Stress and Magnetic Field Bimode Detection Sensors Based on Flexible CI/CNTs-PDMS Sponges

Multifunctional cross-sensitive magnetic alginate-chitosan-polyethylene oxide nanofiber sensor for human-machine interaction

Flexible nanofiber membranes are compelling materials for the development of functional multi-mode sensors; however, their essential features such as high cross-sensitivity, reliable stability and signal discrimination capability have rarely been realized simultaneously in one sensor. Here, a novel multi-mode sensor with a nanofiber membrane structure based on multiple interpenetrating networks of bidisperse magnetic particles, sodium alginate (SA), chitosan (CHI) in conjunction with polyethylene oxide hydrogels was prepared in a controllable electrospinning technology. Specifically, the morphology distributions of nanofibers could be regulated by the crosslinking degree of the interpenetrating networks and the spinning process parameters. The incorporation of SA and CHI endowed the sensor with desirable flexibility, ideal biocompatibility and skin-friendly property. Besides, the assembled sensors not only displayed preferable magnetic sensitivity of 0.34 T-1 and reliable stability, but also exhibited favorable cross-sensitivity, quick response time, and long-term durability for over 5000 cycles under various mechanical stimuli. Importantly, the multi-mode stimuli could be discriminated via producing opposite electrical signals. Furthermore, based on the signal distinguishability of the sensor, a wearable Morse code translation system assisted by the machine learning algorithm was demonstrated, enabling a high recognizing accuracy (>99.1 %) for input letters and numbers information. Due to the excellent multifunctional sensing characteristics, we believe that the sensor will have a high potential in wearable soft electronics and human-machine interactions.

Functional magnetic alginate/gelatin sponge-based flexible sensor with multi-mode response and discrimination detection properties for human motion monitoring

The functional flexible sensors that can simultaneously detect multiple external excitations have exhibited great potential in the human-machine interaction and wearable electronics. However, it is still a primary challenge to develop a multi-mode sensor that can achieve sensitivity equilibrium towards different stimuli, and effectively recognize external stimulus while in a facile and cost-effective material and methodology. This study presented a functional flexible sensor based on natural polymer sodium alginate and gelatin sponge electrode which could detect both external mechanical and magnetic stimuli with superiorities of outstanding sensing capability and stability. With the optimal multilayered structure, it possessed high magnetic responsive sensitivity of 0.45 T-1, excellent stability and recoverability. Its electrical property variations also displayed high sensitivity and durability under cyclic stretching, bending and compressing stimuli for 1000 cycles. More importantly, the sensor could not only respond to magnetic field and compression stimuli with contrary electrical responses, but also recognize the respective input signals to decouple different stimuli in real time. Furthermore, it was developed as electronic skins and smart sensor arrays for human physiological signals and mechanical-magnetic detection. Based on excellent multifunctional response characteristics, the sensor showed significant potential in next-generation intelligent multifunctional electronic system and artificial intelligence.

Magnetically Induced Grid Structure for Enhancing the Performance of a Dual-Mode Flexible Sensor with Tactile/Touchless Perception

As an advanced sensing technology, dual-mode flexible sensing, integrating both tactile and touchless perception, propels numerous intelligent devices toward a more practical and efficient direction. The ability to incorporate multiple sensing modes and accurately distinguish them in real time has become crucial for technological advancements. Here, we proposed a dual-mode sensing system (B-MIGS) consisting of a dual-layer sensing device with a magnetically induced grid structure and a testing device. The system was capable of utilizing mechanical pressure to perceive tactile stimulation and magnetic sensing to simultaneously transduce touchless stimulation simultaneously. By leveraging the triboelectric effect, the decoupling of tactile and touchless signals in the presence of unknown signal sources was achieved. Additionally, the sensing characteristics of the B-MIGS were optimized by varying the curing magnetic induction intensity and magnetic particle concentration. The influence of the temperature and humidity on the sensing signals was also discussed. Finally, the practical value of the B-MIGS as a dual-mode monitoring system was demonstrated on soft petals and sensor arrays, along with exploration of its potential application in underwater environments.

