Wearable Device Analyzes Sweat To Monitor User’s Health

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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.