Ultra-Stretchable Monofilament Flexible Sensor with Low Hysteresis and Linearity based on MWCNTs/Ecoflex Composite Materials

Wearable human–computer interaction equipment is a common technology, which can improve the comfort, convenience, and safety of the human body, and also can monitor human health. The flexible and wearable tensile sensor can be conveniently installed on clothes or directly connected to the body. This provides a convenient, timely, and portable solution for the detection of human motion. Therefore, wearable electronic equipment is gaining more attention. In this paper, a highly stretchable, flexible, and sensitive strain sensor which is based on multi-walled carbon nanotubes/Ecoflex nanocomposites is reported. A monofilament tensile sensor obtains good linearity (10.77%), low hysteresis (1.63%), good stability (6000 cycles under 100% strain), and ultra-high strain range (ε = 1300%). This ultra-stretchable sensor has potential applications in human motion monitoring, medical rehabilitation, health monitoring, human–computer interaction, and soft robots.     

Flexible strain sensors with high sensitivity and large working range prepared from biaxially stretched carbon nanotubes/polyolefin elastomer nanocomposites

Flexible strain sensors are a new generation of flexible and stretchable electronic devices that attracted increasing attention due to their practical applications in many fields. However, maintaining a wide detectable strain range while improving the sensitivity of flexible strain sensors remains challenging. In this study, flexible strain sensors with a large working range based on biaxially stretched carbon nanotubes (CNTs)/polyolefin elastomer (POE) nanocomposites were fabricated. Biaxial stretching was demonstrated to enhance the uniform dispersion and orientation of CNTs, thereby improving the performance of sensors. The optimal stretching ratios (SRs) of nanocomposites were investigated and the data revealed an increment in the sensitivity of sensors with SRs, while the working range first increased after biaxial stretching and decreased at higher SRs. Compared to the 9 wt% CNT/POE-1.0 sensor with a gauge factor (GF) value of 2.37 and a detectable range of 0.5%–230%, the CNT/POE-2.0 sensor exhibited an enhanced sensitivity (GF = 3935.12) coupled with a wider detectable range (0.5%–710%) and better stability. Besides, CNT/POE-2.0 sensor also achieved the monitoring of head movements, mouth opening, facial expression, and physiological signals, showing a potential for use in wearable electronic products.     

Segmented modelling and analysis of disperse dye concentration based on multidimensional spectrum

Since the water-insoluble dispersing dye has both absorption and scattering characteristics, a spatial resolution hyperspectral measurement approach and experimental testing was proposed in this article, which can collect spectral and spatial data from samples simultaneously. The concentration of 81 groups of three-component disperse dye samples were measured. However, the hyperspectral data of dye solutions in the 420–800 nm band is saturated, resulting in the inability for multispectral data processing. A segmented concentration quantitative analysis model was developed. For the unsaturated band (420–510 nm), the partial least squares (PLS), the N-way partial least squares (NPLS), and support vector machine (SVM) models using the data points on the X-axis of a two-dimensional light intensity distribution map were established. The predicted performance of PLS model was worse slightly than that of the other two models, The coefficient of determination (R²) values of concentrations for red, orange and blue disperse dye were 0.888, 0.796 and 0.959, respectively. For saturated band (520–670 nm), the NPLS and SVM models using the data points on the X- and Y-axis were established. Results shows that the prediction accuracy of concentrations of the three-component disperse dye was increased by adding additional data points on the Y-axis, with R² values of 0.944, 0.807, and 0.912, respectively. For the strong scattering band (680–800 nm), a SVM model was established, and R² of concentration of the three dyes reached 0.974, 0.933 and 0.995, respectively. The results showed that multidimensional spectroscopy method can improve the prediction accuracy of component concentration of disperse dye solution, by using more spectral information from X and Y directions.     

Anisotropy of poly(lactic acid)/carbon fiber composites prepared by fused deposition modeling

It is well known that 3D printed parts prepared by fused deposition modeling (FDM) exhibit large anisotropy of mechanical properties. In this article, poly(lactic acid; PLA)/carbon fiber (CF) composites with different built orientations (X, Y, Z) were prepared by FDM. The effects of printing temperature, speed, orientations, and layer thickness on the mechanical properties of the composites were systematically investigated. The mechanical properties of PLA/CF composites show more significant anisotropy. The orientation of the fibers along the printing direction is displayed by scanning electron microscopy. Printing parameters bring almost no effect on mechanical properties of the X-construct oriented specimen, and bring obvious effect on those of the Y-construct oriented specimen and Z-construct oriented specimen. According to the analysis, carbon fiber can amplify this anisotropy from layer fashion, and the key factors from printing parameters are porosity and bond strength between fuses. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48786.     

