Electrode Interface Fixture (EIF)

Contents

Description

[31-MAY-24] The Electrode 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. The connections themselves are wires chosen from those we have found to be compatible with the EIF surgical procedure.

Figure: Electrode Interface Fixture (EIF8-XAAX-15). The leads are 316SS springs insulated with colored-coded silicone. The blue lead is the ground connection. The other four leads are the X1-X4. Two leads are terminated with X-Electrodes. The other three are terminated with A-Coils.

The EIF8-AAAA consists of an eight-way connector with five leads ending in A-Coils. The leads themselves are color-coded, silicone-insulated 316SS springs, which we call C-Leads. The electrodes are 1-mm of bare 316SS helix, which we call A-Coils. We can attach depth electrodes to the end of one or more of these leads, in which case we insulate the solder joints with silicone. The EIF8-XAAX is a fixture with two X-Electrodes and two A-Coils for the analog inputs, plus another A-Coil for the ground potential.

Figure: Electrode Interface Fixture (EIF8-MMMM-12). Silver wires, which are well-suited to recording very slow signals, are strain-relieved with helical springs. The siliver-steel solder joints are insulated with silicone. Solder joints on the connector are insulated with epoxy.

The EIF8-CCCC provides silicone-insulated, annealed 316SS leads. The EIF8-DDDD provides the same annealed 316SS leads, but insulated with clear teflon. The EIF8-SSSS provides teflon-insulated, stranded steel leads with outer diameter 140 μm. The stranded leads may be used flush-cut at the end, or with half a millimeter of insulation removed from the end. Any further insulation removal is likely to result in the stranded leads parting with one another and creating a mess.

For details of surgery, see Surgery section of this manual. For instructions on mounting an HMT on an installed EIF, and un-mounting afterwards, see Mount and Unmount section of HMT manual.

Ordering

[17-SEP-24] There are many possible configurations of the EIF. Each configuration that someone has ordered, or asked us to quote on, graduates from being a mere "configuration" to a "version". We describe and list the available EIF versions in the section below. The minimum order quantity for any particular version is ten pieces.

Versions

[10-NOV-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. Standard lead lengths are 10-40 mm. For a complete list of available terminations, including depth electrodes, see our Electrode Catalog.

We use four letters to specify the leads and their terminations. In some cases, these four letters specify the terminations of the X1-X4 leads.

Figure: Insulated Solder Joint. An X-Electrode with 125-μm 316SS, full-hardened, teflon-insulated wire is connected to a 250-μm diameter 316SS helix insulated in silicone. The insulating heat shrink is McMaster 6699T18.

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

Figure: Insulated Solder Joint. A Y-Electrode with 75-μm 316SS, full-hardened, teflon-insulated wire is connected to a teflon-insulated, 76-μm 316SS stranded lead. The insulating heat shrink is McMaster 6699T15.

We use the smallest diameter heat shrink that will fit over the larger of the two leads we are joining. Sometimes this heat shrink is transparent, sometimes it is black.

Connector

[06-NOV-24] Each EIF is built around a connector. The EIF8 is built around the A79614, also called the PZN-08-AA. This connector is both polarizing and hermaphroditic, in that it mates with another like itself.

Figure: Closeup of Electrode Interface Fixture (EIF8-SSSS). Wires are teflon-insulated, 7-stranded 316SS.

The contacts are on a 0.025" spacing, which is 0.635 mm. To construct an EIF8, we clip the connector leads back, bend them towards the center-line of the connector and solder our leads onto the reduced pins. We wash and dry, then apply DP-460NS epoxy to encapsulate the joints. When we inspect the wires after the epoxy has cured, we reject any EIF with more than half a millimeter of exposed wire outside the epoxy.

Pin Function 1 GND 2 none 3 X3 4 X2 5 none 6 none 7 X1 8 X4

Figure: Electrode Interface Fixture (EIF8) Connector. From left to right: top view, bottom-left view, table of connections, bottom-right view. Pins used by HMT are marked by their color codes.

To make 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-CCCC Connector Pinout and Color Coding. These are silicone-insulated, annealed, 316SS wires. They hold their shape, but are also tough and strong.

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.

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

[10-FEB-25] The EIF must be cemented to the skull of the subject animal. For a description of the surgery in detail, consult the HMT surgical protocol provided by Kate Hills of University of Manchester. If you have additional questions, feel free to e-mail us. We provide summary information below.

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. When 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.

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

[25-OCT-24] For a chronological account of the development of the electrode interface fixture, see our EIF Development page.