Wearable Potentiometric Chloride Sweat Sensor: The Critical Role of

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A wearable potentiometric chloride sweat sensor: the critical role of the salt bridge Dong-Hoon Choi, Jin Seob Kim, Garry R. Cutting, and Peter C Searson Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b03391 • Publication Date (Web): 14 Nov 2016 Downloaded from http://pubs.acs.org on November 21, 2016

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Analytical Chemistry

A wearable potentiometric chloride sweat sensor: the critical role of the salt bridge Dong-Hoon Choia, Jin Seob Kimb, Garry R. Cuttingc, and Peter C. Searson*a,d a

Institute for Nanobiotechnology, John Hopkins University, 3400 North Charles Street,

Baltimore, USA b

Department of Mechanical Engineering, John Hopkins University, 3400 North Charles Street,

Baltimore, USA c d

Department of Pediatrics, John Hopkins University, Baltimore, USA Department of Materials Science and Engineering, John Hopkins University, 3400 North

Charles Street, Baltimore, USA *

Corresponding author

Peter C. Searson, 120 Croft Hal, Johns Hopkins University, Baltimore, MD, 21218 E-mail: [email protected]

Abstract. The components of sweat provide an array of potential biomarkers for health and disease. Sweat chloride is of interest as a biomarker for cystic fibrosis, electrolyte metabolism disorders, electrolyte balance, and electrolyte loss during exercise. Developing wearable sensors for biomarkers in sweat is a major technological challenge. Potentiometric sensors provide a relatively simple technology for on-body sweat chloride measurement, however, equilibration between reference and test solutions has limited the time over which accurate measurements can be made. Here we report on a wearable potentiometric chloride sweat sensor. We performed parametric studies to show how the salt bridge geometry determines equilibration between the reference and test solutions. From these results we show a sweat chloride sensor can be designed to provide accurate measurements over extended times. We then performed on-body tests on test subjects to establish the feasibility of using this technology as a wearable device.

Keywords: sweat chloride, salt bridge, chloride ion, potentiometric sensor

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1. Introduction Sweat is a non-invasively accessed bio-fluid containing electrolytes (sodium, potassium, and chloride ions), urea, lactate, and glucose 1,2 that can provide information on health state 3-5. The components of sweat can serve as biomarkers for various diseases, and to assess electrolyte balance and hydration level

6-9

diagnosis of cystic fibrosis

10-12

. For example, chloride sweat testing is the gold standard for . The analysis of the chloride ion concentration in sweat is

usually performed primarily by laboratory-based methods such as coulometric titration. The development of wearable sensors to measure biomarkers in sweat is recognized as a major technological challenge.13,14 Candidate technologies for wearable sweat chloride measurement include titration devices, conductivity measurements, and potentiometric sensors. Potentiometric measurements rely on the relationship between the ion concentration and the electrochemical potential of an electrode. This is a well-established analytical technique that can be readily miniaturized. Wearable potentiometric sensors using ion selective membranes have been developed for measurement of sodium and potassium ions in sweat 6,15, and wearable patchtype potentiometric sensors for chloride ions have been also developed 16,17. Potentiometric cells consisting of two electrodes separated by a salt bridge and a reference solution are widely used for measurement of ion concentrations. Miniaturization of potentiometric cells for wearable sensors is, in principle, relatively straightforward.

With

appropriate device design the cell potential is directly related to the ion concentration. However, a major challenge is in designing wearable ion sensors for extended measurements. In a classical potentiometric cell, the purpose of the salt bridge is to minimize equilibration between the reference solution and the test solution. In miniaturized potentiometric sensors the reference solution is usually a gel. However, with no salt bridge and a relatively large contact area between the reference solution and test solution, equilibration between the two solutions results in a decrease in measured potential.

Therefore, these devices are suitable for accurate

measurements only for a relatively short period of time. Here we report a wearable potentiometric chloride sweat sensor. We show that the decrease in cell potential with time due to equilibration follows predictions based on the diffusion coefficient and geometry of the salt bridge. We then show how optimization of device design can attenuate equilibration between the reference solution and test sample and allow accurate

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measurements for more than 24 h. We present results from on-body measurements of human subjects while exercising.

