Breath analysis is an attractive strategy that holds tremendous potential to achieve non-implantable physical health management enabled by flexible humidity sensors for breathing behaviors (e.g., continuity, frequency) related to disease monitoring and chemiresistive gas sensors related early disease diagnosis. Compared to other techniques for breath component detection, non-invasive breathing diagnostics based on chemical sensors can offer several advantages like miniaturization, low power consumption, simple structure and cost-saving, which is helpful to enhance the portability of practical tests. Although extensive research has been carried out over the past two decades to improve sensing performances of breath gas sensors, many problems need to be further addressed when it comes to clinical disease diagnosis. Developing integrated gas sensor arrays have become one of the efficient solutions to improve detection accuracy. To get rid of external power supply, various novel sensors combining with self-powered technology are designed to exhibit a desirable development prospect in breath analysis. Thus, this review aims to summarize the latest research advances on wearable humidity-enabled breathing behaviors monitoring and typical biomarker gases-based disease screening, and also provides the prospects of future development from individual sensors to integrated devices and self-powered health monitoring systems.

Evolution of breath analysis based on humidity and gas sensors: Potential and challenges

In mammals, physiological respiration involves respiratory cycles of inhaled and exhaled breaths, which has traditionally been an underutilized resource potentially encompassing a wealth of physiologically relevant information as well as clues to potential diseases. Recently, triboelectric nanogenerators (TENGs) have been widely adopted for self-powered respiration monitoring owing to their compelling features, such as decent biocompatibility, wearing comfort, low-cost, and high sensitivity to respiration activities in the aspect of low frequency and slight amplitude body motions. Physiological respiration behaviors and exhaled chemical regents can be precisely and continuously monitored by TENG-based respiration sensors for personalized health care. This article presents an overview of TENG enabled self-powered respiration monitoring, with a focus on the working principle, sensing materials, functional structures, and related applications in both physical respiration motion detection and chemical breath analysis. Concepts and approaches for acquisition of physical information associated with respiratory rate and depth are covered in the first part. Then the sensing mechanism, theoretical modeling, and applications related to detection of chemicals released from breathing gases are systemically summarized. Finally, the opportunities and challenges of triboelectric effect enabled self-powered respiration monitoring are comprehensively discussed and criticized.

Self‐Powered Respiration Monitoring Enabled By a Triboelectric Nanogenerator

Development of flexible devices for continuous respiration monitoring has been fueled up recently due to their great importance in constructing wearable healthcare systems. However, there is no report on silk fabric-based devices for precise detection of the human respiratory rate and depth. In this work, a silk fabric-based respiration sensor was fabricated by successive electroless plating of conductive interdigital electrodes and spray-coating of a graphene oxide (GO) sensing layer. Surface morphologies of the devices were characterized to show the construction of the sensor. The uniform surface distribution of GO nanosheets was analysed using the Raman mapping method. After optimization, the as-prepared device could accurately detect the human respiration rate and effectively differentiate normal, deep and fast breathing. The flexibility test indicates that the sensor can tolerate 2500 repetitions of bending and twisting without influencing its performance. Moreover, by designing a spray-coating mask, GO layers with various patterns could be deposited to achieve both functionality and aesthetics. This work not only develops a silk fabric-based sensor for potential application in assessing the basic human health-status, but also provides a facile approach to preparing textile-based wearable electronic devices.

A flexible humidity sensor based on silk fabrics for human respiration monitoring

High-performance flexible and wearable strain sensors for human motion detection have been widely investigated recently. Here, we report a flexible strain sensor with ultra-high stretchability, large sensitivity and excellent durability based on common elastic bands from every day life. The strain sensor was fabricated by embedding carbon nanotubes (CNTs) into elastic bands (EB) through swelling-ultrasonication treatment. It is found that the CNTs with a large aspect ratio migrated into the inner regions of the EB successfully, generating a conductive CNT/EB shell with a thickness of about 30 μm. The CNT/EB bands were then coated with a polydopamine (PDA) layer by self-polymerization of dopamine to stabilize the sensitive regions. Interestingly, the specific strength and specific elongation of PDA/CNT/EB are enhanced by about 55% and 23% respectively after the coating of PDA, compared to the pure EB. Strain sensing tests indicated that our strain sensors had desirable integration of an ultra-high sensing range (920% strain), large sensitivity (a gauge factor (GF) of 129 under a strain of 780–920%), superior stability (10 000 cycles at 100% strain) and a fast response speed. Additionally, there was a direct correlation between the mechanical hysteresis HM value and the weak shoulder peak. The sensing mechanism of the strain sensors were investigated. Monitoring of human body motions (such as finger bending, muscle movements and knee-joint movements) indicates that our stretchable PDA/CNT/EB sensor has broad application prospects for human-machine interfaces in the artificial intelligence (AI) field.

