Carbon Nanotube Strain Research Articles

Stretchable Wearable Wireless Bioelectronics Using All Printed Pressure Sensors and Strain Gauges

AbstractThe sense of touch and perception of hand motion form an essential part of the somatic sensory system. Although piezoelectric sensors demonstrate high sensitivity and linearity, they are traditionally not employed for electronic skins because of their limited stretchability. Here, a printed, skin‐conformal, electronic system consisting of stretchable piezoelectric pressure sensors and carbon nanotube strain gauges capable of identifying tactile sensations and joint movements on the hand is introduced. The pressure sensor demonstrates both high sensitivity (19.9 mV kPa−1) and linearity (0.1–1000 kPa) while being reliably stretchable to above 15%. To complement the pressure sensing functionality, a strain gauge is fabricated in a rapid stamp printing process and designed to demonstrate excellent mechanical reliability with up to 70% strain and high sensitivity (36.5‐gauge factor). These sensors can seamlessly conform to numerous anatomical regions and cover wide areas of the skin, making them ideal for deployment in an electronic skin. Finally, the sensors are integrated with a flexible microelectronic device to demonstrate their functionality in human touch and prosthetic hand applications.

Predicting the mechanical properties of pristine and defective carbon nanotubes using a random forest model.

Data-driven models have lately emerged as a faster and less time-consuming method for computing material properties than computationally expensive conventional molecular dynamics and density functional theory-based simulations. Here, we developed a random forest (RF) model for comprehensively predicting mechanical properties such as stress and Poisson's ratio under varying strain and ultimate tensile strain of pristine and defective carbon nanotubes (CNTs). The variations in stress and Poisson's ratio with the strain of CNTs with a 0.4-2 nm diameter range were calculated by classical molecular dynamics simulations and characterized using parameters extracted from fitting polynomial equations. The fitting parameters and ultimate tensile strength showed distinct dependency on chiral indices, chiral angles, radii, and the presence of defects in CNTs, which constituted the target dataset. The dataset features were selected through principal component analysis, and the correlation with targets was scrutinized. We performed a comparative analysis of different machine learning algorithms for predicting mechanical properties, revealing the RF model as the best-performing algorithm. The RMSE for the stress-strain curve had a maximum value of 0.013 and 0.0143 for pristine and defective CNTs, respectively, while the correlation coefficients were ≫ 0.99 for all CNTs, showcasing the excellent predictive power of the model. The model made excellent predictions of properties for CNTs with diameters >2 nm, which is beyond the training dataset range, demonstrating the robustness of the model as a substitute for MD simulation. The insight gained from this study will benefit the research of nanocomposites, nanoelectronics, and nanomechanical systems incorporating CNTs.

Experimental studies on carbon nanotube strain sensors

Conventional metal foil strain sensors only measure strain on the structural surface in predetermined directions and locations. These are made of Nichrome (Ni-Cr alloy) and Constantan (Cu-Ni alloy) and have gauge factor of 2. A carbon nanotube (CNT) as a strain sensing material is gaining research interest due to its outstanding electrical and mechanical properties. These sensors apart from multidirectional sensing are capable of making additional contributions such as structure strengthening or structure damping. Bucky paper (BP) alternatively called CNT film or CNT network made from single-walled carbon nanotube or multiwalled carbon nanotubes (SWCNT or MWCNT) has been used by some researchers in the past as strain sensor. However, they have observed nonlinear relationship between strain and change in resistance, and lower gauge factor. Literature review in the field indicates that there is scope for improvement in the performance of Bucky paper-based strain sensors. The objective of this experimental work is to investigate effect of CNT film constituents (MWCNT, SWCNT and mix of both), purification of CNTs and effect of preparation method on sensitivity. The work involves preparation of film from CNTs by vacuum filtration method and preparing the strain sensors from the same. These sensors are loaded along with conventional foil strain gauges on opposite side. The change in resistance of sensor due to applied load is measured to evaluate sensitivity or gauge factor of sensor for various cases. It was observed that the sensitivity of CNT strain sensor is dependent on the preparation and purification methods as well as on the aspect ratio.

