Abstract Sweat is a biofluid with rich information that can reflect an individual’s state of health or activity. But the real-time in situ sweat sensors lack the ability of long-term monitoring. Against this background, this article provides a holistic review on the necessary process and methods for sweat sensing, including sweat collection, composition analysis, energy supply, and data processing. The impacts of the environment in stimulating sweat production, providing energy supply, and intelligent health monitoring are discussed. Based on the review of previous endeavors, the future development in material, structure and artificial intelligence application of long-term sweat monitoring is envisioned. 

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A review of sampling, energy supply and intelligent monitoring for long-term sweat sensors

Recent technological innovations, such as material printing techniques and surface functionalization, have significantly accelerated the development of new free-form sensors for next-generation flexible, wearable, and three-dimensional electronic devices. Ceramic film sensors, in particular, are in high demand for the production of reliable flexible devices. Various ceramic films can now be formed on plastic substrates through the development of low temperature fabrication processes for ceramic films, such as photocrystallization and transferring methods. Among flexible sensors, strain sensors for precise motion detection and photodetectors for biomonitoring have seen the most research development, but other fundamental sensors for temperature and humidity have also begun to grow. Recently, flexible gas and electrochemical sensors have attracted a lot of attention from a new real-time monitoring application that uses human breath and perspiration to accurately diagnose presymptomatic states. The development of a low-temperature fabrication process of ceramic film sensors and related components will complete the chemically stable and reliable free-form sensing devices by satisfying the demands that can only be addressed by flexible metal and organic components.

https://www.mdpi.com/1424-8220/22/5/1996/pdf?version=1646311933

Flexible Ceramic Film Sensors for Free-Form Devices

Skin-mountable electronics are considered to be the future of the next generation of portable electronics, due to their softness and seamless integration with human skin. However, impermeable materials limit device comfort and reliability for long-term, continuous usage. The recent emergence of permeable skin-mountable electronics has attracted tremendous attention in the soft electronics field. Herein, we provide a comprehensive and systematic review of permeable skin-mountable electronics. Typical porous materials and structures are first highlighted, followed by discussion of important device properties. Then, we review the latest representative applications of breathable skin-mountable electronics, such as bioelectrical sensors, temperature sensors, humidity and hydration sensors, strain and pressure sensors, and energy harvesting and storage devices. Finally, a conclusion and future directions for permeable skin electronics are provided.

Toward a new generation of permeable skin electronics.

Growing health awareness triggers the public's concern about health problems. People want a timely and comprehensive picture of their condition without frequent trips to the hospital for costly and cumbersome general check-ups. The wearable technique provides a continuous measurement method for health monitoring by tracking a person's physiological data and analyzing it locally or remotely. During the health monitoring process, different kinds of sensors convert physiological signals into electrical or optical signals that can be recorded and transmitted, consequently playing a crucial role in wearable techniques. Wearable application scenarios usually require sensors to possess excellent flexibility and stretchability. Thus, designing flexible and stretchable sensors with reliable performance is the key to wearable technology. Smart composite hydrogels, which have tunable electrical properties, mechanical properties, biocompatibility, and multi-stimulus sensitivity, are one of the best sensitive materials for wearable health monitoring. This review summarizes the common synthetic and performance optimization strategies of smart composite hydrogels and focuses on the current application of smart composite hydrogels in the field of wearable health monitoring. 

https://link.springer.com/content/pdf/10.1007/s40820-023-01079-5.pdf

Engineering Smart Composite Hydrogels for Wearable Disease Monitoring

Cortisol is involved in regulating many human physiological functions, with cortisol imbalance capable of causing adverse mental and physical health conditions. Existing tools for monitoring of cortisol can assist in alerting the need for mediation strategies, but these typically require time consuming processes such as sample preparation followed by laboratory analysis. This work demonstrates a miniature flexible sensor patch incorporating electrode structures produced by high throughput roll-to-roll rotary screen-printing process, which are subsequently functionalized by electropolymerization of a polypyrrole/Prussian blue molecularly imprinted polymer for measuring cortisol by chronoamperometry. The developed single use sensor patch provides acceptable selectivity for cortisol and operates in artificial eccrine perspiration across a broad concentration range of 0.1–10 000 ng ml−1 (R 2 = 0.916). In the application of measuring thermally induced sweat, the sensor provided results in agreement with the natural circadian rhythm of fluctuating cortisol.

