Polydimethylsiloxane (PDMS), one type of silicone elastomers, has been widely used in the fabrication and prototyping of microfluidic and micro-engineering devices due to its high transparency, ...nontoxicity, and high chemical inertness. Considering its applications in precisely controlled conditions, e.g., wearable strain sensors, the mechanical properties of PDMS can critically influence the sensor performance. However, due to its viscoelastic properties and flexibility, the mechanical behavior of PDMS is generally difficult to measure accurately. In this study, the digital image correlation (DIC) technique has been adopted to accurately measure full-field strains generated in various quasi-static tension tests, including the cyclic tension test, the uniaxial tension test, and stress relaxation test, and critical material properties of PDMS (Sylgard 184), which are directly related to the performance of wearable strain sensors, were accurately measured. Experimental results indicate that the PDMS (Sylgard 184) elastomer exhibits very consistent and stable mechanical properties under quasi-static loading conditions, which is beneficial to the application of wearable strain sensors. However, the Poisson’s ratio, as a function of stain, needs to be carefully considered into this application.
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•PDMS (Sylgard 184) is characterized under different loading conditions experienced in using wearable sensors.•Full-field measurement technique was utilized based on the DIC technology.•PDMS (Sylgard 184) shows negligible Mullins effect and elastic hysteresis.•Mechanical behavior of the PDMS (Sylgard 184) exhibits insensitivity to the deformation rate.•PDMS (Sylgard 184) shows very limited relaxation effect and reaches steady state rapidly.
A data glove enables fine motion control in virtual reality (VR). This work presents a cost‐effective data glove made of a commercial nylon‐based glove with integrated polyethylene‐carbon composites ...(Velostat). The resistance of the Velostat varies when a finger is bent. This piezoresistive behavior was explored by designing the Velostat as the strain sensor. The strain sensor was characterized for its flexure sensing by connecting it to a data acquisition circuit. The circuit is designed to process the output of the joint angles and feed it to the computer. A linear resistance output was measured with a sensitivity/gauge factor of 1.45 % per degree with a response time of 0.0158 seconds when the strain sensor bends from 0° to 30°. To control the 3D virtual hand's movement, the data glove was coupled with two inertial measurement unit sensors at the forearm and upper arm to identify its coordinates. The fabricated data glove successfully performs a proof‐of‐concept by picking‐and‐placing multiple objects in a VR environment.
Seamless control of virtual reality with an innovative glove‐based wearable strain sensor system is experienced, featuring innovative Velostat sensors. The design and fabrication of the sensors, calibration and testing, and analysis of performance are described. This groundbreaking technology offers exciting possibilities for the future of immersive interaction in virtual reality.
Peripheral nerve injuries cause various disabilities related to loss of motor and sensory functions. The treatment of these injuries typically requires surgical operations for improving functional ...recovery of the nerve. However, capabilities for continuous nerve monitoring remain a challenge. Herein, a battery‐free, wireless, cuff‐type, implantable, multimodal physical sensing platform for continuous in vivo monitoring of temperature and strain from the injured nerve is introduced. The thin, soft temperature, and strain sensors wrapped around the nerve exhibit good sensitivity, excellent stability, high linearity, and minimum hysteresis in relevant ranges. In particular, the strain sensor integrated with circuits for temperature compensation provides reliable, accurate strain monitoring with negligible temperature dependence. The system enables power harvesting and data communication to wireless, multiple implanted devices wrapped around the nerve. Experimental evaluations, verified by numerical simulations, with animal tests, demonstrate the feasibility and stability of the sensor system, which has great potential for continuous in vivo nerve monitoring from an early stage to complete regeneration.
A battery‐free, wireless, cuff‐type, implantable, multimodal physical sensing platform for continuous in vivo monitoring of temperature and strain from the injured nerve is proposed. This device provides great potential for continuous in vivo nerve monitoring from an early stage to complete regeneration, and advanced therapeutic protocols and clinical paradigms in the neuromedical field.
