Skin-Interfaced Wearable Sweat Sensors for Precision Medicine

Jihong Min, Ph.D.

1,†

Jiaobing Tu, B.Eng.

1,†

Changhao Xu, Ph.D.

1,†

Heather Lukas,

Ph.D.

1,†

Soyoung Shin, PhD

Yiran Yang, BS

Samuel A. Solomon

Daniel Mukasa, PhD

Wei Gao, Ph.D.

1,*

Andrew and Peggy Cherng Department of Medical Engineering, Division of Engineering and

Applied Science, California Institute of Technology, Pasadena, California, 91125, USA.

Abstract

Wearable sensors hold great potential in empowering personalized health monitoring, predictive

analytics, and timely intervention toward personalized healthcare. Advances in flexible electronics,

material sciences, and electrochemistry have spurred the development of wearable sweat sensors

that enable the continuous and noninvasive screening of analytes indicative of health status.

Existing major challenges in wearable sensors include: improving the sweat extraction and sweat

sensing capabilities, improving the form factor of the wearable device for minimal discomfort

and reliable measurements when worn, and understanding the clinical value of sweat analytes

toward biomarker discovery. This review provides a comprehensive review of wearable sweat

sensors and outlines state-of-the-art technologies and research that strive to bridge these gaps. The

physiology of sweat, materials, biosensing mechanisms and advances, and approaches for sweat

induction and sampling are introduced. Additionally, design considerations for the system-level

development of wearable sweat sensing devices, spanning from strategies for prolonged sweat

extraction to efficient powering of wearables, are discussed. Furthermore, the applications, data

analytics, commercialization efforts, challenges, and prospects of wearable sweat sensors for

precision medicine are discussed.

Graphical Abstract

weigao@caltech.edu.

†

These authors contributed equally to this work.

Competing interests

The authors declare no competing interests.

HHS Public Access

Author manuscript

Chem Rev

. Author manuscript; available in PMC 2023 October 26.

Published in final edited form as:

Chem Rev

. 2023 April 26; 123(8): 5049–5138. doi:10.1021/acs.chemrev.2c00823.

Author Manuscript

1. Introduction

Wearable sensors hold the promise of providing noninvasive and continuous insight into the

biochemical landscape of our body.

–

From their simple origin as pedometers, wearable

sensors have evolved tremendously into the more complex field of health monitoring.

Fueled by increasing urbanization, improved lifestyle, and increasing awareness toward

health and safety, the wearable sensor industry has witnessed an exponential growth in the

demand for technologies that offer continuous health monitoring in the past decade.

Current

state-of-the-art commercialized wearable devices primarily focus on monitoring biophysical

signals (temperature, heartrate) that indicate the physical manifestations of an underlying

health state or condition which constrain the application of these devices within well-

being services. Owing to the complexity and multidimensional nature of various diseases,

deeper, multiplexed information acquired at the molecular level is needed before wearable

sensors can be adopted for disease monitoring. From smart watches to e-skins, innovations

in wearable sweat sensors promise to address this technological gap by expanding the

biometrics accessible non-invasively through the skin.

Sweat contains a wealth of biochemical information that can be noninvasively and readily

accessed on-demand or even continuously.

–

Compared with the complexities and

discomforts associated in the sampling of other biofluids like blood, interstitial fluid,

tear, saliva, and urine, sweat sampling can be conveniently and unobtrusively achieved

by placing a sensor patch on accessible locations of the skin. Molecular biomarkers

unveiled by wearable sweat sensors through continuous and non-invasive monitoring can

provide a more detailed understanding of the biochemical processes that govern our health,

enabling precision medicine through personalized monitoring of an individual’s fitness

and health conditions, as well as disease diagnosis and prognosis. Furthermore, the large

amounts of biochemical profiles collected by sweat sensors from patients and healthy

populations during the daily activities can be processed through predictive algorithms to

realize personalized therapeutics and preventative care. At the same time, large datasets

collected at the population-level can improve real-time epidemiological surveillance and

enhance the precision of public health responses.

Advances in sensor technologies, materials sciences, and electronics lead to the advent of

the first fully integrated multiplexed wearable sweat sensor in 2016.

Since then, numerous

wearable sweat sensing systems have been developed, typically consisting of a flexible

sweat sensor array for conformal contact, a flexible printed circuit board (FPCB) with

rigid electronic components for signal processing and wireless communication, and a power

source such as a lithium-ion battery to power the electronics. However, for the widespread

commercial adoption of wearable sweat sensors, several challenges need to be addressed.

Rigid or thick elements in sweat-sensing systems often impede the device from achieving

a stable, conformal, and breathable interface with the skin, potentially leading to motion-

induced artifacts, discomfort, and skin irritations. Furthermore, effective sweat sampling

often requires airtight contact with the skin which can be achieved by straps on wristwatches

and headbands, or by novel deformable adhesives. Breakthroughs in elastic wearable

materials can gradually replace rigid and bulky parts of wearable sweat sensing systems

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with lightweight and deformable counterparts to seamlessly interface the skin, evolving from

semi-rigid wristband sensors to e-textile sensors or e-skin sensors.

Next, the continuity and reliability of sweat sensor data are fundamental for achieving

continuous health monitoring. Effective sweat sampling is the first step toward achieving

continuous and accurate biomarker analysis. Early sweat sampling methods for analyzing

biomarkers in sweat were often confounded by discrepancies due to skin contamination,

sweat evaporation, sweat stimulation methods, and sweat rate effects. In addition, sweat

stimulation was primarily achieved physically through exercise or thermal stress, leading

to large variations in sweat rate and limiting sweat collection to very specific scenarios.

