# A Brief Review on Challenges in Design and Development of Nanorobots for Medical Applications
## 2 Challenges in the Design and Development of Nanorobots
### Actuators  
Various types of actuators are currently being researched to be employed in the field of nanorobotics [36]. One of the proposed actuators is constructed using biologically based components and has a mobile member that moves substantially linearly within the actuator as a result of biomolecular interaction between these components [37]. For nanoscale mechanical systems, these actuators may be used to pump fluids, open and close valves, or to provide translational movement [6; 7; 8; 9; 17; 18]. A flagella motor is one popular device that is used for the propulsion of a molecular machine, and an alternative for nanomotors is Adenosine triphosphate (ATP) [38]. In other scenarios, the development of transitional devices by DNA and RNA has also been demonstrated. The use of Carbon Nano Tubes (CNTs) as conductive structures allows for electrostatically driven motions that provide the necessary forces for nano-manipulation [36]. Polymers and gels that can respond to external signals such as light or temperature are known as smart soft materials. These materials, being an integral part of microrobots, use external control signals to actuate a response and are known as controlled actuators [39]. Mobile microrobots use Liquid-crystal elastomers (LCEs), which are a type of thermresponsive and/or photoresponsive soft materials [40]. LCEs transition from an orderly (often nematic) phase to a disordered (isotropic) phase as the temperature rises above the critical value and/or the light stimulates a molecular photoswitch. The temperature-dependent or light-dependent molecular reorientation causes a transition in the material's isovolumetric structure [41]. Therefore, stimulus-responsive soft materials have advanced functionalities and locomotion strategies for microrobots [42]. Smart soft materials can be used as onboard actuators to boost a microrobots control or to introduce different locomotion strategies. For example, a layer of thermoresponsive Poly(N-isopropylacrylamide) (PNIPAM) hydrogel can induce a change in shape in response to infrared light and propulsion characteristics of magnetic microrobots [43]. Microobots based on smart materials will simulate how micro-organisms travel through changes in body shape, with external fields influencing the internal actuating forces. Repeated changes in temperature around the LCST trigger an asymmetric cycle of shrinkage and swelling, allowing temperature-driven walking [44]. However, the temperature change of the whole environment creates a significant issue for this technique of actuation. Light can power microrobots made of photoresponsive soft materials; for example, light-powered motion can be attained by microribbons made up of a thin sheet of PNIPAM hydrogel loaded with nanorods with a 2 nm gold layer [45]. Excitation of plasmonic nanoparticles by near-infrared light heats the PNIPAM microstructures, and this brings about irreversible deformations and disrupts their non-equilibrium actuation [46]. On the other hand, locomotion at a small scale is achieved by actuating photoresponsive LCEs [47]. For example, an LCE can be microfabricated into a structure with asymmetric rigid legs that are driven by light to walk directionally on pre-patterned surfaces with asymmetric friction [48]. Control mechanisms that incorporate the help of light exhibit spatial and temporal resolution by selective control and actuation of various parts of the nanorobot [49]. In this way, biomimetic locomotion techniques are made possible with the use of responsive materials that can also be used for cell and drug delivery functions. The delivery of cargo can be achieved by the response of PNIPAM and various other smart gels. For example, cells can be caught and released by self-folding microstructures by means of temperature-controlled folding and unfolding, and the microstructures can shift by magnetic field control [50]. Drugs and molecules may be loaded into the microrobots and released in response to an external signal or an environmental cue at a location [51].