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Principles Of Optics Electromagnetic Theory Of Propagation Interference And Diffraction Of Light

Advances in wireless technologies, low-power electronics, the internet of things, and in the domain of connected health are driving innovations in wearable medical devices at a tremendous pace. Wearable sensor systems composed of flexible and stretchable materials have the potential to better interface to the human skin, whereas silicon-based electronics are extremely efficient in sensor data processing and transmission. Therefore, flexible and stretchable sensors combined with low-power silicon-based electronics are a viable and efficient approach for medical monitoring. Flexible medical devices designed for monitoring human vital signs, such as body temperature, heart rate, respiration rate, blood pressure, pulse oxygenation, and blood glucose have applications in both fitness monitoring and medical diagnostics. As a review of the latest development in flexible and wearable human vitals sensors, the essential components required for vitals sensors are outlined and discussed here, including the reported sensor systems, sensing mechanisms, sensor fabrication, power, and data processing requirements.

Monitoring of Vital Signs with Flexible and Wearable Medical Devices

Recent progress in electronic skin or e-skin research is broadly reviewed, focusing on technologies needed in three main applications: skin-attachable electronics, robotics, and prosthetics. First, since e-skin will be exposed to prolonged stresses of various kinds and needs to be conformally adhered to irregularly shaped surfaces, materials with intrinsic stretchability and self-healing properties are of great importance. Second, tactile sensing capability such as the detection of pressure, strain, slip, force vector, and temperature are important for health monitoring in skin attachable devices, and to enable object manipulation and detection of surrounding environment for robotics and prosthetics. For skin attachable devices, chemical and electrophysiological sensing and wireless signal communication are of high significance to fully gauge the state of health of users and to ensure user comfort. For robotics and prosthetics, large-area integration on 3D surfaces in a facile and scalable manner is critical. Furthermore, new signal processing strategies using neuromorphic devices are needed to efficiently process tactile information in a parallel and low power manner. For prosthetics, neural interfacing electrodes are of high importance. These topics are discussed, focusing on progress, current challenges, and future prospects.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/adma.201904765

Electronic Skin: Recent Progress and Future Prospects for Skin‐Attachable Devices for Health Monitoring, Robotics, and Prosthetics

In the past two decades, artificial skin-like materials have received increasing research interests for their broad applications in artificial intelligence, wearable devices, and soft robotics. However, profound challenges remain in terms of imitating human skin because of its unique combination of mechanical and sensory properties. In this work, a bioinspired mineral hydrogel is developed to fabricate a novel type of mechanically adaptable ionic skin sensor. Due to its unique viscoelastic properties, the hydrogel-based capacitive sensor is compliant, self-healable, and can sense subtle pressure changes, such as a gentle finger touch, human motion, or even small water droplets. It might not only show great potential in applications such as artificial intelligence, human/machine interactions, personal healthcare, and wearable devices, but also promote the development of next-generation mechanically adaptable intelligent skin-like devices.

A Bioinspired Mineral Hydrogel as a Self-Healable, Mechanically Adaptable Ionic Skin for Highly Sensitive Pressure Sensing.

Wearable biosensors have received tremendous attention over the past decade owing to their great potential in predictive analytics and treatment toward personalized medicine. Flexible electronics could serve as an ideal platform for personalized wearable devices because of their unique properties such as light weight, low cost, high flexibility and great conformability. Unlike most reported flexible sensors that mainly track physical activities and vital signs, the new generation of wearable and flexible chemical sensors enables real-time, continuous and fast detection of accessible biomarkers from the human body, and allows for the collection of large-scale information about the individual's dynamic health status at the molecular level. In this article, we review and highlight recent advances in wearable and flexible sensors toward continuous and non-invasive molecular analysis in sweat, tears, saliva, interstitial fluid, blood, wound exudate as well as exhaled breath. The flexible platforms, sensing mechanisms, and device and system configurations employed for continuous monitoring are summarized. We also discuss the key challenges and opportunities of the wearable and flexible chemical sensors that lie ahead.

