Piezoresistive Nanocomposites Research Articles

Optimized silk fibroin piezoresistive nanocomposites for pressure sensing applications based on natural polymers.

Environmental issues promote the development of sensors based on natural polymers which are becoming an area of increasing interest. Piezoresistive sensors based on silk fibroin with carbon nanotubes (CNTs) as fillers were produced by solvent-casting in order to tune their electrical conductivity and electromechanical responses. It is shown that the carbonaceous fillers are well dispersed in the polymer matrix and the thermal and mechanical properties are independent of the CNT content. On the other hand, the inclusion of CNTs reduces the β-sheet content of silk fibroin and the electrical properties of the composite strongly depend on the filler content, the percolation threshold being around 1 wt% CNTs. The piezoresistive response demonstrates good reproducibility during cyclic loading without hysteresis with a piezoresistive sensitivity of ∼4 MPa−1, regardless of the CNT content. Overall, the results confirm that polymer composites based on natural polymers exhibit excellent piezoresistive responses, also demonstrated by the implementation and testing of a pressure sensor with the corresponding readout electronics. Thus, it is shown that natural polymers such as silk fibroin will allow the development of a new generation of multifunctional force and deformation sensors.

Polydimethylsiloxane nanocomposite filled with 3D carbon nanosheet frameworks for tensile and compressive strain sensors

Elastomeric matrix filled with carbon nanotubes or graphene has been focused for making resistive-type strain sensors. Here, we report the low-cost synthesis of 3D carbon nanosheet frameworks by exploiting the chemical reaction between magnesium powder and carbon disulfide vapor, and prepared piezoresistive nanocomposites of as-prepared carbon nanomaterials and polydimethylsiloxane elastomer for tensile and compressive strain sensors with different device configurations. The optimal carbon content in PDMS for achieving high sensitivity is obtained. The tensile strain sensor shows fast response and high gauge factor in the order of 102. The compressive strain sensor that is elaborated designed as in-plane and miniature concept also shows striking response to external loads.

Development of Flexible Robot Skin for Safe and Natural Human⁻Robot Collaboration.

For industrial manufacturing, industrial robots are required to work together with human counterparts on certain special occasions, where human workers share their skills with robots. Intuitive human–robot interaction brings increasing safety challenges, which can be properly addressed by using sensor-based active control technology. In this article, we designed and fabricated a three-dimensional flexible robot skin made by the piezoresistive nanocomposite based on the need for enhancement of the security performance of the collaborative robot. The robot skin endowed the YuMi robot with a tactile perception like human skin. The developed sensing unit in the robot skin showed the one-to-one correspondence between force input and resistance output (percentage change in impedance) in the range of 0–6.5 N. Furthermore, the calibration result indicated that the developed sensing unit is capable of offering a maximum force sensitivity (percentage change in impedance per Newton force) of 18.83% N−1 when loaded with an external force of 6.5 N. The fabricated sensing unit showed good reproducibility after loading with cyclic force (0–5.5 N) under a frequency of 0.65 Hz for 3500 cycles. In addition, to suppress the bypass crosstalk in robot skin, we designed a readout circuit for sampling tactile data. Moreover, experiments were conducted to estimate the contact/collision force between the object and the robot in a real-time manner. The experiment results showed that the implemented robot skin can provide an efficient approach for natural and secure human–robot interaction.

Piezoresistive nanocomposites for sensing the effectiveness of composite patch repair

Abstract In this paper, piezoresistive nanocomposite is used to detect the effectiveness of bonded composite repair. The hybrid nanocomposites fabricated using wet-layup method with carbon nanotube sheet and graphite platelets as fillers exhibits enhanced piezoresistivity. The nanocomposite is incorporated as outer ply of repair patch in a scarf-repair process of damaged carbon fiber composite. Flexural test of repaired composite with intentionally placed disbond exhibits a different electromechanical response compared to defect-free repair; this concept can be used to detect the repair quality.

Fabrication and characterization of conductive poly(dimethylsiloxane)-carbon nanotube nanocomposites for potential microsensor applications

