Wearable devices for continuous monitoring of biosignals: Challenges and opportunities

Abstract

The ability for wearable devices to collect high-fidelity biosignals continuously over weeks and months at a time has become an increasingly sought-after characteristic to provide advanced diagnostic and therapeutic capabilities. Wearable devices for this purpose face a multitude of challenges such as formfactors with long-term user acceptance and power supplies that enable continuous operation without requiring extensive user interaction. This review summarizes design considerations associated with these attributes and summarizes recent advances toward continuous operation with high-fidelity biosignal recording abilities. The review also provides insight into systematic barriers for these device archetypes and outlines most promising technological approaches to expand capabilities. We conclude with a summary of current developments of hardware and approaches for embedded artificial intelligence in this wearable device class, which is pivotal for next generation autonomous diagnostic, therapeutic, and assistive health tools.

FIG. 1.

Introduction into current wearable systems: (a) illustration showing key aspects for chronic user compliance with wearable medical devices. (b) Graph showing metrics of user retention for fitness wearables. Reproduced with permission from Chandrasekaran et al., J. Med. Internet Res. 22, e22443 (2020). Copyright 2020 Authors, licensed under a Creative commons Attribution (CC BY) license. (c) Graph showing user compliance with wearable monitors over a 2-year period ledger. Reproduced with permission from D. Ledger and McCaffrey, Endeavour Partners 200, 1 (2014). Copyright 2014 Authors, licensed under a Creative commons Attribution (CC BY) license. (d) Illustrations outlining wireless power transfer techniques' battery-free or chronic wearable devices. (e) Summary of power harvesting techniques used for autonomous wearable devices.

FIG. 2.

Wireless battery-free biophysical sensors. (a) Schematic illustration of wireless, battery-free multimodal wireless skin-mounted sensors powered by near-field-enabled clothing. Reproduced with permission from Lin et al., Nat. Commun. 11, 1–10 (2020). Copyright 2020 Authors, licensed under a Creative Commons Attribution (CC BY) license. (b) Illustration of conductive thread to relay power and data transfer for operation of the skin mounted sensors. Reproduced with permission from Lin et al., Nat. Commun. 11, 1–10 (2020). Copyright 2020 Authors, licensed under a Creative Commons Attribution (CC BY) license. (c) Graph showing continuous monitoring of axillary temperature and running gait during exercise using epidermal sensors, compared with the gold standard wired system. Reproduced with permission from Lin et al., Nat. Commun. 11, 1–10 (2020). Copyright 2020 Authors, licensed under a Creative Commons Attribution (CC BY) license. (d) Photographic image of the electrocardiogram (ECG) device capable of measuring HR, SpO₂, and RR along with central-skin and peripheral-skin temperature to monitor neonatal intensive care (NICU). Reproduced with permission from Chung et al., Nat. Med. 26, 418–429 (2020). Copyright 2020 Springer Nature. (e) Photographic image showing the preterm infant in NICU bed equipped with the NFC antenna system to power the battery-free ECG and photophlethysmographgy (PPG) sensors placed on the chest and the foot of a newborn, respectively. Reproduced with permission from Chung et al., Science 363, eaau0780 (2019). Copyright 2019 Authors, licensed under a Creative Commons Attribution (CC BY) license. (f) Graph showing in vivo data collection including HR, SpO₂, Temp, and acceleration. Reproduced with permission from Chung et al., Nat. Med. 26, 418–429 (2020). Copyright Springer Nature. (g) Photographic image of an epidermal wireless thermal sensor (eWTS) for thermal conductivity measurements. Reproduced with permission from Krishnan et al., Small 14, 1–13 (2018). Copyright 2018 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim. (h) Optical image showing readout from the eWTS placed on the forearm using a smartphone. Reproduced with permission from Krishnan et al., Small 14, 1–13 (2018). Copyright 2018 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim. (i) Graph of thermal conductivity measured continuously using the eWTS for 1 week and compared to the gold standard electromagnetic method evaluating skin hydration. Reproduced with permission from Krishnan et al., Small 14, 1–13 (2018). Copyright 2018 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim.

FIG. 3.

