Sweat pH Research Articles

Donor–π bridge-acceptor dyes featuring dual chromophoric groups for enhanced sensitivity in wearable sweat sensor

The pH value of human sweat serves as a crucial biomarker for disease diagnosis, rendering wearable sweat sensors an innovative tool for monitoring human health. However, the commercialization of these devices is seriously impeded by their low sensitivity, poor wearing comfort, and limited reusability. In this work, a reactive dye capable of color change under pH change has been synthesized. Due to the unique molecular structure, the dye molecule exhibits remarkable color change recognition and high sensitivity. To fabricate a wearable pH sensor, screen printing technology was employed for depositing the color-changing dye onto cotton fabric. The resulting sensor demonstrated excellent performance attributes including high color sensitivity, robust cyclic stability in response to pH changes, and exceptional resistance against washing, friction, and sunlight-induced fading. Therefore, this study presents a highly promising approach for achieving real-time monitoring of human sweat pH levels.

Hybrid microstructure-based stretchable biosensors for multi-physiological signal sensing

Wearable biosensors provide continuous, real-time physiological monitoring of biochemical markers in biofluids such as sweat, tears, saliva, and interstitial fluid. However, achieving high stretchability and stable biochemical signal monitoring remains challenging. Here, we propose a hybrid microstructure (HMS) strategy to fabricate highly stretchable multifunctional biosensors capable of detecting sweat electrolyte concentrations, pH levels, and surface electromyography (EMG) signals. By integrating a HMS, stable conductivity under large strains is ensured. Stretching tests up to 5000 cycles demonstrated the electrodes’ stretchable stability and reliability. The high-performance electrodes were used for EMG monitoring on human skin. Additionally, active materials were coated onto the stretchable electrodes to create multifunctional sweat sensors capable of monitoring pH as well as calcium, sodium, and potassium ions (Ca2+, Na+, K+). The electrodes reliably maintained their functionality under 60% strain, providing new insights into the fabrication of stable, highly stretchable biosensors.

Development of a 3D Hydrogel SERS Chip for Noninvasive, Real-Time pH and Glucose Monitoring in Sweat.

Traditional diagnostic methods, such as blood tests, are invasive and time-consuming, while sweat biomarkers offer a rapid physiological assessment. Surface-enhanced Raman spectroscopy (SERS) has garnered significant attention in sweat analysis because of its high sensitivity, label-free nature, and nondestructive properties. However, challenges related to substrate reproducibility and interference from the biological matrix persist with SERS. This study developed a novel ratio-based 3D hydrogel SERS chip, providing a noninvasive solution for real-time monitoring of pH and glucose levels in sweat. Encapsulating the probe molecule (4-MBN) in nanoscale gaps to form gold nanoflower-like nanotags with internal standards and integrating them into an agarose hydrogel to create a 3D flexible SERS substrate significantly enhances the reproducibility and stability of sweat analysis. Gap-Au nanopetals modified with probe molecules are uniformly dispersed throughout the porous hydrogel structure, facilitating the effective detection of the pH and glucose in sweat. The 3D hydrogel SERS chip demonstrates a strong linear relationship in pH and glucose detection, with a pH response range of 5.5-8.0 and a glucose detection range of 0.01-5 mM, with R2 values of 0.9973 and 0.9923, respectively. In actual sweat samples, the maximum error in pH detection accuracy is only 1.13%, with a lower glucose detection limit of 0.25 mM. This study suggests that the ratio-based 3D hydrogel SERS chip provides convenient, reliable, and rapid detection capabilities with substantial application potential for analyzing body fluid pH and glucose.

A microfluidic patch for wireless wearable electrochemical detection of sweat metabolites

