Sweat Lactate Concentrations Research Articles

(Invited) towards Continuous Health Monitoring Platforms By Enzyme-Free Electrochemical Sensors and Triboelectric Sensors for Noninvasive Lactate Detection

Emerging wearable devices with non-invasively biosensing technics have drawn considerable attention to continuously monitor several metabolites in body fluids, such as a tear, saliva, and sweat, to diagnose human health conditions. Most importantly, wearable sensors could offer unique possibilities for online, real-time and non-invasive monitoring of health compared to traditional invasive biosensors. Among lots of analytes, lactic acid concentration in the human body exhibits a high relationship with several diseases such as acute heart diseases, hypoxia, muscle fatigue, meningitis, and cystic fibrosis, and it could also cause muscle pain in athletes. To further boost up the reproducibility and reliability of wearable biosensors to detect lactate concentration levels from human sweat, Ni-based layered double hydroxide (LDH) with various secondary transition metals (Fe and Co) was proposed as electrocatalysts in an enzyme-free electrochemical lactate sensor. According to the mechanism of lactate oxidation on the transition metal-based electrocatalyst, secondary transition metal of Co could serve as the active site for lactate oxidation and facilitate the adsorption of OH- in the alkaline electrolyte. To further increasing the active surface area for enhancing the sensitivity of Ni-based LDH, ZIF-67 derived NiCo LDH was synthesized as the electrocatalyst for non-enzymatic lactate detection. Co-based ZIF-67 served as self-sacrificial templates to fabricate hierarchically structural NiCo LDH with uniform porosity and high electrochemically active surface area to achieve outstanding electrocatalytic performance for lactate sensing. After optimizing the particle size of ZIF-67 and transformation times, ZIF-67 derived NiCo LDH reached the ultrahigh sensitivity of 83.98 μA mM- 1 cm- 2 at an applied potential of 0.55 V (vs. Ag/AgCl KCl sat’d) in the concentration range from 2 to 26 mM. On the other hand, pioneering works in biosensors for lactate detection in sweat has been encountered major challenges such as noble material usage, immobile power supply, and complicated circuit connection to realize the compact sustainable sensing systems. To solve these restrictions, herein, the self-powered molecular imprinted polymers based triboelectric sensor (MIP-TES) was designed to offer a multifunctional noninvasive approach for specific and simultaneous lactate detection. Free-standing PVDF/graphene flexible electrode modified poly(3-aminophenyl boronic acid) imprinted lactate molecule demonstrated the change of the surface properties afterlactate adsorption. MIP-modified electrode revealed the selective lactate sensing over non molecular imprinted polymers (NIP) electrode through the superior and stable signal change with variation of lactate concentration in human sweat. Moreover, MIP modified lactate sensor was further introduced in the triboelectric nanogenerator system to harvest mechanical energy from contact and separation into electrical output. The more adsorbed lactate led to lower energy barriers and decreasing electrical potential when detecting higher lactate concentration. Self-power triboelectric lactate sensor could directly power the number of LED lights without an external energy supply. Eventually, it was validated the feasible application of wearable sensors on human skin. After introducing noninvasive enzyme-free biosensors and triboelectric sensors, an innovatively continuous non-invasive health monitoring platforms can be achieved for practical applications, especially in the areas of home medical examination and wearable personal biosensors.

Fatigue assessment of construction equipment operators using a sweat lactate biosensor

This study investigated the accuracy and usefulness of a sweat-based biosensor for measuring the fatigue levels of construction equipment operators. Specifically, the paper first elaborated on the suitability of selecting sweat lactate as a biomarker for measuring combined fatigue (i.e., a combination of physical and mental fatigue) that construction equipment operators often experience. Then, the mechanism of a sweat-based sensor was explained. An experiment was then presented in which the sweat lactate levels of five subjects engaged in simulated equipment operating tasks were measured. The reliability of the sweat-based lactate sensor was also evaluated by conducting a between-day test-retest experiment. The lactate measurements of the sweat sensor were compared to measurements of blood lactate to assess the accuracy of the sweat sensor. The Fatigue Assessment Scale, a subjective fatigue method, validated sweat lactate as a fatigue assessment biomarker. Analytical results indicate that the lactate measurements from the sweat-based sensor do reflect the fatigue level of equipment operators, and the device has good reliability for measuring sweat lactate concentration.

