2.4.10. Scanning Electron Microscopy (SEM)

Collagen hydrogels cross-linked with GA or EA for morphological characterization were prepared as described by Liu et al. [22] with slight modifications. Briefly, freshly prepared hydrogels were cut into fragments (~2 × 2 mm) and then immersed in 2.5% glutaraldehyde in 0.2 M phosphate (pH 7.2) overnight and dehydrated in graded ethanol solution with a series of concentrations (30%, 50%, 70%, 80%, 95%, and 100%). Subsequently, the samples were then treated with isoamyl acetate for a period of 15 min. The hydrogels were then freeze-dried with the use of a lyophilizer. After that, the hydrogels were coated with a layer of gold. An ultrahigh-resolution field emission scanning electron microscope was used to observe the microstructure, and SEM images were observed using a JSM-5610 SEM (JEOL, Tokyo, Japan), with 20 kV acceleration voltage. The magnification was 3 K, 8 K, and 15 K times.