Wearable Device Analyzes Sweat To Monitor User’s Health | January 28, 2016 Issue - Vol. 94 Issue 5 | Chemical & Engineering News
Volume 94 Issue 5 | p. 11 | News of The Week
Issue Date: February 1, 2016 | Web Date: January 28, 2016

Wearable Device Analyzes Sweat To Monitor User’s Health

Biomarkers: New head-, wrist-, and armbands go beyond Fitbits by detecting levels of glucose, lactate, sodium, and potassium in sweat
Department: Science & Technology
News Channels: Biological SCENE, Materials SCENE, Nano SCENE, Analytical SCENE
Keywords: sweat, analysis, sensor, wearable
Wearable sweat-analyzing wristband (top) includes (bottom) a sensor array (left) and onboard circuitry (right) for real-time calibration, analysis, and transmission of chemical and temperature data.
Credit: Adapted from Nature
Wearable sweat-analysis system includes a sensor array and onboard circuitry for real-time calibration, analysis, and transmission of concentration and temperature data.
Wearable sweat-analyzing wristband (top) includes (bottom) a sensor array (left) and onboard circuitry (right) for real-time calibration, analysis, and transmission of chemical and temperature data.
Credit: Adapted from Nature
Ali Javey discusses his group’s design of the wearable sweat and temperature sensor.
Credit: Berkeley News video edited for length by C&EN

“Never let them see you sweat,” a slogan from 1980s deodorant commercials, is an admonition volunteers in a recent research study happily agreed to ignore. The subjects worked up a good sweat while exercising to test newly developed wristbands, armbands, and headbands that monitor skin temperature and compounds found in sweat.

Fitbit and other popular wearable fitness trackers can measure walking distances and heart rates. But the new system goes far beyond that to analyze multiple aspects of a person’s health through sweat compounds, possibly creating alerts for conditions such as dehydration, muscle cramps, or even improper glucose metabolism caused by diabetes.

The new wireless devices were developed by electrical engineering professor Ali Javey of the University of California, Berkeley, and coworkers (Nature 2016, DOI: 10.1038/nature16521) and tested on 26 men and women, who wore the sensors while either pedaling stationary bikes indoors or running outdoors. Wearable sweat-analysis systems have been developed before but could monitor only a single analyte at a time or lacked onboard circuitry and calibration mechanisms needed to accurately analyze the data to provide insight into a user’s physiological state, the researchers say.

“The multimodal operation and full wireless capabilities represent particularly notable features,” comments bioanalytical device specialist John A. Rogers of the University of Illinois, Urbana-Champaign. He says potential applications include health care diagnostics that can monitor user health continuously and in real time.

The device combines a sensor array and circuit board on a flexible polyethylene terephthalate substrate. The array has sensors that detect two metabolites, glucose and lactate, by monitoring current generated when immobilized enzymes oxidize the compounds; ion-selective electrodes that detect sodium and potassium ions; and a resistance-based skin temperature sensor. The device calibrates and calculates the concentrations and broadcasts the data wirelessly to a smartphone or computer.

The skin temperature readings are used to calibrate the sensors, which generate temperature-dependent responses. Sweat glucose levels correlate with blood glucose levels, which patients with diabetes must monitor regularly. Lactate concentrations in sweat can serve as a marker of restricted blood flow to tissues. Sodium and potassium levels in sweat can indicate conditions such as muscle cramps, dehydration, electrolyte imbalance, and cystic fibrosis.

The system could be extended to other analytes. In a published Nature commentary, electrical engineer Jason Heikenfeld of the University of Cincinnati notes that “small-molecule drugs and their metabolites come out in sweat, so this body fluid might one day be used to monitor the amount of active drug in a patient’s blood.”

Javey’s team has filed a patent for the device. He thinks it would cost some tens of dollars to manufacture.

Chemical & Engineering News
ISSN 0009-2347
Copyright © American Chemical Society
Kafantaris  (Thu Jan 28 15:09:33 EST 2016)
There may be a use for the Apple Watch after all -- beside it helping us find our phone.
Branch, MD (Thu Jan 28 15:21:55 EST 2016)
certain medical conditions manifest with increased amounts of perspiration. One wonders if a divice such as this could be used in ths study of such conditions.
Darahani (Sun Jan 31 23:43:28 EST 2016)
Awesome! I would like to know the functions involved in.the signals transmition.....thanks
Bill Herz (Wed Feb 03 12:39:49 EST 2016)
Cool. All kinds of potential interest here. How can we get in an upcoming trial?
Dajing Yuan (Thu Jun 09 12:01:11 EDT 2016)
I really want to know what is the material used to fabricate sodium and potassium selective electrode.
Stu Borman (Mon Jun 13 09:31:52 EDT 2016)
Hope this helps. From the paper:

A Ag/AgCl electrode serves as a shared reference electrode and counter electrode for both sensors. The use of Prussian blue dye as a mediator minimizes the reduction potentials to approximately 0 V (versus Ag/AgCl), and thus eliminates the need for an external power source to activate the sensors. These enzymatic sensors autonomously generate current signals proportional to the abundance of the corresponding metabolites between the working electrode and the Ag/AgCl electrode. The measurement of Na+ and K+ levels is facilitated through the use of ion-selective electrodes (ISEs), coupled with a polyvinyl butyral (PVB)-coated reference electrode to maintain a stable potential in solutions with different ionic strengths. By using poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) as an ion-to-electron transducer in the ISEs and carbon nanotubes in the PVB reference membrane25, robust potentiometric sensors (with voltage output) can be obtained for long-term continuous measurements with negligible voltage drift.

Fabrication of electrode arrays
Briefly, the sensor arrays on PET were patterned by photolithography using positive photoresist (Shipley Microposit S1818) followed by 30 nm Cr/50 nm Au deposited via electron-beam evaporation and lift-off in acetone. A 500-nm parylene C insulation layer was then deposited in a SCS Labcoter 2 Parylene Deposition System. Subsequently, photolithography was used to define the final electrode area (3 mm diameter) followed by O2 plasma etching for 450 s at 300 W to remove the parylene completely. Electron-beam evaporation was then performed to pattern 180-nm Ag onto the electrode areas, followed by lift-off in acetone. The Ag patterns on working electrode area were dissolved in a 6-M HNO3 solution for 1 min. The Ag/AgCl reference electrodes were obtained by injecting 10 μl 0.1-M FeCl3 solution on top of each Ag reference electrode using a micropipette for 1 min.

Design of electrochemical sensors
For amperometric glucose and lactate sensors, a two-electrode system where Ag/AgCl acts as both reference and counter electrode was chosen to simplify circuit design and to facilitate system integration. The two-electrode system is a common strategy for low-current electrochemical sensing34, 35. The output currents (between the working electrode and the Ag/AgCl reference/counter electrode) of the glucose and lactate sensors could be converted to a voltage potential through a transimpedance amplifier. It is known that amperometric sensors with larger area provide larger current signal. Considering the low concentration of glucose in sweat, we designed the sensors to be 3 mm in diameter to obtain a high current.

Preparation of Na+ and K+ selective sensors
The Na+ selective membrane cocktail consisted of Na ionophore X (1% weight by weight, w/w), Na-TFPB (0.55% w/w), PVC (33% w/w), and DOS (65.45% w/w). 100 mg of the membrane cocktail was dissolved in 660 μl of tetrahydrofuran17. The K+-selective membrane cocktail was composed of valinomycin (2% w/w), NaTPB (0.5%), PVC (32.7% w/w), and DOS (64.7% w/w). 100 mg of the membrane cocktail was dissolved in 350 μl of cyclohexanone. The ion-selective solutions were sealed and stored at 4 °C. The solution for the PVB reference electrode was prepared by dissolving 79.1 mg PVB and 50 mg of NaCl into 1 ml methanol36. 2 mg F127 and 0.2 mg of multiwall carbon nanotubes were added into the reference solution to minimize the potential drift25.

Poly(3,4-ethylenedioxythiophene) PEDOT:PSS was chosen as the ion–electron transducer to minimize the potential drift of the ISEs37 and deposited onto the working electrodes by galvanostatic electrochemical polymerization with an external Ag/AgCl reference electrode from a solution containing 0.01-M EDOT and 0.1-M NaPSS. A constant current of 14 μA (2 mA cm−2) was applied to produce polymerization charges of 10 mC onto each electrode.

Ion-selective membranes were then prepared by drop-casting 10 μl of the Na+-selective membrane cocktail and 4 μl of the K+-selective membrane cocktail onto their corresponding electrodes. The common reference electrode for the Na+ and K+ ISEs was modified by casting 10 μl of reference solution onto the Ag/AgCl electrode. The modified electrodes were left to dry overnight. The sensors could be used without pre-conditioning (with a small drift of ~2–3 mV h−1). However, to obtain the best performance for long-term continuous measurements such as dehydration studies, the ion-selective sensors were covered with a solution containing 0.1-M NaCl and 0.01-M KCl through microinjection (without contact to glucose and lactate sensors) for 1 h before measurements. This conditioning process was important to minimize the potential drift further.

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