Liquid-based Material Extrusion of Flexible Silver Electrodes onto Electrospun Poly(vinylidene fluoride) Microfibers for Soft Piezoelectric Pressure Sensors: Towards Fully Three-dimensional Printed Functional Materials

The creation of soft piezoelectric pressure sensors presents significant challenges. We introduce an all-additive manufacturing technique that combines liquid-based material extrusion (MEX) with electrospinning. This innovative approach marks the first use of electrospun poly(vinylidene fluoride) (PVdF) fibers as the substrate for the direct production of piezoelectrically active materials without the need for electric poling as a post-processing step. The application of MEX enables the direct printing of electrode patterns without the need for mask fabrication. The surface morphology of electrospun PVdF fibers, which was significant for determining the yield of the fabricated sensors, was characterized by using scanning electron microscopy (SEM). We found that finer fibers with a uniform size distribution and fewer beads were preferred to improve the yield and the β-phase content of the electrospun fibers. The β-phase content, critical for the piezoelectric output, was calculated based on Fourier-transform infrared spectroscopy (FTIR) characterization results. The optimized parameters (i.e., 15.0 kV as the voltage, 15.0 wt.% as the solution concentration, and 0.51 mL/h as the flow rate) were determined to achieve a yield of 20.0% and a β-phase content of 71.8%. A theoretical model was developed to explain how porosity, controlled by electrospinning parameters, in combination with MEX-related parameters, can influence ink penetration depth, a critical factor for yield optimization. A representative sensor was evaluated to show a sensitivity of 17.2 mV/kPa. The sensor output signal decreased by approximately 1.9% when subjected to cyclic loading of 17.8 kPa at a frequency of 0.5 Hz for a duration of 21.0 minutes to prove the stability of the sensor performance. We demonstrated the application of the sensors by creating an all-additively manufactured stretchable sensing matrix, showcasing the stretchability and pressure sensing functionalities. The all-additive manufacturing technique simplifies the manufacturing of piezoelectrically active material-based devices, offering flexibility in engineering designs and a cost-effective solution for streamlined device manufacturing.