# Novel Resistive Sensor Design Utilizing the Geometric Freedom of Additive Manufacturing
## 4 Discussion  
The results show that the novel sensor design successfully created a resistive path that is physically shortened under mechanical compression, thereby providing a strong signal response.  
A schematic overview of the relationships between the different variables and the sensor resistive behavior is shown in Figure 12. The final resistive response of the sensors is related to the deformation distance, and affected by the gap size. Deformation dictates how much of the shortening resistive path mechanism is engaged. The initial gap size modulates this behavior both through the number of windings that will be engaged at a certain deformation level, and through the distance required to close the first gap and thereby the delay in response.  
The deformation distance in turn is related to the applied force, and affected by the gap size and the stiffness of the substructure variant used. The gap size influences the amount of physical interference, and thereby force, required for deformation, while a stiffer substructure directly resists deformation and affects the force-displacement curve. The actuation velocity has little to no influence on sensor behavior and is omitted from the figure.  
For the successful design of a sensor utilizing this working principle, this means that gap size and substructure should be selected in a combination suitable for the force range to be measured. The combination should lead to the forces causing gradual sense element engagement over the whole range.  
There are opportunities for further research and improvement on the presented design. The parts with gap size of 0.5 mm allowed a larger measurement range, but had a delayed response to force being applied, whereas the 0.3 mm variants responded more quickly, but nearly all their resistive response occurred in the first millimeter of deformation, limiting the measurement range. A possible improvement might therefore be found in the use of a non-constant gap size that combines both a quick response and a large measurement range. Alternatively, an optimized substructure of different height or graded stiffness could achieve similar results. The application of foaming materials in the substructure could enable use in very low force scenario.  
There may also exist more possibilities to optimize the conductive spiral or the substructure, e.g., by using other freeform surfaces or graded lattice structures (see [34]). The spiral sense element can be modified by laying it out on a 3D cone path, rather than as a 2D spiral. This could improve the engagement of the outer windings of the sense element, and allow further design freedom in sensor customization. Another possibility is the use of substructures with auxetic properties, enabling a further amplification of the resistive path shortening mechanism under load. The observed effects of stair-stepping in the side walls of the sense element leading to discontinuous deformation may be improved upon by optimizing the side wall angle, as well as reducing the layer thickness selected during part production.  
Further research opportunities also exist in the effects of materials and process parameters. Different polymers and different conductive fillers or combinations thereof may allow for further customization of the sensor performance. In addition, by varying the process parameters, the resistivity, geometry, and mechanical behavior of the sensors could  
Figure 12: Chart showing the relationship between influencing factors in sensor behavior.  
all be influenced and controlled. Lastly, establishing a low-resistance contact to outside equipment currently requires the application of a silver paste, silver ink, or similar medium. Further investigation might eliminate the need for this extra handling after the build.