Flexible Multifunctional Sensor Research Articles

An ultra-thin transparent multi-functional sensor based on silk hydrogel for health monitoring

With the gradual improvement of people’s health awareness, wearable devices occupy an important position in daily health care and human physiological activity monitoring. As traditional silicon-based electronic products face problems such as interface mismatch, silk fibroin has gradually become an alternative choice for next-generation wearable electronic devices due to its excellent performance. Herein, an ultra-thin transparent and flexible multi-functional sensor based on silk hydrogel with self-patterned microstructure is proposed. The silk hydrogel exhibits superior transparency (>82%) and thin thickness (∼100 μm). Furthermore, the self-patterned microstructure on the silk hydrogel surface is beneficial for the high sensitivity of pressure sensing response (1.6 kPa−1). This device exhibits advantageous performance on temperature (top sensitivity of 6.25% °C−1) and humidity (top sensitivity of 0.16% RH−1) sensing response. It also shows fast response (0.16 s) and durable stability (over 2000 dynamic cycles). Moreover, this device can be applied to monitor human facial expression, joint movements, temperature change, breathing and other health indicators. It is worth mentioning that this multi-functional sensor can monitor the signal of breathing and throat, so it can be applied to the clinical physiological activity monitoring of patients with upper respiratory tract infection. In addition, we also demonstrate a grasping and relaxing experiment of intelligent manipulator to verify the pressure and temperature sensing performance, providing a possibility for its application in the field of prosthetics. According to these advantages, the reported ultra-thin transparent multi-functional sensor based on silk hydrogel has broad prospects in the fields of health monitoring, intelligent prosthetics, and electronic skin, etc.

An environmentally tolerant, highly stable, cellulose nanofiber-reinforced, conductive hydrogel multifunctional sensor

The application of flexible multifunctional sensors based on conductive hydrogels in human health detection has been widely studied. Herein, a facile one-pot method is proposed to prepare ionic conductive hydrogels by dissolving polyvinyl alcohol (PVA), cellulose nanofiber (CNF), and aluminum chloride hexahydrate (AlCl3·6H2O) in a dimethyl sulfoxide (DMSO)/water binary solvent. The resulting ionically-conductive organohydrogels have high stretchability (up to 696%), fast response (130 ms), wide operating temperature (−50 °C to 50 °C), and long-term stability (30 days). The hydrogel sensor exhibits excellent signal sensing capability (human motion and sound detection signals) and cycling stability (1000 cycles) under extreme temperature and long-term storage conditions. Notably, the organohydrogel displays high sensitivity to both compressive deformation and temperature, resulting in multifunctional sensing performance. This work provides a viable approach for the long-term use of hydrogels as wearable devices in extreme environments and daily life.

Anisotropic Double-Network Hydrogels Integrated Superior Performanceof Strength, Toughness and Conductivity for Flexible Multi-Functional Sensors

Anisotropic Double-Network Hydrogels Integrated Superior Performanceof Strength, Toughness and Conductivity for Flexible Multi-Functional Sensors

Rational Design of All Resistive Multifunctional Sensors with Stimulus Discriminability

AbstractA rational approach is proposed to design soft multifunctional sensors capable of detection and discrimination of different physical stimuli. Herein, a flexible multifunctional sensor concurrently detecting and distinguishing minute temperature and pressure stimuli in real time is developed using electrospun carbon nanofiber (CNF) films as the sole sensing material and electrical resistance as the only output signal. The stimuli sensitivity and discriminability are coordinated by tailoring the atomic‐ and device‐level structures of CNF films to deliver outstanding pressure and temperature sensitivities of −0.96 kPa−1 and −2.44% °C−1, respectively, enabling mutually exclusive sensing performance without signal cross‐interference. The CNF multifunctional sensor is considered the first of its kind to accomplish the stimulus discriminability using only the electrical resistance as the output signal, which is most convenient to monitor and process for device applications. As such, it has distinct advantages over other reported sensors in its simple, cost‐effective fabrication and readout system. It also possesses other invaluable traits, including good bending stability, fast response time, and long‐term durability. Importantly, the ability to simultaneously detect and decouple temperature and pressure stimuli is demonstrated through novel applications as a skin‐mountable device and a flexible game controller.