Wearable and flexible electronics for continuous molecular monitoring.

A flexible, transparent iontronic film is introduced as a thin-film capacitive sensing material for emerging wearable and health-monitoring applications. Utilizing the capacitive interface at the ionic-electronic contact, the iontronic film sensor offers a large unit-area capacitance (of 5.4 μF cm(-2) ) and an ultrahigh sensitivity (of 3.1 nF kPa(-1) ), which is a thousand times greater than that of traditional solid-state counterparts.

Flexible Transparent Iontronic Film for Interfacial Capacitive Pressure Sensing

The study of wearable devices has become a popular research topic recently, where high-sensitivity, noise proof sensing mechanisms with long-term wearability play critical roles in a real-world implementation, while the existing mechanical sensing technologies (i.e., resistive, capacitive, or piezoelectric) have yet offered a satisfactory solution to address them all. Here, we successfully introduced a flexible supercapacitive sensing modality to all-fabric materials for wearable pressure and force sensing using an elastic ionic-electronic interface. Notably, an electrospun ionic fabric utilizing nanofibrous structures offers an extraordinarily high pressure-to-capacitance sensitivity (114 nF kPa-1 ), which is at least 1000 times higher than any existing capacitive sensors and one order of magnitude higher than the previously reported ionic devices, with a pressure resolution of 2.4 Pa, achieving high levels of noise immunity and signal stability for wearable applications. In addition, its fabrication process is fully compatible with existing industrial manufacturing and can lead to cost-effective production for its utility in emerging wearable uses in a foreseeable future.

Supercapacitive iontronic nanofabric sensing

Recent development of epidermal electronics provides an enabling means to continuous monitoring of physiological signals and close tracking of physical activities without affecting quality of life. Such devices require high sensitivity for low-magnitude signal detection, noise reduction for motion artifacts, imperceptible wearability with long-term comfortableness, and low-cost production for scalable manufacturing. However, the existing epidermal pressure sensing devices, usually involving complex multilayer structures, have not fully addressed the aforementioned challenges. Here, the first epidermal-iontronic interface (EII) is successfully introduced incorporating both single-sided iontronic devices and the skin itself as the pressure sensing architectures, allowing an ultrathin, flexible, and imperceptible packaging with conformal epidermal contact. Notably, utilizing skin as part of the EII sensor, high pressure sensitivity and high signal-to-noise ratios are achieved, along with ultralow motion artifacts for both internal (body) and external (environmental) mechanical stimuli. Monitoring of various vital signals, such as blood pressure waveforms, respiration waveforms, muscle activities and artificial tactile sensation, is successfully demonstrated, implicating a broad applicability of the EII devices for emerging wearable applications.

Imperceptible Epidermal-Iontronic Interface for Wearable Sensing.

An iontronic microdroplet array (IMA) device, using an ultra-large interfacial capacitance at the highly elastic droplet–electrode contact, has been proposed for flexible tactile sensing applications. The transparent IMA sensors consist of an array of nanoliter droplets sandwiched between two polymeric membranes with patterned transparent electrodes, forming the electrical double layers with remarkable unit-area capacitance. Under external loading, the membrane deformation results in the circumferential expansion at the highly elastic droplet–electrode contact, which offers a completely new capacitive sensing scheme with a dramatic increase in sensitivity. Under the simple device architecture, the IMA has achieved device sensitivity of 0.43 nF kPa−1 and a minimal detectable pressure of 33 Pa, the highest reported values for its dimension. In addition, the hysteresis of the droplet deformation has been reduced by introducing a layer of hydrophobic coating to the conductive electrode surface, ensuring a fast mechanical response (on the order of several milliseconds). To demonstrate the utility of the transparent flexible IMA sensor, it has been successfully mounted onto a fingertip setting to map different surface topologies and embedded into a wristband to resolve dynamic pressure waves throughout cardiovascular cycles.

