molecules, drugs, genes), the cargo loading or capture process is needed. The loading of cargos is often conducted by encapsulating them inside the MagRobot structure or by attaching them to the MagRobot surface. The encapsulation process can be directly carried out during MagRobot fabrication while the surface attachment (or adhesion) process can be made using superficial functional groups of biohybrid or synthetic MagRobots. Various organic or inorganic artificial nanomaterials (e.g., Au/Ni/Si nanospears, 310 hydrogel-based helical microswimmers, 127 Janus Au/Ni/SiO 2 microparticles, 311 etc.) and biogenic materials (such as pollen grains, 288 sperm cells, 176 bacteria, 35,305,312 erythrocytes, 313 and microalgae 246,293 ) have been developed as functional or structural carriers to encapsulate or carry molecules, drugs, genes, or cells. For example, Fe-coated biotubes, which exhibit a drill-like motion under high-angular frequency magnetic fields, were capable of transporting camptothecin (i.e., an anticancer model drug) and delivering it to specific sites, killing the targeted HeLa cells in vitro ( Figure 14A). 314 Chemical Reviews pubs.acs.org/CR Review Considering the complexity of the human body's environments, it is key to investigate the propulsion mechanisms of MagRobots and strategies for cargo delivery and release under complicated physiological conditions in different body fluids such as gastric juice, saliva, and blood. Recently, a cell-sized Janus micromotor loaded with antibodies as receptors for the recognition of target cells and anticancer drugs was able to navigate in a simulated blood circulation system ( Figure  14B). 311 Although the propulsion of MagRobots was weakened under dynamic flow conditions, the ability of active upstream locomotion in the bloodstream was confirmed in flat and 3D surfaces. Furthermore, the utilization of biohybrid micromotors combining sperm cells and synthetic magnetic micro and nanoarchitectures to deliver anticoagulant agents (i.e., heparin) in the bloodstream was reported ( Figure  14C), 176 which is promising for treating diseases of the circulatory system such as thrombotic clots. In addition to drugs, targeted transport of genes (e.g., plasmid DNA) to a single cell and subsequent transfection was achieved by the utilization of helical micromotors under the actuation and navigation of low-strength rotating magnetic fields ( Figure  14D). 218 Recently, Peer Fischer's group reported targeted transfection and gene delivery by using biocompatible FePt nanopropellers under rotating millitesla fields. 315 After delivering payloads to a specific location, cargo molecules can be released naturally via diffusion or via specific stimuli (such as pH, 265 temperature, 266 light irradiation, 67 or chemical changes at the disease site) according to the practical application requirement. For example, because the concentration of matrix metalloproteinase-2 (MMP-2) enzyme at the tumor site is higher than that at normal physiological conditions, hydrogel-based helical microswimmers demon- Chemical Reviews pubs.acs.org/CR Review strate a quicker response to the evaluated concentration of MMP-2 enzyme, resulting in a boost-release of embedded cargo (i.e., antibody-tagged Fe 3 O 4 nanoparticles) through the swell behavior of the hydrogel. 127 The released antibodytagged payloads from the micromotors can be further used for active labeling of targeted tumor cells ( Figure 14E).