Bioinspired design and assembly of a multilayer cage-shaped sensor capable of multistage load bearing and collapse prevention

Flexible bioinspired mesostructures and electronic devices have recently attracted intense attention because of their widespread application in microelectromechanical systems (MEMS), reconfigurable electronics, health-monitoring systems, etc. Among various geometric constructions, 3D flexible bioinspired architectures are of particular interest, since they can provide new functions and capabilities, compared to their 2D counterparts. However, 3D electronic device systems usually undergo complicated mechanical loading in practical operation, resulting in complex deformation modes and elusive failure mechanisms. The development of mechanically robust flexible 3D electronics that can undergo extreme compression without irreversible collapse or fracture remains a challenge. Here, inspired by the multilayer mesostructure of Enhydra lutris fur, we introduce the design and assembly of multilayer cage architectures capable of multistage load bearing and collapse prevention under large out-of-plane compression. Combined in situ experiments and mechanical modeling show that the multistage mechanical responses of the developed bionic architectures can be fine-tuned by tailoring the microstructural geometries. The integration of functional layers of gold and piezoelectric polymer allows the development of a flexible multifunctional sensor that can simultaneously achieve the dynamic sensing of compressive forces and temperatures. The demonstrated capabilities and performances of fast response speed, tunable measurement range, excellent flexibility, and reliability suggest potential uses in MEMS, robotics and biointegrated electronics.

Flexible ferroelectric wearable devices for medical applications.

SummaryWearable electronics are becoming increasingly important for medical applications as they have revolutionized the way physiological parameters are monitored. Ferroelectric materials show spontaneous polarization below the Curie temperature, which changes with electric field, temperature, and mechanical deformation. Therefore, they have been widely used in sensor and actuator applications. In addition, these materials can be used for conversion of human-body energy into electricity for powering wearable electronics. In this paper, we review the recent advances in flexible ferroelectric materials for wearable human energy harvesting and sensing. To meet the performance requirements for medical applications, the most suitable materials and manufacturing techniques are reviewed. The approaches used to enhance performance and achieve long-term sustainability and multi-functionality by integrating other active sensing mechanisms (e.g. triboelectric and piezoresistive effects) are discussed. Data processing and transmission as well as the contribution of wearable piezoelectric devices in early disease detection and monitoring vital signs are reviewed.

Flexible self-powered multifunctional sensor for stiffness-tunable soft robotic gripper by multimaterial 3D printing

Proprioceptive sensation via embedded flexible sensors is essential for soft robotic grippers to realize safe and dexterous grasping. We demonstrate a self-powered, flexible multifunctional sensor by combining a polyvinylidene difluoride film with a microstructured jamming layer. The jamming layers and the finger body are fabricated by a multimaterial 3D printing technology. The sensor built into the soft finger can provide passive proprioceptive sensations of curvature and stiffness, with maximum sensitivity of 0.55 mV m and 0.09 Vm/N respectively. It also acted as an active jamming element to tune finger stiffness (15–44 N/m) while not affecting the dynamics of the robotic movement. We demonstrate the multifunctional sensor with a three-fingered robotic gripper to successfully record finger bending and stiffening in the ‘pick and place’ process. This work shows potential of greatly enhancing the way in which a gripper interacts with objects that are manipulated and provide a route to achieve anthropomorphic grippers. • Sensitive enhancing layer was printed with soft finger by multimaterial 3D printing. • The self-powered sensor can measure curvature and stiffness up to 10 m -1 and 44 N/m. • The multifunctional sensor is able to sense finger curvature and stiffness. • This work suggests a new way of designing sensors in robotic applications.