Electrochemical sensing monitoring of blood lactic acid levels in sweat during exhaustive exercise

Carbohydrate anaerobic metabolism in the human body produces lactic acid, which is of great importance in the diagnosis and treatment of diseases and the scientific management of sports. Consequently, the development of a rapid, low-cost, and disposable lactate sensor is essential. To this end, a graphene-modified lactate sensor was created. This work concluded with the optimization of the sensor's operating parameters and the evaluation of its performance under the optimal conditions. Following the testing, the proposed lactic acid sensor demonstrated high sensitivity, excellent consistency and linearity, with a linear detection range of 3–32 mM. The detection limit was determined to be 1.4 mM, the lowest value achieved. The current performance of the biosensor is reaching its peak when presented with physiological sweat lactate concentration. To monitor sweat lactate concentration, wearable electronic devices with high precision were incorporated to provide real-time remote monitoring. By engaging in exhaustive physical exercise, the amount of sweat lactate can be determined.

Impact of varying lactate concentration in sweat on thermo-physiological comfort of multi-layered ensembles

PurposeSweating is thermo-regulatory behaviour that occurs when a person performs vigorous activity even in cold climatic condition. One of important component of sweat is the presence of lactate. Based on climatic condition, age, gender, maturity and nature of activity level, the change in lactate concentration is inevitable. Hence, the present study is focussed on the impact of change in the lactate concentration on the moisture transmission behaviour through the clothing. The purpose of this paper is to investigate the impact of changing lactate concentration on the moisture vapour transmission behaviour through multi-layered clothing ensembles.Design/methodology/approachFor the investigation, sweat solution representing male and female sweat were taken for present study. Two different multi-layered ensembles consisting of either spacer or fleece as middle layer were considered. The water vapour permeability and drying rate test were done at standard atmospheric conditions. After testing, ANOVA analysis was done in order to determine the most significant parameters.FindingsFabric structure (constituent layers) behaved differently when tested individually and as the layered component with different sweat solutions. Water vapour permeability of sweat solution with higher lactate concentration was lower as compared to sweat solution with lower lactate concentration. Individual layers showed higher rate of vapour permeability with sweat solution containing lower lactate concentration as compared to multi-layered ensembles. Role of PU coated nylon fabric was predominant in case of multi-layered ensembles. Difference in transmission of sweat solution was found higher in case of uni-directional stitched multi-layer spacer ensembles whereas marginal difference was observed in case of bi-directional seamed multi-layer spacer ensemble. Drying rate of sweat containing lower concentration of lactate was higher as compared to the other sweat solution for all the selected fabrics. Density of liquid and amount of the water available for drying influenced the drying behaviour and thus accounted for difference in drying rate of sweat solution differing in the lactate concentration. The contribution percentage of layers, i.e. type of structure was higher (nearly 93–96%) compared to that of solution type (3.3–4.9%) in case of individual layers whereas in the case of the multi-layer ensembles; type of seam had maximum contribution percentage (71–77%) followed by solution type (10–15%). Type of layers had least contribution percentage (nearly 7–9%).Practical implicationsThe findings from the study are expected to be realistic and important in designing and development of cold weather garment ensemble for different gender type depending on their activity level especially in case of military personnel and those performing combat activities.Originality/valueThis experimental work based will provide the insight about the behaviour of actual sweat transmission through the layered fabric ensembles and ways to prevent the accumulation of moisture near to human skin surface by manufacturing suitable design structures (in terms of layering composition and seam patterns) per the morphology and requirement of specific consumers.

