Sweat pH Research Articles

Dissolvable Polymer Valves for Sweat Chrono-Sampling in Wearable Paper-Based Analytical Devices.

Paper sensors with colorimetric signal transduction mechanisms are promising for developing single-use wearable patches that only require a smartphone to quantify signals. However, measuring biomarker fluctuations with colorimetric wearable sensors requires implementing a chrono-sampling method for performing sequential measurements. In this article, we report on a chrono-sampling method that enables the fabrication of wearable devices made entirely of filter paper. It consists of using dried polymers as closed valves that deflect the flow of liquids to different transducers of a multisensor. As time passes by, the polymer dissolves and the valve opens. The sequential opening of the valves results in a succession of measurements that reveals fluctuations in the concentration of the target analyte. This concept was demonstrated with a paper multisensor capable of performing nine consecutive pH measurements. The device was also adapted for developing a urea biosensor that detects pH measurements generated by the hydrolysis of the analyte catalyzed by urease. The proposed analytical platform could monitor the pH of sweat with an accuracy and precision comparable to a laboratory-based method when worn during an exercise routine. The results shown here pave the way for developing colorimetric wearable biosensors that measure variations in the concentration of biomarkers such as glucose, lactate, creatinine, or uric acid over time.

Highly Sensitive Physiological Sensor Based on Tapered Fiber-Optic Interferometer for Sweat Ph Detection

Highly Sensitive Physiological Sensor Based on Tapered Fiber-Optic Interferometer for Sweat Ph Detection

Synergistic Effect in a Graphene Quantum Dot-Enabled Luminescent Skinlike Copolymer for Long-Term pH Detection.

The alluring properties of a luminescent graphene quantum dot (GQD)-based nanocomposite are unquestionable to realize many advanced applications, such as sweat pH sensors. The well-suited hydrophilic polymers to host GQDs can face an unavoidable swelling behavior, which deteriorates the mechanical stability, whereas the hydrophobic polymers can prevent swelling but at the same time barricade the analyte pathways to GQDs. To resolve the two aforementioned obstacles, we develop a nanocomposite film containing nitrogen-doped GQDs (NGQDs) incorporated into a transparent, elastic, and self-healable polymer matrix, composed of a hydrophobic n-butyl acrylate segment and a hydrophilic N-(hydroxymethyl)acrylamide segment for wearable healthcare pH sensors on the human body. Besides serving as the fluorescence source, NGQDs are also designed as a nano-cross-linker to promote abundant chemical and physical interactions within the nanocomposite network. This synergetic effect gives rise to a 10-fold higher mechanical strength, 7-fold increment in Young's modulus, 4-fold increment in toughness, and 15-fold more sensitivity in pH detection (pH 3-10) compared to those of the pristine copolymer and NGQDs, respectively. Moreover, the mechanically enhanced nanocomposite possesses a high self-healing efficiency (94%) at room temperature even under water and demonstrates a stable sensing performance after repetitive usage for 30 days. Our work provides insights into the simple preparation of human skinlike nanocomposite elastomers usable for wearable pH sensors.

3D-printed low-cost fabrication and facile integration of flexible epidermal microfluidics platform

Skin-mounted microfluidics devices have shown distinct advantages in sweat collection and analysis platform due to their multiple sweat treatment including sample collection, flow control, storage, analysis and expelling by special design in microfluidic networks. However, the fabrication of most reported wearable microfluidic devices with mechanical flexibility and elasticity mainly rely on soft-lithography technology whose implementation is limited by necessitating access to resource-intensive laboratory and costly facilities. Herein, we propose an accessible, low-cost fabrication and integration scheme, which renders flexible microfluidic architecture capable of performing reliable sweat collection and sensing. We utilize low-cost 3D-printing technology to rapidly yield positive mold of facile printable material (malt syrup) for personalized customization of microfluidic channels. An ingenious approach is also proposed to encapsulate microfluidic channel layer and integrate pH sweat sensor into the microfluidic device for dependable sweat collection and pH analysis. The proposed fabrication strategy of flexible and stretchable epidermal microfluidic sensor represents a low cost, reachable alternative to recently prevailing soft-lithography method, and hence significantly reduces the threshold of microfluidic development and manufacture.

