Wearable multifunctional sensors using printed stretchable conductors made of silver nanowires

Multifunctional conductive hydrogel-based flexible wearable sensors

The development of sensors with high sensitivity, good flexibility, low cost, and capability of detecting multiple inputs is of great significance for wearable electronics. Herein, we report a napkin-based wearable capacitive sensor fabricated by a novel, low-cost, and facile strategy. The capacitive sensor is composed of two pieces of electrode plates manufactured by spontaneous assembly of silver nanowires (NWs) on a polydimethylsiloxane (PDMS)-patterned napkin. The sensor possesses high sensitivity (>7.492 kPa−1), low cost, and capability for simultaneous detection of multiple signals. We demonstrate that the capacitive sensor can be applied to identify a variety of human physiological signals, including finger motions, eye blinking, and minute wrist pulse. More interestingly, the capacitive sensor comfortably attached to the temple can simultaneously monitor eye blinking and blood pulse. The demonstrated sensor shows great prospects in the applications of human–machine interface, prosthetics, home-based healthcare, and flexible touch panels.

Wearable sensors with multiple functions are attracting significant attention due to their broad applications in health monitoring and human-computer interaction. Despite significant progress in wearable sensors, it is a significant challenge to monitor temperature and stress simultaneously with a single sensor. A wearable multifunctional optical sensor based on Er3+/Yb3+ co-doped Gd2O3 nanoparticles and a tapered U-shaped fiber is proposed to monitor both temperature and stress in this paper. Temperature resolution of about 0.16℃ is achieved by monitoring the fluorescence intensity ratio (FIR) around 562 nm and 522 nm emitted by Er3+/Yb3+ co-doped Gd2O3 phosphors, which are integrated in a single-mode fiber (SMF). The stress measurement is obtained by monitoring the fluorescence intensity change around 522 nm, which is insensitive to temperature. The results show that the pressure sensitivity and low detection limit are 7% kPa-1 and 127 Pa, respectively. In addition, the response time of 20 ms are achieved for stress sensing. As a proof-of-concept, human skin temperature and heart and respiratory rates are detected before and after exercise by positioning the sensing probe on the wrist. Furthermore, heart and respiratory rates in different parts of the body are also monitored, which are in good agreement with one another. The results demonstrate that the proposed wearable multifunctional optical sensor has huge potential for health monitoring.

Highly stretchable and mechanically robust silver nanowires on surface-functionalized wavy elastomers for wearable healthcare electronics

Revolutionizing human healthcare with wearable sensors for monitoring human strain

Extensive studies have been carried out on silver nanowires (AgNWs) in view of their impressive conductivity and highly flexible one-dimensional structure. They are seen as a promising choice for producing deformable transparent conductors. Nonetheless, the widespread adoption of AgNW-based transparent conductors is hindered by critical challenges represented by the significant contact resistance at the nanowire junctions and inadequate interfacial adhesion between the nanowires and the substrate. This study presents a novel solution to tackle the aforementioned challenges by capitalizing on liquid metal microcapsules (LMMs). Upon exposure to acid vapor, the encapsulated LMMs rupture, releasing the fluid LM which then forms a metallic overlay and hybridizes with the underlying Ag network. As a result, a transparent conductive film with greatly enhanced electrical and mechanical properties was obtained. The transparent conductor displays negligible resistance variation even after undergoing chemical stability, adhesion, and bending tests, and ultrasonic treatment. This indicates its outstanding adhesion strength to the substrate and mechanical flexibility. The exceptional electrical properties and robust mechanical stability of the transparent conductor position it as an ideal choice for direct integration into flexible touch panels and wearable strain sensors, as evidenced in this study. By resolving the critical challenges in this field, the proposed strategy establishes a compelling roadmap to navigate the development of high-performance AgNW-based transparent conductors, setting a solid foundation for further advancement in the field of deformable electronics.

Recently, wearable sensors, due to their ability to exhibit characteristics, have been appealing for health monitoring through detection of human motions and vital signals. The development of strain sensors with high sensing performance and wearability has been a great challenge to date. In this study, a textile-based strain sensor with good skin affinity was fabricated through a simple fabrication process of dip-coating 2D triaxial-braided fabrics using carbon ink and then drying. The macro crack aligned on the 2D triaxial-braided fabric with a high-density structure and good recovery force. The sensitivity of textile-based strain sensor can be enhanced due to aligned macro crack formed by prestrained fabricating process and characteristic of the 2D triaxial braided fabric with high dense structure. The optimized sensor exhibits high sensitivity (gauge factor: 128) in a strain range of 0-30%, durability (5000 cycles), washability, low hysteresis, and fast response time (90 ms). Therefore, it can be applied as a wearable sensor that can monitor human motions (large strain) and biosignals (subtle strain).