Impact of varying lactate concentration in sweat on liquid moisture transmission behaviour of layered ensembles

The present study is focussed on the impact of change in the lactate concentration (43 mM and 22 mM) in sweat solution on liquid moisture transmission behaviour through the clothing. The sweat solution with higher concentration of lactate (43 mM) shows delayed wetting at the top surface both in case of individual layer and multi-layer fabric ensembles, i.e. it takes longer time to wet the top surface in spite of the lower contact angle made by it. Significant difference is observed in in-plane transmission behaviour of both the sweat solutions in the case of multi-layered ensembles. In case of multi-layered ensembles, wetting time reduces drastically, even though both the ensembles consist of polyester knit as the inner surface possessing wetting time is 50 s approximately. Sweat solution with higher lactate concentration also shows higher cross-planar transmission rate as compared to in-plane transmission. Uni-directional seamed multi-layered spacer fabric exhibits better overall moisture management coefficient as compared to bi-directional seamed spacer ensembles with sweat solution containing higher lactate concentration. Middle layer also plays a vital role in altering the overall liquid moisture transmission behaviour.

The effect of heat acclimation on the sweat lactate concentration vs. sweat rate relationship

It is well known that there is a high concentration of lactate in sweat. Interest in measuring sweat lactate has arisen from its potential role in several clinical and sport performance applications. However, the effect of heat acclimation on sweat lactate concentration is still under debate. This is partly because sweat lactate concentration is greatly affected by sweat rate, which is known to increase during heat acclimation. Thus, to better understand this issue it is necessary to account for sweat rate – which has not been done previously in the literature. Therefore, the purpose of the current study was to determine the effect of heat acclimation on the relationship between sweat rate vs. sweat lactate concentration. Six subjects completed a 7-day heat acclimation protocol. The daily 2-h exercise bout was split into three 40-min intervals with exercise intensity increasing with each successive interval. This was done to induce three different sweat rates to determine the sweat rate vs. sweat lactate concentration relationship before and after heat acclimation for each participant. A 2 (heat acclimation) x 3 (sweat rate) repeated measures ANOVA was conducted to determine statistical significance. There was a significant (p < 0.05) decrease in the grand mean sweat lactate concentration over the course of seven days of heat acclimation from 17.0 ± 5.0 to 11.3 ± 1.1 mmol/L (p < 0.05). A significant (p < 0.05) heat acclimation x sweat rate ordinal interaction was also found. The results of the current study show that heat acclimation significantly decreases the sweat lactate concentration. In addition, there was a significant ordinal interaction which suggests that the impact of sweat rate on sweat lactate concentration is decreased following heat acclimation.

Smartphone embedded deep learning approach for highly accurate and automated colorimetric lactate analysis in sweat

Here, a microfluidic paper-based analytical device (μPAD) was first combined with a deep learning-based smartphone app called “DeepLactate” and then applied for quantitative and selective determination of lactate concentration in sweat. The μPAD was made using wax printing protocol and the detection area was modified with horse radish peroxidase, lactate oxidase and the chromogenic agent 3,3′,5,5′-tetramethylbenzidine for enzymatic detection. The images of μPADs taken by smartphones of several brands in different lighting conditions were used to train various deep learning models to make the system more robust and adaptable to lighting changes. The top-performing model, Inception-v3, was then embedded into a smartphone app, offering easy-operation for non-expert users. Deep learning models, unlike machine learning classifiers, can automatically extract features and be embedded in a smartphone app, enabling analysis without internet access. According to the results, the current system showed a classification accuracy of 99.9 % with phone-independent repeatability and a processing time of less than 1 sec. It also showed excellent selectivity towards lactate over different interfering species. Finally, μPAD was turned into a patch to determine the level of sweat lactate in two volunteers after resting and 15 min of jogging. The system successfully detected lactate in human sweat and confirmed that the level of lactate in sweat increased after jogging. Since the μPAD was designed to first absorb a sample and then transfer it to the detection area, avoiding direct contact with the skin, the system reduces the possibility of skin irritation and has great potential for practical use in a variety of fields including self-health monitoring and sports medicine.

Ultrastable Lactate Biosensor Linearly Responding in Whole Sweat for Noninvasive Monitoring of Hypoxia.

