## 6 Discussion and Future Directions  
Even though the microorganisms such as bacteria and cilia do not have a central nervous system, they demonstrate a form of intelligence during locomotion. Therefore, the locomotion techniques used by such living organisms are mimicked to propel microscale robots. Since bacteria can move their flagellum on a helical path, helical microstructures are used to mimic the motion inside a rotating magnetic field. Sperm can also move its flagellum on a helical path in addition to its oscillation on a 2D plane, but the mechanism used to move the flagellum is different from that of the bacteria. Cilia has a similar working principle to sperm, but its locomotion is performed by generating a metachronal wave. Even though the fish is a macro-scale living organism, its propulsion pattern can be mimicked in the microscale. Therefore, nanowires have adopted the BCF locomotion pattern inside an oscillating magnetic field. The length of the microrobot, frequency of the rotation or oscillation of the microrobot, and the magnitude of the magnetic field are analyzed with the swimming speed of the microrobot. In addition to these parameters, the helix angle is analyzed with the swimming speed of the helical microrobots. Swimming speed has shown an upward trend with all the parameters and both frequency and magnetic flux density have higher steepness. Apart from the analyzed parameters, the viscosity of the fluid and the wettability of the microrobot can also affect the swimming speed of the microrobot. In terms of fabrication, nickel, cobalt, and pure iron have been widely used to fabricate the structure of the microrobot and these materials also have magnetic properties. PDMS and PMMA are used as the matrix material and PDMS is popular in microfluidic applications due to its flexibility, transparency, and biocompatibility. For nanowires, flexible hinges are very important to generate undulatory motion. Porous silver is a good candidate for fabricating these hinges.  
Due to the challenges in fabrication techniques on the micro- and nanoscale, the exact mechanism used by the living organisms cannot be mimicked, but their propulsion method or pattern can be mimicked using an alternative actuating principle. For example, actuation using a magnetic field has ensured the untethered actuation inside the human body while reaching targeted locations. Helmholtz coil systems are widely used to generate the magnetic field due to their capability to generate a uniform magnetic field. In terms of magnetically actuated microrobots, multiple challenges have been identified. Namely lack of materials to fulfill biocompatibility and biodegradability, difficulty in selective control of multiple microrobots, and challenges in implementing a closed-loop control system are a few. At present, the microrobot is fabricated or coated with titanium to maintain biocompatibility, and hydrogels are used to fabricate biodegradable microrobots. In addition, implementing fully autonomous microrobots along with a closed-loop control system is currently being investigated and that challenge has not yet been addressed successfully. Therefore, novel research directions are still being introduced to address these challenges in the field of microrobotics amongst which most of these directions tend towards biomedicine.  
Advancements in the microrobotic field have directly influenced the medical sector. As a result, microrobots are most commonly designed to perform in vivo treatments and their locomotion is inside a fluidic environment. The movement of microrobots inside a fluidic environment involves a low Reynold's number fluidic regime. Therefore, non-reciprocal motion is essential to propel these fluidic environments. In conclusion, nature is a good resource to search for new reciprocal motions, and mimicry of such motions in microrobots has provided successful solutions for microrobotic propulsion inside microscale fluidic environments.