Discussion and Future Scope

Although there exist several sophisticated micro-robots, and many of them are currently used in practical applications, there are still many challenges to overcome in the existing micro-robots. The challenges include precise and safe locomotion and targeting and minimizing the power consumption for sensing, locomotion, data transfer, and computation. The micro-robot should not harm tissues and organs, should be biocompatible, and should have robust operation in flowing against bloodstream, respiration, peristalsis, etc. [31].

The use of a magnetic field for control and navigation of a microrobot is prevalent as wireless transmission of high force, and torque is possible with all 6 degrees of freedom. Still, there are some limitations of the magnetic field that need to be overcome. For example, the magnetic force decays exponentially with the distance from the field source. Therefore, the use of a high-strength magnetic field is required to control the micro-robot deep inside the human body. However, the use of a maximum static magnetic field is limited to 2.5 T and for a pregnant woman to 2 T by standard regulation, which limits the use of magnetic steering of a micro-robot inside the human body. The equipment required to generate a magnetic field for medical use of a micro-robot is generally bulky and costly. This bulky equipment is required to be kept closer to the patient, which may reduce the maneuverability of the operator and patient as well. The helical tail and flexible tail micro-robots generate a flow field during their propulsion. The flow field generated by the propulsion of the flexible tail is found to influence the accuracy of positioning of the payload [33]. Therefore, further studies are required for characterizing the frequency-dependent flow field for flexible tail as well as helical tail micro-robots. Research shows that gradient pulling is efficient than the propulsion of a helical propeller, but when limitations of the magnetic field and its generation are considered, then the helical propeller is far better than gradient pulling [51].

Bubble propulsion is not suitable for in vivo applications. As H202 is the main fuel for the bubble-propelled micro-robots, there are no in vivo applications as H202 is not a biocompatible fuel. The self-propulsion of a bubble-propelled micro-robot is limited by the availability of fuel. It can only be propelled until the fuel lasts. The propulsion of a catalytic micro-robot is limited to low-ionic-strength fluids only [60]. There is a requirement of finding alternative fuels and corresponding catalytic materials and reactions. It would be beneficial if in situ fluids can be used as fuel. Water and acids are readily present in a biological media; therefore, these can be used as in situ fuels. Materials like Mg-, A1-, and Znbased catalytic micro-robots propel by depleting protons in an acidic medium. So they can be readily used in an acidic environment without any external fuel injection. Also, these micro-robots can be used for measuring the pH of the fluid domain. However, on the other hand, the reactive material like Mg and Zn used in micro-robots dissolve in the working biological fluid; therefore, their working life is very short and cannot be used for long-time operations. The use of enzymes to decompose the fuel is also an alternative way to replace the metal catalyst from the micro-robot, to make it more efficient and biocompatible for medical applications [19, 63].

Electrophoretic propulsion is a good alternative for chemical- free, bubble-free directional propulsion of a micro-robot [79]. However, the propulsion mechanism of the phoretic effect is not clearly understood [82]. A biohybrid system faces a challenge to maintain the viability of the living cells attached to the micro-robots. Biocompatibility and proper adherence of the synthetic part to the biomicroswimmers are an essential requirement. In the case of taxis of a micro-robot, maintaining a driving gradient of the external stimuli near the micro-robot in a fluidic environment is a tough task to achieve.

Biodegradability and biocompatibility of micro-robot materials are challenges for their potential use in the medical field. If the material is biodegradable, then the micro-robot can dissolve in the biological media and disappear after completion of the operation. However, if the material is nonbiodegradable, then retrieving the micro-robot after the completion of the operation is necessary and is a major challenge. The biological environment inside the human body is very crowded; therefore, precise control of a microrobot is required. The micro-robot components should be soft and deformable so that they do not cause any injury to the components inside the body. Rigid and hard micro-/nanorobots are generally nonbiodegradable and can also cause problems like rupture of blood vessels and tissue injury inside the body. Soft materials like synthetic polymers and plant tissues have potential use in the fabrication of a soft and biocompatible micro-robot. Also, the coupling of natural biological material with synthetic micro-robots reduces the immune evasion and biofouling effects of the biological environment, thus maintaining the efficiency and longer working life of a micro-robot.

Swimming against the dynamic blood flow is challenging with limited power. A consistent power source for continued operation is still a milestone to be achieved. On the other hand, the TDD system is challenged by the low controllability on the release rate of the therapeutics.

Now it is possible to control a micro-robot with a wireless remote or just by the mouse of a workstation [27]. Challenges for future- generation miniaturized robots are to perform collective swarming and task completion by interacting with other micro-robots. Future micro-robots will mimic the natural intelligence of their biological counterparts with adaptable and sustainable operation and group behavior with swarm intelligence. In the future, micro-robots will be able to perform any task in coordination with thousands of other micro-robots in a swarm. This collective dynamics of microrobots is required to treat any large-size body parts, to deliver a large quantity of therapeutics to a target location, and to reduce the overall operation time. This kind of group communication and coordination is challenging, which needs to be overcome by future micro-robots. Although the collective dynamics of the micro-robots and microswimmers [108-111] is an active research area, further studies are required to understand the behavior of microswimmers, both theoretically and experimentally.

New studies on micro-robots would lead to the evolution of therapies and applications that are not yet conceived. For example, Janus micro-robots can be coated with stem cell membranes for cancer cell recognition for targeted photothermal therapy [14]. The development of micro-robots for medical applications will find its way from the lab to the real working environment if the safety of the patient is ensured. It needs to be shown that micro-robots can work in a dynamic environment inside the human body reliably and robustly.

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