Wearable biochemical sensing devices: (a) graphical illustration showing device function and motion harvested by triboelectric nanogenerator (TENG) to power electrolyte sweat analysis. Reproduced with permission from Yu et al., Sci. Adv. 6, eaay9842 (2021). Copyright 2021 Authors, licensed under a Creative Commons Attribution (CC BY) license. (b) Image showing device applied to the subject's torso. Reproduced with permission from Yu et al., Sci. Adv. 6, eaay9842 (2021). Copyright 2021 Authors, licensed under a Creative Commons Attribution (CC BY) license. (c) Real time pH and sodium concentrations measured with the FWS device compared to the system charged with the lithium battery. Reproduced with permission from Yu et al., Sci. Adv. 6, eaay9842 (2021). Copyright 2021 Authors, licensed under a Creative Commons Attribution (CC BY) license. (d) Image of multimodal performance biomarker sweat analysis device interfacing with a smartphone for analysis readout. Reproduced with permission from Bandodkar et al., Sci. Adv. 5, eaav3294 (2019). Copyright 2019 Authors, licensed under a Creative Commons Attribution (CC BY) license. (e) Images showing device operation during perspiration. Reproduced with permission from Bandodkar et al., Sci. Adv. 5, eaav3294 (2019). Copyright 2019 Authors, licensed under a Creative Commons Attribution (CC BY) license. (f) Data correlation between seat-based glucose and lactate readings and measured values from blood level measurements. Reproduced with permission from Bandodkar et al., Sci. Adv. 5, eaav3294 (2019). Copyright 2019 Authors, licensed under a Creative Commons Attribution (CC BY) license. (g) Schematic illustration showing the electrical device operation principle of contact lens based electrochemical sensing of tear fluid. Reproduced with permission from Ku et al., Sci. Adv. 6, eabb2891 (2020). Copyright 2020 Authors, licensed under a Creative Commons Attribution (CC BY) license. (h) Illustration displaying device and sensor composition integrated in contact. Reproduced with permission from Ku et al., Sci. Adv. 6, eabb2891 (2020). Copyright 2020 Authors, licensed under a Creative Commons Attribution (CC BY) license. (I) In vivo cortisol concentrations collected simultaneously from both eyes of a subject. Reproduced with permission from Ku et al., Sci. Adv. 6, eabb2891 (2020). Copyright 2020 Authors, licensed under a Creative Commons Attribution (CC BY) license. (j) Illustration of biofuel-powered energy harvesting from perspiration to power electrochemical analysis of sweat. Reproduced with permission from Yu et al., Sci. Robot. 5, 1–14 (2020). Copyright 2020 AAAS. (k) Image of the device located on the subject's arm. Reproduced with permission from Yu et al., Sci. Robot. 5, 1–14 (2020). Copyright 2020 AAAS. (l) Real-time data collection of urea and NH₄⁺ concentrations from a forehead mounted device. Reproduced with permission from Yu et al., Sci. Robot. 5, 1–14 (2020). Copyright 2020 AAAS.

FIG. 4.

Limitations of current wearable devices for biosensing applications. (a) Illustration showing conventional wrist worn wearable systems during operation. (b) Illustration of wearables utilizing adhesive interfaces during operation with key limitations in epidermal turnover.

FIG. 5.

Long-term wearable sensing devices: (a) image showing biosymbiotic device for the biceps with inset illustrating the personalized digital design and fabrication process. (b) Layered schematic of biosymbiotic devices. (c) Functional diagram of the operating principle. (d) Realtime recording of body temperature collected from the axilla region during sitting (white) and walking (red). (e) Graph showing cyclic response of the 3D printed strain gauge during 16 N of applied stress (red) and corresponding change in resistance (black). (f) Graph showing lower leg acceleration of the biosymbiotic device (black) and gold standard (red) during a stationary jump. (g) Continuous data and sample global positioning system (GPS) mapping from a 7-day recording of body temperature and muscular strain captured by a biosymbiotic device worn on the proximal bicep. Adapted from Tucker et al., Sci. Adv. 7, eabj3269 (2021). Copyright 2021 Authors, licensed under a Creative Commons Attribution (CC BY) license.

FIG. 6.

Embedded artificial intelligence. (a) Advantages of embedded AI biosensors. (b) Block diagram of functional components of wearable devices with embedded AI. (c) Workflow for training and testing Machine learning models. (d) Graph showing the motor recovery trajectory using wearable smart biosensors. Reproduced with permission from Adans-Dester et al., npj Digital Med. 3, 1–10 (2020). Copyright 2020 Authors, licensed under a Creative Commons Attribution (CC BY) license.