Thin-Film Grid Electrodes: Setup, Handling, Cleaning and Care

A thin-film grid ↗ is a flexible polyimide array, 12 µm thick, that records from the surface of the cortex, the skull, the heart or a peripheral nerve. Nothing holds it to the surface but its fit, and almost everything that goes wrong with one is mechanical: a crease, an edge picked up with the wrong tool, a site abraded by sliding, air under the array. This guide collects the handling, setup, cleaning and storage rules in one place. The surgical procedure is in the ECoG and EEG surgical guides.

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.

Five rules#

These are copied in full from the ECoG and EEG surgical guides and repeated where they belong below.

Handle the array with flat-tipped forceps, never sharp ones; if flat-tipped forceps are not available, cut small pieces of silicone tubing and put them on the tips of sharp forceps.

A polyimide grid creases if it is picked up by an edge. Store flat and protected; a crease in a polyimide array is permanent.

Do not reposition by sliding. Lift, re-wet and re-place. Sliding a polyimide array across bone abrades the sites.

Do not press the array flat. Contact should come from the fit and from surface tension. Pressure on cortex produces a lesion and a slow drift in the signal.

Rinse immediately after use with sterile saline.

What you need#

The grid, in its case. The matching headstage and any adapter the package ↗ requires; a grid mounts on a package and mates the same headstages as a silicon probe. Cables, and the acquisition system powered and running. A dissecting microscope with a cold light source. Flat-tipped forceps. Sterile saline.

Handle the grid#

Handle the array with flat-tipped forceps. Do not use sharp forceps on a polyimide grid: the tips cut the film and the traces in it. If flat-tipped forceps are not available, cut small pieces of silicone tubing and put them on the tips of sharp forceps. A polyimide grid creases if it is picked up by an edge, and a crease in a polyimide array is permanent. Keep the array flat whenever it is not on the surface it records from.

Inspect the grid#

Inspect under magnification before each use and check channel continuity. Note which corner of the array corresponds to which channel and record it before the array goes anywhere near the animal; a grid placed at 180 degrees produces a plausible and wrong map.

Connect the headstage#

Attach the headstage to the package connector while supporting the package, with the array flat and unloaded. If an adapter is in the path, get its channel map now and fold it into the site map; site 1 is rarely channel 1 through an adapter. Route and strain-relieve the cable so that nothing pulls on the array or the connector.

Place the grid#

Placement is where a grid recording is made or lost, and the surgical guides give it in full. The rules that belong to handling are these. An ECoG grid goes onto a wet surface with a wet array: place from one edge and roll it into contact so fluid, not air, is displaced ahead of it, then look for bubbles under magnification, because air under a site is an open channel. An EEG array goes onto dry, smoothed bone, laid down from one edge in the same rolling motion. Do not press the array flat; pressure on cortex produces a lesion and a slow drift in the signal. Do not reposition by sliding; sliding a polyimide array across bone abrades the sites. Lift, re-wet and re-place.

Reference and ground#

Every channel is the voltage at its site minus the voltage at the reference, and a reference under the grid subtracts the activity you came to record. Place the reference off the recording area, on a bone screw or in a saline-filled craniotomy as the package and channel count allow; the wiring by site count is in each surgical guide, and the decision is worked through in EEG Referencing Techniques. Decide the reference with the analysis, not at the bench.

Clean the grid#

Rinse immediately after use with sterile saline. Cleaning agents and sterilization methods for polyimide arrays are not specified in current documentation; the enzyme and alcohol sequence written for silicon probes is not written for grids. Contact support before using any agent other than sterile saline.

Store the grid#

Store flat and protected, in the original case, in a clean, dry place at room temperature. Keep the connector covered. A crease in a polyimide array is permanent.

Track condition over time#

Check impedance at implant and at each session, and inspect under magnification before each use. For a chronic implant, inspect the perimeter for cement lifting or fluid accumulation; impedance rising over weeks usually means cement lifting at an edge. Record a baseline before the experimental manipulation begins, because a recording is interpreted against the animal's own baseline. Impedance ↗ explains what the number means.

When something is wrong#

A block of channels flat is contact, not electronics: air, blood, a fold or a fluid layer under that part of the array. All channels noisy is ground and reference first, then shielding, then cable routing. Amplitude falling over the first two weeks is expected in a subdural preparation as the surface reaction develops; a steeper fall suggests fluid under the grid. The full sequence is in Noise Reduction and Troubleshooting, and each surgical guide has a troubleshooting section for its preparation.

Next#

The surgical procedure is in the ECoG or EEG surgical guide; which grid to order is in Thin-Film Grids for Surface Recording. Technical questions: support@neuronexus.com · +1.734.913.8858.