Chemical sweat stimulation methods, as well as efficient sweat collection materials and

microfluidic designs can minimize fouling of sweat samples and extend the use of sweat

sensors to sedentary and everyday scenarios. Ultimately, highly precise, specific, and stable

sweat sensors for detecting a wide range of biomarkers need to be developed or improved

upon. These sensors should also be supported by calibration sensors that simultaneously

analyze variables that can potentially influence sensor readings or sweat content, such as

skin temperature, sweat electrolyte balance, and sweat rate. Lastly, the vast amount of

continuous data collected by sweat sensors can be aggregated through big-data and cloud

computing techniques to better comprehend the meaning of the biomarker levels in terms of

personal health status.

This review provides a comprehensive overview on the field of wearable sweat sensors from

various perspectives including sweat physiology, materials science, sensing mechanism,

power sources, system integration, and data analytics (Fig. 1). In addition to introducing the

latest wearable sweat sensor devices reported in literature, we provide an in-depth summary

of the various engineering aspects that are considered when designing a device. Starting

off by overviewing sweat physiology in terms of sweat gland structure, sweat secretion

mechanisms, and sweat composition, we then highlight the essential material properties

needed for wearable sweat sensors. We then go on to discuss various sweat biomarker

detection mechanisms (not limited to electrochemical) and methods for sweat extraction and

sampling. Next, we describe energy harvesting and energy storage methods for powering

these wearables, as well as system-level integration strategies for integrating sensors,

electronic circuitry, and power sources into a complete wearable device. Furthermore, we

outline the various applications of wearable sweat sensors in terms of fitness monitoring,

disease diagnostics, and precision medicine. Finally, we discuss data post-processing for

wearable sweat sensors and their path to commercialization.

2. Physiology of Sweat

Sweat is produced from glands located deep within the skin, the body’s largest organ by

surface area. The skin has a stratified structure including the stratum corneum, epidermis,

dermis, and hypodermis. The dermis is the major component of the skin containing blood

vessels, nerve endings, and the base of sweat glands, sebaceous glands, and hair follicles

(Fig. 2a). The average eccrine sweat gland density is 200/cm

, but this varies between

individuals and across the body with the highest density among the palms and soles (~400/

The total number of eccrine sweat glands is on the order of 1.6–5 million.

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Sweat plays a very important role in maintaining the body’s core temperature, providing

a means of thermoregulation. Should body core temperatures rise above 40 °C without

modulation, there is a risk of protein denaturation, cell death, and subsequent organ

failure.

Beyond thermal regulation, sweat also participates in skin homeostasis.

Moisturizing factors in sweat, such as lactate and urea, maintain the plasticity and barrier

integrity of the stratum corneum. Secretion of antimicrobial compounds such as dermcidin,

lactoferrin, lysozymes, and immunoglobulin E (IgE) antibodies contributes to the skin’s first

line of defense against infection.

The loss of sweat glands after severe damage as in the

case of burn victims presents new challenges in regenerative wound healing and demands

further research into sweat gland physiology.

Eccrine sweat glands secrete a highly filtered, aqueous fluid composed of electrolytes,

metabolites, and additional molecules. Apocrine sweat glands secrete a viscous fluid

containing lipids, proteins, steroids, and ions, by exocytosis in the apocrine gland coil.

Volatile organic compounds from apocrine secretions act as pheromones.

Apocrine and

eccrine sweat glands are differentially stimulated. The apocrine sweat gland responds

strongly to emotional stimuli and sympathomimetic drugs via adrenergic innervation, but

does not respond to cholinergic or thermal stimulation like the eccrine sweat gland.

The apoeccrine sweat gland shares properties of both eccrine and apocrine glands; it may

develop during puberty in the axillae region from existing eccrine sweat glands. The gland

retains an eccrine-like sweat duct but has an apocrine-like secretory tubule. Apoeccrine

sweat ultimately resembles aqueous eccrine sweat and arises from an intermediate type of

stimulation.

This review focuses on eccrine sweat as eccrine sweat glands are the most

abundant and active source of sweat.

In this section, we present the physiology of eccrine sweat from stimulated innervation

to sweat secretion. We describe sweat gland development and structure. Additionally, we

discuss molecule partitioning into sweat and give an overview of accessible biomarkers in

sweat.

2.1 Structure and Mechanisms

The eccrine sweat tubule is a conduit for sweat and electrolyte exchange 4–8 mm in length.

At the base, the secretory coil is 500–700 μm in size with a lumen inner diameter of 30–

40 μm and a coil outer diameter of 60–120 μm.

The secretory coil is interwoven with

capillaries for vascular exchange and sudomotor nerve fibers for autonomic modulation.

The secretory tubule straightens into the dermal duct with an inner diameter of 10–20 μm

and outer diameter of 50–80 μm composed of two to three layers of epithelial cells.

The

sweat duct is straight from the dermis to the epidermis, and then transitions to a helical

structure in the epidermis that terminates in the stratum corneum. The number of turns of

the helical duct varies from 4–6 and varies proportionally to the stratum corneum thickness,

yet the pitch angle remains constant across sweat glands.

The helical structure makes the

sweat duct act as a helical antenna resulting in resonance behavior. Sweat duct dimensions,

density, distribution, and the dielectric properties of the stratum corneum all determine the

resonant frequency and subsequent skin-THz wave interactions. The duct length varies from

150–600 μm and varies proportionally to the stratum corneum thickness.

