## 3 Results
### In vitro Release Studies  
Figure 5 shows the GA release from the bi- and multilayer PLA films as a function of the immersion time in saline medium for a span time of 1680 h. The in vitro release kinetics data are depicted in terms of the total amount release of GA (Figure 5a) and its cumulative release to the medium based on the theoretical content of GA in the PLA films (Figure 5b). One can observe that the release profile of the PLA films considerably varied according to their structure and, as one could expect, the total amount of GA released to the medium increased with increasing the GA content in the film samples. As a general observation, all bilayer films showed a Fickean diffusion release mechanism. During the first 5 h, a burst release profile was attained, showing cumulative release values of approximately 83%, 46%, and 35% for the PLA films coated by electrospun mats for 1 h, 2 h, and 3 h, respectively. Upon immersion in saline media, it is considered that the PLA fibers' coating is rapidly hydrated and swells, thus allowing the dissolution of the embedded polyphenols and their subsequent release at a relatively high rate due to the highly porosity of the electrospun mat. It is worthy to note that this phenomenon of burst release was observed even though the coated films were immersed in a water-based medium, for which PLA presents a low chemical affinity. It can be considered that the Fickean diffusion process was overlapped by the swelling behavior of the electrospun PLA fibers, leading to high release rates and thus accelerating the process. One can also observe that the GA release rate was higher in the bilayer films processed for shorter electrospinninging deposition times, which can be related to the lower thickness of the electrospun coating and, thus, to a more favored diffusion of the polyphenol to the saline medium. In a second step, the release rates progressively decreased for all PLA films coated at different electrospinninging deposition times. For the bilayer film obtained by 1 h of electrospinninging deposition, the GA release rate stabilized after only \(\sim\)45 h. In the PLA films coated for 2 and 3 h, stabilization was attained after approximately 250 h. At the end of the experiment, all bilayer films completed a theoretical cumulative release of GA in the 82-92% range. The deviation from the ideal value of 100% can be related to the inhomogeneous distribution of GA in different parts of the obtained electrospun PLA layers, partial losses during processing, and also a minor accumulation of GA in the PLA fibers during the whole tested immersion time. The above-described short or short-to-medium release can be of interest for most food packaging materials, based on the fact that the mean shelf life of a food-packaging product is typically below two weeks [44]. In this sense, [PERSON] et al. [32] also studied the release behavior of GA from a single electrospun mat of PLA fibers in saline medium for 48 h. A similar release profile was reported, showing that the cumulative amount of GA released rapidly increased during the initial immersion time. Specifically, at 30 min after immersion, the cumulative amount of GA released into the normal saline was 21%. Further increase in the immersion time resulted in a gradual increase in the cumulative amount up to a plateau value of approximately 90% was reached. This burst release was related to the high water retention behavior of the GA-loaded PLA fibers, as well as potential dissociation of the polyphenol into ionic species due to the difference between the pKa value of GA and the pH of the normal saline, that is, 4.41 vs. 7. Moreover, the dissociation of the carboxylic acid chain ends of PLA into carboxylate groups (and protons) could additionally help promote the diffusion of the gallate ions out of the electrospun fibers due to electrostatic repulsion. Similarly, [PERSON] and [PERSON] [45] reported the release of GA encapsulated at different rations into electrospun cellulose acetate (CA) nanofibers.