## 2 Additively Manufactured Electrically Conductive Structures
### Applications of Additively Manufactured Integrated Electrically Conductive Structures  
Several applications for MEX demonstrate the integration of additively manufactured electrically conductive structures by using composite materials with different fillers, for example, carbon nanotubes (CNT), nano copper wires, carbon black or graphene. In the following, an overview of realized applications regarding electrical components specific to MEX is given.  
[PERSON] et al. (2017) [4] showed the integration of electrical components by means of a 3d printed flashing circuit, for instance, a push button, resistors, and cavities for the integration of a capacitor or an LED. Furthermore, to demonstrate the potential of functional integration of additively manufactured electrical components, [PERSON] et al. (2018) [19] realized a lithium ion battery that is integrated into side temples of a sunglasses and used to power an LCD. They investigated different conductive fillers and degrees with a matrix of PLA (polylactic acid) regarding resistivity and printability. [PERSON] et al. (2018) [PERSON] et al. (2018) [PERSON] et al. (2018) gave an overview of additively manufactured tactile sensors by using material extrusion-based processes such as MEX and direct ink writing. Specific to MEX, they emphasized the challenge between high conductivity and ensuring printability through a correspondingly high proportion of the matrix material. However, the application of different flexible tactile sensors is presented by using a conductive composite in combination with a flexible material. For instance, a capacitive force sensor is realized by using a copper filled thermoplastic elastomer. In addition, strain sensors with piezo-resistive and elastic properties and multiaxial force sensors were additively manufactured by usage of a combination of TPU (thermoplastic polyurethane) and a CNT filled composite material. In comparison, [PERSON] et al. (2012) [PERSON] et al. (2012) [PERSON] et al. (2012) developed a piezo-resistive and capacitive sensor using a carbon black filled PCL (polycaprolactone) filament to detect a change of electrical resistance through pressure applied on the structure. [PERSON] et al. (2016) [PERSON] et al. (2016) printed highly conductive and flexible 2D circuits made of PLA filled with graphene flakes to investigate mechanical properties and resistivity. They reached an electrical conductivity from of up to 4.76 S/cm for a composite material filled with 6 wt % of graphene flakes. Moreover, they concluded that a deformation of the printed structure sharply increases its resistance. An additively manufactured temperature sensor made of PP (polypropylene) filled with carbon black was developed by [PERSON] et al. (2017) [PERSON] et al. (2017). They also presented a wearable flex sensor integrated in a TPU structure that change the resistance depending on the bending radius.  
Furthermore, [PERSON] et al. (2017) [PERSON] et al. (2017) presented an integrated heating panel by combining a carbon black filled composite with a PLA that changes the color depending on temperature. Thus, besides a heating function, they integrated an optical signal as a design element. However, the electrical power is limited by the high resistance of both the filler (CB) and the electrical bonding as they paused the manufacturing process to embed wires. In comparison to this, [PERSON] et al. (2018) [PERSON] et al. (2018) [PERSON] et al. (2018) used an ABS (acrylonitrile butadiene styrene) material filled with 6 wt % CNT to design an additively manufactured heating surface.