Fig. 1. Molecular structures of monomer and photoinitiator for BSA-GMA precursors

Fig. 2. Electrical properties of BSA and BSA-GMA materials with different degrees of methacrylation (D15, D35, and D52). (a) Zeta potentials of BSA and BSA-GMA with different degrees of methacrylation; (b) isoelectric points of BSA and BSA-GMA with different degrees of methacrylation

Fig. 3. Macroscopic pH-responsive properties of BSA-GMA hydrogels with different concentrations and degrees of methacrylation. (a) Images of five BSA-GMA hydrogels in different pH solutions; (b) effect of concentration and degree of methacrylation of BSA-GMA hydrogels on the macroscopic pH response

Fig. 4. Schematic design and fabrication of pH-responsive BSA-GMA hydrogel microstructures. (a) Schematic diagram of the femtosecond laser direct writing system; (b) schematic diagram of the response mechanism of the pH-responsive suspended BSA-GMA structures

Fig. 5. pH-responsive properties of suspended BSA-GMA hydrogel microstructures fabricated by femtosecond laser direct writing. (a) SEM images of suspended BSA-GMA hydrogel microstructures fabricated by femtosecond laser direct writing; (b) bright field images of suspended BSA-GMA microstructures in air; (c)‒(f) bright field images of swelling behavior of suspended BSA-GMA microstructures in different pH solutions

Fig. 6. pH-responsive and cycling properties of suspended BSA-GMA hydrogel microstructures. (a) Influence of concentration and methacrylation on the swelling degree of the hydrogel microstructures in different pH solutions; (b) reversible swelling behavior of hydrogel microstructures

Fig. 7. BSA-GMA 3D cell scaffold and the confocal fluorescence microscopy images of chondrocytes. (a) SEM top view of the 3D cell scaffold fabricated by femtosecond laser direct writing; (b) SEM oblique view of the 3D cell scaffold fabricated by femtosecond laser direct writing; (c) confocal fluorescence microscopy image of the nucleus (Hoechst); (d) confocal fluorescence microscopy image of the cell scaffold; (e) confocal fluorescence image of the mitochondria (Mito-Tracker Deep Red); (f) overlayed confocal fluorescence image of chondrocytes on BSA- GMA cell scaffolds

Fig. 8. Proliferation of chondrocytes in BSA-GMA hydrogel extract by CCK-8 characterization. (a) Proliferation of chondrocytes cultured in control and BSA-GMA hydrogel extract on 1st, 7th, and 14th days; (b) relative cell growth rate of hydrogel extracts