Carbon Nanotube‐Based Strain Sensors: Structures, Fabrication, and Applications

AbstractFlexible strain sensors have received widespread attention because of their great potential in many fields. Carbon nanotubes (CNTs) have been used as conductive materials for flexible strain sensors due to their excellent electrical and mechanical properties, and the fabricated flexible strain sensors have excellent sensing performance. This paper systematically summarizes the advances in flexible resistance‐type strain sensors based on CNTs. The strain sensing mechanisms are introduced, including crack extension, tunneling effect, and disconnection of overlapping materials. The performance parameters of the sensors, including sensitivity, stretchability, linearity, hysteresis, dynamic durability, and transparency, are discussed comprehensively. The coating methods, 3D printing techniques, chemical vapor deposition, transfer methods, and spinning processes used to fabricate CNT strain sensors are highlighted. The effect of isolated and porous internal conductive structures, folded and microcracked surface structures, films and fabrics macroscopic structures on sensor performance were systematically analyzed. The applications of the sensors in medical health, motion monitoring, gesture recognition, human–computer interaction, and soft robotics are provided in detail. Finally, the future challenges of CNT flexible strain sensors are summarized and the outlook is presented. Although CNT strain sensors have made great progress so far, there are still many problems that need researchers’ attention and solutions.

A low-power stretchable neuromorphic nerve with proprioceptive feedback.

By relaying neural signals from the motor cortex to muscles, devices for neurorehabilitation can enhance the movement of limbs in which nerves have been damaged as a consequence of injuries affecting the spinal cord or the lower motor neurons. However, conventional neuroprosthetic devices are rigid and power-hungry. Here we report a stretchable neuromorphic implant that restores coordinated and smooth motions in the legs of mice with neurological motor disorders, enabling the animals to kick a ball, walk or run. The neuromorphic implant acts as an artificial efferent nerve by generating electrophysiological signals from excitatory post-synaptic signals and by providing proprioceptive feedback. The device operates at low power (~1/150 that of a typical microprocessor system), and consists of hydrogel electrodes connected to a stretchable transistor incorporating an organic semiconducting nanowire (acting as an artificial synapse), connected via an ion gel to an artificial proprioceptor incorporating a carbon nanotube strain sensor (acting as an artificial muscle spindle). Stretchable electronics with proprioceptive feedback may inspire the further development of advanced neuromorphic devices for neurorehabilitation.

Fabrication method of flexible strain sensors with CNTs and solvents

Flexible strain sensors have received a lot of attention for their various applications, such as electronic skins, healthcare devices, and human-machine interactions. In many studies of manufacturing flexible strain sensors using carbon nanotubes (CNTs), various solvents are used to disperse CNTs. However, most studies lack explanation of the rationale for using specific solvent, which is mainly used to disperse CNTs and make CNT solution. There are studies dealing with the degree of dispersion in specific solvent of CNTs, but these studies lack information on solvents in the sensor manufacturing process. Therefore, researchers have difficulty in determining which solvent to use for CNT dispersion in their process because solvent selection varies depending on the sensor manufacturing method. Thus, the dispersion stability of CNT solutions with several solvents and the characteristics of CNT sheets made from each CNT solution were quantitatively compared. Then, the solvent suitable for an acrylic mold as well as having high dispersion stability was selected. Moreover, the flexible CNT strain sensor is developed by transferring CNTs to a flexible substrate. This sensor can be stretched up to 150% (linearity R=0.998), and the gauge factor was 26.7. It is expected to be used in various research of wearable sensors or human motion monitoring using CNT dispersion.

Comparative study on the effect of carbon nanotubes and carbon black on fatigue properties of natural rubber composites

Fatigue property is of great importance for the security and reliability of rubber products. The fatigue performance of natural rubber (NR) filled with carbon nanotube (CNT) and carbon black (CB) with different content but similar hardness was investigated. Results indicated that CNT/NR exhibited the best fatigue performance at low CNT content and fatigue strain, while the crack growth was accelerated with the increase of CNT content and strain. As for CB/NR, the fatigue resistance increased with CB content and strain. The different fatigue performance of NR composites was discussed from the perspective of hysteresis, strain-induced crystallization, and stress concentration effect.

利用机器学习分类含缺陷的单壁碳纳米管轴向应变

利用机器学习分类含缺陷的单壁碳纳米管轴向应变

A DIY Fabrication Approach of Stretchable Sensors Using Carbon Nano Tube Powder for Wearable Device.

