2.7. SEM observations

Each sample tissue was cut into small pieces (approximately 0.4 cm × 0.2 cm × 0.1 cm), and fixed with 4% glutaraldehyde for 24 h. Thereafter, the samples were washed for 3 h with phosphate buffer solution (pH 7.2), 10 min each time. Then, samples were dehydrated with 30%, 50%, 60%, 70%, 80%, 90%, 95%, and 100% alcoholic solution, 15 min per gradient. After that, a critical point dryer (Leica CPD 030, USA) with CO₂ was used for sample drying. Before testing, stick the dried samples to the sample stage and make group records. The samples were sprayed with an ion sputtering instrument (HITACHI MC1000, Japan) and observed with a cold field-emission scanning electron microscope (HITACHI Regulus 8200, Japan).