2.2 Fabrication and characterization of PTMC/TPU scaffolds
Different amounts of PTMC and TPU at specific PTMC:TPU ratios were homogeneously mixed in round-bottom flasks by mechanical stirring after being heated in an oil bath to 170 ℃ under nitrogen protection. Afterwards, PTMC/TPU mixtures were cut into pieces and then transferred to a stainless steel cartridge in a 3D bioprinter (regenHU, Switzerland). 4D printing was conducted using these printing parameters: the inner diameter of the printing nozzle was 410 μm, the printing temperature was 170 ℃, and the printing speed was 8 mm/s. Subsequently, 4D printed PTMC/TPU scaffolds were rolled up to tubular structures using a stainless steel rod (rod diameter: 8mm) in an oven at 80℃ for 30min. The tubular scaffolds were then flattened at room temperature for further use.
A scanning electron microscope (SEM, Hitachi S-4800, Japan) was used to observe the surface morphology and microstructure of 4D printed PTMC/TPU scaffolds. Differential scanning calorimetry (DSC, Pyris 6, PerkinElmer, USA) was conducted to analyze the glass transition temperature of PTMC/TPU scaffolds, and thermogravimetric analysis (TGA, DZ-TGA101, Nanjing Dazhan Testing Instrument CO., Ltd, China) was performed to determine the real PTMC:TPU ratios for PTMC/TPU scaffolds. The mechanical properties of PTMC/TPU scaffolds were studied via tensile tests using a universal testing machine (Model 5848, Instron Ltd., USA) at room temperature and at 37 ℃, respectively. The surface wettability and BSA adsorption of PTMC/TPU scaffolds were studied. Moreover, the shape morphing behavior of PTMC/TPU scaffolds was investigated by immersing scaffolds in a water bath at 37 ℃, with shape morphing processes being recorded by a digital camera.