Wearable sweat sensors provide an innovative detection method for analyzing human physiological and biochemical parameters and play an important role in detecting the potential risks of chronic diseases. Existing commercial wearable sensors (Apple iWatch, HUAWEI smart band) have problems such as single detection index (heart rate or blood pressure), and mainly focus on the physical information. This work studied an integrated microfluidic patch to simultaneously detect several biomarkers (pH, sodium ions, and uric acid) in sweat. We achieved the detection of pH level, sodium, and uric acid in linear ranges of 4–8, 0–160 mM, and 5–160 μM, respectively, covering well the clinically relevant ranges in the healthy human sweat. Laser engraving was chosen to prepare a microfluidic layer, and the sweat collection function was realized by combining hydrophilic and hydrophobic microfluidic synergy. It could efficiently collect the required sweat sample volume (80 μL) within 30 s while avoiding the contamination caused by directly contact with the skin. In addition, a wireless signal acquisition and transmission system was designed with the signal reading, processing, and wireless transmission functions, which can display the testing results in real time on Android phone-specific application software. We combined the wireless system with a microfluidic patch for on-body testing to verify the good wearability of the sweat sensor. This research realized the wireless data processing and transmission while improving sweat collection efficiency. In addition, it could detect various sweat physiological components in real-time, and provides a multiparameter, real-time, and portable sensing platform.

Epidermal secretion-purified biosensing patch with hydrogel sebum filtering membrane and unidirectional flow microfluidic channels

The development of biosensing electronics for real-time sweat analysis has attracted increasing research interest due to their promising applications for non-invasive health monitoring. However, one of the critical challenges lies in the sebum interference that largely limits the sensing reliability in practical scenarios. Herein, we report a flexible epidermal secretion-purified biosensing patch with a hydrogel filtering membrane that can effectively eliminate the impact of sebum and sebum-soluble substances. The as-prepared sebum filtering membranes feature a dual-layer sebum-resistant structure based on the poly(hydroxyethyl methacrylate) hydrogel functionalized with nano-brush structured poly(sulfobetaine) to eliminate interferences and provide self-cleaning capability. Furthermore, the unidirectional flow microfluidic channels design based on the Tesla valve was incorporated into the biosensing patch to prevent external sebum contamination and allow effective sweat refreshing for reliable sensing. By seamlessly combining these components, the epidermal secretion-purified biosensing patch enables continuous monitoring of sweat uric acid, pH, and sodium ions with significantly improved accuracy of up to 12 %. The proposed strategy for enhanced sweat sensing reliability without sebum interference shows desirable compatibility for different types of biosensors and would inspire the advances of flexible and wearable devices for non-invasive healthcare.

Skin-attachable Tb-MOF ratio fluorescent sensor for real-time detection of human sweat pH

The pH of human sweat is highly related with a variety of diseases, whereas the monitoring of sweat pH still remains challenging for ordinary families. In this study, we developed a novel dual-emission Tb-MOF using DPA as the ligand and further designed and constructed a skin-attachable Tb-MOF ratio fluorescent sensor for real-time detection of human sweat pH. With the increased concentration of H+, the interaction of H+ with carbonyl organic ligand leads to the collapse of the Tb-MOF crystal structure, resulting in the interruption of antenna effect, and correspondingly increasing the emission of the ligand at 380 nm and decreasing the emission of the central ion Tb3+ at 544 nm. This Tb-MOF nanoprobe has a good linear response in the pH range of 4.12–7.05 (R2 = 0.9914) with excellent anti-interference ability. Based on the merits of fast pH response and high sensitivity, the nanoprobe was further used to prepare flexible wearable sensor. The wearable sensor can detect pH in the linear range of 3.50–6.70, which covers the pH range of normal human sweat (4.50–6.50). Subsequently, the storage stability and detection accuracy of the sensors were evaluated. Finally, the sensor has been successfully applied for the detection of pH in actual sweat samples from 21 volunteer and the real-time monitoring of pH variation during movement processing. This skin-attachable Tb-MOF sensor, with the advantages of low cost, visible color change and long shelf-life, is appealing for sweat pH monitoring especially for ordinary families.

Advanced Functional Photochromic Wearables with Superior Durability and Stability for Sustainable Applications

