A novel class of carbon nanotube (CNT)-based nanomaterials has been surging since 1991 due to their noticeable mechanical and electrical properties, as well as their good electron transport properties. This is evidence that the development of CNT-reinforced polymer composites could contribute in expanding many areas of use, from energy-related devices to structural components. As a promising material with a wide range of applications, their poor solubility in aqueous and organic solvents has hindered the utilizations of CNTs. The current state of research in CNTs—both single-wall carbon nanotubes (SWCNT) and multiwalled carbon nanotube (MWCNT)-reinforced polymer composites—was reviewed in the context of the presently employed covalent and non-covalent functionalization. As such, this overview intends to provide a critical assessment of a surging class of composite materials and unveil the successful development associated with CNT-incorporated polymer composites. The mechanisms related to the mechanical, thermal, and electrical performance of CNT-reinforced polymer composites is also discussed. It is vital to understand how the addition of CNTs in a polymer composite alters the microstructure at the micro- and nano-scale, as well as how these modifications influence overall structural behavior, not only in its as fabricated form but also its functionalization techniques. The technological superiority gained with CNT addition to polymer composites may be advantageous, but scientific values are here to be critically explored for reliable, sustainable, and structural reliability in different industrial needs.

Fabrication, Functionalization, and Application of Carbon Nanotube-Reinforced Polymer Composite: An Overview.

In the last decade, significant developments of flexible and stretchable force sensors have been witnessed in order to satisfy the demand of several applications in robotic, prosthetics, wearables and structural health monitoring bringing decisive advantages due to their manifold customizability, easy integration and outstanding performance in terms of sensor properties and low-cost realization. In this paper, we review current advances in this field with a special focus on polymer/carbon nanotubes (CNTs) based sensors. Based on the electrical properties of polymer/CNTs nanocomposite, we explain underlying principles for pressure and strain sensors. We highlight the influence of the manufacturing processes on the achieved sensing properties and the manifold possibilities to realize sensors using different shapes, dimensions and measurement procedures. After an intensive review of the realized sensor performances in terms of sensitivity, stretchability, stability and durability, we describe perspectives and provide novel trends for future developments in this intriguing field.

https://www.mdpi.com/1424-8220/21/2/341/pdf

Review on Conductive Polymer/CNTs Nanocomposites Based Flexible and Stretchable Strain and Pressure Sensors.

Advance on flexible pressure sensors based on metal and carbonaceous nanomaterial

Metal-Oxide Nanomaterials Synthesis and Applications in Flexible and Wearable Sensors

Many modern user interfaces are based on touch, and such sensors are widely used in displays, Internet of Things (IoT) projects, and robotics. From lamps to touchscreens of smartphones, these user interfaces can be found in an array of applications. However, traditional touch sensors are bulky, complicated, inflexible, and difficult-to-wear devices made of stiff materials. The touch screen is gaining further importance with the trend of current IoT technology flexibly and comfortably used on the skin or clothing to affect different aspects of human life. This review presents an updated overview of the recent advances in this area. Exciting advances in various aspects of touch sensing are discussed, with particular focus on materials, manufacturing, enhancements, and applications of flexible wearable sensors. This review further elaborates on the theoretical principles of various types of touch sensors, including resistive, piezoelectric, and capacitive sensors. The traditional and novel hybrid materials and manufacturing technologies of flexible sensors are considered. This review highlights the multidisciplinary applications of flexible touch sensors, such as e-textiles, e-skins, e-control, and e-healthcare. Finally, the obstacles and prospects for future research that are critical to the broader development and adoption of the technology are surveyed.

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Recent Advances in Touch Sensors for Flexible Wearable Devices

Sensors based on carbon nanomaterials are gaining importance due to their tunable properties and their potentially outstanding sensing performance. Despite their advantages, carbon-based nanomaterial sensors are prone to cross-sensitivities with environmental factors like temperature. Thus, to reduce the temperature influence on the sensing material, compensation and correction procedures are usually considered. These methods may require the use of additional sensors which can themselves be subject to residual errors. Hence, a more promising approach consists of synthesizing a material that is capable of self-compensating for the influence of temperature. In this study, a hybrid nanocomposite based on multi-walled carbon nanotubes (MWCNT) and graphene is proposed, which can compensate, by itself, for the influence of temperature on the material conductivity. The hybrid nanocomposite material uses the different temperature behavior of MWCNTs, which have a negative temperature coefficient, and graphene, which has a positive temperature coefficient. The influence of the material ratio and dispersion quality are investigated in this work. Material composition and dispersion quality are analyzed using Raman spectroscopy and scanning electron microscopy (SEM). A composition of 70% graphene and 30% MWCNT exhibits a nearly temperature-independent hybrid nanocomposite with a sensitivity of 0.022 Ω/°C, corresponding to a resistance change of ~1.2 Ω for a temperature range of 25 to 80 °C. Additionally, a simple investigation of the strain sensing behavior of the hybrid material is also presented. The hybrid nanocomposite-based, thin-film strain sensor exhibits good stability over 100 cycles and a significantly high gauge factor, i.e., 16.21.