The sweat duct

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widens into the acrosyringium, a pore on the outer surface. The acrosyringium is composed

of epithelial cells with no clear distinction or border to the epidermis. The lumen has a

diameter of 20–60 μm and may also contain cornified cells.

Humans are born with almost all their sweat glands, with gland development occurring

mostly during the first two trimesters. This is one explanation for higher observed duct

densities in children than adults.

The sweat gland develops from a group of multipotent

K14

progenitors, descendants of epidermal stem cells. It grows downward as a straight

duct, stratifying in the lower half to proliferative K14

low

/K18

suprabasal progenitors.

The K14

low

/K18

suprabasal progenitors develop into luminal cells, while the remaining

K14

progenitors give way to myoepithelial cells.

Although sweat glands have limited

turnover and proliferation capabilities, there is some promise of regeneration. Stem cells

associated with secretory luminal and myoepithelial cells were found to promote epidermis

and sweat gland regeneration when amplified and seeded in the wound bed.

Additionally,

the use of three-dimensional (3D) bioprinting matrices has been studied for sweat gland

morphogenesis with tissue-level self-organization.

The secretory coil and duct define the two major steps of sweat generation: isotonic

secretion and salt reabsorption (Fig. 2b). Ductal cells facilitate transcellular reabsorption

with mitochondria-rich basal cells contributing to uptake. The secretory coil is made up

of basal myoepithelial cells and luminal clear and dark cells, named for their appearance

in eosin, toluidine blue, and methylene blue stains.

Myoepithelial cells strengthen

the structure of the secretory coil and create a microenvironment for gland stem cell

differentiation.

Clear cells contain many mitochondria suggesting that they facilitate most

of the active sweat secretion and osmotic flow.

Dark cells are granular, containing many

vesicles. Dark cells are more involved in the secretion of proteins, including periodic acid-

Schiff (PAS)-positive diastase-resistant glycoproteins, dermicidin, and sialomucin.

The

interdependent relationship between clear and dark cells requires further investigation.

Sweat secretion is stimulated by adrenergic and cholinergic innervation (Fig. 2c). The

sudomotor response involves several adenosine triphosphate (ATP)-dependent steps, and is

suppressed by ouabain and metabolic inhibitors.

When the secretory cell is stimulated,

a signaling cascade occurs involving Ca

or cyclic adenosine monophosphate (cAMP) as

second messengers to trigger the efflux of Cl

−

into the lumen of the secretory coil. Na

pumped out at the basolateral membrane and diffuses down its electrochemical gradient into

the lumen. The buildup of electrolytes in the lumen renders it hypertonic with respect to the

cytosol; this osmotic gradient drives the primary sweat solution out of the cell and into the

secretory lumen (Fig. 2d). Advective mass transport drives fluid up the eccrine sweat duct.

Along the sweat duct, luminal cells reabsorb ions to produce a hypotonic sweat solution. We

describe this process in further detail below.

2.2 Sweat Stimulation

Thermoregulatory sweating is an autonomic response to signals from thermoreceptors in the

preoptic-anterior hypothalamus area. Upon an increase in core temperature, thermoreceptors

send through efferent pathways to postganglionic sympathetic neurons in the dermis.

Cholinergic nerve fibers around the secretory coil release acetylcholine, thus activating

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muscarinic receptors on the membrane of the eccrine secretory cell. Activation of muscarinic

G-protein-coupled receptors (GPCRs) increases intracellular inositol trisphosphate (IP

binds to receptors on the endoplasmic reticulum (ER) membrane to release Ca

into the cytosol.

Stromal interaction molecule protein, stromal interaction molecule 1

(STIM1), monitors the ER Ca

levels, and when Ca

stores are depleted STIM1 induces

store-operated Ca

entry by binding to and activating Orai, a Ca

channel on the plasma

membrane.

This influx of Ca

mediates the exchange of electrolytes resulting in sweat

secretion.

Sweating is also adrenergically stimulated under the “fight or flight” response. The

physical reaction to stress, anxiety, fear, and pain occurs mostly in the palms, soles,

and axillary region and may have the selective advantage of increasing palmoplantar

friction for fleeing.

“Emotional” sweating is controlled by the limbic system and

efferent signals are sent to adrenergic nerve fibers in the sweat secretory coil. Release of

epinephrine and norepinephrine in signaling stimulates

- and

-adrenoreceptors in sweat

secretory cells. A synthetic sympathomimetic drug, isoproterenol, selectively stimulates

-adrenoreceptors and has been used to further differentiate the two pathways.

-adrenergic

stimulation is the dominant pathway in emotional sweating. The magnitude of stimulated

sweat secretion (measured by secretory rate) is 4:2:1 for cholinergic,

-adrenergic, and

adrenergic pathways, respectively.

-adrenergic stimulation results in Ca

influx similar

to cholinergic pathways.

-adrenergic GPCRs activate adenylyl cyclase and increase the

intracellular concentration of cAMP. cAMP activates protein kinase A (PKA), which in turn

mediates Cl

−

secretion by opening the cystic fibrosis transmembrane conductance regulator

(CFTR).

In the case of cystic fibrosis, CFTR is defective or absent, resulting in blocked

CFTR Cl

−

secretion during

-adrenergic stimulation and inhibited Cl

−

reabsorption. A

“ratiometric” sweat rate test comparing adrenergic and cholinergic sweat rates may be used

to assess CFTR functional activity.

