Bio-integrated wearable systems can measure a broad range of biophysical, biochemical, and environmental signals to provide critical insights into overall health status and to quantify human performance. Recent advances in material science, chemical analysis techniques, device designs, and assembly methods form the foundations for a uniquely differentiated type of wearable technology, characterized by noninvasive, intimate integration with the soft, curved, time-dynamic surfaces of the body. This review summarizes the latest advances in this emerging field of “bio-integrated” technologies in a comprehensive manner that connects fundamental developments in chemistry, material science, and engineering with sensing technologies that have the potential for widespread deployment and societal benefit in human health care. An introduction to the chemistries and materials for the active components of these systems contextualizes essential design considerations for sensors and associated platforms that appear in f...

Bio-Integrated Wearable Systems: A Comprehensive Review

Recent advances in soft materials and system integration technologies have provided a unique opportunity to design various types of wearable flexible hybrid electronics (WFHE) for advanced human healthcare and human-machine interfaces. The hybrid integration of soft and biocompatible materials with miniaturized wireless wearable systems is undoubtedly an attractive prospect in the sense that the successful device performance requires high degrees of mechanical flexibility, sensing capability, and user-friendly simplicity. Here, the most up-to-date materials, sensors, and system-packaging technologies to develop advanced WFHE are provided. Details of mechanical, electrical, physicochemical, and biocompatible properties are discussed with integrated sensor applications in healthcare, energy, and environment. In addition, limitations of the current materials are discussed, as well as key challenges and the future direction of WFHE. Collectively, an all-inclusive review of the newly developed WFHE along with a summary of imperative requirements of material properties, sensor capabilities, electronics performance, and skin integrations is provided.

Advanced Soft Materials, Sensor Integrations, and Applications of Wearable Flexible Hybrid Electronics in Healthcare, Energy, and Environment.

Wearable sweat sensors rely either on electronics for electrochemical detection or on colorimetry for visual readout. Non-ideal form factors represent disadvantages of the former, while semiquantitative operation and narrow scope of measurable biomarkers characterize the latter. Here, we introduce a battery-free, wireless electronic sensing platform inspired by biofuel cells that integrates chronometric microfluidic platforms with embedded colorimetric assays. The resulting sensors combine advantages of electronic and microfluidic functionality in a platform that is significantly lighter, cheaper, and smaller than alternatives. A demonstration device simultaneously monitors sweat rate/loss, pH, lactate, glucose, and chloride. Systematic studies of the electronics, microfluidics, and integration schemes establish the key design considerations and performance attributes. Two-day human trials that compare concentrations of glucose and lactate in sweat and blood suggest a potential basis for noninvasive, semi-quantitative tracking of physiological status.

Battery-free, skin-interfaced microfluidic/electronic systems for simultaneous electrochemical, colorimetric, and volumetric analysis of sweat