Cell Manipulation
Cell manipulation is the practice of maneuvering the physical position of cells to separate them from the milieu of other phenotypically different cells (i.e., cell-based screen), guiding them into a specific target position (e.g., for fertilization), or organizing themselves in vitro. With the rapid advance of proteomics and genomics, it is of great significance to develop sophisticated tools for single-cell manipulation, especially massively parallel single-cell manipulation. 316 Magnetically powered miniaturized robots are capable of 3D manipulation of a single cell in terms of capture, transport, sorting, isolation, and pattering, with excellent maneuverability and high precision at the nano-and microscale in complex physiological environments without changing the intrinsic properties of the cells. 317,318 For instance, trapping of breast cancer cells was reported by tosyl-functionalized superparamagnetic microbeads due to the instantaneous strong binding between the tosyl groups from the surface of microswimmers and the −NH 2 groups from the membrane proteins of cancer cells. Manipulation of single or multiple cell-laden microrobots was achieved by the propulsion of oxygen bubbles and manual direction guidance using a neodymium magnet (Figure Chemical Reviews pubs.acs.org/CR Review 15A). 209 Arranging cells to achieve predetermined patterns with the assistance of an arrayed substrate was implemented through single-cell pick-up and subsequent delivery using magnetically propelled peanut-like micromotors ( Figure  15B). 169 To aid sperm cells with defective locomotion features to complete their fertilization task, Oliver G. Schmidt's group designed several motile nano/micromotors as assisted tools 302 such as magnetic microcarriers with a cylindrical cavity and a helical body 319 and a magnetic helix 247 ( Figure 15C). Moreover, magnetically driven micromotors provide an invasive way to transfer zygotes through the uterus and fallopian tube ( Figure 15D), and magnetic microrobots with spiral shapes exhibit higher maneuverability in terms of capture and transfer of the zygotes between different physiological environments than those with helical shapes. 320 Transportation of neural progenitor cells was conducted by the corkscrew-like motion of magnetically powered soft microswimmers containing piezoelectric polymer and CoFe 2 O 4 magnetic nanoparticles under a rotating magnetic field. Subsequent neuronal differentiation of PC12 cells was induced by the acoustic stimulation due to the utilization of piezoelectric polymer as a stimuliresponsive cell electrostimulation platform ( Figure 15E). 216 Furthermore, Kim et al. 321 precisely manipulated a neuronloaded magnetic microrobot to a gap between two neural clusters to connect broken neural networks. Recently, successful trials of magnetically powering microrobots toward a target site (such as a liver tumor micro-organ, ventricle of mouse brain, blood vessel of rat brain, and live mouse) using in vitro, ex vivo, and in vivo experimental models, indicate the Chemical Reviews pubs.acs.org/CR Review feasibility of adopting MagRobots for the purpose of targeted stem cell transport and transplantation ( Figure 15F). 121

Minimally Invasive Surgery
Miniaturized machines that are capable of precisely opening specific cell membranes to kill abnormal cells and even achieve intracellular delivery of various drugs (including DNA) are promising candidates for noninvasive surgery. 322,323 Nano/ microrobots that project sharp tips or have the ability to perform a corkscrew-like movement can execute drilling under the application of a rotating magnetic field. The drilling feature can be harnessed to penetrate tissue with high precision, holding great promise to perform untethered microsurgeries. As shown in Figure 16A, microdrillers (tubular Ti/Cr/Fe microdrillers with sharp tips) were able to penetrate into a section of porcine liver tissue via magnetically driven mechanical drilling. To make the microdriller "stand up" to drill, a specific angular frequency threshold of the rotating field (in correlation with the viscosity of media) is required to transform the horizontal rotation mode into a vertical rotation mode. 41 Other representative microdrillers are Fe-coated calcified biotubes containing pointed ends, which are extracted from Dracaenea marginata leafs. Upon magnetic actuation, the microdagger stabbed into the cellular membranes of HeLa cells with a drill-like motion, finally resulting in cell death. In addition, the ability to drill into a target cell can be utilized for subsequent drug delivery because the porous structures of calcified biotubes endow the microdriller with the capacity of drug loading. 314 A millimeter-sized magnetic driller can be navigated in a 3D vascular channel and perforate a blood clot in a simulated thrombosis model environment, providing an application potential for cardiovascular disorders ( Figure 16B). 324 Besides, surface walkers also can open the cell membrane. Recently, we developed Au/Ag/Ni microwires that display walking movement under a transversal rotating magnetic field. Because of the rigidness of the microwires, they can only perform a drilling movement. To make the structure of microwires slightly bent, an Ag segment was partly etched by concentrated H 2 O 2 solution. As a consequence, a surface tumbling motion can be achieved. The surface walkers, functioning as microscalpels, can penetrate cancer cells, capture a piece of the cytosol, and exit the cells while leaving the cytoplasmic membrane intact, thus demonstrating excellent minimally invasive microsurgery capabilities 172 ( Figure 16C). Au/Ni/Si nanospears functionalized with plasmid were able to penetrate U87 glioblastoma cells by means of rotating magnets, and deliver the gene (i.e., eGFP expression-plasmid) within the cells over large areas ( Figure 16D). Such intracellular cargo delivery in a high-throughput manner paves the way for translation to new clinical cellular therapies. 