## 4 Materials and Fabrication Methods  
When microrobots are actuated using a magnetic field, it is essential to fabricate at least one part of the microrobot using a magnetically active material [120]. Apart from that, other non-magnetic materials are also required to maintain other features of the microrobot such as the structural rigidity, flexibility of fins, transparency, etc. Therefore, several materials used to fabricate the microrobots are summarized with their key features in Table 3.  
Photolithography and rapid prototyping are the most common fabrication techniques used in fabricating microrobots [124]. Expensive equipment, complex facilities, and tedious tasks are required to perform photolithography processes whereas rapid prototyping techniques have a less complicated approach [125; 126]. In terms of mimicking the bacteria flagella, helical microrobots are used. Due to the 3D nature of the helical structure, conventional 2D photolithographic methods are not suitable [127]. However, the self-scrolling technique enables the creation of 3D helical structures using 2D fabrication techniques [20]. That apart, glancing angle deposition [28] and direct laser writing [15] can be used to fabricate helical structures at the microscale. Glancing angle deposition is a physical vapor deposition method and can be performed in one step [69] whereas direct laser writing is performed on a negative photoresist where it is required to remove the unpolymerized materials using a developer [51]. In glancing angle deposition, magnetic materials can be deposited but in direct laser writing, magnetic materials should be coated [55]. In addition, helical microrobots are created by modifying the naturally available helical structures by coating magnetic nanoparticles, and this method is known as the bio-template method [60; 61]. Electrochemical co-deposition using molds is also used to fabricate helical microrobots in the nanoscale. These microrobots need to be coated with magnetic nanoparticles at a later stage of the process [128]. Nanowires are another type of frequently used microrobots that mimick the propulsion mechanism of living organisms. Molding is widely used to fabricate these microrobots at large scale due to their 2.5D shape [37]. For magnetically actuated microrobots, magnetically active materials are required to generate a propulsive force or torque. Therefore, there are two ways to embed magnetic materials in microrobots. The first method is to fabricate the structure of the microrobot using magnetic materials. The second method is to coat the microrobot using magnetic nanoparticles. E-beam deposition [89] and electroless deposition [60] methods are two methods used to deposit soft magnetic materials on non-magnetic materials. In addition to coating, fabrication of hinges to combine the linkages of nanowires is also important \begin{table}
\begin{tabular}{c p{142.3 pt}} \hline \hline
**Material/s** & **Key Features** \\ \hline Pure iron, ferric oxide, ferro-ferric oxide, nickel, cobalt & Used as the magnetic particles or magnetic material to generate a magnetic moment [38]. \\ \hline Polydimethylsiloxane (PDMS) & Used as the matrix material [121]. \\  & Flexible and transparent material used for biomedical applications [122]. \\ \hline Polyurethane (PU), polystyrene (PS), polymethyl methacrylate (PMMA), epoxy & Used as the matrix material [122]. \\ \hline Titanium & Improves biocompatibility [63]. \\  & Prevents oxidation of nickel [119]. \\  & Facilitates the surface functionalization of the structures with other medical and biological substances such as liposomes [57]. \\ \hline Silver & Used to fabricate hinges by partially dissolving in hydrogen peroxide [37]. \\ \hline Polypyrrole (PPy) & Flexible material and used to fabricate tails of nanowires [113]. \\  & Biocompatible [123]. \\ \hline Gold & Used to fabricate solid linkages in nanowires [84]. \\ \hline Hydrogels & These are used to fabricate biodegradable microrobots [114]. \\ \hline \hline \end{tabular}
\end{table}