## 2 Materials and Methods  
### Bio-Plastic Compounds Fabrication  
The new bio-plastic compounds can be essentially defined as composite materials. These can be described as materials constituted by two or more different phases with a distinct interface: (i) the matrix or continuous phase and (ii) the filler/reinforcement phase, surrounded by matrix material [20; 21]. Although preserving the individual phases for their own features in the final product, when combined, the resulting composite material displays enhanced mechanical/physical characteristics when compared to pristine components [22; 23]. Composite materials can show remarkable anisotropy, according to fibres' arrangement, namely, their properties change considerably in different directions, but also nearly isotropic configurations can be obtained [24]. Generally, in artificial and industrial uses, the matrix phase is based on synthetic thermosetting or thermoplastic polymers, depending on manufacturing processes and final technological applications. On the other hand, the filler phase can be based on inorganic fibers or particulates (e.g., glass, carbon, etc.) in order to ensure strength and stiffness to the resulting composite material [22]. When compared to common composite materials, these compounds are entirely natural-based. The matrix consists of Polylactic Acid (PLA), a 100% bio-based plastics (purchased from Ingeo Biopolymer 3D450 by NatureWorks), and the reinforcements are based on different particles (more than 20% in weight) from agricultural waste, which makes these products into particulate bio-composite materials. In particular, the by-product particles used as filler have been standardized, by controlling the respective humidity values, spherical shape, and caliber, as illustrated in Table 1.  
The different filaments are made with fillers, which are randomly dispersed in a PLA matrix. Thus, they can be considered as composites with short fibres (such as short fiber thermoplastics, which are well-known) that can be treated as nearly isotropic. This assumption is also enforced by the shape of the fillers, which was characterized by an adimensional parameter of sphericity. It was found that this parameter ranged between 0.8 and 1.2, where the value 1 corresponds to a perfect sphere. Sphericity also promotes the homogeneity of the material since it improves the adhesion between the filler and the matrix minimizing the presence of micro-voids, which could produce microcracks and internal stresses. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Material** & **Relative Humidity** & **Shape** & **Particle Size (\(\upmu\)m)** \\ \hline HEMP & 6\% & Spherical-like & 130 \\ WEED & 4\% & Spherical-like & 150 \\ TOMATO & 4\% & Spherical & 100 \\ CAROB & 4\% & Spherical & 130 \\ PRUNED & 4\% & Spherical & 100 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Main characteristics of the particles used as a filler phase from agricultural waste. The production process begins with the selection of industrial plant by-products, which are selected for their particle size and chemical properties. In detail, the standardized particles, illustrated in Table 1, are joined to the thermoplastic PLA matrix in a two-screw extruder (Figure 1) while acting as a mixer/compounder in the temperature range of the PLA melting point. Moreover, before being inserted into the hopper, the organic by-products are pretreated by means of a micronizer and a dehumidifier in order to maximize the compatibility between the matrix and the filler. The injection parameters used by the two-screw extruder are shown in Table 2. The final product is a biodegradable thermoplastic polymer. Upon leaving the mixer/compounder, the compound is placed in a granulator for the production of granules or in an extruder in order to produce filaments for 3D-printing. The five different bio-plastic compounds filaments, obtained by varying the organic by-product fillers used, are:  
(1) HEMP (Figure 2a): filament of natural and compostable origin containing hep (ranging from 15% to 25%).