# Carbon Nanotube-Based Composite Filaments for 3D Printing of Structural and Conductive Elements
## 1 Introduction  
Additive manufacturing (AM), also known as 3D printing, widespread in the 90s and continuously improved [1], is one of the most promising areas in the fabrication of elements from prototypes to final products with complex geometries. Nowadays, this technique has been implemented in many industries such as biomedicine, construction, fashion, space industries, automotive, aerospace, electronics, and many more [2; 3; 4; 5; 6; 7; 8; 9]. Additive manufacturing has a unique advantage over traditional manufacturing methods due to the possibility of obtaining complex shapes with the flexibility to modify them easily, without generating much waste [10; 11], which has a positive impact on the environment [12]. Among many AM techniques, the FDM (fused deposition modeling) technique using thermoplastic polymer material has gained the most significant attention due to its availability, low cost of the material used, and relatively high quality of the manufactured elements. FDM as a filament-based process consists of melting filament in a heated nozzle. The printer nozzle moves and extrudes material onto a substrate or previously deposited layer to directly fabricate the desired 3D components [13]. Obtaining spatial structures consists of applying the material layer by layer, with subsequent thermal bonding of the deposited layers. In the FDM technology, it is possible to use any thermoplastic material. However, to ensure appropriate thermal and rheological properties of the extruded material, the most frequently used materials are ABS (acrylonitrile butadiene styrene), PLA(polylactic acid), and PETG (polyethylene terephthalate glycol-modified). It is also possible for specialized engineering applications to use polymers with increased mechanical and thermal properties such as PEEK (polyether ether ketone), nylon, PEI (polyetherimide), and many others [14]. Along with the development of FDM technology, researchers have begun working on new materials to enable the production of elements with improved mechanical, electrical, thermal, and other functional properties. Despite their numerous advantages over traditional manufacturing methods, FDM printed elements often have deteriorated mechanical properties, significantly reducing their use in final load-bearing products. Deteriorated mechanical properties of objects produced in FDM technology result from the very nature of the materials used. In the most basic version of FDM technology, it is possible to make elements only from thermoplastic polymers. Numerous studies have been performed to improve the mechanical properties of printed parts by optimizing printing parameters, such as layer height, printing speed, or orientation of the element [15; 16; 17]. However, regardless of the optimization of the printing process itself, the printed parts have several limitations resulting from the adhesion between subsequent paths and layers and internal porosity of the element, which makes it impossible to obtain a printout with 100% filling. One of the methods to improve the mechanical properties of FDM printouts is to use composite material. To develop the composite material, it is necessary to determine the suitable polymer matrix, which is primarily responsible for the mechanical and rheological properties of the filament, and the functional phase that will improve the mechanical properties while maintaining appropriate processing properties of the composite. In the literature, there are examples of using functional phases in the form of continuous carbon, glass and other fibers [18; 19; 20; 21; 22], short glass fibers [23], carbon fibers [24], and nanocomposite materials [25; 26; 27; 28] in order to improve the mechanical properties of FDM printed parts. The application of the appropriate functional phase allows not only the enhancement of mechanical properties but also the attainment of other unique properties, for example, electrical conductivity [8; 29; 30; 31].