A portable optical human sweat sensorMahmoud Al-omari, Gengchen Liu, Anja Mueller, Adam Mock, Ruby N. Ghosh, Kyle Smith, and Tolga Kaya Citation: Journal of Applied Physics 116, 183102 (2014); doi: 10.1063/1.4901332 View online: http://dx.doi.org/10.1063/1.4901332 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/116/18?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Evaluation of efficiency of laser systems on removal of debris and smear layer in maxillary canine root canals J. Laser Appl. 26, 032005 (2014); 10.2351/1.4870878 Measurement of compressed breast thickness by optical stereoscopic photogrammetry Med. Phys. 36, 569 (2009); 10.1118/1.3065066 Assessing the future of diffuse optical imaging technologies for breast cancer management Med. Phys. 35, 2443 (2008); 10.1118/1.2919078 A portable, low coherence interferometry based instrument for fine needle aspiration biopsy guidance Rev. Sci. Instrum. 76, 064301 (2005); 10.1063/1.1921509 Optical instrument for measurement of vaginal coating thickness by drug delivery formulations Rev. Sci. Instrum. 76, 034302 (2005); 10.1063/1.1863832

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A portable optical human sweat sensor

Mahmoud Al-omari,1 Gengchen Liu,2 Anja Mueller,3 Adam Mock,1,2 Ruby N. Ghosh,4

Kyle Smith,2 and Tolga Kaya1,2,a)

1Science of Advanced Materials Program, Central Michigan University, Mt. Pleasant, Michigan 48859, USA2School of Engineering and Technology, Central Michigan University, Mt. Pleasant, Michigan 48859, USA3Department of Chemistry, Central Michigan University, Mt. Pleasant, Michigan 48859, USA4Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824, USA

(Received 14 August 2014; accepted 29 October 2014; published online 10 November 2014)

We describe the use of HNQ (2-hydroxy-1,4-naphthoquinone or Lawsone) as a potential sweat

sensor material to detect the hydration levels of human beings. We have conducted optical

measurements using both artificial and human sweat to validate our approach. We have determined

that the dominant compound that affects HNQ absorbance in artificial sweat is sodium. The

presence of lactate decreases the reactivity of HNQ while urea promotes more interactions of

sodium and potassium ions with HNQ. The interactions between the hydroxyl group of HNQ and

the artificial sweat components (salts, lactic acid, and urea) were investigated comprehensively.

We have also proposed and developed a portable diode laser absorption sensor system that

converts the absorbance at a particular wavelength range (at 455 6 5 nm, where HNQ has an

absorbance peak) into light intensity measurements via a photocell. The absorbance intensity

values obtained from our portable sensor system agrees within 10.4% with measurements from a

laboratory based ultraviolet-visible spectrometer. Findings of this research will provide significant

information for researchers who are focusing on real-time, in-situ hydration level detection. VC 2014AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4901332]

I. INTRODUCTION

As biosensors become ubiquitous and receive endorsement

from the general public, research and development on biologi-

cal sensing is experiencing a rapid rise.1,2 The persistent con-

sumer demand for wearable and portable devices has shifted

the research of biosensors toward more application-oriented

processes. This particular shift has resulted in the development

of biological sensors that are non-invasive and have thus

eclipsed the traditional blood analysis methods. To this point,

the development of portable, non-invasive, and biocompatible

sensors has focused mostly on temperature, relative humidity,

glucose, pH, and blood pressure.3–6 Recently, there has been a

growth in research involving the use of sweat as a sensor

analyte to measure certain body functions. Hydration status,

cystic fibrosis diagnosis, and kidney functions analyses are con-

sidered to be particularly enhanced by investigating an individ-

ual’s sweat content.7–9 HNQ (2-hydroxy-1,4-naphthoquinone

or Lawsone) has recently drawn attention in sensor research, as

it has been identified as a promising material for use in skin-

contact biological sensors. HNQ was used as a biological

marker to detect latent fingermarks on paper surfaces.10

Aqueous HNQ solutions were also used to detect pH changes.11

A cyanide and acetate sensor was implemented using HNQ

optical properties.12

It has been well established that sweat can be considered

the diluted version of blood in terms of constituents such as

electrolytes and minerals.13,14 Although contents in sweat

appear in lower concentration than in blood, the sensitivity

of current sensor technology has made it possible to detect

such changes. Sweat contains several hundred different

chemicals.13,14 A good representation of sweat, however, can

be standardized into four major constituents: sodium ions, po-

tassium ions, lactate, and urea.15,16 Combinations of these

compounds are used to create artificial sweat in order to inves-

tigate mainly corrosion and nanoparticle release effects on tex-

tiles and metal implants.16–20 In recent years, the electrolyte

content of sweat has been used to determine the physiological

status of a person, such as hydration or tiredness.8,21–23

In-situ sweat analysis has been centered mainly on pH

level detection and sodium ion concentration monitoring.