Wearable Magnetic Field Sensor with Low Detection Limit and Wide Operation Range for Electronic Skin Applications

Flexible electronic devices extended abilities of humans to perceive their environment conveniently and comfortably. Among them, flexible magnetic field sensors are crucial to detect changes in the external magnetic field. State-of-the-art flexible magnetoelectronics do not exhibit low detection limit and large working range simultaneously, which limits their application potential. Herein, a flexible magnetic field sensor possessing a low detection limit of 22 nT and wide sensing range from 22 nT up to 400 mT is reported. With the detection range of seven orders of magnitude in magnetic field sensor constitutes at least one order of magnitude improvement over current flexible magnetic field sensor technologies. The sensor is designed as a cantilever beam structure accommodating a flexible permanent magnetic composite and an amorphous magnetic wire enabling sensitivity to low magnetic fields. To detect high fields, the anisotropy of the giant magnetoimpedance effect of amorphous magnetic wires to the magnetic field direction is explored. Benefiting from mechanical flexibility of sensor and its broad detection range, its application potential for smart wearables targeting geomagnetic navigation, touchless interactivity, rehabilitation appliances, and safety interfaces providing warnings of exposure to high magnetic fields are explored.

Investigation into a Lightweight Polymeric Porous Sponge with High Magnetic Field and Strain Sensitivity

Recently, flexible sensors have gained significant attention due to their potential applications in soft robotics and biomimetic intelligent devices. However, the successful production of favorable flexible sensors integrated with high flexibility, sensitivity and excellent environment adaptability toward multiple external stimuli is still an enormous challenge. Herein, a lightweight polymeric porous sponge capable of detecting an external magnetic field and strain excitations is proposed by assembling a sodium alginate/chitosan (SA/CHI) porous sponge with micron carbonyl iron and nanoscale Fe3O4 magnetic particles (MPs). Based on the double network structure, the SA/CHI sponge possesses preferable mechanical strength and hydrophilicity, demonstrating its high flexibility and deformability. More importantly, the electrical response of the SA/CHI sponge sensors can display remarkable variation under external magnetic and mechanical stimuli due to their superior magnetic characteristics and electrical conductivity. Meanwhile, their sensing properties can maintain relatively stable recoverability and repeatability towards the periodic excitations and releases. Additionally, a potential mechanism is provided to investigate their stimuli-sensitive behavior. It is highly dependent on the microstructure variations in MPs and conductive multi-walled carbon nanotube (MWCNTs) networks. Due to its exceptional magnetic controllability and appropriate electrical sensitivity, the proposed sensor shows high potential in wearable multi-sensing electronics and intelligent transport devices.

Highly Compressible and Sensitive Flexible Piezoresistive Pressure Sensor Based on MWCNTs/Ti3C2Tx MXene @ Melamine Foam for Human Gesture Monitoring and Recognition

Flexible sensing devices provide a convenient and effective solution for real-time human motion monitoring, but achieving efficient and low-cost assembly of pressure sensors with high performance remains a considerable challenge. Herein, a highly compressible and sensitive flexible foam-shaped piezoresistive pressure sensor was prepared by sequential fixing multiwalled carbon nanotubes and Ti3C2Tx MXene on the skeleton of melamine foam. Due to the porous skeleton of the melamine foam and the extraordinary electrical properties of the conductive fillers, the obtained MWCNTs/Ti3C2Tx MXene @ melamine foam device features high sensitivity of 0.339 kPa−1, a wide working range up to 180 kPa, a desirable response time and excellent cyclic stability. The sensing mechanism of the composite foam device is attributed to the change in the conductive pathways between adjacent porous skeletons. The proposed sensor can be used successfully to monitor human gestures in real-time, such as finger bending and tilting, scrolling the mouse and stretching fingers. By combining with the decision tree algorithm, the sensor can unambiguously classify different Arabic numeral gestures with an average recognition accuracy of 98.9%. Therefore, our fabricated foam-shaped sensor may have great potential as next-generation wearable electronics to accurately acquire and recognize human gesture signals in various practical applications.

Anisotropic conductive networks for multidimensional sensing

In the past decade, flexible physical sensors have attracted great attention due to their wide applications in many emerging areas including health-monitoring, human-machine interfaces, smart robots, and entertainment. However, conventional sensors are typically designed to respond to a specific stimulus or a deformation along only one single axis, while directional tracking and accurate monitoring of complex multi-axis stimuli is more critical in practical applications. Multidimensional sensors with distinguishable signals for simultaneous detection of complex postures and movements in multiple directions are highly demanded for the development of wearable electronics. Recently, many efforts have been devoted to the design and fabrication of multidimensional sensors that are capable of distinguishing stimuli from different directions accurately. Benefiting from their unique decoupling mechanisms, anisotropic architectures have been proved to be promising structures for multidimensional sensing. This review summarizes the present state and advances of the design and preparation strategies for fabricating multidimensional sensors based on anisotropic conducting networks. The fabrication strategies of different anisotropic structures, the working mechanism of various types of multidimensional sensing and their corresponding unique applications are presented and discussed. The potential challenges faced by multidimensional sensors are revealed to provide an insightful outlook for the future development.

