Fig. 1. Schematics of manufacturing techniques commonly used in micro/nano 4D printing. (a) PμSL ^[32]; (b) DIW ^[23]; (c) TPP ^[33]; (d) 3D EBL^[31]; (e) comparison of processing resolution range of above technologies^[25,27]

Fig. 2. Common materials for micro/nano 4D printing ^[51,59-61]

Fig. 3. Shape-morphing microstructures actuated by magnetic field. (a) Bionic ciliary microrobots driven by nonreciprocal magnetic field^[65]; (b) specific shape transformation of programmed micromachines under applied magnetic field^[16]; (c) highly elastic microcolumn and patterned microcolumn arrays with adjustable distribution of magnetic nanoparticles^[67]

Fig. 4. Shape-morphing microstructures actuated by solvent. (a) Hydrogen bonding between material and water molecules to endow gel material excellent water absorption and swelling properties^[69]; (b) self-driving shape conversion of micro-nano structures by switching environmental humidity state^[70]; (c) (d) double-layer structure manufactured by changing forming parameters to realize controllable deformation of six-leaf petals and mimosa-like bionic microstructures in water environment^[70]; (e) (f) shape conversion of microstructure realized by switching solvents^[19]

Fig. 5. Shape-morphing microstructures actuated by pH. (a) Reversible swelling of BSA-based 3D panda relief in changing pH value^[71]; (b) shape-morphing microcrab based on AAc^[72]; (c) 4D-printed microcrawlers with controllable microjoint structures^[74]

Fig. 6. pH actuated microstructures fabricated by different femtosecond laser direct writing processes. (a) Asymmetric femtosecond Bessel beam dynamic holography method used for generating structured light field with uneven energy distribution to rapidly fabricate anisotropic actuator^[76]; (b) fabrication of heterogeneous polymeric microcolumns with pH response by femtosecond laser two-step scanning method^[77]; (c) bionic complex micro-spider constructed　using two-material system^[78]

Fig. 7. Shape-morphing microstructures actuated by temperature field. (a) Self-folding and unfolding phenomena of hydrogel actuator ^[21]; (b) liquid crystal orientator and rotation behavior of bi-layered radial beams of LCE ^[81]; (c) bending deformation and temperature field controlled rollover phenomenon of 3D Au NC/Ti heterostructure in solution ^[82]

Fig. 8. Shape-morphing microstructures by photothermal response. (a) Photothermal response driving of PNIPAM microstructure^[84]; (b) photothermal response driving of double-layer column and double-layer spiral with programmed printing density^[85]; (c) near-infrared light driving of Fe₃O₄ hydrogel micro-actuator^[86]; (d) photothermal response driving of wooden pile structure with doped gold nanorod liquid crystal material^[87]; (e) activation of PNIPAM- carbon nanotube composite hydrogel under near infrared irradiation^[88]

Fig. 9. Shape-morphing microstructures by compound light response. (a) Photothermal response combined with photochemical response for traveling wave actuation ^[89]; (b) photothermal response combined with photochemical response for non-reciprocating motion of cantilever beam ^[90]; (c) photothermal effect combined with Marangoni effect to achieve 3D motion ^[91]; (d) photoinduced surface electrochemical response ^[15]

Fig. 10. Applications of micro/nano 4D printing technology in biomedicine. (a) Soft magnetic helical robots used as cell culture media to induce cell differentiation ^[93]; (b) enzyme-driven drug-loading microrobot based on recombinant spider silk protein ^[31]; (c) microrichthys driven by magnetic fields and controlled to release drugs with change of pH value ^[72]

Fig. 11. Microrobot based on micro/nano 4D printing technology. (a) Miniature artificial walkers based on liquid crystal elastomers ^[51]; (b) quadruped microrobot prepared by standard semiconductor technology ^[15]; (c) artificial synthetic trichiasis strap based on biomimetic principle^[94]; (d) microgripper based on liquid crystal elastomer^[43]; (e) process of packing, transporting, and releasing magnetic nanoparticles by starfish-like gripper ^[95]

Fig. 12. Responsive micro-optical devices . (a) Tunable two-dimensional grating used for beam control ^[96]; (b) tunable whispering gallery mode resonator based on LCE substrate ^[97]; (c) artificial compound eye structure with adjustable focal length and field of view ^[59]