AbstractThe rewritable wearables based on photochromism have emerged as attractive candidates in inkless printing applications. Among various photochromic materials, polyoxometalates have great potential for rewritable wearables, with major advantages in fast response upon UV irradiation, long‐term photochemical stability, and excellent fatigue resistance. However, the development of rewritable wearables based on polyoxometalates is limited by low combining fastness, weak stability, and poor scalability. Here, a scalable strategy is reported to fabricate an ideal rewritable wearable based on fundamental charge balance mechanism. The pristine cotton substrate is grafted by cationic polymer brushes to incorporate photochromic phosphomolybdic acid (PMoA) anions through electrostatic attraction, and then the cationic surfactants with alkyl chains are introduced to encapsulate the PMoA anions to achieve charge balance subsequently. The resultant rewritable wearables display the long‐awaited properties, such as high color contrast, favorable reversibility (>10 cycles), long color retention (>15 days) and high stability against detergent and sweat (pH 6.5–8.0) during repeated washing (50 cycles) and wearing. As a demonstration, a rewritable T‐shirt is scalably fabricated, and excellent antibacterial activity and biocompatibility are demonstrated as well, which is expected to be a sustainable solution for regular fabric printing in household products and public display.

Sweat-enhanced adhesive hydrogel enables interfacial exchange coupling for wearable strain sensor

PVA-based wearable hydrogels have emerged as promising candidate for motion sensors due to their skin-like softness and unique biocompatibility. However, an open question is whether and how the strain-sensitive hydrogel designing can prevent adhesive performance loss caused by sweat between devices and human-skin and reduce interfacial failure. Herein, a versatile sweat-enhanced adhesive hydrogel-based motion sensor (VDBA) paradigm is engineered consisting of sweat sensitivity and self-adhesive function components. Taking advantage of catechol chemistry inspired mussel, the VDBA hydrogel enhances substrate adhesion behavior. The borate ions (B(OH)4−) are found to efficient condensation form borate ester groups with phenolic hydroxyl. Interestingly, human sweat pH serves as a trigger for borate ester bonds protection-deprotection, whereby partial catechol primarily functions in non-sweating states and significantly activates during sweating. The VDBA hydrogel achieves a maximum adhesive strength of 13.45 kPa on skin at non-sweat, however, the maximum bonding strength increases to 24.0 kPa when sweating. The resultant VDBA hydrogel exhibits integration of exceptional mechanical properties and anti-freezing properties. Also, desirable conductivity appears in the hydrogel, allowing outputting accurate and repeatable signals for body motion sensing in various scenarios. Furthermore, VDBA hydrogel is demonstrated for use as a sports training times counter, which can achieve sensitive statistics for stretching and bending movements. This work provides a great hydrogel candidate for various motion sensor applications, particularly beneficial for people with sweaty skin and athletes.

Dual-Function Wearable Hydrogel Optical Fiber for Monitoring Posture and Sweat pH.

In recent years, wearable devices have been widely used for human health monitoring. Such monitoring predominantly relies on the principles of optics and electronics. However, electronic detection is susceptible to electromagnetic interference, and traditional optical fiber detection is limited in functionality and unable to simultaneously detect both physical and chemical signals. Hence, a wearable, embedded asymmetric color-blocked optical fiber sensor based on a hydrogel has been developed. Its sensing principle is grounded in the total internal reflection within the optical fiber. The method for posture sensing involves changes in the light path due to fiber bending with color blocks providing wavelength-selective modulation by absorption changes. Sweat pH sensing is facilitated by variations in fluorescence intensity triggered by sweat-induced conformational changes in Rhodamine B. With just one fiber, it achieves both physical and chemical signal detection. Fabricated using a molding technique, this fiber boasts excellent biocompatibility and can accurately discern single and multiple bending points, with a recognition range of 0-90° for a single segment, a detection limit of 0.02 mm-1 and a sweat pH sensing linear regression R2 of 0.993, alongside great light propagation properties (-0.6 dB·cm-1). With its extensive capabilities, it holds promise for applications in medical monitoring.

Preparation of Conductive Cellulose Coated with Conductive Polymer and Its Application in the Detection of pH and Characteristic Substances in Sweat.

Rich biological information in sweat provides great potential for health monitoring and management. However, due to the complexity of sweat, the development of environmentally friendly green electronic products is of great significance to the construction of ecological civilization. This study utilized a simple combination of polystyrene sulfonate sodium (PSS) and filter paper (FP) to prepare cellulose materials coated with conductive polymers, developing an electrochemical sensor based on the modified materials. The mechanical and electrochemical properties of the fabricated PSS/FP membrane were optimized by adjusting the feeding dosage of PSS. The realized PSS/FP composite containing 7% PSS displayed good conductivity (9.1 × 10-2 S/m), reducing electric resistance by 99.2% compared with the original FP membrane (6.7 × 10-4 S/m). The stable current of the membrane in simulated sweat under different pH environments is highly correlated with the pH values. Additionally, when the membrane is exposed to simulated sweat with varying ion concentrations, the current signal changes in real time with the concentration variations. The response time averages around 0.3 s.

