Actuation of Mobile Microbots: A Review

creates a magnetic, optical, or acoustic ﬁ eld. Other types of actuators, such as piezoelectric, electrostatic, thermal and elastomeric actuators, accept electrical input as a power source. Wiring is generally used to power the ﬁ rst prototypes of these microbots for simplicity. [102,216] Power can be also delivered to these types of actuators by storing energy in micro-batteries or supercapacitors, or by remote power transmission using inductive coupling or directing light onto onboard photovoltaic cells. [101] On-board powering and control of microbots are the most appropriate solutions for emerging applications that require microbots to ﬂ y, walk, or swim autonomously. Yet, the progression toward this goal is subject to innovations in the ﬁ elds of electrical storage micro-technologies and ultralow power micro-electronic design. In conclusion, microbotics is an


Introduction
Microbots are submillimeter footprint devices that employ a diverse range of materials and microstructures to perform a variety of locomotion, sensing, and functional operations. [1,2]lthough they are typically below the millimeter scale, sizing can vary depending on the researcher and application meaning that microbots cover a range from micrometers up to a few centimeters.They have emerged to expand the exploration and micromanipulation potentials beyond human and macro robots' capabilities and have been utilized in a wide range of applications in various scientific disciplines.For instance, several researchers have demonstrated the aptitude of microbots for the navigation of confined spaces and hazardous environments, [3][4][5][6][7][8][9][10] searching underneath obstacles after natural disasters, [4,5,7,[10][11][12] and for environmental inspection and monitoring. [3,4,8,11,13]dditional interesting applications of microbotics include the picking and placing of micro-sized objects, [14][15][16] micro-assembly, [4,17,18] and micromanipulation. [4,16,18,19]Microbots have also attracted significant attention in the field of biomedical engineering, [20] where they have been employed in vivo for treatments, [21,22] drug delivery, [22][23][24][25] during noninvasive micro-surgeries [17,19,[22][23][24] to deliver cargo such as stem cells, and to sample tissues in inaccessible locations. [23,24,26,27]he miniaturization and incorporation of all the actuation, sensing and controlling components and the power supply within the body of a mobile microbot allows fully autonomous and untethered functionality, and performance capacity closer to that of insects and organisms of similar size scale. [28]owever, due to the challenges of miniaturization and complexity of integration, a partially on-board strategy is usually considered for mobile microbots, where some of the functional components are miniaturized and integrated within the body, while other components, mostly the power source and computational circuits, are off-board and tethered to the microbot. [2,29,30]ence, the development of micro-sized functional components that can be easily incorporated and integrated in the system is a key step for advancement in the field of microbotics.
As the micro scale regime includes the average size of insects and microorganisms, a vast number of microbots have been designed and engineered to mimic bio-inspired kinematics, such as crawling, [15,31,32] walking, [4,7,9,10,33] swimming, [27,34,35] jumping, [11,12,36] and flapping locomotion. [8,12,37,38]The locomotion of those microbots was possible with the development of many types of actuators, the main components that induce the motion.The different actuations technologies have their own limitations and beneficial advantages.
The field of mobile microbots is quickly developing and novel actuation methods are continually appearing.The actuation is a crucial aspect of mobile microbots as it directly influences their mobility, manipulation, and other capabilities.This review study aims to give a thorough overview of the many actuator types that have been employed in mobile microbots, their advantages and disadvantages, and potential future directions for research in this field.[41] This analysis offers a current and deep assessment of the state of the art.This thorough approach enables readers to have a holistic grasp of the wide range of actuation techniques and how they are used in mobile microbots, emphasizing the value of actuation in mobile microbots, the technological diversity within the field and the wide range of applications.This review covers a wide spectrum of actuators with a scale of microbots varying from 1 to 5 cm.The graph in Figure 1 shows the scale of those types of actuators reported in publications in the past few years.The distribution shows that each actuator has a certain range of size in which it can operate in with miniaturized motors, and piezoelectric actuators being the largest ranging from few to tens of millimeters followed by thermal, electrostatic, dielectric elastomer, acoustic, optical, and finally biological and chemical, respectively.
This paper provides an overview on the actuation methods that have been frequently used for the development of mobile microbots in recent years along with relative applications and future developments, including miniaturized motors, piezoelectric actuators, magnetic actuators, electrostatic actuators, thermal actuators, biological actuators, optical actuators, acoustic actuators, chemical actuators, and dielectric elastomer actuators.This overview highlights the advantages and disadvantages of the different actuation mechanisms to provide suggestions for the direction of future research and developments in this field.A supplementary video shows many of the recent mobile microbots in motion, classified according to their types of actuators.

Miniaturized Motors
The miniaturization of macroscale actuators is limited by their energy efficiency and the fabrication requirements.The energy sources for macroscale actuators, such as hydraulic, combustion, and nuclear motors and propellers, are mostly proportional to the volume, while the sources of energy losses, such as friction, dragging forces, and heat dissipation, are mostly proportional to the surface area or the length of these actuators.This results in an inefficient power density to energy losses ratio when scaling down the size of these actuators.Besides, the fabrication of macroscale actuators requires the assembly of numerous components, which frequently results in a bulky and complicated structure.Of the existing macroscale actuators, electric motors have the highest potential for miniaturization and are the preferred option for robotics actuation due to their high torque density, light weight, ease of integration, robustness, and commercial availability.Several researchers have presented a variety of nano/micro scale motors for in vivo biomedical applications, [42][43][44] environmental applications, [45] and for nano/micro engineering. [45,46]Millimeter-scale miniaturized motors are commercially available, though have limited applications in the field of microbotics.They were employed to actuate flapping-wings microbots due to their ability to produce high torque, [12,37,47] to drive rolling microbots, [48] and to create swarm microbots as they require high force control to produce precise movement. [49,50]In the following section, we will describe several microbots that utilize miniaturized motors for actuation.
is achieved by integrating sliding gear and dog-clutch mechanisms for the leg structures, while utilizing a rack-and-pinion mechanism for the wings.This 23-g insect-like microbot, shown in Figure 2b, operates at 19 Hz frequency and achieves a jumping height of 0.9 m. [12] Chen et al. [37] presented a bio-inspired flapping-wing rotor that is driven using an ultrasonic motor.The combination of a flapping mechanism and rotary motion significantly increases the efficiency and the aerodynamic lift of flapping microbots.An ultrasonic motor drives the flapping wings of the microbot, shown in Figure 2c, which allows the flapping frequency during the stroke to be rapidly varied.Application of variant flapping frequency led to a higher propulsion efficiency and less power consumption than the constant flapping frequency. [37]n addition to flapping microbots, DeMario et al. [51] introduced a crab-like 3D-printed walking milli-robot that employs a miniaturized DC motor for actuation.Miniaturization of the multilegged robot down to 49 mm Â 28 mm Â 25 mm was achieved by using 3D printing technology to create the body and leg parts, including several revolute joins and linkages, from soft materials and with low fabrication and assembly complexity.The milli-robot, shown in Figure 2d, utilizes a Klann linkage mechanism to actuate its four legs using a single micro-planetary gear DC motor.The milli-robot weighs 14.5 g and achieves a walking speed of 5.7 cm s À1 .
Karshalev et al. [52] introduced an interetsing micromotor that acts as a chemical platform to enhance chemical reactions.Hutama et al. [48] presented a rolling microbot that is driven by a micro ultrasonic motor attached to a micro planetary gear system with a total gear ratio of 64 to amplify the torque magnitude of the rotor.This micro motor can produce a torque of 60 μNm and a speed that exceeds 4500 rpm.The microbot is 14 mm in length, 10 mm in width, 7 mm in height, and weighs 640 mg.This microbot has bevel gears to transfer torque from the micro ultrasonic motor to the front wheels that are attached to the chassis by microbearings.
Klingner et al. [49] presented an omnidirectional low-cost microbot, named the droplet, that employs three miniaturized vibration motors for actuation.The microbot has three legs, and the motors are placed next to each leg, as shown in Figure 2e.The locomotion scheme requires the activation of a single motor at a time to produce body rotation around the leg opposite to that motor, while a sequence of rotation produces the required stick-slip movement.The microbot has a diameter of 2.2 cm and is designed for swarm applications.Rubenstein et al. [50] designed a low-cost and easily assembled microbot swarm, named Kilobot, that uses a miniaturized vibration motor.Figure 2f shows the components of a single Kilobot including two coin-shaped sealed vibrational motors. [53]he motors can generate clockwise/anticlockwise rotations if activated independently, and a forward locomotion utilizing the slipstick principle if activated simultaneously.The main advantage of the Kilobot lies in an algorithm that enables the control, charging, and programming of a swarm that reaches thousands of decentralized microbots by a single user. [50]Subsequent research by Rubenstein et al. [53] presented a system that allows a programmable self-assembly of the Kilobot swarm to form complex 2D shapes.

