Humans rely increasingly on sensors to address grand challenges and to improve quality of life in the era of digitalization and big data. For ubiquitous sensing, flexible sensors are developed to overcome the limitations of conventional rigid counterparts. Despite rapid advancement in bench-side research over the last decade, the market adoption of flexible sensors remains limited. To ease and to expedite their deployment, here, we identify bottlenecks hindering the maturation of flexible sensors and propose promising solutions. We first analyze challenges in achieving satisfactory sensing performance for real-world applications and then summarize issues in compatible sensor-biology interfaces, followed by brief discussions on powering and connecting sensor networks. Issues en route to commercialization and for sustainable growth of the sector are also analyzed, highlighting environmental concerns and emphasizing nontechnical issues such as business, regulatory, and ethical considerations. Additionally, we look at future intelligent flexible sensors. In proposing a comprehensive roadmap, we hope to steer research efforts towards common goals and to guide coordinated development strategies from disparate communities. Through such collaborative efforts, scientific breakthroughs can be made sooner and capitalized for the betterment of humanity.

Technology Roadmap for Flexible Sensors.

Wearable sensors hold great potential in empowering personalized health monitoring, predictive analytics, and timely intervention toward personalized healthcare. Advances in flexible electronics, materials science, and electrochemistry have spurred the development of wearable sweat sensors that enable the continuous and noninvasive screening of analytes indicative of health status. Existing major challenges in wearable sensors include: improving the sweat extraction and sweat sensing capabilities, improving the form factor of the wearable device for minimal discomfort and reliable measurements when worn, and understanding the clinical value of sweat analytes toward biomarker discovery. This review provides a comprehensive review of wearable sweat sensors and outlines state-of-the-art technologies and research that strive to bridge these gaps. The physiology of sweat, materials, biosensing mechanisms and advances, and approaches for sweat induction and sampling are introduced. Additionally, design considerations for the system-level development of wearable sweat sensing devices, spanning from strategies for prolonged sweat extraction to efficient powering of wearables, are discussed. Furthermore, the applications, data analytics, commercialization efforts, challenges, and prospects of wearable sweat sensors for precision medicine are discussed.

Skin-Interfaced Wearable Sweat Sensors for Precision Medicine.

The management of the COVID-19 pandemic has relied on cautious contact tracing, quarantine, and sterilization protocols while we await a vaccine to be made widely available. Telemedicine or mobile health (mHealth) is well-positioned during this time to reduce potential disease spread and prevent overloading of the healthcare system through at-home COVID-19 screening, diagnosis, and monitoring. With the rise of mass-fabricated electronics for wearable and portable sensors, emerging telemedicine tools have been developed to address shortcomings in COVID-19 diagnostics, monitoring, and management. In this Perspective, we summarize current implementations of mHealth sensors for COVID-19, highlight recent technological advances, and provide an overview on how these tools may be utilized to better control the COVID-19 pandemic.

Emerging Telemedicine Tools for Remote COVID-19 Diagnosis, Monitoring, and Management.

Wearable sweat sensors have spearheaded the thrust toward personalized health monitoring with continuous, real-time, and molecular-level insight in a noninvasive manner. However, effective sweat sampling still remains a huge challenge. Here, we introduce an intelligent Janus textile band that bridges the gap between self-pumping sweat collection, comfortable epidemic microclimate, and sensitive electrochemical biosensing via an integrated wearable platform. The dominant sweat sampling configuration is a textile with Janus wettability, which is fabricated by electrospinning a hydrophobic polyurethane (PU) nanofiber array onto superhydrophilic gauze. Based on a contact-pumping model, the Janus textile can unidirectionally and thoroughly transport sweat from skin (hydrophobic side) to embedded electrode surface (hydrophilic side) with epidemic comfort. On-body experimentation reveals that the sensitive detection of multiple biomarkers including glucose, lactate, K+, and Na+ is achieved in the pumped sweat. Such smart Janus textile bands can effectively drain epidermal sweat to targeted assay sites via interface modifications, representing a reinforced and controlled biofluids analysis pathway with physiological comfort.

