2.8 Scanning electron microscopy
The samples were placed in DI water at 37 ℃ for two days, cooled at -80 ℃ for 2 hours, and then lyophilized. The cross section of the structure was sputter coated with gold at 15 mA for 2 min (EM Ace600, Leica, Germany) and observed under a scanning electron microscope (SEM, Flex SEM1000, Hitachi, Japan) at 10kV. The diameters of different samples were analyzed using image J.