Ultrathin Ion-Sensitive Field-Effect Transistor Chips with Bending-Induced Performance Enhancement.

Flexible multifunctional sensors on skin or wearables are considered highly suitable for next-generation noninvasive health care devices. In this regard, the field-effect transistor (FET)-based chemical sensors such as ion-sensitive FETs (ISFETs) are attractive as, with the ultrathin complementary metal oxide semiconductor technology, they can enable a flexible or bendable sensor system. However, the bending-related stress or strain could change the output of devices on ultrathin chips (UTCs), and this has been argued as a major challenge hindering the advancement and use of this technology in applications such as wearables. This may not be always true, as with drift-free ISFETs, we show that bending could also enhance the performance of UTCs. Through fine control of bending radius in the micrometer scale, the mechanically flexible RuO2-based ISFETs on UTCs (44.76 μm thickness) are shown to reproducibly enhance the performance even after 1000 bending cycles. The 1.3 orders of magnitude improved stability (the drift rate changed from −557 nA/min to −28 ± 0.16 nA/min) is observed over a time period of 417.3 s (∼7 min) at fixed biasing and temperature conditions and under different pH conditions. Finally, a compact macromodel is developed to capture the bending-induced improvements in flexible ISFETs. The performance enhancement by controlled bending of devices could generally benefit the rapidly growing field of flexible electronics.

An integrated flexible multifunctional sensing system for simultaneous monitoring of environment signals

With the rapid development of portable devices and internet of things, the requirement of system wearability and integration accelerates the investigation of flexible multifunctional sensors. In this study, we developed an integrated flexible sensing system with four nanowire-based sensors and a Ni microwire-based temperature sensor. The four nanowire-based sensors are three kinds of photodetectors responding to lights with different wavelengths and a gas sensor. Due to the large surface volume ratio and considerable sub wavelength effect, all the nanowire-based sensors show good sensing response and excellent linear relationship between sensitivity and temperature. The as-fabricated flexible sensing system can simultaneously detect environmental parameters, including temperature change, light intensities from UV-Visible to near infrared regions, and harmful gas concentration. Our flexible multifunctional sensing system therefore opens up a new way for the emerging portable and wearable electronics.

A water destructible SnS2QD/PVA film based transient multifunctional sensor and machine learning assisted stimulus identification for non-invasive personal care diagnostics

We demonstrate, for the first time, a transient, flexible multifunctional sensor (strain, pressure, and breath) using a water soluble SnS2-QD/PVA film.

Highly Precise Multifunctional Thermal Management-Based Flexible Sensing Sheets.

Elaborate manipulation of heat transfer renders proper operation of diverse thermal-related technologies. However, accurate implementation of thermal-based or transduction sensing on a thin flexible film over unusual surfaces remains challenging. Herein, efficient thermal management realizes highly accurate flexible multifunctional sensor sheets using a low thermal conductive medium as a thermal barrier. An approximately 50-fold enhancement in the thermal sensing accuracy, which is nearly independent of the changes in the external surroundings, is achieved. Such rational control of heat convection and conduction allows to not only dynamically monitor air flow, but also sight the large-scale air flow distribution on curved surfaces using a flexible thermal flow sensor array. Additionally, accurate wearable skin temperature monitoring independent of the sudden surrounding variations is achieved. This work addresses the formidable challenge of untethered heat transfer induced imprecise thermal related sensing, which universally exists in skin-inspired Internet of Things (IoT) applications.

Superhydrophobic Ultraflexible Triple-Network Graphene/Polyorganosiloxane Aerogels for a High-Performance Multifunctional Temperature/Strain/Pressure Sensing Array

Recently, many efforts have been made to develop various smart sensors. However, achieving flexible multifunctional sensors combining excellent sensing of temperature, strain, and pressure with a s...

Carbon Nanotube/Graphene Nanoplatelet Hybrid Film as a Flexible Multifunctional Sensor.