We report on the lactate biosensor with linear calibration range from 0.5 to 100 mM, which encircles possible levels of this metabolite concentration in both human sweat and blood. The linear calibration range at high analyte concentrations, which exceeds the Michaelis constant of lactate oxidase by several orders of magnitude, is provided by an additional perfluorosulfonated ionomer diffusion membrane. In contrast to the known lactate biosensors, which retain their response within less than a couple of hours, the reported system displays 100% response for dozens of hours even upon high analyte concentrations. The biosensors with an additional diffusion-limiting membrane have been validated for lactate detection both in human blood serum and in undiluted human sweat shortly after its secretion. Both linear response in the entire range of blood and sweat lactate concentrations and ultrahigh operational stability would provide the use of the elaborated biosensor in wearable devices for the monitoring of hypoxia.

Self-powered molecular imprinted polymers-based triboelectric sensor for noninvasive monitoring lactate levels in human sweat

Recent progress in wearable sweat sensors had overcome numerous limitations of conventional sensors and provided methods of collecting molecular-level insight into the dynamics of human bodies. Lactate, a major metabolite in human sweat, can serve as the critical limiting factor for continuing physical activity. However, pioneering works in biosensors for lactate detection in sweat has been encountered major challenges such as noble material usage, immobile power supply, and complicated circuit connection to realize the compact sustainable sensing systems. To solve these restrictions, herein, the self-powered molecular imprinted polymers-based triboelectric sensor (MIP-TES) was designed to offer a multifunctional noninvasive approach for specific and simultaneous lactate detection. Free-standing PVDF/graphene flexible electrode modified poly(3-aminophenyl boronic acid) imprinted lactate molecule demonstrated the change of the surface properties after lactate adsorption. MIP-modified electrode revealed the selective lactate sensing over non molecular imprinted polymers (NIP) electrode through the superior and stable signal change with variation of lactate concentration in human sweat. Moreover, MIP modified lactate sensor was further introduced in the triboelectric nanogenerator system to harvest mechanical energy from contact and separation into electrical output. The more adsorbed lactate led to lower energy barriers and decreasing electrical potential when detecting higher lactate concentration. Self-power triboelectric lactate sensor could directly power the number of LED lights without an external energy supply. Eventually, it was validated the feasible application of wearable sensors on human skin. The proposed self-powered lactate has created a fascinating horizon in portability, compact, and the real-time analysis of people’s perspiration in the future.

Simultaneous monitoring of sweat lactate content and sweat secretion rate by wearable remote biosensors

We report on the simultaneous monitoring of sweat lactate concentration and sweat secretion rate. For this aim lactate oxidase-Prussian Blue enzyme-nanozyme type lactate biosensors were elaborated. The use of siloxane-perfluorosulfonated ionomer composite membrane for enzyme-nanozyme immobilization results in the biosensor displaying flux independence in the whole range of physiological sweat secretion rates (0.025–2 μl cm-2 min-1). On the contrary, current response of the biosensor based on solely siloxane membranes becomes saturated at physiological sweat lactate concentration, depending mostly on the flow rate. Accordingly, for simultaneous monitoring of sweat lactate concentration and its secretion rate both flow-through biosensors were integrated with high-accuracy wearable electronic devices allowing real-time remote monitoring. As found, during exhaustive physical exercise sweat secretion rate and lactate content are independent of each other, thus, confirming that this excretory liquid is suitable for non-invasive diagnostics.

A novel device for detecting anaerobic threshold using sweat lactate during exercise

The lactate threshold (LT1), which is defined as the first rise in lactate concentration during incremental exercise, has not been non-invasively and conveniently determined in a clinical setting. We aimed to visualize changes in lactate concentration in sweat during exercise using our wearable lactate sensor and investigate the relationship between the lactate threshold (LT1) and ventilatory threshold (VT1). Twenty-three healthy subjects and 42 patients with cardiovascular diseases (CVDs) were enrolled. During exercise, the dynamic changes in lactate values in sweat were visualized in real-time with a sharp continuous increase up to volitional exhaustion and a gradual decrease during the recovery period. The LT1 in sweat was well correlated with the LT1 in blood and the VT1 (r = 0.92 and 0.71, respectively). In addition, the Bland–Altman plot described no bias between the mean values (mean differences: − 4.5 and 2.5 W, respectively). Continuous monitoring of lactate concentrations during exercise can provide additional information for detecting the VT1.

