Thermoplastic Elastomer Systems Containing Carbon Nanofibers as Soft Piezoresistive Sensors

Soft Wearable Piezoresistive Sensors Based on Natural Rubber Fabricated with a Customized Vat-Based Additive Manufacturing Process

Piezoresistive sensors for monitoring human motions are essential for the prevention and treatment of injury. Natural rubber is a material of renewable origin that can be used for the development of soft wearable sensors. In this study, natural rubber was combined with acetylene black to develop a soft piezoresistive sensing composite for monitoring the motion of human joints. An additive manufacturing technique based on stereolithography was used, and it was seen that the sensors produced with the method could detect even small strains (<10%) successfully. With the same sensor composite fabricated by mold casting, it was not possible to detect low strains reliably. TEM microscopy revealed that the distribution of the filler was not homogeneous for the cast samples, suggesting a directionality of the conductive filler network. For the sensors fabricated through the stereolithography-based method, a homogeneous distribution could be achieved. Based on mechano-electrical characterization, it was seen that the samples produced with AM combined the ability to endure large elongations with a monotonic sensor response. Under dynamic conditions, the sensor response of the samples produced by 3D printing showed lower drift and lower signal relaxation. The piezoresistive sensors were examined for monitoring the motion of the human finger joints. By increasing the bending angle of the sensor, it was possible to increase the sensitivity of the response. With the renewable origin of natural rubber and manufacturing method, the featured sensors can expand the applicability of soft flexible electronics in biomedical applications and devices.

Wearable and Stretchable SEBS/CB Polymer Conductive Strand as a Piezoresistive Strain Sensor

A wearable and stretchable strain sensor with a gauge factor above 23 was prepared using a simple and effective technique. Conducting nanocomposite strands were prepared from styrene-b-(ethylene-co-butylene)-b-styrene triblock copolymer (SEBS) and carbon black (CB) through a solvent-processing method that uses a syringe pump. This novel nanocomposite preparation technique is a straightforward and cost-effective process and is reported in the literature for the first time. The work included two stages: the flexible nanocomposite preparation stage and the piezoresistive sensor stage. Depending on its molecular structure, the thermoelastic polymer SEBS is highly resilient to stress and strain. The main aim of this work is to fabricate a highly flexible and piezoresistive nanocomposite fibre/strand. Among the prepared composites, a composite corresponding to a composition just above the percolation threshold was selected to prepare the strain sensor, which exhibited good flexibility and conductivity and a large piezoresistive effect that was linearly dependent on the applied strain. The prepared nanocomposite sensor was stitched onto a sports T-shirt. Commercially available knee and elbow sleeves were also purchased, and the nanocomposite SEBS/CB strands were sewn separately on the two sleeves. The results showed a high sensitivity of the sensing element in the case of breathing activity (normal breathing, a 35% change, and deep breathing at 135%, respectively). In the case of knee and elbow movements, simultaneous measurements were performed and found that the sensor was able to detect movement cycles during walking.

Graphene and Its Derivatives: Synthesis and Application in the Electrochemical Detection of Analytes in Sweat

Wearable sensors and invasive devices have been studied extensively in recent years as the demand for real-time human healthcare applications and seamless human-machine interaction has risen exponentially. An explosion in sensor research throughout the globe has been ignited by the unique features such as thermal, electrical, and mechanical properties of graphene. This includes wearable sensors and implants, which can detect a wide range of data, including body temperature, pulse oxygenation, blood pressure, glucose, and the other analytes present in sweat. Graphene-based sensors for real-time human health monitoring are also being developed. This review is a comprehensive discussion about the properties of graphene, routes to its synthesis, derivatives of graphene, etc. Moreover, the basic features of a biosensor along with the chemistry of sweat are also discussed in detail. The review mainly focusses on the graphene and its derivative-based wearable sensors for the detection of analytes in sweat. Graphene-based sensors for health monitoring will be examined and explained in this study as an overview of the most current innovations in sensor designs, sensing processes, technological advancements, sensor system components, and potential hurdles. The future holds great opportunities for the development of efficient and advanced graphene-based sensors for the detection of analytes in sweat.

