Applications In Wearable Systems Research Articles

Soft Multimodal Sensors with Decoupled Multimodality and Minimal Wiring for Wearable Systems

AbstractSoft multimodal sensors represent a promising approach for applications in wearable systems and skin‐adhesive devices, benefiting from their flexibility and body compatibility. However, the complexity of sensor wiring design poses challenges while employing multimodal decoupling. Although recent studies have applied various methods, such as adjusting the electrical properties of materials, these methods encounter limitations in signal decoupling when multichannel measurements are implemented. In this study, a signal measurement method utilizing frequency response analysis is developed using a multi‐band band‐stop filter (MBBSF) for multiple multimodal sensors. This approach enables the decoupling of different types of sensors and allows for the simultaneous collection of sensor data through a two‐wire connection. Consequently, it facilitates the concurrent and independent real‐time assessment of strain and force in each sensor. A sensor glove is developed incorporating multiple multimodal sensors with a two‐wire connection to measure grip posture and force. This glove is used effectively to classify unknown objects, thereby demonstrating its practical utility. This practical application provides a promising solution to the complex wiring challenges commonly encountered in multi‐modal sensor wearable systems.

Enabling a Paper-Based Flexible Sensor to Work under Water with Exceptional Long-Term Durability through Biomimetic Reassembling of Nanomaterials from Natural Wood

Paper-based sensors have many distinguishing advantages; however, how to improve their working durability especially under water is a critical issue in many applications and unfortunately remains a huge challenge. In this work, we design and develop an innovative strategy enabling paper-based strain and pressure sensors to work under water with exceptional long-term working durability by biomimetic reassembling of nanomaterials from natural wood. A composite paper consisting of softwood fibers and 22 wt % graphite nanoplates was prepared based on a papermaking protocol. Cellulose nanofibers were added to strengthen the composite paper, and lignin nanoparticles were applied onto the paper surface via the paper coating technology to obtain its superhydrophobicity. Subsequently, the as-obtained superhydrophobic composite paper was assembled into a flexible sensor that can be used to detect strain and pressure changes both in air and under water. As the strain sensor, its gauge factor was 10.9 and 14.6 in air and under water, and the corresponding response time was 0.3 and 0.15 s, respectively. Surprisingly, an exceptional working stability was achieved even after more than 10,000 bending–unbending cycles under water. As the pressure sensor, its sensitivity (S) was 0.02 and 0.38 kPa–1 in air and under water, and the corresponding response time was 0.46 and 0.3 s, respectively. Its long-term durability can also exceed 10,000 pressing–releasing cycles. This novel strategy developed in this work can be an effective approach for biomimetic re-engineering of biomass-derived nanomaterials, aiming to develop highly stable paper-based flexible sensors that can find applications in wearable systems and underwater equipment.

A flexible resistive switching device for logical operation applications in wearable systems

As an emerging micro/nano device, memristor integrates functions, such as memory, sensing, and computing, and has similar internal dynamics to synapses, showing great potential in neuromorphic computing and logical operations. In this work, a flexible memristor with Al/TiOx/Al/polyethylene terephthalate structure was developed to investigate the synergistic effect from the oxygen vacancy, H2O molecule, and NaCl. A forming-free resistive switching (RS) behavior observed in the flexible memristor can be ascribed to the migration and accumulation of the oxygen vacancy and Schottky emission of the interfaces. This forming-free RS is sensitive to the NaCl and H2O molecule because the Na+ migration disturbs the oxygen vacancy filament and interplay between oxygen vacancy and H2O molecules. The variation conductance modulated by the concentration of NaCl and H2O molecules can represent the weight update incrementally that the online learning required. Therefore, the memristor has the potential to be used as a biosensor in human tissues or to realize the brain-like chip, which points out the research direction for the extension function of the memristor in wearable systems.

Flexible and Stretchable Dry Active Electrodes With PDMS and Silver Flakes for Bio-Potentials Sensing Systems

Flexible and stretchable dry active electrodes for multi bio-potentials sensing based on Ag flakes / polydimethylsiloxane (PDMS) electrically conductive composite (ECC) are developed and characterized. The proposed dry active electrode consists of soft substrate, electrode, and simple circuits with an amplifier and a capacitor. The soft substrate is made of silicone with a molding process, while the electrode is fabricated by bar coating the ECC on the patterned substrate. Furthermore, circuits interconnect, soldering of chip and other components, and connection with flexible PCB (FPC) are all implemented with ECC directly, which simplifies the fabrication process. A portable bio-potential sensing system is also designed and implemented to work with the proposed electrodes. Various experiments were carried out to verify the proposed electrodes as well as the whole sensing system. The connectivity and proper functionality of the electrodes and cables remain stable during the stretching test. The system and the sensors yield good signal quality for multiple bio-potentials. Compared with conventional Ag/AgCl wet passive electrodes in electrocardiogram (ECG) sensing, the proposed dry active electrodes showed comparable noise floor and less sensitivity to power line interferences and motion artifacts. In electroencephalogram (EEG) sensing, the observed alpha rhythm was 12.66 dB higher than the baseline in the eye-close state. The proposed sensors exhibited potential applications in wearable systems in the electromyogram (EMG) grip force measurement and classification test. Overall, the proposed sensors could provide better comforts during long-time wearing according to its better matching modulus with skin and can meet the requirements of high quality multi bio-potential sensing.

A Full Printed Flexible Pressure Sensor with Improved Temperature Performance based on Optimized Curing Mechanism

Printable flexible pressure sensors have many important applications in wearable systems. One major challenge of such a sensor is to maintain sensing properties in high temperature. By optimizing the curing mechanism of the flexible pressure sensor functional materials, this paper proposes a new method of achieving high temperature properties for a full printed sensor. The establishment of curing theory is mainly studied. The printing process of this kind of sensor is systematically stated and tested to check whether it can continue to function at high temperatures. Ultimately a fully-printed flexible pressure sensor with good temperature performance is achieved. The paper focuses around the technical route of “material selection—theoretical analysis —function material preparation—design and preparation of device—device performance evaluation”. Suitable materials are used in flexible pressure sensors and the curing mechanism is established. This proposed technique can be extended to the development of other printable flexible sensors, which can lead to a huge impact on future applications of the flexible electronics.

A Watch Strap Antenna for the Applications of Wearable Systems

A wearable antenna based on the metal watch strap is proposed. A prototype of a TISSOT classic watch is used for the demonstration. The width of the strap has only a little impact on the radiation patterns. However, the variations of the feeding location would dramatically influence the resonance frequency and the matching performance. The shapes and the graphic patterns of the wristband have contributions on the reflection coefficients as well as the radiation patterns. A fractional bandwidth of 77.8% could be obtained when feeding to the right of the dial at 2.46 GHz, which falls into the ISM (industrial, scientific, and medical) band. The main lobe is fairly wide in both $E$ - and $H$ -planes, which would enable the strap antenna to have a good adaptation for the various postures of the arm. The measurement results show good agreement with that of the simulations, which indicates it a potential option for the applications of wearable systems.