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

FABRICATION OF MAGNETIC TWO-DIMENSIONAL AND THREE-DIMENSIONAL MICROSTRUCTURES FOR MICROFLUIDICS AND MICROROBOTICS APPLICATIONS

Micro-electro-mechanical systems (MEMS) technology has had an increasing impact on industry and our society. A wide range of MEMS devices are used in every aspects of our life, from microaccelerators and microgyroscopes to microscale drug-delivery systems. The increasing complexity of microsystems demands diverse microfabrication methods and actuation strategies to realize. Currently, it is challenging for existing microfabrication methods—particularly 3D microfabrication methods—to integrate multiple materials into the same component. This is a particular challenge for some applications, such as microrobotics and microfluidics, where integration of magnetically-responsive materials would be beneficial, because it enables contact-free actuation. In addition, most existing microfabrication methods can only fabricate flat, layered geometries; the few that can fabricate real 3D microstructures are not cost efficient and cannot realize mass production. This dissertation explores two solutions to these microfabrication problems: first, a method for integrating magnetically responsive regions into microstructures using photolithography, and second, a method for creating three-dimensional freestanding microstructures using a modified micromolding technique. The first method is a facile method of producing inexpensive freestanding photopatternable polymer micromagnets composed NdFeB microparticles dispersed in SU-8 photoresist. The microfabrication process is capable of fabricating polymer micromagnets with 3 µm feature resolution and greater than 10:1 aspect ratio. This method was used to demonstrate the creation of freestanding microrobots with an encapsulated magnetic core. A magnetic control system was developed and the magnetic microrobots were moved along a desired path at an average speed of 1.7 mm/s in a fluid environment under the presence of external magnetic field. A microfabrication process using aligned mask micromolding and soft lithography was also developed for creating freestanding microstructures with true 3D geometry. Characterization of this method and resolution limits were demonstrated. The combination of these two microfabrication methods has great potential for integrating several material types into one microstructure for a variety of applications

High deformation multifunctional composites: materials, processes, and applications

Structural health monitoring (SHM) is a non-destructive process of collecting and analysing data from structures to evaluate their conditions and predict the remaining lifetime. Multifunctional sensors are increasingly used in smart structures to self-sense and monitor the damages through the measurements of electrical resistivity of composites materials. Polymer-based sensors possess exceptional properties for SHM applications, such as low cost and simple processing, durability, flexibility and excellent piezoresistive sensitivity. Thermoplastic, thermoplastic elastomers and elastomer matrices can be combined with conductive nanofillers to develop piezoresistive sensors. Polymer, reinforcement fillers, processing and design have critical influences in the overall properties of the composite sensors. Together with the properties of the functional composites, environmental concerns are being increasingly relevant for applications, involving advances in materials selection and manufacturing technologies, In this scenario, additive manufacturing is playing an increasing role in modern technological solutions. Stretchable multifunctional composites applications include piezoresistive, dielectric elastomers (mainly for actuators), thermoelectric, or magnetorheological materials [1]. In the following, piezoresistive materials and applications will be mainly addressed based on their increasing implementation into applications.Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Funding UID/FIS/04650/2019 and UID/EMS/00151/2019. The authors thank the FCT for financial support under SFRH/BPD/110914/2015 (P. C) and SFRH/BPD/117838/2016 (J. Pereira) grants. Financial support from the Basque Government Industry and Education Departments under the ELKARTEK, HAZITEK and PIBA (PIBA-2018-06

Wearable and Stretchable Strain Sensors: Materials, Sensing Mechanisms, and Applications

Recent advances in the design and implementation of wearable resistive, capacitive, and optical strain sensors are summarized herein. Wearable and stretchable strain sensors have received extensive research interest due to their applications in personalized healthcare, human motion detection, human–machine interfaces, soft robotics, and beyond. The disconnection of overlapped nanomaterials, reversible opening/closing of microcracks in sensing films, and alteration of the tunneling resistance have been successfully adopted to develop high-performance resistive-type sensors. On the other hand, the sensing behavior of capacitive-type and optical strain sensors is largely governed by their geometrical changes under stretching/releasing cycles. The sensor design parameters, including stretchability, sensitivity, linearity, hysteresis, and dynamic durability, are comprehensively discussed. Finally, the promising applications of wearable strain sensors are highlighted in detail. Although considerable progress has been made so far, wearable strain sensors are still in their prototype stage, and several challenges in the manufacturing of integrated and multifunctional strain sensors should be yet tackled

A possible scenario for volumetric display through nanoparticle suspensions

We discuss on the potential of suspensions of gold nanoparticles with variable refractive index for the possible physical realization of in-relief virtual dynamic display of plane images. A reasoning approach for a vision system to display in real-time volumetric moving images is proposed based on well-known properties of optical media, namely the anomalous dispersion of light on certain transparent media and the virtual image formed by a refracting transparent surface. The system relies on creating mechanisms to modify the refractive index of in-relief virtual dynamical display (iVDD) bulbs that ideally would contain a suspension of gold nanoparticles each and that might be ordered in an array filling up a whole screen.Comment: 15 pages. To appear Momento - Revista de Fisica (June 2001

Development of elastomeric composite materials for the realization of piezoresistive sensors

