## Materials and Methods
### Characterization of Chitosan
#### 2.3.6 Differential Scanning Calorimetry (DSC)  
Differential scanning calorimetry was performed using DSC823e differential scanning calorimeter (Mettler Toledo, Shanghai, China). The sample was heated from \(-\)30 \({}^{\circ}\)C to 220 \({}^{\circ}\)C at a heating rate of 10 \({}^{\circ}\)C /min under N\({}_{2}\) atmosphere.  
2.3.7. Fourier Transform Infrared Spectroscopy Analysis (FT-IR)  
All samples were previously dried in a vacuum oven at 40 \({}^{\circ}\)C for 24 h under reduced pressure. The characteristic infrared spectra of samples were obtained by 64 scans at resolution of 4 cm\({}^{-1}\) between the wavelength range of 400-4000 cm\({}^{-1}\) in Nicolet is 10 spectrometer (Thermal Fisher Scientific, Waltham, MA, USA).  
2.3.8. Circular Dichroism Spectroscopy (CD)  
The CD spectroscopy of the chitosan was recorded using J-815 circular dichroic spectrometer (JASCO, Tokyo, Japan). 0.7 mL chitosan solution with concentration of 50 mg\(\cdot\)L\({}^{-1}\) prepared by 2% acetic acid was analyzed for CD scanning under scanning rate 100 nm-min\({}^{-1}\) between the wavelength range of 260-340 nm in the optical path 2.00 mm quartz cuvette. The data of HT (voltage of photomultiplier tube) greater than 800 in the range of test wavelength were omitted, and the rest data were analyzed for circular dichroism spectrum.  
2.3.9. Scanning Electron Microscopy (SEM)  
S3400N scanning electron microscope (Hitachi, Japan) was used to observe the surface and microstructure of the chitosan samples. The samples were placed on the double-sided carbon tape and mounted on an aluminum specimen metal disk for vacuum gold spraying. The images were photographed at 400\(\times\) magnification 10 kV accelerating voltage.  
2.3.10. Polarized Optical Microscopy (POM)  
The chitosan powder was coated on the glass slide, and the crystal morphology of the chitosan samples was observed under the color background via DM4500p polarizing microscope (Leica, Wetzlar, Germany).  
2.4. Weighted Composite Index Evaluation  
2.4.1. Index Selection, Classification and Determination of Weight Coefficient  
According to the principle of scientifically, practicability and sensitivity of statistical data, and referring to relevant literature, three dimensions of chitosan characteristic indexes measured by the above methods were selected, including seven indexes which have significant influence on the biochemistry performance of chitosan: M\({}_{\text{W}}\), DD, WBC, FBC, TS, primary structure (PS) and secondary structure (SS).  
Analytic hierarchy process (AHP) and expert consultation were used to calculate the weight coefficient. Relevant literatures and reports were searched [14; 18; 19; 26; 27], experts and scholars in the field of chitosan were organized to discuss and design questionnaires. The relative importance of each layer of indicators was judged according to the hierarchical scaling method, and the weight coefficient of each layer of indicators were calculated.  
2.4.2. Data Standard Normalization Processing  
The original data table of n rows (n evaluation objects) and m columns (m evaluation indexes) were listed. To eliminate the unit limitation of the original data, all index data were standardized and converted into the values of 0-10 interval. For getting rid of the multi variable order of magnitude, root operation was performed on the data with order of magnitude difference.  
Xi is the actual value of each index, and M is the standard value of the index. The purpose of evaluation is to evaluate the biochemistry performance of CS, crab chitosan (CSC) and squid chitosan (CSS), so the average level of three kinds of chitosan was selected as the standard value (M). Then, M was taken as the reference value, Xi was compared with M, in which the M\({}_{\text{W}}\) is inverse indexes, and the rest are positive indexes. The positive index refers to the index that the larger Xi, the better the evaluation, which is reflected by Xi/M. Inverse index refers to the index that the smaller Xi, the better, which is affirmed in M/Xi.