Advances in Non-Electrochemical Sensing of Human Sweat Biomarkers: From Sweat Sampling to Signal Reading

Sweat, commonly referred to as the ultrafiltrate of blood plasma, is an essential physiological fluid in the human body. It contains a wide range of metabolites, electrolytes, and other biologically significant markers that are closely linked to human health. Compared to other bodily fluids, such as blood, sweat offers distinct advantages in terms of ease of collection and non-invasive detection. In recent years, considerable attention has been focused on wearable sweat sensors due to their potential for continuous monitoring of biomarkers. Electrochemical methods have been extensively used for in situ sweat biomarker analysis, as thoroughly reviewed by various researchers. This comprehensive review aims to provide an overview of recent advances in non-electrochemical methods for analyzing sweat, including colorimetric methods, fluorescence techniques, surface-enhanced Raman spectroscopy, and more. The review covers multiple aspects of non-electrochemical sweat analysis, encompassing sweat sampling methodologies, detection techniques, signal processing, and diverse applications. Furthermore, it highlights the current bottlenecks and challenges faced by non-electrochemical sensors, such as limitations and interference issues. Finally, the review concludes by offering insights into the prospects for non-electrochemical sensing technologies. By providing a valuable reference and inspiring researchers engaged in the field of sweat sensor development, this paper aspires to foster the creation of innovative and practical advancements in this domain.

Research on sweat metabolomics of athlete's fatigue induced by high intensity interval training

Objective: Sweat is an important specimen of human metabolism, which can simply and non-invasively monitor the metabolic state of the body, and its metabolites can be used as biomarkers for disease diagnosis, while the changes of sweat metabolites before and after exercise-induced fatigue are still unclear. Methods: In this experiment, high-performance chemical isotope labeling liquid chromatography-mass spectrometry (LC-MS) was used to metabolomic 28 sweat samples before and after exercise-induced fatigue of 14 long-distance runners, also IsoMS PRO and SPSS22.0 software were used to analyze the metabolite changes and differential metabolic pathways. Results: A total of 446 metabolites with high confidence were identified, and the sweat metabolome group before and after high-intensity interval exercise-induced fatigue was obvious, among which the upregulated differential metabolites mainly included hypoxanthine, pyruvate, several amino acids, etc., while the downregulated differential metabolites mainly included amino acid derivatives, vitamin B6, theophylline, etc. Conclusion: The change of hypoxanthine concentration in sweat can be used as a good biomarker for the diagnosis of exercise-induced fatigue, while the change of pyruvate content in sweat can be used as a discriminant index for the energy metabolism mode of the body before and after exercise. The main metabolic pathways involved in differential metabolites produced before and after HIIT exercise-induced fatigue are purine metabolism and amino acid metabolism.

Bilayer Actuator Film for Urea Biosensing with Dual Responsiveness: Bending Actuation and Photonic Color Change

A noninvasive sweat-based biosensor was developed for urea detection using a photonic bilayer actuator film (BAF) consisting of an interpenetrating polymer network (IPN) as the active layer and a flexible poly(ethylene terephthalate) (PET) substrate as the passive layer (IPN/PET). The active IPN layer comprises intertwined solid-state cholesteric liquid crystal and poly(acrylic acid) (PAA) networks. Urease was immobilized in the PAA network in the IPN layer of the photonic BAF. The interaction with aqueous urea altered the curvature and photonic color of the photonic urease-immobilized IPN/PET (IPNurease/PET) BAF. The curvature (and wavelength of the photonic color) of the IPNurease/PET BAF increased linearly with urea concentration (Curea) in the range of Curea = 20-65 (and 30-65) mM with a limit of detection value of 1.42 (and 1.34) mM. The developed photonic IPNurease/PET BAF exhibited high selectivity toward urea and excellent spike test results with real human sweat. This novel IPNurease/PET BAF is promising because it enables battery-free, cost-effective, and visual detection-based analysis without the use of sophisticated instruments. Furthermore, the application of this photonic IPN/PET BAF can be easily extended to other biosensors by immobilizing other receptors on the IPN.

