Spectracarb 2050a 0550

Protocatechuic aldehyde (A‐o, Carl Roth), vanillin (van, Sigma–Aldrich), 3,5‐dihydroxybenzaldehyde (A‐m, Fischer), an aqueous solution (15 %) of polyallylamine with an average molecular weight of 15 000 g mol⁻¹ (PAAm, Polysciences, Inc.), carbon EQ‐AB‐520Y (C, MTI Corporation), carbon paper Spectracarb 2050A‐0550 (Fuel Cell Store), and the solvents ethanol (Fischer) and N‐methyl‐2‐pyrrolidone (NMP, PanReac AppliChem) were used as received.

The electrical characteristics of our device were evaluated using both electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). We performed each measurement with the VersaSTAT 3 potentiostat from Princeton Applied Research. In both modalities, our electrochemical cell comprised three electrodes immersed in room temperature phosphate buffered saline (1 X PBS, pH 7.4): one of the cuff’s cathodes as the working electrode, an Ag |AgCl| 3.0 M KCl cell as the reference electrode (grounded to enclosing Faraday cage to reduce noise), and a 1 × 2 cm² section of carbon paper (Spectracarb 2050A-0550, Fuel Cell Store) as the counter electrode. According to typical EIS procedure, impedance magnitude and phase were evaluated for sinusoidal waveforms (10 mV RMS, biased to the cell’s open-circuit potential) at 60 frequencies logarithmically spaced between 1 Hz and 1 MHz. CV was performed with a scan rate of 100 mV/s between –0.7 V and 1.3 V (centered on 300 mV equilibrium potential). We chose this range based on the cathodic and anodic potentials that generated irreversible redox processes—the potentials at which the current rises precipitously.