DOI: 10.1002/admt.201800628 applications (e.g., soft robotics, medical devices).[1–6] Despite state-of-the-art bulkbased planar integrated-circuit devices, their rigid and brittle nature gives rise to the incompatibility with curvilinear and soft human bodies, restricting the development of newborn human-friendly interactive electronics. In contrast, the bendable and flexible wearable electronics could be conformally attached onto human bodies almost without discomfort and succeed in performing a great deal of sensing functionalities. Realization of such promising goals requires the flexible sensor platforms provided with crucial characteristics of light weight, ultrathinness, superior flexibility, stretchability, high sensitivity as well as rapid response.[7–10] Inspired by the perceptive features of human skins, the wearable sensor systems are capable of acquiring abundant information from the external environment with the assistance of sensing modules, such as pressure sensors, strain sensors, temperature sensors, etc.[11] A typical example is their application in prosthetics that could afford the capacity to perceive touch or temperature for the disabled.[12] Additionally, the wearable sensor systems are able to identify physical or chemical signals produced by the human body, providing promising opportunities to evaluate health states.[5,13–15] Conventional skin-like sensor platforms primarily comprise one or two sensing modules, data processing units, and power supplies. Their unitary functionality, however, cannot satisfy the increasing demands of IoTs. Recently, the rapid advances in novel sensing materials, fabrication strategies, and innovative electronic constitution contribute to the development of versatile integration of multimodal sensors, which could synchronously distinguish diverse stimuli from the complex environment and monitor multiple vital signs from the human body.[16,17] In spite of several attempts done in terms of such multimodal sensor systems, one of the cumbersome issues originates from the crosscoupling effect among different categories of signals simultaneously generated by various sensors. Furthermore, the skin-like multiple sensor systems usually suffer from the limited number of repeated use, resulting in their high use-cost. The development of separable versatile devices may address this issue with one layer realized by costeffective materials and fabrication manners for disposable use and the other composed of relatively expensive components for repeatable applications.[18] Additionally, the multiple bending or Skin-inspired wearable devices hold great potentials in the next generation of smart portable electronics owing to their intriguing applications in healthcare monitoring, soft robotics, artificial intelligence, and human–machine interfaces. Despite tremendous research efforts dedicated to judiciously tailoring wearable devices in terms of their thickness, portability, flexibility, bendability as well as stretchability, the emerging Internet of Things demand the skininterfaced flexible systems to be endowed with additional functionalities with the capability of mimicking skin-like perception and beyond. This review covers and highlights the latest advances of burgeoning multifunctional wearable electronics, primarily including versatile multimodal sensor systems, self-healing material-based devices, and self-powered flexible sensors. To render the penetration of human-interactive devices into global markets and households, economical manufacturing techniques are crucial to achieve large-scale flexible systems with high-throughput capability. The booming innovations in this research field will push the scientific community forward and benefit human beings in the near future.

Multifunctional Skin-Inspired Flexible Sensor Systems for Wearable Electronics

While over half a century has passed since the introduction of enzyme glucose biosensors by Clark and Lyons, this important field has continued to be the focus of immense research activity. Extensive efforts during the past decade have led to major scientific and technological innovations towards tight monitoring of diabetes. Such continued progress toward advanced continuous glucose monitoring platforms, either minimal- or non-invasive, holds considerable promise for addressing the limitations of finger-prick blood testing toward tracking glucose trends over time, optimal therapeutic interventions, and improving the life of diabetes patients. However, despite these major developments, the field of glucose biosensors is still facing major challenges. The scope of this review is to present the key scientific and technological advances in electrochemical glucose biosensing over the past decade (2010-present), along with current obstacles and prospects towards the ultimate goal of highly stable and reliable real-time minimally-invasive or non-invasive glucose monitoring. After an introduction to electrochemical glucose biosensors, we highlight recent progress based on using advanced nanomaterials at the electrode-enzyme interface of three generations of glucose sensors. Subsequently, we cover recent activity and challenges towards next-generation wearable non-invasive glucose monitoring devices based on innovative sensing principles, alternative body fluids, advanced flexible materials, and novel platforms. This is followed by highlighting the latest progress in the field of minimally-invasive continuous glucose monitoring (CGM) which offers real-time information about interstitial glucose levels, by focusing on the challenges toward developing biocompatible membrane coatings to protect electrochemical glucose sensors against surface biofouling. Subsequent sections cover new analytical concepts of self-powered glucose sensors, paper-based glucose sensing and multiplexed detection of diabetes-related biomarkers. Finally, we will cover the latest advances in commercially available devices along with the upcoming future technologies.

Electrochemical glucose sensors in diabetes management: an updated review (2010-2020).

As part of increased efforts to develop wearable healthcare devices for monitoring and managing physiological and metabolic information, stretchable electrochemical sweat sensors have been investigated. In this study, we report on the fabrication of a stretchable and skin-attachable electrochemical sensor for detecting glucose and pH in sweat. A patterned stretchable electrode was fabricated via layer-by-layer deposition of carbon nanotubes (CNTs) on top of patterned Au nanosheets (AuNS) prepared by filtration onto stretchable substrate. For the detection of glucose and pH, CoWO4/CNT and polyaniline/CNT nanocomposites were coated onto the CNT–AuNS electrodes, respectively. A reference electrode was prepared via chlorination of silver nanowires. Encapsulation of the stretchable sensor with sticky silbione led to a skin-attachable sweat sensor. Our sensor showed high performance with sensitivities of 10.89 μA mM–1 cm–2 and 71.44 mV pH–1 for glucose and pH, respectively, with mechanical stability up to 30% s...

