The sweat rate as a digital biomarker in clinical medicine beyond sports science

18 Sweating is an important physiological reaction and a clinical symptom in a variety of 19 diseases. However, sweating remains underrated in clinical use. Gold standards to 20 measure the sweat rate are neither continuous, nor easily or lab-independently 21 applicable. With the emergence of novel wearable devices, using sweat rate as a novel 22 digital biomarker shows promise for clinical monitoring and diagnostics. In this 23 Commentary, we discuss the potential and importance of the sweat rate as a digital 24 biomarker beyond sport science.


INTRODUCTION
Sweating is an important physiological reaction to maintain the body's thermoregulation during exposure to environmental heat stress or during rigorous exertion.This is one of the main reasons why the sweat rate assessment has been investigated in sport science and occupational health since the emergence of wearable sweat sensing.Reports from diseases like the "Sudor Anglicus" in the 15 th century but as well common medical knowledge from oncology and infectious diseases have shown that sweating is not only a thermoregulatory, but moreover a clinical symptom in general [1][2][3] .Therefore, sweat analysis provides valuable information about health, disease, and even age [4,5] .Still, the sweat rate remains underexplored and underrated in clinical medicine.However, with the emergence and continuous advancements of sweat analyzing wearables, sweat rate analysis holds the potential to become a clinically, broadly available digital biomarker.In this Comment, we introduce and discuss sweat rate as a promising novel digital biomarker to monitor health and disease in clinical medicine beyond sports science.

Basics on sweat rate
The human body houses approximately two to four million sweat glands [6,7] .Eccrine, apocrine, and apoeccrine sweat glands can be distinguished [8,9] .While eccrine sweat glands are responsible for the highest volume of sweat excretion [10] , secretion of apocrine and apoeccrine glands can also impact the sweat composition at the skin surface [11] .During sweating, up to 2426 J of heat per gram of evaporated sweat can be dissipated from the body [12] .Aiming at maintaining thermoregulation, sweating is the most efficient way to dissipate heat from the body.Next to heat stress, sweat glands can be stimulated by emotional stress [13] , mechanical vibration [14] , eating spicy food (gustatory hyperhidrosis) [15] , or by chemical substances such as carbachol and local current [16] .

Neurological control of the sweat rate
Many thermosensitive neurons can be found in the preoptic area and the anterior hypothalamus [17] .To keep the body temperature constant, they initiate appropriate responses when detecting changes in body temperature [17] .Augmented local preoptic temperature or a rise in afferent impulses from the cutaneous and spinal thermoreceptors caused by elevated skin temperature can both result in increased sweating [17] .However increased core body temperature stimulates the sweat rate nine times more efficiently than increased mean skin temperature [18][19][20] .

Body map of sweat rates
Local sweat rate for the mid-front, sides and mid later back was found to be significantly higher in males compared to females [21] .In both sexes the highest sweating was observed along the spine, whereas sweat rate on the upper arm was lowest.Furthermore, total sweating on the back exceeded the total sweating of the chest.
For older males gross sweat loss and regional sweat rate were significantly lower compared to the young [22] .During rest, significantly lower regional sweat rates at almost all body regions were observed, whereas during exercise significant difference was found for the hands, legs, ankles, and feet [22] .

Influencing variables such as age, heat acclimatization
Environmental factors such as ambient temperature, air velocity and radiant load, but also clothing and the physical activity level influence the sweating rate [23] .
Heat acclimatization of five to eight days results in thermoregulatory adaptions such as increased sweat rate and earlier onset of sweating [24] .However, dependent on humid or dry heat exposure, the adaptation of eccrine sweat gland differs: the sweat rate in a hothumid environment is greater than in a hot-dry environment [24] .In healthy unacclimatized men, a sweating capacity of maximally 1.5 liters per hour has been reported, whereas in highly trained acclimatized soldiers a sweat rate of 2 to 3 liters per hour were reached [25] .Sweat rates of one liter per hour occur frequently, however, sweat rates can vary considerably [26] .Not only heat acclimatization, but also training can lead to an increase of sweat rate of 200 to 300 ml per hour [26] .

