Flexible Strain-Sensitive Silicone-CNT Sensor for Human Motion Detection

This article describes the manufacturing technology of biocompatible flexible strain-sensitive sensor based on Ecoflex silicone and multi-walled carbon nanotubes (MWCNT). The sensor demonstrates resistive behavior. Structural, electrical, and mechanical characteristics are compared. It is shown that laser radiation significantly reduces the resistance of the material. Through laser radiation, electrically conductive networks of MWCNT are formed in a silicone matrix. The developed sensor demonstrates highly sensitive characteristics: gauge factor at 100% elongation −4.9, gauge factor at 90° bending −0.9%/deg, stretchability up to 725%, tensile strength 0.7 MPa, modulus of elasticity at 100% 46 kPa, and the temperature coefficient of resistance in the range of 30–40 °С is −2 × 10−3. There is a linear sensor response (with 1 ms response time) with a low hysteresis of ≤3%. An electronic unit for reading and processing sensor signals based on the ATXMEGA8E5-AU microcontroller has been developed. The unit was set to operate the sensor in the range of electrical resistance 5–150 kOhm. The Bluetooth module made it possible to transfer the received data to a personal computer. Currently, in the field of wearable technologies and health monitoring, a vital need is the development of flexible sensors attached to the human body to track various indicators. By integrating the sensor with the joints of the human hand, effective movement sensing has been demonstrated.

Graphene as a Piezoresistive Material in Strain Sensing Applications

High accuracy measurement of mechanical strain is critical and broadly practiced in several application areas including structural health monitoring, industrial process control, manufacturing, avionics and the automotive industry, to name a few. Strain sensors, otherwise known as strain gauges, are fueled by various nanomaterials, among which graphene has attracted great interest in recent years, due to its unique electro-mechanical characteristics. Graphene shows not only exceptional physical properties but also has remarkable mechanical properties, such as piezoresistivity, which makes it a perfect candidate for strain sensing applications. In the present review, we provide an in-depth overview of the latest studies focusing on graphene and its strain sensing mechanism along with various applications. We start by providing a description of the fundamental properties, synthesis techniques and characterization methods of graphene, and then build forward to the discussion of numerous types of graphene-based strain sensors with side-by-side tabular comparison in terms of figures-of-merit, including strain range and sensitivity, otherwise referred to as the gauge factor. We demonstrate the material synthesis, device fabrication and integration challenges for researchers to achieve both wide strain range and high sensitivity in graphene-based strain sensors. Last of all, several applications of graphene-based strain sensors for different purposes are described. All in all, the evolutionary process of graphene-based strain sensors in recent years, as well as the upcoming challenges and future directions for emerging studies are highlighted.

Bio-Inspired Hierarchical Carbon Nanotube Yarn with Ester Bond Cross-Linkages towards High Conductivity for Multifunctional Applications

The cross-linked hierarchical structure in biological systems provides insight into the development of innovative material structures. Specifically, the sarcoplasmic reticulum muscle is able to transmit electrical impulses in skeletal muscle due to its cross-linked hierarchical tubular cell structure. Inspired by the cross-linked tubular cell structure, we designed and built chemical cross-links between the carbon nanotubes within the carbon nanotube yarn (CNT yarn) structure by an esterification reaction. Consequently, compared with the pristine CNT yarn, its electrical conductivity dramatically enhanced 348%, from 557 S/cm to 1950 S/cm. Furthermore, when applied with three voltages, the electro-thermal temperature of esterified CNT yarn reached 261 °C, much higher than that of pristine CNT yarn (175 °C). In addition, the esterified CNT yarn exhibits a linear and stable piezo-resistive response, with a 158% enhanced gauge factor (the ratio of electrical resistance changing to strain change ~1.9). The superconductivity, flexibility, and stable sensitivity of the esterified flexible CNT yarn demonstrate its great potential in the applications of intelligent devices, smart clothing, or other advanced composites.

