Bidirectional and Stretchable Piezoresistive Sensors Enabled by Multimaterial 3D Printing of Carbon Nanotube/Thermoplastic Polyurethane Nanocomposites

Fabricating complex sensor platforms is still a challenge because conventional sensors are discrete, directional, and often not integrated within the system at the material level. Here, we report a facile method to fabricate bidirectional strain sensors through the integration of multiwalled carbon nanotubes (MWCNT) and multimaterial additive manufacturing. Thermoplastic polyurethane (TPU)/MWCNT filaments were first made using a two-step extrusion process. TPU as the platform and TPU/MWCNT as the conducting traces were then 3D printed in tandem using multimaterial fused filament fabrication to generate uniaxial and biaxial sensors with several conductive pattern designs. The sensors were subjected to a series of cyclic strain loads. The results revealed excellent piezoresistive responses with cyclic repeatability in both the axial and transverse directions and in response to strains as high as 50%. It was shown that the directional sensitivity could be tailored by the type of pattern design. A wearable glove, with built-in sensors, capable of measuring finger flexure was also successfully demonstrated where the sensors are an integral part of the system. These sensors have potential applications in wearable electronics, soft robotics, and prosthetics, where complex design, multi-directionality, embedding, and customizability are demanded

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3D printing of strain sensors

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

Bidirectional and Stretchable Piezoresistive Sensors Enabled by Multimaterial 3D Printing of Carbon Nanotube/Thermoplastic Polyurethane Nanocomposites

Fabricating complex sensor platforms is still a challenge because conventional sensors are discrete, directional, and often not integrated within the system at the material level. Here, we report a facile method to fabricate bidirectional strain sensors through the integration of multiwalled carbon nanotubes (MWCNT) and multimaterial additive manufacturing. Thermoplastic polyurethane (TPU)/MWCNT filaments were first made using a two-step extrusion process. TPU as the platform and TPU/MWCNT as the conducting traces were then 3D printed in tandem using multimaterial fused filament fabrication to generate uniaxial and biaxial sensors with several conductive pattern designs. The sensors were subjected to a series of cyclic strain loads. The results revealed excellent piezoresistive responses with cyclic repeatability in both the axial and transverse directions and in response to strains as high as 50%. It was shown that the directional sensitivity could be tailored by the type of pattern design. A wearable glove, with built-in sensors, capable of measuring finger flexure was also successfully demonstrated where the sensors are an integral part of the system. These sensors have potential applications in wearable electronics, soft robotics, and prosthetics, where complex design, multi-directionality, embedding, and customizability are demanded

Chemically Active, Porous 3D-Printed Thermoplastic Composites

Metal–organic frameworks (MOFs) exhibit exceptional properties and are widely investigated because of their structural and functional versatility relevant to catalysis, separations, and sensing applications. However, their commercial or large-scale application is often limited by their powder forms which make integration into devices challenging. Here, we report the production of MOF–thermoplastic polymer composites in well-defined and customizable forms and with complex internal structural features accessed via a standard three-dimensional (3D) printer. MOFs (zeolitic imidazolate framework; ZIF-8) were incorporated homogeneously into both poly­(lactic acid) (PLA) and thermoplastic polyurethane (TPU) matrices at high loadings (up to 50% by mass), extruded into filaments, and utilized for on-demand access to 3D structures by fused deposition modeling. Printed, rigid PLA/MOF composites display a large surface area (SA_avg = 531 m² g^–1) and hierarchical pore features, whereas flexible TPU/MOF composites achieve a high surface area (SA_avg = 706 m² g^–1) by employing a simple method developed to expose obstructed micropores postprinting. Critically, embedded particles in the plastic matrices retain their ability to participate in chemical interactions characteristic of the parent framework. The fabrication strategies were extended to other MOFs and illustrate the potential of 3D printing to create unique porous and high surface area chemically active structures