Wearable Sweat Biosensors on Sports Analysis

1.   Introduction

Sports analytics is a process of using sports-related data to discover valuable features and facilitate adjustment of training programs and promotes the implementation of scientific training. In the past few years, however, problems in sports analytics got worse such as difficult continuous monitoring, long inspection cycle, the dependence on laboratories, and athletes’ protest against invasive blood detection. With the rapid rise and development of lab-on-a-chip technology, superior performance wearable sensors are growing. And these lately emerging wearable biosensors are exploited to overcome the difficulties in sports analytics as they are capable of real-time monitoring, good biocompatibility, continuous, mechanical flexibility, and non-invasive detection^[1], by miniaturization of electronics and integration with microfluidics. Despite reducing the time for coaches and sports researchers to obtain high-quality and comprehensive data, the possibilities for improvement still exist.

This review concludes with the latest developments of wearable biosensors in materials and sensing modalities as well as a summary of unresolved challenges and opportunities researchers may be interested in for the future. With a deep understanding of wearable biosensing technologies, advanced wearable biosensors would have a significant impact on athletic monitoring and sports analytics.

2.   Materials of Substrate

As a wearable biosensor for sports analysis, the attachability, and softness of the substrate are the main concerns for the sensors’ stretchability and have an extreme impact on the comfort of the biosensor and monitoring life span. Polymers such as Polydimethylsiloxane (PDMS), Polyimide (PI), and Polyethylene terephthalate (PET) are the organic materials that are widely applied in traditional biosensors to be a substratum for traditional sweat sensors due to their high inherent tensile strength, hydrophobic, non-toxic, and good workability^[2-4].

In quest of the excellent performance of substratum suitable for sports analysis, biopolymer nanofibrils, such as nanocellulose, chitin nanofibrils, and chitin nanowhiskers were utilized to reinforce the mechanical properties and nanostructures of the biomaterials with a drawback of the unaffordable cost (Fig. 1a and 1f). Natural silk is a soft material with a history of more than two thousand years, as an alternative, naturally occurring silk was isolated and used as the assembly element for substrate, featuring well pressure resistance, high strength, and elongation as well as thermomechanical stability, of which, the remarkable elongation was famous for thousands of years (Fig. 1b-1e). To obtain silk nanofibrils with the original nanostructure and mechanical properties, Cai et al. pretreat natural silk with calcium salt before mechanical disintegration treatment to weaken the force between nanofibrils without excessively damaging the crystallization area of silk^[5].

Fig. 1  (a) Young’s moduli of various substrate materials. The modulus can vary depending on the type of crosslinker, the fraction of polymer component, and curing temperature. The moduli of nanocomposites increase when nanomaterials are embedded in pure elastomer substrates^[16]. Copyright 2019, Royal Society of Chemistry. (b,c) Scanning electron microscopy images of the top surface of the chitosan-based films with different Silk nanofibrils/Chitosan ratios. Scale bars: 5 μm. (b-e) cross-section images of the films. Scale bars: 1 μm. (b,d) Pure chitosan, (c,e) Silk nanofibrils/Chitosan ratios = 1/2^[37]. Copyright 2021, Elsevier. (f) After electroless nickel plating, the whole acetate fabric was tightddly covered by a nickel layer^[34]. Copyright 2019, American Chemical Society.

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Hydrogel is another material with high strength and elongation that is suitable for wearable biosensors for sports analysis, as it is generally soft and elastic, while the internal structures are relatively durable because of the stable and interconnected frameworks. They are composed of flexible materials such as elastomer, polymer, carbohydrates, biocompatible molecules, and additives. Macroscopically, hydrogels are characterized by elasticity, viscoelasticity, and creep and relaxation, analogous to those of polymer materials such as rubber. Also, if an appropriate sensing medium like nanocrystal or graphene and carbon nanomaterials is successfully introduced, signal transduction would be accessible through the internal space of hydrogels, improving the biosensors’ performance, and even producing a benign environment for the operation of hyper-structured sensor systems^[6]. Recently, sophisticated hydrogel structures have been reported to demonstrate self-healing properties as well as enhanced mechanical^[7,8]. Self-healing materials with the ability to restore their electrical and mechanical properties after physical damage resolve the problems of other excellent properties materials as they can be scratched or broken apart by violent movement while sports analysis.

3.   Materials of Conductive Trace

To achieve real-time monitoring, wireless transmission equipment such as Bluetooth components or WiFi components was attached to the substrate and connected with detecting devices by conductive trace (Table 1). The conductive trace for common wearable sensors can be the patterns of graphite drawn on substrates^[9], carbon nanotube (CNT) mixed to polymer substrates^[10], and graphene with better electrical conductivity and mechanical properties compared with graphite^[11]. Metal with excellent electrical conductivity can be serpentined, weaved, or coiled to get flexible or stretchable structures (Fig. 2g and 2h)^[12,39]. Nanowires (NWs) or nanoparticles (NPs) of metal can often be used to prepare conductive ink^[13].