Diamond’s group, as a part of the BIOTEX project,24 devel-

oped textile sensors for sweat pH analysis using optical tech-

niques.25,26 The analysis of the sodium ion concentration of

sweat has been investigated using skin contact electrodes to

detect the real-time electrolyte-level changes.27,28 In other

research, the sweat lactate levels have been determined using

electrochemical tattoos.29 Sweat ethanol levels have been

monitored via an electrochemical transdermal alcohol con-

centration sensor.30

Besides electrochemical methods, the detection of opti-

cal property variation in response to changes in the concen-

tration of sweat content has shown great promise. We have

previously demonstrated that a natural dye known as HNQ

can be used as a sweat sensing material based on color

changes.31,32 Our previous work focused on the optical prop-

erties of HNQ under various hydration conditions where the

optical absorbance is linearly related to the amount of ions

that are present in a liquid media along with HNQ.32

In the current work, optical absorbance versus concen-

tration of sweat components is analyzed. The effects of

a)Author to whom correspondence should be addressed. Electronic mail:

kaya2t@cmich.edu

0021-8979/2014/116(18)/183102/7/$30.00 VC 2014 AIP Publishing LLC116, 183102-1

JOURNAL OF APPLIED PHYSICS 116, 183102 (2014)

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increasing the concentration of four primary chemical com-

ponents of sweat both in aggregate as well as individually

are presented and discussed. The sensitivity of the sensor

technique is presented and shown to be sufficient for the pro-

posed application. Finally, we have successfully imple-

mented a portable diode laser absorption sensor (PDLAS)

system that would eliminate the need for bulky laboratory

instruments in testing the optical properties of sweat. This is

a first step toward a deployable, wearable sweat sensor. Our

investigation of sweat chemistry sensing is particularly

groundbreaking as it will shed light on the effect of sweat

content on the real-time physical condition of subjects such

as athletes and soldiers.

II. ARTIFICIAL SWEAT

Artificial sweat is defined as a water solution with four

primary dissolved compounds: sodium chloride (NaCl), po-

tassium chloride (KCl), urea, and lactic acid. It is important

to note that human sweat contains additional compounds in

low concentrations besides the four compounds in artificial

sweat. However, for many applications and for the purposes

of this work, measuring these four compounds is sufficient.

The artificial sweat used in this study was prepared accord-

ing to the European Standard number EN1811: 2012 by

dissolving the following compounds in 1 liter of ultra-

purified deionized water (resistivity of 18.3 MX cm): 0.5%

of NaCl, 0.1% of KCl, 0.1% of lactic acid, and 0.1% of urea

(weight/volume ratio). NaCl, KCl, and lactic acid were

acquired from VWR International, Inc. Urea was acquired

from Fisher Scientific, Inc. The resulting constituent concen-

trations are 85 mM NaCl, 13 mM KCl, 17 mM lactic acid,

and 16 mM urea. In this study, these concentrations of artifi-

cial sweat represent “normal” human levels. The target

application of this work is dehydration sensing, which is

manifested as an increase in the concentration of these 4

principal sweat components from their base values.

It has been reported that the HNQ dye has redox proper-

ties allowing the binding of cations at pH values between 5

and 7.33 For this reason, pH was carefully controlled in these

experiments. The Eco Testr PH2 pH meter was used to deter-

mine the pH value for the samples; the pH of the artificial

sweat solutions was adjusted to 5.4 6 0.3 by drop-wise addi-

tion of 0.01 M ammonium hydroxide (NH4OH, Fischer

Scientific, Inc.).