A Wearable Autonomous Colorimetric Sweat Induction System for Sweat Analysis.

The wearable biochemical sweat sensor's capability to provide insight into molecular information of health dynamics ignites sweat analysis as a promising noninvasive diagnosis scheme for precision medicine. Here, we demonstrate, for the first time, a colorimetric sweat induction microfluidic patch, which facilitates on-demand sweat glands activation by agonist coupled electrode and capillary action-based fluidics to collect microliter volumes (~5 μL) of sweat for monitoring its analytes by digital image analysis. The system's clinical utility demonstrated on a healthy volunteer for sweat pH monitoring flags the way towards other important sweat markers analysis for personalized healthcare.Clinical Relevance- Sweat analysis based on wearable technologies and its correlation with blood analytes pave the way towards non-invasive point-of-care monitoring, as an alternative to blood analysis.

Fabrication of a Wearable Flexible Sweat pH Sensor Based on SERS-Active Au/TPU Electrospun Nanofibers.

Development of wearable sensing platforms is essential for the advancement of continuous health monitoring and point-of-care testing. Eccrine sweat pH is an analyte that can be noninvasively measured and used to diagnose and aid in monitoring a wide range of physiological conditions. Surface-enhanced Raman scattering (SERS) offers a rapid, optical technique for fingerprinting of biomarkers present in sweat. In this paper, a mechanically flexible, nanofibrous, SERS-active substrate was fabricated by a combination of electrospinning of thermoplastic polyurethane (TPU) and Au sputter coating. This substrate was then investigated for suitability toward wearable sweat pH sensing after functionalization with two commonly used pH-responsive molecules, 4-mercaptobenzoic acid (4-MBA), and 4-mercaptopyridine (4-MPy). The developed SERS pH sensor was found to have good resolution (0.14 pH units for 4-MBA; 0.51 pH units for 4-MPy), with only 1 μL of sweat required for a measurement, and displayed no statistically significant difference in performance after 35 days (p = 0.361). Additionally, the Au/TPU nanofibrous SERS pH sensors showed fast sweat-absorbing ability as well as good repeatability and reversibility. The proposed methodology offers a facile route for the fabrication of SERS substrates which could also be used to measure a wide range of health biomarkers beyond sweat pH.

Lattice Proton Intercalation to Regulate WO3‐Based Solid‐Contact Wearable pH Sensor for Sweat Analysis

AbstractSweat pH monitoring is a routine indicator in wearable biotechnology. The state‐of‐the‐art wearable pH sensors mostly rely on organic materials but face the risk of biological toxicity. WO3 is a typical H+‐sensitive inorganic material with chemical stability, biocompatibility, and low cost but low sensitivity and slow response. Lattice H+ intercalation is herein proposed as an efficient approach that can greatly improve the sensitivity and selectivity of WO3‐based pH sensors. Specifically, lattice H+ intercalation can promote WO3 from the monoclinic phase to cubic phase, which enhances the ion exchange capacity between WO3 and H+. The resistance decreases more than two orders of magnitudes, which improves the interfacial charge transport. The occupancy of lattice H+ leads to ion exchange only with H+, thus increasing the H+ recognition. The intercalated HxWO3 exhibits much improved sensitivity, reversibility, and response time. Additionally, the HxWO3 is integrated with a solid reference electrode on a miniaturized chip for wearable sweat pH monitoring. The pH sensor exhibits good potential response even at curving over 270°. On‐body sweat pH measurments show high accuracy compared with ex situ analyses. This work emphasizes the concept of lattice proton intercalation to regulate the H+ recognition of solid contacts.