Sweat may be generated at the periphery of a stimulated region via the sudomotor axon

reflex (Fig. 2c). Nicotinic agonists interact with receptors on postganglionic sudomotor

terminals at the base of the sweat gland, causing antidromic axonal conduction towards a

branch point followed by orthograde conduction down the branching fibers. Acetylcholine

is then released at the nerve terminals and binds to muscarinic receptors on the eccrine

sweat gland, resulting in sweat secretion similar to the direct iontophoretic response.

The spatial extension of this sweating could be millimeters beyond the periphery of the

stimulation region.

The sudomotor axon reflex may be used to assess autonomic nervous

system disorders, such as diabetic neuropathy.

The sudomotor axon reflex may also

be used to separate drug-induced sweat stimulation and sweat sampling regions to prevent

cross-contamination.

The sudomotor axon response has a longer latency than the direct

cholinergic response by about 5 s, which accounts for axonal conduction and neuroglandular

transmission. The sudomotor axon response and direct response produce similar sweat

volumes in the presence of nicotinic agonists. In contrast to the direct stimulated sweat

response, which continues over an hour after cessation of the stimulus, the sudomotor axon

response returns to baseline 3–5 minutes after stimulus cessation.

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The sweat rate is modulated in part by non-uniform, localized activation. Under mental

stress, sweat production of adjacent sweat glands varied strongly.

The cumulative sweating

response controlled by the sympathetic nerve is discretized into active and inactive sweat

glands.

The sweat rate in healthy individuals ranges from 0.2–1 μL/cm

/min.

an average sweat gland density of 200/cm

, this equals 1–5 nL/gland/min. Sweat rate

is affected by local skin temperature.

Sweat stimulated pharmacologically may also

further increase the sweat rate to approximately 10 nL/gland/min.

Sweat rate decay

and cessation occur in part due to the subcutaneous elimination of the sweat stimulant

(e.g. acetylcholinesterase).

Interindividual variations in sweat rate are likely due to

differences in the function and responsiveness of the sweat gland.

Many factors may

influence the sweat response including gender, physical fitness, menstrual cycle, and

circadian rhythm.

Intraindividual regional variations in observed sweat rate may be

associated with variations in sweat gland density and distribution.

For example, the

forehead has a high density of sweat glands and has the highest tested sweat rate region

during both active and passive thermal sweating.

2.3 Sweat Secretion and Electrolyte Reabsorption

Upon stimulation, Ca

and cAMP act as intracellular messengers for sweat secretion.

activates transmembrane K

and Cl

−

channels. TMEM16A and bestrophin 2 are Ca

activated chloride channels (CaCCs) located on the apical membrane of secretory gland

cells. Bestrophin 2 is expressed only in dark cells, yet it is necessary for sweat generation.

CFTR is the active Cl

−

channel in cAMP-mediated

-adrenergic sweat secretion. PKA-

independent CFTR activation via calmodulin-mediated Ca

signaling results in cross-talk

between cAMP and Ca

signaling for CFTR regulation.

It is possible CFTR may be

involved in both sweat secretion pathways.

As Cl

−

diffuses into the lumen at the apical membrane, Cl

−

enters the cell via basolateral

Na-K-Cl cotransporter 1 (NKCC1), a Na

-K

−2Cl

−

electroneutral co-transporter. Excess

accumulated Na

is then actively pumped out via Na

exchanger 1 (NHE1) and Na

-K

ATPase.

is passively transported paracellularly down the electrochemical gradient

established in the lumen. A buildup of electrolytes in the lumen of the secretory coil

results in an osmotic gradient driving transcellular fluid flow via aquaporin 5 (AQP5) and

paracellular flow from the interstitial fluid (ISF).

As a result, the aqueous fluid in the

secretory coil becomes isotonic with respect to ISF, blood, and cytosol.

Continued sweat secretion drives flow up the sweat duct, where reabsorption of electrolytes

results in a hypotonic final sweat secretion. CFTR is necessary for Cl

−

reabsorption. Unlike

in the secretory coil, CFTR in the sweat duct is constitutively active. CFTR activity is

complexly regulated by intracellular cAMP, ATP, and K

levels. CFTR conduction of Cl

−

is transcellular, but CFTR is present at a greater surface density on the apical membrane.

In contrast to the secretory coil, Na

transport in the duct is transcellular rather than

paracellular. Na

is reabsorbed passively by the epithelial sodium channel (ENaC) at the

apical membrane and actively pumped at the basolateral membrane by Na

ATPase.

ENaC and CFTR interact with each other in complex ways.

reabsorption is reduced

by increases in luminal Ca

ENaC is regulated by Ca

in other reabsorption cells,

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suggesting that inhibition of ENaC by increases in Ca

reduces the membrane permeability

and passive Na

flux at the apical luminal cell membrane. As sweat rate increases, Na

reabsorption increases; but the Na

secretion rate increases relatively more, resulting in

higher salt concentrations at higher sweat rates.

Bicarbonate (HCO

−

) is involved in both sweat secretion and reabsorption yet the

mechanisms remain unclear. HCO

−

exchange is mediated by both CFTR and Bestrophin

2 channels.

Various carbonic anhydrase isoforms also regulate HCO

−

by reversibly

converting CO

to HCO

−

. Carbonic anhydrase II (CA2) operates intracellularly in secretory

coil clear cells and ductal cells.

Carbonic anhydrase XII (CA12) is a transmembrane

protein also broadly expressed in the sweat gland. Defective CA12 results in excessive Na

secretion in sweat.

