## 3 Manipulation of Micro Actuator
### Optically Manipulated Micro-Actuator/Robot  
In addition to micro-actuator/robot actuation by an external chemical environment and magnetic field, the optical manipulation of micro-actuators/robots has been studied (Table 1). As a focused laser beam is irradiated onto an object, an optical force is generated in the momentum exchange between light and matter by the law of momentum conservation [79]. By making use of the light-matter interaction, optical forces can be used to trap, move, and rotate microstructures [80], and they have been successfully used in a variety of applications in physics, chemistry, and biology [81]. As the representative optical manipulation of micro-actuators/robots, optical tweezers are created from an electric field gradient provided by a highly focused laser beam, which allows objects of various sizes ranging from several nanometers to tens of micrometers to be moved and controlled in a non-contact, frictionless, and precise manner. For single-beam trapping, optical trapping regimes depend on the particle's size, thus affecting light wavelength. If an object is much smaller than the wavelength of the laser beam, it is usually called the Rayleigh regime, and an electromagnetic model could be used to better understand the trapping mechanism. This regime is regarded as a point dipole to satisfy Rayleigh scattering. In contrast, if an object is larger than the wavelength of the laser, it is referred to as Ray optics or geometric optics regimes. In this case, trapping force is independent of particle size. If the object size is compatible with the wavelength of the laser, the Mie scattering theory helps to bridge the gap between the two regions [82; 83; 84]. [PERSON] et al. used laser trapping force to realize the sophisticated operation of micro-oscillators such as the capture of beads, spring pulling, and displacement release [88]. Optical trapping provides an opportunity for the ultra-precise manipulation of micro-actuators/robots fabricated by TPP [92; 93]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Catalytic** & **Magnetic** & **Optical** & **Acoustic** \\ \hline \multirow{4}{*}{**Principle**} & Conversion & \multirow{4}{*}{Control of magnetized structures by external magnetic force.} & Control of transparent structures using optical force by light-matter interaction. & Acoustic streaming flow generation by acoustically oscillation. \\  & & & & \\ \cline{3-4}  & Simple fabrication. & Speed control. & Direction control. & Simple fabrication. \\  & Speed control. & Direction control. & Various movements. 1  & Speed control. \\  & High speed motility. & Applicability in vivo. & Remote control & High speed motility. \\  & No need for & Precise manipulation, & hard-to-reach areas. 2  & High speed motility. \\  & complex/bulky devices. & Various movements. 1  & Applicability in vivo. \\  & Real-time tracking. & Biocompatibility. & Real-time tracking. & Real-time tracking. \\ \hline \multirow{4}{*}{**Limitation and Requirement**} & \multirow{4}{*}{Low directional control.} & Limitation of materials. 3  & Micro-turbine, rotor, and actuator [85; 86; 87; 88; 89; 90]. & Actuator [91]. \\  & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Characteristics of actuation method of micro-actuator/robot. When an object trapped in a laser beam has a distinctive shape such as helical, the object tends to rotate due to optical torque derived from the light itself. The light carries both linear and angular momentum, and trapped objects can rotate through interaction with light due to the transmission of angular momentum. It has been proven through many studies that various movements (e.g., translation, rotation, sliding, and swing) of micro-actuators/robots are possible using optical tweezers [85; 86; 87; 88; 94]. For example, a micro-rotor was fabricated through TPP, and it could be optically manipulated using optical tweezers. The optical tweezers exerted flux for rotation, as well as a collecting force to trap the micro-rotor.