## 4 Perspectives  
Due to rapid technological progress, research is currently being conducted under ideal conditions, and it is necessary to study motion control in environments similar to actual environments. For example, to realize an actuator that operates in vivo, further studies on the actuation of micro-actuators in artificial microfluidic chips with high viscosity and flow are needed in order to properly prepare for real-life applications. Another new approach suggests the possibility of solving issues that may arise in existing actuation through multi-material TPP processing [106]. Multi-material-based actuators utilizing materials ranging from hard to soft/elastic materials are able to move in complex environments. Actuators/robots that are operated at larger scales by utilizing both soft and rigid materials can improve performance, handle uncertainty, and reduce control requirements, thus enabling efficient actuator movement in real environments. Micro-actuators/robots with soft materials that are combined with rigid materials can facilitate large movements in narrow spaces due to increased compliance [107; 108]. Hence, the concept of multi-material-based fabrication for a diverse range of materials has shown potential for the improvement of robot functionality and complexity. It is expected that 3D multi-material micro-actuators can ultimately move closer to the complexity found in living organisms at the same size scales. The use of multiple materials with a wide range of moduli can create complex mechanisms in small quantities. More complex designs are possible by increasing the number of printing steps, and they can be expanded according to the number of materials instead of the number of alternating layers due to the ability to print inside existing structures [107; 109; 110]. Hence, the fabrication of 3D multi-material-based micro-actuators is still challenging because it requires multiple steps in addition to the general fabrication method. However, TPP makes it possible to overcome this limitation because its structure fabrication is based on laser-direct writing. In previous studies, this method was used to fabricate artificial scaffolds for cell culture and printing [111; 112] and to induce local mechanical/chemical property changes by controlling the degree of crosslinking of different proteins on hydrogels, which leads to cell adhesion and differentiation on TPP-fabricated scaffolds [113; 114; 115]. As an example applied to an actuator, [PERSON] et al. printed two materials with three orders of difference of Young's modulus in a consecutive cycle using the TPP technique [106]. They fabricated a high-strength adhered hybrid structure with a layer accuracy of less than three microns by integrating a rigid material  
Figure 4: Acoustically manipulated micro-actuators/robots. (**a**) SEM image of the device; (**b**) experimental setup for acoustic manipulation; (**c**) schematic of the bubble-first vibration mode; (**d**) SEM image of fabricated spinner; (**e**) image of armed micro-bubble (AMB) spinning at 175 Hz; and (**f**) image of moving AMB [91] (reproduced from [91] with permission from American Physical Society). and a soft elastic material, ultimately making a structure capable of more than 200% deformation. The fabrication of a 3D-printed, 2-mm wingspan flapping wing with a large deformation potential was possible using a multilink multi-material mechanism. [PERSON]'s group developed femtosecond, laser-programmed, 3D, micro, artificial musculoskeletal system robot fabrication composed of multiple materials [116]. This structure was made by two materials of different stiffness values--SU-8 (stiffer) as a skeleton and a pH-responsive BSA (bovine serum albumin) protein (softer) as a smart muscle. The successive on-chip TPP strategy enabled the sequential fabrication of two photosensitive materials within a predesigned configuration.