## 2 Microbots
### Classification  
The classification of microbots is not straightforward, and, for simplification, it can be made based on their fabrication and/or propulsion method. In this sense, most of the microbots can be sorted in one of the following categories. * Electrostatic microbots As it is well known, an electric field, which is created by an electric charge, applies a force to any other charged particles. [PERSON]'s law states that the electrical force between two charged objects is directly proportional to the product of the quantity of charge on the objects and inversely proportional to the square of the separation distance between the two objects. This is the principle on which some Micro Electromechanical Systems (MEMS) are based for their control and propulsion. Specifically, electrostatic actuators have been highly explored to convert electrical energy into mechanical deformation for the propulsion of microbots [36]. For instance, [PERSON] et al. [37] designed and fabricated MEMS microbots with an untethered scratch drive actuator used for their propulsion in order to allow the microbot to move freely without wires and rails. Other work carried out by [PERSON] et al. [38] used electrostatic adhesion to perform robotic insets able to perch and take off. * Magnetic microbots A magnetic field is the magnetic effect of electric currents and magnetic materials. Magnetic fields have been used to apply forces and torques on magnetic microrobots, providing a valuable actuation method. The controlled magnetic fields can be generated in a variety of ways, and [PERSON] et al. [9] published a review article in which they detailed the power and locomotion of different microbots with special attention to magnetic microbots. Several works have been developed in this area. [PERSON] et al. [39] reported the control of multiple robots by an array of electrostatic anchoring pads on the surface using also some fixed microbots to prevent translation. Later on, [PERSON] and [PERSON] [40] demonstrated magnetic three-degree-of-freedom closed-loop position and two degree-of-freedom open-loop orientation control of a capsule used for endoscopy with a single permanent magnet. [PERSON] et al. [41] achieved the control of multiple microbots in 3D using magnetic gradient. More recently, [PERSON] et al. [42] proposed a theoretical framework and design for six degree-of-freedom actuation of a magnetic micro robot, and [PERSON] et al. [43] developed methods for controlling multiple untethered magnetic micro-robots. * Optically actuated microbots It is very common to use optical tweezers to trap particles at the scale of nanometers, and, therefore, this technique has also been being used as an actuator in microbots. However, several issues are limiting its applicability as they are low force generated, and a high aperture lens is needed to focus the laser, allowing only a very small workspace and the unsuitability for its use in vitro and in vivo [44]. [PERSON] et al. [45] developed microrobot actuation and control using optically induced thermocapillary effects. The thermocapillary effect has been used to manipulate gas bubbles in oil using optically induced heating. [PERSON] et al. [46] demonstrated experimentally the orientationalresponse and phototactic motion of spherical active colloids in an inhomogeneous laser field. [PERSON] et al. [47] reported that soft microrobots consisting of photoactive liquid-crystal elastomers can be driven by structured monochromatic light to carry out specific motions. * Catalytic microbots The idea of catalytic microbots is to mimic the biological motors able to convert chemical into mechanical energy [18]. [PERSON] et al. [48] reported the use of catalytic microbots able to transport cells in a controllable manner and their delivery on desired targets. Moreover, [PERSON] et al. [49] successfully used catalytic micro motors for the degradation of nitroaromatic pollutants in water, and other authors such as [PERSON] et al.