A Practical Strain Sensor Based on Ecoflex/Overlapping Graphene/Ecoflex Sandwich Structures for Vocal Fold Vibration and Body Motion Monitoring

Graphene-based stretchable and flexible strain sensors are one of the promising “bridges” to the biomedical realm. However, enhancing graphene-based wearable strain sensors to meet the demand of high sensitivity, broad sensing range, and recoverable structure deformation simultaneously is still a great challenge. In this work, through structural design, we fabricated a simple Ecoflex/Overlapping Graphene/Ecoflex (EOGE) strain sensor by encapsulating a graphene sensing element on polymer Ecoflex substrates using a drop-casting method. The EOGE strain sensor can detect stretching with high sensitivity, a maximum gauge factor of 715 with a wide strain range up to 57%, and adequate reliability and stability over 1,000 cycles for stretching. Moreover, the EOGE strain sensor shows recoverable structure deformation, and the sensor has a steady response in the frequency disturbance test. The good property of the strain sensor is attributed to the resistance variation induced by the overlap and crack structure of graphene by structural design. The vibrations caused by sound and various body movements have been thoroughly detected, which exhibited that the EOGE strain sensor is a promising candidate for wearable biomedical electronic applications.

Carbon nanotube/glycerol embedded low cost flexible sensor for large deflection sensing of continuum manipulators

Large deflection sensing is highly crucial for proper positioning and control of continuum robots during robot assisted minimally invasive surgery. Existing techniques suffer from eletromagnetic noise susceptibility, harmful radiation exposure, limited range, bio-incompatibility and necessity of expensive instruments. In the present study, we propose a Multi-Walled Carbon Nano-Tube (MWCNT)/polyglycerol based low cost, flexible and biocompatible sensor which could allow safer, faster and accurate angular deflection measurement of continuum robots for biomedical applications. Experimental results demonstrate that the sensor is stretchable upto 100% , provides a gauge factor upto 11.65, have response time around 8 ms, durability of -0.14% for cyclic loading and unloading and show very small creep upto ±0.0008 ( ±2.88%). Furthermore, the sensor can measure continuum robot deflection upto ±150 o with a sensitivity of 666.67 ohms/degree, with a maximum error of 1.67% and maximum hysteresis of 1.41%. Thus, wide range, low cost, fast response, and biocompatibility justify the potential of the proposed sensor for large deflection sensing of continuum robots during robot assisted minimally invasive surgery.

High Effective Sensitivity, Wide Working Range of Melt-blown Nonwoven Conductive Substrates for Smart Wearable Strain Sensors

High sensitivity, wide working range and flexible portability of strain sensors are crucial for smart wearable applications. To obtain these performances, several elastic melt-blown nonwoven substrates with excellent flexibility and high conductivity were developed by loading with polypyrrole through a double-dipping and double-rolling finishing method. The structure and conductivity are characterized by scanning electron microscope, infrared spectrometer, digital source meter and so on. The results indicate that the conductivity of prepared substrates is affected by the pyrrole concentration and polypyrrole amount deposited in nonwovens. Obviously, the conductivity and sensitivity of substrates as strain sensors are highly and positively correlated to the fiber orientation in nonwovens, and the effective working range and corresponding sensitivity of sensors are determined by the elastic deformation interval of melt-blown substrate. When the pyrrole concentration is 5.5%, the nonwoven substrate with 45.30% porosity possesses the anisotropic optimal conductivity with 23.491 S m-1 along winding direction and 15.063 S m-1 along width direction. Moreover, the as-prepared flexible conductive substrate exhibits the characteristics of wide working strain range (0-24.2%), high sensitivity with 1.94 gauge factor at the range, fast response (0.023 s), tiny hysteresis (0.011s), high durability and stability after 1000 cycles. Furthermore, the as-prepared sensor provides an effective method to prepare smart wearable strain sensors used as the monitor of finger bending in details and the precise recognition of human voice changes.