97,310 Realistic biological environments are substantially complex. The microscopic propulsion of micro/nanorobots in biofluid environments (e.g., bloodstream, 24,250,325−327 saliva, 328 semen, 329 mucus, 221 vitreous humor, 126,330−332 brain vasculature, 333 cerebrospinal fluid in the spine or brain, urinary fluid, gastrointestinal fluid, 334,335 etc.) is different from that in Newtonian fluid. Physicochemical and histological barriers (e.g., cell membrane, 322 blood−brain barrier, 336 intestinal mucosal barrier), interactions with boundaries, crowded biological environments, complex rheology (e.g., viscoelasticity, shear-thinning), and other factors impact the locomotion behaviors and application performance of micro/nanorobots in biological environments. 337−340 Attempts have been made to exploit the actuation of MagRobots in complex biofluids. For example, to overcome the mucus barrier, Peer Fischer's group 221 developed a helical microdriller surface-functionalized with urease as shown in Figure 16E. Such microdrillers can penetrate the viscoelastic mucin gel in an acidic environment in the presence of urea and swim freely inside under a rotating magnetic field. This idea is inspired by Helicobacter pylori bacteria, which are capable of decreasing the viscosity of mucin gel via a gel−sol transition caused by the release of ammonia through an enzyme-catalyzed procedure that raises the local pH. To move further toward clinical application, the same group created magnetic helical micropropellers that were able to penetrate the biopolymeric network of porcine vitreous humor and swim inside over a centimeter distance under navigation by a rotating magnetic field and using clinical optical coherence tomography as shown in Figure 16F. 126 The smooth propulsion of the micropropellers in the dense biopolymeric network lies in the slippery liquid layer on the surface of micropropeller, which minimizes the adhesion force to the surrounding environment. More mechanisms, actuation approaches, and applications of micro/nanorobots in complex biofluids that resemble real-world scenarios are required to be explored.

Biopsy
MagRobots have been proved to be wireless biopsy tools to capture a single cell or collect tissue samples from healthy or diseased organs, including breast, lung, liver, skin, prostate, and so forth, with high specificity and selectivity for further disease diagnosis. These functional magnetic miniaturized robots, normally in the microscale, are called microgrippers. To have the ability to pick up an object and lay it down, analogous to the function of human hands, most of the magnetically driven microgrippers 40,341−343 explored to date are flexible (see Section 4.3). Thermoresponsive flexible MagRobots have been widely used as grippers due to their temperature-induced opening and closing capacities 39,344−346 ( Figure 17A). For instance, a thermoresponsive magnetic microrobot, having a tip-to-tip size of 70 μm in its open state and 15 μm in its folding state, was able to conduct single-cell biopsy ( Figure  17B). The thermally responsive layer of the microgripper is made from paraffin wax, whose phase-transition temperature is in close proximity to biological temperatures, including humans. After being navigated to the position of a fibroblast cluster, the untethered microgripper grasped one cell or a few cells when it transformed from open to closed state with the increase of field temperature. Cell separation from the cluster and retrieval of the microrobot can be easily fulfilled by adjusting the direction of magnetic field. Metin Sitti's group 342 utilized hundreds of thermosensitive microgrippers that had been pre-encapsulated in the chamber of a centimeter-scaled magnetically actuated capsule endoscope (MASCE), to grab stochastically tissue inside the stomach ex vivo for further analysis. Retrieval of distributed magnetic microgrippers was conducted by strong wet-adhesive force from the retrieval unit of MASCE. This multiscale robotic system provides a novel multiagent collaboration strategy not only for gastrointestinal capsule biopsy but also for other biopsy tasks in complex physiological structures and environments. An in vivo tissue excision of the porcine biliary tree was conducted using thermal-induced self-folding microgrippers as shown in Figure  17C. More than 1000 microgrippers were delivered to the position of interest (i.e., the biliary orifice) through a standard catheter with the assistance of the endoscopic camera. The thermosensitive magnetic microrobots, initially in the open state, spontaneously transformed into closed state in order to excise tissue samples when they are exposed to body temperature (37°C) for 10 min. Retrieval was carried out by using a catheter containing a magnetic tip. Subsequent PCR (polymerase chain reaction) results indicated that the excised tissue piece was sufficient for genetic or epigenetic diagnosis in terms of quantity and quality.