III. SWEAT CHEMICAL ANALYSIS USING HNQ DYE

The main goal of this paper is to validate the use of

HNQ as a sweat sensor material to quantify the hydration

levels of human beings in-situ. One drop of real/artificial

human sweat solution into 15 ml HNQ solution causes a sig-

nificant change in the intensity of the absorbance peak cen-

tered at 455 nm as shown in Figure 1. We have previously

shown that the sweat itself does not have an absorbance in

the region of 400 nm and 500 nm.32 Sweat contains 98%

water, and the remaining is mainly electrolytes, urea, and

lactic acid. Figure 1 shows the UV-Vis spectrum of HNQ

dye by itself, with artificial sweat as described in Sec. II and

real human sweat collected from subjects (approved by

Central Michigan University’s Institutional Review Board).

The measurements were performed with a standard Varian

CARY 1 Ultraviolet absorbance spectrometer with 1 nm

resolution. The HNQ (Aldrich Chemical Company) is dis-

solved in a solution of 4:1 DI: Acetone (volumetric ratio).

In earlier research, we found that HNQ changes color

when it is doped with sodium or potassium ions.31 It turns

from pale orange to dark orange (see Figure 2(a) inset). The

main reason behind the color change is that the hydroxyl

group of HNQ (see Figure 1 inset) is deprotonated, and the

negatively charged oxygen atom is attached to positively

charged sodium or potassium ions, forming a doped HNQ

structure.31 This effect changes the electron density of the

FIG. 1. UV-Vis spectrum of HNQ solutions with and without real/artificial

sweat. Human/artificial sweat data were obtained by adding a drop of sweat

into 15 ml of HNQ solution. Our focus is on the absorbance peak centered

around 455 nm, which is not present in a pure sweat sample.32 The inset

shows the chemical scheme of the HNQ molecule.

FIG. 2. (a) UV-Vis spectra at 5 repre-

sentative artificial sweat constituent

concentrations. Inset shows photo-

graph of the 5 samples arranged from

lowest concentration (left) to highest

(right). (b) Average absorbance versus

concentration at center wavelengths

ðkcenterÞ 455 nm, 555 nm, and 655 nm.

The averaging was performed over a

10 nm bandwidth.

183102-2 Al-omari et al. J. Appl. Phys. 116, 183102 (2014)

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benzene, which enhances the absorption peak at 455 nm. As

more blue/green light at 455 nm is absorbed, only the red/

orange regions of the spectrum pass through the sample giv-

ing its characteristic color.

Figure 2(a) shows the optical absorption spectra meas-

ured by conventional UV-Vis spectroscopy for 5 representa-

tive artificial sweat þ HNQ samples. Various concentrations

of artificial sweat were prepared by increasing the concentra-

tions of the 4 primary compounds while keeping the relative

ratios the same. The goal is to demonstrate the ability to

measure a wide range of hydration levels. The broad peak

centered at 455 nm grows in amplitude with increasing artifi-

cial sweat concentration, whereas in the wavelength region

500 nm–800 nm, the absorbance is relatively unchanged.

Figure 2(a) inset illustrates that the effect of absorption

due to large differences in concentration of artificial sweat is

visible to the naked eye. To ensure that the optical absorption

results from HNQ molecular absorption and not simply from

increasing conductivity of the liquid due to a higher concen-

tration of dissolved charged species, the average absorbance

centered at 455 nm, 555 nm, and 655 nm is plotted in Figure

2(b). These are absorbance values averaged over a 10 nm

region centered at the wavelength of interest. The wavelength

value at which the maximum near 455 nm occurs can shift by

as much as 65 nm due mostly to the breadth of the peak and

the limited spectrometer dynamic range. The 10 nm averaging

procedure removes any ambiguity in concentration measure-

ment due to small spectral shifting. In a deployed and/or wear-

able sweat sensor, a light source with a sufficiently broad

spectrum will automatically perform this averaging procedure.

In Figure 2(b), it is clear that the strongest absorbance

occurs at 455 nm, whereas the absorbance at 655 nm is barely

above the measurement noise floor. The continuous lines are

least squares fits of the formula ABSavg ¼ affiffiffiffiCpþ b, where

ABSavg is the average absorbance, C is the concentration, and

a and b are fit parameters. The values of the fit parameters

are (a, b)¼ (6.98, �0.57); (0.674, 0.059); (0.061, 0.005) for

455 nm, 555 nm, and 655 nm, respectively. The excellent

agreement between the trend lines and the experimental data

suggests that the dependence of ABSavg on C is indeed of the

form ABSavg ¼ affiffiffiffiCpþ b. On the other hand, Beer’s law

states that the absorbance is linearly related to the concentra-

tion of the media C, the attenuation coefficient or molar

absorptivity e, and the path length l:

ABS ¼ elC: (1)

However, the law is only valid for low concentration

samples, particularly for concentrations below 10 mM.34 As

the concentration increases, the spacing between ions or mol-

ecules in the medium are reduced so that neighboring par-

ticles contribute to the charge distribution, ultimately

altering the absorption.34 In other words, solutions that con-

tain electrolytes can alter the molar absorptivity due to elec-

trostatic interactions.34 This effect is apparent in Figure 2(b)

as the concentration range is from 10 mM to 250 mM. In

fact, derivation of Beer’s law excludes the interaction

between particles.35

Here, sensitivity is defined in terms of the minimum

measureable change in transmitted light intensity for a given

change in component concentration DCmin ¼ 1jdI=dCjDImin,

where I ¼ Ioe�ax (Beer-Lambert law36) and

dI

dC¼ Ioe�ax d �axð Þ

dC¼ �2:3I

d ABSavgð ÞdC

¼ �2:3Ia= 2ffiffiffiffiCp� �

:

(2)

Here, DCmin and DImin are defined as the minimum de-

tectable concentration and light intensity changes, respec-

tively, Io is the nominal intensity, a is the absorption

coefficient, and x is the distance that the light travels in the

medium (i.e., the size of the sample container). The factor 2.3

appears because ABS ¼ �log10ðI=I0Þ and ax ¼ �lnðI=I0Þ,and 2:3 � 1= log10e converts between natural log and log

base 10.

If a 10% relative intensity change can be reliably meas-

ured, then DImin

I ¼ 0:1 and DCmin ¼ 0:2ffiffiffiffiCp

=ð2:3aÞ. Figure

3(a) shows DCmin calculated from the trend lines. Because of

the sublinear increase in ABSavg with C, the minimum reliable

sweat component concentration increases as C increases. If

the dependence of ABSavg on C were linear or superlinear,

then DCmin would remain constant or decrease, respectively.

While the sensitivity trend depicted in Figure 3(a) is inferior

to a linear or superlinear dependence of ABSavg on C, it does

not limit the performance of the sensor in the present applica-

tion. Figure 3(b) shows the percent concentration uncertainty

(100% x DCmin=C) assuming 10% confidence in light inten-

sity changes. The uncertainty percentage decreases as concen-

tration increases with the most uncertainty occurring at low

sweat component concentrations. The uncertainty is well

below 10% in the neighborhood of “normal” human sweat

concentration of 0.1 M. From this analysis, we conclude that

the proposed sensor technology is sufficiently accurate for

reliable measurement of human sweat concentration.

FIG. 3. (a) DCmin versus C at kcenter ¼45565 nm using the fitted trend lines

from Figure 2(b). The assumed mini-

mum reliably measured intensity

change is 10%. (b) 100% � DCmin=Cversus C for kcenter ¼ 45565 nm. This

shows that the largest percent uncer-

tainty occurs at low concentrations,

and that for kcenter ¼ 45565 nm, the

uncertainty in measured C is actually

lower than 10% for sweat concentra-

tions larger than 0.03 M.

183102-3 Al-omari et al. J. Appl. Phys. 116, 183102 (2014)

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IV. SWEAT CHEMISTRY

In order to understand the effect of individual compounds

in sweat, different combinations of urea, lactic acid, NaCl,

and KCl were added to HNQ and the resulting optical spectra

were analyzed. All possible combinations of the four artificial

sweat components were prepared as shown in Table I.

Because normal human sweat has a pH in the range 5 to 7, the

pH of all samples was set to 5.4 6 0.3. The standardized artifi-

cial sweat concentration values were used. The data are

arranged in a binary setting where “1” representing the exis-

tence of the compound in the solution, as shown in Table I.

“0000” means there is no compound added to the HNQ.

The main reason of the increase in the absorbance peak at

455 nm is that sodium and potassium ions are attached to the

deprotonated HNQ as illustrated in Figure 1. It is evident that

83% of HNQ will be in deprotonated (forming the HNQ ion,

HNQ�, as shown in Figure 4) since the pKa value of HNQ is

4.0 (percentage of deprotonation can be calculated using the

Henderson-Hasselbach equation; pH ¼ pKa � logn

HNQ½ �HNQ�½ �

o)

and the pH is 5.6. Positively charged sodium and potassium

ions in the artificial sweat solution can then be bonded to

HNQ�, forming either HNQ-Na or HNQ-K, which changes

the electron density and thus energy of the ring, as detailed in

Figure 4.

