Fig. 1. Principle of micro-nano 3D printing based on one/two-photon polymerization. (a) Schematic of one/two-photon absorption energy level transitions and polymerization process, where hν represents photon energy, S₀ represents ground state, S₁ represents excited singlet state, T₁ represents triplet state, and ISC represents intersystem crossing; (b) schematic of feature scales of one/two-photon polymerization, where d represents the minimum print feature scale; (c) schematic of one/two-photon polymerization and crosslinking process of photoresist

Fig. 2. A typical laser direct writing micro-nano 3D printing system. (a) Schematic of optical path; (b) schematic of a parallel laser beam generation scheme

Fig. 3. A typical laser projection micro-nano 3D printing system. (a) Schematic of optical path; (b) schematic of a“digital mask”generation scheme for surface projection

Fig. 4. Micro-nano 3D printing based on one-photon polymerization and resolution. (a) 3D printing equipment based on stereolithography and the first 3D printed object^[95]; (b) microgear structure and 1.2 μm feature size structure^[96]; (c) microrotor structure and nanowire with linewidth of 0.43 μm^[97-98]; (d) woodpile structure and nanowire with linewidth of 85 nm^[99]; (e) spiral photonic crystal structure and nanodots with diameter of 190 nm^[100]; (f) woodpile photonic crystals, cage structure and nanodot array with diameter of 85 nm^[101]; (g) spiral photonic crystals and nanowire with linewidth of 0.6 μm^[102]; (h) high-precision projection lithography system and 180 nm feature size structure^[103]

Fig. 5. Micro-nano 3D printing based on two-photon polymerization and resolution. (a)(c) Schematic of two-photon polymerization printing and“nano cattle”prepared by the technology^[64]; (d)(f) two-photon polymerization area, voxel morphology and the structure with 120 nm feature size^[64,77]; (g)(j) nanowires with the minimum linewidth of 100, 80, 50, and 35 nm on the glass substrate^[107-111]; (k)(m) dangling nanowires with the minimum linewidth of 30, 23, and 7 nm^{[105,112-113]}

Fig. 6. Super-resolution imaging technology-assisted two-photon polymerization micro-nano 3D printing. (a)(c) Schematic of STED two-photon printing and the minimum longitudinal size of 40 nm^[116]; (d)(f) nanowires fabricated by STED single-photon lithography with the minimum size of 64 nm and 35 nm^[117-118]; (g)(h) nanowires fabricated by STED two-photon lithography with the minimum size of 54 nm and 9 nm^[124]; (i)(j) schematic of 4Pi　multiphoton polymerization and nanowires fabricated by the technique with the minimum axial size of 150 nm^[127]

Fig. 7. Multifocus parallel printing and microstructure array. (a)(b) The letter“N”and microspring array fabricated by parallel MLA-TPP^[129]; (c)(d) microgear and micron“cattle”array fabricated by parallel DOE-TPP^[131]; (e)(h) multifocus TPP system and prepared three-dimensional mechanical metamaterial by this system^[133]

Fig. 8. Holographic multifocal printing and microstructure arrays. (a) Dynamic holographic TPP method and fabricated 3D microtube by this method^[138]; (b) multi-dodecahedron microstructure exposed by 5 foci of holographic TPP^[139]; (c) SLM holographic multifocus 3D TPP method and fabricated 3-layer and 6-layer 3D structures by the method with holographic 20-focus^[140]; (d) DMD holographic multifocus 3D TPP method and printed high-resolution“bridge”structure by the method, and woodpile structures printed by single-focus, double-foci, and three-foci TPP generated with the binary holography^[146]

Fig. 9. Fast surface projection 3D printing. (a) 3D printing method for continuous liquid interface production and prepared“Eiffel Tower”model by the method^[48]; (b) large-area fast printing based on mobile-liquid interface and prepared mechanical metamaterials by the method^[150]; (c) rapid continuous stereolithography based on STED and printed hollow structures by the method^[151]; (d) dynamic conformal 3D printing method and prepared real-time bending conversion model of“Eiffel Tower”by the method^[152]

Fig. 10. DMD surface projection stereolithography. (a) Femtosecond projection TPL technology based on DMD and prepared millimeter-scale 3D structure with submicrometer features, and micro-nano bridge structure by the technology^[49]; (b) DMD projection multi-photon 3D printing with spatiotemporal focusing technology and prepared suspension line structures, micro-nano suspended lines structures, and macro-metamaterial structures by the technology^[50]; (c) femtosecond projection nanolithography technology based on DMD and prepared nanowires and nanodots structures, and cross-scale micro-nano structures by the technology^[158]

Fig. 11. Volume projection stereolithography. (a)(b) One-step volumetric additive manufacturing and printed arbitrary 3D structures by the method^[159]; (c)(d) dual-color light induced-suppressed photopolymerization volumetric 3D printing and printed 3D objects by the method^[160]; (e)(g) dual-color xolography volumetric 3D printing and prepared spherical cage with free-floating ball and anatomical model structure by the method^[161]

Fig. 12. Volume projection stereolithography based on tomographic reconstruction. (a)(b) Printing principle and equipment of volume 3D printing based on tomographic reconstruction^[163]; (c)(d) preparation process and 3D geometric structures^[163]; (e)(f) high-resolution tomographic 3D printing with integrated feedback system, prepared high-fidelity structures in the modes with and without feedback^[164]

Fig. 13. Comparison of process performance of different types of photopolymerization micro-nano 3D printing/lithography technologies

Fig. 14. Minimum line spacing structure prepared by micro-nano 3D printing. (a) Diffraction-limited light intensity (red) of line array for parallel exposures, where the incident wavelength is 800 nm and NA is 1.4^[120]; (b) two adjacent lines fabricated by STED lithography with different line spacing values^[124]; (c) two adjacent lines fabricated by STED lithography with the different intensities of inhibition laser beam^[125]; (d) two suspension lines fabricated by TPP lithography with the different line spacing values^[105]

Fig. 15. Large-area splicing structures prepared by micro-nano 3D printing. (a)(b) Macroscopic foam disk (1.5 mm in diameter, 100150 μm in thickness) constructed from individual layers consisting of stitched 100 μm×100 μm×16 μm logpile blocks^[177]; (c)(d) 3D gyroid structures printed with scanners only and using continuous writing via synchronized linear stages and galvo-scanners^[178]; (e)(f) Yggdrasil structure printed with high-magnification (63×， NA of 1.4) and low-magnification (20×， NA of 0.5) microscope objectives, respectively, and the stitching scheme used for low-magnification objective^[179]; (g)(h) printed structures using only galvanometric scanners, and continuous scanning via synchronization of both linear translation stages and galvanometric scanners^[166]; (i) a mesoscale butterfly created by continuous scanning via synchronization of both linear translation stages and galvanometric scanners^[166]

Fig. 16. Low-cost micro-nano 3D printing systems and microstructures. (a) 3D printer using high-definition digital versatile discs (HD DVDs) optical-pickup-unit (OPU) as core optical module, laser beam of 405 nm wavelength was focused to cure photopolymer^[190]; (b)(e) 3D printed nanoscale line structures with the widths of 992, 879, 769, and 385 nm, and tower structures^[190]; (f) semiconductor laser diode used in two-step absorption 3D nanoprinter, operating at 405 nm wavelength^[192]; (g) printed 2D line gratings^[192]; (h)(l) various 3D-printed nanostructures^[192]