Iontronic microdroplet array for flexible ultrasensitive tactile sensing

© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1807343 (1 of 11) Paper has been utilized as an ideal platform for chemical, biological, and mechanical sensing for its fibrous structures and properties in addition to its low cost. However, current studies on pressure-sensitive papers have not fully utilized the unique advantages of papers, such as printability, cuttability, and foldability. Moreover, the existing resistive, capacitive, and triboelectric sensing modalities have long-standing challenges in sensitivity, noiseproofing, response time, linearity, etc. Here, a novel flexible iontronic sensing mechanism, referred to as iontronic sensing paper (ISP), is introduced to the classic paper substrates by incorporating both ionic and conductive patterns into an all-in-one flexible sensing platform. The ISP can then be structured into 2D or 3D tactile-sensitive origamis only by the paper-specific manipulations of printing, cutting, folding, and gluing. Notably, the ISP device possesses a device sensitivity of 10 nF kPa−1 cm−2, which is thousands of times higher than that of the commercial counterpart, a resolution of 6.25 Pa, a single-millisecond response time, and a high linearity (R2 > 0.996). Benefiting from the unique properties of the fibrous paper structures and its remarkable performances, the ISP devices hold enormous potential for the emerging human–machine interfaces, including smart packaging, health wearables, and pressure-sensitive paper matrix. disposable, degradable material at a low cost, paper has long been utilized as a flexible platform for chemical and biological sensing. For instance, the pH papers, blood glucose-sensing strips, and early pregnancy detection kits are the most notable ones, along with the recent developments of organic gas sensors, DNA and protein sensors, and heavy ion detection devices.[1–15] Furthermore, benefiting from their fibrous structure, papers can be modified with functional additives, such as carbon-derived materials (e.g., carbon nanotube (CNT) and graphene), conductive polymers, and metallic nanocomposites, leading to new functionalities and sensing modalities.[16–21] Among these emerging functional papers, pressuresensitive papers can be configured with a simple device architecture due to its straightforward sensing principles.[22–25] Although recent studies have successfully exhibited the device flexibility, low-cost manufacturability, and disposability, the additional unique natural advantages of paper have not been fully utilized, such as printability, cuttability, and foldability. The previously reported pressure-sensitive papers, along with the pressure sensors made of them are primarily based on three existing sensing mechanisms, i.e., resistive, capacitive, and triboelectric.[22,26,27] The resistive pressure-sensing papers detect the variations of the electrical resistance induced by the change of the contact area between two resistor structures upon the applied pressure, which can be prepared by dip coating and spray coating of a particular conductive material. Ren’s group has reported a novel graphene paper prepared by immersing tissue papers into a graphene oxide solution, and consecutively, this paper pressure sensing device has exhibited a sensitivity of 17.2 kPa−1 within the range of 2 kPa.[22] Cheng’s group has published a gold nanowire coated paper as a functional sensing material, the prepared pressure sensing device shows a device sensitivity of 1.14 kPa−1 within the range of 5 kPa.[25] However, the nonlinearity between the resistance measurements and the pressure readings can lead to substantial reduction in the device sensitivity as pressure increases. For instance, the sensitivity of the graphene-paper pressure sensor dramatically declines from 17.2 to 0.012 kPa−1 beyond its range limit of 2 kPa, thus restricting its practical utilities and applications.[22] Alternatively, the capacitive pressure-sensitive papers typically utilize parallel electrodes sandwiching a compressible dielectric layer.[28–30] As the loading pressure increases, the distance between two parallel All-in-One Iontronic Sensing Paper

Ruya Li

Papers

Flexible Transparent Iontronic Film for Interfacial Capacitive Pressure Sensing

Supercapacitive iontronic nanofabric sensing

Imperceptible Epidermal-Iontronic Interface for Wearable Sensing.

Iontronic microdroplet array for flexible ultrasensitive tactile sensing

All-in-One Iontronic Sensing Paper