## 2 Bio-Inspired Mechanisms Used in Locomotion
### Bacteria Flagella
These microrobots can rotate their flagella separately whereas the mimicking helical microrobots can only rotate all the flagella at once inside a rotating magnetic field. Even though the motion is not identical, having multiple helical structures increases the torque applied to the microrobot [46]. Figure 2: Helical microrobots: (**a**) With a soft magnetic head; (**b**) having a head with a microholder; (**c**) with a spherical head; (**d**) with a circular head; (**e**) without a head. Apart from the above types, helical microswimmers which have mastigonemes (flagella covered with appendages perpendicular to the main flagella) have been presented and the effect of the length/space ratio of mastigonemes on the speed of the microrobot has been investigated [64]. In addition, nanowires are used to mimic the motion of the Monotrichous bacteria. They are also maneuvered inside a rotating magnetic field similar to the helical microrobots [65]. A swarm of microrobots that mimic the motion of Monotrichous bacteria as shown in Figure 3b has been successfully tested in vitro and in vivo in the intraperitoneal cavity of a 4-week-old Bulb-C mouse [63]. The geometry of the helix, properties of the coated magnetic layer, viscosity of the fluid, wettability of the helix in the fluid, and step-out frequency are several factors that affect the swimming performance of helical microrobots [66]. Mathematical modeling and computer-based simulations have been widely used to investigate the effects of geometric parameters of the helix on the microrobot's swimming performance [67; 68]. When increasing the helix angle and number of turns, the velocity of the microrobot also increases [44]. Similarly, the velocity of the microrobot increases with the thickness of the magnetic layer [66] and decreases with the viscosity of the fluid [48]. When increasing the frequency of the magnetic field, the rotating speed and linear velocity of the microrobots increase up to a certain point and then start to decrease. For microrobots, the frequency which gives the highest velocity is taken as the step-out frequency [69]. The wettability of the helix mainly depends on the contact angle. Therefore, hydrophilic surfaces have a low step-out frequency compared to hydrophobic surfaces in water [70]. In addition to these factors, the biodegradability of the microrob was investigated to avoid material accumulation inside the organs [71]. To achieve biodegradability, helical microstructures are first fabricated using biodegradable materials and then, coated with magnetic [72] or magnetoelectric [73] nanoparticles to control them using a rotating magnetic field. In terms of controlling the microrobots, most of the helical microrobots utilize an open-loop controlling system [74]. This controlling method can be used to follow a straight path with velocity controlling and so avoid lateral motion due to gravity [75]. In addition, a millimeter scale helical microrobot was studied with a closed-loop controlling system using position-based visual servoing which requires multiple calibrated cameras [76]. Due to the refractions caused at low Reynold numbers, the accuracy of the 3D reconstruction procedure drops in this method [77]. This challenge is addressed by introducing a closed-loop in controlling a millimeter scale helical microrobot using image-based visual servoing which, directly calculates the control sequence using image features [78]. Even though it is  
Figure 3: Helical microrobots: (**a**) With multiple flagella; (**b**) swarm of helical microrobots manipulated in different directions [63]. possible to manipulate a single microrobot using a magnetic field, selective manipulation of multiple microrobots is difficult. This was investigated by manipulating two types of helical microrobots that had a bar-shaped head and a cross-shaped head, using an external magnetic field [79]. Another way to perform selective control is the use of hydrophobic surfaces which exhibit larger step-out frequencies and hydrophilic surfaces which exhibit lower step-out frequencies.