**1. Introduction**

The packaging industry requires the development of new plastic materials with active properties, based on the demand by consumers for safer and more nutritive food [1]. Moreover, the growing concern over the environmental problems caused by petroleum-derived materials has led to the search for new renewable raw materials for the development of compostable packaging [2,3]. Polyhydroxyalkanoates (PHAs) are amongs<sup>t</sup> the most promising biopolymers,

being a group of totally renewable, biodegradable, and biocompatible aliphatic polyesters. PHAs are synthesized in the cytoplasm of a wide range of bacteria from glucose-rich substrates [4,5]. Some PHAs, such as poly(3-hydroxybutyrate) (PHB), poly(3-hydroxybutyrate-*co*-3- hydroxyvalerate) (PHBV), poly(3-hydroxybutyrate-*co*-4-hydroxybutyrate) (P(3HB-*co*-4HB)), and poly(3-hydroxybutyrate-*co*-3-hydroxyhexanoate) (PHBH) are currently being employed to develop bioplastic packaging articles, such as injection-molded pieces, compression-molded sheets, and films [6–9].

Active packaging technology is mostly related to materials and articles that are intended to extend food shelf life, and also to improve packaged food conditions by interacting with the food product and/or with its internal packaging environment. Active packaging materials are usually designed to deliberately incorporate components, which would then release and/or absorb substances into or from the packaged food or the environment surrounding the food [10]. Active packaging systems can therefore extend the shelf life of food products and reduce food waste by maintaining the quality of food products for longer, increasing product safety by securing the foods against pathogens, and enhancing the convenience of food processing, distribution, retailing, and consumption [11]. Concerning the active packaging materials, these are classified as either active scavenging types (e.g., oxygen scavengers) [12] or active releasing types (e.g., antioxidants) [13]. Active releasing-type packaging can provide novel "extra" functions, such as aromatic, antioxidant, and long-term antimicrobial properties [14]. In particular, active-releasing antimicrobial packaging applications are directly related to food microbial safety, as well as to shelf life extension, by preventing the growth of spoilage and/or pathogenic microorganisms [15,16]. The growth of spoilage microorganisms can not only reduce the food shelf life, but it can also endanger public health (particularly in the case of pathogenic microorganisms).

Active properties can be conferred by the incorporation into the packaging materials of substances with inherent antioxidant and antimicrobial properties, such as essential oils (EOs) [17], natural extracts (NEs) [18], and/or inorganic and metal nanoparticles [19]. EOs are volatile compounds obtained from aromatic plants that produced them naturally as secondary metabolites [20]. EOs and NEs are mainly composed of terpenoids, phenolic, and aromatic compounds, and their composition can widely vary depending on the edaphoclimatic characteristics of the plant, the part of the plant (i.e., flower, seed, leaves, fruits, stems, and others), and the extraction procedure [21]. There is grea<sup>t</sup> interest in the use of these natural products because they are classified as generally recognized as safe (GRAS) food additives by the Food and Drug Administration (FDA) [22].

In line with this, over the last few years, different EOs and NEs have been proposed as alternative sources of antimicrobials in packaging materials. Within the grea<sup>t</sup> variety of EOs, oregano essential oil (OEO) from *Origanum vulgare* is well known for its antioxidative and antimicrobial activities [23]. The EO content in the oregano plant fluctuates from 0.5–2% [24] up to 7% [25]. Its main constituents are the isomer phenols, carvacrol and thymol, which represent up to 80% and 64%, respectively [26]. In addition, up to 52% of each of their precursor monoterpenes, p-cymene and γ-terpinene, as well as terpinen-4-ol, linalool, β-myrcene, trans-sabinene hydrate, and β-caryophyllene, are also present [27]. Rosemary extract (RE), which is obtained from *Rosmarinus officinalis*, is an aromatic plant belonging to the Lamiaceae family [28], and it also presents strong antimicrobial and antifungal properties [29]. The active properties of RE are primarily conferred by its phenolic, and the volatile constituents carnosol, carnosic acid, and rosmarinic acid [30]. Its minor components may have a potential influence on biological activity due to the possibility of synergistic effects amongs<sup>t</sup> their components [31]. Finally, green tea tree extract (GTE) obtained from *Camellia sinensis* has gained significant attention in recent years. GTE is mainly composed of gallic acid, theobromine, chlorogenic acid, and caffeic acid [32]. In view of the potential uses of these natural products as effective antimicrobial and antioxidants for food preservation, they can be grea<sup>t</sup> candidates for incorporation into PHA films to generate active packaging articles.

Since most EOs and NEs are volatile compounds, they require the use of manufacturing methods that are carried out at room temperature to preserve their original properties. In this sense, the electrospinning technique is an emerging technology in the food packaging field [33,34], which is based on the application of electrostatic forces to polymer solutions to generate polymer fibers with diameters ranging from below 100 nm to several micrometers. Owing to the high surface-to-volume ratio of the electrospun fibers and the controllable pore size of the electrospun mats, several active and bioactive applications have been proposed in recent years [35], including the development of novel antimicrobial systems [36]. Since the electrospinning technique is frequently performed at room temperature, it facilitates the processing of thermolabile substances [37]. In addition, in a packaging application context, the ultrathin electrospun PHA fiber mats can be further converted into continuous films through the application of a thermal post-treatment below the polymer's melting temperature (*Tm*), i.e., the so-called annealing [38,39].

The objective of this research was to develop, for the first time, electrospun PHBV films containing OEO, RE, and GTE, in order to obtain active packaging layers with antioxidant and antimicrobial properties. Likewise, the morphological, optical, and thermal properties of the electrospun biopolymer films were also evaluated.

#### **2. Materials and Methods**
