## 3 Manipulation of Micro Actuator
### Magnetically Manipulated Micro-Actuator/Robot
The micro-swimmer, which is composed of four rigid segments that are each connected to the next segment by springs, was found to be able to return to its initial state after disturbance, thereby maintaining strong structural robustness. This micro-swimmer was able to achieve undulatory locomotion along with the directions guided by an external magnetic field in a low Reynolds number regime, and the swimming direction of the micro-swimmer was well-controlled. In addition, when fabricating a magnetic-driven actuator/robot for remote control, it is possible to produce an actuator capable of real-time tracking. A micro-actuator can colored after printing for tracking purposes, but multiple functionalization steps are required using the general method, and there is are limitation factors such as quenching that cause the color to decrease over time [75]. It is possible to fabricate a one-step, magnetic-driven, helical micro-actuator using TPP, and this structure can include structural colors by adding arrays of blocks with dimensions of approximately 1 \(\upmu\)m or less on the flat surface of the head part [76]. A metallic or dielectric material is coated onto a patterned surface for a distinct color response under visible light, and it provides a permanent and qualitative method for visually locating and identifying actuators when driven under a magnetic field. The real-time tracking of a rotating helical actuator at a low frequency is possible based on a color change with an angular orientation with respect to a light source, and rotational angular changes in horizontal and vertical planes can be detected and quantitatively identified by observing the expressed colors. Despite its various strengths, magnetically-manipulated fabrication has many requirements that must be considered in order to make efficient structures. The consideration of mechanical strength is essential in fabricating 3D magnetic structures using ferrofluidic photoresists [68]. A structure with a low mechanical strength cannot achieve a desired performance in harsh environments such as viscous and narrow-gap conditions. Hybrid micro-actuators/robots have shown excellent performance in remote operation while maintaining high mechanical performance. A nickel and phosphorus (Ni-P) alloy has the advantages of a high hardness, wear/corrosion resistance, and good magnetic sensitivity [70] (Figure 2k,l). An Ni-P composite was introduced into a micro-actuator/robot using an electroless plating process, and it not only allowed for the movement of the robot to be remotely controlled but also gave a favorable mechanical performance. In addition, a modification to structural strength and recovery can be induced through the diversification of coatings method, resulting in the expansion of the functionalization of the structure. A hybrid structure with a thin alumina layer on the outer surface by atomic layer deposition (ALD) was shown to have better mechanical strength and recoverability than that without alumina coating [77]. By removing the backbone of the polymer after the ALD process, a hollow metallic structure with high strength and ductility but small size and weight can be obtained [78]. As the intensity of the magnetic field is considered relatively low and biologically harmless, the remote control of a micro-actuator using a magnetic field has been regarded as a good candidate in bio-related fields among various actuation methods. However, regarding Ni/Ti bilayer coatings, Ni is harmful heavy metal to living organisms, and biocompatibility issues may arise. Therefore, the treatment of biocompatible alternatives (e.g., ones that are biodegradable or bioresorbable) is still in demand for in vivo applications.