Highly flexible fabric strain sensor based on graphene nanoplatelet–polyaniline nanocomposites for human gesture recognition

Flexible and wearable smart fabrics are becoming increasingly popular in healthcare and motion monitoring because of their potential applications in flexible and stretchable electronics. The integration of ordinary fabric with conductive fillers provides the fabric with new and intriguing functions, such as sensation. In this study, a low‐cost and efficient manner was used to fabricate a highly reliable conductive composite on fabric as an effective sensing material for gesture recognition. A strain sensor was fabricated by the incorporation of the highly conductive polyaniline (PANI) polymer, graphene nanoplatelets (GNPs), and a handful of silicon rubber (SR) onto elastic Lycra fabric via a spin‐coating method. We demonstrated that the fabric strain sensor was able to detect and monitor the bending angle of a human finger. By means of the covered structure of the PANI and GNPs, the composite fabric could bear a 40% maximum strain and possess the pleasant characteristic of stretching and bending. The gauge factor of the fabric strain sensor reached 67.3; this was an improvement of approximately four times compared to sensors without PANI microparticles. Finally, the superior performance of our strain sensor through the integration of five strain sensors on a glove for the motion detection of fingers was demonstrated. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45340.     

Double Network Hydrogel Sensors with High Sensitivity in Large Strain Range

Owing to their preferable flexibility and facilitation to integrate with various apparel products, flexible sensors with high sensitivity are highly favored in the fields of environmental monitoring, health diagnosis, and wearable electronics. However, great challenges still remain in integrating high sensitivity with wide sensing range in one single flexible strain sensor. Herein, a new stretchable conductive gel-based sensor exhibiting remarkable properties regarding stretchability and sensitivity is developed via improving the ionic conductivity of the PVA/P(AM-AANa) double network hydrogel. Specifically, the strain sensor developed exhibits an excellent elongation of 549%, good fatigue resistance, and recovery performance. Simultaneously, the hydrogel strain sensor shows a high conductivity of 25 mS cm⁻¹, fast response time of 360 ms, and a linear response (gauge factor = 4.75) to external strain (≈400%), which endow the sensor with accurate and reliable capacities to detect various human movements. Integrating the merits of flexibility, environment friendliness, and high sensitivity, the conductive gel-based sensor has promising application prospects in human–machine interfaces, touchpads, biosensors, electronic skin, wearable electronic devices, and so on.     

不同模式多功能柔性传感器研究进展

柔性传感器能够实现压力、应变、温度、湿度及气体等与人体健康相关信号多功能识别及监测,在可穿戴人工智能设备的开发中展现出巨大的应用前景。本文综述了具有多种模式监测功能的柔性电化学式传感器领域最新研究成果,包括双模式传感器、三模式传感器和多模式传感器;重点介绍了传感器实现多功能监测的途径和传感机理。研究表明,多模式传感性的实现方法主要包括结构设计和多功能材料制备两种。而基于先进功能材料(包括纳米金属、纳米碳及导电聚合物)和柔性基体材料(如水凝胶、气凝胶及弹性聚合物)所制造的柔性多功能复合材料可有效降低多模式传感器的复杂性。最后,对比并指出了不同类型的功能材料在制造多功能柔性传感器中的特点与优势,为多功能柔性传感器的研究提供借鉴意义。     

Anisotropic thermal expansion in polypropylene/poly(ethylene-co-octene) binary blends: influence of arrays of elastomer domains

In injection molded specimens consisting of isotactic polypropylene (iPP)/poly(ethylene-co-octene) (EOR) blends with different viscosity ratio of η(EOR)/η(iPP), the coefficient of linear thermal expansion (CLTE) was investigated by thermal mechanical analysis (TMA). It was found that the blend with a smaller viscosity ratio showed the larger anisotropy of CLTE depending upon the directions. TEM observations revealed that the shape of rubber domains varied from slabs, cylinders to ellipsoids in shape, by increasing η(EOR)/η(iPP). The crystal orientation analysis by WAXD have revealed that the blend with ‘slab’ EOR domains showed the orientation of the c-axis of iPP crystals was preferably oriented to FD (flow direction) and TD (transverse to FD), and that the b-axis was exclusively oriented to ND (thickness direction). The CLTE of each FD and TD was in good agreement with the rules-of-mixing for CLTE by introducing the effect of the arrays of the elastomer domains and the PP crystal orientation. On the other hand, the CLTE in ND showed massive discrepancy between the calculation and observation. It was found that the incorporation of the retraction effect could explain the discrepancy to some extent.     

