2.8.1. DPPH Assay

The assay was performed as previously described [39]. Briefly, HGel-B and HGel-C, loaded with DHICA or MeDHICA at 10% *<sup>w</sup>*/*<sup>w</sup>*, were immersed at 37 ◦C (10 mg/mL) in PBS medium. Then, 60 μL aliquots were periodically withdrawn over 6 h and added to 200 μM DPPH solution in methanol (2 mL) with rapid mixing. The reaction was followed by spectrophotometric analysis measuring the absorbance at 515 nm after 10 min. Values are expressed as DPPH decay over time. The experiments were run in triplicates. In other experiments DHICA or MeDHICA loaded gelatins that had been taken in air over one week were immersed in 200 μM DPPH solution (using a 0.04 *w*/*v* ratio) and the antioxidant power was evaluated by UV–Vis recording spectra over time up to 7 days. In control experiments the DPPH assay was run on the materials soon after loading of the indoles.

#### 2.8.2. Ferric Reducing/Antioxidant Power (FRAP) Assay

The assay was performed as previously described [40]. Briefly, the ferric reducing/antioxidant power (FRAP) reagen<sup>t</sup> was prepared by sequentially mixing 0.3 M acetate buffer (pH = 3.6), 10 mM TPTZ in 40 mM HCl, and 20 mM ferric chloride in water, at a 10:1:1 *v*/*v*/*v* ratio. To a solution of the FRAP reagent, 12 μL aliquots of the PBS medium, in which 10% HGel-B and HGel-C had been immersed at 37 ◦C, were added and rapidly mixed. After 10 min, the absorbance at 593 nm was measured. The assay was repeated on the samples over 6 h.