MicroSweat: A Wearable Microfluidic Patch for Noninvasive and Reliable Sweat Collection Enables Human Stress Monitoring

Stress affects cognition, behavior, and physiology, leading to lasting physical and mental illness. The ability to detect and measure stress, however, is poor. Increased circulating cortisol during stress is mirrored by cortisol release from sweat glands, providing an opportunity to use it as an external biomarker for monitoring internal emotional state. Despite the attempts at using wearable sensors for monitoring sweat cortisol, there is a lack of reliable wearable sweat collection devices that preserve the concentration and integrity of sweat biomolecules corresponding to stress levels. Here, a flexible, self-powered, evaporation-free, bubble-free, surfactant-free, and scalable capillary microfluidic device, MicroSweat, is fabricated to reliably collect human sweat from different body locations. Cortisol levels are detected corresponding to severe stress ranging from 25 to 125 ng mL^-1 averaged across multiple body regions and 100-1000 ng mL^-1 from the axilla. A positive nonlinear correlation exists between cortisol concentration and stress levels quantified using the perceived stress scale (PSS). Moreover, owing to the sweat variation in response to environmental effects and physiological differences, the longitudinal and personalized profile of sweat cortisol is acquired, for the first time, for various body locations. The obtained sweat cortisol data is crucial for analyzing human stress in personalized and clinical healthcare sectors.

Palladium Hydroxide (Pearlman's Catalyst) Doped MXene (Ti3C2Tx) Composite Modified Electrode for Selective Detection of Nicotine in Human Sweat

High concentrations of nicotine (40 to 60 mg) are more dangerous for adults who weigh about 70 kg. Herein, we developed an electrochemical transducer using an MXene (Ti₃C₂Tx)/palladium hydroxide-supported carbon (Pearlman's catalyst) composite (MXene/Pd(OH)₂/C) for the identification of nicotine levels in human sweat. Firstly, the MXene was doped with Pd(OH)₂/C (PHC) by mechanical grinding followed by an ultrasonication process to obtain the MXene/PHC composite. Secondly, XRD, Raman, FE-SEM, EDS and E-mapping analysis were utilized to confirm the successful formation of MXene/PHC composite. Using MXene/PHC composite dispersion, an MXene/PHC composite-modified glassy carbon electrode (MXene/PHC/GCE) was prepared, which showed high sensitivity as well as selectivity towards nicotine (300 µM NIC) oxidation in 0.1 M phosphate buffer (pH = 7.4) by cyclic voltammetry (CV) and amperometry. The MXene/PHC/GCE had reduced the over potential of nicotine oxidation (about 200 mV) and also enhanced the oxidation peak current (8.9 µA) compared to bare/GCE (2.1 µA) and MXene/GCE (5.5 µA). Moreover, the optimized experimental condition was used for the quantification of NIC from 0.25 µM to 37.5 µM. The limit of detection (LOD) and sensitivity were 27 nM and 0.286 µA µM^-1 cm², respectively. The MXene/PHC/GCE was also tested in the presence of Na⁺, Mg²⁺, Ca²⁺, hydrogen peroxide, acetic acid, ascorbic acid, dopamine and glucose. These molecules were not interfered during NIC analysis, which indicated the good selectivity of the MXene/PHC/GCE sensor. In addition, electrochemical determination of NIC was successfully carried out in the human sweat samples collected from a tobacco smoker. The recovery percentage of NIC in the sweat sample was 97%. Finally, we concluded that the MXene/PHC composite-based sensor can be prepared for the accurate determination of NIC with high sensitivity, selectivity and stability in human sweat samples.

