Transduction Mechanisms, Micro-Structuring Techniques, and Applications of Electronic Skin Pressure Sensors: A Review of Recent Advances

Flat-Silk-Cocoon-Based Wearable Flexible Piezoresistive Sensor and Its Performance

Flexible sensors are becoming the focus of research because they are very vital for intelligent products, real-time data monitoring, and recording. The flat silk cocoon (FSC), as a special form of cocoon, has all the advantages of silk, which is an excellent biomass carbon-based material and a good choice for preparing flexible sensors. In this work, a flexible piezoresistive sensor was successfully prepared by encapsulating carbonized flat silk cocoons (CFSCs) using an elastic matrix polydimethylsiloxane (PDMS). The sensing performance of the material is 0.01 kPa-1, and the monitoring range can reach 680.57 kPa. It is proved that the sensor can detect human motion and has excellent durability (>800 cycles). In addition, a sensor array for a keyboard based on CFSCs was explored. The sensor has a low production cost and a simple preparation process, and it is sustainable and environmentally friendly. Thus, it may have potential applications in wearable devices and human-computer interactions.

Recent Advances in Nanowire-Based Wearable Physical Sensors

Wearable electronics is a technology that closely integrates electronic devices with the human body or clothing, which can realize human-computer interaction, health monitoring, smart medical, and other functions. Wearable physical sensors are an important part of wearable electronics. They can sense various physical signals from the human body or the surrounding environment and convert them into electrical signals for processing and analysis. Nanowires (NW) have unique properties such as a high surface-to-volume ratio, high flexibility, high carrier mobility, a tunable bandgap, a large piezoresistive coefficient, and a strong light-matter interaction. They are one of the ideal candidates for the fabrication of wearable physical sensors with high sensitivity, fast response, and low power consumption. In this review, we summarize recent advances in various types of NW-based wearable physical sensors, specifically including mechanical, photoelectric, temperature, and multifunctional sensors. The discussion revolves around the structural design, sensing mechanisms, manufacture, and practical applications of these sensors, highlighting the positive role that NWs play in the sensing process. Finally, we present the conclusions with perspectives on current challenges and future opportunities in this field.

Tungsten-Doped Indium Tin Oxide Thin-Film Transistors for Dual-mode Proximity Sensing Application

Technologies for human-machine interactions are booming now. In order to achieve multifunctional sensing abilities of electronic skins, further developments of various sensors are in urgent demand. Herein, a dual-mode proximity sensor based on an oxide thin-film transistor (TFT) is reported. Although InSnO (ITO) is featured with high mobility, the inherent high carrier concentration limits its use as a channel material for thin-film transistors. Herein, the tungsten element was introduced as a carrier suppressor to develop ITO-based semiconducting materials and devices. TFTs with amorphous tungsten-doped ITO (ITWO) channel layers were fabricated. As for a flat panel display application, the TFT device from 250 °C-annealed ITWO layer with an atomic ratio of In/Sn/W = 86:9:5 presented the optimal device performance with carrier mobility of 11.53 cm2 V-1 s-1, swing subthreshold of 0.66 V dec-1, threshold voltage of -2.18 V, and Ion/Ioff ratio of 3.33 × 107 and much small hysteresis of transfer characteristic. ITWO TFT devices were further developed as dual-mode proximity sensors that could work with both extended-gate and compact configurations, where the drain current was directly related to the surface potential of a charged object and the distance between the sensing end and the object, enabling the proximity sensing of charged stimuli. For extended-gate-configured proximity sensing, a charged object modulated the formation of a conductive channel at the semiconductor/SiO2 interface, while this conductive channel occurred at the semiconductor/air interface for compact-configured sensing. Formation of the conductive channel of the compact transistor was modulated by the electric field component in the direction perpendicular to the interface, and the drain current was sensitive to the orientation of the approaching object, which implied the capacity of angle sensing to the approach of a charged object. This work further emphasizes that the basic device performance should be optimized according to its specific application scenarios rather than only considering the requirements of the panel display.