Multifunctional hybrid skin patch for wearable smart healthcare applications

Recent advances in wearable patches have included various sensors to monitor either physiological signs, such as the heart rate and respiration rate, or metabolites. Nevertheless, most of these have focused only on a single physiological measurement at a time, which significantly inhibits the calibration of various biological signals and diagnostic facilities. In this study, a novel multifunctional hybrid skin patch was developed for the electrochemical analysis of sweat glucose levels and simultaneous monitoring of electrocardiograms (ECGs). Furthermore, pH and temperature sensors were co-integrated onto the same patch for the calibration of the glucose biosensor to prevent inevitable inhibition and weakening of enzyme activity due to changes in the sweat pH and temperature levels. The fabricated electrochemical glucose biosensor exhibited excellent linearity (R2 = 0.9986) and sensitivity (29.10 μA mM−1 cm−2), covering the normal range of human sweat. The potentiometric pH sensor displayed a good response with an excellent sensitivity of −77.81 mV/pH and high linearity (R2 = 0.991), indicating that it can distinguish variations in the typical pH range for human sweat. Furthermore, the P, QRS complex, and T peaks in the measured ECG waveforms could be clearly distinguished, indicating the reliability of the fabricated flexible dry electrodes for continuous monitoring. The fabricated skin patch overcomes the inconvenience of the mandatory attachment of multiple patches on the human body by fully integrating all the electrochemical and electrophysiological sensors on a single patch, thus facilitating advanced glycemic control and continuous ECG monitoring for smart management of chronic diseases and healthcare applications.

Preparation of green and sustainable colorimetric cotton assay using natural anthocyanins for sweat sensing

Herein, we develop a novel smart cotton swab as a diagnostic assay for onsite monitoring of sweat pH changes toward potential applications in monitoring human healthcare and drug exam. Anthocyanin (Ac) can be extracted from Brassica oleracea var. capitata f. rubra using a simple procedure. Then, it can be used as a direct dye into cotton fibers using potash alum as mordant (M) to fix the anthocyanin dye onto the surface of the cotton fabric (Cot). This was monitored by generating mordant/anthocyanin nanoparticles (MAcNPs) onto the fabric surface. The cotton sensor assay demonstrated colorimetric changes in the ultraviolet-visible absorbance spectral analysis associated with a blueshift from 588 to 422 nm with increasing the pH of a perspiration simulant fluid. The biochromic performance of the dyed cotton diagnostic assay depended essentially on the halochromic activity of the anthocyanin spectroscopic probe to demonstrate a color change from pink to green due to intramolecular charge transfer occurring on the anthocyanin chromophore. After dyeing, no significant defects were detected in air-permeability and bend length. High colorfastness was investigated for the dyed cotton fabrics.

PANI-Based Wearable Electrochemical Sensor for pH Sweat Monitoring

Nowadays, we are assisting in the exceptional growth in research relating to the development of wearable devices for sweat analysis. Sweat is a biofluid that contains useful health information and allows a non-invasive, continuous and comfortable collection. For this reason, it is an excellent biofluid for the detection of different analytes. In this work, electrochemical sensors based on polyaniline thin films deposited on the flexible substrate polyethylene terephthalate coated with indium tin oxide were studied. Polyaniline thin films were abstained by the potentiostatic deposition technique, applying a potential of +2 V vs. SCE for 90 s. To improve the sensor performance, the electronic substrate was modified with reduced graphene oxide, obtained at a constant potential of −0.8 V vs. SCE for 200 s, and then polyaniline thin films were electrodeposited on top of the as-deposited substrate. All samples were characterized by XRD, SEM, EDS, static contact angle and FT-IR/ATR analysis to correlate the physical-chemical features with the performance of the sensors. The obtained electrodes were tested as pH sensors in the range from 2 to 8, showing good behavior, with a sensitivity of 62.3 mV/pH, very close to a Nernstian response, and a reproducibility of 3.8%. Interference tests, in the presence of competing ions, aimed to verify the selectivity, were also performed. Finally, a real sweat sample was collected, and the sweat pH was quantified with both the proposed sensor and a commercial pH meter, showing an excellent concordance.

Comparison of Synthetic Sweat and Influence of Sebum in the Permeation of Bioaccessible Metal(loid)s from Contaminated Soils through a Synthetic Skin Membrane.