Effect of GNPs on the Piezoresistive, Electrical and Mechanical Properties of PHA and PLA Films

Sustainability has become the primary focus for researchers lately. Biopolymers such as polyhydroxyalkanoate (PHA) and polylactic acid (PLA) are biocompatible and biodegradable. Introducing piezoresistive response in the films produced by PLA and PHA by adding nanoparticles can be interesting. Hence, a study was performed to evaluate the mechanical, electrical and piezoresistive response of films made from PHA and PLA. The films were produced by solvent casting, and they were reinforced with graphene nanoplatelets (GNPs) at different nanoparticle concentrations (from 0.15 to 15 wt.%). Moreover, cellulose nanocrystals (CNC) as reinforcing elements and polyethylene glycol (PEG) as plasticizers were added. After the assessment of the nanoparticle distribution, the films were subjected to tests such as tensile, electrical conductivity and piezoresistive response. The dispersion was found to be good in PLA films and there exist some agglomerations in PHA films. The results suggested that the incorporation of GNPs enhanced the mechanical properties until 0.75 wt.% and they reduced thereon. The addition of 1% CNCs and 20% PEG in 15 wt.% GNPs’ tensile values deteriorated further. The PHA films showed better electrical conductivity compared to the PLA films for the same GNPs wt.%. Gauge factor (GF) values of 6.30 and 4.31 were obtained for PHA and PLA, respectively.

Rational Design of a Novel Highly Folding-tolerable Edgewo-rthia Chrysantha Lindi-derived Substrate for Versatile Flexible Pressure Sensors

Cellulose-based composites with superior mechanical and electrical properties are highly desirable for a sustainable and multifunctional substrate of flexible electronics. However, their practical application is hindered by the lack of superflexible cellulose-based composites to fabricate ingenious flexible electronics with considerable robustness. Here, cellulose derived from underutilized biomass (Edgewo-rthia chrysantha Lindi, ERCL) was composited with highly-conductive silver nanowires (AgNWs) through a general papermaking process. Benefiting from the interactions between cellulose and AgNWs including hydrogen bonding and van der Waals force, the composite presented superb electrical conductivity (> 27000 S/m) and flexibility (folding times ≥1110). By employing it as the substrate of flexible pressure sensors (FPSs) through layer-by-layer assembly, improved sensitivity (Gauge Factor=846.4), rapid response (0.44 s), and excellent stability (≥2000 folding cycles) were demonstrated. Impressively, the novel FPS could monitor human motions, including finger bending, elbow flexion, speaking, and pulse, suggesting its great potentials in emerging flexible electronics.

Thermoresponsive Lignin-Reinforced Poly(Ionic Liquid) Hydrogel Wireless Strain Sensor

To meet critical requirements on flexible electronic devices, multifunctionalized flexible sensors with excellent electromechanical performance and temperature perception are required. Herein, lignin-reinforced thermoresponsive poly(ionic liquid) hydrogel is prepared through an ultrasound-assisted synthesized method. Benefitting from the electrostatic interaction between lignin and ionic liquid, the hydrogel displays high stretchability (over 1425%), excellent toughness (over 132 kPa), and impressive stress loading-unloading cyclic stability. The hydrogel strain sensor presents excellent electromechanical performance with a high gauge factor (1.37) and rapid response rate (198 ms), which lays the foundation for human body movement detection and smart input. Moreover, owing to the thermal-sensitive feature of poly(ionic liquid), the as-prepared hydrogel displays remarkable thermal response sensitivity (0.217°C-1) in body temperature range and low limit of detection, which can be applied as a body shell temperature indicator. Particularly, the hydrogel can detect dual stimuli of strain and temperature and identify each signal individually, showing the specific application in human-machine interaction and artificial intelligence. By integrating the hydrogel strain sensor into a wireless sensation system, remote motion capture and gesture identification is realized in real-time.

Charge completeness and the massless charge lattice in F-theory models of supergravity

We prove that, for every 6D supergravity theory that has an F-theory description, the property of charge completeness for the connected component of the gauge group (meaning that all charges in the corresponding charge lattice are realized by massive or massless states in the theory) is equivalent to a standard assumption made in F-theory for how geometry encodes the global gauge theory by means of the Mordell-Weil group of the elliptic fibration. This result also holds in 4D F-theory constructions for the parts of the gauge group that come from sections and from 7-branes. We find that in many 6D F-theory models the full charge lattice of the theory is generated by massless charged states; this occurs for each gauge factor where the associated anomaly coefficient satisfies a simple positivity condition. We describe many of the cases where this massless charge sufficiency condition holds, as well as exceptions where the positivity condition fails, and analyze the related global structure of the gauge group and associated Mordell-Weil torsion in explicit F-theory models.