Soft robotics and wearable devices are promising technologies due to their flexibility. As human-soft robot interaction technologies advance, the interest in stretchable sensor devices has increased. Currently, the main challenge in developing stretchable sensors is preparing high-quality sensors via a simple and cost-effective method. This study introduces the do-it-yourself (DIY)-approach to fabricate a carbon nanotube (CNT) powder-based stretchable sensor. The fabrication strategy utilizes an automatic brushing machine to pattern CNT powder on the elastomer. The elastomer ingredients are optimized to increase the elastomer compatibility with the brushing method. We found that polydimethylsiloxane-polyethyleneimine (PDMS-PEIE) is 50% more stretchable and 63% stickier than previously reported PDMS 30-1. With these improved elastomer characteristics, PDMS-PEIE/multiwalled CNT (PDMS-PEIE/MWCNT-1) strain sensor can realize a gauge factor of 6.2–8.2 and a responsivity up to 25 ms. To enhance the compatibility of the powder-based stretchable sensor for a wearable device, the sensor is laminated using a thin Ecoflex membrane. Additionally, system integration of the stretchable sensors are demonstrated by embedding it into a cotton-glove and a microcontroller to control a virtual hand. This cost-effective DIY-approach are expected to greatly contribute to the development of wearable devices since the technology is simple, economical, and reliable.

Analysis of quantum effects of fine scaling on the axial buckling of MWCNTs based on the density functional theory and molecular mechanics method

In this paper, quantum and molecular mechanics are used to study the quantum effects of fine scaling on the buckling strength of multi-walled carbon nanotubes under axial loading, as well as the effects of changes in length, diameter, chirality, wall number and length-to-diameter ratio of the structure. To this end, the total potential energy of the system is calculated with the consideration of both bond stretching and bond angular variations. The density functional theory along with the generalized gradient approximation function is used to obtain the relevant elastic constants of the nanotubes. An excellent agreement is found between the present numerical results and those found in the literature which confirms the validity as well as the accuracy of the present closed-form solution. The results show that in the effective longitudinal range for quantum effects of fine scaling, any change that leads to a change in the size of the structure has significant effects on the buckling strength of the structure. By increasing the diameter due to the increase in the number of walls or chirality and increasing the length of the structure, the critical buckling strength experiences a decreasing trend and this decrease is highly dependent on the increasing of the diameter due to the increase in the number of walls. In addition, zigzag multi-walled carbon nanotubes are more resistant than armchair multi-walled nanotubes, and the critical buckling strain of multi-walled carbon nanotubes with different chiralities is in the range of zigzag and armchair nanotubes. In other words, it can be said that quantum effects of fine scaling cause more buckling strengthening of the structure against external axial loads and with each longitudinal change that reduces the quantum effects of fine scaling, the strength of the structure decreases sharply.

Shape-Recognizable Origami Sheet Device With Single Walled Carbon Nanotube Strain Sensor

This article describes a shape-recognizable sheet device using origami structures with single-walled carbon nanotubes (SWCNTs) strain sensors. Recently, a sheet-like measurement system that is directly attached to an object has been attracting attention for obtaining the shape, motion, and deformation of the object as alternative systems with external cameras. However, previous sheet-like measurement systems are difficult to distinguish between the stretch and the bend of the object appropriately. Here we propose a sheet-like measurement system that can distinguish the stretch and the bend individually by using origami structures. We formed a strain sensor using SWCNTs that was deposited on a substrate to form a 5-7 nm thin film. By patterning the SWCNTs strain sensors, origami-based sheet devices were fabricated. We confirmed that our origami sheet device obtained the bending and stretching information separately by attaching the device to cylinders. We believe that the origami system could contribute to the observation of complex deformations such as artificial skin, human interface, and biomedical applications. [2020-0324].

Modeling of current generation process in an aligned carbon nanotube under the action of strain gradient

The results of theoretical and experimental studies of the current generation process in an aligned carbon nanotube (CNT) during its deformation are presented. It is theoretically established that there is a strain gradient equal to 8 in the aligned nanotube under the influence of the external field strength of 1010 V/m. The piezoelectric coefficient of the CNT at a diameter of 92 nm and a length of 2.2 µm is empirically determined and amounted to 0.107 C/m2. It is theoretically established that a CNT strain gradient of 8 leads to a current generation of up to 12 nA. It is shown that the modeling results correlate with experimental data. The obtained results can be used to develop the nanopiezotronics devices based on aligned carbon nanotubes.

