Electrode Interface Fixture (EIF)

© 2023-2024, Kevan Hashemi, Open Source Instruments Inc.

Contents

Description
Versions
Connector
Surgery
Development

Description

[10-APR-24] The Electrod Interface Fixture (EIF) mounts permanently on the skull of a subject animal. The EIF mates with a Head-Mounting Transmitter (HMT) such as the Head-Mounting Transmitter (A3040). It provides two or more electrical connections between the HMT and animal biopotentials, most often EEG, ECoG, and LFP. Each connection is terminated with an electrode chosen from our Electrode Catalog


Figure: Electrode Interface Fixture (EIF8-AAAA). Five silicone-insulated leads each end in 1 mm of helical steel wire. The leads can be anything from 10-40 mm long.

The EIF8-AAAA consists of an eight-way connector with five leads ending in A-Coils. Each A-Coil is a 1-mm long spring made of bare 50-μm diameter 316 stainless steel. We can attach depth electrodes to the end of one or more of these leads, in which case we insluate the solder joints with silicone.


Figure: Electrode Interface Fixture (EIF8-XAAX). The electrode solder joint is insulated with heat shrink. The joints at the base of the connector are encapsulated with epoxy.

The EIF8-SSSS provides teflon-insulated, stranded steel leads with outer diameter only 140 μm, which is much thinner than the 500-μm nominal diameter of our silicon-insulated springs. We recommend that stranded leads be used flush-cut at the end with no lenth of wire stripped of insulation. The EIF8-MMMM provides teflon-insulated silver leads, which can be stripped at the end for a larger electrode surface area, or left flush-cut for a smaller electrode surface area.

For details of surgery, see below. For instructions on mounting an HMT on an installed EIF, and unmounting afterwards, see elsewhere.

Versions

[10-MAY-24] Each EIF consists of a connector and a number of wires leading from the connector. Each wire has a termination. The EIF part number takes the form EIFn-W-L, where n is the number of contacts on the connector, W is a string of letters that identifies the channels used, the leads, and their terminations, L is the length of the leads in millimeters, not including the terminations. For a complete list of available terminations, including depth electrodes that we can attach directly to the EIF leads, see our Electrode Catalog.

Version Connector Leads Terminatios
(X1, X2, X3, X4, GND)
Compatible Transmitter
EIF8-AAAA A79614 316SS, helix of 50-μm wire, helix 250 μm dia, silicone 500 μm dia. 5 × 1-mm bare helix A3040D
EIF8-XAAX A79614 316SS, helix of 50-μm wire, helix 250 μm dia, silicone 500 μm dia. X-Electrode, 2 × 1-mm bare helix,
X-Electrode, 1 × 1-mm bare helix
A3040D
EIF8-XAAA A79614 316SS, helix of 50-μm wire, helix 250 μm dia, silicone 500 μm dia. X-Electrode, 4 × 1-mm bare helix A3040D
EIF8-YAAA A79614 316SS, helix of 50-μm wire, helix 250 μm dia, silicone 500 μm dia. Y-Electrode, 4 × 1-mm bare helix A3040D
EIF8-BBBB A79614 316SS, solid wire 50 μm dia, teflon 115 μm dia. 5 × 2-mm bare, solid wire A3040D
EIF8-SSSS A78914 316SS, 7 strands of 25-μm wire, stranded 76 μm dia, teflon 140 μm dia. 5 × square-cut tip A3040D
EIF8-MMMM A78967 Silver, solid wire 125 μm dia, silicone 400 μm dia. 5 × 2-mm bare solid wire A3040D
Table: Electrode Interface Fixture Versions.

You will note that the the strings of letters that specify the lead terminations consist of four letters, each specifying the termination of one of the X1-X4 leads. We do not have a letter for the C lead, which is the reference potential. Our assumption is that the reference potential will always be a bare-wire or square-cut end to the same type of lead we use for X1-X4.


Figure: Electrode Interface Fixture (EIF8-AAAA). Five silicone-insulated leads each end in 1 mm of helical steel wire. The leads can be anything from 10-40 mm long.

When we connect our flexible, helical leads to depth electrodes, as in the EIF8-XAAX, we insulate the joints between the helical leads and the solid steel wire of the X-Electrode with flexible heat shrink, as shown above.

Connector

[19-MAR-24] Each EIF is built around a connector. Sometimes we buy the connector with the wires loaded, and cut off the wires we don't need. Sometimes we solder the wires onto the connector ourselves and terminate them with depth electrodes. Here are data sheets for the connectors we use in our fixtures.