Molecularly imprinted polymer on roll-to-roll printed electrodes as a single use sensor for monitoring of cortisol in sweat

Continuous monitoring of the blood glucose levels is necessary for its diagnosis and treatment. In recent years, researchers have discovered a correlation between sweat glucose and blood glucose. The methodology of obtaining the value of blood glucose by detecting glucose in sweat is expected to replace conventional diabetes detection. Flexible sweat glucose biosensor owns the characteristics of stretchability, comfortability, and the capability of non-invasive and is a promising candidate for non-invasive diabetes monitoring. In this review, we summarize the latest developments in research and applications of the flexible sweat glucose biosensors based on enzyme and non-enzymatic detection methodologies. The correction methodologies, challenges and outlooks of the wearable sweat glucose biosensor technology are also be discussed

Recent Advances in Flexible Sweat Glucose Biosensors

Electronic skin (E‐skin) is widely used in bionic prosthesis, soft robotics, and wearable electronic devices. Waterproof and breathable membranes that provide a high level of protection and comfort are promising core materials for meeting the pressing demand for E‐skin. However, creating such materials exhibiting waterproofness, breathability, and ease of fabrication has proven to be a great challenge. In this paper, waterproof, breathable Modal‐based knitted fabric E‐skin is proposed by using a simple, low‐cost three‐step process including the graphene oxide (GO) loading, GO‐reducing, and polydimethylsiloxane loading. The prepared E‐skin is insensitive to common liquids in daily life including water, tea, milk, and coffee, showing superior waterproof capability. The breathability is also tested with a water vapor transmission rate of ≈13.6 mg cm−2 h−1. Besides, this E‐skin can achieve reliable human motion monitoring. The authors believe this E‐skin can provide a reliable principle for further design and development of waterproof and breathable electronic skin.

Waterproof and Breathable Graphene‐Based Electronic Fabric for Wearable Sensors

Hydrogels are considered as promising materials in developing next-generation wearable sensors due to their intrinsic biocompatibility and low Young's modulus. However, there are few reports on their breathability which is very important for wear comfortability of wearable sensors. Here graphene-based hydrogel is presented with regular pores using microneedles as templates endowing them with excellent breathability (water vapor transmitting rate 2.8 kg d–1 m–2), which is near ten times that of human skin (water vapor transmitting rate less than 0.3 kg d–1 m–2). A high-performance strain sensor based on this breathable hydrogel is developed. This sensor not only possesses high sensitivity but also has an excellent linearity with strain ranging from 0% to 110%. Based on these performances, this sensor can be applied in monitoring various joint movements and muscle movements. Moreover, it can achieve barrier-free communication through Morse code. 

Graphene‐Based Hydrogel Strain Sensors with Excellent Breathability for Motion Detection and Communication

Regulation of crystallinity and defects on CoNiRuOx nanocages for enhanced oxygen evolution reaction

High sensitivity, excellent linearity, and wireless monitoring are strongly desired for the practical application of flexible temperature sensors in real‐time wearable health care. Especially the multichannel body temperature monitoring system has high requirements on the performance of the sensor because it needs to monitor a large amount of data at the same time. Herein, a flexible temperature sensor based on a porous graphene/polydimethylsiloxane sensing layer is developed. The prepared sensor exhibits high sensitivity of 5.203% °C−1 for temperature sensing between 30 and 70 °C, and excellent linearity (R2 = 0.996) in the temperature range from 30 to 70 °C. Based on its excellent performance, the proposed high‐performance temperature sensors can be applied in practical applications such as body temperature monitoring and human breath monitoring under different states including slow breath, normal breath, and fast breath. Moreover, a high‐throughput wireless body temperature monitoring system including wireless sensor modules, a cloud server, and a portable electronic device is constructed, which can achieve a remote and multichannel body temperature monitoring efficiently.

Changqing Ye

Papers

Recent Advances in Flexible Sweat Glucose Biosensors

Waterproof and Breathable Graphene‐Based Electronic Fabric for Wearable Sensors

Graphene‐Based Hydrogel Strain Sensors with Excellent Breathability for Motion Detection and Communication

Regulation of crystallinity and defects on CoNiRuOx nanocages for enhanced oxygen evolution reaction

High Throughput In–Situ Temperature Sensor Array with High Sensitivity and Excellent Linearity for Wireless Body Temperature Monitoring