Thermo-luminescence Response of Carbon Nanotubes and Some Other Familiar TL Materials Using Medical LINAC

Strain and impurity defects in carbon nanotubes (CNTs) particularly their potentiality as a new TL material has been studied over the years. In this research, our main objective is to explore the suitability of using CNTs and its composites in the area of TL dosimeter. For this purpose, a study was carried out between the TL responses of the dosimeters TLD-100, TLD-7000 and NaI-LiF pellets. To carry out this research, equivalent irradiations were performed with these pellets using clinical linear accelerator (LINAC) under 6 MV X-ray photon beam. The dose range was from 0.5 to 5 Gy. During irradiation, the dose rate was kept constant at 300 MU/min. TLD reader was used to readout the samples in a flowing N2 atmosphere to reduce surface oxidation.During readout, pre-heat temperature was set initially at 50 °C, acquired temperature rate 10 °C/s and maximum annealed temperature was 300 °C. Response of TLD-100 under varying dose was typically linear for any doses but other dosimeters TLD-7000 showed supra-linearity beyond 2 Gy and NaI-LiF pellets showed sub-linearity response after 2 Gy. The TL glow peak of CNTs indicated that it was lying somewhere away from 300 °C.

A Fully Inkjet-Printed Strain Sensor Based on Carbon Nanotubes

A fully inkjet-printed strain sensor based on carbon nanotubes (CNTs) was fabricated in this study for microstrain and microcrack detection. Carbon nanotubes and silver films were used as the sensing layer and conductive layer, respectively. Inkjet-printed CNTs easily undergo agglomeration due to van der Waals forces between CNTs, resulting in uneven films. The uniformity of CNT film affects the electrical and mechanical properties. Multi-pass printing and pattern rotation provided precise quantities of sensing materials, enabling the realization of uniform CNT films and stable resistance. Three strain sensors printed eight-layer CNT film by unidirectional printing, rotated by 180° and 90° were compared. The low density on one side of eight-layer CNT film by unidirectional printing results in more disconnection and poor connectivity with the silver film, thereby, significantly increasing the resistance. For 180° rotation eight-layer strain sensors, lower sensitivity and smaller measured range were found because strain was applied to the uneven CNT film resulting in non-uniform strain distribution. Lower resistance and better strain sensitivity was obtained for eight-layer strain sensor with 90° rotation because of uniform film. Given the uniform surface morphology and saturated sheet resistance of the 20-layer CNT film, the strain performance of the 20-layer CNT strain sensor was also examined. Excluding the permanent destruction of the first strain, 0.76% and 1.05% responses were obtained for the 8- and 20-layer strain sensors under strain between 0% and 3128 µε, respectively, which demonstrates the high reproducibility and recoverability of the sensor. The gauge factor (GF) of 20-layer strain sensor was found to be 2.77 under strain from 71 to 3128 µε, which is higher than eight-layer strain sensor (GF = 1.93) due to the uniform surface morphology and stable resistance. The strain sensors exhibited a highly linear and reversible behavior under strain of 71 to 3128 µε, so that the microstrain level could be clearly distinguished. The technology of the fully inkjet-printed CNT-based microstrain sensor provides high reproducibility, stability, and rapid hardness detection.

Mapping the residual strain of carbon nanotubes in DWCNT/epoxy nanocomposites after tensile load using Raman microscopy

The toughness mechanisms of epoxy-based nanocomposites reinforced by double-walled carbon nanotubes (DWCNT) are investigated using Raman microscopy mapping. Maps of the residual strain of the carbon nanotubes were generated for the fracture surfaces obtained after tensile loading. By imaging through different microscope objectives, Raman mapping allowed to determine the distribution of the nanotube residual strain, indicating tension or compression, in different sampling volumes near the fracture surface. The G′ band with its high sensitivity to nanotube diameter variations, and not overlapping with the characteristic epoxy Raman lines, was selected for the mapping. Results provide a quantitative analysis of the nanotube bridging and pull-out mechanisms identified by scanning electron microscopy (SEM) inspection of the fracture surfaces.

Directional Response of Randomly Dispersed Carbon Nanotube Strain Sensors.

Tests on a double lap bonded joint, with transverse strips of randomly oriented carbon nanotubes (CNT) sprayed onto an epoxy adhesive film, showed a positive increment in electrical resistance under tensile load, even though the transverse strains were negative. Other experiments included in this work involved placing longitudinal and transversal CNT sensors in a tensile loaded aluminum plate, and, as reported by other authors, the results confirm that the resistance change is not only dependent on the strains oriented with the electrode line, while the other strain components also influence the response. This behavior is quite different to that of conventional strain gages which have a near zero sensitivity to strains not aligned to the sensor direction. The dependence of the electrical response on all the strain components makes it quite difficult, possibly unfeasible, to experimentally determine the individual strain components with this kind of sensors; however, the manufacturing of aligned CNT sensors could deal with this issue.