2. Materials and methods 2.1 Parametric studies To assess the role of salt bridge geometry on device performance, we performed parametric studies using a custom designed sensor (Supplementary Fig. S1). The housing was formed by casting polydimethylsiloxane (PDMS, Sylgard 184, Dow Corning) in an aluminum mold containing a template rod (Fig. S1A). The rods were either 100 or 150 µm diameter Nitinol wires (McMaster-Carr) or 380 or 660 µm diameter stainless steel wires (Small Parts). The PDMS was cured at 75 °C for 1 hour using a convection oven. Following curing the rod was removed and the PDMS block was removed from the aluminum mold (Fig. S1B). The reference and test chambers were formed manually using a hole punch (Fig. S1C).

The length of salt

bridge is determined by the distance between the holes. Next, the PDMS housing was plasma bonded to a glass slide (Fig. S1D). After plasma bonding, two Ag/AgCl wire electrodes were inserted into the PDMS housing (Fig. S1E).

The Ag/AgCl electrodes were prepared by

chloridization of 625 µm diameter Ag wire (A-M systems) in hydrochloric acid solution (Fisher Science) using a previously published protocol 18; the thickness of AgCl layer was about 20 µm. The electrodes were fixed in the housing using PDMS and cured at 75 °C for 1 hour (Fig. S1F). Next, the salt bridge was filled with 6.8 vol.% agarose gel (Invitrogen) containing 1 M KCl solution (Sigma-Aldrich) (Fig. S1G). The 100 and 150 µm diameter salt bridges were filled using a vacuum. The 380 and 660 µm diameter salt bridges were manually filled using a syringe. After filling the salt bridge with the agarose gel, 300 µL reference and test solutions were introduced into the respectively chambers (Fig. S1H). In all cases the test solution was 10 mM NaCl (Fisher Science). The reference solution was 100 mM, 500 mM, or 1 M KCl solution. To prevent the evaporation, the chambers were sealed with Kapton tape (Uline). All devices were calibrated by introducing 10 mM, 50 mM, and 100 mM NaCl solution into the test chamber and measuring the voltage for 5 minutes. The calibration curve for each sensor was obtained from the measured voltages using a linear least squares fit to plots of V - logC. During parametric studies, cell voltages were recorded every 30 s for at least 1 day using a DAQ (USB-6363, National Instruments) and Labview software (National Instruments).



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Table 1. Summary of conditions for parametric studies. To assess the role of the salt bridge geometry on sensor performance we evaluated three parameters, salt bridge length (l), salt bridge diameter (d), and the concentration of the reference solution (Cref). #1 (Fig. 2A-C)

#2 (Fig. 2D-F)

#3 (Fig. 2G-I)

parameter

salt bridge length (l)

salt bridge diameter (݀)

Cref - Ctest

l (mm)

0.9, 2.8, 4.8, 10.4

1.3

5

d (µm)

660

100, 150, 380, 660

660

Cref

1 M KCl

1 M KCl

0.1, 0.5, 1 M KCl

Ctest

10 mM NaCl

10 mM NaCl

10 mM NaCl

Cref - Ctest

0.99 M

0.99 M

0.09, 0.49, 0.99 M

‫ݒ‬௥௘௙ & ‫ݒ‬௧௘௦௧

300 µL

300 µL

300 µL

hydrogel

6.8 vol.% agarose

6.8 vol.% agarose

6.8 vol.% agarose

2.2 Simulations of ion transport To analyze the time dependent distribution of chloride ions in the salt bridge we performed finite element simulations (“Transport of diluted species physics” module, COMSOL). This module solves Fick’s second law (∂c/∂t = ∇·(D∇C)) for the transport of chemical species in a fixed geometry. In the simulations the volume of solution in the reference and test chambers was 300 µL, with a 1 M reference solution and 10 mM test solution. The diameter of the salt bridge was 100, 150, 380, or 660 µm, and the length was 3, 5, 7.5, or 10 mm. To account for the porous agarose gel, we used the effective salt bridge area determined from the cross-sectional area and the gel porosity. The purpose of the simulations was to validate analytical expressions for the change in concentration in the test solution.