Unimorph Piezoelectric Actuators
Unimproh piezoelectric micro actuators are composed of one piezoelectric layer bonded to an inactive elastic layer, as shown in Figure 3a. [65,66]A wide variety of designs, layer sizes and thicknesses have been adopted. [55]Application of an electric field across the two-layer stack induces strain in the piezoelectric film and reshaping resistance in the elastic membrane, which in turn lead to out-of-plane buckling toward the direction of the applied electric field, as depicted in Figure 3b. [55,67]Deflection in the opposite direction can be obtained by reversing the electric current. [55,67]ang et al. [68] incorporated unimorph piezoelectric actuators to drive the motion of the four-winged insect-sized flapping microbot shown in Figure 3c.The unimorph actuators achieved a displacement of 300 μm upon application of a 260-V sinusoidal signal at a frequency of 100 Hz.In addition to its simplicity of fabrication, the lightweight actuators contribute to the efficiency of the microbot by significantly reducing the wing-loading to facilitate the lift-off process. [68]hoi et al. [62] took advantage of the compatibility of piezoelectric thin films with conventional micro-fabrication processes and utilized unimorph piezoelectric and parylene-C thin films to produce leg joints that can produce in-plane and out-of-plane motions to actuate millimeter-sized silicon microbots.The resulting hexapod microbot utilizes a piezoelectric cantilever design for out-of-plane rotation and a thin parylene-C polymer beam for in-plane rotation.It is worth mentioning that the transverse load bearing can be enhanced by using materials with a high Young's modulus and/or high aspect ratio structures. [62]

Bimorph Piezoelectric Actuators
Bimorph piezoelectric actuators are the most widely used type of piezoelectric actuator in microbotics.These types of actuator conventionally consist of two active piezoelectric layers sandwiching a thin elastic inactive layer, as shown in Figure 4a. [67,69,70][73] The Harvard Ambulatory series of multi-legged micro robots, such as HAMR, [10,33] HAMR-Jr, [71] HAMR-F, [7] and HAMR-E, [6] are among the most highperformance insect-like bimorph piezoelectrically actuated microbots.These microbots demonstrate remarkable features such as high speed, low weight, and a high energy density, [7] and have been implemented at various sizes, as shown in Figure 4a.In addition to their compatibility with the PC-MEMS process that facilitates their fabrication and assembly, bimorph actuators have been incorporated into the HAMR series due to their large bandwidth, wide range of stride frequencies, and ability to achieve high speed locomotion up to 13.9 body length per second by only increasing the stride frequency. [7,10,64]All versions of HAMR incorporate six bimorph piezoelectric actuators, except for HARM-E which uses eight bimorph piezoelectric actuators, [6] to provide electrical connection and mechanical support to the legs.Bimorph piezoelectric actuation is characterized by its high blocking force and precise control, therefore, it was also incorporated to build microgrippers [54] and to produce the high-speed climbing microbot (HAMR-E) shown in Figure 4b. [6]ia et al., [74] presented a novel design and manufacturing methods for a quadruped microrobot "HAMR-F" based on biomorph piezoelectric actuators with enhanced base.It operates on a working voltage of 200 V and a frequency of 20 Hz.It weighs 1.51 g and measures 5.1 cm Â 4.5 cm Â 2.3 cm and is able to reach a speed of 14.9 cm s À1 which is about 3 body-lengths per second.Compared with motors, piezoelectric actuators show better prospects in the field of centimeter-scale microbots.It has a five-layer structure with glass fiber added on the FR 4 carbon fiber base which improves the rigidity of the actuator base resulting in improving the dielectric breakdown voltage and mechanical energy of the actuators.Wu et al. reported a flexible PVDF-based soft meso-robot with a high robustness and mobility.This microbot can climb slopes, carry loads, withstand high loads, and move at a rapid rate of 20 cm s À1 (20 body length per second), as shown in Figure 4c. [4]ha et al. combined bimorph and unimorph piezoelectric actuators to create a soft-mobile microbot with outstanding mobility and resilience, in which a bimorph actuator is utilized for the main body while unimorph actuators are employed to drive the legs, as shown in Figure 4d.This microbot achieved a speed of 35.5 mm s À1 , which represents a travel mobility of 70% of its length per second. [59]imorph piezoelectric actuators are extensively used in wingflapping insect-like micro robots due to their compatibility with MEMS fabrication methods and ability to generate high bi-directional forces. [60,75,76]Fuller et al. utilized four bimorph piezoelectric actuators to build a flapping insect-sized robot that was actuated by interchanging the stroke amplitudes and speeds to mimic the motion of a four-blade rotorcraft.This flapping microbot, shown in Figure 4e, achieved a maximum payload capability of 262 mg at a flapping frequency of 170 Hz. [77] Chopra et al. incorporated strain sensing into the bimorph piezoelectric actuator of a flapping microbot to study the wings collision when flying near an obstacle, in addition to detection of the wings degradation that causes amplitude variation. [60]gure 3. a) The stack layers of unimorph and bimorph piezoelectric actuators.b) The resultant out-of-plane bending of the actuator upon application of an electric field.Reproduced (adapted) with permission. [55]Copyright 2006.c) The flapping-wing microbot that utilizes twinned unimorph piezoelectric actuators.Reproduced (adapted) with permission. [68]Copyright 2019.
Chen et al. presented a hybrid flapping microbot that can travel in aerial and aquatic environments.The microbot, shown in Figure 4f, utilizes a pair of bimorph piezoelectric actuators to drive the flapping motion.It takes advantage of an electrochemical reaction to transit between aerial and aquatic mediums, thus it requires unique components, such as a gas collection chamber, copper sparker and electrolytic plates.Once the microbot is in water, it collects oxyhydrogen in the chamber.The increase in the buoyancy force drives the motion of the wings to take-off while the robot maintains its stability via surface tension. [8]gure 4. a) Various sizes and versions of the HAMR microbot.Reproduced (adapted) with permission. [64]Copyright 2014.b) The HAMR-E climbing microbot with adhesive pads.Reproduced (adapted) with permission. [6]Copyright 2018.c) A meso-scale PVDF-based robot, rapidly climbing a 7.5 slope.Reproduced (adapted) with permission. [4]Copyright 2019.d) The locomotion mechanism of the PVDF-based flexible microbot that combines unimorph and bimorph piezoelectric actuators.Reproduced (adapted) with permission. [59]Copyright 2019.e) A flapping microbot that utilizes four bimorph piezoelectric actuators.Reproduced (adapted) with permission. [77]Copyright 2019.f ) A hybrid flapping microbot that uses two bimorph piezoelectric actuators.Reproduced (adapted) with permission. [8]Copyright 2017.g) The stick-slip 3D-printed microbot with the piezoelectric actuator on the top.Reproduced (adapted) with permission. [78]Copyright 2019.h) The correlation between vertical displacement and time along with the frictional motion mechanism of the microbot.Reproduced (adapted) with permission. [78]Copyright 2019.