WEARABLE SWEAT RATE ANALYSIS
Diagnostic instruments that rely on sweat samples collected using absorbent pads yield a wealth of information related to physiological status and athletic performance [27,28] .
The protocols and the benchtop systems required for this purpose are, however, incompatible with real-time monitoring in the field.This is due to the bulk and expense of the hardware as well as the time and effort required for sample collection and preparation.Recent advances in flexible, hybrid electronics, soft microfluidics, and electrochemical sensors serve as foundations for emerging classes of skin-mounted systems for measuring the properties of sweat, each with features that overcome key limitations of conventional technologies [29][30][31] .
The measurement of the sweat rate in skin-mounted systems occurs through microfluidic devices that capture the sweat directly from the glands (Fig. 1).The pressure that drives fluid flow arises from the action of the sweat glands themselves, assisted by capillary effects in the microchannels.The microfluidic system usually consists of a thin polydimethylsiloxane (PDMS) layer embossed with appropriate relief geometry with a top-capping layer of PDMS that serves as a seal.The resulting overall thickness (usually smaller than 1 mm) and the addition of adhesive films enable intimate contact with the epidermis.Sweat rate strongly depends on the body location and the intensity of the exercise and it may range from 39 g*h -1 *m -2 to 614 g*h -1 *m -2 [22]   .Therefore, microfluidic devices are typically designed to accommodate tens of uL.
The inlet opening of a few mm in diameter enables sweat collection of multiple glands.
The measurement of the sweat rate can be implemented through various strategies that can be roughly categorized by transduction method: (i) optical/visual; (ii) electrical (impedance, resistive).The former consists of fully passive devices (do not need a battery) in which the volume change is estimated visually or with the help of a camera [32,33]   .More sophisticated designs offer quantitative information.They usually see the embedding of conductive traces or pads into the microfluidic channels.The flowing of sweat induces changes in the electrical impedance.Initial designs suffered from the interdependence of the impedance on the volume and ionic concentration of the sweat making the estimation of the rate difficult [34][35][36] .More recent designs have overcome this problem by (i) introducing differential measurements through two microfluid systems on the same patch (one for the ionic concentration and one for the rate) [37] and by (ii) patterning an array of pads along the channel to register discrete/digital changes of the impedance that enable time-volume synchronization independently from the ionic concentration [38] .It is worth mentioning that both methods require AC measurements to avoid the accumulation of ionic charge in the channels as well as the fouling of the impedance readout.This requirement complicates the circuit design of the wearable patch.While the previously mentioned devices rely on the indirect estimation of the rate via the measurement of the sweat volume and the passage time between some markers, another recent solution relies on the implementation of a flowmeter.It consists on the reading of the electrical resistance of two thermistors positioned on top of the microchannel and spaced out by a heater [39] .The flow of the sweat establishes a temperature change between the two thermistors whose dynamic response correlates with the flow rate.The simplicity of this strategy requires design optimization and intermittent operation to not incur high power consumption due to the heater.It is worth mentioning that all devices listed above are for single use: once the microfluidic is filled up, the device must be replaced by an empty one.Therefore, modular multi-layer designs are exploited to re-use the expensive layer that contains the electronic components and to dispose the microfluidic layer that can be produced with biodegradable polymers [40] .An overview on typical specifications of wearable sweat rate sensors and a list of reported designs can be found in Table 1.

Table 1. Typical Specifications of Wearable Sweat Rate Sensors
• Sweat rate: 39 to 614 g*h -1 *m -2 • Total capacity of the device: 10-50uL • Sweat opening with a mm-range diameter to collect the sweat from more than 10 sweat glands • The pressure that drives fluid flow arises from the action of the sweat glands themselves, assisted by capillary effects in the microchannels and the materials embedded within them

Device Materials and Design of µ-fluidics Sweat rate measurement
Ref.