Integrated Smart Janus Textile Bands for Self-Pumping Sweat Sampling and Analysis.

Recent advances in materials, mechanics and design have led to the development of ultrathin, lightweight electronic devices that can conformally interface with human skin. With few exceptions, these devices rely on electrical power to support sensing, wireless communication and signal conditioning. Unfortunately, most sources of such power consist of batteries constructed using hazardous materials, often with form factors that frustrate incorporation into skin-like, or epidermal, electronic devices. Here we report a biocompatible, sweat-activated battery technology that can be embedded within a soft, microfluidic platform. The battery can be used in a detachable electronic module that contains wireless communication and power management systems, and is capable of continuous on-skin recording of physiological signals. To illustrate the practical utility of our approach, we show using human trials that the sweat-activated batteries can operate hybrid microfluidic/microelectronic systems that simultaneously monitor heart rate, sweat chloride and sweat pH. Sweat-activated, biocompatible batteries can be used to power flexible on-skin electronic systems that monitor and wirelessly transmit physiological signals.

Sweat-activated biocompatible batteries for epidermal electronic and microfluidic systems

Implantable devices capable of targeted and reversible blocking of peripheral nerve activity may provide alternatives to opioids for treating pain. Local cooling represents an attractive means for on-demand elimination of pain signals, but traditional technologies are limited by rigid, bulky form factors; imprecise cooling; and requirements for extraction surgeries. Here, we introduce soft, bioresorbable, microfluidic devices that enable delivery of focused, minimally invasive cooling power at arbitrary depths in living tissues with real-time temperature feedback control. Construction with water-soluble, biocompatible materials leads to dissolution and bioresorption as a mechanism to eliminate unnecessary device load and risk to the patient without additional surgeries. Multiweek in vivo trials demonstrate the ability to rapidly and precisely cool peripheral nerves to provide local, on-demand analgesia in rat models for neuropathic pain. Description Cooling away the pain After injury due to an accident or medical procedure, various forms of pain relief may be required. These can include analgesic medications or local injections to dull the pain receptors, but may also be as simple as applying something cold to the location causing the pain, such as ice packs for sore or bruised joints or muscles. Reeder et al. developed a soft, miniaturized, implantable cooler to temporarily block nerve conduction using a liquid-to-gas phase transition as the cooling mechanism (see the Perspective by Jiang and Hong). They borrowed the design of electrical nerve cuff and substituted electrical wires with a microfluidic channel carrying a microliter volume of bioinert coolant. A thermal thin-film sensor integrated within the cuff enabled monitoring of the temperature in real time, thus enabling closed-loop control. —MSL Implantable nerve coolers with onboard temperature monitoring enable targeted, on-demand pain relief.

Soft, bioresorbable coolers for reversible conduction block of peripheral nerves

Recently introduced classes of thin, soft, skin-mounted microfluidic systems offer powerful capabilities for continuous, real-time monitoring of total sweat loss, sweat rate and sweat biomarkers. Although these technologies operate without the cost, complexity, size, and weight associated with active components or power sources, rehydration events can render previous measurements irrelevant and detection of anomalous physiological events, such as high sweat loss, requires user engagement to observe colorimetric responses. Here we address these limitations through monolithic systems of pinch valves and suction pumps for purging of sweat as a reset mechanism to coincide with hydration events, microstructural optics for reversible readout of sweat loss, and effervescent pumps and chemesthetic agents for automated delivery of sensory warnings of excessive sweat loss. Human subject trials demonstrate the ability of these systems to alert users to the potential for dehydration via skin sensations initiated by sweat-triggered ejection of menthol and capsaicin.

/pdf/resettable-skin-interfaced-microfluidic-sweat-collection-15th4k4vy6.pdf

Resettable skin interfaced microfluidic sweat collection devices with chemesthetic hydration feedback.

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Soft, bioresorbable coolers for reversible conduction block of peripheral nerves

Resettable skin interfaced microfluidic sweat collection devices with chemesthetic hydration feedback.