## 1 Introduction  
In the last few years, 3D printing has emerged as a leading manufacturing technology all over the world for a variety of applications. The success of 3D printing depends on fine-tuned materials to the needs of each application. There are many thermoplastic and thermosetting polymeric materials that can be selected for 3D printing depending on the application [1]. Polylactic acid (PLA)--a thermoplastic polymer derived from natural sources, completely biodegradable, and bio-absorbable--is considered a green alternative to petrochemical commodity plastics. It is highly demanded due to versatility in applications such as packaging, pharmaceuticals, textiles, engineering, chemical industries, automotive composites, and biomedical and tissue engineering; lately material for 3D printing has been promising [2; 3]. However, its mechanical performance as well as electrical and thermal properties must be improved in order to expand the application fields. The incorporation of nanofillers is a common approach to attain this goal. Carbon-based nanomaterials, offer the potential to combine PLA properties with several of their unique features, such as high mechanical strength, electrical conductivity, thermal stability and bioactivity. Carbon nanotubes (CNT) and graphene are state of the art and very promising carbonaceous materials. Their high specific surface area allows low loadings in order to tune polymer key properties concerning mechanical, thermal, electrical, and biological performance. CNT have exceptional mechanical properties, aspect ratio, electrical and thermal conductivities, and chemical stability. These characteristics make them excellent candidates for the creation of multifunctional materials either in the field of polymer composites or other application [4; 5; 6; 7; 8; 9; 10; 11; 12; 13]. Based on the unique properties of graphene, the graphene-based polymer composites are expected to offer superior mechanical properties, enhanced electrical and thermal conductivity, improved dimensional stability, higher resistance to microcracking, and increased barrier properties above the matrix polymer [14; 15; 16; 17; 18]. Recently, graphene and functionalized graphene were utilized in composites containing different fillers, and the combination of fillers have shown synergy effect in terms of the mechanical properties, thermal and electrical conductivities, and super capacitance [19]. It was found that combining together two nanofillers, such as carbon nanotubes and graphene, leads to the formation of a co-supporting network of both fillers. Among the different technologies applicable for graphene-polymer products fabrication, the additive manufacturing (e.g., 3D printing) is the most promising since it is generally applicable to different types of polymers and is a versatile and low-cost technology. The development of new conductive polymer nanocomposite materials for 3D printing is highly desirable to achieve better printability, mechanical properties, electrical and thermal conductivity [20; 21; 22; 23; 24; 25; 26; 27; 28]. Three methods most frequently used to obtain a dispersion of nanofillers into a polymer matrix in order to produce a filament are: solution mixing, melt blending, and in situ polymerization [29; 30]. Melt blending is a very attractive method from an economical point of view; it is environmentally friendly and highly scalable for preparing nanocomposites. This method involves direct addition of the nanofillers into the molten polymer, and thus allows optimization of the dispersion state by adjusting processing parameters such as mixing speed, time and temperature. The main idea and the novelty in the present work are related with investigation of the synergetic (combined) effects of various mono and bi-filler combinations of low-cost industrial graphene and multi-walled carbon nanotubes on the structure, electrical and thermal conductivity of biodegradable PLA-based nanocomposites.