Fig. 1. Two-photon polymerization principle. (a) Schematic of single-/two-photon absorption transition; (b) schematic of two-photon polymerization

Fig. 2. Schematic of basic design types of two-photon initiators

Fig. 3. Molecular structure of the compounds with distyrene benzene core^[30]

Fig. 4. Chemical structure of multipolar styrene-1,3, 5-triazine heterocyclic derivatives^[38]

Fig. 5. Chemical structure of A-π-D-π-A carbazole derivative with low fluorescence quantum yield^[40]

Fig. 6. Chemical structure of water-soluble two-photon initiator with ionic groups^[45]

Fig. 7. Chemical structure of water-soluble two-photon initiator and simulated configuration obtained from host guest chemical interaction of quantum chemical^[49]

Fig. 8. Femtosecond laser processing system

Fig. 9. SEM images of 3D hydrogel structure prepared by two-photon polymerization. (a) Adenovirus hydrogel microstructure^[48]; (b) 3D wood pile structure^[47]; (c) erythrocyte microstructure^[60]; (d) microstructure of high-viscose-protein Morning star chain^[58]; (e) PEGDA humidity responsive pattern and micr

Fig. 10. Modified gelatin scaffolds prepared by two-photon polymerization. (a) GelMOD 3D scaffold promoting calcium deposition of porcine mesenchymal stem cells^[70]; (b) cell adhesion and invasion on GelMA scaffolds with different degrees of polymerization^[71]

Fig. 11. Cell viability and adhesion on PEGDA hydrogel scaffold^[49]. (a) Oblique SEM image of a complete hexagonal grid scaffold; (b) confocal fluorescence image and bright field image; (c) confocal fluorescence image of HeLa cells co-culturing with the grid scaffold; (d) bright field image of HeLa cells co-culturing with the grid scaffold

Fig. 12. Cell viability and adhesion on PEGDA hydrogel scaffold^[74]. (a) SEM image of 3D PEGDA hydrogel scaffold; (b) SEM image of neuro2A cells transplanting on a 3D PEGDA hydrogel scaffold; (c) magnified image of neuro2A cells transplanting on the surface of hydrogel scaffold; (d) connection interaction between neuro2A cells on the planar substrate and neuro2A on the scaffold