Other Types of Piezoelectric Actuators
The concept of utilizing the vibration produced from piezoelectric actuators has been employed to drive the locomotion of microbots.Kim et al. presented a 3D-printed multilegged microbot that carries a piezoelectric actuator, as shown in Figure 4g.The robot is designed to be actuated by piezoelectric vibration that generates vertical oscillations and uses friction to generate locomotion by successive stick-slip cycles.Each vertical oscillation period from the actuator induces one step of displacement of the micro bristle-bot, as shown in Figure 4h.The bristle-bot can achieve an average speed of 8 mm s À1 at an excitation frequency of 6.3 kHz, using an input amplitude of 17 V. [78] Wang et al. [79] introduced a novel microrobot based on piezoelectric actuators which is the T-phage Mimic MicroRobot (TMMR).T-phages are viruses that enable recognition of host bacteria and can move on the surface of bacteria.Inspired by this function, TMMR was designed to mimic the core and the tail fibers of the T-phage in the micro-world.It is composed of a triangular prism core with piezoelectric elements and legs attached on bases and it measures 2.5 Â 2.5 Â 2.6 mm 3 and weighs 1.74 g.TMMR is able to move forward and backward and steering at a voltage of 25 V only.Its speed can reach 120 mm s À1 which is 4.8 body lengths per second and can carry an object weighing 10.6 g.
In essence, piezoelectric actuators with their various types and designs are most commonly used for microbots that require relatively high frequency and precise motion speed.Moreover, due to their fabrication simplicity and the availability of materials, it is preferred to rely on these types of actuators for applications such as flapping wing microbots and quadruped microbots rather than resorting to DC miniaturized motors that can be relatively more expensive in terms of fabrication.However, they require high voltage supply which makes it difficult to provide power supply through miniaturized batteries that should be mounted on the microbots, and the tether would be unavoidable in this case.

Magnetic Actuators
Magnetic actuation is significant for the advancement of microbotics due to the fast response and wireless control, which is a critical attribute for in vivo biomedical applications, such as tissue repair and diagnosis, [23,26,32,[80][81][82] and for maneuvering in confined spaces. [9,83]Magnetic actuation is powered through a magnetic field, which is created by either a magnet [84] or Helmholtz coils. [26,83]Although Helmholtz coils can achieve more control over microbots, the external control system is also more complicated and takes up much space than magnets.
A number of magnetically actuated microbots incorporate rigid magnets to drive locomotion. [9,23,80,85]In addition, magnetic micro/nano particles can be patterned within various polymer composites to produce highly controllable and flexible microbots. [32,80,81,86,87]From this perspective, magnetically actuated microbots are categorized into two subsections based on the type of the magnetic material: hard magnets and micro/nano magnetic particles.

Microbots Actuated Using Hard Magnets
Permanent neodymium magnets of the N50, [9] N55, [9] and N52 [23,32] types are frequently integrated within untethered microbots, and are also utilized as an external source to generate a rotational magnetic field that drives locomotion of microbots. [84,88]Many studies have utilized Helmholtz coils as external driving magnetic field source; this system is used to generate a uniform, highly directable magnetic field to align the robot motion. [83,89]Vogtmann et al. presented an untethered microbot that was actuated via the interaction of an N50 magnet embedded in each leg with an external rotating magnetic field, which was generated by mounting an N55 neodymium magnet on a DC motor, as shown in Figure 5a.The maximum speed achieved was 25.5 mm s À1 , which represents 6.4 body length per second, at a frequency of 21.5 Hz. [9] Jeon et al. developed a submillimeter magnetically controlled soft-microbot with the aim to increase the guidewire steerability of submillimeter catheters.The microbot is composed of a rod-shaped PDMS beam with a 1-mm diameter that embraces two N52 neodymium magnets and a spring is used to connect the PDMS beam to the guidewire, as shown in Figure 5b.The microbot is placed at the tip of the guidewire, and steering is achieved via controlling the intensity and direction of the externally applied magnetic field. [23]Wang et al. developed a magnetically actuated capsulelike microbot for applications that requires maneuvering within narrow spaces.Figure 5c shows the structure of this microbot, which consists of a screw-shaped body, an O-ring type neodymium magnet, a shaft, and an outer shell that aims to reduce friction with the human intestine.The speed and frequency of the screw-jet motion of the microbot can be precisely controlled by modifying the applied magnetic field, which is externally generated using three Helmholtz coils. [83]

Micro Robots Actuated Using Magnetic Micro/Nano Particles
The use of nano/micro magnetic particles for the synthesis of magnetically actuated microbots enables highly sophisticated motions, deformations, and functionalities. [26,32,81]This is due to the ability of using the conventional lithography processes to generate a variety of programmable magnetic films with precise magnetic particles patterns within various elastomeric materials.Microbots actuated by nano/micro magnetic particles are mainly used in medical and surgical fields where a certain drug delivery would be required to be transmitted to a specific membrane in regions that are practically difficult to reach by any medical tool.Xu et al. [32] utilized patterned magnetization to trigger the transformation of a millimeter-scale flexible robot into complex 3D shapes to achieve locomotion.They developed a technique based on UV lithography to pattern permanent magnetic microparticles with an average diameter of 5 μm to encode 3D magnetization in planar elastomeric composites.Figure 5e shows the motion of the millirobot in response to the magnetization pattern, indicated by the purple arrow, and the direction of the external magnetic field generated via a N52 rigid permanent magnet (indicated by the green arrow).A physical design model to predict the shape change under magnetic actuation was also presented. [32]Gyak et al. [26] presented a magnetically actuated biocompatible silicon carbonitride (SiCN)-based crawling microbot that is capable of repairing cells tissue in vivo.3D laser lithography was used to fabricate the cylindrical body of the robot, shown in Figure 5f, while the surface of the microbot was coated with magnetic nanoparticles to drive the actuation.A rotating magnetic field generated by an electromagnetic coil system is applied to drive the rolling motion of the microbot. [26]Khalil et al. [86] fabricated a sperm-shaped magnetically actuated microbot using electrospinning.A magnetic dipole moment is achieved by mixing iron oxide particles with polystyrene solution, which was used to fabricate the head of the sperm shape.The swimming direction of the microbot, shown in Figure 5g is controlled by the influence of a 2-mT magnetic field that drives the head motion, allowing the microbot to achieve an average speed of 112 μm s À1 , equivalent to 1 bodylength per second, at a frequency of 10 Hz. [86] Therapeutic drug delivery needs microbots that are capable of accurate targeting using electromagnetic actuation (EMA) system.Lee et al. [90] proposed a biocompatible and hydrolysable PEDGA-based drug delivery helical microbot capable of The untethered microbot along with the setup of the permanent magnets on the legs.Reproduced (adapted) with permission. [9]Copyright 2017.b) Structure of the steering rod-shaped soft microbot.Reproduced (adapted) with permission. [23]Copyright 2019.c) The components of the capsule-shaped microbot, which consists of a. an outer shell, b. a screw-like body, c. a shaft, and d. an O-ring magnet.Reproduced (adapted) with permission. [83]Copyright 2019.d) Position of the permanent magnets in the screw-like steering microbot.Reproduced (adapted) with permission. [89]Copyright 2011.e) The crawling locomotion and response of the milli-robot to application of an electric field.Reproduced (adapted) with permission. [32]Copyright 2019.f ) SEM image of the fabricated SiCN-based crawling microbot.Reproduced (adapted) with permission. [26]Copyright 2020.g) The sperm-shaped microbot.Reproduced (adapted) with permission. [86]Copyright 2016.h) SEM image of fabricated drug-encapsulated microstructure by using the TPL process.Reproduced (adapted) with permission. [90]Copyright 2021.i) SEM image of a printed helix.Reproduced (adapted) with permission. [91]Copyright 2021.magnetic nanoparticle retrieval in which an anticancer drug is encapsulated and magnetic nanoparticles are conjugated by a disulfide bond.An eight-coil EMA system was used to apply a rotating magnetic field for the helical microbot.Figure 5h shows an SEM image of the helical microbot. [90]iltinan et al. [91] presented a ferromagnetic and biocompatible iron platinum (FePt) nanoparticle-based microbot using the two photon polymerization technique.The shape of the microbot is helical and is 30 μm long and is able to swim at speeds exceeding five body-lengths per second at a frequency of 200 Hz which makes them the fastest in the microscale with a relatively low magnetic field intensity of 10 mT. Figure 5i represents an SEM image of the 3D printed helical microbot. [91]