Biofilm Disruption/Eradication
Different from planktonic (free-swimming) bacterial cells, the interaction of cell masses (i.e., community of microorganisms) produces a matrix called "extracellular polymeric substances" (EPS). 348 The embedded cells and the viscoelastic matrix that constitute the biofilm on the surface of a subject are notoriously difficult to eliminate. 349 The nature of bacterial biofilms' resistance to antimicrobial agents makes them a source of some recalcitrant infections. Magnetically powered nano/microrobots manifest themselves in the competence to penetrate into the matrix and disrupt the biofilm formation or eradicate already-formed biofilm due to their small size as well as high magnetically driven mechanical force. A biohybrid microrobot based on nonpathogenic magnetotactic bacteria has been used to penetrate into the island of Escherichia coli by the external actuation of magnetic field 350 as shown in Figure  18A. Although this invasion can temporarily cause the elastic formation of the biofilm, the microrobot was almost trapped in it, presenting restrained movement ability. How to make nano/ microrobots swim in a viscous media is a common challenge. A magnetic microrobot made from tea buds, called "T-Budbots", was able to precisely fragment and remove bacteria biofilm. 351 As demonstrated in Figure 18B, T-Budbots left a clear trail on the surface of P. aeruginosa biofilm after their movements, indicating that the biofilm had been effectively swept away. Moreover, antibiotic encapsulated in T-Budbots of the biofilm exhibited a pH-triggered release behavior around the acidic microenvironment of the biofilm. Once the biofilm was disrupted, the dislodged bacterial cells were exposed to the drugs and finally killed. One of the most outstanding advantages of using MagRobots to execute the task of biofilm elimination lies in their function to be directed to a confined and hard-to-access position. A recent study demonstrated that magneto-catalytic iron oxide nanorobots (called "CARs") are capable of the degradation and removal of biofilms in the isthmus of human teeth due to the catalytically induced generation of reactive antibiofilm molecules and the external shear forces from magnetic actuation ( Figure 18C). 352

Imaging-Guided Delivery/Therapy/Surgery
To translate medical micro/nanorobots from the bench to the bedside, imaging technologies are of vital importance to achieve real-time tracking of the MagRobots in vivo. 201,353−360 Clinically established imaging modalities, including but not limited to optical imaging, magnetic resonance imaging (MRI), 53,197,325,361−363 374 and their combined imaging techniques (e.g., MR/CT, 375 PET/ CT, 376 PET/MRI 377 ) can be integrated into miniaturized robotics systems. Although many challenges remain, many researchers have attempted to use these imaging techniques as powerful tools to assist the tracking of MagRobots for sitespecific drug delivery, targeted therapy, and precision surgery.