Recently, flexible wearable electronic devices have attracted immense interest as an alternative for conventional rigid metallic conductors in personal healthcare monitoring, human motion detection, and sensory skins, owing to their intrinsic characteristics. However, the practical applications of most wearable sensors are generally limited by their poor stretchability and sensitivity, unsatisfactory strength, lower conductivity, and single sensory function. Here a hydrogen bond cross-linked network based on carboxylic styrene butadiene rubber (XSBR) and hydrophilic sericin (SS) non-covalently modified carbon nanotubes (CNTs) is rationally designed and then fabricated into multi-functional sensors. The resultant versatile sensors are able to detect both weak and large deformations, which owns a low detection limit of 1% strain, high stretchability up to 217%, superior strength of 12.58 MPa, high sensitivity with a gauge factor up to 25.98, high conductivity of 0.071 S m-1 , and lower percolation threshold of 0.504 wt%. Moreover, the prepared sensors also possess an impressively thermal response (0.01636 °C-1 ) and realize the application in the measurement of human body temperature. The multifunctional and scalable XSBR/SSCNT sensor with the integrated tracking capabilities of real-time and in situ physiological signals, providing a promising route to develop wearable artificial intelligence in human health and sporting applications.

A low-modulus, adhesive, and highly transparent hydrogel for multi-use flexible wearable sensors

The beginners are hard to use traditional remote controllers to operate sophisticated moving machines like robot, helicopter, airplane, and so on. Thus, only well-skilled operator has controlled them conventionally. This fact disturbs the spread of such sophisticated moving machines. Therefore, it is important to make their operation simple. In this paper, we propose a flexible touch panel with a single resistive film and some point electrodes. Any form of touch panel can be realized by using the flexible touch panel. That is, our flexible touch panel will realize an intuitive controller to operate sophisticated machine, by providing a simple operation interface. Through the experiments on a prototype system with a circular touch panel, the axis of the finger touching can be detected by using three point electrodes.

Silver nanowire mesh electrodes represent a possible mass-manufacturable route toward transparent and flexible electrodes for plastic-based electronics such as organic photovoltaics (OPVs), organic light emitting diodes (OLEDs), and others. Here we describe a route that is based upon spray-coated silver nanowire meshes on polyethylene terephthalate (PET) sheets that are treated with a straightforward combination of heat and pressure to generate electrodes that have low sheet resistance, good optical transmission, that are topologically flat, and adhere well to the PET substrate. The silver nanowire meshes were prepared by spray-coating a solution of silver nanowires onto PET, in air at slightly elevated temperatures. The as-prepared silver nanowire electrodes are highly resistive due to the poor contact between the individual silver nanowires. Light pressure applied with a stainless steel rod, rolled over the as-sprayed silver nanowire meshes on PET with a speed of 10 cm s(-1) and a pressure of 50 psi, results in silver nanowire mesh arrays with sheet resistances of less than 20 Ω/□. Bending of these rolled nanowire meshes on PET with different radii of curvature, from 50 to 0.625 mm, showed no degradation of the conductivity of the electrodes, as shown by the constant sheet resistance before and after bending. Repeated bending (100 times) around a rod with a radius of curvature of 1 mm also showed no increase in the sheet resistance, demonstrating good adherence and no signs of delamination of the nanowire mesh array. The diffuse and direct transmittance of the silver nanowires (both rolled and as-sprayed) was measured for wavelengths from 350 to 1200 nm, and the diffuse transmission was similar to that of the PET substrate; the direct transmission decreases by about 7-8%. The silver nanowires were then incorporated into OPV devices with the following architecture: transparent electrode/PEDOT:PSS/P3HT:PC61BM/LiF/Al. While slightly lower in efficiency than the standard indium tin oxide substrate (ITO), the rolled silver nanowire electrodes had a very good device yield, showing that short circuits resulting from the silver nanowire electrodes can be successfully avoided by this rolling approach.

Ultrasensitive wearable sensor with novel hybrid structures of silver nanowires and carbon nanotubes in fluoroelastomer: Multi-directional sensing for human health monitoring and stretchable electronics

3D double-faced interlock fabric triboelectric nanogenerator for bio-motion energy harvesting and as self-powered stretching and 3D tactile sensors

Automation Highlights from the Literature

Wearable sensors have drawn considerable interest in the recent research world. However, simultaneously realizing high sensitivity and wide detection limits under changing surrounding environment conditions remains challenging. In the present study, we report a wearable piezoresistive pressure sensor capsule that can detect pulse rate and human motion. The capsule includes a flexible silicon cover and is filled with different PVA/MXene (PVA-Mx) composites by varying the weight percentage of MXene in the polymer matrix. Different characterizations such as XRD, FTIR and TEM results confirm that the PVA-Mx silicon capsule was successfully fabricated. The PVA-Mx gel-based sensor capsule remarkably endows a low detection limit of 2 kPa, exhibited high sensitivity of 0.45 kPa−1 in the ranges of 2–10 kPa, and displayed a response time of ~ 500 ms, as well as good mechanical stability and non-attenuating durability over 500 cycles. The piezoresistive sensor capsule sensor apprehended great stability towards changes in humidity and temperature. These findings substantiate that the PVA/MXene sensor capsule is potentially suitable for wearable electronics and smart clothing.