Dermal exposure to metal(loid)s from contaminated soils has received less attention than oral and inhalation exposure. Still, it can be a relevant pathway for some contaminants. Comparison of synthetic sweats (donor solutions), the influence of sebum, and the characterization of diffusion parameters through a synthetic membrane (acting as skin surrogate) in the permeation of metal(loid)s (As, Cr, Cu, Ni, Pb, and, Zn) from polluted soils is missing. The dermal bioaccessibility tests were performed using two sweat compositions [EN 1811, pH 6.5 (sweat A) and NIHS 96-10, pH 4.7 (sweat B)]. Diffusion parameters of soluble metal(loid)s using the Franz cell methodology were calculated using the Strat-M membrane. The influence of synthetic sebum in the permeation of metal(loid)s was also investigated. The metal(loid) bioaccessibility percentage was higher for sweat B (pH 4.7) compared to sweat A (pH 6.5), attributed to lower pH of sweat B. Among the six elements tested, only chromium and copper permeated the membrane. Permeation coefficient (Kp) was higher for chromium in sweat A (0.05-0.11 cm h-1) than sweat B (0.0007-0.0037 cm h-1) likely due to a higher pH and thus more permeable Cr species. The presence of sebum increased lag times for copper permeation. Additional studies regarding speciation of metal(loid)s following extractions in synthetic sweat and comparison of synthetic membrane Strat-M and human skin in the permeation of metal(loid)s are recommended.

Wearable microneedle dual electrochemical sensor for simultaneous pH and cortisol detection in sweat

AbstractWe report herewith an inexpensive flexible dual target electrochemical sensor for simultaneous detection of pH and cortisol in human sweat. The sensor was fabricated by printing layer by layer (LbL) on a conductive microneedle polydimethylsiloxane (PDMS) flexible substrate. The dual sensor integrates two detection chambers comprising polyaniline (PANi) and cortisol imprinted poly (glycidylmethacrylate‐co ethylene glycol dimethacrylate) (poly (GMA‐co‐EGDMA)). The dual wearable sensor rapidly (< 1 min) responded linearly to pH in the range of 3–9, while the cortisol sensor chamber had a linear range of 0–100 ng/mL. The cortisol sensing region had an excellent limit of detection (LOD) of 1.4 ± 0.3 ng/mL, with intra‐batch reproducibility of 2.4% relative standard deviation (%RSD). The inter‐batch precision (%RSD for three different sensors) was determined to be 4.7%. Demonstrating excellent stability and reusability, a single patch of cortisol sensor was used for 15 times over a 30‐day period, with minimal change in response. The dual analyte wearable sensors were effective for detection of pH and cortisol in real human sweat.

Bacterial cellulose-based re-swellable hydrogel: Facile preparation and its potential application as colorimetric sensor of sweat pH and glucose

Direct deposition of the negatively charged polyelectrolyte, carboxymethyl cellulose (CMC), into a bacterial cellulose (BC) matrix was used as a simple route to fabricate a re-swellable and biocompatible cellulose-based hydrogel. As a result of this non-destructive approach, the physical and mechanical property of the original BC were well-preserved within the resulting BC/CMC hydrogel. As a BC/CMC-based colorimetric pH sensor, it exhibited a rapid response with an easy color differentiation between each pH by the naked eye, and wide linear range of pH 4.0–9.0 with good linearity. For the detection of glucose in sweat, the BC/CMC-based colorimetric glucose sensor provided a low limit of detection (25 μM) with a wide linear detection range (0.0–0.5 mM) and high accuracy. These BC/CMC based sensors could potentially be applied as non-invasive semi-quantitative sensors for on-skin health monitoring.

Textile sensors platform for the selective and simultaneous detection of chloride ion and pH in sweat

The development of wearable sensors, in particular fully-textile ones, is one of the most interesting open challenges in bioelectronics. Several and significant steps forward have been taken in the last decade in order to achieve a compact, lightweight, cost-effective, and easy to wear platform for healthcare and sport activities real-time monitoring. We have developed a fully textile, multi-thread biosensing platform that can detect different bioanalytes simultaneously without interference, and, as an example, we propose it for testing chloride ions (Cl−) concentration and pH level. The textile sensors are simple threads, based on natural and synthetic fibers, coated with the conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) and properly functionalized with either a nano-composite material or a chemical sensitive dye to obtain Cl− and pH selective sensing functionality, respectively. The single-thread sensors show excellent sensitivity, reproducibility, selectivity, long term stability and the ability to work with small volumes of solution. The performance of the developed textile devices is demonstrated both in buffer solution and in artificial human perspiration to perform on-demand and point-of-care epidermal fluids analysis. The possibility to easily knit or sew the thread sensors into fabrics opens up a new vision for a textile wearable multi-sensing platform achievable in the near future.