Because of limited penetration depth of biological tissues, optical imaging is not suitable for the visualization of MagRobots across tissues in vivo. For magnetically driven micro/nanorobots, MRI is an efficient tool to track the position of MagRobots both in vitro and in vivo. 356  As one representative example shown in Figure 19A, in vivo MRI tracking of a swarm of microalgae-based helical microrobot inside the subcutaneous tissues of a rodent stomach was reported by Zhang's group. 248 Felfoul and coworkers 378 reported real-time positioning and tracking of a microrobots magnetically propelled by MRI gradients in the carotid artery of a pig in a closed-loop control scheme. MPI, first proposed by Bernhard Gleich and Jurgen Weizenecker, 379 is a three-dimensional tomographic imaging method. The MPI scanner comprises two permanent magnets in a Maxwell configuration. Larger field gradients in the MPI scanner workspace provide a strong propulsion force to drive magnetic objects. 380−382 However, in terms of the spatial resolution of MPI (a few millimeters) in the current platform,   Figure 19B).
Fluorescence imaging, with the advantages of excellent planar resolution (≈ 100 nm) and high sensitivity, has become another widely used medical imaging modality. Under the guidance of fluorescence imaging, the utilization of spore-based magnetic microrobots functionalized with carbon quantum dots for effective targeted delivery was demonstrated by Zhang's group. 383 They designed an automated control system that can help microrobots avoid obstacles and find the optimal path based on a particle swarm optimization algorithm with the assistance of vision feedback. 383,384 However, fluorescent Chemical Reviews pubs.acs.org/CR Review probes (e.g., organic dyes, 385 quantum dots, 386 metal−organic frameworks, 387,388 etc.), which usually have poor biocompatibility and biodegradability, are required to label the micro/ nanorobotic materials or cells. Because of the intrinsic fluorescence feature, excellent biocompatibility, and biodegradable performance of Spirulina microalgae, microalgaebased magnetic microrobots allow for in vivo fluorescent imaging without the use of probes and concern for biosafety ( Figure 19C). 248 Ultrasound imaging, as a conventional clinical imaging technique, mainly has two different modalities, namely, Bmode and Doppler. 389,390 The former is based on pulse−echo technique while the latter relies on the Doppler effect. The main advantages of US imaging lie in high spatial and temporal resolution, large penetration depth, minimal damage to tissues, and relatively lower setup cost. A magnetically driven microrobot swarm was visualized and tracked in a bovine eyeball via US imaging 124 as shown in Figure 19D. Sitti's group used the color Doppler mode of US imaging to track the "hairbots" in ex vivo chicken breast. 267 Recently, Zhang's group adopted US Doppler for real-time guidance of a swarm of magnetic microrobots for endovascular delivery. 369 Photoacoustic imaging, first proposed by Alexander Graham Bell 391 in 1881, is a "light-in, sound-out" approach. A light source (i.e., IR laser) and US transducer are two fundamental elements for a PA imaging setup. Utilization of PA imaging to track microalgae-based magnetic microswimmers for killing pathogenic bacterial was reported. 225 A more advanced PA imaging technique, multispectral optoacoustic tomography, was adopted for real-time monitoring of the migration of single magnetically driven conical micromotors with the length of 100 μm in phantom as well as ex vivo chicken tissue 392 as show in Figure 19E.
