## 2 Materials and Methods
### Characterization of Decellularized Matrices and Hydrogels  
Freshly isolated tissues and decellularized matrices were fixed in 4% paraformaldehyde-hyde, washed three times with distilled water, embedded, frozen, sectioned at low temperature, and stained with hematoxylin and eosin (H&E). The stained sections were photographed with a Nikon eclipse E100 microscope.  
The DNA from freshly isolated tissues and decellularized matrices was extracted using a DNA extraction kit (Thermo Fisher Scientific, USA), and analyzed with Quant-iT(tm) PicoGreen(tm) dsDNA Assay Kit (Thermo Fisher Scientific, USA) according to the instruction manual provided by the manufacturer, using 485 nm as the excitation wavelength and 528 nm as the emission wavelength on a Synergy HTX microplate reader (Biotek, Santa Barbara, CA, USA), with triplicates for each group.  
Total glycosaminoglycan (GAG) content was isolated from freshly isolated tissues and decellularized matrices by high salt extraction, and quantified with a kit (Genmed Scientifics Inc., USA) based on a dimethyl methylene blue colorimetric assay, according to the manufacturer's instructions.  
Collagen content was determined indirectly by measuring hydroxyproline concentration, based on the fact that hydroxyproline represents 13.4% of the total amino acids contained in collagen. Freshly isolated tissues and decellularized matrix samples were hydrolyzed by alkali, hydroxyproline oxide, and dimethylaminobenzaldehyde, to produce a purplish red compound quantified by colorimetry.  
Theepsin-digested porcine decellularized myocardial and intestinal submuc matrices and the collagen solutions were analyzed and compared by SDS-PAGE, in 12% polyacrylamide separation gel and 5% polyacrylamide concentration gel. The electrophoresis was performed in a XCell SureLock Mini-Cell electrophoresis system (Thermo Fisher Scientific) and the proteins were detected by silver staining.  
A scanning electron microscope (SEM, HITACHI S-4800, Japan) was used to analyze the hydrogel scaffolds. pDMYO-gel, pDSIS-gel, and COL1-gel at a concentration of 5 mg/mL were fixed in 2.5 wt% glutaraldehyde solution for 6 h. The samples were washed three times with deionized water for 30 min. Then, they were dehydrated successively in 30%, 50%, 75%, and 100% ethanol solutions for 30 min at each step. The samples were soaked in deionized water for 6 h, with the deionized water being changed every 2 h. The final hydrogel scaffolds were lyophilized. The samples were coated by platinum and observed by SEM. The SEM images were analyzed in the software ImageJ (version No. 1.53c). Three samples per experimental group were selected, and two SEM images/sample were analyzed. Ten fibers were randomly selected from each SEM image for diameter measurement and statistics.  
The elastic storage modulus of the hydrogel scaffolds was tested in a Rotational Rheometer (Haake MARS, Thermo Fisher). The concentration of each tested sample was 5 mg/mL. The operation steps were as follows: 0.2 mL of hydrogel solution was placed on the parallel sample stand. Analysis was performed in flow mode and in conjunction with parallel steel plate geometry (20 mm diameter) with a gap of 0.5 mm. The temperature of the sample table was set at 4 \({}^{\circ}\)C. The test parameters were set at 1% strain and 1 Hz frequency. The temperature of the sample table rapidly raised from 4 \({}^{\circ}\)C to 37 \({}^{\circ}\)C (the heating rate was 200 \({}^{\circ}\)C per min) and maintained at 37 \({}^{\circ}\)C for 6 min. The elastic storage modulus (g) of each group was recorded.