Wearable flexible sweat sensors for healthcare monitoring: a review

Abstract

The state-of-the-art in wearable flexible sensors (WFSs) for sweat analyte detection was investigated. Recent advances show the development of integrated, mechanically flexible and multiplexed sensor systems with on-site circuitry for signal processing and wireless data transmission. When compared with single-analyte sensors, such devices provide an opportunity to more accurately analyse analytes that are dependent on other parameters (such as sweat rate and pH) by improving calibration from in situ real-time analysis, while maintaining a lightweight and wearable design. Important health conditions can be monitored and on-demand regulating drugs can be delivered using integrated wearable systems but require correlation verification between sweat and blood measurements using in vivo validation tests before any clinical application can be considered. Improvements are necessary for device sensitivity, accuracy and repeatability to provide more reliable and personalized continuous measurements. With rapid recent development, it can be concluded that non-invasive WFSs for sweat analysis have only skimmed the surface of their health monitoring potential and further significant advancement is sure to be made in the medical field.

Keywords: biosensor; electrolyte; healthcare; sweat sensor; wearable flexible sensor.

Figure 1.

Analyte partitioning pathway from interstitial fluid and blood to sweat through lipophilic cell membranes. Adapted from [24]. (Online version in colour.)

Figure 2.

Schematic of the main components of a biosensor. (a) The desired analyte from a sample interacts with the analyte-specific bioreceptor; (b) bioreceptor outputs a signal with defined sensitivity and (c) transducer transforms bioreceptor output into a readable signal for amplification and data processing. (Online version in colour.)

Figure 3.

(a) Schematic of enzymatic amperometric biosensor with mediator and (b) schematic applied to example using glucose oxidase to detect glucose. (Online version in colour.)

Figure 4.

Schematic of transducer output signal processing and transmission to external monitoring device. Adapted from [55]. (Online version in colour.)

Figure 5.

(a) Epidermal microfluidic colorimetric sweat sensor with and without artificial sweat applied and (b) process using smartphone software for image capture and analysis of colorimetric sensor [84]. (Online version in colour.)

Figure 6.

(a) FISA and FPCB worn on the wrist; (b) sensor array worn on the forehead, arm and wrist during cycling exercise and (c) on-body analysis yielded very similar results to ex situ analysis for sodium and glucose [3]. (Online version in colour.)

Figure 7.

Schematic of iontophoresis with agonist delivered to skin aided by electrical current. Adapted with permission from [18]. Copyright © 2016 American Chemical Society. (Online version in colour.)

Figure 8.

(a) Schematic of hyaluronic acid hydrogel microneedles, with phase change material (PCM) coatings, containing metformin-loaded PCNs and (b) SEM image of the microneedles [58]. (Online version in colour.)

Figure 9.

Schematic for the measurement and display of ethanol levels in sweat. Adapted with permission from [18]. Copyright © 2016 American Chemical Society. (Online version in colour.)

Figure 10.

Integrated wearable sweat ethanol sensor with (a) system assembly and (b,c) photos of assembled devices [16]. (Online version in colour.)