3D printing of strain sensors

Josef F. Christ

As the desire for advanced wearable electronics increases and the soft robotics industry advances, the need for new sensing materials has also increased. Recently, there have been many attempts at producing novel materials which exhibit piezoresistive behavior. However, one of the major shortcomings in strain sensing technologies is in fabricating such sensors. While there is significant research and literature covering the various methods for developing piezoresistive materials, fabricating complex sensor platforms is still a manufacturing challenge. This research aims to bridge this gap between the design and fabrication of stain sensors through the integration of fused deposition modeling (FDM) and piezoresistive nanocomposites. In this work, a 3D-printable, flexible, and electrically conductive thermoplastic-based filament was successfully developed for strain sensing applications. Thermoplastic polyurethane/multiwall carbon nanotube composites were compounded, their filaments were extruded and 3D printed using fused deposition modeling. The mechanical, electrical, and piezoresistivity behaviors of bulk and 3D printed TPU/MWCNT were investigated under single strain and cyclic strain loadings. It was found that the printed samples demonstrated lower mechanical strength and lower elastic modulus when compared to their extruded counterparts by approximately 16%. Further, in both printed and extruded samples, the piezoresistive behavior showed very similar responses within the tested range for both single strain loads as well as for cyclic loading for loadings at or greater than 3wt. % MWCNTs. The gauge factor also demonstrated similar results. Following the filament characterization, pure thermoplastic polyurethane (TPU) and TPU containing 3wt. % multiwall carbon nanotubes (MWCNT) were printed in tandem using a low-cost multi-material FDM printer to fabricate uniaxial and biaxial embedded strain sensor platforms with various patterns of conductive paths. The sensors were then subjected to a series of cyclic strain loads. They demonstrated strong piezoresistive behavior over a range of cyclic strains. It was also demonstrated that the response could be adjusted by altering the printed pattern within the sensor. This work demonstrates the potential application of 3D printed strain sensing platforms with high impact to fields like wearable electronics, prosthesis design, and other fields where rapid fabrication of strain sensors could decrease fabrication complexities and increase design feasibility.

3D printing of strain sensors

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