## 3 Manipulation of Micro Actuator
### Magnetically Manipulated Micro-Actuator/Robot
(**a**) Fabrication of superparamagnetic hydrogel micro-robots by TPP [60]; (**b**) SEM image of helical micro-swimmer robot [60] (reproduced from [60] with permission from John Wiley & Sons, Inc.); (**c**) procedures of remotely controllable micro-turbine fabrication [61]; (**d**) optical microscopy images of the micro-turbine in a circumgyration cycle [61] (reproduced from [61] with permission from John Wiley & Sons, Inc.); (**e**) ciliary micro-robots fabrication process [63]; (**f**) SEM images of fabricated ciliary micro-robots with and without a mask structure (scale bar: 100 \(\mu\)m) [63]; (**g**) time-lapse images of the cilia micro-robot manipulation (translational motion and rotational motion) [63]; (**h**) schematics of magnetic actuation for the ciliary micro-robot with power stroke (Tp) and recovery stroke (Tr) [63] (reproduced from [63] with permission from Springer Nature); (**i**) electroless magnetite plating of a polymeric micro-rotor fabricated by TPP [69]; (**j**) subsequent frames recorded at 50 fps with different rotation speeds (200 and 400 rpm) [69] (reproduced from [69] with permission from MDPI); (**k**) fabrication process of 3D magnetically driven micromachine [70]; and (**l**) SEM image of polymeric micromachine with an electroless magnetic nickel–phosphorus layer [70] (reproduced from [70] with permission from American Chemical Society). a breakthrough for the fabrication and remote control of components inside microchannels for microfluidic applications. However, this process may cause complexity in the synthesis of a ferromagnetic photocatalytic resin or may cause extra issues during the polymerization. As a magnetic nanoparticle could not be maintained for a long fabrication time, the stability and homogeneous dispersion of nanoparticles in a photocurable resin should be considered for material preparation. In addition, the surfaces of structures manufactured using ferromagnetic photocurable resins have a relatively higher roughness than structures processed with photocurable resins without magnetic particles. This can cause low resolution and unexpected disturbances under external forces. By using an appropriate solvent, it is possible to overcome these limitations through the uniform dispersion and stabilization of nanoparticles. Instead of preparing a ferrofluidic photocurable resin by mixing magnetic materials into a photocurable resin, there is another method of imparting a magnetic function: magnetically coating the outer surface after the 3D structure's fabrication using a deposition method. In the initial form of a magnetic-driven actuator, a simple porous structure can be coated with a nickel/titanium (Ni/Ti) bimetallic layer as a magnetic material to induce actuation in a magnetic field for a targeted cell transportation system or a drug delivery system [65]. Cylindrical and hexahedral micro-robots can be manipulated by an external magnetic field gradient in the x-direction when they are aligned with the \(z\)-axis. This simple porous structure can be expanded into a complicated 3D shape like, a helical structure. [PERSON] et al. coated an Ni/Ti bimetallic layer onto an ABF structure using e-beam evaporation, thereby giving magnetic properties to a structure for remote control [64]. They encapsulated an ABF inside a droplet using a flow-focused droplet generator, and they remotely manipulated the cells and droplets within the microfluidic system with an external magnetic field, thereby suggesting the possibility of expansion to microfluidic system applications. The effect of an anisotropic magnetic shape on ABF structures under magnetic fields has been studied [50; 66]. It has been found that as the helix angle of a structure becomes smaller, the angle between the external magnetic field and the axis of the ABF structure tends to decrease, and a higher magnetic field frequency allows for a smaller angle between the axes of the ABF and rotation.