A wearable chemical–electrophysiological
hybrid biosensing system for real-time health and fitness monitoring
Somayeh Imani
, Amay J. Bandodkar , A. M. Vinu Mohan , Rajan Kumar , Shengfei Yu
, Joseph Wang & Patrick P. Mercie
Abstract
Flexible, wearable sensing devices can
yield important information about the underlying physiology of a human subject
for applications in real-time health and fitness monitoring. Despite
significant progress in the fabrication of flexible biosensors that naturally
comply with the epidermis, most designs measure only a small number of physical
or electrophysiological parameters, and neglect the rich chemical information
available from biomarkers. Here, we introduce a skin-worn wearable hybrid
sensing system that offers simultaneous real-time monitoring of a biochemical
(lactate) and an electrophysiological signal (electrocardiogram), for more
comprehensive fitness monitoring than from physical or electrophysiological
sensors alone. The two sensing modalities, comprising a three-electrode
amperometric lactate biosensor and a bipolar electrocardiogram sensor, are
co-fabricated on a flexible substrate and mounted on the skin. Human
experiments reveal that physiochemistry and electrophysiology can be measured
simultaneously with negligible cross-talk, enabling a new class of hybrid
sensing devices.
Introduction
Wearable sensors present an exciting
opportunity to measure human physiology in a continuous, real-time and
non-invasive manner1,2. Recent advances in hybrid fabrication techniques have
enabled the design of wearable sensing devices in thin, conformal form factors
that naturally comply with the smooth curvilinear geometry of human skin,
thereby enabling intimate contact necessary for robust physiological measurements1,3,4.
Development of such epidermal electronic sensors has enabled devices that can
monitor respiration rate5,6,7, heart rate8,9, electrocardiograms4,10,11,12,
blood oxygenation13, skin temperature14,15, bodily motion16,17,18,19,20, brain
activity21,22,23 and blood pressure24,25. To date, most systems have targeted
only a single measurement at a time, and most such sensors measure only
physical and electrophysiological parameters, significantly limiting monitoring
and diagnostic opportunities. For example, the human body undergoes complex
physiological changes during physical activities such as exercise26,27, and
monitoring the physiologic effect of physical activity can be important for a
wide variety of subjects ranging from athletes to the elderly28,29,30. However,
current wearable devices that only measure heart rate, motion and
electrocardiogram provide an incomplete picture of the complex physiological
changes taking place. As a result, further progress in the area of wearable
sensors must include new, relevant sensing modalities, and must integrate these
different modalities into a single platform for continuous, simultaneous
sensing of multiple parameters relevant to a wide range of conditions,
diseases, health and performance states.
Inclusion of chemical measurements can
provide extremely useful insights not available from physical or
electrophysiological sensors31. Chemical information can be conventionally
acquired via clinical labs or point-of-care devices32,33,34; unfortunately,
such approaches do not support continuous, real-time measurements, therefore
limiting their utility to applications where stationary, infrequent tests are
sufficient. While recent work, including our own, has demonstrated that
chemicals such as electrolytes and metabolites can be measured continuously
using epidermal electronics on the skin35,36,37,38, or through non-invasive
monitoring of other body fluids38,39,40, these devices measure only a single
parameter at once, and are not integrated with other sensing modalities.
Recently, Gao et al.41 demonstrated a wearable patch that can simultaneously
track levels of metabolites and electrolytes in human sweat. However,
electrophysiology sensors were not included, and such multimodal sensor fusion
is crucial to obtain a more comprehensive knowledge about a wearer’s
well-being.
Here, we introduce a wearable device that
can simultaneously measure chemical and electrophysiological parameters in the
form factor of a single epidermal patch. The hybrid wearable, termed here as a
Chem–Phys patch, comprises a screen-printed three-electrode amperometric
lactate biosensor and two electrocardiogram electrodes, enabling concurrent
real-time measurements of lactate and electrocardiogram. When used in
physical-exertion monitoring, electrocardiogram measurements can help monitor
heart health and function, while sweat lactate can be used to track an
individual’s performance and exertion level, and is also an important biomarker
for tissue oxygenation and pressure ischaemia42,43,44,45,46,47. Although prior
work has demonstrated separate wearable electrocardiogram and lactate sensors,
these devices were fabricated on separate platforms and thus mandate applying
multiple patches on the human body, which is inconvenient and can deter long-term
use. By combining a lactate biosensor and an electrocardiogram sensor, the new
Chem–Phys hybrid wearable patch represents a powerful platform capable of
simultaneously tracking both physicochemical and electrophysiological
attributes, thus providing a more comprehensive view of a person’s health
status than current wearable fitness monitors.
The Chem–Phys hybrid patch was fabricated
by leveraging screen-printing technology on a thin, highly flexible polyester
sheet that conforms well with the complex three-dimensional (3D) morphology of
human skin to provide a low-noise signal. The working electrode of the lactate
biosensor was functionalized and coated with a biocompatible biocatalytic layer
(lactate oxidase (LOx)-modified prussian blue). The three amperometric
electrodes were separated from the Ag/AgCl electrocardiogram electrodes via a
printed hydrophobic layer to maximize sensor stability and signal-to-noise
ratio even in the presence of significant perspiration. The dimensions of the
electrodes and the inter-electrode distances have been optimized based on the
human trials to acquire a clean electrocardiogram signal and lactate response
with minimal interference between the two sensors. The two sensors were
interfaced to a custom-printed circuit board (PCB) featuring a potentiostat, an
electrocardiogram analogue front-end (AFE), and a bluetooth low-energy (BLE)
radio for wireless telemetry of the results to a mobile platform, such as a
smartphone or laptop. The hybrid sensing system was tested on three human
subjects during exercise on a stationary bicycle, showing that lactate and
electrocardiogram can be measured simultaneously with negligible
co-interference. Electrocardiogram data was found to be similar to the data
collected from standard electrode types, and extracted heart rate correlated
well to commercial heart rate detectors. A control experiment, where an
enzyme-free amperometric sensor was applied to a perspiring human subject,
corroborated the lactate sensor’s sensitivity and selectivity towards on-body
detection of physiologic lactate levels. The promising data obtained in this
work thus supports the possibility of developing more advanced hybrid wearable
sensors that involve complex integration of several physical and chemical
sensors on the same platform for monitoring many relevant modalities.