Thermal Actuators
Thermal actuation uses the shape memory effect of certain materials or thermally induced expansion and contraction of elastic materials to generate motion.Thermal actuators are characterized by their large force density and simple design, though they suffer from low electrical efficiency. [92]Shape memory alloys (SMA) are commonly used for thermal microbotic actuation due to their remarkable miniaturization ability, large deformation, [3,[93][94][95][96][97] and motion stability. [15]Saito et al. [18] and Sugita et al. [98] presented an ant-like fast-walking microbot that utilizes SMA to drive a hexapod locomotion.All the microbot parts were constructed on a silicon wafer using conventional MEMS microfabrication processes, and four shape memory alloy wires were connected to the rotor, shown in Figure 6a, using a conductive paste. [18,94]The motion is initiated by applying electric current to selected shape memory alloy wires connected to the rotary actuator via a link mechanism, which converts the rotary motion of the actuator into forward or backward locomotion, as shown in Figure 6b. [18,94,98,99]The robot achieves a locomotion speed of 90.8 mm min À1 with a step size of 2.8 mm. [98]Shi et al. [15] introduced an inchworm-like crawling soft microbot that utilizes two SMA wires to mimic the muscle fibers of inchworms.The SMA wires are integrated longitudinally within the elastomeric body, as shown in Figure 6c.The inchworm microbot achieved a speed of 48 mm min À1 at a maximum wire temperature of 69 °C. [15] SMA-actuated hexapod microbot was efficiently fabricated by Asamura et al. [3] using MEMS processes for the purpose of compatibility with mass production.The mobility of the microbot is achieved via the rotation of several joints, noted as the CF and FT joints in Figure 6d, which are composed of hinges driven by twelve SMA wires.The hexapod microbot is powered using an ion-lithium polymer battery and can walk for 13 min with a power consumption of 287 mW. [3]In addition to SMA, the combination of polyamide with V-groove-shaped silicon heaters has been utilized to form the legs of thermally actuated silicon-based microbots.[17,100] Bolotnik et al. [100] utilized the thermal expansion of polyamide patterned in silicon V-grooves to actuate the legs of the microbot shown in Figure 6g.This microbot demonstrated movement in four directions at a speed of 10 mm min À1 .[100] U-shaped, [101] V-shaped, [102][103][104] and Z-shaped [105] electrothermal actuators are widely used in MEMS [106][107][108][109][110][111][112] due to their simple structure, large force and displacement, and compatibility with common fabrication processes.The planar displacement of these actuators limits their direct incorporation into mobile microbots requiring out-of-plane deformation of certain elements.Zhang et al. used eight sets of V-shaped actuators fabricated on a single chip to create stick-and-slip displacement for eight legs in different directions.The legs were assembled perpendicularly to the main structure using a z-plug mechanism.The power is delivered to these actuators by directing light onto photovoltaic cells on the top of the mobile microbot.[102,113] Moreover, Zhong et al. [105] presented a laser-driven microbot that utilizes two Z-shaped silicon actuators.The movement of this microbot is induced by using a pulsing laser beam to heat up the serpentine microstructures, as shown in Figure 6i, while the driving force is controlled by tuning the beam frequency.[105] Hussein et al. [104] developed a microbotic leg driven using two electrothermal actuators that induce the forward and reverse locomotion.The leg, in addition to its standard parallel planar actuator setup, possesses an amplifying mechanism, as shown in Figure 6j, that allows for a greater motion and larger displacement.
W. Huang et al. [93] also presented an inchworm like locomotion mechanism microbot actuated by SMA materials.The microbot is composed of three main elements: the body, the front leg, and the back leg.The body serves the role of the actuator with SMA wires embedded inside of it that bend in a contractionexpansion manner which induces locomotion.The microbot can reach a speed of approximately 29.87 mm s À1 .

Dielectric Elastomer Actuators (DEAs)
Dielectric elastomer actuators (DEAs) consist generally of a thin layer of electrostatic electroactive polymers sandwiched between two flexible electrodes, as shown in Figure 7a.The application of an electric potential across the electrodes results in compression of the thickness of the stack due to the Maxwell force. [36]Despite their ease of fabrication, [114,115] lightweight, large actuation strain, and large power density, [13,36,114,116,117] the applications of DEAs are limited due to their low output force and high voltage requirements. [36,118]Chen et al. [13] presented a highly resilient, flying insect-size microbot that utilizes a multilayer DEA to actuate the wings.The actuator is composed of several dielectric elastomer layers rolled around a light cylindrical frame that is attached to the wings via a four-bar transmission, as shown in Figure 7b.Axial expansion and contraction of the cylindrical DEA generate the rotation stroke of the wings.This flying microbot generates a power density of 600 W Kg À1 and can reach an elevation of 23.5 cm within only 0.83 s. [13] Luo et al. [36] presented a DEA-driven jumping microbot that incorporates a 20-layer DEA, as shown in Figure 7c.The microbot utilizes a tension spring to store the energy produced by pre-stretching the DEA, and the jumping locomotion occurs by activating the DEA.The microbot produces an output force of 30 N and reaches a jumping height of 45 mm at a voltage of 7 kV. [36]As the high-voltage requirement of DEA limits its utilization for the development of soft microbotics, Ji et al. [114] developed an insect-inspired 40-mm long soft microbot driven by a DEA that operates at a voltage of 450 V.The low-operating voltage was achieved by building a low-voltage stack DEA (LVSDEA) that incorporates three PDMS layers and four thick, Figure 6.a) The silicon hexapod microbot.Reproduced (adapted) with permission. [99]Copyright 2016.b) Illustration of the forward and backward of the ant-like hexapod microbot.Reproduced (adapted) with permission. [18]Copyright 2016.c) The SMA-actuated inchworm-like microbot.Reproduced (adapted) with permission. [15]Copyright 2019.d) The MEMS SMA-actuated hexapod microbot with CF and FT joints.Reproduced (adapted) with permission. [3]Copyright 2020.e) The polyamide joint structure of the silicon-based walking microbot.Reproduced (adapted) with permission. [17]Copyright 1999.f ) The silicon-based microbot walking, holding a load of 2500 mg.Reproduced (adapted) with permission. [17]Copyright 1999.g) The thermomechanically actuated silicon-based microbot.Reproduced (adapted) with permission. [100]Copyright 2015.h) The silicon-based jumping microbot that utilizes a Chevron-type thermal actuator.Reproduced (adapted) with permission. [92]Copyright 2013.i) Silicon-based Z-shaped electrothermal actuators.Reproduced (adapted) with permission. [105]Copyright 2020.j) V-shaped electrothermally actuated microbotic leg.Reproduced (adapted) with permission. [104]Copyright 2022.
yet stretchable, single-walled carbon nanotube (SWCNT) electrode layers, as shown in Figure 7d, which enable a high strain level at low resistance.Three LVSDEA were incorporated to drive the locomotion of three independent legs, wherein in-plane expansion of the LVSDEA creates a vibrational friction force that drives the forward locomotion of the legs.The soft microbot weighs 190 mg and operates at a frequency of 1 kHz to reach a speed of 30 mm s À1 , as illustrated in Figure 7e. [114] Wei et al. [119] developed a different type of inchworm microbot using magnetic active elastomer with micropillars.This microbot, as shown in Figure 7f, has a straightforward design with two legs acting as the longitudinal muscle in a biological inchworm and supported by micropillars that allow locomotion on smoother, rougher, and more inclined terrains. Thosemicropillars with a size around 200 μm are shown in Figure 7g.The stretched elastomer has a similar role to a pre-loaded muscle  [36] Copyright 2020.b) The DEA-based insect-sized flying microbot, along with an illustration of its design and kinematics.Reproduced (adapted) with permission.[13] Copyright 2019.c) The DEA-based jumping microbot and its jumping mechanism.Reproduced (adapted) with permission.[36] Copyright 2020.d) The microbot with an illustration of the Low Voltage Stack DEA (LVSDAE) layers.Reproduced (adapted) with permission.[114] Copyright 2019. e) he forward locomotion test of the microbot, demonstrating its high speed.Reproduced (adapted) with permission.[114] Copyright 2019.f ) Magnetic active elastomer with micropillars microbot. Copyright 2023.g) Micropillars.Reproduced (adapted) with permission.[119] Copyright 2023.
that contracts and expands based on a magnetic field to enable the locomotion of the inchworm microbot while achieving a speed of 0.125 body length per second.