Screen-Printed Lactate Biosensor Based on a Lox-Immobilized MgO-Templated Carbon for Continuously Monitoring of Lactate Concentration in Sweat

Lactate concentration in sweat is closely related to exercise intensity. Thus, real-time monitoring of lactate concentration is extremely important for improving athlete training and health management. There are many reports for monitoring the change in the lactic acid concentration in sweat over a long period of time using a wearable lactate biosensor 1,2). For practical application of a wearable lactate biosensor for long-term monitoring, it is important to suppress elution of the enzyme from the electrode by simple enzyme immobilization scheme. Recently, we have reported that long-term stability of a glucose oxidase (GOD)-immobilized electrode was dramatically improved by introducing a poly(glycidyl methacrylate) on a mesoporous carbon, namely MgO-templated carbon (MgOC), which binds to a GOD by coavalent bonding scheme. In this study, we applied the method mentioned above to a printed lactate biosensor. A lactate oxidase (LOx) was immobilized on the electrode by covalent bonding with a poly(glycidyl methacrylate)-modified carbon surface. Firstly, radicals were introduced on the MgOC surface by electron beam irradiation, and glycidyl methacrylate (GMA) having an epoxy group was polymerized according to the previously reported scheme3). We referred the poly(GMA)-immobilized MgOC was “GMgOC” in this study. GMgOC and polyvinylidene fluoride (PVdF) were dispersed in N-methyl pyrrolidone (NMP) to prepare a graft polymerized carbon ink for printing. A printed electrode tips were fabricated by screen printing. Then, an enzyme-immobilized electrode was prepared by modifying 1,2-naphthoquinone as a mediator and lactate oxidase as an enzyme. The stability of the enzyme-immobilized electrode was evaluated by 10 cycles of cyclic voltammetry. The measurement was performed in a 1 mol dm-3 phosphate buffer (pH 7.0) containing 25 mmol dm-3 lactic acid with a three-electrode system. A platinum wire as a counter electrode, and a sat. KCl/Ag/AgCl electrode as a reference electrode were used. The maximum current density was observed at about 0.13 V vs. Ag/AgCl. Figure 1 shows the maximum current of the cyclic voltammogram plotted against the number of cycles during multiple sweeps. In case of the GMgOC, the current value retained 95.7% after 10 cycles, but it was 68.4% in the case of unmodified MgOC. By performing graft polymerization on the MgOC surface to fabricate the electrode, no significant change in the current value was observed. These results indicated that LOx was stably immobilized on the GMgOC surface without degeneration. Acknowledgements This work was partially supported by JST-ASTEP Grant Number JPMJTS1513, JSPS Grant Number 17H02162 and Private University Research Branding Project (2017-2019) from Ministry of Education, Culture, Sports, Science and Technology, and Tokyo University of Science Grant for President's Research Promotion． References 1) W. Jia, A. J. Bandodkar, G. Valdés-Ramírez, J.R. Windmiller, Z. Yang,J. Ramírez ,G. Chan, J. Wang, Analytical Chemistry, 85 (2013), 6553.2) R.A.Escalona-Villalpando,E.Ortiz-Ortega,J.P.Bocanegra-Ugalde,Shelley,D.Minteer,J.Ledesma-García, L.G. Arriaga, Journal of Power Sources, 412 (2019),496-504.3) I. Shitanda ,T. Kato ,R. Suzuki ,T. Aikawa ,Y. Hoshi ,M. Itagaki , and S. Tsujimura , Bulletin of the Chemical Society of Japan, 93 (2019), 32-36． Figure 1

Fiber organic electrochemical transistors based on multi-walled carbon nanotube and polypyrrole composites for noninvasive lactate sensing.