PurposeThe purpose of the study is to explore the potential possibility of using the conductive and piezoresistive nanocomposites that consist of insulating poly(dimethylsiloxane), a very popular silicone polymer, and the amazing properties of carbon nanotubes (CNT) in sensing applications. This nanocomposite is prepared by an optimized process to achieve a high-quality nanocomposite with uniform properties.Design/methodology/approachThe optimized process achieved in this study to provide PDMS/CNT nanocomposite includes the appropriate use of ultrasonic bath, magnetic stirrer, molding and curing in certain circumstances that results in obtaining high-quality nanocomposite with uniform properties. Experiments to characterize the influence of some factors such as pressure, temperature and the impact of CNT’s concentration on the electrical properties of the prepared nanocomposite have been designed and carried out.FindingsThe obtained preparing method of this nanocomposite is found to have better homogeneity in comparison to other methods for CNT/PDMS nanocomposite. This nanocomposite has both desirable properties of the PDMS elastomer and the additional conductive CNT, and it can be used to create all-polymer systems. Furthermore, the conductivity values of these nanocomposites can be changed by varying some factors such as temperature and pressure, so that those can be used in temperature- and pressure-sensoring applications.Originality/valueIn the present research, a convenient, inexpensive and reproducible method for preparing CNT/PDMS nanocomposite was investigated. These nanocomposites with the unique properties of both PDMS elastomer and CNTs and also with high electrical conductivity, piezoresistive properties and temperature dependent resistivity can be used in different sensoring applications.

Enhanced imaging of piezoresistive nanocomposites through the incorporation of nonlocal conductivity changes in electrical impedance tomography

Electrical impedance tomography is a method of noninvasively imaging the internal conductivity distribution of a domain. Because many materials exhibit piezoresistivity, electrical impedance tomography has considerable potential for application in structural health monitoring. Despite its numerous benefits such as being low cost, providing continuous sensing, and having the ability to be employed in real time, electrical impedance tomography is limited by several important factors such as the ill-posed nature of the inverse problem and the requirement for large electrode arrays to produce quality images. Unfortunately, current methods of mitigating these limitations impose upon the benefits of electrical impedance tomography. Herein, we propose a multi-physics approach of enhancing electrical impedance tomography without sacrificing any of its benefits. This approach is predicated on coupling global conductivity changes with the electrical impedance tomography inversion process thereby adding additional constraints and rendering the problem less ill-posed. Additionally, we leverage physically motivated global conductivity changes in the context of piezoresistive nanocomposites. We demonstrate this proof of concept with numerical simulations and demonstrate that by incorporating multiple conductivity changes, the rank of the sensitivity matrix can be improved and the quality of electrical impedance tomography reconstructions can be enhanced. The proposed method, therefore, has the potential of easing the implementation burden of electrical impedance tomography while concurrently enabling high-quality images to be produced without imposing on the major advantages of electrical impedance tomography.

Dispenser printing of piezo-resistive nanocomposite on woven elastic fabric and hysteresis compensation for skin-mountable stretch sensing

Recently, piezo-resistive nanocomposites have emerged as an important smart material for realizing less obtrusive and more comfortable stretch sensing applications. To manufacture cost-effective and skin-mountable stretch sensor, dispenser printing is advantageous method because piezo-resistive nanocomposites can be directly printed on a woven elastic fabric in various patterns. However, both electrical and mechanical properties of the nanocomposites need to be modulated to achieve favorable sensing performance as well as strong adhesion between the nanocomposite and the fabric to sustain large strains. Moreover, inherent hysteretic behavior of the soft nanocomposite should be compensated to obtain consistent stretch sensing. This paper presents silicone rubber mixed with long multi-walled carbon nanotubes (Long-MWCNTs) composites as a piezo-resistive transducing material for dispenser printing. High aspect ratio of the Long-MWCNTs resulted in low viscosity of a liquid state nanocomposite and high electrical conductivity. Due to the low viscosity, the liquid state nanocomposite could permeate into gaps of the woven elastic fabrics and ensured strong bonding force in large strains up to 35%. In addition, a modified Prandtl-Ishilinskii (MPI) model was adopted to compensate for piezo-resistive hysteresis of the nanocomposite. For validation, the skin-mountable sensor was applied to estimate rotation angle of a wrist. The sensor system estimated the rotation angle of the wrist with an estimation error of 1.93 degrees within 65 degrees range (2.9%) for the step increment and decrement test, and 7.15 degrees within 75 degrees range (9.5%) for the arbitrary movement test. Thus, the experimental results show that the dispenser printing method incorporated with hysteresis compensation can provide a guideline to implement skin-mountable smart fabrics for stretch sensing using various nanocomposites