Spectral power distributions for the CIE stimuli

The CIE reference colour stimuli, X, Y, and Z, were derived by constructing a triangle outside the R,G,B triangle and outside the area bounded by the spectrum locus and the purple line. By this means, all colours, including monochromatic ones, have positive tristimulus values. The colour‐matching functions are the relative quantities of these stimuli required to be mixed additively to match the equal energy monochromatic colours. The stimuli are not realizable as light sources, and the CIE has not specified their spectral power distributions. There is an infinite number of spectral power distributions whose properties meet the prerequisites for X (X = 100, Y = 0, Z = 0), Y (0, 100, 0), and Z (0, 0, 100), and two possible sets have been calculated by different methods. These curves could be used as primary red, green, and blue lights in additive mixing to produce synthetic reflectance curves, which are useful in the specification of on‐screen colours, and as a means of producing colour constant standards. © 2001 John Wiley & Sons, Inc. Col Res Appl, 26, 478–482, 2001     

Synthesis,Characterization of Fulvic Acid–Poly(Methylmethacrylate) Graft Copolymers Based on Surface-Initiated Atom Transfer Radical Polymerization and its Effect on Performance of Poly(Lactic Acid)

Fulvic acid–poly(methylmethacrylate) graft copolymers were synthesized by surface-initiated atom transfer radical polymerization with fulvic acid. The result demonstrated that the hydrophobicity of fulvic acid–poly(methylmethacrylate) was improved after modification by surface-initiated atom transfer radical polymerization. Furthermore, poly(lactic acid)/fulvic acid–poly(methylmethacrylate) composites were prepared to improve the performances of poly(lactic acid) by blend melting. Compared to poly(lactic acid) with X_c of 5.38%, the X_c of poly(lactic acid)/fulvic acid–poly(methylmethacrylate) composites was 19.94%. Moreover, the impact strength of poly(lactic acid)/fulvic acid–poly(methylmethacrylate) composites was increased by 5.19% compared to poly(lactic acid). In all, this study provided an effective and feasible method for optimizing interface performance and enhancing the thermal stability of poly(lactic acid).     

Highly Stretchable and Sensitive Flexible Strain Sensor Based on Fe NWs/Graphene/PEDOT:PSS with a Porous Structure

With the rapid development of wearable smart electronic products, high-performance wearable flexible strain sensors are urgently needed. In this paper, a flexible strain sensor device with Fe NWs/Graphene/PEDOT:PSS material added under a porous structure was designed and prepared. The effects of adding different sensing materials and a different number of dips with PEDOT:PSS on the device performance were investigated. The experiments show that the flexible strain sensor obtained by using Fe NWs, graphene, and PEDOT:PSS composite is dipped in polyurethane foam once and vacuum dried in turn with a local linearity of 98.8%, and the device was stable up to 3500 times at 80% strain. The high linearity and good stability are based on the three-dimensional network structure of polyurethane foam, combined with the excellent electrical conductivity of Fe NWs, the bridging and passivation effects of graphene, and the stabilization effect of PEDOT:PSS, which force the graphene-coated Fe NWs to adhere to the porous skeleton under the action of PEDOT:PSS to form a stable three-dimensional conductive network. Flexible strain sensor devices can be applied to smart robots and other fields and show broad application prospects in intelligent wearable devices.     

Highly stretchable and sensitive strain sensors using nano-graphene coated natural rubber

Flexible, stretchable and wearable sensors are needed for the human motion detection. Here, a highly stretchable and sensitive strain sensor is fabricated based on the coating of nano-graphene platelets on natural rubber by simple dry coating process. The gauge factors are adjustable in the ranges of 0.78–52.53 depended on the preparation conditions and strain state. The sensors showed a high stretchability up to 750% and high durability of 1500 stretching–releasing cycles. The stretchable strain sensors are capable of detecting a bending fingers and the pulse of radial artery on the wrist. In addition, a smart glove made form five independent strain sensors was created. The data of the glove finger motions are used to control an avatar robotic hand.     

