Fig. 1. Photochemical reaction and direct laser writing process. (a) Schematic diagram of the preparation of positive and negative photoresists; (b) schematic diagram of direct laser writing

Fig. 2. Basic two-photon polymerization micro-nano processing system^[28]

Fig. 3. Study on the mechanism and application of two-photon polymerization of olefin photoresist. (a) Polymerization mechanism of olefin photoresist^[40]; (b) manufacturing process of magnetic control micro-nano rotor^[42]; (c) schematic illustration of the 3D nanofabrication based on two-photon lithography^[43]; (d) the main polymerization reaction for pH responsed hydrogel involved in the printing process and the schematic of the 4D-DLW process^[44]; (e) a switchable capillary-force self-assembly strategy based on a temperature-responsive hydrogel is proposed to construct 2D and 3D microstructures^[45]; (f) a density distribution controllable femtosecond Bessel beam is generated, and then a pH-responsive microactuator composed of bendable hydrogel microtubes has been successfully fabricated^[46]

Fig. 4. Study on polymerization mechanism and application of thiol‒olefin photoresist. (a) Cycles showing reactions of thiol‒ene‒acrylate polymerization^[52]; (b) a model of ternary mercaptan/acrylate copolymerization of acrylate, mercaptan olefin and mercaptan acrylate^[53]; (c) schematic diagram of photocrosslinking and photodegradation of 4-arm PEG-SH and Bimane norbornene, and photodegradation of 3D printed boxing ring structure^[54]

Fig. 5. Research on processing and application of inorganic monomer photoresist. (a) Photoreduction synthesis of different metal materials is realized by different lasers^[71]; (b) nanoscale additive manufacturing process of metal and two-photon lithography (TPL) process^[72]; (c) schematic diagram of the AM by a two-photon polymerization and subsequent 650 ℃ heat treatment^[80]; (d) micrographs of the printed fused silica structure^[80]; (e) construction of glass microstructure by laser 3D printing PDMS network^[81]

Fig. 6. Research on processing and application of heterogeneous monomer photoresists. (a) Schematic illustration showing 3D nanoprinting of MPA-capped CdSe/ZnS QDs using photoexcitation-induced chemical bonding^[84]; (b) scheme illustrating 3D printing of inorganic nanomaterials from a solution containing NCs (blue spheres) and bisazides using a femtosecond laser^[85]; (c) two-photon additive manufacturing schematic diagram and chemical structure of BTMST and ZrO₂ hybrid photoresist^[86]

Fig. 7. Common organic photoinitiators. (a) Two modes of organic photoinitiator to produce free radicals after absorbing photons; (b) coumarin-based organic photoinitiator^[72]; (c) organic photoinitiator based on phosphine oxide^[100]; (d) benzophenone-based organic photoinitiator^[101]

Fig. 8. Applications of inorganic photoinitiators. (a) Multifunctional photopolymer nanocomposites based on ZnO quantum photoinitiator^[104]; (b) TEM images of ZnO nanorods^[104]; (c) the formula of 3D printed PEGDA hydrogel^[104]; (d) schematic diagram of different types of photopolymerization in two-photon AM based on nanoclusters (Ag₂₈Pt)^[110]; (e) comparison of photoexcitation mechanisms between nanocluster photoinitiators (left) and typical organic photoinitiators (right)^[110]

Fig. 9. Study on two-step absorption (TSA) mechanism and photoinitiators. (a) TSA mechanism based on benzoyl derivatives as photoinitiators^[115]; (b) photoinitiators that can be excited by 405 nm blue light after screening for additive manufacturing^[90]; (c) a photoinitiators that can be used for additive manufacturing by simultaneous excitation of 405 nm blue light and 640 nm red light after screening^[90]; (d) schematic diagram of triplet‒triplet annihilation (TTA) processing, and formulation and UV-vis absorption spectra of the photoresist using in the TTA laser AM process^[120]

Fig. 10. A simple illustration of the concept of 4D printing^[126]

Fig. 11. Schematic diagram of super-resolution processing and characteristics after processing^[140]

Fig. 12. Multi-focus additive manufacturing technology. (a) Printing process of a 3×3 octahedral truss structure via multi-focus scanning^[142]; (b) optical configuration and fabrication principle of the multi-foci fast scanning system^[143]