Self-sensing performance of MWCNT-low density polyethylene nanocomposites

Carbon nanotubes (CNTs) based polymer nanocomposites offer a range of remarkable properties. Here, we demonstrate self-sensing performance of low density polyethylene (LDPE)-multiwalled carbon nanotubes (MWCNTs) nanocomposites for the first time. The dispersion of the CNTs and the morphology of the nanocomposites was investigated using scanning electron microscopy, x-ray diffraction and Raman spectroscopic techniques. The thermal properties were measured using thermal gravimetric analysis and differential scanning calorimetry and were found to increase with increasing wt% of MWCNTs in LDPE matrix. An overall improvement in ultimate tensile strength, yield strength and Young’s modulus was found to be 59.6%, 48.5% and 129.3%, respectively for 5.0 wt% loading of MWCNTs. The electrical percolation threshold was observed at 1.0 wt% of MWCNTs and the highest electrical conductivity of 2.8 × 10−2 Scm−1 was observed at 5.0 wt% loading of MWCNTs. These piezo-resistive nanocomposites offer tunable self-sensing capabilities with gauge factors in the ranges of 17–52 and 42–530 in linear elastic (strain ∼3%) and inelastic regimes (strain ∼15%) respectively. Our demonstration would provide guidelines for the fabrication of low cost, self-sensing MWCNT-LDPE nanocomposites for potential use as civil water pipelines and landfill membranes.

The effect of error and regularization norms on strain and damage identification via electrical impedance tomography in piezoresistive nanocomposites

Electrical impedance tomography (EIT) has recently emerged as a promising tool for the structural health monitoring (SHM) and non-destructive evaluation (NDE) of piezoresistive nanofiller-modified composites. With few exceptions in SHM and NDE, the vast majority of studies have formulated the EIT problem such that it endeavors to minimize a voltage error vector in the least-squares sense while simultaneously using an additional least-squares term for regularization. Unfortunately, the use of least-squares minimization and regularization has important limitations especially in the context of SHM and NDE. For example, the least-squares error minimization is extremely sensitive to outlier data. Such outlier data is a very real possibility in SHM applications due to damaged electrodes. Additionally, utilizing a least-squares term for regularization renders EIT incapable of resolving discontinuous conductivity changes such as would be induced by fracture events. More sophisticated minimization and regularization techniques that obviate these limitations are well known to medical and mathematical practitioners of EIT; however, these methods have not been explored in SHM and NDE applications. This is an important oversight because the needs of SHM and NDE differ considerably from the needs of medical imaging. Therefore, we herein explore the use of different minimization and regularization norms on EIT for experimental strain identification, detection of artificially induced through-hole damage, and detection of impact damage.

Electrical, Elastic, and Piezoresistive Properties of Nanocomposites of Poly(dimethylsiloxane) and Poly(phenylmethylsiloxane)-Functionalized Graphene Nanoplatelets

High-performance piezoresistive materials can detect pressures in the finger-sensing regime (0 kPa to 100 kPa). Piezoresistive nanocomposites of poly(phenylmethylsiloxane) (PPMS)-functionalized graphene nanoplatelets (P-GNPs) as conductive filler and polydimethylsiloxane (PDMS) as polymer matrix have been prepared and their electrical, elastic, and piezoresistive properties investigated. GNPs were π–π stacked with PPMS by noncovalent functionalization, and P-GNPs/PDMS nanocomposites were prepared by solution casting. The results showed that P-GNPs with sandwiched nanostructures (PPMS/GNPs/PPMS) exhibited improved dispersibility and compatibility in the PDMS matrix. Compared with GNPs/PDMS nanocomposites, low percolation threshold (2.96 vol.%) was obtained for the P-GNPs/PDMS nanocomposites. P-GNPs/PDMS nanocomposite with 3.00 vol.% P-GNPs showed remarkable negative piezoresistivity with high sensitivity of −105.22 × 10−3 kPa−1 (0 kPa to 10 kPa), low Young’s modulus of 408.26 kPa, and high electrical conductivity of 1.28 × 10−6 S/m. These results demonstrate a simple and low-cost method for preparation of high-performance nanocomposites and facilitate wide application of such piezoresistive materials, especially in cheap and flexible tactile sensors.

An inverse methodology for calculating strains from conductivity changes in piezoresistive nanocomposites

Carbon nanofiller-modified composites have incredible potential for self-sensing, structural health monitoring (SHM), and damage identification because they are piezoresistive. This means that conductivity changes can be monitored to detect mechanical perturbations such as damage and strain. Recently, considerable research effort has been dedicated to understanding the effect of strain on nanocomposite conductivity. However, from a SHM perspective, it is more desirable to know the strain state giving rise to an observed conductivity change. Therefore, we herein develop an inverse relation for nanocomposite piezoresistivity. That is, given a conductivity change, we seek to determine the underlying strain state of the nanocomposite. This is done by formulating an inverse problem wherein we seek the strain state that minimizes the difference between the observed conductivity and the conductivity predicted by a piezoresistivity model. This approach is verified through simulations by making use of a piezoresistivity model published in the literature. Our results show that strains can indeed be reconstructed from known conductivity changes. When coupled with conductivity-based SHM such as electrical impedance tomography, these results have the potential to provide unprecedented insight into the mechanical response of nanofiller-modified materials and structures.