Piezoresistive materials, able to sense geometrical deformation through variations of the electrical resistance, attracted an increasing interest in the scientific and industrial comparts during the last forty years, which increased significantly with the advent of nanostructured carbon-based conductive materials. These light, highly conductive and easy-to-obtain fillers have broadened the spectrum of materials that had been used up to that time, opening up the possibility of greater development of multifunctional materials. In particular, the carbonaceous fillers, homogeneously dispersed within a polymer matrix, immediately represented a valid alternative to the metals used in the field of piezoresistive systems. In the context of polymer nanocomposites and piezoresistive materials, a significant challenge for the scientific community is represented the by the achievement of an effective percolation pathway, which allows the passage of an electric current at the lowest percentage of filler (percolation threshold), and provides a direct correlation of the external forces in with the electrical resistance variations. Generally, the piezoresistive materials based on the exploitation of the polymer-based composites are designed by homogeneously dispersing the carbonaceous filler in the polymeric matrix. However, it is well known that a simple approach to reduce the content of filler and realize a conductive composite can be obtained by exploiting the concept of segregation of filler in the polymeric matrix. When the filler is not randomly dispersed, but segregated to build up a three-dimensional network, the electrical conductivity can be obtained with a significantly lower content of the carbonaceous filler. Amongst the several techniques for the realization of piezoresistive systems, in the last decade, the Additive Manufacturing (3D printing) technologies have aroused the greatest interest. The 3D printing processes lead to a considerable reduction in costs and times as compared with the traditional technologies of processing of polymers. Furthermore, as regards prototyping, they allow an almost total freedom to create even complex shapes and geometries in an automated and effective way. In particular, Selective Laser Sintering (SLS) is one of the most interesting technology, able to build up easily the segregated filler network, starting from polymeric powder adequately prepared. It is focused on the sintering of polymeric particles by a laser in the classic layer-by-layer mode. Many polymers can be used, from elastomeric to thermosetting, as well as conductive fillers. In this PhD research project, it was investigated the possibility of obtaining piezoresistive materials printed with 3D SLS using thermoplastic polyurethane (TPU) as a polymer matrix and graphene nanoparticles (GE) and multiwalled carbon nanotubes (MWCNTs) as conductive filler. The main objective of the doctoral research was to investigate the potential of SLS to create porous conductive materials with segregated distribution of the conductive filler, by evaluating the effect of different geometries and porosities (from 20% to 80%) and different shape of the conductive filler (i.e. 1D filler and 2D filler). Again, the aim was to evaluate, based on the complete characterization of the materials, what is the effect of the technology used, finding a possible correlation with the printed geometries. Thus, in the first part of the project, porous systems were printed using TPU modified with 1wt% of GE and starting from Diamond (D), Gyroid (G) and Schwarz (S) geometries for the building up of systems with regular porosity. The resulting three-dimensional porous structures show an effective conductive network due to the segregation of the graphene nanoplatelets previously assembled on the TPU powder surface in between the sintered elastomeric particles. The results confirm that GE nanoplatelets improve the thermal stability of the TPU matrix, while also increasing its glass transition temperature. Furthermore, porous structures made from S geometry show higher elastic modulus values in comparison with D and G based structures. After cyclic compression tests, all porous structures show robust negative piezoresistive behavior, regardless of their porosity and geometry, with exceptional sensitivity to deformation. Gauge Factor (GF) values of 12.4 at 8% deformation are obtained for S structures with 40 and 60% porosity, while GF values up to 60 are obtained for deformations lower than 5%. The thermal conductivity of TPU/GE structures significantly decreases with increasing porosity, while the effect of the structure architecture is less relevant. The second part of the project focused on the characterization of 3D printed TPU products with MWCNTs and a mixture of the two fillers, again at 1wt% but with a proportion of 70/30 wt/wt MWCNTs/GE with geometries D and G, in order to investigate a possible synergistic effect of the two conductive fillers. The results showed that the porous structures based on TPU with 1wt% MWCNTs/GE exhibit excellent electrical conductivity and mechanical strength. In particular, all the porous structures show a robust negative piezoresistive behavior, as demonstrated by the GF values that reach values of about -13 at 8% deformation. Moreover, the G20 porous structures (20% porosity) show microwave absorption coefficients ranging from 0.70 to 0.91 in the 12-18 GHz region and close 1 in the THz (300 GHz - 1 THz) frequency region. The results show that the simultaneous presence of MWCNT and GE brings a significant improvement in the specific functional properties of porous structures, which are proposed as potential piezoresistive actuators with relevant electromagnetic interference (EMI) shielding properties

Conductive textiles for signal sensing and technical applications

Conductive textiles have found notable applications as electrodes and sensors capable of detecting biosignals like the electrocardiogram (ECG), electrogastrogram (EGG), electroencephalogram (EEG), and electromyogram (EMG), etc; other applications include electromagnetic shielding, supercapacitors, and soft robotics. There are several classes of materials that impart conductivity, including polymers, metals, and non-metals. The most significant materials are Polypyrrole (PPy), Polyaniline (PANI), Poly(3,4-ethylenedioxythiophene) (PEDOT), carbon, and metallic nanoparticles. The processes of making conductive textiles include various deposition methods, polymerization, coating, and printing. The parameters, such as conductivity and electromagnetic shielding, are prerequisites that set the benchmark for the performance of conductive textile materials. This review paper focuses on the raw materials that are used for conductive textiles, various approaches that impart conductivity, the fabrication of conductive materials, testing methods of electrical parameters, and key technical applications, challenges, and future potential