Electrostatic Actuation
Electrostatic actuators are frequently used for MEMS and NEMS devices due to their power efficiency, fast response and ease of microfabrication. [120]Electrostatic actuation is driven by the force generated between two conductive electrodes when a voltage is applied.[122][123] Electrostatic actuators are mainly characterized by their ability to store and release energy over time, which can enable exceptional microbot locomotion capabilities such as repeated jumping. [11,124]chindler et al. [11] introduced a 90-mg MEMS SOI-based jumping microbot that employs an electrostatic motor for actuation and inchworm springs for energy storage and actuation amplification. [11,124]The jumping electrostatic force of the microbot shown in Figure 8a depends on the voltage applied and the generated capacitance, which can be improved by reducing the separation between the electrodes and increasing the area of the electrodes.The microbot jumps when the energy stored in the compressed inchworm springs, which can reach 100 μJ, is released to produce vertical jumps up to 4 cm. [11]ang et al. [125] presented the design, fabrication, and analysis of an ultra-compact power electronic interface (PEI) that enables electrostatic actuation without the need for an external bulky power source.This PEI is characterized by an ultralight weight of 63 mg, high power density of 500 mW and large gain, and was used experimentally to drive a 110 V electrostatic inchworm motor at 1 kHz to actuate the jumping microbot shown in Figure 8b. [125]n addition to jumping microbots, electrostatic actuators have been employed to drive the locomotion of insect-like crawling [31] and flying [38] microbots.Qi et al. [31] presented a fast-crawling insect-sized microbot that was actuated via electrostatically generated vibrations.The microbot utilizes super elastic Nickle Titanium (NiTi) wires with a diameter of 56 μm, a 10 nF capacitor, two foil-coated carbon fibre electrodes, and two carbon fiber plates to support the structure from both ends, as shown in Figure 8c.Actuation is generated by applying a high DC voltage to the electrodes, which makes the cantilever beam vibrate and subsequently shake the body structure due to the momentum conversion principle, resulting in forward crawling motion, as illustrated in Figure 8c.The microbot achieves a crawling speed of 30 mm s À1 , which represents 1.5 body lengths per second.Furthermore, the capacitor was used in self-excitation experiments, and achieved untethered operation for 10 s at a speed of 2 mm s À1 . [31]an et al. [38] introduced insect-like, electrostatically-actuated flapping microbot wings that utilize two parallel-metal beam actuators to form the body structure, as shown in Figure 8d.The microbot is powered via an external DC power source, and the vertical take-off system shown in Figure 8e is utilized to supply the electrostatic driving force to drive the wing motion.When operated at a frequency of 70 Hz and voltage of 5 kV, the flapping wings lead to uplift at a speed of 2 mm s À1 . [38]kayanagi et al. [126] studied a legged type microrobot that is actuated by an electrostatic micromotor.This micromotor shown in Figure 8g consists of two pairs of electrostatic actuators, a central shuttle, arms, and multiple springs.A miniaturized neural networks integrated circuit (NNIC) is implemented to power the motor.The latter produces a linear motion of the shuttle with 1 μm steps.Figure 8f shows a schematic of the microbotic system proposed in this paper.

Biological and Bio-Hybrid Actuators
The realm of nature encompasses an extensive array of motility mechanisms and natural actuators that facilitate movement.The diversity of bio-mechanical machines, whether natural or artificial, is remarkably broad, making it challenging to present a comprehensive catalog of all the different types of bio-mechanical actuators. [127]Notably, humans alone possess approximately 100 different genes of kinesin, myosin, and dynein, which are molecular motors responsible for transporting biological payloads within the human body.Molecular motors, which are crucial for movement in living organisms, belong to a class of proteins that drive intracellular trafficking.However, as their scale is typically at the nanometer level, they fall outside the scope of this review.An extensive overview of these actuators can be found in specialized reviews. [127,128]n a larger scale, biological and biohybrid actuators are based on cells such as bacteria or muscle cells and hold potential for biomedical applications like drug delivery and cargo transport.These bio-actuators take advantage of the diverse sizes, speeds, propulsion mechanisms, and living environments of living cells.The actuating cells can be mainly divided into two types, bacteria and other motile cells that are propelled by flagella or cilia, including prokaryotes such as E. coli and S. marcescens, and eukaryotic cells whose motion is realized through a contraction, such as muscle cells (also called myocytes) like cardiomyocytes.

Bacteria and Other Motile Cells
Bacteriabots are microbots composed of bacteria serving as the actuator and a nanoparticle serving as cargo.These microbots can be propelled through low Reynold number fluids by cilia or flagella.Cilia or flagella are hair-like appendages that extend from the surface of living cells, as shown in Figure 9a.Bacteria's high moving speed makes them attractive for bio-actuators, while their small size makes them compatible with human capillaries and shows promise in future biomedical applications such as drug delivery.[135] The taxi response for these bacteria represents a "passive" robotic system with low human control over their operations. [136]Although the taxi response has shown some autonomy in tumour targeting, the further disposal of these microbots requires a higher level of controllability.Copyright 2019.b) The jumping microbot prototype that employs PEI.Reproduced (adapted) with permission. [125]Copyright 2014.c) The electrostatically actuated, insect-like crawling microbot along with an illustration of its vibration-based motion mechanism.Reproduced (adapted) with permission. [31]Copyright 2017.d) The body structure of the insect-like, electrostatically actuated flapping microbot.Reproduced (adapted) with permission. [38]Copyright 2015.e) The external vertical take-off system of the flapping microbot.Reproduced (adapted) with permission. [38]Copyright 2015.f ) The NNIC electrostatically actuated microbot.Reproduced (adapted) with permission. [126]Copyright 2022.g) Micromotor of NNIC microbot.Reproduced (adapted) with permission. [126]Copyright 2022.

