State of Sweat: Emerging Wearable Systems for Real-Time, Noninvasive Sweat Sensing and Analytics

Abstract

Skin-interfaced wearable systems with integrated colorimetric assays, microfluidic channels, and electrochemical sensors offer powerful capabilities for noninvasive, real-time sweat analysis. This Perspective details recent progress in the development and translation of novel wearable sensors for personalized assessment of sweat dynamics and biomarkers, with precise sampling and real-time analysis. Sensor accuracy, system ruggedness, and large-scale deployment in remote environments represent key opportunity areas, enabling broad deployment in the context of field studies, clinical trials, and recent commercialization. On-body measurements in these contexts show good agreement compared to conventional laboratory-based sweat analysis approaches. These device demonstrations highlight the utility of biochemical sensing platforms for personalized assessment of performance, wellness, and health across a broad range of applications.

Keywords: biosensors; eccrine sweat; epidermal microfluidics; flexible electronics; health monitoring; lab-on-chip; sweat analysis; wearable sensors.

Figure 1.

Methods for sweat stimulation. Typical methods for sweat stimulation include (A) physical activity or (B) pharmacological stimulation. Adapted with permission from ref (), Copyright 2019 American Association for the Advancement of Science, and ref (), Copyright 2018 John Wiley and Sons, respectively. Alternative approaches seek to collect sweat passively using (C) thermal stimulation via showering or (D) wicking materials. Adapted with permission from ref (), Copyright 2019 The Royal Society of Chemistry, and ref (), Copyright 2021 American Chemical Society, respectively.

Figure 2.

Technology foundations for wearable sweat sensing. Fluid Handling. Networks of (A) passive or (B) active valves enable sophisticated routing of harvested sweat in a programmatic manner. Adapted with permission from ref (), Copyright 2017 John Wiley and Sons, and ref (), Copyright 2020 Springer Nature, respectively Timing. Nuanced designs integrate sensing features such as sweat-activated galvanic cells shown in (C) to enable temporal analysis of sweat constituents. Adapted with permission from ref (). Copyright 2019 John Wiley and Sons. In large arrays (D) such biofuel cells support battery-free electrochemical sensing of sweat. Adapted with permission from ref (). Copyright 2020 American Association for the Advancement of Science. Advanced designs. Optimization of mechanical properties (E) address sensing challenges in high-impact environments. Adapted with permission from ref (). Copyright 2020 John Wiley and Sons. Utilization of customized extraction hardware (F) assists in reducing sample contamination. Adapted with permission from ref (). Copyright 2021 Elsevier B.V.

Figure 3.

Performance health management. (A) A recent large-scale study validates the performance of a wearable microfluidic patch for estimating whole-body sweat parameters from regional sweat analysis. Many sweat biomarkers exhibit a concentration dependence on rate of sweat loss. Adapted with permission from ref (). Copyright 2020 American Association for the Advancement of Science. Recent embodiments utilize (B) conductivity, (C) capacitive, or (D) thermal sensing strategies to continuously measure real-time sweat rate. Adapted with permission from ref (), Copyright 2019 The Royal Society of Chemistry, ref (), Copyright 2020 American Chemical Society, ref (), Copyright 2021 Springer Nature, respectively. (E) Emerging device architectures integrate chemesthetic sensors and user-activated valves to alert wearers to anomalous physiological conditions during exercise. Adapted with permission from ref (). Copyright 2019 Springer Nature.

Figure 4.

Clinical diagnostics. Sweat chloride. Sweat chloride is a longstanding clinically validated diagnostic biomarker used for confirmatory diagnosis of cystic fibrosis (CF). Recent reports demonstrate (A) the first large-scale study of a soft, flexible epidermal platform (“sweat sticker”) for clinical diagnosis and (B) use of wearable sweat sensors for monitoring sweat chloride levels outside of a clinical setting. Adapted with permission from ref (), Copyright 2021 American Association for the Advancement of Science, and ref (), Copyright2020 Springer Nature, respectively. Emerging sweat biomarkers. Use of sweat glucose as a noninvasive replacement for blood glucose monitoring in diabetes management is of academic and commercial interest with recent efforts demonstrating sensors for monitoring sweat glucose levels (C) at rest and (D) during exercise. Adapted with permission from ref (), Copyright 2020 American Chemical Society, and ref (), Copyright 2020 American Chemical Society, respectively. (E) One embodiment demonstrates glucose monitoring during exercise in wireless, battery-free form factor. Adapted with permission from ref (). Copyright 2019 American Association for the Advancement of Science. Other targets of interest include the concentration of (F) uric acid in sweat (for gout), (G) various cytokines (inflammation, fever), (H) vitamins (nutrition monitoring), and (I) illicit drugs. Adapted with permission from ref (), Copyright 2020 Springer Nature; ref (), Copyright 2021 John Wiley and Sons; ref (), Copyright 2020 American Association for the Advancement of Science; ref(), Copyright 2021 American Association for the Advancement of Science, respectively. (J) Device designs exploiting wicking materials enable passive (i.e., absence of active sweating) multiparameter monitoring of disease biomarkers or the concentration of drug therapeutics. Adapted with permission from ref (). Copyright 2021 Springer Nature.

Figure 5.

Integrated devices and commercially available systems. (A) An integrated device strategy for long-term sweat analysis via on-demand sweat stimulation. Adapted with permission from ref (). Copyright 2020 The Royal Society of Chemistry. (B) A recent effort details a strategy to utilize roll-to-roll manufacturing to produce epidermal microfluidic sensors in a scalable manner suitable for mass manufacture. Implementation of such fabrication strategies enables further maturation of wearable sweat sensing platforms and offers opportunities for broad consumer adoption. Adapted with permission from ref (). Copyright 2019 American Association for the Advancement of Science. (C) Gx Sweat Patch and (D) Connected Hydration System, both developed by Epicore Biosystems, represent the vanguard of the emerging commercial sensing platforms. Reprinted with permission from ref (). Copyright 2021 Epicore Biosystems.