Project C: Make your own electrodes
Silver/silver-chloride by chloriding a silver wire, dry electrodes from conductive silicone or printed pins, and an active electrode with a buffer on it. Then measure the impedance of each versus frequency and find out why gel exists.
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The electrodeElectrodeThe conductor that turns ion currents in tissue into electron currents in a wire; its chemistry and contact quality set the noise you live with. Glossary entry is where ions in the body become electrons in a wire, and its chemistry decides how much noise and drift you will live with. Commercial gold cups cost several dollars each and are excellent. Making your own teaches you what excellent means. In this project you chloride a piece of silver wire into a proper Ag/AgClSilver/silver-chloride (Ag/AgCl)The standard electrode material for biopotentials, because its chemistry produces a stable, low-noise, low-drift interface with salty gel. Glossary entry electrode, make a dry electrode two ways, and measure each one’s impedanceElectrode impedanceHow hard it is to push current through the skin-electrode contact, typically 5 to 50 kilohms with gel and megohms dry, higher at low frequencies. Glossary entry from 1 Hz to 1 kHz.
A dry electrode reads 5 kΩ on a 1 kHz impedance meter. What will its impedance be at 10 Hz, where EEG lives?
Ten to a hundred times higher. The skin’s outer layer acts like a capacitor in parallel with a large resistor. At 1 kHz the capacitor shorts it out; at 10 Hz it does not. Impedance meters that test at 1 kHz flatter dry electrodes, which is why the professional ones test at 10 or 30 Hz.
Parts
| Part | Where | Qty | Approx. |
|---|---|---|---|
| Digital multimeter You will use this more than any other tool. Auto-ranging saves time. | Amazon, Harbor Freight, Micro Center | 1 | $25 |
| 3D printing access (campus makerspace) or a Bambu A1 Mini Free on campus after the safety certification. Buying your own is ~$250 and is worth it by Phase 2 if you use it weekly. | Marriott Library or Lassonde makerspace; Bambu Lab if buying | 1 | free |
| Conductive silicone or carbon-loaded TPU filament for dry electrodes | Amazon, Protopasta | 1 | $25 |
| Silver wire (0.5 mm) and household bleach for Ag/AgCl plating Chloriding in bleach or by electrolysis in saline turns bare silver into a proper Ag/AgCl electrode. | Jewelry suppliers, Amazon | 1 | $15 |
| Total (prices drift; treat as a ceiling) | $65 | ||
Ag/AgCl, two ways
Bare silver in contact with salty gel forms a metal-electrolyte junction whose voltage wanders with every fluctuation in ion concentration: noisy and drifty. A silver surface coated with silver chloride has a stable, reversible chemistry, which is why every serious biopotential electrode is Ag/AgCl.
Chemical chloriding. Clean a 5 cm length of silver wire with fine sandpaper and alcohol. Immerse the end you want to use in household bleach for 15 to 30 minutes. It darkens to grey-brown as a chloride layer forms. Rinse in distilled water, dry, and do not touch the coated surface. The coating is soft; treat it gently.
Electrolytic chloriding. Two silver wires in 0.9 percent saline. Connect the one you want to coat to the positive terminal of a 9 V battery through a 4.7 kΩ resistor (about 1 mA) and the other to the negative. Run for ten minutes. Chlorine from the saline deposits on the anode. This method gives a more even, thicker coating and is how the good commercial electrodes are made.
Make three of each. You will use pairs in the measurements.
Dry electrodes, two ways
Conductive silicone or filament. 3D-print a small pad with rounded pins (to reach through hair) in carbon-loaded TPU, or cast conductive silicone in a printed mold. Attach a wire with conductive epoxy or a crimped ring.
Spring pins. Half a dozen gold-plated spring-loaded test pins (the kind used in test fixtures) soldered to a small disc of copper-clad board, so that each pin finds the scalp independently.
Dry electrodes have high, unstable impedance and are sensitive to pressure. They exist because gel is slow and messy, and every consumer headset uses them. Feeling their limitations yourself is the point.
An active electrode
Take one of your electrodes and mount a unity-gain buffer (half a TL072, or a dedicated low-noise buffer chip) on a coin-sized board directly at the electrode, powered from the amplifier’s rails through the cable. Now the high-impedance path is a few millimetres long instead of a metre, and the cable carries a low-impedance signal that cable movement and room fields barely disturb. This is what the active electrodeActive electrodeAn electrode with a small amplifier or buffer built into it, so the high-impedance signal travels only millimetres before being made robust. Glossary entry in every research dry-electrode system does.
Measure impedance versus frequency
Two electrodes on the same forearm, ten centimetres apart, on clean skin. Drive a sinusoid from the signal generator through a 100 kΩ resistor into one electrode and out the other; the resistor makes it a known current (1 V through 100 kΩ is 10 µA). Measure the voltage between the electrodes with the scope or your amplifier. Impedance is voltage over current. Repeat at 1, 3, 10, 30, 100, 300, and 1000 Hz. Plot on log-log axes for each electrode type: gel cup, your Ag/AgCl with gel, dry silicone, dry pins, and Ag/AgCl on skin prepared with Nuprep.
If you built the ADS1299 board, use its lead-off detection: it injects a small known current at 31 Hz and the resulting voltage on each channel is the electrode’s impedance at that frequency, which is how the OpenBCI GUI’s impedance check works.
- Chloride six silver wires, three by each method. Photograph the coating.
- Print or assemble two dry electrodes. Build one active electrode on a small board.
- Set up the current-injection measurement and verify it with a 10 kΩ resistor in place of the body: you should read 10 kΩ at every frequency.
- Measure each electrode pair at the seven frequencies. Record everything in a table.
- Plot impedance versus frequency, log-log, one line per electrode type. Note the impedance at 10 Hz for each in your notebook.
- Record two minutes of EEG at O1 with each electrode type and compare the noise floor and the drift.
Where this leads
The interface explainer is the chemistry behind the plot you just made. Phase 6’s electrode fabrication takes the same measurements into the nanofab, where the electrodes are a hundred micrometres across and the coatings are conductive polymers.