Self-Testing of Ketone Bodies, along with Glucose, Using Touch-Based Sweat Analysis

β-Hydroxybutyrate (HB) is one of the main physiological ketone bodies that play key roles in human health and wellness. Besides their important role in diabetes ketoacidosis, ketone bodies are currently receiving tremendous attention for personal nutrition in connection to the growing popularity of oral ketone supplements. Accordingly, there are urgent needs for developing a rapid, simple, and low-cost device for frequent onsite measurements of β-hydroxybutyrate (HB), one of the main physiological ketone bodies. However, real-time profiling of dynamically changing HB concentrations is challenging and still limited to laboratory settings or to painful and invasive measurements (e.g., a commercial blood ketone meter). Herein, we address the critical need for pain-free frequent HB measurements in decentralized settings and report on a reliable noninvasive, simple, and rapid touch-based sweat HB testing and on its ability to track dynamic HB changes in secreted fingertip sweat, following the intake of commercial ketone supplements. The new touch-based HB detection method relies on an instantaneous collection of the fingertip sweat at rest on a porous poly(vinyl alcohol) (PVA) hydrogel that transports the sweat to a biocatalytic layer, composed of the β-hydroxybutyrate dehydrogenase (HBD) enzyme and its nicotinamide adenine dinucleotide (NAD⁺) cofactor, covering the modified screen-printed carbon working electrode. As a result, the sweat HB can be measured rapidly by the mediated oxidation reaction of the nicotinamide adenine dinucleotide (NADH) product. A personalized HB dose-response relationship is demonstrated within a group of healthy human subjects taking commercial ketone supplements, along with a correlation between the sweat and capillary blood HB levels. Furthermore, a dual disposable biosensing device, consisting of neighboring ketone and glucose enzyme electrodes on a single-strip substrate, has been developed toward the simultaneous touch-based detection of dynamically changing sweat HB and glucose levels, following the intake of ketone and glucose drinks.

Artificial Human Sweat as a Novel Growth Condition for Clinically Relevant Pathogens on Hospital Surfaces

The emergence of biofilms on dry hospital surfaces has led to the development of numerous models designed to challenge the efficacious properties of common antimicrobial agents used in cleaning. This is in spite of limited research defining how dry surfaces are able to facilitate biofilm growth and formation in such desiccating and nutrient-deprived environments. While it is well established that the phenotypical response of biofilms is dependent on the conditions in which they are formed, most models incorporate a nutrient-enriched, hydrated environment dissimilar to the clinical setting. In this study, we piloted a novel culture medium, artificial human sweat (AHS), which is perceived to be more indicative of the nutrient sources available on hospital surfaces, particularly those in close proximity to patients. AHS was capable of sustaining the proliferation of four clinically relevant multidrug-resistant pathogens (Acinetobacter baumannii, Staphylococcus aureus, Enterococcus faecalis, and Pseudomonas aeruginosa) and achieved biofilm formation at concentration levels equivalent to those found in situ (average, 6.00 log₁₀ CFU/cm²) with similar visual characteristics upon microscopy. The AHS model presented here could be used for downstream applications, including efficacy testing of hospital cleaning products, due to its resemblance to clinical biofilms on dry surfaces. This may contribute to a better understanding of the true impact these products have on surface hygiene. IMPORTANCE Precise modeling of dry surface biofilms in hospitals is critical for understanding their role in hospital-acquired infection transmission and surface contamination. Using a representative culture condition which includes a nutrient source is key to developing a phenotypically accurate biofilm community. This will enable accurate laboratory testing of cleaning products and their efficacy against dry surface biofilms.

Wearable Electronic Tongue for Non-Invasive Assessment of Human Sweat

Sweat is a promising biofluid in allowing for non-invasive sampling. Here, we investigate the use of a voltammetric electronic tongue, combining different metal electrodes, for the purpose of non-invasive sample assessment, specifically focusing on sweat. A wearable electronic tongue is presented by incorporating metal electrodes on a flexible circuit board and used to non-invasively monitor sweat on the body. The data obtained from the measurements were treated by multivariate data processing. Using principal component analysis to analyze the data collected by the wearable electronic tongue enabled differentiation of sweat samples of different chemical composition, and when combined with ¹H-NMR sample differentiation could be attributed to changing analyte concentrations.