https://www.mdpi.com/2504-477X/3/4/96/pdf

Temperature Self-Compensated Strain Sensors based on MWCNT-Graphene Hybrid Nanocomposite

Multiwalled carbon nanotubes (MWCNTs) are attractive materials for realizing sensors, owing to their high aspect ratio associated with excellent mechanical, electronic, and thermal properties. Moreover, their sensing properties can be tuned by introducing functional groups on their framework and adjusting the processing conditions. In this paper, we investigate the potential of functionalized CNTs for humidity and temperature sensing by optimization of the functionalization, the processing conditions and the printing conditions. The morphology of the differently functionalized MWCNTs is investigated by infrared spectroscopy (IR), scanning electron microscopy, thermogravimetry (TG) and TG-coupled mass-spectrometric studies. Using the functionalized MWCNTs, films were fabricated with different numbers of layers (4, 6, 8, 10 layers) via inkjet printing on a flexible polyimide substrate containing an interdigital microelectrode. The influence of hydrothermal effects was investigated. The sensitivity to humidity is higher for films prepared with MWCNTs functionalized with a high sonication amplitude and a bigger number of layers due to enhancements of hydrophilicity and water mobility. A higher sensitivity to temperature is achieved by a low sonication amplitude and a small number of layers. For the encapsulation of the temperature sensor against humidity, a Bectron layer is proposed, which reduces also the hysteresis effect. This study demonstrates the efficiency of carboxylic functionalized MWCNTs deposit by inkjet printing for realization of sensitive and cost-effective humidity and temperature sensors. It provides a real example for the interesting contribution of functionalization procedures to the sensing properties of MWCNTs films.

Customizing hydrothermal properties of inkjet printed sensitive films by functionalization of carbon nanotubes

Recently, the demand for flexible and wearable nanogenerators (NGs) have been paid great of attention to convert mechanical energy such as human motion into electrical energy. NGs are known by their simple structure and excellent piezoelectric performance after polarization. However, the polarization is very long and unsafe. In order to avoid that, the proposed solution was to dope multiwalled carbon nanotubes (MWCNTs) for better distribution of piezoelectric nanoparticles. In this work, piezoelectric nanocomposite generators were prepared using solution mixing and mold casting. A comparative study was performed to investigate the impact of MWCNTs via morphological and mechanical testing. Both nanogenerators shows good performance due to high uniformity of nanoparticle distribution within the polymer matrix showing the efficiency of the adapted fabrication process. In addition, doping multiwalled carbon nanotube (MWCNTs) within the piezoelectric nanocomposite leads to increase the output voltage by about 20% comparing to the NG without carbon nanotube (CNTs) which corresponds to 6.6 V and a power of ~37.87 μW at a load resistance of about 660 kΩ due to the enhanced distribution of the nanoparticle and better electro-mechanical coupling.

Carbon nanotubes for high performance flexible piezoelectric polymer composite nanogenerators

In this work, nanocomposite filament-based sensors made from polystyrene-block-polybutadiene-block-polystyrene (SBS) and graphene-based electrically conductive thermoplastic polyurethane (C-TPU) have been fabricated using a small extrusion process. Because of the high flexibility of the polymer matrix selected and the processing method employed, it is possible to reach elongations above 150% with an adjustable length and diameter making them good candidates for applications where flexible and adaptable sensors are needed. The electrical response of the filaments is measured at different elongations exhibiting good reproducibility and linearity. Strain gauge factors up to 26 were obtained showing up higher sensitivity than commercial metallic strain gauges. This kind of sensors can be applied in many fields including wearable electronics and structural health monitoring.

Jose Roberto Bautista-Quijano

Papers

Review on Conductive Polymer/CNTs Nanocomposites Based Flexible and Stretchable Strain and Pressure Sensors.

Temperature Self-Compensated Strain Sensors based on MWCNT-Graphene Hybrid Nanocomposite

Customizing hydrothermal properties of inkjet printed sensitive films by functionalization of carbon nanotubes

Carbon nanotubes for high performance flexible piezoelectric polymer composite nanogenerators

Flexible strain sensing filaments based on styrene-butadiene-styrene co-polymer mixed with carbon particle filled thermoplastic polyurethane