# A Brief Review on Challenges in Design and Development of Nanorobots for Medical Applications
## 3 Challenges in the Application of Nanorobots  
The applicability of micro/nanomotors for in vivo purposes, such as drug delivery and cargo transport, must overcome immune system clearance so as not to be lost as foreign material. Therefore, aside from increasing drug delivery and therapeutic effectiveness, overcoming the host's immune system is crucial [22]. Furthermore, nanorobots need to provide therapy with a high degree of precision when contacting the site; therefore, the motor must be able to penetrate through barriers provided by cells and tissues to distant areas of the host organism [126]. In order to be used effectively in various biomedical applications, the design of micro/nano vehicles is of vital importance in performing the given mission. Consequently, the structure, form, and propulsion source must be carefully considered in order to project the nanorobot successfully as an effective sensor or delivery system for theranostics (combination of the terms therapeutics and diagnostics) applications [60]. In traditional drug delivery systems, the small release of therapeutic molecules--ideally to unhealthy cells--is a severe restriction. Due to their directional movement, and tissue retention and penetration ability, autonomous micro/nanomotors open up possibilities for active delivery to the site of interest. For this reason, the technique generally involves encapsulation of the drug/gene/protein into micro/nanorobots, propelling them to the specified destination and triggering their release using physiological or external parameters [127]. Some medical applications of nanorobots are shown in Figure 5.  
[PERSON] and colleagues demonstrated an initial instance of using an artificial catalytic micromotor for the delivery and release of drugs; the group showed the transport and release of the drug molecule to the required targets [128]. Although these systems are promising, H2O2 is not biocompatible. Self-propelling nanotools have the immense ability to deliver surgical operations at an organism's remote position. The use of ultrasound-triggered  
Figure 5: Medical applications of nanorobots. microbullets has been demonstrated in biolistic circulation and precision nanosurgery [51]. Such advances illustrate the power achieved in micro/nanomotor propulsion and signify a noteworthy phase in the realization of prominent nano surgical devices. The most notable use of such micro/nanomotors may be their use in vivo. This can be said based on the enormous uncertainty involved and the challenges that must be overcome. In vivo applications require extremely small motors for an enhanced capacity to penetrate. Furthermore, addressing the biocompatibility problems related to artificial motors is of utmost importance [129]. As mentioned in previous sections, fuel-based and organism-based nanorobots were developed for biomedical applications. Nevertheless, the in vivo application of micro/nanomotors is in the budding process, and relentless effort has been devoted to the steering, storage, imaging, and tracing of micro/nanomotors in this setting [130]. In dentistry, nanorobots can be applied to improve dentine hypersensitivity and teeth durability and can be used to carry out orthodontic treatments such as realigning and straightening irregular teeth. Significant applications of nanorobots in the dental field include anesthesia and osseointegration; carrying dentine tubules for blocking tooth hypersensitivity; contributing to orthodontic treatments by allowing painless tooth up-righting, rotating, and vertical repositioning; or acting as nanoneedles [17]. These applications have been possible due to the polymeric materials available today, which include bio nanocomposites prepared with polymers, such as biocompatible collagen, alginate, silk, poly (lactic acid), poly (glycolic acid), poly (lactic-co-glycolic acid), and poly (caprolactone).