Methods for real-time reconstruction of structural displacements using measured strain data is an area of active research due to its potential application for Structural Health Monitoring (SHM) and ...morphing structure control. The inverse Finite Element Method (iFEM) has been shown to be well suited for the full-field reconstruction of displacements, strains, and stresses of structures instrumented with discrete or continuous strain sensors. In practical applications, where the available number of sensors may be limited, the number and sensor positions constitute the key parameters. Understanding changes in the reconstruction quality with respect to sensor position is generally difficult and is the aim of the present work. This paper attempts to supplement the current iFEM modeling knowledge through a rigorous evaluation of several strain-sensor patterns for shape sensing of a rectangular plate. Line plots along various sections of the plate are used to assess the reconstruction quality near and far away from strain sensors, and the nodal displacements are studied as the sensor density increases. The numerical results clearly demonstrate the effectiveness of the strain sensors distributed along the plate boundary for reconstructing relatively simple displacement patterns, and highlight the potential of cross-diagonal strain-sensor patterns to improve the displacement reconstruction of more complex deformation patterns.
Flexible strain sensors are of great importance in many emerging applications for human motion monitoring, implanted devices, and human–machine interactive systems. However, the dual‐channel sensing ...systems that enable both strain‐dependent electronic and visually optical signal responses still remain underdeveloped, but such systems are of great interest for human–machine interactive uses. Here, inspired by the mechanically modulated skin color changes of squids via muscle contracting/releasing movements, a class of mechanofluorescent and conductive hydrogel laminates for visually flexible electronics is presented. The sensing laminates consist of interfacially bonded red fluorescent hydrogel, polydimethylsiloxane and carbon nanotubes (CNTs) film. Since the densely stacked microscopic CNTs film can be precisely stretched to induce the formation of network microcracks, the developed hydrogel laminates are endowed with simultaneous fluorescence‐color and resistance changes, which can function as dual‐channel flexible sensors for real‐time human motion monitoring. These properties make the bioinspired soft hydrogel laminate electronics quite promising in the flexible electronics field.
Inspired by the mechanically modulated skin color changes of squids via muscle contracting/releasing movements, a powerful kind of mechanofluorescent and conductive hydrogel laminates is developed. The materials have the properties of the desirable strain‐dependent continuous and synergistic optical/electronic signal changes. They can be used as dual‐channel conformal biomimetic soft sensing skins for real‐time human motion monitoring.
This article describes the implementation and characterization of a new self‐contained large‐area wireless strain sensor, operating around 1.5 GHz, based on the concept of multi‐layer microfluidic ...stretchable radiofrequency electronics (μFSRFEs). Compared to existing solutions, the presented integrated strain sensor is capable of remotely detecting repeated high tensile dynamic strains of up to 15% over very large surfaces or movable parts, and gets rid of all hardwiring to external storage or data processing equipment. Unlike conventional electronic devices, the major part of the sensor is a mechanically reconfigurable and reversibly deformable patch antenna, which consists of two layers of liquid metal alloy filled microfluidic channels in a silicone elastomer. A simplified radiofrequency (RF) transmitter composed of miniaturized rigid active integrated circuits (ICs) associated with discrete passive components was assembled on a flexible printed circuit board (FPCB) and then heterogeneously integrated to the antenna. The elastic patch antenna can withstand repeated mechanical stretches while still maintaining its electrical function to some extent, and return to its original state after removal of the stress. Additionally, its electrical characteristics at frequency of operation are highly sensitive to mechanical strains. Consequently, not only is this antenna a radiator for transmitting and receiving RF signals like any other conventional antennas, but also acts as a reversible large‐area strain sensor in the integrated device. Good electrical performance of the standalone antenna and the RF transmitter sub‐module was respectively verified by experiments. Furthermore, a personal computer (PC)‐assisted RF receiver for receiving and processing the measured data was also designed, implemented, and evaluated. In the real‐life demonstration, the integrated strain sensor successfully monitored periodically repeated human body motion, and wirelessly transmitted the measured data to the custom‐designed receiver at a distance of 5m in real‐time.
A novel concept of self‐contained large‐area wireless strain sensors, based on multi‐layer microfluidic stretchable radiofrequency electronics (μFSRFEs), is proposed and evaluated. Compared to existing solutions, the presented device is capable of remotely detecting repeated high tensile dynamic or static strains of up to 15% over very large surfaces or movable parts.
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•Highly conductive wool yarns (CWYs) through stirring in a conductive ink were achieved.•Effect of geometry changes of active materials on the performance of the strain sensors based ...on CWYs was thoroughly studied.•The wearable strain sensors show good sensitivity, high durability, and stability based on electromechanical test results.•The fabricated strain sensors detected a wide range of human body movements with high sensitivity.