E-Skin: The Dawn of a New Era of On-Body Monitoring Systems

Real-time "on-body" monitoring of human physiological signals through wearable systems developed on flexible substrates (e-skin) is the next target in human health control and prevention, while an alternative to bulky diagnostic devices routinely used in clinics. The present work summarizes the recent trends in the development of e-skin systems. Firstly, we revised the material development for e-skin systems. Secondly, aspects related to fabrication techniques were presented. Next, the main applications of e-skin systems in monitoring, such as temperature, pulse, and other bio-electric signals related to health status, were analyzed. Finally, aspects regarding the power supply and signal processing were discussed. The special features of e-skin as identified contribute clearly to the developing potential as in situ diagnostic tool for further implementation in clinical practice at patient personal levels.

Exploring Reversible Thermochromic Behavior in a Rare Ni(II)-MOF System

Thermochromic metal-organic frameworks (MOFs) are promising functional materials for a wide range of applications due to their ability to exhibit color variation under external temperature stimuli, yet the development of them with high cyclability and efficient regeneration processes remains challenging. Here, presented is a rare example of an ultrastable Ni(II)-MOF exhibiting an unprecedented reversible four-step color change between two complementary colors in a wide temperature range, which could be repeated for at least 500 cycles without losing crystallinity and thermochromic performance. Notably, the regeneration can be achieved within 1 min by simply letting the crystals cool naturally in the air, facilitated by the unique nature of the channels' inner surface. The reversible thermochromic behavior is owing to a series of reversible crystal structure changes with temperature, including the stepwise dehydration/rehydration process, and structural changes. This work facilitates the future development of more MOF-based reversible thermochromic materials with excellent performance and improved practical applicability.

Constitutive Model of Isotropic Magneto-Sensitive Rubber with Amplitude, Frequency, Magnetic and Temperature Dependence under a Continuum Mechanics Basis

A three-dimensional nonlinear constitutive model of the amplitude, frequency, magnetic and temperature dependent mechanical property of isotropic magneto-sensitive (MS) rubber is developed. The main components of MS rubber are an elastomer matrix and magnetizable particles. When a magnetic field is applied, the modulus of MS rubber increases, which is known as the magnetic dependence of MS rubber. In addition to the magnetic dependence, there are frequency, amplitude and temperature dependencies of the dynamic modulus of MS rubber. A continuum mechanical framework-based constitutive model consisting of a fractional standard linear solid (SLS) element, an elastoplastic element and a magnetic stress term of MS rubber is developed to depict the mechanical behavior of MS rubber. The novelty is that the amplitude, frequency, magnetic and temperature dependent mechancial properties of MS rubber are integrated into a whole constitutive model under the continuum mechanics frame. Comparison between the simulation and measurement results shows that the fitting effect of the developed model is very good. Therefore, the constitutive model proposed enables the prediction of the mechanical properties of MS rubber under various operating conditions with a high accuracy, which will drive MS rubber’s application in engineering problems, especially in the area of MS rubber-based anti-vibration devices.

Magnetic Field Triggerable Macroporous PDMS Sponge Loaded with an Anticancer Drug, 5-Fluorouracil

This study aims to prepare, optimize, and characterize magnetic-field-sensitive sugar-templated polydimethylsiloxane (PDMS) sponges for localized delivery of an anticancer drug, 5-fluorouracil (FLU). For this purpose, different concentrations of carbonyl iron (CI) and magnetite Fe₃O₄ nanopowders were embedded as magnetosensitive materials in PDMS resins for the fabrication of macroporous sponges via a sugar-template process. The process is environmentally friendly and simple. The fabricated interconnected macroporous magnetic particles loaded PDMS sponges possess flexible skeletons and good recyclability because of their recoverability after compression (deformation) without any breakdown. The prepared magnetic PDMS sponges were evaluated for their morphology (SEM and EDS), porosity (absorbency), elastic modulus, deformation under a magnetic field, thermostability, and in vitro cell studies. All physicochemical and magnetomechanical analysis confirmed that the optimized magnetic-field-sensitive PDMS sponge can provide an efficient method for delivering an on-demand dose of anticancer drug solutions at a specific location and timing with the aid of controlled magnetic fields.