Wireless wearable potentiometric sensor for simultaneous determination of pH, sodium and potassium in human sweat

This paper reports on the development of a flexible-wearable potentiometric sensor for real-time monitoring of sodium ion (Na+), potassium ion (K+), and pH in human sweat. Na0.44MnO2, polyaniline, and K2Co[Fe(CN)6] were used as sensing materials for Na+, H+ and K+ monitoring, respectively. The simultaneous potentiometric Na+, K+, and pH sensing were carried out by the developed sensor, which enables signal collection and transmission in real-time to the smartphone via a Wi-Fi access point. Then, the potentiometric responses were evaluated by a designed android application. Na+, K+, and pH sensors illustrated high sensitivity (59.7 ± 0.8 mV/decade for Na+, 57.8 ± 0.9 mV/decade for K+, and 54.7 ± 0.6 mV/pH for pH), excellent stability, and good batch-to-batch reproducibility. The results of on-body experiments demonstrated that the proposed platform is capable of real-time monitoring of the investigated ions.

Hydrodynamics-engineered polyaniline nanofibers on graphene nanosheets for high-performance pH sensors

Real-time sweat monitoring in the human body using wearable sensors is promising for noninvasive and continuous healthcare applications. For reliable and accurate sweat analysis, further improvement in the response time and long-term stability of sensors is required. This paper reports an electrochemical pH sensor based on polyaniline/graphene (PANi-Gr) with rapid electron and ion transfer. The implementation of the Taylor–Couette flow approach allowed the efficient exfoliation of graphite into Gr nanosheets and the rapid polymerization of aniline monomers on the Gr surface, resulting in nanofiber PANi-deposited Gr nanocomposites. PANi-Gr had a large surface area and porous structure, which are favorable for rapid and stable electrochemical reactions. The screen-printed PANi-Gr-based pH sensor exhibited a Nernstian sensitivity of 61.91 mV/pH, rapid response time of <1 s, and long-term stability upon monitoring with minimal potential drift (0.04 mV h−1) under strong acidic and basic conditions. Furthermore, the on-body performance of the wearable pH sensor integrated with a wireless electronic device was analyzed via the noninvasive real-time monitoring of human sweat pH during stationary cycling. The developed sensor can be used in wearable real-time sweat monitoring devices for medical and healthcare applications.

Stretchable and Sweat-Wicking Patch for Skin-Attached Colorimetric Analysis of Sweat Biomarkers.

Stretchable sweat sensors are promising technology that can acquire biomolecular insights for health and fitness monitoring by intimate integration with the body. However, current sensors often require microfabricated microfluidic channels to control sweat flow during lab-on-body analysis, which makes effective and affordable sweat sampling a significant practical challenge. Here, we present stretchable and sweat-wicking patches that utilize bioinspired smart wettable membranes for the on-demand manipulation of sweat flow. In a scalable process, the membrane is created by stacking hydrophobic elastomer nanofibers onto soft microfoams with predefined two-dimensional superhydrophobic and superhydrophilic patterns. The engineered heterogeneous wettability distribution allows these porous membranes to achieve enhanced extraction and selective collection of sweat in embedded assays. Despite the simplified architecture, the color reactions between sweat and chemical indicators are inhibited from directly contacting the skin to achieve a largely improved operation safety. The sensing patches can simultaneously quantify pH, urea, and calcium in sweat through digital colorimetric analysis with smartphone images. The construction with all compliant materials renders these patches soft and stretchy to achieve conformal attachment to the skin. Successfully analyzing sweat compositions after physical exercises illustrates the practical suitability of these skin-attachable sensors for health tracking and point-of-care diagnosis.

