Skin is the largest organ of the human body, and it offers a diagnostic interface rich with vital biological signals from the inner organs, blood vessels, muscles, and dermis/epidermis. Soft, flexible, and stretchable electronic devices provide a novel platform to interface with soft tissues for robotic feedback and control, regenerative medicine, and continuous health monitoring. Here, we introduce the term “lab-on-skin” to describe a set of electronic devices that have physical properties, such as thickness, thermal mass, elastic modulus, and water-vapor permeability, which resemble those of the skin. These devices can conformally laminate on the epidermis to mitigate motion artifacts and mismatches in mechanical properties created by conventional, rigid electronics while simultaneously providing accurate, non-invasive, long-term, and continuous health monitoring. Recent progress in the design and fabrication of soft sensors with more advanced capabilities and enhanced reliability suggest an impending t...

Lab-on-Skin: A Review of Flexible and Stretchable Electronics for Wearable Health Monitoring

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.

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

Tactile sensors, most commonly referred as strain and pressure sensors, can collect mechanical property data of the human body and local environment, to provide valuable insights into the human health status or artificial intelligence systems. The introduction of a high level of wearability (bendability and stretchability) to tactile sensors can dramatically enhance their interfaces with the contact objects, providing chronically reliable functions. Therefore, the developed wearable tactile sensors are capable of conformably covering arbitrary curved surface over their stiff counterparts without incurring damage, emerging as a promising development direction toward the Internet of Things (IoT) applications. Fundamental parameters of the wearable tactile sensors such as sensitivity and stretchability have experienced unprecedented advancement, owing to the progress of device fabrication techniques and material structural engineering. Moreover, novel smart materials and mechanically durable sensor design concepts endow these sensors with multi-functionality integration (e.g., simultaneous force, temperature and humidity detection, simultaneous pressure and strain discrimination) and stirring properties (e.g., biocompatibility, biodegradability, self-healing, self-powering and visualization), further broadening the application scope of current wearable tactile sensors. Besides, it is desirable that a tactile sensor is compatible with a printing process that presents a new era of feasible wearable technology due to its large-area and high-throughput production capability. In addition to the development of sensors, packaging, and integration of the rest of the tactile device system (data memory, signal conversion, power supply, wireless transmission, feedback actuator, etc.) to build a wearable platform also emerge as major research frontiers in recent years. This review attempts to summarize the current state-of-the-art wearable tactile sensors concerning basic concepts, functional materials, sensing mechanism, promising applications, performance optimization strategies, multifunctional sensing, and system integration. Finally, the discussion will be presented regarding potential challenges, pathways, and opportunities.

Recent advances in wearable tactile sensors: Materials, sensing mechanisms, and device performance

Flexible electronics, as an emerging and exciting research field, have brought great interest to the issue of how to make flexible electronic materials that offer both durability and high performance at strained states. With the advent of on-body wearable and implantable electronics, as well as increasing demands for human-friendly intelligent soft robots, enormous effort is being expended on highly flexible functional materials, especially stretchable electrodes, by both the academic and industrial communities. Among different deformation modes, stretchability is the most demanding and challenging. This review focuses on the latest advances in stretchable transparent electrodes based on a new design strategy known as kirigami (the art of paper cutting) and investigates the recent progress on novel applications, including skin-like electronics, implantable biodegradable devices, and bioinspired soft robotics. By comparing the optoelectrical and mechanical properties of different electrode materials, some of the most important outcomes with comments on their merits and demerits are raised. Key design considerations in terms of geometries, substrates, and adhesion are also discussed, offering insights into the universal strategies for engineering stretchable electrodes regardless of the material. It is suggested that highly stretchable and biocompatible electrodes will greatly boost the development of next-generation intelligent life-like electronics.

Flexible Electronics: Stretchable Electrodes and Their Future

Continuous monitoring of variations in blood flow is vital in assessing the status of microvascular and macrovascular beds for a wide range of clinical and research scenarios. Although a variety of techniques exist, most require complete immobilization of the subject, thereby limiting their utility to hospital or clinical settings. Those that can be rendered in wearable formats suffer from limited accuracy, motion artifacts, and other shortcomings that follow from an inability to achieve intimate, noninvasive mechanical linkage of sensors with the surface of the skin. We introduce an ultrathin, soft, skin-conforming sensor technology that offers advanced capabilities in continuous and precise blood flow mapping. Systematic work establishes a set of experimental procedures and theoretical models for quantitative measurements and guidelines in design and operation. Experimental studies on human subjects, including validation with measurements performed using state-of-the-art clinical techniques, demonstrate sensitive and accurate assessment of both macrovascular and microvascular flow under a range of physiological conditions. Refined operational modes eliminate long-term drifts and reduce power consumption, thereby providing steps toward the use of this technology for continuous monitoring during daily activities.

Epidermal devices for noninvasive, precise, and continuous mapping of macrovascular and microvascular blood flow

Continuous infrared imaging revealed transient changes in forearm temperature during arterial occlusion, reperfusion, and recovery in a healthy subject group. Processing the imaging data with the k-means algorithm further revealed reactive vascular sites in the skin with rapid or delayed temperature amplification. The observed temporal and spatial diversity of blood-flow-derived forearm temperature allow consideration of thermal-imaging guided placement of skin sensors to achieve enhanced sensitivity in monitoring of skin hemodynamics.

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Stephen Yoon

Papers

Epidermal devices for noninvasive, precise, and continuous mapping of macrovascular and microvascular blood flow

Rapid vs. delayed infrared responses after ischemia reveal recruitment of different vascular beds.