3D-printing of embedded resistive flex sensor, process development, and application to the custom rhythm game

Sensors are devices that can respond to an input with a measurable output, providing information about variations in the input source. Additive Manufacturing (AM) plays a pivotal role in the fabrication on novel sensors, offering new opportunities for sensor design and performance optimization using a wide range of materials at a relatively low cost. While the fabrication by the material extrusion technique (MEX) of commercial materials has been investigated in the literature, including the use of conductive PLA filament for strain gauge sensors, the systematic understanding of how printing parameters should be optimized based on the material and its intended function is still limited. Moreover, an analysis of how this printing process and the design affect the final printing, hence sensor performance outcomes, is still lacking. These aspects take on greater importance when a process is used in a less common context, as in the case of sensor printing. In this work, a robust and repeatable MEX process for the fabrication of PLAc strain gauge-based embedded sensors is proposed. Protopasta (TM) conductive PLA (PLAc) was adopted to print the sensor within an insulating PLA material (PLA). Key process parameters, including pre-printing idle time, ironing, and the number of sensors printed on the same substrate, were thoroughly evaluated. Then, strain gauge-based samples were fabricated under different printing conditions (idle pre-printing time for PLAc, optimal printing values, optimal design) to investigate and assess the robustness of the process. The evaluation was carried out by measuring, comparing, and analysing the relationship between the cross-section and electrical resistance of the samples. The optimal conditions that enhance the repeatability of the process were identified. Furthermore, the strain gauge-based sensors were characterised under bending conditions and showed a linear behaviour of the electrical resistance with the increasing curvature radius. Finally, the developed sensors were implemented in a rhythm game demonstrator, validating their practical functionality. Overall, the research provides a robust methodology to optimize the printing of a flexible sensor, leveraging the advantages of AM to produce reliable, ready-to-use devices.