Synthesis, characterisation and performance of piezo-resistive cementitious nanocomposites

Smart material reinforced non-destructive structural health monitoring technique has been evolving as the most predominated route for assessing the performance of the civil structures. In the present study, multiwalled nanotubes (MWNT) were suitably incorporated into the cement matrix, which act as actively embedded sensor for monitoring real-time flaws in structures. Initially, the stable homogenous MWNT dispersion was prepared by using ionic surfactant technique with high-intensity ultrasonic agitation process. Since, a suitable and adequate synthesis procedure to incorporate MWNT in cement matrix is essential, but complicated, the role of amplitude and frequency of sonication on dispersion of nanotubes was categorically evaluated. Further, this paper focuses to find out the effect of surfactant on MWNT dispersion by using the UV Visible spectroscopy and by evaluating the effective hydro-dynamic diameter. Based on micromechanics based analytical model, the influence of the interface layer thickness and geometrical configuration of nanotubes on the electrical conductivity of cement nano-composite are also analyzed. Further, the electrical conductivity of MWNT incorporated cement system, as developed in the present study, is measured using four probe method. Piezo-resistivity of the oven dried samples is measured to evaluate the change in potential drop under cyclic loading regime. It is found that the efficiency of the piezo-resistive strain sensors greatly depends on synthesis process and the circuit system. Appropriately proportioned and properly synthesized MWNTs incorporated in cement matrix were capable of providing consistent and steady response under the variable external stress. Thus, the material can be used as embedded sensor for health monitoring and identifying initiation of any damage in reinforced concrete structure.

Tuning the Network Structure in Poly(vinylidene fluoride)/Carbon Nanotube Nanocomposites Using Carbon Black: Toward Improvements of Conductivity and Piezoresistive Sensitivity

Piezoresistive poly(vinylidene fluoride) (PVDF) nanocomposites are very intriguing for strain sensor applications in structural health monitoring (SHM) systems. In general, high piezoresistive sensitivity combined with broad measurable strain ranges are greatly favored in those sensors. Here, a facile strategy, i.e. constructing strain susceptible conductive networks using hybrid filler systems consisting of carbon nanotubes (CNTs, 0.5-1 wt %) and carbon black (CB, 0.5-4 wt %), was introduced to tune both electrical conductivity and piezoresistive sensitivity of melt mixed PVDF nanocomposites. At the same filler content CNTs, due to their larger aspect ratio, contribute more to electrical conductivity improvements of nanocomposites than CB, while contacts between CB particles are more sensitive to tensile strain. With retained ductility of PVDF, tunable electrical conductivity and ΔR/R0-strain sensitivity can be achieved by combining the advantages of CNTs and CB by adjusting the conductive network structure. Conductivity improvement is more remarkable if the mass ratio of CNTs to CB (mCNTs/mCB), varied between 1:1 and 1:4, is higher in hybrid filler compositions. Lower mCNTs/mCB ratios result in higher ΔR/R0 values in PVDF nanocomposites whether they have the same content of total filler or similar/the same initial electrical resistivity. At 10% tensile strain, the highest ΔR/R0 of 0.65 was obtained for the nanocomposite filled with 0.5 wt % CNTs and 0.5 wt % CB, while that for the counterpart containing 1 wt % CNTs is 0.35 at the same strain. The concept of using hybrid fillers provides a low-cost and effective way to fabricate piezoresistive polymer nanocomposites toward SHM applications.

Piezoresistive nanocomposite as an embedded stress sensor in instrumented knee prosthesis.

We characterize the electrical properties of a biocompatible nanocomposite which will be used as a stress sensing material in an instrumented knee implant. The composite is fabricated from multi-walled carbon nanotubes and ultra high molecular weight polyethylene. Experimental cyclic compression loading shows that the composite's resistance exponentially decreases with increasing compression stress, proving its potential for application as a piezoresistive stress sensing material. An analytical model is built to estimate the optimal depth from the tibio-femoral contact surface at which an embedded stress sensor could achieve the highest stress resolution and lowest distortion energy inside the tibial insert.