Designing of multifunctional graphene quantum dot-polyvinyl alcohol-polyglycerol luminescent film for fluorescence detection of pH in sweat

BackgroundWound infection, skin disease, renal failure, cancer, cystic fibrosis, and other pathologies may induce obvious pH changes in sweat. Thus, tracking skin pH changes can help monitor human health in a convenient manner. Owing to their biocompatibility, easy preparation, and sensitive response to pH changes, graphene quantum dots (GQDs) have received increased attention in the optical detection of pH changes. However, their poor luminescent efficiency under visible light excitation and lack of functional diversification limit their application in skin pH monitoring. Therefore, the development of GQDs with excellent ultraviolet protection ability and antibacterial and luminescence performance is essential. ResultsFolic acid-, histidine-, and serine-functionalized boron-doped graphene quantum dots (FHSB-GQDs) were designed and synthesized via thermal treatment. The resulting FHSB-GQDs exhibit strong yellow fluorescence emission under excitation with 490-nm visible light and sensitive pH responsiveness. The peak fluorescence intensity linearly decreases with increasing pH from 4 to 9. Furthermore, the FHSB-GQDs were integrated with polyvinyl alcohol and polyglycerol to form a luminescent film via hydrogen bond interactions. The film exhibits high transparency, mechanical flexibility, ultraviolet protection ability, and antibacterial activity. The presence of polyvinyl alcohol and polyglycerol restricts the free movement of the FHSB-GQDs and improves fluorescence behavior. The film was successfully applied in an intelligent pH-sensing system for monitoring pH changes in human sweat. SignificanceThe graphene quantum dot–polyvinyl alcohol–polyglycerol luminescent film offers excellent transparency, mechanical flexibility, ultraviolet protection ability, antibacterial activity, and luminescence performance. It was successfully applied in an intelligent pH sensing system for the detection of pH changes in human sweat. This study provides a new strategy for the design and construction of wearable sensing systems for health monitoring, facial masks, and medical dressings.

Development of a wearable surface enhanced Raman scattering sensor chip based on silver nanowires for rapid detection of urea, lactate and pH in sweat

A wearable sweat sensor chip based on surface-enhanced Raman spectroscopy (SERS) is here presented. The plasmonic core of the chip, obtained by deposition of silver nanowires on a polytetrafluoroethylene (PTFE) porous membrane, permits the direct and label-free detection of urea and lactate at physiologic concentrations in combination with the pH measurement in the range between 5 and 9. Once integrated with commercial biocompatible and adhesive tape layers, the as produced SERS-active chip provides a low-cost, flexible and wearable sensing device for rapid and reliable human sweat analysis. The chip was finally tested on real sweat samples to estimate lactate and urea during medium-intense exertions.

SweatSkin

Sweat sensing affords monitoring essential bio-signals tailored for various well-being inspections. We present SweatSkin, the fabrication approach for customizable sweat-sensing on-skin interfaces. SweatSkin is unique in exploiting on-skin microfluidic channels to access bio-fluid secretes within the skin for personalized health monitoring. To lower the barrier to creating skin-conformable microfluidics capable of collecting and analyzing sweat, four fabrication methods utilizing accessible materials are proposed. Technical characterizations of paper- and polymer-based devices indicate that colorimetric analysis can effectively visualize sweat loss, chloride, glucose, and pH values. To support general to extreme sweating scenarios, we consulted five athletic experts on the SweatSkin devices' customization guidelines, application potential, and envisioned usages. The two-session fabrication workshop study with ten participants verified that the four fabrication methods are easy to learn and easy to make. Overall, SweatSkin is an extensible and user-friendly platform for designing and creating customizable on-skin sweat-sensing interfaces for UbiComp and HCI, affording ubiquitous personalized health sensing.

Wax screen-printable ink for massive fabrication of negligible-to-nil cost fabric-based microfluidic (bio)sensing devices for colorimetric analysis of sweat

Fabric-based microfluidic analytical devices (μADs) have emerged as a promising material for replacing paper μADs thanks to their superior properties in terms of stretchability, mechanical strength, and their wide scope of applicability in wearable devices or embedded in garments. The major obstacle in their widespread use is the lack of a technique enabling their massive fabrication at a negligible-to-nil cost. In response, we report the development of a wax ink with proper thixotropic and hydrophobic properties, fully compatible with automatic screen-printing that allows the one step massive fabrication of microfluidics on a cotton/elastane fabric, with a printing resolution 400 μm (hydrophilic channel) and 1000 μm (hydrophobic barrier), without being necessary any post curing. The cost of the ink (50 g) and of each microfluidic device is ca. 2.3 and 0.007 €, respectively. The active component of the ink was a refined beeswax in a matrix based on ethyl cellulose in 2-butoxy ethyl acetate. Screen-printed fabric μADs were used for the simultaneous colorimetric determination of pH and urea in untreated human sweat by using multivariate regression analysis. This method enabled the direct measurement of urea using urease, regardless of the sweat’s pH, and shows strong agreement with a reference method.

