## 3 Manipulation of Micro Actuator
### Optically Manipulated Micro-Actuator/Robot
In [PERSON]'s research, a micro-gear was kept untethered after fabrication, while the shaft was connected to the substrate, and a continuous laser was not required to keep the structure to a fixed position [95]. The rotational motion of the gear was achieved by trapping a tooth of the micro-gear with optical tweezers and making the laser beam have a controllable circular path. A microstructure with a metalized surface using electroless plating was driven with ultralow-power due to ablation and repulsive force with the substrate [86] (Figure 3b-d). The net repulsive force exerted on a metallic surface of micro-rotor blades is larger than the attractive force applied to a dielectric micro-rotor. A metalized micro-rotor can be driven efficiently by optimizing the radiation pressure exerted on the tilted blade and topology of the blades. This micro-rotor is expected to have a wide range of applications due to its low energy consumption, high performance, and ability to be grafted to a lab-on-a-chip. [PERSON] and [PERSON] succeeded in tweezing and rotating a micro-rotor using an optical tweezer [96]. They found that the amount of rotational momentum depended on where the micro-rotor was focused, and the rotational speed depended on the light scattering. In addition, the shape of the micro-rotor affected efficiency, and the efficiency of the sprinkler-shaped micromachine exceeded that of helical and propeller-shaped micro-rotors. A turbine-like symmetrical micro-rotor was driven using a spiral phase plate (SPP) [87] (Figure 3e,f). Using an SPP, a plane wave was converted into a helical wave that carried orbital angular momentum while giving the SPP an inverse angular momentum that could exert a torque to actuate a rotation of the SPP and induce symmetrical micro-rotor rotation. Under an optical tweezer system, a micro-actuator/robot not only stabilizes to a fixed position but also generates high torque when non-absorbing and non-spherical micro-actuators/robots are incorporated by the orbital angular momentum into a Gaussian laser beam. By integrating the TPP technique with temperature-responsive materials, the fabrication of a structure that responds to stimuli differently depending on the amount of light is possible. [PERSON] et al. fabricated a 3D hetero-microstructure using poly(N-isopropyl acrylamide) (pNIPAM) that exhibited a substantial response to changes in temperature close to its low critical solution temperature [97]. During the laser direct writing process, it was found that the material parameters could be changed on-demand in a single resist formulation through the variation of the local exposure dose, and a sophisticated 3D architecture with a large amplitude and complex response under light exposure was fabricated. [PERSON] et al. successfully fabricated a 3D Near-infrared (NIR) light-driven hydrogel micro-actuator with a fast response using TPP and macroscopic pNIPAM/nano-Fe\({}_{3}\)O\({}_{4}\) hydrogels. This micro-actuator showed fast (response time \(\approx\) 0.033 s) and reversible responses that could \"close\" or \"open\" in response to light. By changing the laser focus and power, the distance between the two arms of the micro-actuator could also be controlled. Moreover, photoreactive hydrogel micro-actuators can have numerous potential uses in biomedical applications due to their biocompatibility. Considering that Fe\({}_{3}\)O\({}_{4}\) nanoparticles can also work magnetically, this actuator can be expected to operate under optical and magnetic control systems [98]. Microstructures of various shapes can be used in microfluidic applications such as microscale pumps and mixers. A micro-pump was fabricated based on the generation of net flow using two-lobed micro-rotors and dual optical tweezers [85] (Figure 3a). The net flow rate is proportional to the rotational speed of a micro-rotor, and this proportion was utilized through a micro-pump by trapping a micro-screw using optical tweezers and rotating a blade connected to a micro-screw, which served as a microscale mixer [94].