Table 1.  Performance parameters of some conductive traces.

Material	Repeatability (Cycles)	Electrical Conductivity (S cm−1)	Respons Time (ms)	Application	Reference
Multi-walled carbon nanotubes (MWCNTs)	104			Resistive strain sensor	[42]
MXene			50	Piezoresistive pressure sensor	[43]
Silver nanowires (AgNWs)	2.5 × 103	3.1	49	Resistive strain sensor	[44]
Gallium-indium alloy (EGaIn)	8 × 103	103	220	Strain sensor	[45]
Ag nanoparticles (AgNPs)	6.99	>104	83	Capacitive pressure sensor	[39]
Graphene (G)		103	33	Resistive strain sensor	[2]

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Fig. 2  (a-c) Liquid metal was plated on a silicon dioxide surface and embedded in PDMS as a conductive trace for flexible substrate and a liquid-metal-based stretchable pulse sensor, (a) crumpling deformation, (b) stretching conditions, (c) attached to finger^[18]. Copyright 2013, Li. (d-f) An enhanced liquid-metal-based microfluidic strain sensor, (f) bendability of the enhanced strain sensor^[36]. Copyright, 2020, American Chemical Society. (g,h) Schematic illustration and Cross-sectional scanning electron microscopy image of MXene and zinc-coated fibers and braided coaxial hybrid fiber supercapacitors. Ti₃C₂T_x MXene cathode as core electrodes and zinc fiber anode shell was braided on the surface of the Ti₃C₂T_x fibers across the solid electrolytes^[38]. Copyright 2022, Springer Nature. (i) Scanning electron microscopy image of a partially dried conductive ink showing the high-magnification microstructure of Ag aggregates. Ag aggregates surrounded by a layer of polymers (CMC and PAA) are embedded sporadically in the glycerol solvent. (j-l) The LED circuit with self-healing ability^[39]. Copyright 2021, American Chemical Society.

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These traces show high durability and sensitivity from external loading, however, they exhibited large hysteresis and a lack of stretchability. Large hysteresis can cause significant errors during the analytic processes, which leads to inaccurate results^[14]. For precisely identifying deterministic physiological status, it is important to reduce the hysteresis as same as ensuring the stretchability. In long-term detection, furthermore, plastic deformation of rigid conductive materials would lead to accumulated damage in the form of fatigue cracks over time, which increase the electrical resistivity, eventually generate open circuits, and ends the life span of the sensor^[15,16].

Thus, the conductive trace should be stretchable and fatigue-resistant to withstand repetitive strains due to the motion of sports. Combining with microfluidic technology, liquid metal achieves the fabrication of electrodes that are less than two micrometers by the selective-liquid metal plating (SLMP) process, which is mostly based on the selective wetting behavior of hydrochloric acid (HCl)-treated Galinstan (gallium-indium-tin liquid alloy) on various surfaces like metal patterns (Au/Cr pattern) and the polydimethylsiloxane (PDMS) substrate (Fig. 2a-2c)^[17,18]. A group of researchers proposed eutectic-gallium-indium (EGaIn) as a soft electrode material (Fig. 2d-2f) for flexible electronics for self-healing substrate^{[19-21, 36]}, as its high conductivity, deformability, and ability of self-healing (Fig. 2i-2l)^[22].

4.   Strain Sensing

So far, conventional sensors based on rigid conductive materials such as electrochemical sensing, optical sensing, and volumetric sensing were widely applied in wearable sensors as their inexpensive and high performance. As an important subfield of wearable electrics, strain sensors can translate mechanical deformations (compression, stretching, twisting, bending, etc.) into recordable signal (Fig. 3c-3e). Conventional strain sensing, however, was developed by incorporating functional materials into a stretchable support substrates^[16] and limited by the detecting range of conventional rigid strain sensors (below 5%)^[23] which was far below the biological skin strain (above 75%)^[24]. With the rapid development of flexible materials, high-performance wearable strain biosensors have become more numerous.

Fig. 3  (a) Schematic diagrams of a superabsorbent hydrogel of a wearable strain sensor for real-time sweat volume monitoring, which shows the swelling process of dry hydrogel to the final equilibrium from left to right. (b) Cross-section view of the structure of sweat glands, the wearable strain sensor is placed across the skin’s surface to absorb the sweat^[1]. Copyright 2020, Royal Society of Chemistry. (c) Picture of the spiderweb-like tactile sensor and the inset shows good flexibility. (d) Overall structure image of the sensing device. (e) Image of equivalent circuit of the sensor^[40]. Copyright 2021, American Chemical Society.

