Abstract
Wearable lactate sensors for sweat analysis are highly appealing for both the sports and healthcare fields. Electrochemical biosensing is the approach most widely used for lactate determination, and this technology generally demonstrates a linear range of response far below the expected lactate levels in sweat together with a high influence of pH and temperature. In this work, we present a novel analytical strategy based on the restriction of the lactate flux that reaches the enzyme lactate oxidase, which is immobilized in the biosensor core. This is accomplished by means of an outer plasticized polymeric layer containing the quaternary salt tetradodecylammonium tetrakis(4-chlorophenyl) borate (traditionally known as ETH500). Also, this layer prevents the enzyme from being in direct contact with the sample, and hence, any influence with the pH and temperature is dramatically reduced. An expanded limit of detection in the millimolar range (from 1 to 50 mM) is demonstrated with this new biosensor, in addition to an acceptable response time; appropriate repeatability, reproducibility, and reversibility (variations lower than 5% for the sensitivity); good resiliency; excellent selectivity; low drift; negligible influence of the flow rate; and extraordinary correlation (Pearson coefficient of 0.97) with a standardized method for lactate detection such as ion chromatography (through analysis of 22 sweat samples collected from 6 different subjects performing cycling or running). The developed lactate biosensor is suitable for on-body sweat lactate monitoring via a microfluidic epidermal patch additionally containing pH and temperature sensors. This applicability was demonstrated in three different body locations (forehead, thigh, and back) in a total of five on-body tests while cycling, achieving appropriate performance and validation. Moreover, the epidermal patch for lactate sensing is convenient for the analysis of sweat stimulated by iontophoresis in the subjects’ arm, which is of great potential toward healthcare applications.
Highlights
Wearable lactate sensors for sweat analysis are highly appealing for both the sports and healthcare fields
The saturation of the lactate oxidase (LOx) is expected at a higher lactate concentration in the sample. This outer layer is traditionally called as ′′diffusion limiting layer′′ and has been proposed in the form of polymeric films with thicknesses ranging from 10 to 50 μm for other analytes.[26,36−38] we have evaluated polyvinyl chloride (PVC)-based membranes deposited on top of the Prussian blue (PB)-LOx biosensor
We demonstrated a new strategy for lactate biosensing that provides an extended linear range of response covering expected levels in sweat while preserving the biosensor response toward pH and temperature changes
Summary
Very similar lactate concentration values were observed for both subjects (i.e., 15.1 and 13.4 mM), and variability against IC values was similar to that previously observed in on-body cycling tests (see Figure 4f, black circles) Overall, these results demonstrate that the developed wearable lactate biosensor is compatible with measurements via sweat induction by iontophoresis, which opens up the possibility of full integration of the two steps (i.e., iontophoresis and lactate detection) in a single device for further implementation in the healthcare field
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