Electromechanical Performance of Strain Sensors Based on Viscoelastic Conductive Composite Polymer Fibers

Flexible conductive polymer composite (CPC) fibers that show large changes in resistance with deformation have recently gained much attention as strain-sensing components for future wearable electronics. However, the electrical resistance of these materials decays with time during dynamic cyclic loading, a deformation performed to simulate their real application as strain sensors. Despite the extensive research on CPC fibers, the mechanism leading to this decay in the electromechanical response under repetitive cycles remains unreported. Herein, this behavior is investigated using fiber-based strain sensors wet spun from thermoplastic polyurethane (TPU) consisting of a carbonaceous hybrid conductive filler system of carbon black (CB) and carbon nanotubes (CNTs). We found electrical viscosity to predict the observed electromechanical resistance decay. This implies that cycling these materials enables the relaxation of both the polymer chains and the conductive network. In addition, the resulting piezoresistive fibers are sensitive to deformation in the region of low strain (gauge factor of 6.0 within 3.0% strain), remain conductive under 280.5% deformation, and are stable for more than 2000 cycles. Finally, we demonstrate the potential of TPU/CB-CNT fibers as strain sensors for monitoring human motion.

Development of an Innovative Soft Piezoresistive Biomaterial Based on the Interconnection of Elastomeric PDMS Networks and Electrically-Conductive PEDOT:PSS Sponges

A deeply interconnected flexible transducer of polydimethylsiloxane (PDMS) and poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) was obtained as a material for the application of soft robotics. Firstly, transducers were developed by crosslinking PEDOT:PSS with 3-glycidyloxypropryl-trimethoxysilane (GPTMS) (1, 2 and 3% v/v) and using freeze-drying to obtain porous sponges. The PEDOT:PSS sponges were morphologically characterized, showing porosities mainly between 200 and 600 µm²; such surface area dimensions tend to decrease with increasing degrees of crosslinking. A stability test confirmed a good endurance for up to 28 days for the higher concentrations of the crosslinker tested. Consecutively, the sponges were electromechanically characterized, showing a repeatable and linear resistance variation by the pressure triggers within the limits of their working range (∆RR0 max = 80% for 1-2% v/v of GPTMS). The sponges containing 1% v/v of GPTMS were intertwined with a silicon elastomer to increase their elasticity and water stability. The flexible transducer obtained with this method exhibited moderately lower sensibility and repeatability than the PEDOT:PSS sponges, but the piezoresistive response remained stable under mechanical compression. Furthermore, the transducer displayed a linear behavior when stressed within the limits of its working range. Therefore, it is still valid for pressure sensing and contact detection applications. Lastly, the flexible transducer was submitted to preliminary biological tests that indicate a potential for safe, in vivo sensing applications.

Recent advance in the fabrication of carbon nanofiber-based composite materials for wearable devices

Carbon nanofibers (CNFs) exhibit the advantages of high mechanical strength, good conductivity, easy production, and low cost, which have shown wide applications in the fields of materials science, nanotechnology, biomedicine, tissue engineering, sensors, wearable electronics, and other aspects. To promote the applications of CNF-based nanomaterials in wearable devices, the flexibility, electronic conductivity, thickness, weight, and bio-safety of CNF-based films/membranes are crucial. In this review, we present recent advances in the fabrication of CNF-based composite nanomaterials for flexible wearable devices. For this aim, firstly we introduce the synthesis and functionalization of CNFs, which promote the optimization of physical, chemical, and biological properties of CNFs. Then, the fabrication of two-dimensional and three-dimensional CNF-based materials are demonstrated. In addition, enhanced electric, mechanical, optical, magnetic, and biological properties of CNFs through the hybridization with other functional nanomaterials by synergistic effects are presented and discussed. Finally, wearable applications of CNF-based materials for flexible batteries, supercapacitors, strain/piezoresistive sensors, bio-signal detectors, and electromagnetic interference shielding devices are introduced and discussed in detail. We believe that this work will be beneficial for readers and researchers to understand both structural and functional tailoring of CNFs, and to design and fabricate novel CNF-based flexible and wearable devices for advanced applications.

