Bench modeSteps, parts, and safety only. Big type for a phone at the bench.
Microvolt
Phase 6: Beyond the scalpProjectA semester in a labLab-fundedTier 3Needs a lab

Project A: Electrode fabrication

PEDOT:PSS coatings that drop a microelectrode's impedance tenfold, electroplating, impedance spectroscopy, carbon fibre microelectrodes, and flexible polyimide arrays in the nanofab. The materials spine, hands on, in a lab.

AssumesProject C: Make your own electrodesThe electrode-electrolyte interfaceSpineMaterials / microfabricationAnalog / mixed-signal hardware

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In Phase 2 you chlorided a wire and measured its impedance. Now the electrode is a hundred micrometres across or less, the impedance is in the hundreds of kilohms to megohms because the area is tiny, and the goal is to record single neurons or inject stimulation charge safely. The tools are a potentiostat for , a conductive polymer bath, an electroplating setup, and, for flexible arrays, the Utah Nanofab’s cleanroom. Every step is the same chemistry as Phase 2 at a scale where it matters more.

Predict before you look

A platinum microelectrode 50 µm in diameter has an impedance around 1 MΩ at 1 kHz. You coat it with PEDOT:PSS. What happens to the impedance and why?

Drops by about an order of magnitude. is a conductive polymer that grows as a rough, porous film, multiplying the electrochemically active surface area many times over, and it conducts both electronically and ionically, so charge crosses the interface more easily. Lower impedance means lower thermal noise for recording and higher safe charge injection for stimulation. It is the most widely used coating in the field for those reasons.

Impedance spectroscopy, properly

A potentiostat applies a small sinusoidal voltage (10 mV) across the electrode in saline against a large counter electrode and a reference, sweeps from 0.1 Hz to 100 kHz, and records magnitude and phase. Plot as a Bode plot (magnitude and phase versus frequency) and a Nyquist plot (imaginary versus real impedance). Fit the equivalent circuit from Phase 2 (a constant-phase element for the double layer in parallel with a charge-transfer resistance, in series with the solution resistance) and extract the parameters. The 1 kHz magnitude is the single number everyone quotes; the full spectrum is what tells you why it is what it is.

Measure bare electrodes first. Then after each treatment. The before-and-after Bode plot is the artifact.

PEDOT:PSS electrodeposition

A solution of the EDOT monomer with a PSS counter-ion in water. The electrode to be coated is the anode, driven at constant current (a few hundred microamps per square centimetre of electrode area, so nanoamps for a microelectrode) for tens of seconds to minutes; the deposited charge per area controls the film thickness. The film grows blue-black. Too little charge and the coating is thin and fragile; too much and it cracks and delaminates. Find the window by depositing a series and measuring each with EIS and under a microscope.

Then test durability: cycle the coated electrode through a million biphasic stimulation pulses at a clinically relevant charge density and re-measure. Coatings that survive are the ones worth publishing.

Electroplating platinum black or iridium oxide

The older alternatives. Platinum black is electroplated from a chloroplatinic acid solution and gives a rough, high-area surface that is fragile. Iridium oxide, activated by cycling an iridium electrode in saline, gives excellent charge injection and is on many clinical stimulating electrodes. Deposit each on a set of electrodes and compare to PEDOT:PSS on impedance, charge injection capacity, and durability. The comparison table is a real contribution.

Carbon fibre microelectrodes

A single carbon fibre, seven micrometres in diameter, insulated with parylene and exposed at the tip, records single units with a fraction of the tissue damage of a silicon shank. Making them is a benchtop craft: thread the fibre into a glass capillary or a printed holder, insulate, cut or laser-expose the tip, coat with PEDOT:PSS. Labs that use them are usually happy to teach a student who then makes the lab’s supply.

Flexible arrays in the nanofab

Polyimide or parylene substrates, thin-film gold or platinum traces patterned by photolithography, a top insulation layer opened over the electrode sites. The nanofab’s process engineers run the tools; a trained student user does the design in a layout program, the lithography steps, and the characterization. Getting through the training and producing one working array, however simple, is a line on a resume that opens doors at every implant company.

  1. Get trained on the lab’s potentiostat. Measure EIS on a set of bare microelectrodes and fit the circuit.
  2. Deposit PEDOT:PSS at three deposition charges. Measure EIS on each. Image under the microscope.
  3. Stimulation durability: a million pulses at a set charge density; re-measure.
  4. Plate platinum black and activate iridium oxide on other electrodes; build the comparison table.
  5. Make a carbon fibre electrode and measure it.
  6. If the lab can sponsor it, complete nanofab user training and fabricate a simple flexible array.
Recall
Why does PEDOT:PSS lower a microelectrode's impedance?
It grows as a porous, rough film that multiplies the electrochemically active surface area and conducts both electronically and ionically, easing charge transfer at the interface.
Recall
What does the full impedance spectrum give you that the 1 kHz value does not?
Fitting it to an equivalent circuit separates double-layer capacitance, charge-transfer resistance, and solution resistance, which explains why the impedance is what it is and how a coating changed it.
Recall
Why test coatings with a million stimulation pulses?
Coatings often lower impedance initially and then degrade or delaminate under use; long-term stability under realistic stimulation is the property that matters for an implant.