# Additively Manufactured Polymers for Electronic Components
## 3 Polymers Used for AM in Electronic Applications
### Extrinsically Conductive Polymers (ECPs)
#### 3.2.1 Carbon Nanotubes  
Theoretically, carbon nanotubes have great mechanical properties due to the strong s bonds that characterise them. To make a comparison, multiwall CNTs (MWCNTs) have a tensile strength of 11-63 GPa [91], while Kevlar is characterised by a tensile strength of 3 GPa [92]. MWCNTs may present some percentage of defects due to their growing mechanism, so their mechanical properties could be experimentally lower than their theoretical ones. Their Young's modulus is independent of chirality but not of diameter, and the highest value of 1 TPa can be obtained for diameters of 1 nm and 2 nm. CNTs have been widely investigated in the literature and processed both in various forms and with different technologies [93; 94; 95]. In analogy with ICPs, the fabrication of CNT-based materials with AM has also gained wide interest in industrial applications [96]. Electrical conductivity was enhanced by coupling PEDOT with CNTs. In fact, a \(\pi\)-\(\pi\) interaction is established between the thiophene rings of the PEDOT backbone and MWCNTs, allowing the charge to be more delocalised [48]. PEDOT is a highly conductive polymer that is suitable for use in biosensors and energy storage applications. Its electrical conductivity arises from the delocalisation of \(\pi\)-electrons within its chemical structure, as well as the presence of sulfonated polystyrene (PSS) [97]. Three-dimensional samples were obtained and showed good mechanical properties and considerable electrical conductivity (0.05 S/cm\({}^{1}\)). Epoxy and acrylic CNT composites have been largely investigated due to the possibility of being directly printed through vat photopolymerisation processes. SL can effectively print functional polymeric materials with tailored chemical-physical properties by optimising the formulation [98]. Nevertheless, a lot of critical aspects should be taken into account when UV-curing these systems. [PERSON] et al. [99] developed a photocurable acrylic resin with CNTs for SL printing using PEGDA and poly(ethylene glycol) methyl ether methacrylate (PEGMEMA) as monomers, with CNTs dispersed via sonication. PEGMEMA was used to decrease the viscosity of the resin, enhanced by the addition of the filler, since low viscosities are required for the material to flow inside the vat for this 3D printing technology. Good electrical properties were obtained, reaching a value of 2 \(\times\) 10\({}^{-5}\) S/cm for a filler content of up to 0.5 wt%. The problem related to UV-curing of these types of systems is the shielding effect of CNTs, which makes polymerisation much more difficult. In fact, CNTs contribute to the absorption of UV light, competing with the photoinitiator. This results in a decrease in the cross-linking density, which reduces both resolution and printability. This drawback can be overcome by increasing the intensity of the SL light source or by increasing the photoinitiator content. Hence, 3D printing can be implemented to directly fabricate electrically conductive systems, such as microelectromechanical systems (MEMS), as illustrated in Figure 7. They are devices that combine electric and mechanical functions, such as pressure sensors, accelerometers, and gyroscopes. They are commonly produced via integrated circuit (IC) batch processing techniques and can have dimensions ranging from a few micrometres to millimetres. These devices can sense, control, and actuate on a microscale and have an impact on the macroscale. IC processing involves photolithography and chemical etching to add or subtract two-dimensional layers on a substrate (typically silicon). Among all AM techniques, SL, DLP, and two-photon lithography (2 PL) are suitable for printing these microstructures, as they offer high resolution and improved surface finish. However, they can work with a limited number of materials, and there is a need for a metallic finish on the printed object to provide electrical conductivity to the device since conventional MEMS are made of silicon and glass [100].