Multi-Transduction-Mechanism Technology, an Emerging Approach to Enhance Sensor Performance

Conventional sensor systems employ single-transduction technology where they respond to an input stimulus and transduce the measured parameter into a readable output signal. As such, the technology can only provide limited corresponding data of the detected parameters due to relying on a single transformed output signal for information acquisition. This limitation commonly results in the need for utilizing sensor array technology to detect targeted parameters in complex environments. Multi-transduction-mechanism technology, on the other hand, may combine more than one transduction mechanism into a single structure. By employing this technology, sensors can be designed to simultaneously distinguish between different input signals from complex environments for greater degrees of freedom. This allows a multi-parameter response, which results in an increased range of detection and improved signal-to-noise ratio. In addition, utilizing a multi-transduction-mechanism approach can achieve miniaturization by reducing the number of required sensors in an array, providing further miniaturization and enhanced performance. This paper introduces the concept of multi-transduction-mechanism technology by exploring different candidate combinations of fundamental transduction mechanisms such as piezoresistive, piezoelectric, triboelectric, capacitive, and inductive mechanisms.

Research Progresses in Microstructure Designs of Flexible Pressure Sensors

Flexible electronic technology is one of the research hotspots, and numerous wearable devices have been widely used in our daily life. As an important part of wearable devices, flexible sensors can effectively detect various stimuli related to specific environments or biological species, having a very bright development prospect. Therefore, there has been lots of studies devoted to developing high-performance flexible pressure sensors. In addition to developing a variety of materials with excellent performances, the microstructure designs of materials can also effectively improve the performances of sensors, which has brought new ideas to scientists and attracted their attention increasingly. This paper will summarize the flexible pressure sensors based on material microstructure designs in recent years. The paper will mainly discuss the processing methods and characteristics of various sensors with different microstructures, and compare the advantages, disadvantages, and application scenarios of them. At the same time, the main application fields of flexible pressure sensors based on microstructure designs will be listed, and their future development and challenges will be discussed.

Artificial Neurons on Flexible Substrates: A Fully Printed Approach for Neuromorphic Sensing

Printed electronic devices have demonstrated their applicability in complex electronic circuits. There is recent progress in the realization of neuromorphic computing systems (NCSs) to implement basic synaptic functions using solution-processed materials. However, a fully printed neuron is yet to be realised. We demonstrate a fully printed artificial neuromorphic circuit on flexible polyimide (PI) substrate. Characteristic features of individual components of the printed system were guided by the software training of the NCS. The printing process employs graphene ink for passive structures and In2O3 as active material to print a two-input artificial neuron on PI. To ensure a small area footprint, the thickness of graphene film is tuned to target a resistance and to obtain conductors or resistors. The sheet resistance of the graphene film annealed at 300 °C can be adjusted between 200 Ω and 500 kΩ depending on the number of printed layers. The fully printed devices withstand a minimum of 2% tensile strain for at least 200 cycles of applied stress without any crack formation. The area usage of the printed two-input neuron is 16.25 mm2, with a power consumption of 37.7 mW, a propagation delay of 1 s, and a voltage supply of 2 V, which renders the device a promising candidate for future applications in smart wearable sensors.

Force-Sensitive Interface Engineering in Flexible Pressure Sensors: A Review

Flexible pressure sensors have received extensive attention in recent years due to their great importance in intelligent electronic devices. In order to improve the sensing performance of flexible pressure sensors, researchers are committed to making improvements in device materials, force-sensitive interfaces, and device structures. This paper focuses on the force-sensitive interface engineering of the device, which listing the main preparation methods of various force-sensitive interface microstructures and describing their respective advantages and disadvantages from the working mechanisms and practical applications of the flexible pressure sensor. What is more, the device structures of the flexible pressure sensor are investigated with the regular and irregular force-sensitive interface and accordingly the influences of different device structures on the performance are discussed. Finally, we not only summarize diverse practical applications of the existing flexible pressure sensors controlled by the force-sensitive interface but also briefly discuss some existing problems and future prospects of how to improve the device performance through the adjustment of the force-sensitive interface.