Monitoring of Sweat pH and Dual‐Mode Anti‐Counterfeiting from Metal‐Organic Framework‐Based Multifunctional Gel

Comprehensive SummaryMonitoring of sweat pH plays important roles in physiological health, nutritional balance, psychological stress, and sports performance. However, the combination of functional MOFs with phosphorescent material to acquire the real‐time physiological information, as well as the application of dual mode anti‐counterfeiting, has seldom been reported. Herein, we developed multifunctional gel films based on MOFs and phosphorescent dyes which responded to H+ ions and the related mechanism was studied in detail. Upon exposure to H+, the composite gel film exhibited decreased fluorescent signal but enhanced room temperature phosphorescence (RTP), which could be utilized for sweat pH sensing through a dual‐mode. Moreover, multifunctional gel films exhibited a potential application in information encryption and anti‐counterfeiting by designing of stimulus responsive multiple patterns. This research provided a new avenue for portable and non‐invasive sweat pH monitoring methods while also offering insights into stimulus‐responsive multifunctional materials.

A novel strategy for the preparation of multifunctional and robust polyhydroxyalkanoates/GQDs nanocomposites with UV-resistance, pH-responsive and antibacterial properties

Biosynthesized polyhydroxyalkanoates (PHAs) offer several compelling advantages, including biodegradability, biocompatibility, and suitability for use in environmentally friendly packaging materials. However, poor crystallization ability and lack of functionality (e.g., UV-resistance and antibacterial properties) significantly restrict the practical applications of PHAs. In this study, graphene quantum dots (GQDs) containing with rich amino groups, carboxy groups and conjugate structure were tailor-made as multifunctional nanofillers to prepare PHAs/GQDs composites. The GQDs not only exhibited excellent dispersion within the PHAs matrix at the nanoscale but also act as nucleating agents to dramatically increase the nucleation density of PHAs. Remarkably, the incorporation of just 0.5 wt% GQDs led to outstanding antibacterial performance (99.99%), UV resistance (90.50%), transparency (>80%) and mechanical properties in the PHAs composite. ​Moreover, these PHAs composites displayed strong fluorescence stability and demonstrated sensitivity to pH variations. Building upon these findings, we propose a novel concept for a wearable device capable of intelligently detecting changes in sweat pH during human exercise. This work presents a straightforward and effective method for preparing multifunctional GQDs, thereby expanding the potential applications of PHAs in the realm of green packaging and intelligent sensing.

Smart Wearable Nanopaper Patch for Continuous Multiplexed Optical Monitoring of Sweat Parameters.

Notwithstanding the substantial progress in optical wearable sensing devices, developing wearable optical sensors for simultaneous, real-time, and continuous monitoring of multiple biomarkers is still an important, yet unmet, demand. Aiming to address this need, we introduced for the first time a smart wearable optical sensor (SWOS) platform combining a multiplexed sweat sensor sticker with its IoT-enabled readout module. We employed our SWOS system for on-body continuous, real-time, and simultaneous fluorimetric monitoring of sweat volume (physical parameter) and pH (chemical marker). Herein, a variation in moisture (5-45 μL) or pH (4.0-7.0) causes a color/fluorescence change in the copper chloride/fluorescein immobilized within a transparent chitin nanopaper (ChNP) in a selective and reversible manner. Human experiments conducted on athletic volunteers during exercise confirm that our developed SWOS platform can be efficiently exploited for smart perspiration analysis toward personalized health monitoring. Moreover, our system can be further extended for the continuous and real-time multiplexed monitoring of various biomarkers (metabolites, proteins, or drugs) of sweat or other biofluids (for example, analyzing exhaled breath by integrating onto a facemask).