Spider-Inspired Ultrasensitive Flexible Vibration Sensor for Multifunctional Sensing

Flexible vibration sensors can not only capture broad classes of physiologically relevant information, including mechano-vibration signatures of body processes and precision kinematics of core-body motions, but also detect environmental seismic waves, providing early warning to wearers in time. Spider is one of the most vibration-sensitive creatures because of its hairlike sensilla and lyriform slit structure. Here, a spider-inspired ultrasensitive flexible vibration sensor is designed and fabricated for multifunctional sensing. The vibration sensitivity of the flexible sensor is increased over 2 orders of magnitude from 0.006 to 0.5 mV/g, and the strain sensitivity is hugely enhanced from 0.08 to 150 compared to a plain sensor counterpart. It is shown that the synergistic effect of cilium arrays and cracks is the key for achieving the greatly enhanced vibration and strain sensitivity. The dynamic sensitivity of 0.5 mV/g outperforms the corresponding commercial vibration sensors. The flexible sensor is demonstrated to be generally feasible for detecting vibration signals caused by walk, tumble, and explosion as well as capturing human body motions, indicating its great potential for applications in human health-monitoring devices, posture control in robotics, early earthquake warning, and so forth.

Three-Dimensional Structured Dual-Mode Flexible Sensors for Highly Sensitive Tactile Perception and Noncontact Sensing

This work reports a three-dimensional (3D) structured multifunctional sensor by connecting a magnetowhisker with a superflexible patterned skin film. Composed of percolation networks of silver nanowires, the patterned skin film is integrated via a simple template manufacturing method without increasing the complexity and sacrificing the flexibility. The as-prepared 3D structured sensor can realize the multimodal detection of out-of-plane tactile stimuli and details of noncontact environmental obstacles in multiple directions. Here, the sensor's perception behaviors on compression, pulling, magnetic field, sound waves, airflow, water level, water flow, and backwash are presented. Furthermore, the 3D structured sensor obtains outstanding mechanical robustness and stability for 8000 cycles, excellent sensitivity (12 800% when the applied pulling displacement was 3.5 mm; 152% T^-1 when the magnetic flux density variation was 40.6 mT), ultrahigh response time, and ultrahigh recovery time (∼5 ms), which may meet the industrial sensing requirement for artificial tactile electronics. Facile manufacturing processes and outstanding multimodal sensing characteristics make the 3D structured sensor to possess great potential to be implemented in the next-generation intelligent bionic equipment or systems.

A review of electronic skin: soft electronics and sensors for human health

This article reviews several categories of electronic skins (e-skins) for monitoring signals involved in human health. It covers advanced candidate materials, compositions, structures, and integrate strategies of e-skin, focusing on stretchable and wearable electronics. In addition, this article further discusses the potential applications and expected development of e-skins. It is possible to provide a new generation of sensors which are able to introduce artificial intelligence to the clinic and daily healthcare.

A Hydrophobic, Self-Powered, Electromagnetic Shielding PVDF-Based Wearable Device for Human Body Monitoring and Protection

With the rapid development of the electronics, information technology, and wearable devices, problems of the power crisis and electromagnetic radiation pollution have emerged. A piezoelectric wearable textile combined with electromagnetic shielding performance has become a favorable solution. Herein, a multifunctional PVDF-based wearable sensor with both electromagnetic shielding function and human body monitoring performance is proposed by incorporating silver nanowires (Ag NWs) and multiwall carbon nanotubes (MWCNTs) hybrid-networks into PVDF-casted commercial nonwoven fabrics (NWF). The coordination of Ag NWs and MWCNTs networks ensures the ideal electrical conductivity and mechanical strength. The maximum shielding value of the developed sensor reaches up to 34 dB when the area densities of the Ag NWs and MWCNT are kept at 1.9 and 2.0 mg/cm², respectively. Additionally, the hydrophobicity of the as-proposed sensor (water contact angle of ∼110.0°) ensures the self-cleaning function and makes it resistive against water and dirt. Moreover, the sensor possesses a force-sensing property by generating different piezoelectric voltages (0, 0.4, 1.0, and 1.5 V) when stimulated by various forces (0, 20, 44, and 60 N). Not only can it respond to different external stress in a timely manner (response sensitivity of ∼0.024 V/N, response time of ∼35 ms), but it can also monitor different body movements, such as joint bending, running, and jumping. This work opens up a new prospect of monitoring the human body as well as protecting human health from electromagnetic radiation surroundings.