## 3 Results
### Conductive Fillers
#### 3.2.1 Carbon-Based Solutions  
Mainly, carbon-based fillers such as carbon black, graphene, carbon nanofibers and carbon nanotubes are widely used to increase conductivity [73; 74]. * _Carbon Black_--Carbon black fillers are preferred over metal fillers as they do not undergo oxidation, whereas, metal fillers get oxidized and create an insulation layer on particles surface [75]. Other advantages of hybrid composites made from carbon black fillers include: flexibility, light weight process capabilities, absorption of mechanical shock, and low production costs [76]. In [77], carbon black nanoparticles have been used to develop an electric responsive scaffold for cardiomyocytes-related experiments, showing good adhesion and viability. * _Carbon Nanotubes_--Carbon nanotubes (CNTs) possess extremely high strength together with low density and high electrical conductivity. CNTs are widely used in scaffold-related applications, as they support cell adhesion and their dimensions are comparable with extracellular matrix (ECM) molecules [78]. In [79; 80], CNTs are used as carbon filler showing that these fillers has remarkably increased the conductivity and mechanical strength of designed scaffolds. A 3D-structure with tunable porosity for cardiac tissue engineering was developed, showing reinforced properties in terms of mechanical and electrical properties thanks to DWCNTs [81]. Also nerve tissue  
Figure 1: (**a**) Experimental setup for electrode-based device with scaffold; (**b**) impedance magnitude and phase shift change with and without cells (reprinted with copyright permission [60] Copyright © 2018 American Chemical Society). engineering applications, regarding both central nervous system (CNS) and peripheral nervous system (PNS), were studied in presence of CNTs showing cells proliferation and viability [78; 82]. * _Carbon Fibers and Nanofibers_--Carbon nanofibers (CNFs) could mechanically strengthen scaffolds and have the availability of higher amount of sites on the outer wall respect CNTs, thus facilitating electron transfer for electroactive analytes such as proteins and enzymes [83]. In [83], CNFs were successfully used in a chitosan matrix to fabricate an electrically conductive scaffold with a highly porous and interconnected structure for cardiac tissue engineering exhibiting high electrical properties and an elastic modulus similar to that of rat myocardium. CNFs were also incorporated in a biocompatible scaffold made up of cotton, which was carbonized, oxidized, and coated with polydopamine (PDA) to study nerve cells proliferation and differentiation, obtaining a final material similar to native cellular physiological environment facilitating cell adhesion, proliferation, differentiation, and other biofunctions [84]. * _Graphene_--Graphene is a single layer of carbon atoms in a hexagonal lattice, capable of high mechanical properties combined with high electrical conductivity and biocompatibility. Graphene showed an important versatility since it was employed as filler in different base materials like chitosan/gelatin matrices, hydrogels and Poly(trimethylene carbonate) (PTMC). Biological properties like better adhesion, spreading and proliferation of cells on the conductive graphene make it a suitable material for scaffold based applications in tissue engineering [85; 86; 87; 88; 89; 90; 91]. A summary of the specific properties of each carbon-based filler are reported in Table 2.  
In [78], CNTs/PLLA sensing scaffolds were microfabricated using Pressure-Activated Microsyringes (PAMs), a rapid prototyping microfabrication system which extrudes polymer solutions for biomedical and biosensing applications. Studies were carried out in presence of hepatocytes C3A cells and an electric cell-substrate impedance sensing (ECIS) method has been used, in which the impedance change of scaffold is recorded and correlated with corresponding cellular activities.