Wearable strain sensors for various applications, such as human motion detection, soft robotics, and healthcare have recently received extensive attention. Although a number of strain sensors based on various active materials have been proposed, a simple and practical method to obtain both high stretchable and sensitive strain sensors remains challenging. This paper presents a simple, scalable and environment-friendly fabrication method for wearable strain sensors, based on wool yarns, as abundant, lightweight, and stretchable natural materials. A simple coating technique was used to achieve highly conductive wool yarns using a conductive ink. Different types of strain sensors were then fabricated by changing the shape of the active material within the elastomer to tune their sensitivity. In detail, the conductive yarns were sandwiched within the Ecoflex in the form of a straight line (CWY-1 strain sensors) or serpenoid curves (CWY-2 and CWY-3 strain sensors). The strain sensors were fully characterized up to 200% of applied tensile strain. Gauge factors of 5 and 7.75 were found within the percentage stretch ranges of 0–127 and 127–200 %, respectively, for the CWY-1 strain sensors. We have also demonstrated the ability of the strain sensors to monitor human muscle and joints movements.
•A novel DES stabilized polymerized α-lipoic acid ionogel is facilely fabricated.•PolyLA-DES ionogel exhibits multifunctional and unique properties.•Excellent stretchability, UV resistance, and ionic ...conductivity, etc. are obtained.•Flexible strain sensors can be assembled via coating polyLA-DES on substrates.•The assembled sensors exhibit excellent and stable R/R0 towards external stimulus.
With the rapid development of “Internet of Things” and human-computer interaction techniques, it is essential and urgent to develop facile and scalable fabrication platforms for stretchable flexible sensor. Herein, we report a facile strategy of using the green choline chloride–acrylamide deep eutectic solvent (CC-AM DES) to guide the in-situ ring-opening polymerization of α-lipoic acid (LA), leading to the successful development of a stretchable ionogel material. The as-prepared ionogel from CC-AM DES system exhibits multifunctional merits including the super stretchability (>9000%), 100% UV-blocking ability, tunable adhesiveness (29–414 kPa), high ionic conductivity (4.45 × 10−4 S/cm), and ideal anti-freezing (–27 °C). In addition, this outstanding ionogel can be readily coated on various material substrates with designable shapes and patterns. Owning to these promising properties and performances, a scalable flexible strain sensor is assembled from the ionogel and exhibits stable resistance variations (R/R0) towards multiple external mechanical stimulus. This study provides a green, cost effective, and scalable strategy to fabricate ionogel materials and multifunctional flexible strain sensors, showing a great potential in the fast-emerging highly stretchable wearable/flexible electronics.
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Different from traditional rigid sensors based on silicon substrates, flexible sensors are suitable for irregularly shaped surfaces and complex measurement situations that need to be embedded in ...curved surfaces. Due to high flexibility and stretchability, flexible sensors are extensively served as personalized health surveillance and implantable biomedical devices. This paper reviews the latest breakthrough in flexible electronics, including new materials, fabrication strategies, and properties, emphasizing various applications of flexible sensors based on different material designs in health monitoring systems. In addition, the simultaneous transmission and optimization of signals from the human body to the detector, excellent stretchability, transparency, and biocompatibility give attractive prospects for wearable electronics with functions that infinitely tend to resemble real human skin or other skin. Finally, we discuss challenges the flexible sensing systems must rise to and look forward to the development prospect. With the rapid development in the field of flexible strain sensors, the practicality of their large-scale manufacture with a certain commercial and practical value is becoming a trend and an urgent need.
Flexible strain sensors based on conductive fillers and flexible polymers possessed significant advantages in human motion detection. Preparing a strain sensing layer with high electrical ...conductivity and excellent mechanical property under high content of conductive filler contributed to the stability of flexible strain sensors. In this study, MWCNTs/PDMS composite film was prepared by the organic solvent method. The microstructure, electrical conductivity, mechanical property, and piezoresistive characteristics of the composite film at different MWCNTs contents were characterized and discussed. When the mass fraction of MWCNTs in the composite film was 5%, the composite film exhibited a high electrical conductivity of 9.56 S/m while maintaining ideal mechanical properties, and the film thickness was just about 180 μm. The relationship between electrical signals and film strain was performed. The piezoresistive characteristic results demonstrated that the prepared composite film could be used as flexible strain sensor for human motion detection. The prepared thin MWCNTs/PDMS composite film in this paper illustrated high conductive and desired flexibility, and was an alternative material for human motion detection.
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