## 5 Conclusions  
TPP is in the spotlight as a new field of micro/nanofabrication processes in that it enables the fabrication of precise and complicated structures with a high spatial resolution and a flexible design. Much effort has been made to develop and improve innovative micro-actuators/robots that can be remotely driven. To impart a specific function to a micro-actuator/robot, structural and material modifications can be performed. Various types of micro-actuators (e.g., transporter, mixer, pump, rotor, and swimmer) have been fabricated using TPP, and efficient activation is possible when using diverse driving methods such as chemical, magnetic, optic, and acoustic forces. First of all, a catalytic micro-tubular actuator converts chemical energy into mechanical energy for jet propulsion. However, the commonly used rolled-up method is not able to make an asymmetric tubular microstructure [8; 9; 54; 55; 56; 57], and this can be resolved by laser-direct writing (LDW) processes like TPP. A rocket-shaped asymmetric micro-tubular actuator was found to possesses successful dynamic motion generation through the chemical decomposition of an external chemical, resulting in actuation with a high speed [58]. In addition to chemically catalytic actuation, the remote control of a microscale actuator/robot can be achieved using magnetic, optical, and acoustic manipulation. To manipulate a micro-actuator/robot using a magnetic force, magnetic nanoparticles (e.g., Fe\({}_{3}\)O\({}_{4}\)) can be added to a photocurable resin in order to control the movement of a 3D microscale structure (e.g., microturbine, micro-rotor, and actuator) via an external magnetic field [60; 61; 62; 66; 67; 68]. However, this material has issues including its stability, homogeneous dispersion during long fabrication processes, and scattering during the processing. In another way, by imparting ferromagnetic properties to a microstructure is possible when using an Ni/Ti bilayer coating, which is the most widely used structural treatment for magnetic manipulation [63; 64; 65]. However, Ni is generally regarded as inappropriate to be applied to bio-related applications due to its non-biocompatibility, and it should also be considered that a bulky operating system is required for magnetic manipulation. When adjusting a micro-actuator using optical manipulation, unlike magnetic manipulation, no additional treatment is required. It is possible to successfully and optically control an actuator's movement in a non-contact/non-invasive manner, and the actuator, pump, and mixer can be grafted into a microfluidic system [85; 86; 87; 88]. In addition, it is able to fabricate other optical components (e.g., waveguide) near an actuator by making use of the advantages of TPP, thus making the efficient actuation and control of a micro-actuator possible [90]. However, this requires the optical transparency of a structure, so the range of applications is limited. Therefore, the development of a photocurable resin that could fabricate a transparent feature is required, and it is also critical to consider the enormous cost and effort, due to complicated optical systems, that is required for optics-based manipulation. The acoustic-based actuation of a micro-actuator is considered to be promising technology due to its non-invasive and contactless nature [91]. However, due to the weak mechanical connection between voxels, it has limited operation capabilities under an acoustic field. Though critical challenges have remained for stable manipulation under acoustic forces, it is expected that potential applications will be broadened by solving issues such as the reinforcement of mechanical strength. Though there are still issues to be resolved for the efficient manipulation of a micro-actuator. However, due to the merits of 3D precision processing including efficient actuation and control through functionalization, TPP-based micro-actuators will soon be established as next-generation, highly efficient, and ultra-precise actuators.