# 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
#### 2.3.1 Fuel-Based Discussion  
Fuel-propelled motors work in the presence of a specific fuel (aqueous solution) that allows them to propel themselves using motor composition-dependent surface reactions. The propulsion mechanism varies based on the reactions that can lead to concentration, electrical potential, and bubbles generated at local gradients. Despite various forms of motion being achieved using fuel-based propulsion, defining a proper trajectory has been a key challenge. Defined navigation has been attained by external chemical sources; however, due to their incomplete degradation in the environment, research has shifted towards using fuels that are biocompatible, such as glucose, water, and acid.  
Interactions between the fuel and the motor generate chemical energy, which aids in the propulsion of chemically powered motors, and this is done with the help of a catalyst and an inert material. The catalyst decomposes the fuel, which acts as the source of power for the nanorobot. The process of self-electrophoresis (see Figure 3) involves the generation of motion without the usage of external factors, which is possible as chemical gradients in the system develop a field for themselves [67]. Bimetallic nanomotors typically display self-electrophoresis, and the first of its kind was Au/Pt, created by [PERSON] et al. [68]. By adopting a tubular configuration and using Au/Ag alloy instead of Au in the cathodic section, the speed of these motors was dramatically improved. A speed of 150 mm/s was demonstrated by [PERSON] et al. [69]. Using a nanowire-based motor incorporating carbon nanotubes, the speed was further increased to 200 mm/s [70]. The [PERSON] group also developed methods for controlling the movement of tubular motors by modulating the platform's temperature and electrical potential [67; 71]. Alternate fuels, such as bromide and iodide for self-propulsion, were also demonstrated by the [PERSON] group [72]. Another study by [PERSON] et al. presented spherical-shaped robots and demonstrated that their speed was dependant on fuel concentration and diameter [73].  
In addition to chemically active motors, synthetic motors have also been developed based on diffusiophoresis mechanisms (see Figure 4). The catalytic part generates the reaction products, and this provides a repulsive force that propels away from the catalyst [74]. Bubble-induced motion is another standard propulsion method that uses gas bubbles to provide the force for movement [75; 76]. In this scenario, the decomposition of fuel by the catalyst results in the formation of gas bubbles, which provides the required thrust for the motor. Bubble propulsion has been achieved by various geometries like Janus motors, micro/nanotubular motors, and micro/nanoshells [72; 73]. Motors based on Pt, Ag, and MnO\({}_{2}\) were also manufactured to achieve bubble propulsion in H\({}_{2}\)O\({}_{2}\)[77]. Lastly, buoyancy-driven motion is another propulsion mechanism developed by [PERSON] et al. [78; 79]. However, the above-mentioned motors depend on H\({}_{2}\)O\({}_{2}\) for their propulsion, which is incompatible with biological environments. To overcome this challenge, water-driven bubble propulsion was developed by [PERSON] et al. [80]. The motion of the motor takes place when H\({}_{2}\)O\({}_{2}\) breaks into water and O\({}_{2}\). Plant-tissue-based motors demonstrated  
Figure 3: Self-Electrophoresis.  
self-propulsion due to the catalase present; this was achieved using various materials like potato tubers, mushrooms, carrot roots, and millet seeds [81]. [PERSON] et al. recently demonstrated chemotherapy drug delivery with magnetite-coated mushroom-based nanomotors based on their catalytic and magnetic properties [82]. [PERSON] et al. confirmed the possibility of the nanomotor application of enzymes, such as catalase itself [83]. This result is a ground-breaking step in the direction of biocompatible nanomotors for applications such as catalyst isolation and drug delivery in the biomedical field.