Wearable microfluidic-based e-skin sweat sensors

Electronic skins (e-skins) are soft (deformable and stretchable) state-of-the-art wearable devices that emulate the attributes of human skin and act as a Human-Machine Interface (HMI). Recent advances in e-skin for real-time detection of medical signals such as pulse, temperature, electromyogram (EMG), electroencephalogram (EEG), electrooculogram (EOG), electrocardiogram (ECG), and other bioelectric signals laid down an intelligent foundation for early prediction and diagnosis of diseases with a motive of reducing the risk of the ailment reaching to the end stage. In particular, sweat testing has been employed in diverse applications ranging from medical diagnosis of diabetes, cystic fibrosis, tuberculosis, blood pressure, and autonomic neuropathy to evaluating fluid and electrolyte balance in athletes. Typically, sweat testing techniques are done by trained experts and require off-body measurements, which prevent individuals from de-coding health issues quickly and independently. With the onset of soft electronics, wearable sweat sensors overcome this disadvantage via in situ sweat measurements with real-time feedback, timely diagnosis, creating the potential for preventive care and treatment. Over the past few decades, wearable microfluidic-based e-skin sweat sensors have paved a new way, promising sensing interfaces that are highly compatible with arranging medical and electronic applications. The present review highlights the recent research carried out in the microfluidic-based wearable sweat sensors with a critical focus on real-time sensing of lactate, chloride, and glucose concentration; sweat rate, simultaneously with pH, and total sweat loss for preventive care, timely diagnosis, and point-of-care health and fitness monitoring.

The status and perspectives of nanostructured materials and fabrication processes for wearable piezoresistive sensors

The wearable sensors have attracted a growing interest in different markets, including health, fitness, gaming, and entertainment, due to their outstanding characteristics of convenience, simplicity, accuracy, speed, and competitive price. The development of different types of wearable sensors was only possible due to advances in smart nanostructured materials with properties to detect changes in temperature, touch, pressure, movement, and humidity. Among the various sensing nanomaterials used in wearable sensors, the piezoresistive type has been extensively investigated and their potential have been demonstrated for different applications. In this review article, the current status and challenges of nanomaterials and fabrication processes for wearable piezoresistive sensors are presented in three parts. The first part focuses on the different types of sensing nanomaterials, namely, zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) piezoresistive nanomaterials. Then, in second part, their fabrication processes and integration are discussed. Finally, the last part presents examples of wearable piezoresistive sensors and their applications.

Review: Sensors for Biosignal/Health Monitoring in Electronic Skin

Skin is the largest sensory organ and receives information from external stimuli. Human body signals have been monitored using wearable devices, which are gradually being replaced by electronic skin (E-skin). We assessed the basic technologies from two points of view: sensing mechanism and material. Firstly, E-skins were fabricated using a tactile sensor. Secondly, E-skin sensors were composed of an active component performing actual functions and a flexible component that served as a substrate. Based on the above fabrication processes, the technologies that need more development were introduced. All of these techniques, which achieve high performance in different ways, are covered briefly in this paper. We expect that patients' quality of life can be improved by the application of E-skin devices, which represent an applied advanced technology for real-time bio- and health signal monitoring. The advanced E-skins are convenient and suitable to be applied in the fields of medicine, military and environmental monitoring.

A fully 3D printed electronic skin with bionic high resolution and air permeable porous structure

The bionic application of electronic skin (e-skin) requires a high resolution close to that of human skin, while its long-term attachment to human body or robotic skin requires a porous structure that is air permeable and enables hair growth. To simultaneously meet the requirements of high resolution and porous structure, as well as improve the sensing performance, we propose a fully 3D printed e-skin with high-resolution and air permeable porous structure. The flexible substrate and electrodes are 3D printed by a direct ink writing extrusion printer. The sensitive material is 3D printed by a self-made low-viscosity liquid extrusion 3D print module. This e-skin has a high sensor density of 100/cm², which is close to the resolution of the human fingertip skin. The piezoresistive sensor units of e-skin exhibit a highly linear resistance response and a relatively performance consistency between devices. Owing to the porous and breathable structure, better human comfort and mechanical heat dissipation are realized. This high-resolution e-skin is successfully applied to identify small-sized objects with complex contours.

Advanced materials-integrated electrochemical sensors as promising medical diagnostics tools: A review

Electrochemical sensors have increasingly been linked with terms as modern biomedically effective highly selective and sensitive devices, wearable and wireless technology, portable electronics, smart textiles, energy storage, communication and user-friendly operating systems. The work brings the overview of the current advanced materials and their application strategies for improving performance, miniaturization and portability of sensing devices. It provides the extensive information on recently developed (bio)sensing platforms based on voltammetric, amperometric, potentiometric and impedimetric detection modes including portable, non-invasive, wireless, and self-driven miniaturized devices for monitoring human and animal health. Diagnostics of selected free radical precursors, low molecular biomarkers, nucleic acids and protein-based biomarkers, bacteria and viruses of today's interest is demonstrated.