X-ray-CT, PET, and SPECT belong to the category of ionizing radiation-based techniques that employ high-frequency radiation with wavelength ranging 10−100 nm. As a consequence, these techniques endow high penetration depth and spatial resolution, but the harm radiation does to living (human) tissues must be taken into consideration. In comparison with the widely used X-ray CT technique used in clinics, PET and SPECT techniques based on γ-rays have been developed in the last decades. Although the two state-ofthe-art imaging techniques exhibit excellent spatial resolution and molecular selectivity, the utilization of PET and SPECT (usually in conjunction with CT imaging) for the localization and tracking of MagRobots is still in its infancy. For both techniques, interested materials or micro/nanorobots are often conjugated with radiotracers (such as 64 Cu, 124 I, 18 F, 68 Ga, 99m Tc, etc.). 373,377,393−396 SPECT imaging for individual microrobots with diameter as low as 100 μm was reported by Nelson's group 373 as shown in Figure 19F. To track the shape transition (e.g., from tubular to planar configuration) of microrobots, they used 99m Tc [Tc]-based radioactive compounds to label the magnetically driven thermoresponsive hydrogel-based microrobots. More research is expected to explore the combination between biomedical imaging techniques and locomotive micro/nanorobots, and aimed at targeting individual MagRobots or a swarm of MagRobots to a specific location with high temporal and spatial precision, and executing certain diagnostic or therapeutic tasks in an invasive and visualizable fashion. Because of the restriction of small size, clear observation of a single miniaturized robotic in the nanoscale and microscale using current biomedical imaging techniques is still a big challenge.

Pollution Removal for Environmental Remediation
In addition to the biofriendliness, recoverability of magnetically driven micro-and nanorobots, and the toxin-free nature of magnetic manipulation, MagRobots can also actively swim around waterborne pollutants (e.g., dyes, oil, heavy metals, 291 microplastics, microbial pathogens, estrogenic, 397,398 etc.) and remove them by capture (adsorption/absorption) or degradation. As such, small-scale MagRobots constitute a technology with great potential for water remediation. In the future, sophisticated magnetic manipulation systems could be used to externally guide MagRobots to pollution sites (i.e., canalizations, industrial reactors, tanks, pools) in a contactless fashion. Additionally, magnetic fields can be used to accelerate reaction kinetics or recognition efficiency due to the robust dynamic intermixing (i.e., magnetic stirring function) and to retrieve the nano/microrobots once the cleaning procedure has been finalized. 399 Eventually, the cleaning agents can be reused or recycled if their constituent components have remained unaltered. The treatment of six representative pollutants using miniaturized magnetic motors is summarized in Figure 20.
The autonomous movement of a walnut-like microrobot composed of polycaprolactone, Fe 3 O 4 nanoparticles, and catalase in H 2 O 2 -included solution is ascribed to the oxygen bubbles from the enzyme-catalytic degradation of H 2 O 2 , exhibiting a spiral trajectory. 400 The direction of the microswimmers could be controlled using external magnetic fields. Because of the hydrophobic nature, the motile walnut-like micromotor was capable of collecting spilled oil ( Figure 20A). Because of the incorporation of Fe 3 O 4 component, the recycling of the micromotor was realized by using a magnetic field. 400 A magnetic hollow microsubmarine, using natural sunflower pollen grains as a template, was reported to remove leaked oil and microplastics pollutants simultaneously ( Figure  20B). 401 High removal efficiency of heavy metal ions was found in porous biohybrid microrobots consisting of fungi spore and Fe 3 O 4 nanoparticles. The collective behaviors of the microrobots and magnetically steered agitation could further enhance the pollutant adsorption ability compared with static microrobots ( Figure 20C). 291 The excellent antibacterial ability of Pd/Ni/Ag nanocoils and high magnetic maneuverability at low magnetic strength (8 mT; 10 Hz) allows for precise locomotion of nanorobots toward the target location of bacterial infection to efficiently fight against the drug-resistant bacteria ( Figure 20D). 402 The dual actuation of micromotors prepared from carbon soot by using a magnetic field and oxygen microbubbles facilitated efficient on-the-fly degradation of MB dye pollution 403 ( Figure 20E). In addition, the use of functional magnetic micromotors for the absorption or removal of antibiotics, such as erythromycin ( Figure 20F) 289 and doxycycline 404 in contaminated water, has also been investigated.