HCO

−

plays an important role in regulating the acid-base chemistry

of sweat secretion both intracellularly and extracellularly. Cytosolic pH affects ion channel

activity. For example, ENaC becomes inhibited at acidic cytosolic pH in ductal cells.

pH-sensitive phosphatases occur in the intercellular canaliculi of secretory cells. HCO

−

may also be secreted in coordination with acidic proteins, such as sialomucin, to neutralize

the pH in the lumen.

Final sweat pH can range from roughly 5 to 7 and is positively

correlated with sweat rate. Sweat pH in the secretory coil has a neutral pH, like ISF; but pH

decreases as it moves through the sweat duct. This suggests that ductal HCO

−

reabsorption

at low sweat rates contributes to the acidification of sweat.

Fluctuations in sweat pH

represent a challenge in sweat sensing, both because pH may affect partitioning of detected

molecules and also because pH may directly affect biosensor performance.

Acclimatization to thermal (and physical) stimuli markedly affects sweat generation.

Physically fit individuals have higher glandular functions and sweat rates per gland after

methacholine stimulation.

Over a multi-week exercise series, acclimatization due to

increased fitness resulted in a reduced lactic acid concentration in sweat.

Additionally,

thermal acclimatization increases the Na

reabsorption capacity of the human eccrine sweat

gland.

2.4 Biomarkers in Sweat

Sweat is an information-rich biofluid containing many molecules that can serve as

biomarkers. Sweat is composed of various electrolytes, metabolites, hormones, proteins,

and peptides (Table 1). Sweat samples may be analyzed using metrics such as biomarker

concentrations, biomarker flux, sweat rate, sweat pH, and ionic strength to provide important

information as they correlate to health. In some cases, biomarker flux may represent a

better metric of analysis since it accounts for the dynamic water flux, which may affect

concentration measurements. Biomarker flux may be calculated using the product of sweat

rate and biomarker concentration.

Recent reports have shown promising correlations

between the levels of a number of sweat and blood analytes,

indicating the great potential

of using sweat as an alternative source for personalized healthcare. Since sweat is readily

available for noninvasive sampling, sweat is an attractive biofluid for point-of-care (POC),

at-home, and continuous diagnostics. Moreover, new biomarker discovery for precision

medicine can be greatly facilitated by the continuous, large sets of data collected through

non-invasive sweat analysis in daily activities.

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2.4.1 Analyte Transport and Partitioning—

Prior to electrolyte reabsorption in the

sweat duct, initial sweat secretion is isotonic and resembles filtered ISF. Passive sweat

secretion may result in reduced concentrations 10- to 1000-fold lower than in ISF

and blood plasma. However, blood-to-sweat correlations vary based on the analyte and

its subsequent partitioning. Analyte partitioning occurs primarily via transcellular and

paracellular transport. Small, uncharged analytes readily enter sweat transcellularly via

diffusion through the plasma membrane of capillary endothelial cells.

Large, hydrophilic

molecules enter sweat paracellularly via diffusion and advective transport through the

intercellular canaliculi between adjacent cells.

Transcellular transport of small, lipophilic molecules results in strong blood-sweat

correlations as these molecules freely diffuse across the selectively permeable cell

membrane. This is likely the dominant transport mechanism for several classes of analytes,

including steroid hormones (i.e. cortisol

), ethanol,

and many therapeutic and abused

drugs (e.g., nicotine, fentanyl). Partitioning is limited by the least permeable state. This

results in plasma correlations that hold only for the unbound fractions of the analyte as is

the case with cortisol.

For instance, ionization may impede the molecule from transcellular

transport. The pH of sweat may become an important consideration for weak acids and

weak bases due to the possibility of ion trapping. In the case of ammonia (NH

), which

has a pKa of 9.3, NH

diffuses readily into the secretory lumen but under acidic sweat

conditions (as in the case of exercise), NH

protonates to become ammonium (NH

). In

the protonated form, transcellular exchange is impeded and NH

accumulates in the lumen

of the sweat gland. This phenomenon results in amplified sweat concentrations.

Since

primary sweat pH is 7.2 to 7.3 in the secretory coil,

this phenomenon is likely to mostly

impact reabsorption in the sweat duct. While ion trapping is a common topic of research in

subcellular pharmacokinetics, the role of ion trapping in sweat partitioning warrants further

consideration.

The intercellular canaliculi forms a >10 nm gap for paracellular molecular transport, but

tight junctions adjoining secretory cells act as a roadblock.

Tight junctions are formed by

over 40 different proteins, with the claudin family of transmembrane proteins defining the

structure and selective permeability of the tight junction.

Paracellular sweat partitioning

is likely to occur during tight-junction remodeling allowing for ISF molecules in the

canaliculi to make their way into the lumen. Tight junctions may be modulated using

calcium chelators. For example, citrate addition leads to a >10x increased flux of glucose to

sweat from ISF.

Although paracellular sweat partitioning may result in significant dilution

from blood plasma protein levels, this nonspecific channel for proteins from the ISF may

still result in correlated blood plasma ratios for trend analysis.

The observed lag time between blood and sweat measurements is on the order of ones to

tens of minutes. The secretory coil is highly vascularized, minimizing the lag in circulating

blood changes.

In the simplified case of transcellular transport, the rate of diffusion

determines the time to enter the lumen of the sweat duct. Once in the lumen, advective

transport by osmotic fluid flow (i.e. sweat rate) determines the time from analyte secretion

to analyte elution. When the correlation of blood alcohol and sweat alcohol content was

measured continuously, the lag time for signal onset ranged 2.3–11.4 min and 19.32–34.44

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min for the overall curve.

