## 2 Micro-Actuator Fabrication
### Principle of Two-Photon Polymerization
The PI and monomer or oligomer should not be thermally damaged or ablated during the TPP process, and photocurable resin should be transparent at the TPA wavelength. At the same time, the PI must not only absorb the two-photon wavelength but must also satisfy high active moieties and radical  
Figure 1: The principle of the two-photon polymerization (TPP) and experimental setup. **(a)** Schematic diagram of two-photon absorption (TPA)—sequential and simultaneous TPA; **(b)** schematic diagram of photopolymerization by the transformation process of photon energy; **(c)** schematic of the two-photon polymerization process (S: photosensizer; S*: excited states of the photosensizer; hv: [PERSON]’s equation energy for each adsorbed photon; I: photoinitiator (PI); I*: excited states of the PI; R: radical; and M: monomer); and **(d)** schematic illustration of a typical TPP experimental configuration: galvano mode (**left**) and piezo mode (**right**). generation. In [PERSON]'s research, a photo-polymerization process using material without a PI was reported, and the highest resolution of the 3D structure with the sub-100 nm resolution was experimentally achieved in a non-photosensitive resist [27]. However, the widest processing window was observed for photo-sensitized resists, not non-photosensitive resists. During the irradiation of the tightly focused femtosecond laser beam into the photocatalytic resin, avalanche ionization plays an important role in chemical bond breaking and polymerization [28]. Valence electrons in a PI are excited via the fluorescence induced by a photosensitizer. These excited PIs are partitioned into three types of transformations such as the decay back to an initial state of a PI with light or heat emissions, quenching with oxygen or quenching agents, and a chemical reaction yielding an initiator species (e.g., radical) for polymerization (Figure 1b). In the initiation step, the reactive initiating species such as radicals are generated by absorbing the excitation energy. These reactive free radicals attack the C=C bonds of monomers and form monomers containing free electrons. Additionally, the generated radicals react with monomers or oligomers to generate monomer radicals, and the resulting monomer radicals are polymerized into high-molecular-weight materials by continuing the propagation process until they are terminated with other types of radicals. This process refers to the propagation step. In the termination step, two radicals meet and the reaction stops [29] (Figure 1c). Generally, a TPP fabrication system consists of an ultrafast (femtosecond) laser source and a moving part (piezo mode [30] or galvano mode [31]) that controls the scanning of the focused laser beam within a photocurable resin (Figure 1d). The photopolymerization process is also possible even with low-cost lasers that emit picosecond [32; 33] or nanosecond [34] pulses and continuous waves (CWs) [35] if appropriate exposure energy is delivered to the photopolymerization material. Considering its short pulse duration and high peak power, however, a Ti:sapphire near-infrared fs-laser [36; 37; 38] or Er-doped femtosecond fiber laser [39; 40] system is used as a light source for an efficient TPA process. The beam expander reduces the divergence angle of the laser beam, and this expanded beam can be tightly focused into the photocurable resin using a high N.A. objective lens. The exposure time is controlled by a mechanical shutter, and the processes are real-time monitored by a CMOS (complementary metal-oxide semiconductor)/CCD (charge-coupled device) camera. For the moving part, polymerization through the movement of a laser beam inside a photocurable resin can be realized in two ways. In the piezo mode, the sample is placed on a 3D piezoelectric stage and moves along the x-y-z-axis according to the machining geometry, so the structure is fabricated according to the laser trajectory. It is possible to further reduce the focal volume using a high N.A.