Microfluidic wearable electrochemical sweat sensors for health monitoring

Abstract

Research on remote health monitoring through wearable sensors has attained popularity in recent decades mainly due to aging population and expensive health care services. Microfluidic wearable sweat sensors provide economical, non-invasive mode of sample collection, important physiological information, and continuous tracking of human health. Recent advances in wearable sensors focus on electrochemical monitoring of biomarkers in sweat and can be applicable in various fields like fitness monitoring, nutrition, and medical diagnosis. This review focuses on the evolution of wearable devices from benchtop electrochemical systems to microfluidic-based wearable sensors. Major classification of wearable sensors like skin contact-based and biofluidic-based sensors are discussed. Furthermore, sweat chemistry and related biomarkers are explained in addition to integration of microfluidic systems in wearable sweat sensors. At last, recent advances in wearable electrochemical sweat sensors are discussed, which includes tattoo-based, paper microfluidics, patches, wrist band, and belt-based wearable sensors.

Scheme 1.

Various microfluidic wearable devices for the sweat analysis

FIG. 1.

Evolution of electrochemical wearable devices. Traditional electrochemical instruments commonly used in 1990s (a) electrochemical system with the rotating disk electrode setup and (b) the dropping mercury electrochemical system. Electrochemical handheld systems used in 2000s. (c) Portable electrochemical analyzers and (d) screen printed carbon electrodes. Reproduced with permission from Ainla et al., Anal. Chem. 90, 10 (2018). Copyright 2018 American Chemical Society. (e) Flexible printed electrodes. Reproduced with permission from Varodi et al., Sensors 20, 12 (2020). Copyright 2020 Author(s), licensed under a Creative Commons Attribution (CC BY) License. Emergence of wearable electrochemical devices and (f) tattoo-based wearable sensor for sweat analysis. Reproduced with permission from Jia et al., Anal. Chem. 85, 14 (2013). Copyright 2013 American Chemical Society. (g) Microfluidic based wearable sensor and (h) flexible wearable smart sensors. Reproduced with permission from Manjakkal et al., Biosensors 9, 1 (2019). Copyright 2019 Author(s), licensed under a Creative Commons Attribution (CC BY) License.

FIG. 2.

Classification of healthcare wearable sensors. (a) Smart headband biosensor for electrochemical detection of glucose and sodium ions in sweat. Reproduced with permission from Zhao et al., Biosens. Bioelectron. 188, 113270 (2021). Copyright 2021 Elsevier. (b) Silicon micropillar-based wearable sweat sensor skin patch for continuous monitoring of glucose. Reproduced with permission from Dervisevic et al., ACS Appl. Mater. Interfaces. 14, 1 (2021). Copyright 2021 American Chemical Society. (c) Laser inscribed contact lens sensors for detection of glucose, proteins, pH, and nitrite sensing in tears. Reproduced with permission from Moreddu et al., Sens. Actuators, B 317, 12813 (2020). Copyright 2020 Elsevier.

FIG. 3.

Challenges in attaining wearability, characteristics of skin, and requirements for wearable sensors.

FIG. 4.

(I) (a) Schematic representation of wearable microfluidic sensor device reinforced with PEDOT:PSS hydrogel. (b) Fabrication of PEDOT:PSS hydrogel modified electrodes. Reproduced with permission from Xu et al., Sens. Actuators, B 348, 130674 (2021). Copyright 2021 Elsevier. (II) (a) Image showing various components of wearable microfluidic sweat sensor. (b)–(d) Photo showing the device collecting the sweat from different body positions. Reproduced with permission from Ma et al., Talanta 212, 120786 (2020). Copyright 2021 Elsevier.

FIG. 5.

Overview of integrated smart watch designed for Na⁺ and K⁺ ions in sweat. (I) (a) Structure of paper-based microfluidic sensor and smart watch sensor being investigated on forearm. (b) Detailed description of electrodes and (c) block diagram of smart watch. (II) On body perspiration investigating though fabricated smart watch. Reproduced with permission from Cao et al., Electroanaylsis 33, 3 (2021). Copyright 2021 Wiley.

FIG. 6.

(a) Direct skin contact approach for fabricating wearable sensor and (b) microfluidic layer integrated approach for fabricating wearable sensor.

FIG. 7.

(a) Electrochemical tattoo-based sweat sensors for lactate monitoring. Reproduced with permission from Jia et al., Anal. Chem. 85, 14 (2013). Copyright 2013 American Chemical Society. (b) Real world sensing of Zinc while workout though a temporary tattoo on a human subject. Reproduced with permission from Kim et al., Electrochem. Commun. 51, 41 (2015). Copyright 2015 Elsevier. (c) Design and fabrication of stretchable and wearable biosensor for glucose monitoring. Reproduced with permission from Abellan-llobregat et al., Biosens. Bioelectron. 91, 885 (2017). Copyright 2015 Elsevier. (d) Textile silk carbon reinforced electrochemical sensor patch for simultaneous monitoring of sweat analytes. Reproduced with permission from He et al., Sci. Adv. 5, 11 (2019). Copyright 2019 Author(s), licensed under a Creative Commons Attribution (CC BY) License.

FIG. 8.

(a) Schematic representation of multi-layered configuration of microfluidic wearable device and picture of sensor device integrated with a wireless electronic system. Reproduced with permission from Martin et al., ACS Sens. 2, 12 (2017). Copyright 2017 American Chemical Society. (b) 3D paper-based microfluidic electrochemical sensor and real time monitoring of glucose using a fabricated sensor. Reproduced with permission from Cao et al., RSC Adv. 9, 10 (2019). Copyright 2019 Author(s), licensed under a Creative Commons Attribution (CC BY) License. (c) Roll to roll screen printing of wearable sweat sensor patches. Reproduced with permission from Nyein et al., Sci. Adv. 5, 8 (2019). Copyright 2019 Author(s), licensed under a Creative Commons Attribution (CC BY) License.