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Liquid-state-based strain sensors are the ideal platform for sports analytics since liquid possesses a high degree and momentary deformability and the ability to self-healing. Intrinsic self-healing can be easily achieved by simply rebuilding a connection with separated liquid pieces, therefore, they can undergo tensile elongation of over 200% without failure^[14]. A recent study applied microfluidics for fabricating flexible strain sensors, where conductive liquid was encapsulated into patterned elastomer microstructures^[25-26]. The deformation of the elastomer leads to a change in resistance of an active sensing component, so that, movements of the joint can be speculated and recorded.

Fiber strain sensors, as the convergence of textile platforms and strain sensing materials (conductive materials), have been applied to monitor various motions. Furthermore, combined with super-absorbent hydrogels, fiber strain sensors can be used for real-time sweat volume monitoring (R²=0.9827) (Fig. 3a and 3b)^[1], which overcomes the difficulty of directly obtaining real-time accurate readings of conventional approaches^[27,28]. Further combined with E-Textile, as a soft and textile sensor that can be incorporated into garment construction, can also recognize gestures and movements through different forms of fabric and stitching^[29-30].

5.   Unsolved challenges for future research

Although considerable research efforts and remarkable progress have been made in wearable biosensors for sports analytics, several difficulties remain in technological aspects. In addition, with the rapid rise of flexible electronic devices, the demands for superior performance wearable sensors are growing as well.

For the past few years, most wearable sensors particularly electrochemical sensors rely on reliable and continuous power supplies like coin cells or flexible batteries which have poor performance as the requirement of frequent recharging or large power source carried^[15]. To solve the problem, near-field communication (NFC) is utilized to be the power resource for battery-free systems but may suffer from the short operation distance. Radiofrequency Identification (RFID) platform is applied as well, as it works in Ultra High Frequency (UHF) band to enable a much longer read range than NFC and a better power autonomy than Bluetooth Low Energy (BLE), and it can be also used in a fully battery-less mode for real-time measurements, which allow tracking of sweat loss for up to 6 hours with a maximum error of 23% (Fig. 4b and 4c)^[41], the battery-less pH sensor can obtain a correlation value of R²=0.997^[31]. Recently, a triboelectric nanogenerator (TENG), which converts the mechanical energy created by human motion into electrical energy through triboelectric effects, was applied to drive wearable sensors in sports (Fig. 4d-4f)^[32]. However, there are barriers to the widespread adoption of TENGs that are inefficient power management, low power intensity, and lack of power longevity.

Fig. 4  (a) Schematic description of EMI shielding mechanism for SS@CNTs^[34]. Copyright 2019, American Chemical Society. (b) Photograph of a wearable wireless battery-free hybrid sensor system. (c) Image illustrating the device during sweating^[41]. Copyright 2019, American Association for the Advancement of Science. (d) Optical images of a flexible printed circuit board (FPCB)-based sensor. Scale bars: 4 cm. (e) Schematic illustrating the sensor that integrates human motion energy harvesting, microfluidic-based sweat biosensing, signal processing, and Bluetooth-based wireless data transmission to a mobile device interface for real-time health status tracking. (f) Schematic illustration of the charge distribution and working mechanism of the freestanding triboelectric nanogenerator (FTENG)^[32]. Copyright 2020, American Association for the Advancement of Science.

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The recent tendency of comprehensive evaluation has led to the advent of hybrid biosensors, which can monitor multiple parameters by simultaneously performing electrochemical, colorimetric, and volumetric sensing^[33]. Those sensors of several subsystems lead to the high-cost challenge as the difficulty of separating the reusable components from disposable ones and making the majority of wearable biosensors disposable. In addition, mutual interference between detected signals is still a huge challenge for a wearable hybrid sensor to realize monitoring simultaneously and sensitively. Therefore, materials with excellent sensing performance and high electromagnetic interference (EMI) shielding effectiveness were fabricated such as nickel-plated textile coated with low-loading Carbon Nanotubes Modified by Silk Sericin (SS@CNTs) (Fig. 4a)^[34].

Stress levels that are increased by unadapted wearables^[35] should also be mentioned, as high-stress levels may decrease sports performance. E-Textile can be a solution in the future, especially when clothes carry sensors as a substrate for sweat biosensors.

6.   Conclusions and perspectives

With its characteristics of in-situ, real-time, and non-invasive, wearable biosensors can continuously monitor the biochemistry and physiology status and collect and analyze various human indicators in real-time. Despite the remarkable progress achieved over the past few years, formidable challenges remain in data interpretation, power supply, and the cost of fabrication. However, advanced technology in other fields such as wireless power supply, cloud database, and 5G wireless communication technologies provide robust support to solve the problems above. With these challenges overcome, those wireless wearable biosensors of biocompatible and self-healing with strength and flexibility at once and can almost meet all the needs of human wearable devices through a watch, a cloth, or even a chip, which offers revolutionary capacities for future wearable technologies, along with the fundamental transformation of state-of-the-art athletic monitoring and sports analytics.