The relative contributions of sweat flow rates and analyte

partitioning mechanisms on the sensor response remains obscure. For sweat generation

or refreshing to be the rate-limiting step, the sweat collection designs should be further

optimized. Further work is required to better define the variation in lag of different analyte

partitioning. However, real-time sensing is contextual; the measurement of an analyte whose

concentration changes slowly relative to the lag in transport and sensing is effectively a

real-time measurement.

2.4.2 Sweat Composition Analysis—

Sweat has been surveyed broadly using liquid

chromatography (LC) or gas chromatography (GC), mass spectrometry (MS), and nuclear

magnetic resonance (NMR) techniques. NMR requires minimal sample preparation but

achieves a lower sensitivity. MS is often preceded by chromatographic techniques to

enhance detection quality.

A high coverage LC-MS technique based on chemical

isotope labeling was used to identify over 2707 unique metabolites across 54 sweat

samples.

Subsequently, 83 metabolites were identified with high confidence. With such

a diverse dataset, LC-MS may be used to characterize the sweat submetabolome and draw

statistically significant observations based on gender and activity duration.

LC-MS and

GC-MS represent the gold standard of trace concentration sweat biomarker identification

and quantification. The disadvantage of these techniques is that they require expensive

equipment along with complex protocols that require thorough validation for use in

metabolite identification and quantification.

Regional variations in sweat composition have been studied using a variety of assays,

recently including NMR and multiplexed immunoassays.

In general, there are minimal

variations in sweat composition when sampling from different body locations. No significant

difference was observed for sweat cytokine composition at different arm locations, and

metabolic profiles are generally conserved across the body.

Sweat from the upper chest,

upper back, arms, and forehead exhibited similar NMR spectra.

Sweat from the lower

back, axillary, and inguinal regions contained a higher fat content, but this may be due to

sweat mixing with sebum since these areas also contain a high density of sebaceous glands.

Forehead sweat exhibited high levels of lactate, pyruvate, glycerol, and serine relative to the

arm sweat. Serine content was also high on the hands and feet.

Since serine is active in

skin regeneration, this is indicative that these regions may undergo more epithelial turnover.

The hands appear to have a lower content of natural moisturizing factors, such as glycerol

and urea.

For electrolytes, regional sodium chloride concentrations are well-correlated

with whole-body sweat concentrations, with the exception of forehead sweat possibly due to

the effects of a significantly higher sweat rate. The forearm, thigh, and calf were all highly

correlated and are potential single-site sweat collection areas. HCO

−

concentration was

high at the forearm despite the average sweat rate. K

and lactate concentrations were higher

at the extremities (foot, hand, and forearm).

Electrolytes:

and Cl

−

, the most copious electrolytes found in sweat, are partitioned

into sweat via active mechanisms that are tied to the osmotic secretion of water. Therefore

and Cl

−

serve as potent biomarkers of electrolyte balance and hydration status for

cystic fibrosis diagnostics and fitness monitoring applications. According to the Na

-K

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−2Cl

−

cotransport model as outlined previously, a series of cascading effects instigated

by the stimulation of cholinergic nerve endings surrounding the sweat gland induce the

influx of NaCl into the secretory coil lumen, which then causes the osmotic influx of

water.

In this primary sweat, Na

levels are isotonic and Cl

−

levels are slightly

hypertonic to plasma. However, as this fluid gets pumped through the duct, Na

and Cl

−

ions are reabsorbed through ENaC and CFTR to prevent rapid electrolyte loss. Despite wide

ranging concentrations, resulting sweat Na

and Cl

−

levels are often hypotonic to plasma

levels. Additionally, as reabsorption of these ions occurs at steady rates, increased sweat

rates correlate with increased Na

and Cl

−

levels in final sweat.

is another electrolyte secreted via the Na

-K

−2Cl

−

cotransport model that is relevant to

the function of nerve and muscle cells.

While understanding of the exact partitioning

mechanism of K

requires further investigation, studies have shown that K

levels of

primary sweat in the secretory coil are isotonic to plasma levels but increase to hypertonic

levels in final sweat exiting the duct.

Furthermore, K

concentrations seem to not have a

strong correlation with sweat rate.

, an electrolyte found in sweat with metabolic origins, is of interest for tracking liver

and kidney function, as well as exercise intensity.

As described previously, NH

is a

small and uncharged polar molecule with a pKa of 9.3 (weakly basic), allowing for passive

diffusion into the sweat gland lumen where weakly acidic conditions result in increased

protonation to ammonium. Due to its charge, the ammonium ion gets entrapped in the lumen

of the sweat gland, yielding sweat ammonium levels to be 20 ~ 50 times higher than plasma

ammonium levels.

Furthermore, sweat ammonium levels have been reported to decrease

with increased sweat pH and sweat rates.

Metabolites:

Blood glucose monitoring is critical for managing diabetes, and sweat glucose

has the potential to serve as a non-invasive surrogate. Some studies have shown positive

correlations between sweat and blood glucose levels, and while the exact partitioning

mechanism is still being studied, the primary source of sweat glucose is likely to be from

blood through paracellular transport.

The rather large size and polarity of glucose

likely limit its passage through the tight junctions of the sweat gland, resulting in sweat

glucose levels being ~ 100 times lower than blood glucose levels.

Lactate is a metabolite found in sweat that has been extensively studied as a potential

biomarker for muscle exertion and fatigue. While the transport mechanism of lactate from

plasma to sweat is obscure and the correlation between lactate levels in sweat and plasma is

weak, sweat lactate is also produced from sweat gland metabolism and can still be reflective

of whole-body exertion. Sweat lactate levels are typically higher than blood lactate levels

and decrease with increased sweat rates, potentially due to dilution.

