## 3 Manipulation of Micro Actuator
### Magnetically Manipulated Micro-Actuator/Robot  
The fabrication of various magnetic-responsive micromachines (e.g., actuators, rotors, and turbines) has been achieved by TPP, and diverse remote/precise manipulation methods have been devised. Since photocurable resin itself does not have ferromagnetic properties, additional modification is required to impart ferromagnetic properties to micro-actuators/robots. In order to introduce magnetism to a structure, Fe\({}_{3}\)O\({}_{4}\) ferromagnetic nanoparticles can be mixed with the photocurable resin [60; 61; 62], or magnetic materials can be coated onto the surface of a pre-fabricated structure [63; 64; 65]. For example, for the magnetic manipulation of micro-actuator/robot fabricated using ferromagnetic photocurable resin, an artificial bacterial flagella (ABF) micro-swimmer with a magnetically self-driven helical structure was fabricated [66]. In a different form, a micro-swimmer based on the corkscrew motion was fabricated using magnetic hydrogel [60] (Figure 2a,b). [PERSON] et al. synthesized photopolymerizable ferromagnetic polyethylene glycol-diacrylate (PEG-DA) and trimethyllopropane ethoxylate triacrylate (TMPE-TA) with an acrylate group at the end by mixing them with superparamagnetic Fe\({}_{3}\)O\({}_{4}\) nanoparticles [67]. Using this material, a hemocompatible micro-swimmer that could be driven by an external magnetic force and absorb/release drugs was fabricated. Moreover, they developed a new method to align doped magnetic nanoparticles along a magnetic field before TPP processing, and an ABF structure that exhibited a wobble-free motion was fabricated. The preparation of a magnetic photocurable resin by mixing magnetic particles has been extended to commercial materials such as SU-8. [PERSON] et al. prepared this by adding Fe\({}_{3}\)O\({}_{4}\) nanoparticles to a solvent of SU-8 and then diluting the mixture with \(\gamma\)-butyrolactone to meet the required viscosity for spin coating and ABF fabrication [68]. This ABF was not only able to be accurately manipulated by an external magnetic field--it also exhibited biocompatibility with a WST-1 proliferation assay. The shape of the fabricated structure was not only helical (e.g., ABF) but also a more complex 3D shape to satisfy various purposes. From [PERSON]'s research, it was found that a ferrofluidic photoresist by doping the synthesized Fe\({}_{3}\)O\({}_{4}\) magnetic nanoparticles into an original photoresist could be, and 6-(methacryloyloxy) hexanoic acid was used for the homogeneous dispersion of magnetic nanoparticles [71]. To examine the effectiveness of a ferrofluidic photoresist, a magnetic micro-spring structure was fabricated, and the performance was confirmed by changing the shape of the spring (e.g., elongation and bending) according to the change of the external magnetic force. [PERSON] et al. also fabricated micro-springs that were connected on one end to a sphere and on the other end to a substrate with an anchor after doping a photoresist with surface-modified Fe\({}_{3}\)O\({}_{4}\) nanoparticles [62]. Various motions such as bending, swinging, and stretching were achieved by applying magnetic gradients to this structure using an external ferromagnet. A rotor-shaped structure was fabricated using ferrofluidic resin composed of methacrylate and Fe\({}_{3}\)O\({}_{4}\) nanoparticles [61] (Figure 2c,d). Since the homogeneous dispersion of nanoparticles is a critical factor for successful rotor rotation in a strong magnetic field, 3-(trimethoxysily) propyl methacrylate (MPS) was used for the uniform dispersion of nanoparticles. Two micro-rotors were easily operated using external magnets, which provided  
Figure 2: Magnetically manipulated micro-actuators/robots.