Sensing and Biosensing
According to sensing mechanisms, there are three main purposes of using magnetically driven micro/nanomotors for sensing and biosensing. First, because the motion behaviors (e.g., velocity, wobbling angle) of MagRobots is related to an applied external magnetic field as well as properties (e.g., temperature, pH, viscosity, ionic strength) of the solution, the detection of these movement parameters of MagRobots provides a novel approach to probing the local microenviron-Chemical Reviews pubs.acs.org/CR Review ment in a heterogeneous medium. 405 For instance, a helical nanomotor was developed as a mobile viscometer capable of monitoring in real-time the surrounding viscosity in homogeneous or heterogeneous media. A mathematical model was developed that establishes a relation between viscosity and the precession angle of the swimmer. High temporal and spatial precision of the viscometer was confirmed by gradually measuring the viscosity of deionized water from the hot state (70°C) to its cool-down state (30°C) and mapping the local viscosity from a reference fluid (e.g., deionized water) to another fluid (e.g., glycerol−water 4:1 v/v) in a microfluidic chamber under the application of homogeneous rotating magnetic fields ( Figure 21A). 157 Second, externally maneuvered MagRobots can act as signal amplifiers and, therefore, provide enhanced detection sensitivity and efficiency for identifying the signals (e.g., fluorescence) triggered by target molecules due to the active stirring and vigorous mass transfer in the solution. 406 Janus micromotors, which contain phenylboronic acid-modified graphene quantum dots, iron oxide nanoparticles, and Pt nanoparticles, were used to detect the bacterial endotoxin in contaminated water. The reaction between graphene quantum dots and the targeted endotoxin results in the fluorescence quenching of the dots while phenylboronic acid tags serve as specific recognition receptors of the endotoxin. Compared with that in the static conditions, the micromotors actuated by external magnetic fields or those autonomously propelled by oxygen bubbles displayed faster fluorescence quenching than those that remained static due to elevated fluid intermixing ( Figure 21B). 407 Similarly, mobile magnetic spore@Fe 3 O 4 @CDs microrobots can remotely detect C. dif f toxins with much more obvious fluorenes quenching in a noninvasive way through the targeting combination of C. dif f toxins and CDs (carbon quantum dots) in comparison with nonactuated microrobots. 295 Third, MagRobots can function as a navigator, precisely guiding payloads (especially biomolecules for the diagnostic purpose) to a user-defined site for chemical/biological interactions or other purposes in an untethered way. Janus magnetic microrobots were capable of loading biotin-functionalized commercially purchased microbeads and transporting them to a specific region under the steering of a uniform electric field and rotating magnet. The dynamic binding between the surface-immobilized probe (i.e., biotin) and the target analyte (i.e., avidin) provides a label-free method for biosensing. The experimental detection limit in a single microfluidic chamber can be as low as 2 μg/mL ( Figure 21C). 408

CONCLUSION AND FUTURE PERSPECTIVES
The last decades have witnessed great advances and breakthroughs in MagRobots, including innovative manufacturing approaches, reconfigurable and programmable navigation techniques, advanced theoretical models, impressive proofs of concept, and clinically oriented application trials. This review introduces basic knowledge of magnetic fields and magnetic materials, offers the experimental setups of magnetic manipulation systems and various field configurations, and proposes the strategies to generate nonreciprocal movement. The movement mechanisms of flagella-inspired helical motion, undulatory motion, and boundary-assisted motion also are presented. Fabrication techniques of (quasi-)spherical, helical, Chemical Reviews pubs.acs.org/CR Review flexible, wire-like, and biohybrid MagRobots are summarized, followed by various state-of-the-art applications in the field of biomedicine and environment. The considerable application potential of micro/nanorobots in the biomedical area, such as targeted drug/gene delivery, localized bioanalysis, cell sorting, microsurgery, biopsy, detoxification, biofilm removal, and biosensing becomes a driving force that attracts an increasing number of scientists to join in this emerging research field. 