2.3 Sensor fabrication The sensor design for on-body tests was modified from the design used for parametric studies (Supplementary Fig. S2).

The sensor consisted of two PDMS components: a housing

containing the reference chamber (Fig. S2A) and a base containing the salt bridge (Fig. S2B). To form the housing, a 660 µm rod was inserted into a mold that was then filled with PDMS (Fig. 3 ACS Paragon Plus Environment

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2A). The metal wire served as a template to form the hole in the PDMS housing into which the Ag/AgCl electrode was inserted. The PDMS was then cured at 75 ˚C for 1 hour. The wire was then removed and the PDMS housing removed from the mold. The thickness of the PDMS housing was about 5 mm, but can be decreased to make the sensor dimensions smaller. After removal of the PDMS housing from the mold, a 4 mm hole punch was used to form the reference solution chamber. An additional hole was formed in the housing that was subsequently filled with PDMS to seal the electrode into the housing. Next, the PDMS base containing the salt bridge was formed by a similar casting process using a 100 µm rod template (Fig. S2B). Following curing, the PDMS was removed from the mold and the template rod was removed from the PDMS. The PDMS was then cut into slices with a thickness that defines the length of the salt bridge. Each slice forms the base of a sensor with a hole that will be the salt bridge channel. The target thickness of the PDMS base was 3 mm in this work, but the variation was about ± 0.5 mm since it was manually sliced. The PDMS housing with the reference electrode chamber and PDMS base with the salt bridge channel were then plasma bonded (Fig. S2C). Ag/AgCl electrodes were prepared by chloridization of FA (perfluoroalkoxy)-insulated Ag wires (A-M systems, bare diameter = 381 µm, coated diameter = 483 µm). The electrode wire was inserted into the hole for the reference electrode (Fig. S2D). The second chamber was then filled with PDMS to fix the reference electrode wire into the housing (Fig. S2E). The PDMS is then cured at 75 °C for 1 hour. After curing PDMS, the reference electrode chamber was filled with a hydrogel containing 1 M KCl solution (Fig. S2F). Vacuum was applied to the chamber through the salt bridge channel to fill the salt bridge with the gel solution. Next, the top of the reference chamber was sealed with PDMS to prevent evaporation (Fig. S2G). The device was cured for 5 hours at 45 ˚C. Finally, a hole was formed in the housing for the working electrode using a 1 mm hole punch, and then a Ag/AgCl working electrode was inserted into the hole (Fig. S2H).

2.4 Calibration and on-body tests Calibration curves and dose response curves for each device were obtained by sequentially immersing the sensor into 10, 50, 100 mM NaCl solutions and measuring the output voltage using a DAQ (USB-6363, National Instruments) and Labview software (National Instruments). The calibration curve was obtained for each sensor prior to on-body tests.



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The on-body tests were conducted in compliance with a protocol approved by the institutional review board (IRB) at Johns Hopkins University. Tests were performed on three subjects: subject 1: female, age = 26, height = 155 cm, and weight = 64.4 kg; subject 2: male, age = 35, height =196 cm, weight = 100 kg; subject 3: female, age = 19, height = 170 cm, weight = 60 kg) while spinning on an exercise bike for about one hour. The sensor was attached using a fitness band (Fitbit Replacement Band) or a commercial transparent adhesive bandage (Nexcare, TegadermTM). Sensor voltages were recorded every 2 s using a DAQ (USB-6363, National Instruments) and Labview software (National Instruments). Subjects were asked to spin on a stationary exercise bike for up to 60 minutes. The ambient temperature and humidity were not controlled.

3. Results and Discussion 3.1 Parametric studies to optimize salt bridge geometry 3.1.1 Equilibration and concentration drift rate (Q) Potentiometric sensors comprise a reference chamber with a solution of known concentration and a test solution (Fig. 1A). The incorporation of a salt bridge is important in controlling the rate of equilibration between the reference and test solutions (Fig. 1B). During sensor use, equilibration between the reference chamber and test solution results in a decrease in the cell voltage and hence an apparent decrease in the ion concentration of the test solution. Therefore controlling equilibration is important in developing accurate and reliable wearable potentiometric sensors. In patch-type sensors, the reference solution gel is in direct contact with the test solution and hence equilibration is relatively fast, limiting the time that the sensor can be used. The changes in chloride ion concentration in the reference and test solutions per unit time can be expressed by the flux (J) of ions and the geometry of salt bridge.

