Comprehensive analysis of sweat chemistry provides noninvasive health monitoring capabilities that complement established biophysical measurements such as heart rate, blood oxygenation, and body temperature. Recent developments in skin‐integrated soft microfluidic systems address many challenges associated with standard technologies in sweat collection and analysis. However, recording of time‐dependent variations in sweat composition requires bulky electronic systems and power sources, thereby constraining form factor, cost, and modes of use. Here, presented are unconventional design concepts, materials, and device operation principles that address this challenge. Flexible galvanic cells embedded within skin‐interfaced microfluidics with passive valves serve as sweat‐activated “stopwatches” that record temporal information associated with collection of discrete microliter volumes of sweat. The result allows for precise measurements of dynamic sweat composition fluctuations using in situ or ex situ analytical techniques. Integrated electronics based on near‐field communication (NFC) protocols or docking stations equipped with standard electronic measurement tools provide means for extracting digital timing results from the stopwatches. Human subject studies of time‐stamped sweat samples by in situ colorimetric methods and ex situ techniques based on inductively coupled plasma mass spectroscopy (ICP‐MS) and chlorodimetry illustrate the ability to quantitatively capture time‐dynamic sweat chemistry in scenarios compatible with field use.

General transfer hydrogenation procedure S6 Conversion monitoring of 4-phenyl-2-butanol dehydrogenation S7 General transfer dehydrogenation procedure S8 Product inhibition experimental details S8-S9 NMR experiments S9-S10 N-Benzylideneaniline reduction procedure S10-S11 References S11 NMR Spectra S12-S25 S2 General Methods. Compound 3 was prepared according to the literature procedure. S1 All commercially available chemicals and anhydrous solvents were used as received, and all reactions were done under an atmosphere of nitrogen unless otherwise noted. Reagent grade isopropanol and acetone (for the transfer hydrogenations and dehydrogenations) were degassed by bubbling N2 through them for at least 15 minutes prior to use, but no attempt was made to remove residual water. NMR spectra were recorded at rt (approx. 22 °C) unless otherwise noted on a Bruker Avance 400 MHz FT-NMR spectrometer. 13 C NMR spectra (100 MHz) were all proton decoupled. Chemical shifts are reported in parts per million (ppm) downfield from tetramethylsilane (TMS) with reference to TMS for 1 H NMR and 13 C NMR spectra. Multiplicities are abbreviated as follows: singlet (s), doublet (d), triplet (t), quartet (q), quintet (quint), sextet (sext), septet (sept), multiplet (m), and broad (br). IR spectra were collected on a Nicolet IR200 attenuated total reflectance FT-IR spectrometer and only diagnostic peaks are given. IR bands are given in cm-1 and peak intensities correspond to weak (w), medium (m), strong (s), and broad (br). High resolution mass spectrometry data were collected at the Johns Hopkins University Mass Spectrometry Facility. Analytical thin-layer chromatography (TLC) was performed using silica gel 60 F254 precoated plates (0.25 mm thickness) with a fluorescent indicator. Visualization was performed with UV light. Flash column chromatography was performed using silica gel 60 (230-400 mesh). Gas chromatograms were collected on a Thermo Scientific Trace 1300 gas chromatograph with an AI 1310 autosampler and an FID. A TR-5 (5% phenyl methylpolysiloxane) column (30 m length x 0.25 mm ID x 0.25 μm film thickness) was used under the following method conditions: 110 °C for 5 min, ramp 20 °C/min to 250 °C, hold at 250 °C for 2 min. The carrier gas was helium, used at a constant flow rate of 1 mL/min. A sample volume of 1 μL was added to the 300 °C injector at a 30:1 split ratio, and the FID temperature was 250 °C. Retention times (4.7 min for acetophenone, 4.5 min for 1-phenylethanol, 7.6 min for 4-phenyl-2-butanol, 7.4 min for 4-phenyl-2-butanone, 11.2 min for Nbenzylideneaniline, 11.5 min for N-benzylamine, and 9.0 min for biphenyl) were determined using pure samples. Synthesis of Iron Compounds Scheme S1. Synthesis of 4. (2,3,4,5-Tetraphenylcyclopentadienone)iron tricarbonyl (4). S2 A solution of 2,3,4,5tetraphenylcyclopentadienone S3 8a (3.0 g, 7.8 mmol) and iron pentacarbonyl (2.0 mL, 3.0 g, 15 mmol) in 35 mL degassed toluene in a thick-walled round-bottom flask with a PTFE screw cap was heated to 140 °C for 24 h. After cooling to rt,...

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