Fused Filament Fabrication, a New Fabrication Paradigm for 3D Printing Capacitive Sensors: Design, Fabrication, and Characterization for Liquid Level Measurements.

In recent years, the exploitation of Additive Manufacturing technologies for the fabrication of different kinds of sensors has abruptly increased: in particular, a growing interest for extrusion-based techniques has emerged. This research proposes the exploitation of Fused Filament Fabrication (FFF) process and two commercial materials (one flexible and one conductive) for the monolithic fabrication of a bendable, coplanar capacitive sensor. The whole sensor, consisting of a flexible substrate and two electrodes, has been fabricated in a single-step printing cycle: Design for Additive Manufacturing approach was used, setting out a methodology to direct 3D print thin and close tracks with conductive materials, in order to obtain high capacitance values measurable by common measurement instrumentations. Despite a huge exploitation of FFF technology for piezoresistive-based sensors, this manufacturing process has never been used for the fabrication of coplanar capacitive sensors since the manufacture of thin and close conductive tracks (key requirement in coplanar capacitive sensors) is a challenging task, mainly due to low manufacturability of extruded conductive beads with a high level of detail. Two versions of the sensor were developed: the first one with an embedded 3D printed coverage (ready to use) and the second one which requires a further manual post-processing to seal the electrodes. The main benefits related to the exploitation of FFF technology for these sensors are: i) the reduction of the number of different manufacturing processes employed, from at least two in traditional manufacturing approach up to one, ii) the exploitation of a cost-effective technology compared to traditional high-cost technologies employed (i.e. lithography, inkjet etc.) iii) the reduction of manual and assembly tasks (one of the proposed versions does not require any further task) , and iv) the cost-effectiveness of the sensors (in a range between 0.27 € and 0.38 €). The two developed prototypes have been tested demonstrating all their potentialities in the field of liquid level sensing, showing results consistent with the ones found in scientific literature: good sensitivity and high linearity and repeatability were proved when different liquids were employed. These 3D printed liquid level sensors have these features: i) flexible sensor, ii) the length is limited only by the machine workspace, iii) they can be either applied outside of the traditional reservoirs or embedded into the reservoirs (by 3D printing both the reservoir and sensor in the same manufacturing cycle), and iv) simple calibration.Finally, the bendability of these sensors paves the way toward their application for liquid level sensing into tanks with non-conventional shapes and for other application fields (i.e. soft robotics, non-invasive monitoring for biomedical applications).