Wireless wearable wristband for continuous sweat pH monitoring

Several studies have shown that the determination of pH in sweat, which is one of the most accessible body fluids, can be an indicator of health and wellness, and even be used for potential disease diagnosis. On that basis, we present herein a wearable wristband for continuous and wireless monitoring of sweat pH with potential applications in the field of personal health assessment. The developed wristband consists of two main parts: a microfluidic cloth analytical device (μCAD) to collect continuously the sweat from skin with a color-based pH sensing area; and a readout and processing module with a digital color sensor to obtain the pH of sweat from the color changes in the μCAD. In addition, the readout module includes a low-power Bluetooth interface to transmit the measurements in real-time to a custom-designed smartphone application. To allow continuous operation, an absorbent pad was included in the design to retire and store thsweat from the sensing area through a passive pump path. It was found that the Hue parameter (H) in the HSV color space can be related to the sweat pH and fitted to a Boltzmann equation (R2 = 0.997). The range of use of the wristband device goes from 6 to 8, which includes the pH range of sweat, with a precision at different pH values from 3.6 to 6.0 %. Considering the typical human sweat rate, the absorbent pad allows continuous operation up to more than 1000 min.

Sodium polyacrylate microparticle containing multifunctional skin patch for sweat analysis

Recent advances in wearable sensor technologies have enormous potential for early diagnosis and personalized medicine. Although wearable sweat sensors have been designed in various approaches, including electronics and electrochemical methods, reliable and low-cost methods for monitoring sweat volume and pH remains a challenge. Here, sodium polyacrylate (SP) microparticle containing multifunctional skin patch was developed for local sweat volume and pH analysis. The wearable skin patch monitors local sweat volume and rate by wicking of sweat into SP hydrogel microparticles. In this approach, the circular geometry of the hydrogel microparticles measures sweat volume and, therefore, sweat rate. Also, this patch contains pH monitoring system which provides colorimetric response for sweat analysis. Moreover, an easy-to-use, smartphone integrated wearable sensor system for local sweat volume analysis is introduced. Multifunctional wearable skin patch showed rapid, reliable and predictable results under various sweat physiological conditions such as pH (pH 4, pH 6, pH 7 and pH 9), NaCI (30, 60 and 90 mM), KCI (5, 10 and 20 mM) and urea (10, 20 and 30 mM). The advantages of the proposed method are seen in simple, fast, and low-cost fabrication of multifunctional wearable sensors that accurately monitor local sweat volume.

Aggregation kinetics of diesel soot nanoparticles in artificial and human sweat solutions: Effects of sweat constituents, pH, and temperature

Soot nanoparticles (SNPs) are airborne contaminants that could potentially penetrate skin, but their aggregation after contact with sweat may lower their health risks. This study investigated SNP aggregation kinetics in 4 artificial sweat standards and 21 human sweat samples. Effects of sweat inorganic (NaCl, Na2HPO4, and NaH2PO4) and organic (L-histidine, lactic acid, and urea) constituents, pH, temperature, and concentrations were examined. Results showed that SNP aggregation rates in 4 standards followed American Association of Textile Chemists and Colorists (AATCC) > British Standard (EN) > International Standard Organization (ISO) pH 5.5 > ISO pH 8.0, and could be described by the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The aggregation rates increased with concentrations of SNPs, inorganic salts, L-histidine, and lactic acid, decreased with increasing pH and concentration of urea, and were weakly influenced by temperature. Systematic characterizations revealed SNP adsorption for organic sweat constituents. SNPs aggregated rapidly to ∼1000 nm in AATCC, but remained stable in ISO pH 8.0 and > 14/21 human sweat fluids over 20 min. The SNP aggregation rates correlated negatively with pH (r = -0.531*) and |ζ potential| (r = -0.464*) of human sweat samples. Sweat evaporation could promote aggregation of SNPs, hence lowering their potential harm via dermal exposure.