A novel,stretchable, silver-coated polyolefin elastomer nanofiber membrane for strain sensor applications

In this study, we prepared a novel, stretchable, and porous polyolefin elastomer (POE) nanofiber membrane (NM) on the basis of a high-throughput, low-cost, environmentally friendly melt-blending method. To obtain an excellent conductivity, we prepared the Ag–POE composite NM via a facile and effective method of electroless plating. Via the control of the pretreatment process and concentration of the reactant, a homogeneous Ag layer formed on the surface of the POE nanofibers without damaging the original thermal and mechanical properties. Because of the good stretchability, conductivity, and porosity, the Ag–POE NM was used as a strain sensor to detect the pressure and airflow. When the silver nitrate concentration was 4 g/L, a good sensitivity and durability were obtained. The current change accurately recorded the deformation of the membrane under different pressures and air flows. The results of this study provide us with an industrial method to prepare Ag–POE NM and indicate its potential application as a strain sensor for outer stimuli detection. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47928.     

Blends of aliphatic polyesters. I. Physical properties and morphologies of solution-cast blends from poly(DL-lactide) and poly(ε-caprolactone)

Effects of the mixing ratio of poly(DL-lactide) (PDLLA) and poly (ε-caprolactone) (PCL) on the thermal and mechanical properties and morphologies of the solution-cast blends were investigated by differential scanning calorimetry (DSC), polarizing microscopy, tensile tests, and dynamic mechanical analysis. The presence of amorphous PDLLA did not disturb crystallization of PCL over the PDLLA content [X_PDLLA = PDLLA/(PCL + PDLLA)] from 0.1 to 0.9 and allowed PCL to form spherulites over X_PDLLA ranging from 0.1 to 0.6. The spherulite radius was larger for the blends than for the nonblended PCL. Phase separation occurred for the blends with X_PDLLA between 0.1 and 0.9 T^m of PCL remained unchanged in the X_PDLLA range up to 0.6 but decreased at X_PDLLA above 0.6, whereas the crystallinity of PCL was constant around 60%, irrespective of X_PDLLA. The tensile strength (σ_B, the yield stress (σ_Y), the Young's modulus(E), and the storage modulus (G′) of the blends increased monotonously with X_PDLLA if σ_B at X_PDLLA = 0 and 0.6 and σ_Y at X_PDLLA = 0.6 were excluded. Elongation-at-break (ε_B) of PDLLA increased dramatically, while ε_B of PCL decreased remarkably when a small amount of the other component was added. Equations and parameters predicting σ_Y, E, and G′ of the PCL-PDLLA blends were proposed as a function of X_PDLLA. © 1996 John Wiley & Sons, Inc.     

Electrical and thermal conductivities of polyethylene composites filled with biaxial oriented short-cut carbon fibers

Polyethylene composites filled with various types of carbon fiber were prepared for electrical and thermal conductivity measurements. By estimation of the anisotropic parameter (Hermans' parameter), the fibers were confirmed to be significantly biaxially oriented in the composites. The critical volume fractions in the electrical conductivity of these composites for the two oriented directions (X and Y) were equal to each other and smaller than that for a direction (Z) vertical to the above. The electrical anisotropy, i.e., ratio of electrical conductivity of the composite for the Z direction to the X and Y directions varied drastically with increase in filler content. The longer the length of carbon fiber was, the higher became the electrical conductivity of the biaxially oriented carbon fiber composites for all directions. But, the thermal conductivity of the composite was almost unchanged for the Z direction, even if fiber length was sufficiently long. Our equation, previously proposed, proved adaptable to these thermal conductivities. The factors of C_p and C_f in the equation are kept unchanged, in spite of increasing fiber length. © 1994 John Wiley & Sons, Inc.     

A Skin-Inspired Triboelectric Nanogenerator with an Interpenetrating Structure for Motion Sensing and Energy Harvesting

Rapid advancements in wearable electronics impose the challenge on power supply devices. Herein, a flexible single-electrode triboelectric nanogenerator (SE-TENG) that enables both human motion sensing and biomechanical energy harvesting is reported. The SE-TENG is fabricated by interpenetrating Ag-coated polyethylene terephthalate (PET) nanofibers within a polydimethylsiloxane (PDMS) elastomer. The Ag coating and PDMS are performed as the electrode and dielectric material for the SE-TENG, respectively. The Ag-coated PET nanofibers enlarge the electrode surface area, which is beneficial to increase sensing sensitivity. The flexible SE-TENG sensor shows the capability of outputting alternating electrical signals with an open-circuit voltage up to 50 V and a short-circuit current up to 200 nA in response to externally applied pressure. It is used to sense various types of human motions and harvest electric energy from body motion. The harvested energy can successfully power wearable electronics, such as an electronic watch and light-emitting diode. Therefore, the as-prepared SE-TENG sensor with a pressure response and self-powered capability provides potential applications in wearable sensors or flexible electronics for personal healthcare and human–machine interfaces.     