1
A coiled tubing kept in position by a round plastic frame whose underneath presents a concave Sensor: Water-responsive chromogenic reagent Read-out: external camera for [33]  surface with an orifice time-stamped picture.

SWEATING -A SYMPTOME IN CLINICAL MEDICINE
Sweating is an unspecific, but very common symptom in clinical medicine.In clinical medicine, the quantification of the sweat rate can currently only be conducted during clinical examination.There are several qualitative measures (the Clinical Opioid Withdrawal Scale and the descriptive observation by the examinator) and quantitative measures (gravimetrical analysis using technical absorbents and determining changes in the body weight) that can be employed [41][42][43] .An easy, continuous, and straightforward measurement of the sweat rate during everyday life is not available, yet.Therefore, the full clinical potential of sweating as a symptom and an indicator for health and diseases has only been vaguely exploited until recently.

Clinical sweat terminology
To objectively assess the symptom "sweating", the sweat rate must be determined.It can be distinguished between local and whole body sweat rate.The local sweat rate refers to the excretion of sweat on a certain skin surface, whereas the whole body sweat rate covers the total loss of sweat from the body.Changes of the sweat rate can result from (i) either physiological and/or pathophysiological changes in the body, referred as non-iatrogenic, or (ii) be induced in the body for example by pharmacological therapy, referred as iatrogenic.The sweat rate may be increased above the physiological need in regard to maintain thermoregulation, named hyperhidrosis, or decreased, named hypohydrosis [44] .When there is no sweat excretion at all, it is called anhidrosis.
Derived from common medical terminology, changes in the sweat rate are classified as focal if a limited area on the body surface is affected or generalized if the whole-body surface is presumed to be affected.Changes in the sweat rate are categorized to be primary if being caused within the functional chain of the sweat gland or secondary, if induced due to external variables of the chain of sweat gland (Table 2) [45] .

Umbrella term Specification Description
Sweat rate local Sweat rate collected from a specific body surface over a specified amount of time

Clinical sweat monitoring
Both the local and the whole body sweat loss assessments are not continuously feasible and need lab infrastructure.Wearable sweat analysis to assess hydration during exertion or in hot environments are convenient and affordable [46] .These wearable sweat sensors enable sweat rate monitoring and can additionally assess changes in electrolyte concentration such as sodium, chloride and additional biomarkers of interest [47,48]   .By being coupled to smartphones, wearable sweat sensors provide a straightforward opportunity to continuously assess the local sweat rate.This in turn makes it possible to extrapolate the whole body sweat loss [47] .Furthermore, learning algorithms enable direct data interpretation and use within predictive models to establish preventive measures or to adapt therapies.

Hyperhydrosis
Primary hyperhidrosis is an idiopathic condition which occurs in 4.8% of the U.S.
population.The lead symptom is excessive sweating.Most affected regions are the plantar, the palmar, and axillary regions of the body [49,50] .While not primarily being life threatening, primary hyperhidrosis directly impacts social life.More severe causes of hyperhidrosis can be assessed during clinical anamnesis and examination.One of the more prevalent symptoms of hyperhidrosis is night sweats, which are assessed by inquiring the patients if night sweats have been observed.The current definition of night sweat is when pajamas had to be changed during the night as being soaked up wet.Due to the binary nature of the question, only the affirmative answer leads to documentation and therefore implementation into the treatment plan.In many instances, however, night sweats are not further investigated.Increased night sweats may have several underlining reasons such as: (i) elevated environmental conditions (temperature/relative humidity), (ii) infectious diseases such as viral infections with Influenza or COVID-19, (iii) bacterial infections such as pneumonia and even tuberculosis.Additionally, night sweats may appear when (iv) suffering from autoimmune diseases or (v) cancer.Also, (vi) hormone changes such as in hyperthyroidism, (vii) genetical changes in the sweat gland function (e.g. in cystic fibrosis), (viii) brain infarction, or (ix) pharmacological treatments with amiodarone or hormone substitution, may lead to hyperhydrosis.