Multi-walled carbon nanotubes (MWCNT) play a synergistic role with conducting polymer in practical applications such as biological sensing. In this paper, multi-walled carbon nanotube and polypyrrole (PPy) composites were prepared on a fiber surface for the first time, and their morphology and electrical properties were characterized. Compared with PPy-coated fiber, the presence of carbon nanotubes induced the growth of large areas of PPy nanowires. In addition, fiber organic electrochemical transistors (FECTs) based on PPy and MWCNT were assembled, showing a higher on/off ratio, better stability, and greater flexibility. The lactate biosensor based on FECTs exhibits high sensitivity, with a correlation coefficient of R = 0.9889, quick response time of 0.6-0.8s, a wide linear response range of 1nM-1mM, and excellent selectivity for lactate. Furthermore, the lactate concentration in human sweat was successfully detected by a FECT-based sensor. The hybrid fibers can be easily woven and placed on fabric simply by stitching. This favorable performance of the FECT-based sensor makes it suitable for noninvasive sensing of lactate. Therefore, it provides a promising platform for future use in healthcare and detection applications. Graphical abstract.

A Self-Powered Biosensor for Monitoring Maximal Lactate Steady State in Sport Training

A self-powered biosensor for monitoring the maximal lactate steady state (MLSS) during exercise has been developed for intelligently assisting training system. It has been presented to create poly (vinylidene fluoride) (PVDF)/Tetrapod-shaped ZnO (T-ZnO)/enzyme-modified nanocomposite film through an efficient and cost-effective fabrication process. This sensor can be readily attached to the skin surface of the tester. Due to the piezoelectric surface coupling effect, this biosensor can monitor/sense and analyze physical information in real-time under the non-invasive condition and work independently without any battery. By actively outputting piezoelectric signals, it can quickly and sensitively detect body movements (changes of joint angle, frequency relative humidity during exercise) and physiological information (changes of lactate concentration in sweat). A practical application has been demonstrated by an excellent professional speed skater (male). The purpose of this study is to increase the efficiency of MLSS evaluation, promote the development of piezoelectric surface coupling effect and motion monitoring application, develop an intelligently assisting training system, which has opened up a new direction for human motion monitoring.

Relationship Between Sweat and Blood Lactate Levels During Exhaustive Physical Exercise

AbstractIn contrast to the generally accepted view, that sweat lactate is irrelevant to blood, we report on the suitability of sweat lactate for diagnostics. We demonstrate a significant increase in lactate concentration in sweat from the working muscle area simultaneously with the rise of blood lactate content during exhaustive physical exercise. The variation rates of lactate concentration in sweat from working and latent muscle areas correlate positively with blood lactate levels (r&gt;0.8 and r=0.7, respectively), thus offering the prospect of a non‐invasive approach for the monitoring of sportsperson training. Moreover, sweat lactate, being the faster indicator of muscle metabolism, would be even more important for sports medicine than the nowadays‐used blood lactate.

Self-Powered Piezoelectric-Biosensing Textiles for the Physiological Monitoring and Time-Motion Analysis of Individual Sports.

Self-powered piezoelectric-biosensing textiles for the physiological monitoring and time-motion analysis of individual sports have been developed. The material system is composed of tetrapod-shaped ZnO nanowires on common textiles. The mechanism is based on the coupling of enzymatic reaction (LOx and lactate) and piezoelectric effect. After conformably attaching the device to the athlete, the device can monitor in real-time the moving speed, frequency, joint angle, and sweat lactate concentration of the athlete. The whole monitoring/analysis process is battery-free. The motor skills and physiological state of two athletes are investigated using the textiles, and different lactate threshold times and maximum lactate release capacities have been obtained. This technique can help them develop distinct training programs. This research is a new direction for the scientific monitoring of kinematics and may also stimulate the development of self-powered wearable sports-related systems.