AI/ML-Enabled 2-D - RuS2 Nanomaterial-Based Multifunctional, Low Cost, Wearable Sensor Platform for Non-Invasive Point of Care Diagnostics

We report the first of its kind artificial intelligence/machine learning (AI/ML) enabled nanomaterial based multifunctional sensing platform for simultaneous and continuous monitoring of certain vital body parameters viz. the hydration levels of the skin, glucose concentration and pH levels in biofluid sweat with high accuracy and speed. RuS 2 nanoparticles were synthesized using a facile hydrothermal method and detailed characterization revealed cubic crystal structure of laurite-RuS 2 with mesoporous morphology that provided enhanced electrocatalytic sites for sensing. The biochemical sensor was fabricated using layer-by-layer spincoating technique of RuS 2 on an interdigitated PDMS substrate. Further, to facilitate human-machine interface that can analyze data from large sample sizes (~ 50 sensors), the sensor was interfaced with the open-source microcontroller board (QueSSence) wherein Artificial Intelligence (AI) based K-Nearest Neighbors (KNN) algorithm enabled precise and faster data acquisition from complex mathematical conjunctures. The response of the sensor towards hydration levels for various skin conditions, glucose concentration, and pH of sweat were examined for both artificial skin and human skin. The RuS 2 based sweat-glucose sensor exhibited a sensitivity of 87.9 ± 0.6 μM -1 cm -2 in a physiologically relevant range of 10 nM - 0.1 mM and limit of detection of 4.87 nM. The pH sensor exhibited a sensitivity of 71.2 ± 0.5 pH -1 cm -2 across the pH range of 4 -8.5. The multifunctional sensor displayed high stability and reusability at room temperature. The optimized response was integrated with a smartphone via a customized application enabling user-friendly, real-time monitoring of the health conditions.

A handheld device to monitor physiological changes using sweat

Sweat is a viscous, multi-parameter fluid that can be used to gain information on various functions of the body. The focus of this paper is to develop a prototype for a hand held that considers factors such as temperature, pH, and conductivity of sweat to arrive at a conclusion about the current physiological condition of the user. Sweat is metabolite rich so it contains salts such as Sodium, chloride and potassium. The concentration of each metabolite varies the general conductivity of sweat. The prototype will contain a temperature Detector, pH sensor and Galvanic Response sensor. Any abnormal change in temperature, pH and the conductivity of sweat indicates an underlying problem. Further the sensor is capable of detecting fluctuation in sodium ion concentration by measuring the changes in the voltage response. The device will use machine learning and IOT to come to a conclusion whether the user is in immediate danger or not and constantly display readings when the sensors come in contact with sweat.

Pencil–paper on-skin electronics

Pencils and papers are ubiquitous in our society and have been widely used for writing and drawing, because they are easy to use, low-cost, widely accessible, and disposable. However, their applications in emerging skin-interfaced health monitoring and interventions are still not well explored. Herein, we report a variety of pencil-paper-based on-skin electronic devices, including biophysical (temperature, biopotential) sensors, sweat biochemical (pH, uric acid, glucose) sensors, thermal stimulators, and humidity energy harvesters. Among these devices, pencil-drawn graphite patterns (or combined with other compounds) serve as conductive traces and sensing electrodes, and office-copy papers work as flexible supporting substrates. The enabled devices can perform real-time, continuous, and high-fidelity monitoring of a range of vital biophysical and biochemical signals from human bodies, including skin temperatures, electrocardiograms, electromyograms, alpha, beta, and theta rhythms, instantaneous heart rates, respiratory rates, and sweat pH, uric acid, and glucose, as well as deliver programmed thermal stimulations. Notably, the qualities of recorded signals are comparable to those measured with conventional methods. Moreover, humidity energy harvesters are prepared by creating a gradient distribution of oxygen-containing groups on office-copy papers between pencil-drawn electrodes. One single-unit device (0.87 cm2) can generate a sustained voltage of up to 480 mV for over 2 h from ambient humidity. Furthermore, a self-powered on-skin iontophoretic transdermal drug-delivery system is developed as an on-skin chemical intervention example. In addition, pencil-paper-based antennas, two-dimensional (2D) and three-dimensional (3D) circuits with light-emitting diodes (LEDs) and batteries, reconfigurable assembly and biodegradable electronics (based on water-soluble papers) are explored.