Aedes aegypti Mosquitoes Detect Acidic Volatiles Found in Human Odor Using the IR8a Pathway

Mosquitoes use olfaction as a primary means of detecting their hosts. Previously, the functional ablation of a family of Aedes aegypti olfactory receptors, the odorant receptors (ORs), was not sufficient to reduce host seeking in the presence of carbon dioxide (CO₂). This suggests the olfactory receptors that remain, such as the ionotropic receptors (IRs), could play a significant role in host detection. To test this, we disrupted the Ir8a co-receptor in Ae. aegypti using CRISPR/Cas9. We found that Ir8a mutant female mosquitoes are not attracted to lactic acid, a behaviorally active component of human sweat, and they lack odor-evoked responses to acidic volatiles. The loss of Ir8a reduces mosquito attraction to humans and their odor. We show that the CO₂-detection pathway is necessary but not sufficient for IR8a to detect human odor. Our study reveals that the IR8a pathway is crucial for an anthropophilic vector mosquito to effectively seek hosts. VIDEO ABSTRACT.

An improved experimental methodology to evaluate the effectiveness of protective gloves against nanoparticles in suspension

Recent studies underline the potential health risks associated to the "nano" revolution, particularly for the workers who handle engineered nanoparticles (ENPs) that can be found in the formulation of several commercial products. Although many Health & Safety agencies recommend the use of protective gloves against chemicals, few studies have investigated the effectiveness of these gloves towards nanoparticle suspensions. Moreover, the data that are available are often contradictory. This study was designed to evaluate the effectiveness of protective gloves against nanoparticles in suspension. For this purpose, a new methodology was developed in order to take into account parameters encountered in the workplace such as mechanical deformations (MD) that simulate hand flexion and sweat. The effects of the precise experimental protocol on the concentrations of nanoparticles that were detected in the sampling suspension were assessed. Several samples of nitrile rubber gloves (73 µm thick), taken from different boxes, were brought into contact with gold nanoparticles (5 nm) in water. During their exposure to ENPs, the glove samples submitted systematic mechanical deformations and were placed in contact with a physiological solution simulating human sweat. Under these conditions, results obtained by inductively coupled plasma mass spectrometry (ICPMS) showed that the 5 nm gold nanoparticles passed through the protective gloves. This result was acquired, in spite of the observation of significant losses during the sampling phase that will be important for future experiments evaluating the effectiveness of these materials.

Behavioral response of the malaria mosquito, Anopheles gambiae, to human sweat inoculated with axilla bacteria and to volatiles composing human axillary odor

The responses of Anopheles gambiae Giles sensu stricto (Diptera: Culicidae) to odors from male and female axillary sweat incubated with human axilla bacteria were recorded in a dual-choice olfactometer. Staphylococcus epidermidis was selected for its low odor-producing pattern, Corynebacterium jeikeium for its strong Nα-acylglutamine aminoacylase activity liberating carboxylic acids including (R)/(S)-3-hydroxy-3-methylhexanoic acid (HMHA) and Staphylococcus haemolyticus for its capacity to liberate sulfur-containing compounds including (R/S)-3-methyl-3-sulfanylhexan-1-ol (MSH). Anopheles gambiae behavioral responses were evaluated under (i) its responsiveness to take off and undertake sustained upwind flight and (ii) its discriminating capacity between the two olfactometer arms bearing a test odor in either one or both arms. Experiments were conducted in the presence of carbon dioxide pulses as a behavioral sensitizer. Anopheles gambiae clearly discriminated for the olfactometer arm conveying odor generated by incubating any of the three bacteria species with either male or female sweat. Whereas An. gambiae did not discriminate between male and female sterile sweat samples in the olfactometer, the mosquito consistently showed a preference for male sweat over female sweat incubated with the same bacterium, independent of the species used as inoculum. Sweat incubated with C. jeikeium rendered mosquitoes particularly responsive and this substrate elicited the strongest preference for male over female sweat. Tested on their own, neither HMHA nor MSH elicited a clear discriminating response but did affect mosquito responsiveness. These findings serve as a basis for further research on the odor-mediated anthropophilic host-seeking behavior of An. gambiae.