Hypohydrosis
Hypohydrosis can be categorized into exogenous, dermatological, and neurological causes [51] .Systemic neurohormonal inhibition of sweating or skin and sweat gland damage run under exogenous reasons.Congenital disorders lead to dermatological disorders, and neurological pathologies can be caused by autonomous dysfunction.
While hyperhidrosis can be an indicator of serious health deterioration, hypohydrosis is less often clinically significant.However, hypohydrosis can be an indicator for peripheral polyneuropathy such as in Diabetes Mellitus.Hypohydrosis can be assessed using one of the few established clinical sweat tests, namely the thermoregulatory sweat test (TST) [52] .For the TST, an indicator powder changing color when getting in contact with sweat is applied to the skin.The person being examined is subsequently exposed to environmental heat that usually leads to increased sweating, thus, enabling to identify hypohydrotic skin areas.

CONCLUSION
Sweating is a common symptom in clinical medicine beyond sports science.Up to date, the absolute quantification of the sweat rate is challenging as the gold standard analysis by gravimetrical analysis is neither continuous, nor feasible outside of a lab setting.
These barriers are main factors why sweat analysis has not been implemented in clinical medicine yet.Novel wearable sweat analysing biosensors enables us to easily monitor the sweat rate continuously and lab-independently.With the emergence of these novel biosensing devices, the sweat rate is accessible for structured clinical investigation and can serve as a novel digital biomarker.Importantly, cyber security, bioethical, and policy makers' considerations need to be taken into account for a successful clinical implementation [53,54] .Clinical investigations are needed to demonstrate added clinical value of wearable sweat rate analysis for all stakeholders in healthcare.The device is formed by three main layers: (i) an adhesive layer to strengthen the contact with the skin, (ii) a PDMS layer with microfluidics and electrodes for sensing; (iii) flexible PCB to connect electronics and communication chips.

DECLARATIONS
Sensor: the sensing mechanism relies on the measurement of the resistance between metal pads patterned onto the wall of the microfluidic channels.Two separate channels allow to solved the interdependence of the resistance on rate and electrical conductivity of the sweat.AC modulation is implemented to avoid the formation of electronic double layer that may foul the reading.Read-out: flexible PCB for signal processing and communication via NFC [37]   6 Design consists of (i) an adhesive layer, (ii) a PDMS microchannel, (iii) a PDMS PCB that connects two thermistors and a heater and (iv) finally a PDMS cover.The design is simpler than the one of device 3 and 5 since it is rely on the direct measurement of the speed of the sweat flow rather than of the volume.The modular assembly facilitate the re-use of the PCB with with the disposable layers connected via magnets to the flexible PCB.
Sensor: differently from all previous devices that assess the rate by measuring the volume and the passage time, this device implement a flowmeter by reading the temperature difference between two thermistors.A heater positioned midway between the thermistors set their temperature which is identical before the flow of the sweat and that is different after.The difference of temperature induce a difference of the resistance of the two thermistors.
Read-out: flexible PCB for signal processing and communication via BLE [39]   Table 2. Summary of the terminology of clinical sweat assessment body surface is presumed to be affected primary caused within the functional chain of the sweat gland or secondary secondary due to external variables of the chain of the sweat gland

FigureFigure 1 .
Figure Sweat rate collected from a specific body surface over a specified amount of time (indicated in g*h -1 *m -2) body surface is presumed to be affected primary caused within the functional chain of the sweat gland or secondary secondary due to external variables of the chain of the sweat gland

Table 1 .
Typical Specifications of Wearable Sweat Rate Sensors • Sweat rate: 39 to 614 g*h -1 *m -2 • Total capacity of the device: 10-50uL • Sweat opening with a mm-range diameter to collect the sweat from more than 10 sweat glands • The pressure that drives fluid flow arises from the action of the sweat glands themselves, assisted by capillary effects in the microchannels and the materials embedded within them