Elucidation of structure-to-property relationships of piezoresistive polymer-carbon nanotube nanocomposites

Polymeric nanocomposites (PNC) filled with carbon nanotubes (CNTs) possess superior multifunctionality, including electrical, thermal, and mechanical properties, making them an emerging family of advanced and multifunctional materials. In recent years, flexible polymer/CNT nanocomposites are increasingly being considered as promising alternatives to conventional smart materials. Their piezoresistive behaviours have led to many potential applications in strain sensing. Despite extensive experimental and theoretical research, the underlying mechanisms for polymer/CNT nanocomposites' piezoresistive behaviours have yet been elucidated. This paper reports comprehensive investigations on the mechanisms and the structure-to-property relationships of these piezoresistive nanocomposites. Quantitative analyses revealed that piezoresistivity of polymer/CNT nanocomposites is predominantly governed by the three mechanisms related to the strain-induced morphological evolution of the CNT network embedded in the polymer matrix. Furthermore, both CNT content and CNT alignment are key structural parameters that affect the contribution of different mechanisms on PNCs' piezoresistivity and the sensitivity of flexible PNCs as strain sensors. For PNC filled with high content of randomly dispersed CNTs, the piezoresistivity was predominantly caused by the breakage of a complex conductive network into two or more simpler conductive paths. For PNC filled with low content of highly aligned CNTs, the piezoresistivity was mainly contributed by the complete disruption of originally interconnected CNTs in electrically conductive pathways.

Correction: A straightforward and “green” solvothermal synthesis of Al doped zinc oxide plasmonic nanocrystals and piezoresistive elastomer nanocomposite

Correction for ‘A straightforward and “green” solvothermal synthesis of Al doped zinc oxide plasmonic nanocrystals and piezoresistive elastomer nanocomposite’ by Andris Šutka et al., RSC Adv., 2015, 5, 63846–63852.

A straightforward and “green” solvothermal synthesis of Al doped zinc oxide plasmonic nanocrystals and piezoresistive elastomer nanocomposite

Ethanol solvothermal method derives plasmonic Al doped ZnO nanocrystals suitable for advanced applications.

Surface-modified piezoresistive nanocomposite flexible pressure sensors with high sensitivity and wide linearity.

Flexible pressure sensors working in a low pressure range (<10 kPa) have become an important part of recent research due to their applications in "artificial skin", foldable electronics and so on. Several efforts have been focused on the high sensitivity of devices with the neglect of linearity which is essential for real applications. Here, we present a device with a new Gaussian random distribution contact surface profile and a novel contact and piezoresistive composite working principle by numerical simulation, which predicts the combination of wide linearity and high sensitivity. With the modified surfaces' contact effect and the piezoresistive capability of these nanocomposite structures, an outstanding linearity can be achieved all along the measuring scale from 0 to 14 kPa, with a high sensitivity around 13.8 kPa(-1). The random distribution surface also provides the device with fine stability and reproducibility, which are validated in the test.

Sensitivity enhancement of a flexible MEMS strain sensor by a field effect transistor in an all organic approach

We report a low-cost piezoresistive nanocomposite based organic micro electro mechanical system (MEMS) strain sensor that has been combined to an organic field effect transistor (OFET) with the objective of amplifying the sensitivity of the sensor. When the MEMS cantilever is strained by a mechanical deflection, the resulting variation of resistivity influences the gate voltage (VGS) of the OFET and, hence, changes the drain current (IDS) of the transistor. The present combination allows an enhancement of sensitivity to strain by a factor 3.7, compared to the direct detection of resistance changes of the nanocomposite. As a consequence, a low limit of detection of 24ppm has been estimated in terms of strain transduction efficiency. Furthermore, the organic microsystem exhibits a short response time and operates reversibly with an excellent robustness.

Positive piezoresistive behavior of electrically conductive alkyl-functionalized graphene/polydimethylsilicone nanocomposites

Piezoresistive nanocomposites using alkyl-functionalized graphene (G-ODA) as a conducting filler and polydimethylsilicone (PDMS) as the polymer matrix were prepared and their piezoresistivity behavior was investigated. One-pot synthesis of G-ODA from graphite oxide and octadecylamine improved its dispersion in nonpolar xylene and PDMS with low surface free energy. Results show that the graphene nanosheets were homogeneously dispersed in the PDMS matrix and an ultra-low percolation threshold (0.63 vol%) of the composites was obtained. The G-ODA/PDMS composites with 1.19 vol% content of G-ODA show a remarkable positive piezoresistivity of high sensitivity (R/R0 > 400 under the pressure of 1.2 MPa), excellent repeatability, small hysteresis, and long-term durability. Under uniaxial compression, the resistance of the composites exponentially increased with the pressure. The resistance–pressure curves remain nearly unchanged after 1000 loading–unloading cycles. The results suggest that the G-ODA/PDMS nanocomposites provide a new route toward fabrication of soft piezoresistive sensors with high performance.