# Light-Controlled Microbots in Biomedical Application: A Review
## 3 Challenges  
For a light-actuated microbot, many challenges are encountered, but they can be mainly classified into two main categories- fabrication limitations and control difficulties. For microlevel fabrication, soft photolithography is used. The conventional method of photolithography involves creating thin masks on top of a photoresist and then exposing, developing, and etching. However, this limits the proper generation of a complex three-dimensional structure, which is very necessary for biomedical applications. On the other hand, in Direct Laser Writing (DLW), the two-photon polymerization (TPP/2 PP) technique is utilized as it gives more flexibility to the scaling of the 3D model. The 2 PP method gives good accuracy and resolution up to a few hundred nanometers [116], but with the growing number of microbots, the time to fabricate them increases rapidly. Moreover, for biomedical applications biocompatibility has to be ensured, otherwise, it could pose a risk of toxicological hazards [70]. The light-actuated microrobots are used in the first place to reduce the toxicity of high-dose drugs, but the carriers themselves can pose the risk of toxicological hazards. There is a limitation in the size of the microcarriers due to the fabrication facility and the requirement of loading enough drugs to come into effect. Moreover, if microrobots are left inside the body after their purpose is fulfilled, they could accumulate in delicate organs, causing chronic inflammatory reactions [117]. To avoid this, there could be some solutions such as the use of biocompatible materials, developing retrieving techniques such as using suction tubes after the operation for reuse or removal of the microrobots, otherwise exposing them to foreign stimuli to degrade [118], such one method could be the irradiation of tightly focused lasers. Sometimes, it is necessary to functionalize the surface of microbots to add features such as biosensing [119] or to carry out specific tasks such as heating [97] the micro tools; especially in magnetically actuated microbots, surface functionalization and selection of magnetic material are very important when it comes to biomedical applications [3]. Proper choice of biomaterial is also critical in designing a light-controlled microbot. The use of light to control microbots could also be tricky. Visible light cannot be used for in vivo applications due to the penetration depth limitations of the wavelength. Moreover, high-powered and concentrated NIR light used in the concurrent studies poses the risk of photodamaging the cells. [PERSON] et al. showed that a laser with 19 mW power irradiated for 1 min hardly affects the appearance of the growth time of _S. cerevisiae_, but when the laser power is increased to 25 mW, a significant difference in time is observed [112]. The predicament here is that if the laser power is reduced, then the maximum velocity of the trap is reduced. This can happen because of the increased drag force with the movement of the particles in the trap. A detailed discussion on the wavelength dependency and fabrication challenges is given in the next section. Some suggestions have been made to integrate alternative control systems into one microbot design to overcome these limitations [29]. A brief comparison between these control mechanisms is presented in Table 1.