A79614: PZN-08-AA, 8-way connector, vertical, through-hole.
A78914: PZN-08-WC, 8-way connector, stranded 316SS leads.
A78967: PZN-08-WC, 8-way connector, solid silver leads.

The PZN-08-AA and PZN-08-WC are miniature, polarized connectors. Here is the pin numbering scheme we use for the connector.

PinFunction
1GND
2none
3X3
4X2
5none
6none
7X1
8X4
Figure: Electrode Interface Fixture (EIF8) Connector. Left: Top View. Center: Bottom view. Right: Pin Assignments.

The EIF8-SSSS starts off as a connector loaded with eight wires. We remove those that are not needed, cut those that remain to length 40 mm and strip the insulation off the final 2 mm of each lead by scraping it off in one movement with a scalpel. We use these bare ends to test the electrical function of the fixture before we ship.


Figure: EIF8-SSSS with Stranded, Stainless Steel, Teflon-Insulated Wires.

The EIF8-XAAX is a complex electrode with five wires, each of which has its own color and termination. The table below presents the color coding.

PinColorFunctionTermination
1blueGNDA-Coil
2nonenonenone
3greenX3A-Coil
4yellowX2A-Coil
5nonenonenone
6nonenonenone
7redX1X-Electrode
8salmonX4X-Electrode
Table: Color Codes, Pin Numbers, and Functions of EIF8-XAAX Wires.

The "GND" connection is "Signal Ground". Once we connect this potential to an animal body, we say the HMT is "grounded".

The EIF8 pin numbers in the sketch match the pin numbers of the mating connector on the HMT circuit. So we have Pin 1 and 2 are GND. We have X1, X2, X3, and X4 on pins 7, 4, 3, and 8 respectively.

Surgery

[15-MAY-24] The EIF must be cemented to the skull of the subject animal. For advice on details of this procedure, please post a question on the private Surgery board of our OSI Forum.


Figure: EIF8-XXXX Being Loaded Onto Mouse Skull. To the right is a 22G cannula for injection.

Loading the EIF8 on a mouse skull is a delicate operation. There is a shortage of space. We want the EIF to sit as close to the skull as possible, so that the HMT to which it attaches will protrude as little as possible from the mouse's head. Whem implanting the EIF8-XAAX, which is equipped with two depth electrodes, space becomes limited for clamp fixtures, making it harder to place all components directly on the skull. Once all the leads, electrodes, and connectors are in place, we cover with dental cement to create a headplate. For a description of the surgery in detail, consult this surgical protocol provided by Kate Hills of University of Manchester.


Figure: EIF8-XAAX Loaded On Mouse Skull. Headplate cement is curing.

When it comes to recordings down to 0.0 Hz (DC recordings), we recommend against using a screw with silver leads, so silver leads should be cemented in place. In the Kate Hills protocol, there are no screws fastening the leads into their holes, only cement. As a result, there is more space for the surgeon to work in. So far as we can tell, the cemented leads are just as stable and quiet as leads held in place by screws.

Development

[24-OCT-23] We have 10 of EIF8-XAAX ready to ship, with 15-mm leads and silicone-insulated solder joints.

[05-JAN-24] We receive five of A78967 connectors with teflon-insulated silver wires.

[09-JAN-24] We have have been studying how to insulate solder joints between our C-Leads (0.5-mm diameter with helical steel wire) and X-electrodes (solid steel wire). For our write-up see EIF Insulation Methods. We will use SS5001 silicone applied with a syringe.

[08-MAR-24] We hear from our collaborators. "In the surgical image the bare steel cannula (for viral injection) is secured to the skull using 3M RelyX unicem 2 dental cement after first protecting the dura with a small amount of 3M Vetbond (cyanoacrylate). We use a similar technique to secure depth electrodes. I haven't used commercially available cannula, since the black seating material takes too much real estate on the skull for multiple electrode placement. The surface electrodes (bare wire hooks) are held in place with a biocompatible acrylate resin with which I am experimenting; again the dura was first protected with a small amount of Vetbond. I haven't yet had a chance to check for irritation or toxicity, so this remains at an early stage. I will share more details once I'm sure of its use."

[25-APR-24] We find the right heat shrink to insulate our soldered joints to X and Y Electrodes, McMaster part 6699T19. The result is a tidy joint, see here. We abandon our silicone dispersion application procedure in favor of the heat shrink.

[15-MAY-24] We have twelve hours recording from EIF8-SSSS with A3040D3Z 0.0-160 Hz four-channel HMT. We see <10 mV drift in first hour, <1 mV drift in the subsequent hours. Baseline swings are <100 μV after first hour. We receive this implantation protocol from Kate Hills.