Electrode Interface Fixture (EIF)

Contents

Description

[31-MAY-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. 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 insluate 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 grount potential.

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

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. The EIF8-CCCC provides silicone-insulated, annealed 316SS leads. The EIF8-DDDD provides the same annealed 316SS leads, but insulated with clear teflon.

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

Versions

[31-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.

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

[31-MAY-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. The contacts are on a 0.025" spacing, which is 0.635 mm. To contruct 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. Left: Top View. Center: Bottom view. Right: Pin Assignments.

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 Connector Pinout and Color Coding. These are silicone-insulated silver wires on the discontinued EIF8-MMMM. Silver wires break easily when manipulated more than one or two times during surgery.

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

[15-MAY-24] 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, we invite you to post a question on the Surgery board of the OSI Forum. 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. 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.

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.

[28-NOV-23] We receive five of A78914, PZN-08-WC, 8-way connector, equipped with stranded 316SS leads by manufacturer. Wire is A-M Systems part number 793200. We remove three wires to create a prototype EIF8-SSSS, see EIF8-SSSS-Omnetics.

[05-JAN-24] We receive five of A78967, PZN-08-WC, 8-way connector, equipped silver wires by manufacturer. Wire is A-M Systems part number 786000. We remove three wires to create a prototype EIF8-MMMM, see EIF8-MMMM-Omnetics

[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.

[25-MAY-24] This past week we hear from Kate Hills that several wires have broken off her EIF8-MMMM fixtures. These wires are 125-μm Ag with silicone insulation. An example is below.

Figure: M-Wire Breaks During Surgery. The red lead has snapped where it emerged from the epoxy. (Kate Hills, University of Manchester)

We load seven wires onto two EIF8 fixtures and apply epoxy. Once cured, we performed destructive fatigue testing on the wires, which consists of pulling with a constant force and moving the far end in a 25-mm diameter circle. We did not measure the force, but the force wasapproximately the same in all cases. We are watching through a loupe while I perform the test. The wire breaks after a certain number of gyrations.

• 2 of 50 um 316SS FH teflon-insulated: 10 and 8 gyrations
• 2 of 125 um Ag teflon-insulated: 4 and 4 gyrations
• 3 of 125 um Ag silicone-insulated: 5, 12, and 5 gyrations

In the 12-gyration Ag result, we felt the wire break after about 5 gyrations, but the silicone did not part until 12 gyrations. We're not pulling hard enough to tear the silicone right away. We assume the silicone-insulated silver lasted 5, 5, and 5 gyrations. It appears that manipulation during surgery is equivalent to 3-5 of our gyrations. We refer to our earlier work on fatigue failure of implantable leads. We found that:

• A solid steel wire lasts at least one hundred times longer than a silver wire of the same diameter.
• A seven-stranded steel wire lasts ten times longer than a solid steel wire of the same diameter.
• A wire of twice the diameter lasts four times as long.

We must abandon silver leads for the EIF. The EIF leads must not break during surgery. At the same time, they must hold their shape so that the surgeon can arrange them and cover them with cement. We load A-M Systems 791900 solid, annealed 200-um steel wires onto an EIF so we can test them for fatigue. We also load A-M Systems 973200 stranded 75-um steel wire onto the same EIF so we can compare. We order bare annealed 316SS wire from A-M Systems.

[31-MAY-24] We construct seven EIF8-SSSS-15 using A-M Systems stranded, teflon-insulated 316SS wire, part number 793200. We solder the wires ourselves and insulate with DP-460NS. We set one aside and perform the same gyration fatigue test we performed on 25-MAY-24. We gyrate one wire one hundred times with tweezers and force. It does not break. We gyrate another with thumb nails and force. After fifty cycles, it shows no sign of breaking. We pull on another wire with tweezers until the tweezers slip. The wire does not break.

[20-JUN-24] We take bare 125-μm diameter annealed 316SS wire and apply one coat of colored silicone. The outer diameter of the resulting lead is 350 μm. We twist one lead until it tangles into a spiral. We immerse in water to check for insulation damage. The insulation remains intact. The steel is resistant to gyration.

Figure: C-Lead, Green. We have 125-μm 316SS annealed wire, one coat of silicone. Twisted into a spiral, bend into a zig-zag.

The green lead shown above we made with SS5001 silicone. The coating is uniform and robust over a 150-mm length of wire. We also made blue leads out of an old batch of MED-6607. The coating was irregular in places, and adhered less well to the wire. Given that these leads will be implanted in dental cement, we see no need to use the MED-6607 unrestricted medical grade silicone for insulation. We will use the more suitable SS5001 with dye, one coat.