143,409 In addition, before implementing MagRobots in real applications, the following aspects should be taken into consideration: (i) MagRobots' materials should meet the standards of practical biomedical and environmental applications, such as biocompatibility and biodegradability, and bring economic and social benefit. For instance, expensive materials and fabrication apparatus or complicated preparation procedures limit the mass production of synthetic microstructures. This is a challenge that researchers face today and should be solved in the future. (ii) To enhance the work efficiency of MagRobots in complex environments, swarms or collective behavior of synthetic MagRobots can be regulated to cooperatively and efficiently execute complex biological or environmental missions that would be insurmountable for a single MagRobot. Moreover, reconfigurability provides another strategy for MagRobots to adapt to variational biological surroundings. For instance, the intriguing collective behavior from the self-assembly of nanoparticles could present a reversible pattern transformation (i.e., reconfigurability) under the steering of an external field, enhancing MagRobots' tasking capabilities and high environmental adaptability. Finally, great endeavors have been made to navigate these untethered microrobots in various complex body fluids such as blood, gastric juice, urine, cerebrospinal fluid, 216 and intracellular medium. However, given the complexity of biological fluids, the relation between movement behaviors of MagRobots and environment parameters (e.g., the components, temperature, viscosity, boundaries, the flow speed of the biological fluids, etc.) are expected to be theoretically and experimentally established in order to obtain better control of MagRobots. (iii) Precise maneuvering of MagRobots on-body and in real-time is very important and their monitoring is essential. This is a challenge confronted by micro/nanorobots researchers. Clinical imaging systems in current use, such as MRI as discussed in Section 5.6, can help in terms of visualization and as an actuation source. However, there is still room to improve MagRobots' programmability in terms of orientation, locomotion, and even morphology. In this way, if MagRobots can be controlled and altered according to actual conditions or occasions such as the patient's health status and physiology, then MagRobots will be able to perform precise and personalized therapy.
In summary, a good understanding of the mechanism of magnetically driven micro/nanorobots and corresponding impact factors (e.g., geometrical shape, field configuration, fluids properties, and boundary) is a precondition for the conceptualization, functionalization, and automation of Ma-gRobots. High spatial maneuverability, fast reconfigurability, and precise programmability are the ultimate research goals of small-scale robots (see Figure 22). Although there is a long way to go to translate robust minimized robots from bench to bedside, considerable advances are bringing fantasy closer to reality.

AUTHOR INFORMATION Biographies
Huaijuan Zhou is currently a Marie Skłodowska-Curie Actions (MSCA) Fellow at the University of Chemistry and Technology Prague, Czech Republic. She received her Ph.D. degree in 2016 from the University of Chinese Academy of Sciences under the supervision of Prof. Ping Jin. She has a broad research interest in designing, preparing, and characterizing functional nanothin films, semiconductor materials, field-induced chromic materials, energy conversion materials, lithium battery materials, biomaterials, and locomotive micro/nanomachines for energy conservation/conversion/storage, biomedical engineering, and environmental remediation.
Carmen C. Mayorga-Martinez is currently the Kralupy Unit Leader and senior scientist at the Center for Advanced Functional Nanorobots, UCT-Prague. She was research fellow in the nanobioelectronics and biosensors group/ICN2, Barcelona-Spain, and in Nanyang Technological University, Singapore. She completed her Ph.D. degree in National University of Tucuman, Argentina, in 2009. Currently, her main research fields include development of bio/ sensors based on 2D materials and nanoparticles platforms functionalized with bioreceptors (enzyme, DNA, and antibodies) as well as micro/nanomotors for biomedical applications and environmental monitoring. Moreover, she is also interested in 2D-materials catalysis for energy application. In 2010, he joined Nanyang Technological University, Singapore, as a tenured associate professor for nearly a decade. He has broad interests in nanomaterials and microsystems and in specific areas of electrochemistry and synthetic chemistry of 2D nanomaterials, nanotoxicity, micro and nanomachines, and 3D printing.