Magnetotactic bacteria (MTB) such as Magnetococcus marinus
MC-1 and Magnetococcus massalia MO-1 [137] possess magnetosomes in their cells, which act like a compass and enable the cells to react to magnetic fields.Another approach is to use magnetic microbeads or deposit microbeads made of magnetic materials such as Nickel to make the bead magnetic. [138]Using a magnetic field as an extra control has several advantages.The microbots react fast to the external magnetic field and thus have a high speed, up to 100 body lengths per second. [139,140]Magnetic fields are widely used in biomedical applications.Since magnetic fields can safely penetrate most biological environments, it is one of the most ideal methods to control bacteriabots remotely.
Electrical control of bacteriabots could also be realized using either the galvanotactic or electrophoretic [142,151] properties of bacteria.The advantages of using electrical methods include the rapid reactions of the microbots and the possibility of parallel control, where several microbots can react to the same field simultaneously.However, the heat and potential chemicals generated by the application of a current remain important problems that need to be solved. [152]ptical control takes advantage of the phototactic property of bacteria.This approach was initially realized with UV light, [142] but recently visible light and even red/far red light-which have a lower energies than UV light and are thus more biocompatiblehave also been proven to work. [141,143,144]However, unlike a magnetic field, the penetrative ability of light is very limited in biological tissues.
There are also chemical methods to control bacteriabots, usually through the control of chemical or O 2 concentration.Studies have shown that bacteriabots propelled by bacteria such as E. coli, S. marcescens or S. typhimurium can follow chemoattractant gradients, [35,135,145,146] such as moving toward cancerous cells [147] and the ability to move away from strongly acidic or alkaline environments.[150] One advantage of chemical control is that no external system is needed.However, the chemical control might not be as sensitive as other methods as the magnetic control and may take a longer time for bacteriabots to react.

Muscle Actuators
Muscles can be divided into three categories: cardiac, smooth and skeletal muscle.Cardiac and smooth muscles are self-beating cells with limited modulation activity, while skeletal muscles are much more controllable with electrical stimulus, making skeletal muscles more favorable as an actuator, although some microrobots have employed cardiac muscle tissues as actuators. [153]uscles can generate larger forces than bacteria and other motile cells.Bio-actuators based on single muscle cells have been reported, [154] but more often muscle cells are employed in the form of cell sheets or 3D structures because the force generated by muscles is proportional to their cross-sectional area. [155]Thus, muscle-powered microbots are usually larger in size, and their scalability in size is one of their advantages.
Optogenetics is one of the most promising genetic modifications of muscles, in which muscle cells are genetically modified to express a light-activated cation channel that when illuminated at proper wavelength, cells start contraction, making it possible to control muscles optically instead of using electrical methods. [156]

Chemical Actuation
Microbots can also be powered and actuated through chemical reactions.To be more specific, these microbots are mostly self-propelled micro-swimmers that harvest energy from their environment.[165][166] In general, microbots powered by chemical reactions are mostly self-propelled microswimmers with limited controllability.The typical applications include killing bacteria in water, [157] removing heavy metals, [162] separating particles, [167] targeting, releasing and sampling. [168]Most studies in this field have focused on self-propulsion mechanisms and more applications are yet to be explored.
Gas-propelled locomotion is propelled by two distinct processes: self-diffusiophoresis and self-electrophoresis.In the self-diffusiophoresis process, chemical reactions occurring on the particle surface establish a concentration and pressure  [224] Copyright 2015.c) Example of a magnetotactic microbot.Reproduced (adapted) with permission. [222]Copyright 2017.d) Example of an optotactic bacteriabot.Reproduced (adapted) with permission. [143]Copyright 2020.
gradient that drives the motion of the particles.Self-electrophoresis, on the other hand, involves the generation of a proton flow, distinguishing it from self-diffusiophoresis. [169]Gas propelled micro-swimmers, usually in the form of particles (Figure 10a, b) or micro-rockets (Figure 10c), are particles decorated or coated with catalytic materials.The most typical particle microswimmers are Janus particles (Figure 10a) which are usually colloids with one side treated differently, commonly by applying catalytic coatings.The catalytic material reacts with media like water or H 2 O 2 to generate O 2 bubbles to push the microbot forward.Researchers developed particle swimmers that exhibit taxi behaviour and follow pH, [158] light [159] and chemical gradients. [160]icro-rockets are conical microbots based on micro/nano tubular structures, they are usually made of carbon nanotubes (CNT), aluminum, titanium, ceramic composites, and polymer materials such as Kevlar and other high strength plastics.Similar to Janus particles, micro-rockets are also powered through catalytic decomposition and propelled by the bubbles generated (Figure 10c).Micro-rockets usually have a larger surface area than particle microswimmers and thus can carry more "cargo". [162]nlike the gas-propelled microswimmers, micro-droplets are propelled through Marangoni stress, which is the stress along an interface between two fluids due to a gradient in surface tension.Micro-droplets typically have radii of several micrometers and speeds in the order of one diameter per second.When chemical reactions, such as hydrolysis, occur between a surfactant and solution, local surface tension is induced.The induced surface tension pushes the droplet forward, as illustrated in Figure 10d. [165]As the Marangoni stress is trivial, the speed is much lower than other chemical-powered methods.Researchers have achieved control of droplets through certain tactical responses.For example, Sun et al. proposed a water droplet robot decorated with Fe 2 O 3 nanoparticles that can self-propel along an optical gradient in oil solvent. [164]in et al. successfully guided droplet microbot swimmers through a microfluidic maze with designed chemical gradients along micellar surfactant gradients. [170]The potential applications of controllable droplet-based microbots include carriers, sensors, and actuators in biological medicine and oil exploration. [164].Optical Actuators Light can also be used as an actuation method for microbots and has several advantages, including remote control, noninvasive Figure 10.a) Illustration of a Janus particle, the fabrication process and how the self-propelled Janus particle works.Reproduced (adapted) with permission. [157]Copyright 2017.b) Illustration of a self-propelled particle decorated with a catalytic material moving along a pH gradient.Reproduced (adapted) with permission. [158]Copyright 2013.c) Illustration of a micro-rocket.Reproduced (adapted) with permission. [162]Copyright 2016.d) Illustration of a droplet propelled through chemical reactions: Stage 1, fresh oil droplet coated with a homogeneous surfactant layer (blue line); Stage 2, hydrolysis of surfactant precursor occurs at different sites (red circles) on the droplet surface; Stage 3, symmetry of surface tension is broken and convection starts.Reproduced (adapted) with permission. [165]Copyright 2016.
nature, high spatiotemporal resolution, and the potential for operation without wires and batteries.Optical control of microbots can be realized through photoresponsive materials, including liquid crystalline elastomers (LCEs), liquid crystalline networks (LCNs), and hydrogels, and using optical or optoelectronic tweezers to manipulate the microbot.

Photoresponsive Materials
Photoresponsive materials provide one way to activate microbots using light.In addition to microorganisms with phototactic properties, as discussed in Section 8.1, inorganic particles such as photoactive semiconductors also exhibit photo-activity.The most typical example is TiO 2 , which can promote the catalytic decomposition of hydrogen peroxide under UV illumination.A Janus particle made of TiO 2 -Au can achieve a moving speed of 25 body lengths per second using low-power UV light (40 mW cm À2 ) as the only power source. [171]olymers, including LCEs/LCNs and hydrogels, are another major group of photoresponsive materials.LCEs and LCNs are smart materials that can transfer into liquid crystalline (LC) phases in response to specific stimuli, such as heat or light, as illustrated in Figure 11.Thus, LCEs and LCNs can contract under illumination.LCEs and LCNs have been successfully applied as inchworms, [172,173] micro-walkers, [174] artificial cilia, [175] micro-swimmers, [176] and micro-grippers. [177]Hao Zeng et al. applied spatial and temporal light modulation and stationary light at subsequent phases to control a microbot via light color or polarization.The ultimate goal is to have the microbot interact with the environment to autonomously execute complex tasks. [178]ydrogels are networks of hydrophilic crosslinked polymers that hold a large amount of water.Hydrogels can shrink in size in response to external stimuli, usually temperature.An important property of hydrogels is their low critical solution temperature (LCST).As the temperature increases, the hydrophobic interactions between the carbon chains increase and, above the LCST, the polymer becomes insoluble in water. [179]Considering direct heating of the water with light does not work well for light-driven microbots.Researchers have doped hydrogels with absorptive compounds such as carbon nanotubes [180] and gold. [181]These doped hydrogels can be actuated under selected wavelengths.
Opto-thermal actuators are also implemented to drive microbots.Martin-Olmos et al. [182] exhibited the opto-thermal actuation capabilities of photostructurable polymers doped with specific nanoparticles.Li et al. [21] proposed a micro-rocket design capable of achieving high speeds of 62 body lengths per second (2.8 mm s À1 ) in viscous media.This microbot moves in a bloodstream where laser pulses irradiate a rocket structure with lightabsorbing materials.This heats up the rocket which converts the light energy into ultrasonic emission to drive the locomotion.