This molecular change is reflected in the optical absorb-

ance spectrum and clearly observed in the experiments pre-

sented in Table I. In the cases where only NaCl and/or KCl

(samples #4, #8, and #12) are present in the solution, ABS(maximum absorption intensity obtained from UV-Vis meas-

urements in the region of 450 nm–460 nm) significantly

increased compared to the HNQ sample alone (sample #0).

As shown in Figure 5, adding KCl increases absorption by

1.5 times, adding NaCl increases the absorption by 2.6 times

and adding both KCl and NaCl increase the absorption by

3.1 times.

On the other hand, the presence of lactic acid (lactate�)

lowers the absorbance (sample #2 vs. #3, #6 vs. #7, #8 vs.

#9, and #12 vs. #13) as shown in Figure 6(a). Lactate (Lactic

acid) will be 96% dissociated (pKa¼ 3.86) to form nega-

tively charged lactate ions (lactate� in Figure 4), which will

prevent the process of HNQ protonation. Therefore, lower

percentage of HNQ� will cause a reduced intensity absorb-

ance. In two specific cases (sample #4 vs. 5 and #10 vs.

#11), lactate has a different effect; we postulate that some of

the Naþ and Kþ ions are consumed by binding to lactate and

thus are unavailable to bind to HNQ. As every negative ion

needs a positive ion, and Naþ and Kþ bind a carboxylate ion

more strongly than NH4þ, the results of sample #4 vs. 5 and

#10 vs. #11 are actually expected.

Urea, on the other hand, has two NH2 groups, which are

capable of accepting protons to form protonated urea, as

illustrated in Figure 4. Due to its pKa (pKa¼ 0.2, i.e.,

pKb¼ 13.8), 98% of the available urea molecules can donate

electrons to form ammonia ions, which can further deproto-

nate the remaining HNQ (17% from the initial solution)

resulting in higher concentration of HNQ-Na and/or

HNQ-K. This theory is well observed in Table I comparing

samples #0 vs. #2, #1 vs. #3, #9 vs. #11, etc., as shown in

Figure 6(b). Finally, the pH of the solutions was adjusted

with NH4OH to 5.4 6 0.3. This compound can react with the

remaining unreacted HNQ and create further negatively

TABLE I. The optical absorbance measured for different combination of ar-

tificial sweat while adjusting pH using NH4OH. The binary representation

corresponds to the presence of the compound (represented as “1”).

Sample

number

NaCl

(85 mM)

KCl

(13 mM)

Urea

(16 mM)

Lactic acid

(17 mM) ABS

0 0 0 0 0 0.16

1 0 0 0 1 0.18

2 0 0 1 0 0.21

3 0 0 1 1 0.18

4 0 1 0 0 0.24

5 0 1 0 1 0.29

6 0 1 1 0 0.30

7 0 1 1 1 0.28

8 1 0 0 0 0.41

9 1 0 0 1 0.37

10 1 0 1 0 0.29

11 1 0 1 1 0.40

12 1 1 0 0 0.49

13 1 1 0 1 0.34

14 1 1 1 0 0.57

15 1 1 1 1 0.58

FIG. 4. HNQ reacts with sweat compo-

nents resulting in doped HNQ (HNQ-

Na and HNQ-K), charged urea (Ureaþ),

and charged lactate (Lactate�). A

by-product of ammonia ions is also

generated due to the pH adjustment

with NH4OH.

183102-4 Al-omari et al. J. Appl. Phys. 116, 183102 (2014)

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charged HNQ ions (HNQ�), which ultimately results in

advance reaction of sodium and potassium ions.

As a result, the main mechanism behind the interaction

between sodium and potassium ions in sweat is the deproto-

nated HNQ. While urea promotes further deprotonation and

thus doping, lactic acid prevents the reaction. Although this

is the general trend, it must be noted that there are some

cases where this theory was not supported. We believe this is

mainly because some complex reactions we have yet to iden-

tify. A deeper study on the each compound’s interactions

with HNQ is needed.

