# Novel Resistive Sensor Design Utilizing the Geometric Freedom of Additive Manufacturing
## 2 Materials and Methods
### The Additive Manufacturing Process and Design Variants  
The sensor designs were manufactured by material extrusion, because of the multi-material capability and the variety of commercial available technical polymers, especially regarding elastomers and electrical conductive polymers [6]. In order to ensure a reliable multi-material design, the compatibility of the material combination had to be taken into account. The compact design allowed no additional measures for improving the interface  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Conductive PLA** & **TPU Variants 1,2, \& 4** & **TPU Variant 3** \\ \hline Extrusion width (mm) & 0.4 & 0.4 & 0.4 \\ Nozzle temperature (\({}^{\circ}\)C) & 230 & 235 & 235 \\ Bed temperature (\({}^{\circ}\)C) & 40 & 40 & 40 \\ Layer height (mm) & 0.2 & 0.2 & 0.2 \\ Infill (\%, pattern) & 100, only shells & 100, only shells & 20, honeycomb \\ Extrusion speed (mm/s) & 30 & 35 & 35 \\ Perimeter shells (-) & 3 & 3 & 2 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Process settings for sample production.  
Figure 1: Render showing the sensor design (**a**). The black top is electrically conductive polymer, the white substructure is flexible elastomer to control the mechanical behavior of the sensor. Details (**b–e**) show substructure variants 1 to 4 respectively, each structure with increasing stiffness. For variant 3, the toolpath from the underside is depicted, as the CAD (computer-aided design) models for variants 3 and 4 are identical. The difference in stiffness is created through lower infill during printing for variant 3, whereas 4 is solid.  
strength between the substructure and the conductive spiral, e.g., interlocking features [37], without a negative impact on the sensor function. In accordance with [PERSON] et al. [37], the material combination of TPU and PLA was chosen because of the good interface strength. The substructure was made using NinjaTek(r) Ninjaflex TPU [38], and the conductive spiral was manufactured using Protopasta Conductive PLA [39], which is a PLA with approximately 21.5 wt% CB conductive particles. Aside from favorable multi-material behavior, the conductive PLA was selected based on its easy processability, low geometrical deviations, and relatively high electrical resistance. As demonstrated in [PERSON] et al. [6], resistivity of the extruded material lies in a range of 0.07-0.11 \(\Omega\)m. Though materials with lower resistivity exist [40], a relatively high initial electrical resistance was considered to be a desirable property in this case, as it enables a larger change in resistance when the resistive path is shortened during sensor operation.  
Parts were sliced using Simplify3D(4.1.2, Simplify3D, LLC, Cincinnati, OH, USA, 2020), and built on the pro-consumer machine X400 by German RepRap GmbH (Feldkirchen, Germany), that comes with a dual extrusion system. The process settings were used as listed in Table 2 (Section 2.1). The build orientation was such that the conductive spiral faces downwards on the build platform. This resulted in only a single tool change per build. Build times ranged from 10 to 15 min per sensor, depending on the density of the substructure.