Microdome-Induced Strain Localization for Biaxial Strain Decoupling toward Stretchable and Wearable Human Motion Detection

Soft strain sensors have attracted significant attention in wearable human motion monitoring applications. However, there is still a huge challenge for decoupled measurement of multidirectional strains. In this study, we have developed a biaxial and stretchable strain sensor based on a carbon nanotube (CNT) film and a microdome array (MA)-patterned elastomeric substrate. The MA structures lead to generating localized and directional microcracks of CNT films within the intended regions under tensile strain. This mechanism allows a single sensing layer to act as a strain sensor capable of decoupling the biaxial strains into axial and transverse terms. The ratio of resistance change between two perpendicular axes is about 960% under an x-directional strain of 30%, demonstrating the biaxial decoupling capability. Also, the proposed strain sensor shows high stretchability and excellent long-term reliability under a cyclic loading test. Finally, wearable devices integrated with the strain sensor have been successfully utilized to monitor various human motions of the wrist, elbow, knee, and fingers by measuring joint bending and skin elongation.

Ionic complexation of endblock-sulfonated thermoplastic elastomers and their physical gels for improved thermomechanical performance

Thermoplastic elastomers (TPEs) composed of nonpolar triblock copolymers constitute a broadly important class of (re)processable network-forming macromolecules employed in ubiquitous commercial applications. Physical gelation of these materials in the presence of a low-volatility oil that is midblock-selective yields tunably soft TPE gels (TPEGs) that are suitable for emergent technologies ranging from electroactive, phase-change and shape-memory responsive media to patternable soft substrates for flexible electronics and microfluidics. Many of the high-volume TPEs used for these purposes possess styrenic endblocks that are inherently limited by a relatively low glass transition temperature. To mitigate this shortcoming, we sulfonate and subsequently complex (and physically crosslink) the endblocks with trivalent Al³⁺ ions. Doing so reduces the effective hydrophilicity of the sulfonated endblocks, as evidenced by water uptake measurements, while concurrently enhancing the thermomechanical stability of the corresponding TPEGs. Chemical modification results, as well as morphological and property development, are investigated as functions of the degree of sulfonation, complexation and TPEG composition.

A highly flexible and multifunctional strain sensor based on a network-structured MXene/polyurethane mat with ultra-high sensitivity and a broad sensing range

Flexible and multifunctional strain sensors with superior properties including high sensitivity, low detection limits, and a wide sensing range are always in high demand for wearable electronics. However, it remains a big challenge to fully satisfy the aforementioned requirements. In particular, there is always a trade-off between high sensitivity and wide sensing range. Here, we developed a multifunctional strain sensor based on a network-structured MXene/polyurethane mat (network-M/P mat) and well balanced the relationship between the sensitivity and sensing range by rationally designing the morphology and microstructures of the sensing device. The network-structured polyurethane mat (network-P mat) was fabricated through a facile and scalable electrospinning technique. The highly conductive MXene sheets were decorated onto the network-P mat through hydrogen bonding or electrostatic interactions. The obtained highly flexible and stretchable network-M/P mat exhibited a superior comprehensive sensing performance that was characterized by high sensitivity (gauge factor up to 228), a low limit of detection (0.1%), a large and tunable sensing range (up to 150%), excellent stability (over 3200 cycles), and multiple functions (lateral strain, vertical pressure, bending and subtle vibration). Based on its superior performance, the network-M/P mat-based strain sensor can detect a full range of body actions and subtle physiological signals (e.g. respirations and pulse waves), demonstrating great potential for applications in artificial electronic skin and wearable health detectors.

1st Symposium on Polydopamine and NanoTech Poland 2018: Conference Report

The NanoTech Poland is an annual international conference with a strong scientific agenda focused on nanotechnology in energy, environment, and biomedicine. The Nanotech Poland 2018 was held at the NanoBioMedical Centre and Department of Physics at Adam Mickiewicz University in Poznań from June 6th to June 9th. The aim of NanoTech Poland 2018 was to bring together the scientific community's principal investigators, scientists, researchers, analysts, clinicians, policy makers, industry experts, and well-established and budding entrepreneurs to discuss the present and future perspectives in nanotechnology and nanoscience research and development. This year, the 1st Symposium on Polydopamine was held on June 6th. This forum was dedicated to the application of polydopamine and related catechol materials in a variety of research fields, both at the nano- and macroscale. The symposium gathered leading scientists from this important research field from top universities and institutions that have been involved in the research revolved around polydopamine. With over 200 national and international participants, NanoTech Poland 2018 and the 1st Symposium on Polydopamine provided a forum to present and discuss the latest scientific news from the field of nanotechnology with a strong interdisciplinary aspect and bioinspired materials.