Along with ammonia, urea, uric acid, and creatinine are nitrogenous compounds produced

from protein metabolism that indicate renal function. As a small polar molecule that can

passively diffuse through the sweat gland through paracellular transport, sweat urea has

been speculated to primarily originate from the blood.

However, reported sweat urea

concentrations are often significantly higher than blood urea concentrations (up to 50 times),

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potentially indicative of additional sources of urea in sweat. A popular hypothesis is that

there is a finite pool of urea in the epidermis that gets depleted during profuse sweating,

supported by studies showing that sweat urea levels trend towards blood urea levels with

increased sweating.

Additionally, studies indicate the potential for active mechanisms

of urea excretion through sweat as an alternative method for excreting excess metabolic

wastes.

–

Uric acid and creatinine are slightly larger molecules that are found in sweat

at micromolar levels and around 5 times lower than in blood.

–

While the partitioning

mechanism of these metabolites has not been studied in detail, a positive correlation between

sweat and serum uric acid levels has been reported.

Minerals:

Trace minerals such as Ca

, Mg

, Fe

, and Zn

are often found in sweat at

concentrations similar to or slightly lower than blood concentrations

–

. Due to their small

size and hydrophilicity, these trace minerals have the potential to be secreted through sweat

via paracellular mechanisms in their free and ionized states. However, approximately 30 ~

45% of plasma Mg

, 50% of plasma Ca

, 70% of plasma Zn

, and above 95% of plasma

are bound to proteins or complexed with anions, likely impeding passive diffusion into

the sweat glands.

Nutrients:

Water-soluble vitamins such as ascorbic acid and thiamine, which are large and

polar molecules, have been reported in sweat at concentrations significantly lower than

in blood

. On the other hand, amino acids, which are the building blocks of protein

in our body, are often found in sweat at concentrations similar to or sometimes even

higher than in blood.

–

The levels of amino acids in sweat are likely attributed to

partitioning from plasma, as well as production of natural moisturizing factors (NMF) and

hydrolysis of the epidermal protein filaggrin in the stratum corneum.

As such, studies have

shown that sweat amino acid concentrations decline with increased sweat rates.

Positive

correlations between sweat and serum levels, as well as increases in sweat concentrations

after supplement intake have been reported for nutrients such as ascorbic acid and branch-

chain amino acids (BCAAs).

Hormones:

Hormones are chemicals that carry signals throughout our body for regulating

physiological processes and behavior. Cortisol is a primary glucocorticoid hormone

produced by the adrenal glands to regulate the body’s stress response. As a large lipid-

soluble molecule that can diffuse through lipid bilayer membranes via intracellular passive

transport, unbound cortisol is found in various body fluids.

However, over 90% of

endogenous cortisol in blood is bound to carrier proteins that hinder intracellular passive

transport.

–

While significantly lower in concentration than serum cortisol levels, cortisol

levels in sweat and saliva have been reported to correlate with unbound cortisol in

serum.

Neuropeptide Y (NPY) is one of the most abundant peptides in the central nervous system

and acts as a hormone that has close ties with stress, appetite, and depression.

When the

levels of various cytokines and neuropeptides were compared between women with and

without major depressive disorder (MDD), elevated sweat NPY levels were observed in

patient subjects.

In addition. a good correlation was found between NPY levels in sweat

and blood.

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Proteins:

Proteins are macro molecules (> 5 kDa) constructed by numerous amino acids.

In sweat, proteins with protective functions for maintaining the epidermal barrier integrity

(dermicidin, apolipoprotein D, clusterin, prolactin-includible protein, and serum albumin)

make up 91% of the secreted proteins.

–

Trace-level proteins such as c-reactive protein

(CRP) and cytokines are of particular interest as they modulate the body’s inflammation

and immune response. While CRP levels in sweat have been reported to be significantly

lower than in blood, many cytokines have been reported to be in sweat at concentrations

similar to or higher than in blood.

100

101

While the exact partitioning mechanism of

cytokines into sweat is uncertain, promising correlations between sweat and blood cytokine

levels have been widely reported.

100

Considering their large size, it is likely that most

of the cytokines found in sweat are produced locally by the eccrine gland. However, their

production is often due to a systemic response throughout the body, and therefore sweat

cytokine levels can still be reflective of systemic levels.

Substances:

Exogenous substances including toxins and drugs are often metabolized by

enzymes and excreted via urine and sweat. When alcohol is ingested, 90% of the ethanol

is broken down sequentially into acetaldehyde, acetate, and acetyl coenzyme A (CoA); and

a portion of the remaining ethanol is excreted through sweat. As ethanol is both soluble in

water and lipids, it can passively diffuse through most membranes in the body, leading to

strong correlations between sweat and blood ethanol levels.

2.5 Sweat Physiology Outlook

There is much yet to learn about sweat gland physiology. Human sweat duct density and

distribution have been investigated using ductal pore counting, colorimetry, and plastic

impression techniques.

The advent of optical coherence tomography has allowed for non-

invasive morphological visualization.

3D sweat gland tissue models are being developed

to better understand sweat physiology for pathology and tissue regeneration.

These models

will contextualize sweat measurements and correlated analyte concentrations by revealing

interdependent pathways. Improved sweat gland models may help in developing algorithms

for calibration.

The development of continuous, compact, on-body collection-to-analysis sweat sensing

platforms will further improve the quality and quantity of data for sweat characterization.