Optical and Optoelectronic Tweezers
Optical tweezers (OT) use lasers to manipulate micro-scale objects, and were initiated by Arthur Ashkin, [183] who was awarded the Nobel Prize in Physics in 2018 for development of the optical tweezer.OT have many potential applications, including tissue growth, [184] translation of cells, [185] pumping, [186] and surface scanning. [187]However, OT have several disadvantages: they can only reliably actuate objects with sizes less than 30 μm; microbots that are manipulated by OT must typically be fabricated using expensive, time-consuming and specialized fabrication tools; and it is hard to manipulate multiple microbots in parallel using OT, [136] However, optoelectronic tweezers provide an alternative to these limitations.
Optoelectronic tweezers (OET) make use of both light and an electric bias to sculpt a potential landscape on a photosensitive substrate.As OET rely on light to control dielectrophoresis Figure 11.a) Illustration of the TiO2-Au Janus particle powered by UV light.Reproduced (adapted) with permission. [171]Copyright 2016.b) Illustration of the transition between LCEs/LCNs and LCs.Reproduced (adapted) with permission. [179]Copyright 2018.c) An inchworm microbot built with a LCE.Reproduced (adapted) with permission. [172]Copyright 2018.d) Use of optoelectronic tweezers for isolation of single cells.Reproduced (adapted) with permission. [136]Copyright 2019.
(DEP) (rather than the forces generated by direct photon momentum), OET systems typically exert a stronger manipulation force for a given intensity of light compared to OT. [136,188] The applications of OET include trapping, [189] bead positioning, [190] and manipulation and assembly of micro particles. [191]More Advanced Trapping techniques have emerged, which use machine learning technologies to automatically localize and trap microrobots in the OT. [192]Applications such as the selection and isolation of single cells for clonal expansion and RNAsequencing, selection and targeting of cell-cell fusion partners, and collection of precious microtissue specimens from complex samples [136] are also being explored.

Acoustic Actuators
Scientists have been exploring alternatives to wires and electricity to actuate microbots, including ultrasound.In general, there are two popular acoustic waves as power sources: standing ultrasound waves and surface acoustic waves.Acoustic waves are usually generated through transducers.There are three potential actuation methods based on acoustic waves.][199][200] The third is actuated through a transducer to transfer the energy from acoustic waves into mechanical energy (Figure 11c). [201,202]ltrasound, especially in the form of standing waves, can move micro and nanoparticles. [195]In 2012, Wang et al. manipulated metallic microrods (2-μm long and 330-nm diameter) in water using ultrasonic standing waves in MHz frequency range, demonstrating autonomous propulsion of individual microparticles. [196]When sound propagates perpendicularly to water, the acoustic pressure creates a node that can trap particles.Different shapes of microbots have been designed to accomplish different movements, such as directional, clockwise, and counterclockwise (Figure 12d). [193]This actuation method can also be combined with chemical [203] and magnetic [204,205] actuation by depositing catalytic or magnetic materials on the microbot.
However, because these nanoparticles must operate at an acoustic pressure node, they must stay close to the source of acoustic power, usually within a few wavelengths (hundreds of millimetres for MHz wavelengths). [206]][199][200] Figure 12c shows a typical design of the oscillating bubble.Each bubble has a resonance frequency at which the oscillation amplitude reaches a maximum.The resonance frequency of the bubble is determined by its shape and size.Ahmed et al. successfully demonstrated selectively Figure 12. a) Illustration of a node generated by acoustic waves that can trap and move microparticles.Reproduced (adapted) with permission. [194]opyright 2016.b) Illustration of the oscillating bubble in a microtube: when an acoustic field is applied, the bubble oscillates and generates a forward or backward propulsive force, depending on the position of the meniscus.Reproduced (adapted) with permission. [225]Copyright 2018.c) Example design of a transducer.Reproduced (adapted) with permission. [201]Copyright 2018.d) Different microbot shapes used to realize different movements such as translational and rotational.Reproduced (adapted) with permission. [193]Copyright 2017.e) Design of a microbot powered by SAW.Reproduced (adapted) with permission. [201]Copyright 2018.manipulable bubbles by controlling the bubble size. [200]Recently, a microbot based on a surface-slipping mechanism containing a trapped bubble achieved 90 body lengths per second with a body length of about 25 μm. [22]nother way to actuate microbots acoustically is to apply surface acoustic waves (SAW). [201,202]SAW (Rayleigh waves) are generated in a solid substrate in contact with fluid by piezoelectric excitation. [207]In general, SAW is mostly used to manipulate cells on microfluidic chips. [208,209]Figure 12d shows the design of an example transducer, including a driving interdigital transducer (D-IDT) and a reflector interdigital transducer (R-IDT) to generate SAW. [201]igure 12e shows an example design of a SAW-driven microbot.The angle θ between the unidirectional interdigital transducer (U-IDT) and the kickboard is set to enlarge the horizontal acoustic propulsion. [201]The kickboard is positioned to make the microbot float on water and the U-IDT is the power source.Acoustic tweezers are another interesting application of SAW.