(

(

2

JAeff Jπ p  d  ∆t = Ctest t + ∆t v test v test  2 

)

()

)

JA Jπ p  d  t + eff ∆t = Ctest t − ∆t v ref v ref  2 

Ctest t + ∆t = Ctest t +

Cref t + ∆t = Cref

()

()

()

(1)

2


(2)

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where Ctest(t) and Cref(t) (M) are the concentrations of reference and test solutions, respectively, J (mole m-2 s-1) is the flux of ions, Aeff is the effective cross-sectional area of the salt bridge, t is time, d is the salt bridge diameter, p is the porosity of the hydrogel, and vtest and vref are the volumes of the test and reference solutions, respectively.

Figure 1. Potentiometric sensor. (A) A potentiometric sensor measures the potential between reference and test solutions.

(B) The salt bridge geometry controls equilibration between

reference and test solutions. (C) Schematic illustration of sensor for parametric studies. (D) Sensor for parametric studies. To assess the kinetics of equilibration we define the concentration drift rate Q (M h-1) through the salt bridge: Qi =

Ci ( t + ∆t ) − Ci ( t ) ∆t

JA Jπ p  d  = ± eff = ± vi v i  2 

2

(3)

where the subscript i indexes the test and reference solutions. Since the ion flux (J) is dominated by diffusion, equation (3) can be written as:  ∂C ( x, t )  π p  d 2 Qi = ±  DCl −  ∂x  v i  2  

(4)



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where D is the diffusion coefficient of chloride ions (2.1 x 10-5 cm2 s-1), and C(x,t) is the concentration distribution of chloride ions within the salt bridge. Under steady state conditions, the concentration gradient in the salt bridge is linear and the concentration drift rate is given by:  C (t) − Ctest (t)  π p  d  Qi = ±  DCl − ref  v  2  ℓ 

2

(5)

i

where l is the length of salt bridge. While equation (5) describes steady state conditions, in practice, the chloride ion concentration in the salt bridge is initially the same as the reference solution. Therefore, on contacting the sensor with a test solution there is transport of chloride ions into the test solution (assuming Cref > Ctest) associated with establishing a linear concentration gradient in the salt bridge. The influence of this contribution to the concentration drift rate is described in detail in Supplementary Information (Section S3: FEM simulations). From equation (5) it is evident that Q is: (1) inversely proportional to the length of the salt bridge, (2) proportional to the square of the diameter of the salt bridge, and (3) proportional to the concentration difference between the reference solution and the test solution.

These

parameters provide design criteria to minimize equilibration of potentiometric sensors. Furthermore, these results highlight a limitation of patch-type devices: the large contact area between the gel and the test solution is equivalent to a large salt bridge diameter, which results in rapid equilibration between the reference and test solutions.

3.1.2 Parametric studies To optimize device performance, and to minimize error due to equilibration, we performed a series of parametric studies.

The objective of these experiments was to measure the time

dependent change in the test solution concentration due to equilibration, and to confirm that equilibration is dependent on the geometry of the salt bridge described by equation (5). The sensor for parametric studies has reference and test chambers connected by a salt bridge of varying diameter and length (Figs. 1C,D). The salt bridge was filled with a hydrogel (6.8 vol% agarose gel) containing the reference solution. From the volume fraction of the gel we obtain a salt bridge porosity of p = 0.932. The reference chamber and salt bridge were filled with 300 µL of 1 M KCl solution and the test camber was filled with 300 µL of 10 mM NaCl solution to


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simulate sweat in a healthy individual. The solution concentrations and salt bridge dimensions are summarized in Table 1. The sensor voltage was converted to test solution concentrations using a calibration curve recorded before the test. We assume that any voltage change was caused by a change in concentration of the test solution. As described above, since the concentration of the reference solution (1 M) is much larger than the test solution (10 mM), voltage changes are dominated by changes in the concentration of the test solution. According to the Nernst equation, the sensor voltage at room temperature and assuming unit activity is given by:

V=

C C 2.303RT log test = 0.059log test Cref Cref F

(6)

Therefore, a sensor with a 1 M reference solution and 10 mM test solution, and negligible equilibration between reference and test chambers is expected to have a voltage of about 118 mV at 25 ˚C. The change in sensor voltage due to ion transport from the reference chamber to the test chamber is given by:   C (t) + ∆Ctest   C (t)    C (t) + ∆Ctest  ∆V = −0.059  log  test − log  test   ≈ −0.059log  test   Ctest (t)  Cref (t)       Cref (t) − ∆Cref 

(7)

Therefore, the increase in chloride ion concentration in the test solution chamber can be calculated from the decrease in sensor voltage. From equation (7) it is seen that a 0.5% change in the reference chamber concentration, which corresponds to a 50% increase in test chamber concentration, results in a 10 mV decrease in the cell voltage. Although the concentration in the test chamber increases linearly with time under steady state conditions, at short times as long as Cref >> Ctest then Q is approximately constant and independent of Ctest. Simulations verifying these assumptions are presented in Fig. S3. 3.1.3 Salt bridge length The influence of salt bridge length on device voltage was tested for a salt bridge diameter of 660 µm. For the shortest salt bridge (0.9 mm), the voltage decreases rapidly to about 40 mV after 24 h, whereas for longer salt bridges, the decrease in voltage is progressively slower (Fig. 2A). The



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decrease in voltage reflects the increase in concentration of the test solution due to equilibration (Fig. 2B). For a 10.4 mm salt bridge, this increase is modest, however, for the 0.9 mm salt bridge, the concentration increases to about 160 mM after 24 h.

The linear increase in

concentration with time is consistent with the linear concentration gradient across the salt bridge and hence a constant flux. The concentration drift rate is obtained from a linear least squares fit to the concentration versus time curves over the first 5 hours (Fig. 2B), and is inversely proportional to the length of the salt bridge and shows excellent agreement with the prediction described in equation (5) (Fig. 2C). The value of Q for the short (0.9 mm) salt bridge deviates from the calculated value due to the additional contribution from the boundary layer in the test and reference solutions. In a static solution, the Nernstian boundary layer thickness is typically around 100 µm, corresponding to 20% of the length of a 0.9 mm salt bridge, close to the 25% difference in Q.

3.1.4 Salt bridge diameter The influence of salt bridge diameter on device voltage was tested for a salt bridge length of 1.3 mm. For a 100 µm salt bridge diameter, the voltage decrease over 24 h is relatively small, but increases significantly with increasing diameter (Fig. 2D). The concentration in the test chamber correspondingly increases more rapidly with larger diameter salt bridges (Fig. 2E). The concentration drift rate increases with the square of the salt bridge diameter (i.e. cross section area) and shows excellent agreement with the equation (5) (Fig. 2F).

3.1.5 Reference solution concentration To assess the role of reference solution concentration on the concentration drift rate, we recorded the sensor voltage with fixed salt bridge geometry (l = 5 mm, d = 660 µm) with Cref = 1 M, 500 mM, and 100 mM. The corresponding theoretical sensor potentials are 118 mV, 100 mV, and 59 mV. The decrease in sensor voltage is largest for Cref = 1 M, decreasing to around 70 mV after 24 h (Fig. 2G). For Cref = 100 mM, the sensor voltage decreased from 55 mV to about 50 mV after 24 h. Correspondingly, the increase in test solution concentration was largest for the 1 M reference solution (Fig. 2H). The concentration drift rate increases monotonically with Cref Ctest and shows excellent agreement with equation (5) (Fig. 2I).


