Using fused filament fabrication (FFF), they studied two commercial hybrid metal/polymer filaments for manufacturing metal parts without using the expensive and complex equipment typically associated with standard additive manufacturing (AM) techniques for metals [8]. The results of the mechanical characterization revealed that considerable gaps still exist compared to metals produced by standard metal AM techniques. However, it was demonstrated by cost modelling referring to a single mold prototype that the use of hybrid metal/polymer filaments processed by FFF has a strong potential to facilitate the printing of metal parts at lower cost. [PERSON] and his collaborators prepared a hybrid composite based on polyvinyl alcohol-chitosan (PVA-Chi) and reinforced it with conductive fillers such as polypyrrole (PPy), Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT: PSS), carbon black (CB), and a multi-wall carbon nanotube (MWCNT) for engineering applications [9]. The obtained hybrid materials showed higher electrical conductivity in the OH's presence and NH\({}_{2}\) groups, which means that they could have possible applications in biopolymer electrodes. [PERSON] et al. investigated the dispersion and interfacial adhesion of both isotropic and anisotropic fillers in based polymer composites [10]. They highlighted the key role of the particles in the filler distribution. Specifically, they observed how non-isotropic particles created eddies in the matrix, resulting in some porosity within the volume, but enhancing filler adhesion with the polymer. On the contrary, isotropic particles showed uniform distribution within the polymer matrix. The mechanism of the filler particles' movement within the matrix was explained in terms of simulation and surface images. Through the sol-gel method, [PERSON] and coworkers designed and synthesized iron (II)-based hybrid materials with silica and polyethylene glycol (PEG) for drug delivery employment [11]. The release kinetics in a simulated body fluid (SBF) were investigated, and the amount of Fe\({}^{2+}\) released was detected via ultraviolet-visible spectroscopy (UV-Vis). They also evaluated the bioactivity of the synthesized hybrid materials and the potential antibacterial properties of seven different materials against two different bacteria--_E. coli_ and _S. aureus_. Continuing their studies on the hybrid materials employed in medical applications, the Italian research group focused their work on the failure of medical devices, such as bones prosthesis, which is generally the result of inflammatory and infectious phenomena. Since entrapping anti-inflammatory and anti-microbial agents inside the biomaterial matrix could avoid these phenomena, they prepared, consistently through sol-gel synthesis, inorganic/organic silica (S)/polyethylene glycol (P)/caffeic acid (A) hybrid systems with different weight percentages of P and A [12]. They verified the formation of a hydroxyapatite layer on hybrid surfaces, and through antimicrobial activity measurements, they evaluated the caffeic acid kinetic release in simulated body fluid (SBF) at 37 \({}^{\circ}\)C.  
[PERSON] and her research group were engaged in developing eco-friendly bio-composites through the copolymerization of a vegetable-oil-based epoxy and epoxidized linseed oil (ELO) with dodecenyl succinin anhydride (DDSA) [13]. The polymer matrix and the natural filler were then characterized showing improvements to the mechanical properties of the composites, fulfilling multiple roles: (i) spruce bark powder (SB) with its hydrochar (HC) acts as a co-reactant in the copolymerization mechanism; (ii) HC acts as reinforcement, consolidating the network and providing stiffness and rigidity; and (iii) SB acts as a plasticizer for reducing the brittle character of the epoxy resins. [PERSON] et al.