Discussion and Conclusion
In this review, we explored various examples of recent microbots utilizing different actuation mechanisms, specifically miniaturized motors, piezoelectric actuators, magnetic actuators, thermal actuators, electrostatic actuators, dielectric elastomer (DEA), optical actuators, acoustic actuators, biological actuators, and chemical actuators.
Several actuators demonstrated successful functionality after being scaled down; however, scaling-down reduces their performance and triggers trade-offs in terms of efficiency, response time and actuation strain. [78,125]Below are two tables; one (Table 1) which summarizes the advantages and disadvantages of each actuator type, and another (Table 2) that summarizes challenges associated with each actuator type and required implementation techniques.
Miniaturized electric motors are an efficient alternative for the actuation of untethered microbots and have successfully been used to produce highly functioning micro-versions of large robots. [47,50]Electric motors specifically are considered to be highly reliable, and their robust design make them a great choice for microbots.Nevertheless, these motors are relatively large compared to other types of actuators, which limits the scale-down of robots beyond the millimeter-scale making them not very compatible with microfabrication, especially since they need a power source to operate.Other types of actuators, mostly with simpler structures and a smaller number of components, have exhibited more efficient performance at the microscale and are compatible with microfabrication techniques.
For instance, piezoelectric actuators are ideal for microbots that require precise control as they can provide very small movements with high precision and respond quickly to changes in voltages allowing for fast locomotion.In addition to that, they are characterized by low power consumption [33,54] and compatibility with PC MEMS fabrication and pop-up assembly techniques. [64,72]However, the majority of piezoelectric-actuated microbots have complex control schemes, thus must be tethered to an off-board system to communicate the control signals [6][7][8] and are considered relatively more expensive when compared to different types of actuators.It is possible that an amplifier or a driver is needed to amplify or modify the electric signal fed into the actuator.It is also worth noting that most piezoelectric actuators are made from brittle materials which makes them very fragile and subject to less durability, and they might exhibit hysteresis errors.In contrast, magnetic actuators can be wirelessly powered via an external magnetic field generator, which reduces the weight of the microbot, and allows highly controlled actuation while eliminating the wiring constraint, [9,83] and avoiding any damage or tear.Additionally, their high force output and fast response time are key to their popularity and success while their versatility is also a major advantage: magnetic actuators can enable a wide range of motion including linear, rotary, and oscillating movement.These characteristics come at a price.In fact, the setup of magnetic actuators is relatively more expensive to manufacture and those actuators have a limited range of displacement compared to the setup dimensions.They also should be equipped with a magnet or a Helmoltz coil.Another issue that must be considered is the possible interference of other magnetic fields that may affect the actuator's performance and reliability.Electrostatic actuators are characterized by their fast response, low voltage consumption, and energy storage/release capability, which are key performance metrics for the kinematics of jumping microbots, [125,210] and microbots powered by small batteries moving at high speed.However, some disadvantages that come with electrostatic actuators include low output force, limited and nonlinear range of motion which might be insufficient for some applications, pull-in instability, wear and degradation that may lead to short lifespan, and operational instability that may result in the failure of micro-devices, thus its use in microbotics is limited. [120,125]DEAs can generate large forces per unit area, therefore providing large forces relative to their size and weight, they are also characterized by their lightweight, flexibility, large deformations which allows them to cover big distances in each actuation, and their ease of fabrication such as molding or stamping.However, DEAs suffer from a high driving voltage typically of the order of 1 kV and low output force which limit their application in certain areas, and they require use of specific material for each of the dielectric and the elastomer.In addition to that, DEAs are very sensitive in cold or humid environments and well be inefficient in such circumstances.DEAs are subject to fatigue and degradation meaning that they may not be the best option for applications that require a great number of actuations.In comparison, thermal actuators have good stability, [15,210] large deflection or force outputs, [92,211] and a simple and low-cost design that do not require complex control and can operate at a wide range of temperatures making them one of the most straightforward types of actuators in microbots.However, thermal actuators suffer from a slow response time due to the time needed for them to heat up and cool down, have low electrical efficiency and consume a significant amount of energy. [92]It also should be mentioned that they require use of a specific type of shape memory alloy, and they need wires to connect them to an energy source.
Chemical actuators receive their power from the environment, they can store high amounts of energy for their size and their design is relatively inexpensive to manufacture, making them a cost-effective option.In addition to that, they can be completely autonomous, suggesting great potential in environmental applications. [157]Yet this autonomy can sometimes be a disadvantage and external control systems may need to be added to achieve better steering control. [212]Other disadvantages include the short lifespan of chemical actuators that are generally limited by the number of chemical reactions that occur and the environment that they should be carried out in, alongside safety concerns toward humans as well as the environment due to some chemicals that if not handled with caution can cause harm.[215] On the other hand, biological actuators possess a complex design and controllability due to their internal structure and functions and can also be regarded as less reliable than nonbiological actuators due to their high sensitivity to changes in the environment.Some other challenges include having to implement a control strategy that considers morphological computation and muscle synergy, in addition to having to integrate a power source and thermoregulation in the actuator.
Optical actuators do not require physical contact and can be controlled remotely leading to a lighter weight for the microbot and since they are powered by light, electrical power is not demanded for them to operate, which could save a lot of energy.They can also achieve a very high level of precision and react quickly to any change in the light intensity.Being powered by light, one must take into consideration the interference of ambient light and must therefore make sure to isolate the light source from the ambient light by shielding the actuator.Optical actuators are more expensive to manufacture than other types of actuators.Acoustic actuators also work remotely with soundwaves and therefore avoid any undesirable physical contact with the microbot, and in addition to that they perform quickly to input soundwaves allowing the microbot to perform at high speed and do not require a high voltage level to function.However, acoustic actuators are not capable of generating large output forces which might make them impractical for some applications and their design is rather complex to design and manufacture especially at a micro scale.Not only that, but external noises might interfere with their performance so one should consider this undesirable factor.
From another perspective, the fact power is required to activate actuators and the lack of structural tethering to a fixed support complicate the power transmission required for the locomotion of mobile microbots.The power delivery method is strongly related to actuation technology.Remote microbot actuators, such as magnetic, optical, or acoustic actuators, have mobile parts made of materials sensitive to such types of fields.These mobile parts are controlled by an external power source that  • Possible use of piezoelectric material creates a magnetic, optical, or acoustic field.Other types of actuators, such as piezoelectric, electrostatic, thermal and elastomeric actuators, accept electrical input as a power source.Wiring is generally used to power the first prototypes of these microbots for simplicity. [102,216]Power can be also delivered to these types of actuators by storing energy in micro-batteries or supercapacitors, or by remote power transmission using inductive coupling or directing light onto onboard photovoltaic cells. [101]n-board powering and control of microbots are the most appropriate solutions for emerging applications that require microbots to fly, walk, or swim autonomously.Yet, the progression toward this goal is subject to innovations in the fields of electrical storage micro-technologies and ultralow power microelectronic design.
In conclusion, microbotics is an area of great interest that has witnessed exponential growth in recent years and is far from stopping there.This growth is directly linked to the development of other technologies that could revolutionize the world of microbotics.Developing smaller power supplies with higher capacitive power, reducing the number of wires, improving microfabrication of actuators, reducing the design's weight are just a few along many other technicalities that need to be addressed in order to further improve microbots and their application.If we can find a way to overcome those hurdles, we would have the key to opening many doors in the microbotics field.

Figure 2 .
Figure2.a) The insect-like microbot that utilizes a miniaturized servo motor to produce a highly controlled actuation.a) Reproduced (adapted) with permission.[47]Copyright 2019.b) The insect-like flapping and jumping microbot.Reproduced (adapted) with permission.[12]Copyright 2019.c) The flapping wing microbot that utilizes an ultrasonic motor to operate at a rapidly varying frequency.Reproduced (adapted) with permission.[37]Copyright 2020.d) The crab-like walking milli-robot.Reproduced (adapted) with permission.[51]Copyright 2018.e) The droplet microbot and the positions of the three miniaturized vibration motors.Reproduced (adapted) with permission.[49]Copyright 2014.f ) The Kilobot microbot, with an illustration of its components and the positioning of the two miniaturized vibration motors.Reproduced (adapted) with permission.[53]Copyright 2014.

Figure 5 .
Figure5.a) The untethered microbot along with the setup of the permanent magnets on the legs.Reproduced (adapted) with permission.[9]Copyright 2017.b) Structure of the steering rod-shaped soft microbot.Reproduced (adapted) with permission.[23]Copyright 2019.c) The components of the capsule-shaped microbot, which consists of a. an outer shell, b. a screw-like body, c. a shaft, and d. an O-ring magnet.Reproduced (adapted) with permission.[83]Copyright 2019.d) Position of the permanent magnets in the screw-like steering microbot.Reproduced (adapted) with permission.[89]Copyright 2011.e) The crawling locomotion and response of the milli-robot to application of an electric field.Reproduced (adapted) with permission.[32]Copyright 2019.f ) SEM image of the fabricated SiCN-based crawling microbot.Reproduced (adapted) with permission.[26]Copyright 2020.g) The sperm-shaped microbot.Reproduced (adapted) with permission.[86]Copyright 2016.h) SEM image of fabricated drug-encapsulated microstructure by using the TPL process.Reproduced (adapted) with permission.[90]Copyright 2021.i) SEM image of a printed helix.Reproduced (adapted) with permission.[91]Copyright 2021.
Miniaturized Motors: • Power source Implementation • Easy to integrate and commercially available Piezoelectric: • Power source • Amplifier or driver • Hysterisis and drift Magnetic: • Use of magnets or Helmoltz coils • Hard magent or magnetic particles Thermal: • Shape memory alloy • Energy source and wires Dielectric: • Power source • Specific material for each of the dielectric and the elastomer Electrostatic: • Nonlinear • Pull-in instability • Feedback system to rectify possible long-term drift Biological: • Complex control schemes to maintain muscle synergy.• Energy source • Thermoregulation Chemical: • Highly dependent on environment • Hard to control independently Optical: • Complex setup and equipment • Limited range and working space Acoustic:

Table 1 .
Advantages and disadvantages of each actuator type.

Table 2 .
Challenges and Implementation of each actuator type.