Real-time multiplexed sweat measurements will also contribute to our understanding of

the physiological sweat response. Tissue level sweat pH regulation remains a hurdle for

pH-dependent sensing platforms. Understanding the acid-base controls in the sweat gland

may aid in designing on-body stimulation and collection platforms at predictable sweat

pH. Elucidating the factors that contribute to dynamic sweat concentrations and analyte

partitioning is necessary to relate noninvasive sweat measurements to system-level changes

both in time and concentration. Increasing our understanding of the physiology of the

sweat gland and surrounding skin tissue may also better explain variations in localized

sweat measurements from systemic trends. Data from biological models and wearable sweat

sensors will complement each other for growth in both fields.

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3. Essential Material Properties for Wearable Sensors

On-body applications and unique operating conditions in complex biofluids require

wearable sweat sensors to have certain essential material properties. Functional materials

in wearables typically include four classes

: natural materials such as textiles and papers,

inorganic conducting materials as well as nanomaterials including metallic and carbon-based

composites, stretchable polymers, and stretchable hydrogels (Table 2). In this section, we

summarize and review essential material properties requisite for wearable sweat sensing

applications.

Biocompatibility.

Biocompatibility is defined as the ability of a material to perform with an appropriate host

response in a specific application.

102

Biocompatibility is one of the key considerations

for wearable sensors, as non-toxic materials as well as resistance to biofouling and

corrosion are prerequisite before on-body human experiments. A number of biomaterials

have been studied, including metals (titanium, gold, stainless steel and alloys), ceramics,

polymers and composite materials.

103

For wearables, natural materials such as cellulose

and fabrics are optimal for long-term wearing,

104

–

106

while a number of synthesized

inert materials have demonstrated similar properties.

107

Depending on their applications,

transient bioresorbable materials have also been developed to meet disposable use.

108

109

Permeability.

Permeability of a material is an advantage for long-term wear as it allows the exchange

of heat, air, and moisture, which affects thermal comfort and wetness discomfort.

110

Most

early pioneering examples of wearables were focused on transitioning rigid wafer-based

materials into flexible ones, which usually applied polyimide (PI) or polydimethylsiloxane

(PDMS) as the substrate, and thus were not gas or sweat permeable.

111

Subsequent research

introduced a number of gas and sweat permeable material substrates, including textiles and

fabrics,

105

106

as well as tattoo-like electrodes without substrates

112

; but, the permeability to

sweat also causes measuring inaccuracies of sweat biomarkers. To meet the recent demands

of sweat monitoring, materials that are comfortable, gas permeable, and sweat impermeable

have been further developed, often using structurally engineered nanomeshes with tiny pores

for gas exchange.

113

114

Conductivity.

Conductive materials are the foundation for wearable sensors and devices. An ideal

conductive material aims to retain stable electrical performance against strain and sweat

interference. One example is to use liquid metals and ionic liquids,

115

–

118

as they can

best offer conductivity and stretchability in their intrinsic fluidic nature. However, liquid

metals need to be encapsulated in channels and typically require complex designs for

integrated electronics. Therefore, a number of conductive polymers and hydrogels have been

introduced that balance cost and stretchability.

119

120

In order to improve the conductivity,

nanomaterials including both nanoparticles and nanowires are often adopted.

121

122

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Transparency.

Considering the wearing comfort and daily user compliance, transparent materials have

gained rising interest. One design strategy is to use ultrathin materials, and build

the wearable sensors into tattoo-like patches for both physiological and biochemical

monitoring.

123

124

Another approach is to apply intrinsically transparent nanomaterials such

as nanomesh and nanowire networks

125

–

127

, and transparent polymers and hydrogels such as

PDMS.

128

Adhesion property.

A strong adhesion to skin can improve signal reliability. However, excessive adhesion may

also make removing the wearable patch difficult after use and may even cause skin irritation.

For ultrathin and ultralight electrode tattoo patches, they can be applied onto human skin

by surface tension.

112

129

For more complicated wearable devices with electronics, external

adhesives are usually introduced, such as using bandages or medical ahsesives.

130

131

But

these adhesive methods usually require additional cleaning as adhesive residue is often

leftover on the skin after the patch is removed. Several recent studies have focused on

adhesive dry electrodes

132

133

, which aim to achieve a robust and reversible adhesion on

sweaty skin.

Scalability.

Fabrication cost and scalability is one of the key considerations when it comes to practical

use. To decrease the cost of conventional cleanroom lithography,

123

a number of fabrication

methods have been carried out, including transfer printing,

134

electrospinning,

112

roll-to-

roll gravure printing,

135

laser engraving,

3D printing,

136

inkjet printing,

137

and screen

printing.

138

Overall, the application of on-body wearable sensors requires building materials to be

biocompatible, conductive for electrical interconnects, comfortable for daily wearing,

and scalable for mass fabrication. In addition to these requirements that are general to

wearable sensing, wearable sweat sensors demand special attention to materials that can

achieve strong adhesion with sweaty skin, high stability in the sweat matrix, and selective

permeability of gas and sweat.

4. Biosensor Mechanisms

Novel sweat sensing platforms based on various detection methods have raised enormous

attention for non-invasive and real-time biomarker detection

in situ

for personalized

healthcare.

–

These sensors are required to be miniaturized for on-body wearing and

label-free for direct measurements.

Not only are they required to have robust performance

in complex and dynamic chemical environments, but also have a sufficient detection limit

and a wide linear range to detect biomarkers at physiologically relevant ranges. Most

wearable sweat sensors are based on electrochemical, optical and bioaffinity detection

mechanisms. In the following sections, we describe in detail the major categories and their

sensing mechanisms.

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