Potentiometric sensor for non invasive lactate determination in human sweat

The present work describes a non invasive lactate sensing in sweat during workout. The sensing system is based on a non-equilibrium potentiometric measure performed using disposable, chemically modified, screen printed carbon electrodes (SPCEs) that can be wetted with sweat during the exercise. The potentiometric signal, which is proportional to lactate concentration in sweat, is produced by a redox reaction activated by UV radiation, as opposed to the enzymatic reaction employed in traditional, blood-based measuring devices. The sensing system exhibits chemical selectivity toward lactate with linearity from 1 mM up to 180 mM. The dynamic linear range is suitable for measurement of lactate in sweat, which is more than 10 times concentrated than hematic lactate and reaches more than 100 mM in sweat during workout. The noninvasive measure can be repeated many times during exercise and during the recovery time in order to get personal information on the physiological and training status as well as on the physical performance.The device was successfully applied to several human subjects for the measurement of sweat lactate during prolonged cycling exercise. During the exercise sweat was simultaneously sampled on filter paper and extracted in water, and the lactate was determined by HPLC for method validation. The lactate concentration changes during the exercise reflected the intensity of physical effort. This method has perspectives in many sport disciplines as well as in health care and biomedical area.

Flat enzyme-based lactate biofuel cell integrated with power management system: Towards long term in situ power supply for wearable sensors

Flat enzyme-based lactate biofuel cell (ELBC) integrated with power management system (PMS) was developed as a potential power supply for wearable sensors. Kinetic models of power output and chemical reagent concentration (lactate oxidase LOx and lactate) were developed to determine the limiting factor of the ELBC performance. Given the lactate concentration (0–40mM) in human sweat, the optimum LOx amount coated on the anode ranged from 6U to 54U based on the experimental maximum power output. In the ELBC-PMS entity simulation test, power discharge/recharge frequency was calculated for self-sustained sensing and data transmission, indicating that the ELBC could support wearable sensors (power requirement: 1–1350mW; signal transfer frequency: 1–1320times/h). The electrochemical activity of LOx coated anode was validated using the cyclic voltammetry (CV). The coefficient of variance and regression statistical analysis revealed the high stability and long lifespan (2weeks) of ELBC without the need of lactate refill.

Effect of Heat Acclimation on Lactate Excretion Rate and Sodium Ion Reabsorption Rate during Exercise

Sweat production requires two Na+/K+-ATPase energy dependent processes: (i) isoosmotic precursor sweat formation in the secretory coil, and (ii) sodium ion reabsorption along the distal sweat duct. Sweat lactate production has been hypothesized to reflect glycolytic ATP-resynthesis for both processes. Research suggests that heat acclimation does not alter sweat lactate concentration ([Lactate]). This is hard to rationalize considering heat acclimation is reported to increase sweat production and sodium ion reabsorption, which are the energy dependent processes. PURPOSE: (1) To determine the relationships during exercise between sweat rate (SR) and sodium ion reabsorption rate (NaRR), and exercise intensity and lactate excretion rate (LER), and (2) to determine whether heat acclimation affects these relationships. METHODS: Six subjects completed an eight-day heat acclimation protocol comprised of 90-minutes treadmill walking in an environmental chamber set at 39°C and 40% relative humidity. Subjects performed three, 30-minute exercise bouts at 30, 40, and 50% of their previously determined VO2max. Sweat rate was measured volumetrically during each bout on the flexor surface of the proximal half of the forearm. [Lactate] was measured and multiplied by forearm SR to calculate LER. Variables of interest were analyzed using non-parametric Friedman tests. RESULTS: LER, and ratios of LER to NaRR, and [Lactate] to SR were not significantly different between the 30, 40, and 50% workloads or between pre and post-heat acclimation based on 95% confidence intervals. Additionally, these relationships were not significantly different between the three workloads on the first or last day of heat acclimation except for LER to NaRR (p<0.05), which increased from 0.05 ± 0.02 to 0.08 ± 0.02 from 30 to 50% of VO2max. CONCLUSION: These data suggest that sweat gland ATPase activity does not increase as a function of exercise intensity, as demonstrated by LER and NaRR remaining relatively constant. Heat acclimation does not appear to affect these relationships.

