## 2 Bio-Inspired Mechanisms Used in Locomotion
### Bacteria Flagella  
Bacteria is a living organism that can move inside a low Reynolds number fluidic environment. A helical-shaped filament called flagellum is used by bacteria for locomotion. This flagellum can be rotated at about 100 Hz by a rotary motor connected to the cell body. The rotary motor and flagellum are connected through a short flexible hook as shown in Figure 1 and the arrangement operates as a universal joint. In general, the diameter and the length of the helical flagellum are about 20 nm [43] and 3-10 \(\upmu\)m [44]. Bacteria have one or many flagella located at the pole or lateral side of the cell body. Accordingly, bacteria are categorized into four categories as shown in Table 1[45]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Type**} & **No. of** & **Placement of the** & **Layout** & **Examples** \\  & **Flagella** & **Flagella** & **Layout** & **Examples** \\ \hline Monotrichous & One & One pole & & Pseudomonas \\ \hline Amphitrichous & Two & Two poles & & Campylobacter \\ \hline Lophotrichous & More than two & One pole & & Helicobacter \\ \hline Peritrichous & More than two & Lateral & & Escherichia coli \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Classification of bacteria according to the arrangement of the flagella. Figure 1: Placement of rotary motor, hook, and flagellum or filament. A helical microstructure is used to mimic the bacteria flagella, but in some cases, a head is attached to the helical structure to act as the magnetically active part or to perform micromanipulations [46]. Therefore, microrobots that mimic Monotrichous bacteria can be again classified as microrobots with heads and without heads. In the literature, rectangular [47], circular [48], spherical [49], and cylindrical [50] shaped heads are used in helical microrobot designs, and several designs are shown in Figure 2a-c. In addition, the microholder shown in Figure 2d was also used as the head of the helical microrobot to perform micromanipulations [51]. It is significant that, when the performance of the microrobots with different rectangular head sizes is compared with the rotating frequency of the magnetic field, microrobots with larger heads can achieve higher maximum velocities when operated at high frequencies [52]. When the helical tail is fabricated with a non-magnetic material, a magnetically active head is essential to control the microrobot [53]. Microrobots having both head and tail fabricated using magnetic materials have also been reported because it improves the magnetic effect [54]. In comparison to helical microrobots with heads, microrobots without heads are less complex in fabrication due to the geometry as shown in Figure 2e [55]. Because of the nature of the helical structure, these microrobots can swim inside a fibrous environment which leads to in vivo experiments [56]. In addition, drugs can be coated on the helical microstructures to perform targeted drug delivery [57]. These capabilities have been further improved by culturing and transporting stem cells on a helical microrobot with holes in its body [58]. Their bodies are fabricated or coated using soft magnetic materials which can be activated using a rotating magnetic field [59]. Artificial helical microrobots pose challenges in fabrication therefore, an alternative approach of coating magnetic particles on naturally available helical structures which are available on Spirulina [60], Rhaphiolepis indica [61], and self-assembled liposomes [28] is performed. In addition, hollow helical microrobots can be fabricated, removing the Spirulina template by annealing, after coating the magnetic particles [62]. A microrobot which has two flagella similar to the geometric appearance of the Amphitrichous bacteria has been reported. However, the propulsion was performed using planar wave propagation which is not similar to the propulsion of a bacteria flagella [47]. A helical microrobot with two, three, or four flagella fixed to its head mimics the shape of the Lophotrichous bacteria as shown in Figure 3a.