The Piezoresistive Highly Elastic Sensor Based on Carbon Nanotubes for the Detection of Breath.

Wearable electronic sensor was prepared on a light and flexible substrate. The breathing sensor has a broad assumption and great potential for portable devices in wearable technology. In the present work, the application of a flexible thermoplastic polyurethane/multiwalled carbon nanotubes (TPU/MWCNTs) strain sensor was demonstrated. This composite was prepared by a novel technique using a thermoplastic filtering membrane based on electrospinning technology. Aqueous dispersion of MWCNTs was filtered through membrane, dried and then welded directly on a T-shirt and encapsulated by a thin silicone layer. The sensing layer was also equipped by electrodes. A polymer composite sensor is capable of detecting a deformation by changing its electrical resistance. A T-shirt was capable of analyzing a type, frequency and intensity of human breathing. The sensitivity to the applied strain of the sensor was improved by the oxidation of MWCNTs by potassium permanganate (KMnO4) and also by subsequent application of the prestrain.

Measurement of finger joint angle using stretchable carbon nanotube strain sensor.

Strain sensors capable of monitoring complex human motions are highly desirable for the development of wearable electronic devices and healthcare monitoring systems. Excellent sensitivity and a wide working range of the sensor material are important requirements for distinguishing dynamic human motion. In this study, a highly stretchable strain sensor was fabricated via inkjet printing of single-walled carbon nanotube (SWCNT) thin films on a stretchable polydimethylsiloxane substrate. The sensor was attached to the metacarpophalangeal (MCP) joint of the hand in 12 healthy male subjects. The subjects placed their hands next to a conventional goniometer and flexed the MCP joint to predetermined angles. A linear relationship was found between the change in the length of the strain sensor and the intended angle of the MCP joint. The fabricated thin films showed high durability during repeated cycling (1,000 cycles) and good sensitivity with a gauge factor of 2.75. This study demonstrates that the newly developed stretchable CNT strain sensor can be used for effectively measuring MCP joint angles. This sensor may also be useful for the analysis of complex and dynamic hand motions that are difficult to measure using a conventional goniometer.

Thermoelectric and Elastic Properties of Carbon Nanotubes Irradiated with High-Energy Electrons

Abstract—The electrical conductivity, thermo-emf, and elasticity coefficient were experimentally studied in multilayered carbon nanotubes (CNTs) exposed to room-temperature irradiation. The initial CNTs with dimensions of 18 ± 7 nm were obtained via chemical vapor deposition using propane–butane as precursor. The irradiation of CNTs was implemented on Mikrotron-ST cyclic electron accelerator at the energy of 21 MeV and electron doses of 0.3 × 1017, 0.7 ×1017, 1.1 ×1017, and 1.5 × 1017 cm–2. The characteristics of initial and irradiated samples were measured in a dielectric cylinder. The following critical parameters of dielectric–metal transition in CNTs were established: array density (ρ1), elastic strain of CNT (ρrel), maximum electrical conductivity (σmax), and elastic strain coefficient (e). The irradiated CNTs exhibit a decrease in thermo-emf (Seebeck coefficient (α)) by 50% or more in comparison with the initial samples, which is due to the formation of radiation defects.

Temperature-Robust Active Vibration Controller Using MWCNT/Epoxy Strain Sensor and PZT-5H Actuator

Active vibration control (AVC) of a cantilevered beam using a multiwalled carbon nanotube (MWCNT)/epoxy nanocomposite strain sensor and piezoelectric actuator at elevated temperature has been performed. The resistance of the nanocomposite strain sensor varies nonlinearly with the change in temperature. To use a carbon nanotube (CNT) nanocomposite strain sensor in AVC applications, the temperature variation of the sensor must be taken into account. A control law for AVC of the first mode of vibration of the cantilevered beam was derived using a negative velocity feedback control law. AVC of the first mode of the smart cantilevered beam instrumented with a collocated CNT nanocomposite strain sensor and piezoelectric actuator was performed at temperatures in the range of 40°C to 60°C under two cases, viz. without and with consideration of the temperature dependence of the CNT strain sensor in the control law, revealing that AVC performance was only maintained at elevated temperature in the latter case. These results suggest a strategy for AVC of a structure using a MWCNT/epoxy strain sensor and PZT-5H actuator that is robust to variations in temperature.