Non-Invasive Sensor for Quantitative Determination of Lactate in Sweat

Lactic acid is a key biomarker of anaerobic respiration and descriptor of an individual’s health state. Currently, lactate monitoring is performed though continuous invasive blood sampling and sample processing which is inconvenient and unpractical when trying to obtain real time measurements of an active individual. Non-invasive sensing methods are needed for monitoring human performance in sports, military and health care fields. Lactate is found in various bodily fluids including sweat and a correlation between blood and sweat lactate concentration has been determined (1). Hence, monitoring sweat lactate levels is a good noninvasive alternative to blood sampling methods. Here we report the development, characterization and optimization of an amperometric lactate sensor based on a lactate dehydrogenase and carbon nanotube chitosan electrode coupling system previously developed by our group (3). Due to the high overpotential of NADH oxidation, the electrocatalyst, polymethylene green (PMG) was electrochemically deposited onto the surface of multi-walled carbon nanotube (CNT) material called Bucky paper for NAD⁺ regeneration. Lactate dehydrogenase was immobilized onto the Bucky paper-PMG electrode with a chitosan-based carbon nanotubes (CNT) mixture. The bioelectrodes were tested in a standard three-electrode polycarbonate cell hardware consisting of enzymatic working electrode, Ag/AgCl reference and platinum wire counter electrode and operated in a chronoamperometric regime. Sweat composition varies between individuals and different parts of the human body resulting in variations in pH, salt concentration (related to conductivity,) and lactate levels before and after exercise with rates of release decreasing over time. In order to address some of these issues, calibration curves at various buffer pHs and buffer concentrations were created (Fig 1). The variation of pH of solutions from 5 to 7 showed an increase in bioelectrode response. The slope of the current/lactate concentration linear dependence was strongly influenced by the solution pH with low pH leading to decreased sensitivity most likely as a result of influenced enzyme activity. A similar trend was observed when the concentration of the electrolyte was varied from 0.01 to 0.245 M. The slope of the calibration curve was strongly influenced by the solution conductivity with low values leading to decreased reproducibility and sensitivity. In all cases the electrode response was linear with lactate concentration allowing one to tailor the system based on individual needs. An artificial sweat was prepared, composed of NaCl, urea, pH 6.5, glucose, NAD⁺, and having a conductivity of 16.6 mS/cm. As it was expected the initial tests of the LDH-electrode with the artificial sweat showed lower sensitivity most likely due to low conductivity of the solution, the generated current form the lactate oxidation followed linear dependence from lactate concentration (data not shown). The strong dependence of the slope of the sensor calibration curve from the electrolyte conductivity and pH can be minimized by buffering the sweat, which in real conditions is performed by impregnation of the sensor sweat collector with a buffer salts. A prototype of a patch lactate sensor was developed composed of a LDH-working electrode, a Ag/AgCl reference electrode and carbon yarn counter electrode. The three electrodes were placed on a medical adhesive and covered with a bandage, preventing a contact between electrodes and the skin and at the same time adsorbing and collecting the sweat. The designed patch sensor displayed an increase in current density with increasing lactate concentration with high sensitivity. To avoid artificial introduction of NAD+ in the sweat, a method for NAD+immobilization of the electrode surface will be implemented along with encapsulation of the enzyme into silica gel matrix, which improve the linearity of the response and prolong the life -time of the sensor. The proposed amperometric enzyme electrode coupling approach along with optimization experiments provides the opportunity to monitor noninvasively and in real-time sweat lactate concentrations. (1) Sakharov, D.A. et al., 2010. Relationship between lactate concentrations in active muscle sweat and whole blood. Bull. Exp. Biol. Med.. 150 (1) 83-5. (2) Nikolaus, N., and Strehlitz B., 2008. Review. Amperometric lactate biosensors and their application in (sports) medicine, for life quality and wellbeing. Microchim Acta. 160, 15-55. (3). Narvaez Villarrubia C.W. et al., 2011. Biofuel cell anodes integrating NAD⁺-dependent enzymes and multiwalled carbón nanotube papers. ACS Appl. Mater Interfaces. 2011, 3(7), p. 2402-9 (7) Figure 1