Fig. 1. Electron excitation of materials in single-photon, multi-photon, and two-photon absorption processes^[62].^{(a) In single-photon absorption process; (b) in multi-photon absorption process; (c) in two-photon absorption process}

Fig. 2. Spatial distributions of laser energy absorbed by materials and two- and multi-photon polymerization induced by femtosecond laser inside the photoresist. (a) Spatial distribution of the actual light intensity (thick dashed line) and the spatial distributions of the laser energy (solid horizontal line represents the reaction threshold) absorbed by the material through single-photon (thick dashed line), two-photon (solid line), and three-photon (thin dashed line) processes^[60]; (b) schematic diagram of two- and multi-photon polymerization induced by femtosecond laser inside the photoresist under the influence of threshold effect, in which the central red dot region is the region where polymerization occurs beyond the threshold value

Fig. 3. Two-photon polymerization (TPP) 3D printed MEMS temperature and humidity sensors. (a) TPP 3D printed on-chip MEMS temperature and humidity sensor with ionic liquid (ILs)-doped photoresist^[66]; (b) TPP 3D printed liquid-filled micro-cavity temperature sensor^[67]; (c) optic fiber Mach‒Zehnder interferometer (MZI) temperature sensor based on integrated polymer waveguide micro-cavity^[68]; (d) TPP 3D printed polymer optic fiber Bragg grating temperature sensor^[69]; (e) TPP 3D printed fluorescent temperature sensor based on Rhodamine B-doped photoresist^[70]; (f) optic fiber temperature and concentration sensor based on TPP 3D printed optical waveguide MZI^[72]; (g) temperature and humidity sensor based on TPP 3D printed supramolecular cholesteric liquid crystal photoresist^[71]; (h) optic fiber humidity sensor based on TPP 3D printed castle-shaped Fabry‒Perot interferometer (FPI) micro-cavity^[73]; (i) optic fiber temperature and humidity sensor based on 3D printed whispering gallery mode (WGM) resonator^[74]; (j) 3D printed micro-ring polymer micro-fiber Bragg grating (FBG) temperature sensor^[75]; (k) optic fiber temperature sensor based on TPP 3D printed asymmetric interferometric arm MZI^[76]

Fig. 4. TPP 3D printed MEMS microforce sensors. (a) TPP 3D printed ultrafine microforce probes^[77]; (b) TPP 3D printed 3D-flexible mechanical gripper with integrated force sensor^[78]; (c) MEMS sensor skin patch with TPP 3D printed microsprings^[79]; (d) TPP 3D printed polymer microbeam optic-fiber-end probe^[81]; (e) TPP 3D printed cantilever beam microforce probe^[82]; (f) TPP 3D printed metamaterial cantilever beam optic-fiber-end microforce probe^[83]; (g) optic-fiber-end gas pressure sensor based on TPP 3D printed spring-tube cavity and hinged mirror^[84]; (h) optic fiber Fabry‒Perot microforce sensor based on TPP 3D finely printed microspring structures^[85]

Fig. 5. TPP 3D printed MEMS vibration sensors. (a) Optical sensor ultrasonic wave sensor based on TPP 3D printed phase-shifted Bragg grating waveguide^[87]; (b) MEMS accelerometer with electron-beam evaporated coating integrated with TPP 3D printing^[88]; (c) optic fiber acoustic wave sensor based on TPP 3D printed membrane and horn-shaped auxiliary structure^[89]; (d) optic fiber acoustic wave sensor based on TPP 3D printed microspring supports and thin film structures^[90]; (e) optic fiber Fabry‒Perot acoustic wave sensor based on TPP 3D printed corrugated membrane structures^[91]

Fig. 6. TPP 3D printed gas sensors. (a) Oxygen sensor based on TPP 3D printed metalloporphyrin photoresist microstructures^[92]; (b) ethanol gas sensor based on TPP 3D printed conductive nanotube-doped NECHs^[93]; (c) optic-fiber-end hydrogen gas sensor based on TPP 3D printed cantilever beams combined with a palladium coating^[94]; (d) ethanol gas sensor based on TPP 3D printed spiral photonic array^[96]

Fig. 7. TPP 3D printed liquid sensors. (a) Liquid refractive index sensor based on TPP 3D printed whispering gallery mode micro-ring resonator^[97]; (b) liquid refractive index sensor based on 3D printed micro-nano optic fiber Bragg grating^[98]; (c) ethanol sensor based on TPP 3D printed micro-ring resonator^[99]; (d) highly sensitive micro-nano organic arsenic 4D sensor with quantum dot-doped photoresist using TPP 3D printing^[100]; (e) optic fiber SPR liquid refractive index sensor based on TPP 3D printed micro three-dimensional helical grating cone^[101]

Fig. 8. TPP 3D printed biosensors. (a) MEMS glucose sensor based on TPP 3D printed electrodes using multi-material-doped photoresist^[102]; (b) optic fiber surface-enhanced Raman scattering (SERS) sensor based on TPP 3D printing for enhanced Raman scattering^[103]; (c) MEMS pH sensor with TPP 3D printed biomimetic venous valve structures^[104]; (d) optic fiber liquid refractive index sensor based on TPP 3D printed Fabry‒Perot micro-cavity^[106]; (e) optic fiber respiratory rate sensor based on TPP 3D printed cantilever beams^[107]; (f) optic fiber SPR sensor for human serum IgG detection based on TPP 3D printed micro-cone structures^[109]

Fig. 9. TPP 3D printed magnetic field sensors. (a) Optic fiber magnetic field sensor based on 3D printed micro-nano waveguide micro-cavity filled with magnetic fluid^[115]; (b) optic fiber magnetic field sensor based on 4D printed magnetic particle-doped photoresist Fabry‒Perot cavity^[116]

Fig. 10. TPP 3D printed flow velocity sensors. (a) MEMS flow velocity sensor based on TPP 3D printed high aspect ratio hair-like structure and flexible membrane^[118]; (b) optic fiber flow velocity sensor based on TPP 3D printed micro helical propellers^[82]

Fig. 11. TTP 3D printed UV MEMS sensor based on ZnO semiconductor nanowire-doped photoresist^[119]

Fig. 12. MEMS mass sensor based on metal salt-doped photoresist formed by two-step fabrication of TPP 3D printing and thermal treatment^[121]