A flexible hybrid film consisting of graphene nanoplatelets (GNPs) and multi-walled carbon nanotubes (MWCNTs) was prepared and employed as a multifunctional sensor to monitor temperature and liquid leakage, based on the piezoresistive effect. The influences of the GNP content on the mechanical, thermal, and sensing properties were investigated. Experimental results showed that both the hardness and Young’s modulus of the hybrid film were decreased with an increasing GNP content, while the thermal conductivity exhibited in an opposite trend. The electrical resistance of the hybrid film decreased was linearly with an increase in temperature. The resistance change increased linearly with an increase of the solvent adsorption. These features demonstrated the potential applications of the hybrid film in the detection of temperature, and liquid leakage. The sensitivity of leakage detection is increasing with the increase of the GNP loading, while temperature sensitivity is in the opposite trend.

Flexible Multifunctional Sensors for Wearable and Robotic Applications

AbstractThis review provides an overview of the current state‐of‐the‐art of the emerging field of flexible multifunctional sensors for wearable and robotic applications. In these application sectors, there is a demand for high sensitivity, accuracy, reproducibility, mechanical flexibility, and low cost. The ability to empower robots and future electronic skin (e‐skin) with high resolution, high sensitivity, and rapid response sensing capabilities is of interest to a broad range of applications including wearable healthcare devices, biomedical prosthesis, and human–machine interacting robots such as service robots for the elderly and electronic skin to provide a range of diagnostic and monitoring capabilities. A range of sensory mechanisms is examined including piezoelectric, pyroelectric, piezoresistive, and there is particular emphasis on hybrid sensors that provide multifunctional sensing capability. As an alternative to the physical sensors described above, optical sensors have the potential to be used as a robot or e‐skin; this includes sensory color changes using photonic crystals, liquid crystals, and mechanochromic effects. Potential future areas of research are discussed and the challenge for these exciting materials is to enhance their integration into wearables and robotic applications.

(Invited) Human Interactive Flexible Multi-Functional Sensor Sheets

Human interactive and friendly sensor sheets have a great attention for internet of things (IoT) concept including healthcare and medical applications to collect multiple information from the variety of surfaces. To cover the device conformally and comfortably in the case of human body, mechanical flexibility with different types of sensors is required to monitor health conditions. Realizing the application needs to develop the device concepts and structures for macro-scale, multi-functionality with low cost process. In this talk, flexible multi-functional sensor sheets using mainly printing methods of inorganic nanomaterials will be introduced especially for healthcare and IoT applications as the future human interactive electronics. In addition to the proof-of-concept applications, stability and reliability of the printed flexible sensors will be discussed for moving forward to building more practical sensors in the market.

Wearable, Flexible, and Multifunctional Healthcare Device with an ISFET Chemical Sensor for Simultaneous Sweat pH and Skin Temperature Monitoring.

Real-time daily healthcare monitoring may increase the chances of predicting and diagnosing diseases in their early stages which, currently, occurs most frequently during medical check-ups. Next-generation noninvasive healthcare devices, such as flexible multifunctional sensor sheets designed to be worn on skin, are considered to be highly suitable candidates for continuous real-time health monitoring. For healthcare applications, acquiring data on the chemical state of the body, alongside physical characteristics such as body temperature and activity, are extremely important for predicting and identifying potential health conditions. To record these data, in this study, we developed a wearable, flexible sweat chemical sensor sheet for pH measurement, consisting of an ion-sensitive field-effect transistor (ISFET) integrated with a flexible temperature sensor: we intend to use this device as the foundation of a fully integrated, wearable healthcare patch in the future. After characterizing the performance, mechanical flexibility, and stability of the sensor, real-time measurements of sweat pH and skin temperature are successfully conducted through skin contact. This flexible integrated device has the potential to be developed into a chemical sensor for sweat for applications in healthcare and sports.