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Figure 2. The role of the salt bridge in sensor performance. All experiments were performed with a 10 mM NaCl test solution and vref = vtest = 300 µL. (A-C) Influence of salt bridge length on equilibration. Salt bridge length l = 0.9 mm (n = 4), 2.8 mm (n = 6), 4.8 mm (n = 6), and 10. 4 mm (n = 6) with d = 660 µm and Cref = 1 M. The change in chloride ion concentration in the test solution was determined from the measured sensor voltage and the calibration curve of each sensor. We assumed that the decrease in voltage is dominated by a change in concentration of the test solution as described in equation (7). The concentration drift rate Q was obtained from a linear least squares fit of the chloride ion concentration in the first 5 hours. The solid line represents the concentration drift rate predicted by equation (5). (D-F) Influence of salt bridge 10 ACS Paragon Plus Environment


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diameter on equilibration. Salt bridge diameter d = 100 µm (n = 3), 150 µm (n = 2), 380 µm (n =3), 660 µm (n = 2) with l = 1.3 mm and Cref = 1 M. (G-I) Influence of reference solution concentration on equilibration. Reference solution concentration Cref = 0.1 M (n = 6), 0.5 M (n = 6), 1 M (n = 6) with l = 5 mm and d = 660 µm. (J) Chloride ion concentration for three sensors with optimized salt bridge (l = 10 mm, ݀ = 100 µm, vref = 300 µL, Cref = 0.1 M) with vtest = 40 µL and Ctest = 10 mM. The dotted lines indicate 10 ± 1 mM. 3.1.6 Salt bridge design for potentiometric sensors The parametric studies described above verify the design rules implied by equation (5) and show that the concentration drift due to equilibration can be minimized by: (1) increasing the length of the salt bridge, (2) reducing the diameter of the salt bridge, and (3) reducing the concentration difference between the reference solution and the test solution. Increasing the length of the salt bridge is straightforward but can increase the footprint of the device. Reducing the diameter of the salt bridge is more challenging since fabrication of high aspect ratio cylinders is difficult but can be solved by adopting microfabrication techniques used for micropores or microfluidic channels

19,20

. The concentration difference (Cref - Ctest) between

the reference and test solutions can be easily reduced independent of device design. However, there are two drawbacks. First, a high concentration minimizes the concentration change in the reference chamber. Second, increasing the concentration difference (Cref - Ctest) between the reference and test solutions increases the dynamic range for measurement.

Therefore a 1 M

reference solution provides a dynamic range of around 60 - 120 mV for sweat chloride concentrations of 10 - 100 mM. To demonstrate how the salt bridge can be designed to minimize equilibration and maintain the initial calibration, we tested a sensor with l = 10 mm, d = 100 µm, Cref = 100 mM KCl (v = 300 µL) and Ctest = 10 mM NaCl (v = 40 µL). The salt bridge was filled with 6.8 vol. % agarose gel containing 100 mM KCl solution. In this test, 40 µL of 10 mM NaCl solution was used to simulate the small sweat volume under a wearable sensor. The small volume of the test solution means that even a small flux of chloride ions from the reference solution can result in a large increase in concentration in the test solution. The chloride ion concentration, obtained from the calibration curve and sensor voltage, was in the range 10 ± 1 mM for the duration of the 30 h experiment (Fig. 2J). The small positive slope in the test from 20 – 30 h reflects the small ion 11 ACS Paragon Plus Environment

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flux from the reference chamber. The chloride ion concentration in the test solution obtained from simulations using the same geometry and experimental conditions was 10.3 mM after 30 h (Fig. S4), in good agreement with the experimental results. These results demonstrate the ability to design potentiometric sensors to minimize equilibration and allow accurate measurement over long periods, essential for wearable sensors.

3.2 Wearable chloride sweat sensor for on-body tests To assess the performance of the sensor to measure the chloride ion concentration in sweat, we fabricated devices suitable for on-body measurements (Fig. 3A,B). The sensor is designed to be low cost, simple to fabricate, and consists of Ag/AgCl reference and test electrodes, and a reference chamber and salt bridge filled with a hydrogel containing 1 M KCl (Fig. 3A). The use of a hydrogel reference solution reduces the potential for leakage but has no influence of device performance (see Fig. S5). In these sensors the salt bridge was 3 mm long and 100 µm in diameter, with a concentration drift rate of 0.06 mM h-1 for a sweat volume of 300 µL (Fig. S6). Calibration curves recorded for these sensors using 10, 50, and 100 mM NaCl solutions were highly reproducible, with a slope around 50 mV decade-1 (Fig. 3C). Dose response curves show a fast response (Fig. 3D).

