# A Brief Review on Challenges in Design and Development of Nanorobots for Medical Applications
## 2 Challenges in the Design and Development of Nanorobots
### Powering and Navigation
#### 2.3.3 Magnetic Approach  
Generally, magnetic motors consist of a magnetic material and, based on the type of applied magnetic field, they can be propelled accordingly. Magnetic motors have gained popularity in biomedical applications due to their low strength, which is harmless on cells and tissues. In addition, magnetic fields can travel unimpeded through water, allowing them to pass unabsorbed into the human body [24]. This is one of the commonly used fuel-free propulsion methods [60]. Nanorobots were widely driven by magnetic propulsion, which has proper navigation features and remote actuation. The principle involved in magnetic propellers, such as helical propellers and surface walkers, is that their function relies on the response to a magnetic field. To achieve navigation in biological environments, they must have low-resistance magnetic fields [55; 84]. Various propulsion mechanisms can be accomplished under a magnetic field through various designs such as self-scrolled tubular helices, corkscrews, and radial magnetized helices [85].  
One of the most straightforward motion principles to mimic is the movement of a flexible flagellum, as it is a one-dimensional structure. A flexible beam or a sheet can be magnetically actuated when it is fixed to a magnetic head [80; 81]. Magnetic surface walkers are one type of magnetically powered nanorobot; they create a hydrodynamic reaction with boundaries near them and break the spatial symmetry to activate propulsion [86]. Magnetic surface walkers run under a spinning magnetic field in a roll or tumble motion [85]. Thin micro/nano magnetic swimmers have been engineered to achieve propulsion through asymmetric deformation of thin filaments embedded in the swimmer, and this was influenced by spermatozoa [87]. A versatile artificial flagella-based swimmer with a paramagnetic bead chain linked to DNA was demonstrated by [PERSON] et al. [88]. A cost-effective approach of using bio templated synthesis was demonstrated; this was  
Figure 4: Diffusiophoresis.  
applied in the fabrication of helical motors by incorporating spiral water-conducting vessels of vascular plants [89]. Magnetic helical swimmers contain a head and a helical body, which were inspired by helical bacterial flagella in which propulsion takes place based on a corkscrew mechanism [90]. A periodic magnetization profile along the sheet can be built to induce the propagation of bending waves through an external rotating magnetic field [91]. This method makes multimodal locomotion possible in millimetre-scale magnetoelastic robots [92]. Magnetic flexible composites at the nano level can be achieved by loading elastomers with superparamagnetic nanoparticles [93]. Artificial cilia were created by fixing rod-like magnetoelastic microstructures to a surface and were used to pump fluids [94]. Cilia are hair-like appendages found in tightly packed arrays on the surface of several eukaryotic cells, for example, in the lungs for the transport of mucus. The propulsion by cilia is attained by continuous beating along with a spatially varying lag phase [95]. However, cilia that are actuated by global magnetic fields beat in ideal synchrony without any lag, and therefore provide the required locomotion. The most important challenge faced in using a magnetically propelled nanorobot is that the magnetic field fades away with increased distance from the source. The source can be an electromagnet or a strong permanent magnet [24]. The strength of a magnetic field relates to many factors, including the size of an electromagnet or permanent magnet, which in turn is proportionally related to the price of the magnet. Besides considerations and preparations needed for such a huge magnet, the price can be in the range of hundreds of thousands of dollars and not economical for some applications.