# Novel Resistive Sensor Design Utilizing the Geometric Freedom of Additive Manufacturing
## 2 Materials and Methods  
### Design Methodology  
AM's new design freedom is often considered too late in the design development process, and therefore, the design potentials are only used selectively [33]. Based on limitations of previous works (see Table 1) a novel sensor design (see Figure 1a) for measuring compressive loads using resistive path adjustment is developed by considering AM's unique design possibilities. The systematic development of the novel resistive sensor with customizable force-resistance-behavior is supported by different design tools in order to overcome thinking barriers and ensure a goal-oriented utilization of the design possibilities provided by AM. In the conceptual design phase, the semantic network of AM design potentials by [PERSON] et al. [34], the design heuristics by [PERSON] and [PERSON] [35], and the design principles for multi-material AM by [PERSON] et al. [36] were used. Thus, different design principles and features were identified to realize a novel sensor design with tailored electrical and mechanical properties. Based on these design possibilities and the different sensor principles shown in Table 1, the following design potentials were identified to develop a novel sensor design for adjusting the force-resistance-behavior at specific deformation under compressive load:  
* Incorporation of electrical functionalities by using different materials in one part [34; 35; 36]
* Tailoring the change in electrical resistance under compressive load by using the principle of shortening the conductor length [31; 36], freeform surfaces, and undercuts [34]
* Tailoring the mechanical properties of the substructure by freeform surfaces, wall thickness combinations, and hierarchical structures (e.g., internal structures) [34; 36]The novel sensor design, as shown in Figure 1, utilizes a conductive spiral-shaped path that shortens under a compressive load, enabling an increased resistive response compared to material piezoresistivity alone. The non-conductive flexible structure underneath the conductor allows for precise control over the sensor's stiffness, and thereby its response under a specific load. The multi-material approach allows the tuning of the mechanical behavior, and thus the force range of the sensor, independent of its resistive range. If, by comparison, a mono-material approach was used, increasing the stiffness would require additional material, which would decrease the electrical resistance. This demonstrates the advantages of both a multi-material solution and of utilizing AM's freedom in design regarding shape complexity. Eight different sensors were designed and manufactured in order to realize specific force-resistance-behaviors. These consisted of four different substructure variants of increasing stiffness, as shown in Figure 1b-e. These substructures were covered by a conductive spiral that had either a 0.3 mm or a 0.5 mm gap between its windings. In all samples, the conductive spiral had a width of 2.4 mm, a thickness of 2.0 mm, and made three windings. The substructure had a height of 5.0 mm and a thickness of 0.8 mm for variant 1 (Figure 1b), a thickness of 0.8 mm--each bar--for variant 2 (Figure 1c), and thickness of 2.4 mm for variants 3 (Figure 1d) and 4 (Figure 1e). However, the outer shapes of variant 3 (Figure 1d) and 4 (Figure 1e) were equal, and a variation of the stiffness of variant 3 (Figure 1d) was achieved by using a honeycomb infill structure with an infill of 20% (see Table 2).