Skin-Attachable, Stretchable Electrochemical Sweat Sensor for Glucose and pH Detection.

Microporous Polypyrrole‐Coated Graphene Foam for High‐Performance Multifunctional Sensors and Flexible Supercapacitors

In this study, we demonstrate the fabrication of a highly sensitive flexible temperature sensor with a bioinspired octopus-mimicking adhesive. A resistor-type temperature sensor consisting of a composite of poly(N-isopropylacrylamide) (pNIPAM)-temperature sensitive hydrogel, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate, and carbon nanotubes exhibits a very high thermal sensitivity of 2.6%·°C–1 between 25 and 40 °C so that the change in skin temperature of 0.5 °C can be accurately detected. At the same time, the polydimethylsiloxane adhesive layer of octopus-mimicking rim structure coated with pNIPAM is fabricated through the formation of a single mold by utilizing undercut phenomenon in photolithography. The fabricated sensor shows stable and reproducible detection of skin temperature under repeated attachment/detachment cycles onto skin without any skin irritation for a long time. This work suggests a high potential application of our skin-attachable temperature sensor to wearable devices for m...

Fabrication of High-Sensitivity Skin-Attachable Temperature Sensors with Bioinspired Microstructured Adhesive.

Body-attachable sensors can be applied to electronic skin (e-skin) as well as safety forewarning and health monitoring systems. However, achieving facile fabrication of high-performance, cost-effective sensors with mechanical stability in response to deformation due to body movement is challenging. Herein we report the material design, fabrication and characteristics of a skin-like stretchable array of multi-functional (MF) sensors based on a single sensing material of polyurethane foam coated with multi-walled carbon nanotube/polyaniline nanocomposite, which enables simultaneous detection of body temperature, wrist pulse and ammonia gas. These sensors exhibit high sensitivity, fast response and excellent durability. Furthermore, the fabricated sensor array shows stable performance under biaxial stretching up to 50% and attachment to skin owing to the use of direct-printed Galinstan liquid metal interconnections. This work proposes a facile method for fabrication of high-performance, stretchable MF sensors via appropriate selection of sensor design and functional materials that are applicable to e-skin and health monitoring systems. Combining stretchy foams with conductive nanomaterials enables real-time temperature, pulse, and gas exposure sensing from a patient's skin. Interactive ‘e-skin’ devices need to resolve different stimuli into clear electronic signals while attached to a deformable substrate. To satisfy these challenging constraints, Jeong Sook Ha and colleagues at Korea University coated polyurethane foams with a nanocomposite made from multiwalled carbon nanotubes and polyaniline. The porous microstructure of this hybrid material imparted mechanical flexibility and high responsiveness to electrical resistance changes and output voltage induced by pressure and heat variations, respectively, while the polyaniline component made it selective to toxic ammonia gas. A prototype device in which the foam is sandwiched between thin-film electrodes and which uses an adhesive gel to attach to the skin showed a rapid response and an excellent durability when attached to the fingertips and the wrist. A skin-like stretchable multi-functional sensor array based on a combination of PU foam and MWCNT-PANI nanocomposite is demonstrated to simultaneously detect pressure, temperature and ammonia gas with high sensitivity. The integrated sensor array with printed liquid metal interconnections operates stably under applied strain due to its attachment to the skin and under a biaxial strain up to 50%.

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Polyurethane foam coated with a multi-walled carbon nanotube/polyaniline nanocomposite for a skin-like stretchable array of multi-functional sensors

Two-dimensional transition metal carbide and nitride nanomaterials, known as MXenes, exhibit low chemical stability in aqueous environments; they tend to oxidize and react with water molecules, res...

Ju Hyun Oh

Papers

Skin-Attachable, Stretchable Electrochemical Sweat Sensor for Glucose and pH Detection.

Microporous Polypyrrole‐Coated Graphene Foam for High‐Performance Multifunctional Sensors and Flexible Supercapacitors

Fabrication of High-Sensitivity Skin-Attachable Temperature Sensors with Bioinspired Microstructured Adhesive.

Polyurethane foam coated with a multi-walled carbon nanotube/polyaniline nanocomposite for a skin-like stretchable array of multi-functional sensors

Annealed Ti3C2Tz MXene Films for Oxidation-Resistant Functional Coatings