Ultra-stretchable, sensitive and durable strain sensors based on polydopamine encapsulated carbon nanotubes/elastic bands

Stretchable respiration sensors: Advanced designs and multifunctional platforms for wearable physiological monitoring.

Wrinkled nitrile rubber films for stretchable and ultra-sensitive respiration sensors

Highly sensitive strain sensors that can detect small strain are in high demand in the fields of displays, robotics, fatigue detection, body monitoring, in vitro diagnostics, and advanced therapies. However, resistive-type sensors that are composed of electrically conductive sensing films coupled with flexible substrates suffer from the limits that their gauge factors (GFs) at small strains (e.g., 0.1–1%) are not high. Herein, through frictional direct-writing of graphite rod on the composite paper substrates, we produced strain sensors with extremely high gauge factor at small strains. The sensors exhibited a gauge factor of 9720 at a small strain of 0.9%, minimum strain detection up to 0.05%, strain resolution of 0.05%, response time of 40 ms, and high stability (>5000 bending–unbending cycles). Compared with the literature results so far, our sensors hold the highest GF value at small strains. Such high sensitivities are due to the precise control of narrow two-dimensional percolative conductive pathwa...

Precise Engineering of Conductive Pathway by Frictional Direct-Writing for Ultrasensitive Flexible Strain Sensors

Respiration is one of the most vital physiological activities of the human body. Abnormalities in the respiratory rates and patterns can ascertain the condition of the human body, such as sleeping, exercising, anaerobic threshold during athletic activity, etc. However, current respiratory sensor materials are often complicated in system and cumbersome in technology. This paper proposes a simple model breathing sensor preparation scheme that only requires electrodes to be attached to common insulating materials used in daily life. The basic working principle is: (a) conductive layer formation (the conductivity of hydrogen bonding induced water layer is mainly due to the proton transition), (b) signal acquisition and (c) health-status-feedback. The effects of the chemical and physical properties of different insulating materials on the respiratory detection caused by surface water condensation were studied for the first time. The different response mechanisms of several types of materials were studied in detail, and the selection criteria for sensing materials were given, which has great reference value for further research and industrial production. Considering the requirements of comfort, long-term hygiene (antibacterial) and excellent performance, etc., a complete human respiratory monitoring system specifically based on nitrile rubber films (NRFs) as wearable respiration sensors is constructed. This sensor achieved a good balance of several parameters. The signal changes by 3 orders of magnitude in cycles of inhalation and exhalation maximally. This universal patterning design scheme for respiratory sensors will be a significant reference for application and industrialization, and will have a profound impact on further fundamental research.

A universal respiration sensing platform utilizing surface water condensation

The invention discloses a method for preparing a flexible breathing sensor. According to the method, complex process steps and various auxiliary materials are not needed, only an insulating flexible substrate with the rough surface needs to serve as a sensitive surface, an electrode is attached to the sensitive surface, so that the flexible breathing sensor can be prepared. Moisture during breathing is utilized to form an instant conductive effect of a water molecule film on the surface of the flexible substrate, so that during expiration, water molecules are condensed to form a film to make resistance between electrodes reduced (a current rises); after expiration is stopped, the water molecule film is volatilized instantly, and resistance between the electrodes is increased (the current descends) to detect the breathing; within a certain roughness range, since water molecule condensation nucleating points are rich, instant formation of the water molecule film is facilitated, and thus a detection effect is remarkable; since the selected substrate is flexible, a detection material has good flexibility; raw materials are low in cost, a preparation process is simple and convenient, and the method has the advantage of being low in cost.

Method for preparing flexible breathing sensor

The invention discloses a method for manufacturing high-strain-factor flexible resistance strain gauge through a friction mode. The method is characterized by tiling a flexible substrate on an elastomer; and then using a graphite block to carry out friction on the flexible substrate so that a graphite micro chip attached to the flexible substrate forms a conducting surface and the flexible resistance strain gauge is acquired. A conductive mechanism of the flexible resistance strain gauge manufactured in the invention is a seepage conducting surface formed by the graphite micro chip attached to the flexible substrate. Because a microscopic conductive pathway of the seepage conducting surface changes significantly along with a strain, a resistance after bending or stretching changes significantly. The selected substrate is flexible so that a conductive film possesses good flexibility. Simultaneously, because raw material cost is low and a manufacturing technology is simple, a characteristic of low cost is possessed.

Song Yongming

Papers

Wrinkled nitrile rubber films for stretchable and ultra-sensitive respiration sensors

Precise Engineering of Conductive Pathway by Frictional Direct-Writing for Ultrasensitive Flexible Strain Sensors

A universal respiration sensing platform utilizing surface water condensation

Method for preparing flexible breathing sensor

Method for manufacturing flexible resistance strain gauge through friction mode