## 3 Manipulation of Micro Actuator
### Optically Manipulated Micro-Actuator/Robot
The screws with smaller pitches and thinner blades, as well as optical tweezers with larger numerical apertures, contributed to the increase in rotational speed. Two chiral propellers were fabricated, and these counter-spinning propellers were trapped by holographic optical trapping [89]. Hydrodynamic synchronization, a fundamental physical phenomenon by which self-sustained oscillators interact through perturbations in the surrounding fluid and converge to a stable synchronized state, was achieved by precisely controlling the relative torque of the two propellers. The precise fabrication of microfluidic components by TPP and the precise flow control of the precisely fabricated structure by optical control were simultaneously achieved as well. A combination of micro-optic components (e.g., waveguide, diffractive, and holography) and actuating parts makes it possible to control the micro-optical component and precisely manipulate the actuating part. A free-standing waveguide fabricated using TPP can be manipulated in any direction, thus making it possible to transmit/receive light in a region that is difficult to reach [99]. By incorporating an optical waveguide into an adjacent micro-rotor, and optical fiber can be coupled to the waveguide and a micro-rotor could be controlled with precision [90]. The precise manipulation of a micro-actuator/robot is also possible via the optical control of microscopic diffractive optical elements. Furthermore, combining optical tweezers with digital holography makes it possible to create a large number of high-quality optical traps, and it is expected that the real-time 3D manipulation of multiple objects at the same time will soon be possible [100; 101; 102]. Figure 3: Optically manipulated micro-actuators/robots. (**a**) SEM image of the lobed micropump and images of optically driven micro-pump [85] (reproduced from [85] with permission from American Institute of Physics (AIP) Publishing); (**b**) schematic of optical driving of a metallized micro-rotor [86]; (**c**) sequential optical images of a rotating micro-rotor (scale bar: 10 \(\upmu\)m) [86]; (**d**) schematic of driving mechanism of a metallized micro-rotor with repulsive force [86] (reproduced from [86] with permission from Japan Laser Processing Society); (**e**) schematic of mechanism of a spiral phase plate (SPP) converting a plane wave into a helical wave [87]; (**f**) design of a turbine-like micro-rotor [87] (reproduced from [87] with permission from AIP Publishing). The control of a micro-actuator/robot's motion using optical force has many strengths, but some considerations must be considered during its application. Optical transparency is required to manipulate a micro-actuator/robot using optical controls such as optical tweezers [82; 103]. Generally, objects involved in vivo applications exhibit a lower optical transmittance, and the application area can be limited in bio-related applications such as biomaterials or prototyping tests. If a transparent structure can be fabricated using a novel photocurable resin with the steady development of photoresists, the scope of the applications of micro-actuator/robot motion control using optics will be broadened. Additionally, the optical force manipulation of large objects in a liquid medium can suffer from high drag, during the dynamic effect of optical force is much less pronounced. These limitations can be improved through the continuous development of optical drives such as plasmon-enhanced forces [82; 104]. [PERSON] et al. fabricated a stimulus-responsive nanocomposite/hydrogel actuator with a defined 3D geometry using programmed printing density via TPP. This structure showed ultrafast/reversible shape morphing through plasmon heating. The dynamic-static bilayer helices of PNIPAm-Au nanorods nanocomposite gels were fabricated using a newly synthesized PNIPAm macro-crosslinker. Light-driven manipulation induced ultrafast/reversible shape changes based on body deformation.