Figure 3. Chloride sensor for on-body sweat tests. (A) Schematic illustration of the device with an optimized salt bridge. (B) Photograph of a fabricated prototype device. (C) Calibration curve of the prototype devices (N = 13). (D) Dose response curves for a chloride sensor.

For on-body tests, the sensor was attached to the body using a wristband or an adhesive bandage (Fig. 4A,B). The chloride concentration in sweat was measured in 3 subjects while



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spinning on an exercise bike for about one hour (Fig. 4C-E). The signal was noisy for the first 10 - 15 minutes until the onset of sweating but then decreased to a stable value for the remainder of the trial. The initial transient response is due to the sensor impedance prior to the onset of sweating, which is greater than the input impedance of the measurement circuit used in these trials (10 MΩ) (Fig. S7). The steady state chloride ion concentration was between 4 mM and 40 mM, within the range for healthy individuals

12

. Increases in skin temperature during exercise

are typically less than 5 °C, 21 and hence from the Nernst equation, the error associated with ignoring any temperature change is ≤ 7% for a chloride ion concentration of 10 mM.

To verify that the optimized salt bridge can minimize the measurement error due to equilibration, we fabricated a planar device with a relatively large contact area (Fig. S8A), mimicking the design of previously reported sweat sensors 16,21. The sensor consists of Ag/AgCl reference and test electrodes, and a hydrogel containing a reference solution (1 M KCl). The reference electrode is covered by the hydrogel, and the hydrogel and the test electrode directly contact the skin when attached on the body. The diameter of the reference solution hydrogel is 4 mm.

The sensor was attached to the forearm of a subject for testing.

The chloride ion

concentration initially decreased with the onset of perspiration, but increased to values in excess of 100 mM after 60 minutes. Three on-body tests with the planar device were performed, and all of the measured concentrations were much higher than the chloride concentration in the sweat of healthy individuals (Fig. S8B-D). The corresponding decrease in sensor potential is due to rapid equilibration enhanced by the large contact area between the hydrogel containing the reference solution and the test solution.


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Figure 4. On-body tests of a wearable chloride sweat sensor.

(A) Sweat sensor attached

wristband. (B) Sensor worn by subject during exercise. The inset shows the sensor on the wrist. Sensor voltage and chloride ion concentration for (C) subject 1, (D) subject 2, and (E) subject 3.

4. Conclusions We have designed a wearable potentiometric chloride ion sensor that is capable of accurate measurement of chloride ion concentration over extended time periods. We show that salt bridge geometry is key to minimizing equilibration between reference and test solutions, and hence minimizing measurement error. We performed a series of parametric tests to optimize the salt bridge geometry, and showed that the steady state flux of chloride ions from the reference chamber to the test chamber can be minimized by appropriate design of the salt bridge geometry. Equilibration through the salt bridge in sensors with variable salt bridge length and diameter



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follows predictions based on ion transport. Based on these studies, a relatively simple, low-cost device was designed and fabricated using a PDMS housing, reference solution gel, and two silver chloride electrodes. The prototype device was easily attached on the body, and on-body tests with the device were performed to measure sweat chloride concentration during exercise. The measured sweat concentration is very constant and within the normal range (below 40 mM) during 1 hour of exercise. In contrast, patch-type devices without a salt bridge show a rapid increase in sweat concentration, with measured values much higher than normal values in healthy individuals.

The prototype device shows promising results for an individualized

healthcare monitoring system for cystic fibrosis patients, as well as fitness and military applications. Acknowledgements The authors gratefully acknowledge support from inHealth, the John Hopkins individualized health initiative.

Supporting Information Fabrication of device for parametric studies. Fabrication of a wearable chloride sweat sensor. Sensor equilibration: comparison of analytical solution and simulations.

Simulations of

concentration change in a sensor with an optimized salt bridge. Comparison of response of sensors with liquid and gel reference solutions. Parametric studies of sensors with a salt bridge diameter of 100 µm.

Comparison between measurement circuits with low and high input

impedance. On-body tests of chloride concentration in sweat using a planar patch-type device without a salt bridge


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Table of Contents Graphic


Wearable Potentiometric Chloride Sweat Sensor: The Critical Role of

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