## 4 Results and Discussion
### Derivation of Design Principles  
Based on the experimental results in Sections 4.1 and 4.2, several design principles for the utilization of both electrically conductive and heat resistive structures manufactured by AM can be derived. By incorporating electrically conductive materials in a standard polymer such as PLA or another functional material, for example, shape memory or color-changing polymers, several functions can be integrated using multi-material MEX. In addition, these functionalities can be adjusted by the choice of matrix polymer and filler as well as geometry and process parameters. Table 6 shows an overview of selected design principles derived based on the experimental investigations. Besides the utilized effect, AM-specific levers and advice for design are indicated to facilitate both an application and an implementation of the design principles in product design. \begin{table}
\begin{tabular}{c c c c} \hline \hline  
Figure 20: Representation of current, electrical power and resistance depending on voltage of M3D. Based on the experimental results in Section 4.1, the resulting resistance of an additively manufactured structure can be adjusted by both geometry and process parameters. Especially the length and cross section as well as the raster angle orientation and the extrusion temperature. In addition, the filler and its content significantly influence the resistivity of the feedstock material, but also change the mechanical properties. Consequently, there are many possibilities to realize the required resistance depending on the application. However, the process parameters interact with the geometry of the part, which is why the determined resistivity of narrow structures (e.g., electrical conductors) is not directly transferable to broader geometries such as a heating surface. Due to its high conductivity and the low melting point of the matrix polymer PCL, M3D is suitable, e.g., for the manufacturing of electrical fuses, as resistance sharply increases when PCL softens. The electrical power that results in a softening of the structure can be adjusted by the raster angle orientation and the structure's dimensions (cross section and length) as well as the filler. However, for a repeatable application, it is required that the softening process is reversible and the initial resistance of the structure does not change, for example, due to a continual damage. The material- and process-specific resistance heating can further be utilized to realize a heating surface with varying heat radiation by using locally different process parameter sets, for example, raster angle and extrusion temperature or combining materials with different fillers. Temperature can also be adjusted by variation of the applied voltage. When incorporating a heating surface into an additively manufactured part, it should be considered that support structures below the conductive part probably decrease the electrical power due to damages. To integrate additional functions based on heat-activation, a conductive material can be combined, for example with a color-changing material, to realize thermal induced optical signals. In addition, resistive heating can also be utilized for the activation of shape memory polymers. Moreover, AM's design freedom allows manufacturing of hierarchical lattice structures that can be utilized to heat air flows. The resulting resistance and, thus, the electrical power can be adjusted by varying cellular geometry (cross section) and process parameters (extrusion temperature and raster angle). However, the filler limits the resolution regarding thin features as a small nozzle diameter increases the risk of clogging. The surface roughness on the overhangs probably improve the heat transfer of the lattice structure and the air flow due to turbulences. Finally, the temperature-dependent resistance of the CB can be used to realize additively manufactured temperature sensors. However, it has to be ensured that the initial resistance of the sensor does not vary, e.g., due to damage caused by heating.