V. PORTABLE SWEAT SENSOR

With a view toward a wearable, commercially available

diagnostic system, a size-scalable single wavelength optical

absorption measurement system (PDLAS) was built using

commercially available, off-the-shelf components. The light

source is an OSRAM PL450B blue-violet laser diode

(k0¼ 450 nm). This laser wavelength targets the maximum

of the HNQ absorbance peak. An Advanced Photonix

PDV-P9003 photoconductive photocell was used for detec-

tion. A schematic diagram and a photograph of the actual

setup are shown in Figure 7. A spherical lens with one-inch

focal length is used to collimate the laser beam. The beam

intensity is controlled by an optical attenuator that consists

of a stationary linear polarizer and a rotatable linear polar-

izer. Intensity outputted from the attenuator is I0 cos2(h),

where h is the angle of rotation between the two linear polar-

izers and I0 is the nominal laser intensity. Standard current

control circuitry was used as a buffer between the DC power

supply (BK Precision 1651 A) and the laser diode. The resist-

ance of the photocell was measured with a BK precision

5492B standard multimeter.

Characterization of the PDLAS is shown in Figure 8(a).

The relationship between the intensity and resistance of a

photocell is �log10ðI=I0Þ ¼ n log10ðR=R0Þ. I0 is the incident

intensity. Ro was experimentally measured to be 1.9 kX.

Figure 8(a) shows the experimental calibration used to deter-

mine n ¼ 1:85, which holds over a large dynamic range.

Figure 8(b) shows the comparison between the absorption

coefficient measured using a conventional UV-Vis spectro-

photometer and the PDLAS system. The absorption coeffi-

cient is related to ABS via ABS ¼ �log10ðI=I0Þ ¼ ax log10e,

so that a ¼ 2:3ABS=x where 2:3 � 1= log10e. The samples

size is x¼ 1.1 cm. The PDLAS system is able to reproduce

the UV-Vis measurements with an average disagreement of

10.4% (minimum and maximum errors are 1.2% and 25.2%,

respectively). While further iterations of the PDLAS imple-

mentation can potentially improve the average measurement

agreement to within 5%, even in its present form the PDLAS

can be used for reliable sweat concentration monitoring.

VI. CONCLUSION

The effect of hydration on the performance of athletes

has been studied over the years. There is a current trend

where non-invasive hydration detection systems are of inter-

est. Sweat analysis provides a viable alternative to blood

analysis towards this goal. However, current techniques of

sweat analysis require time consuming laboratory analysis

(results being obtained even in a longer time than a blood

analysis), hence are not applicable for frequent sampling.

This work has investigated HNQ dye and the role of

four primary sweat components (sodium chloride, potassium

chloride, lactate, and urea) in the ability to detect sweat com-

ponent detection optically. The sensing mechanism is shown

to be robust with sufficient accuracy for reliable detection.

Furthermore, the HNQ optical sensing approach is scalable

to a compact and potentially wearable form. As a step toward

a deployable sensor device, we have successfully imple-

mented a PDLAS that can be used to quantitatively

FIG. 5. Bar chart representation of samples 0, 4, 8, and 12 from Table I.

Adding salt into HNQ increases the absorbance significantly.

FIG. 6. The effect of urea and lactate on the optical absorbance: (a) Lactate

mostly decreases the absorbance; (b) Urea mostly increases the absorbance.

183102-5 Al-omari et al. J. Appl. Phys. 116, 183102 (2014)

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determine the hydration levels of human sweat in-situ by

eliminating the need for a complicated laboratory based mea-

surement setup. We believe that our approach will help

researchers to investigate advanced materials including but

not limited to natural dyes for their physical and chemical

properties towards compact, wearable sweat analysis

systems.

ACKNOWLEDGMENTS

Dr. Kevin Miller and Dr. Jeff Edwards provided valuable

insights on the sweat composition and sweat collection

procedures. Dr. Kivanc Sel provided fruitful discussions. Dr.

Bunyamin Sahin helped with the final reviewing. Funding

was provided by the Science of Advanced Materials program

at Central Michigan University, Central Michigan University

Research Corporation, and National Science Foundation,

grant number EEC-1201095.

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FIG. 8. (a) Characterization of the

PDLAS system. The function �log10

ðI=I0Þ ¼ n log10ðR=R0Þ is fit to the

data (linear in log-log scale). (b) The

absorbance coefficients for the UV-Vis

and PDLAS measurements form a

straight line with a slope of 1.1, which

corresponds to 10% error.

FIG. 7. PDLAS setup; (a) the photo of

the setup, (b) optical circuit schematic.

LP¼ linear polarizer, RLP¼ rotatable

linear polarizer.

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