**1. Introduction**

Active technologies, when applied to packaging refer to the incorporation of certain additives into the packaging structure. These additives may be loose as sachets within the design, attached to the inside part or, more recently, dispersed as an additive within the packaging materials, in order to maintain or even extend product quality and shelf-life [1].

Permeated or head space oxygen in packaged foods, beverages, and pharmaceuticals can promote a range of oxidative degradation reactions and support microbial growth, ultimately impacting on product quality and shelf-life. Oxygen-scavenging active packaging systems have therefore been explored, to control headspace oxygen content [2].

The application of oxygen scavengers is one of the most important active packaging technologies, which aim to remove any residual oxygen that is present in the food packaging. In some cases, the residual levels of oxygen in the package can be reduced to < 0.01 vol %, and actively controlled, which is not possible with other packaging systems [3].

Oxygen scavengers are by far the most commercially important sub-categories of active packaging, and the market has been growing steadily over the last few years. Almost all oxygen scavenger sachets used commercially are based on the principle of iron oxidation. On the other hand, oxygen-scavenging film is a more promising emerging packaging technology, because it contains the active material within the film, and consumers are not in favor of having foreign objects such as sachets in the lining of their product packaging. With oxygen scavenger films, the consumer cannot physically see the oxygen scavenger materials, ye<sup>t</sup> are able to experience its benefits [4–6].

The incorporation of scavengers into packaging films is a better way of resolving sachet-related problems. Scavengers may either be imbedded into a solid, dispersed in the plastic, or introduced into various layers of the package, including adhesive, lacquer, or enamel layers. Multi-layer oxygen scavengers more effectively absorb oxygen than single-layer scavenging systems [7].

Several new oxygen-scavenging technologies have been developed over the last decade, incorporating active substances and metals directly into packaging films or containers [8]. However, only a few of them have been successfully implemented in real food systems, due to, for instance, in the case of metals that function by chemical reduction, low reaction capacities and the need for triggering mechanisms, among other factors. Consequently, real application studies demonstrating the benefits of alternative oxygen-scavenging systems to particular food products are rather rare [9].

Recently, Hutter et al. [10] developed an oxygen scavenging film based on a catalytic system with palladium (CSP), which is able to reduce residual headspace oxygen very quickly. Palladium, in very low dosages, catalyzes the oxidation of hydrogen into water, and thus can remove the residual oxygen in the headspace of a modified atmosphere package containing hydrogen. Catalytic systems based on palladium have also been reported to have other interesting applications, such as the construction of complex molecules [11–14].

The main difficulties of this approach are the dispersion of the scavenger in the matrix, the accessibility of the scavenger to oxygen, and the necessity of an activation system for the oxygen absorption reaction. Without an activation system, the oxygen-scavenging capacity of the active film would be consumed during storage, before the packaging is used [15].

In addition, consumer trends for better quality, fresh-like, and convenient food products have intensified over the last decade. Therefore, a variety of active packaging technologies have been developed to provide better quality, wholesome, and safe foods, and also to limit package-related environmental pollution and disposal problems.

Recently, the environmental impact of persistent plastic packaging wastes is raising general global concern, since disposal methods are limited. Biopolymers have been considered as a potential environmentally-friendly substitute for the use of non-biodegradable and non-renewable plastic packaging materials [16].

Polycaprolactone (PCL) is petroleum-based, but it can be degraded by microorganisms, and the polyhydroxyalkanoate (PHA) homopolymer called poly(hydroxybutyrate) (PHB) is produced from biomass or renewable resources, and it is readily biodegradable [17]. The aim of this emerging and developing field is to change the nature of polymer products and to minimize the environmental impact. Various approaches are currently being investigated for possible polymers that may be utilized to design adequate environmentally friendly packaging [18].

Electrospinning is a feasible, efficient, and convenient technique for obtaining biopolymer-active nanofibers of interest in many application fields, such as active packaging, and since recently, it has also been scaled up for mass production [19–23]. Many factors influence fiber morphology and diameter, including solid concentrations, types of solvent, surface tension, additivation, solution viscosity, polymer molecular weight, flow rate, injector design, spinneret diameter, solution conductivity, injector to collector distance, and applied voltage. Of the many parameters discussed, concentration/solution viscosity, surface tension, and conductivity are probably the most important factors affecting the final fiber morphology and diameter [24–26]. A previous study [15] dealt with the development of a monolayer of oxygen-scavenging electrospun PHB containing palladium nanoparticles (PdNP). This monolayer demonstrated oxygen scavenging, but after annealing of the fibers to reduce porosity and to generate a water barrier, the material reduced the oxygen scavenging capacity to a significant extent.

The present work focuses, for the first time, on the preparation of significantly enhanced oxygen-scavenging bilayered coatings of PHB and PCL electrospun fibers, so-called biopapers, containing PdNPs, so-called nanobiopapers, deposited on a cellulose paper, to derive an optimized passive and active coating of interest in biodegradable fiber-based packaging.

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