5 min read
Updated September 24, 2025
Two separate limits bound an ICMS protocol: what the electrode can deliver without degrading, and what the tissue can absorb without damage. They are computed differently and they do not coincide. The lower of the two is your protocol.
Research use only. NeuroNexus products and the procedures described here are for non-clinical research use, and must not be used in human or veterinary medical procedures.
Three things to get right#
Back voltage must stay inside the water window, or the stimulus electrolyses the fluid around the site.
Charge delivery must stay capacitive. Faradaic reactions dissolve the electrode.
Charge per phase must stay under the tissue-damage threshold, which depends on site area.
Back voltage and electrolysis#
Ohm's law sets the voltage you need for a given current: a higher-impedance site demands a higher voltage for the same current. Past a threshold, that voltage electrolyses cerebrospinal fluid and produces oxygen and hydrogen at the site.
For NeuroNexus iridium oxide sites the thresholds are +0.6 V and −0.8 V.
Monitor back voltage during every stimulation experiment. It is the only direct measurement of whether you are inside the safe window; current and impedance are proxies.
Two ways to keep the voltage down:
- Use a larger site. Sites of 703 µm² or more, with impedance in the 50–300 kΩ range, deliver useful currents at voltages well inside the window.
- Use an activated iridium oxide surface. It raises charge capacity and lowers impedance at the same geometric area — which is what Z-Coat does, specified at order time against a design that supports it.
Capacitive and faradaic delivery#
Capacitive delivery charges the double layer at the interface. Counter-ions accumulate, the extracellular field changes, and neurons depolarize. Nothing leaves the electrode, so the process is reversible and the site is unchanged after the pulse.
Faradaic delivery transfers metal species from the electrode into solution. Some redeposits on the counter-phase and some does not. The result is progressive loss of charge capacity and a site whose behaviour changes over the course of the experiment.
The rest of this guide exists to keep the pulse in the capacitive regime.
Limit 1 — what the electrode can deliver#
Charge capacity is a property of the site material, measured by cyclic voltammetry.
| Site material | Charge capacity |
|---|---|
| Iridium | 100–150 µC/cm² |
| Iridium oxide | 1,200 µC/cm² |
The current the site can carry follows from capacity, area and phase duration:
I_max = (charge capacity × site area) / phase duration
Worked example — iridium oxide, 1,250 µm² (1.250 × 10⁻⁵ cm²), 200 µs phase:
I_max = (1,200 µC/cm² × 1.250 × 10⁻⁵ cm²) / 200 × 10⁻⁶ s = 75 µA
Above 75 µA on this site, delivery goes faradaic and the electrode begins to degrade.
Limit 2 — what the tissue can absorb#
The Shannon criterion relates charge per phase to charge density, with an empirical constant k:
log₁₀(D) = k − log₁₀(Q)
where Q is charge per phase in µC and D is charge density in µC/cm² per phase. Damage is
observed above k = 1.7; work below it, with margin.
Solving for the maximum charge per phase on a site of area A:
log₁₀(Q_max) = (k + log₁₀(A)) / 2
Worked example — same 1,250 µm² site, k = 1.7:
log₁₀(Q_max) = (1.7 + (−4.903)) / 2 = −1.602
Q_max = 0.025 µC per phase
I_max = 0.025 µC / 200 × 10⁻⁶ s = 125 µA
Which limit binds#
On this site, the electrode limit is 75 µA and the tissue limit is 125 µA. The electrode is the binding constraint. A protocol run at 100 µA satisfies the Shannon criterion while steadily destroying the site.
Which limit binds depends on site area and material, so compute both for your own geometry. Neither one alone is a safety margin.
Recommended starting parameters#
For a 1,250 µm² activated iridium oxide site with a 200 µs phase:
| Parameter | Value |
|---|---|
| Site impedance | 50–300 kΩ |
| Site area | ≥ 703 µm² |
| Maximum current | 75 µA (electrode-limited) |
| Maximum charge per phase | 0.015 µC at 75 µA |
| Back voltage window | +0.6 V to −0.8 V |
| Waveform | Charge-balanced biphasic |
Use charge-balanced biphasic pulses. An unbalanced waveform accumulates net charge across a train. A train that is safe pulse by pulse is not necessarily safe overall.
Before and during a session#
Before. Measure impedance on every site to be stimulated. Run a test pulse and confirm back voltage stays inside the window. Compute both limits for the site geometry you actually have.
During. Monitor back voltage continuously. Verify delivered current periodically — a constant-current source in compliance limit is not delivering what it reports. Watch impedance between blocks; a rising value is the electrode degrading.
Log the parameters. Current, phase duration, frequency, train structure, site identity and measured back voltage. Stimulation parameters are part of the result.
Troubleshooting#
Back voltage too high for the current you want. The site impedance is too high. Use a larger site, or an activated one. Do not raise the compliance voltage.
Impedance climbing across a session. The pulses are going faradaic. Reduce current, verify charge balance, and check the phase duration.
Stimulation stops being effective at the same parameters. The site has lost charge capacity. Measure impedance and compare against its baseline.
References#
- Merrill DR, Bikson M, Jefferys JG. Electrical stimulation of excitable tissue: design of efficacious and safe protocols. J Neurosci Methods. 2005 Feb 15;141(2):171–98.
- Marzullo T. Intracortical Microstimulation with Microelectrodes. Report Briefs. NeuroNexus Technologies, 2008.