*2.1. Hydrogels Preparation*

#### 2.1.1. Pristine Gelatin Hydrogel (HGel-A)

A total of 0.2 g of gelatin was dissolved and mixed in 2 mL (10% *w*/*v*) of PBS (pH = 7.4) at 37 ◦C. After 5 min, DHICA or MeDHICA pre-dissolved in the minimal amount of DMSO were added to the gelatin solution up to 1, 5, 10% *w*/*w* with respect to gelatin in the case of DHICA, or 10% *w*/*w* in the case of MeDHICA, and continuously stirred until the solution appeared homogeneous. The hydrogels were set for gelation for 12 h at 4 ◦C and then washed with 5 mL PBS (pH = 7.4) for 30 min to remove not incorporated indole compounds.

#### 2.1.2. Cross-Linked Pristine Gelatin Hydrogel (HGel-B)

For preparation of HGel-B (10% *w*/*v*), gelatin (1 g) was initially dispersed in 10 mL of PBS, pH = 7.4, at 45 ◦C with continuous stirring till complete dissolution (1.5 h). DMTMM was then added (44 mg, 0.16 mmol dissolved in 100 μL of PBS, roughly 20% mmol of gelatin carboxyl groups), and the solution was kept under stirring at 45 ◦C for 30/45 s. Finally, the solution was poured in a 24 multiwell plate (1 mL per well), plugged, and rested till gelation (5–10 min); the gels were then kept at 37 ◦C for 2 h and finally freeze-dried.

#### 2.1.3. Gelatin-Chitosan Blend (HGel-C)

For preparation of HGel-C (gelatin/chitosan 8:1 *w*/*w*), 0.8 g of gelatin was initially dispersed in 8 mL (10% *w*/*v* gelatin solution) of PBS, pH = 7.4, and maintained at 45 ◦C under stirring till complete dissolution (1.5 h). Chitosan (0.1 g) was dissolved in 0.833 mL of 0.5 M acetic acid till complete

dissolution (3 h). Gelatin solution was poured into the chitosan solution and kept under stirring at 45 ◦C. After 24 h, DMTMM (18 mg, 0.064 mmol dissolved in 70 μL of PBS, roughly 10% mmol based on the gelatin carboxyl groups) was added. The solution was poured in cylindric molds (17 mm in diameter with total volume of 2.2 mL, 1.7 mL/mold), plugged, and rested till gelation (5–10 min); the gels were then kept at 37 ◦C for 2 h. Finally, the gels were dried at 4 ◦C.

#### *2.2. Determination of Swelling Degree of HGel-B and HGel-C*

Washed hydrogels were swollen in distilled water up to 5 h and the swollen weight was recorded at 10 min intervals, after dabbing the hydrogels with a filter paper before weighing. Totally, three replicas were run. The degree of swelling (*SDi*) was calculated as the following:

$$SD\_i = \left[ (Mw\_i - Md) / Md \right] \times 100\% \tag{1}$$

where *Mwi* is the swollen weight and *Md* is the dry weight [38].

#### *2.3. Fourier Transform-Infrared Spectroscopy (FT-IR)*

FT-IR analyses of loaded and unloaded gelatins HGel-B and HGel-C were done in the Attenuated Total Reflectance (ATR) mode using a Thermo Fisher Nicolet 5700 spectrophotometer equipped with a Smart Performer accessory (Rodano, Italy) mounting a ZnSe crystal for the analysis of solid samples.

#### *2.4. Scanning Electron Microscopy (SEM)*

The surface morphology of gelatins HGel-B and HGel-C were examined using a SEM Tabletop Microscope-1000 and a Field Emission SEM (FEI corporate, Hillsboro, OR, USA). The solid samples were mounted on a stub using double-sided adhesive tape before being coated with gold.

#### *2.5. Loading of DHICA*/*MeDHICA to HGel-B and HGel-C Gelatins*

HGel-B and HGel-C were swelled in distilled water for 1 h or 4 h, respectively. After that DHICA or MeDHICA, pre-dissolved in the minimal amount of DMSO, were added to the gelatin solution in PBS 1× (pH = 7.4) up to 10% *w*/*w* with respect to gelatin. In the case of HGel-B, DHICA was also used at 5% *w*/*w* with respect to gelatin. The optimum loading time was determined by UV–Vis monitoring of the remaining indole in the solution.

#### *2.6. Kinetics of DHICA*/*MeDHICA Release*