Effect of Solvent on Segregated Network Morphology in Elastomer Composites for Tunable Piezoresistivity

Flexible, light‐weight, and wearable electronics have significant potential for the development of Internet of Things. Flexible sensors with tunable piezoresistive properties are in high demand for various practical applications. Herein, different morphology thermoplastic polyurethane (TPU)/ carbon nanostructure (CNS) composites with segregated network are obtained by swelling the TPU powders using various solvents. The better solvent for TPU, dimethylformamide (DMF), renders the composites with 0.7 wt% CNS stronger polymer‐filler interactions, resulting in significantly improved piezoresistive sensitivity at strain larger than 150%. Also the gauge factors (G_Fs) for these composites are 9.7 in the range 0–60% strain and 19.3 for 60–100% strain. In contrast, the composites with ethanol (EtOH) and tetrahydrofuran (THF) which swell less the TPU show delayed increase in piezoresistivity and G_Fs of 2.2 and 3.5 for strain up to 100%, respectively, suggesting potential applications for stretchable conductors.     

Investigating the effect of silver nanorods embedded in polydimethylsiloxane matrix using nanoindentation and its use for flexible electronics

The stretchable electrodes with excellent flexibility, electrical conductivity, and mechanical durability are the most fundamental components in the emerging and exciting field of flexible electronics. This article proposes a method for fabrication of such a stretchable electrode by embedding silver nanorods (AgNRs) into a polydimethylsiloxane (PDMS) matrix that is grown by a unique glancing angle deposition technique. The surface, mechanical, and electrical properties of PDMS are significantly changed after embedding the AgNRs in it. The results show that surface roughness and polarity increase after AgNRs are embedded in the PDMS matrix. Elastic modulus (E) and hardness (H) decrease with an increase in the indentation load as a result of the indentation depth effect. Due to strong interfacial adhesion of AgNRs embedded in the PDMS matrix, the E and H of nanocomposite are increased by 167.6 and 93.3% compared with PDMS film, respectively. Furthermore, the AgNRs-PDMS film has an electrical resistivity value in the order of 10⁻⁷ Ωm. It remains conductive during various mechanical strains such as bending, twisting, and stretching, which is demonstrated using a light-emitting diode circuit. Simultaneously, the antimicrobial activity of silver could make it a promising candidate for wearable electronics.     

Wide angle X-ray diffraction investigation of crystal orientation in miscible blend of poly(ε-caprolactone)/poly(vinyl chloride) crystallized under strain

The orientation of poly(ε-caprolactone) crystals in miscible poly(ε-caprolactone)/poly(vinyl chloride) (PCL/PVC) blends, melt crystallized under strain, has been studied by wide angle X-ray diffraction (WAXD). At low draw ratios or low PVC contents, all the observable (hk0) crystal reflections orient towards the meridional direction in WAXD patterns, indicating the presence of ring-fibre orientation. With the increase of draw ratio or PVC content, additional crystal orientation with the crystal a-axis parallel to the stretching direction is found to superimpose on the WAXD pattern of ring-fibre orientation. Both the ring-fibre orientation, which dominates the WAXD pattern, and the a-axis orientation are characterized by the perpendicular orientation of the crystal c-axis to the stretching direction. The unusual PCL orientation is a consequence of the combined effects of both the stretching and the presence of PVC in the PCL/PVC blends.     

Highly Stretchable and Sensitive SBS/Graphene Composite Fiber for Strain Sensors

Flexible strain sensors have attracted tremendous interests due to the emergence of intelligent wearable technology. Electrically conductive fibers are desirable candidates for flexible strain sensors, but up til now, there still exist enormous challenges to obtain conductive fibers exhibiting simultaneously high stretchability and high strain sensitivity. This paper introduces a poly (styrene‐butadiene‐styrene) (SBS)/graphene (Gr) composite fiber‐based flexible strain sensor fabricated by a facile and highly scalable wet spinning method. The results demonstrate that the graphene content has significant influence on the morphology, mechanical properties, and electromechanical properties of the composite fibers. The fibers with 5 wt% graphene have a wide response range of up to 100% strain, a high electrical sensitivity with the gauge factor of 10083.98 at 100% strain, and meanwhile, a high level of stability for 2100 stretching–releasing cycles under an applied strain of 20%. Furthermore, the SBS‐5%Gr composite fibers display excellent sensing performance in detecting human upper limb movements at different joints including hand joints, wrist joints, elbow joints, and shoulder joints.