Electrodes

Contents

Introduction

[27-MAR-24] An electrode is a component designed to acquire the electrical potential at some location in an animal's body. A lead termination is an electrically conducting component designed to connect to an electrode or permit the construction of an electrode. Suppose we might have a short length of bare stainless steel helix on the end of an insulated lead. We stretch the wire, straighten it, bend it, cut it short, and fasten it in a skull hole with a screw. The stainless steel helix is the "termination", while the screw and bent wire form the "electrode". In this report we present a catalog of terminations and electrodes that we can supply with our Subcutaneous Transmitters (SCT).

Figure: Pin and Bare Wire Electrodes. Three leads are terminated with two right-angled pins, or D-Pins, and one 1-mm bare wire helix, or A-Coil.

Following the catalog, we describe the various electrodes and terminations in detail. For details of our silicone-insulated electrode leads, see Flexible Wires. For instructions on soldering stainless steel wire and screws, see Solder Joints. For a discussion of the sources of the electroencephalogram (EEG) and local field potentials (LPF), see our essay The Source of EEG. We discuss electrode impedance in the Electrode Impedance section of the same essay.

Catalog

[22-MAR-24] Each lead of the a subcutaneous transmitter (SCT) has its own termination. We specify the termination of each lead with its own letter code: two letters for single-channel transmitters, three letters for a dual-channel transmitters with a common reference potential, and four letters for a dual-channel transmitter with separate reference potentials. The lead terminations is: Red, Yellow, Blue, Green. For a transmitter with Red, Yellow, and Blue leads, the letters "DDA" specify D-Pin termination on the red and yellow leads and an A-Coil termination on the blue lead. For a transmitter with Yellow and Blue leads "PA" specifies a P-Coil termination on the yellow lead and an A-Coil termination on the blue lead.

By default, we solder screws, pins, and sockets perpendicular to leads and wires. But if we precede the electrode letter code with a lower-case "s", the pin, screw, or socket is parallel to the lead or wire, as shown below.

Figure: Perpendicular (Left) and Straight (Right) Attachment of D-Electrode Pins. We specify perpendicular with code "D" and straight with code "sD".

Some lead terminations can act as electrodes themselves. The screws we can push into a skull hole. The bare helix we can straighten and bend to fit in a skull hole.

Figure: Bare Wire Termination of B-Leads. Left: 1-mm A-Coil. Right: 3-mm P-Coil. P-Coil is designed for crimp connections to depth electrodes and is not available for C-Leads.

If we want a more sophisticated electrode, we must use the termination to connect to that electrode during surgery. Whatever we attach to the ends of the SCT leads must be small enough to be drawn up under the skin of the neck to the head. If we want to use a depth electrode with our transmitter, we connect the depth electrode to the transmitter lead during surgery, for which we use a pin and socket contact or a crimp contact. Order depth electrodes at the same time as your transmitters, and we will try to make sure your transmitter leads have matching terminations. The pin and socket connection is slightly easier to perform than the crimp contact, but the crimp contact generates less noise.

See SCT Implantation for an introduction to the implantation procedure. See Bare Wires for video demonstration of stripping silicone from the ends of our electrode leads. See Solder Joints for instructions on tinning the bare wire for soldering to your own screws and pins. Join the OSI Forum to ask questions about implantation.

Coiled Wires

[22-MAR-24] Of all our electrodes, the A-Coil is the most popular. Our electrode leads contain a stainless steel, helical spring that acts as its conductor. The wire itself is 316SS. To make an A-Coil termination we remove 1 mm of the silicone at the end of the lead, revealing the spring helix itself. A P-Coil is 3-mm of bare spring metal.

Figure: The A-Coil Termination. We have 1-mm of bare wire at the end of a B-Lead. To make a bare wire electrode, we stretch out the helix and bend the straightened wire at right angles.

This coil can be useful as it is. But by far the most common use of the A-Coil is to create an angled wire for insertion into a skull hole.

Figure: Angled Wire Made From A-Coil.

The A-Coil has the great advantage that it can be re-created with the help of a scalpel from a freshly-cut lead. When we re-use an implant, the simplest procedure is to cut off the ends of the leads to free them from the cement used to hold them in place in the first implantation. We create a new A-Coil by removing insulation from the cut lead. When you order an SCT, you specify the insulated length of the leads. Add ten or fifteen millimeters for each time you want to be able to cut the leads back and re-implant your telemetry device.

Figure: Subcutaneous Lead Drawing.

We use a scalpel to remove silicone from the tip of the lead so as to expose the bare wire. The tutorial movie below shows how to remove silicone from one of our 0.7-mm diameter leads.

Movie: How to Remove Silicone From the End of a 0.7-mm Diameter Lead.

The procedure for our 0.5-mm diameter leads is similar, but more delicate. We have an additional movie to explain the extra precautions one must take to remove the silicone from these smaller leads.

Movie: How to Remove Silicone From the End of a 0.5-mm Diameter Lead.

To record electrocorticography (ECoG) with a bare wire, we stretch it, bend it, and secure it with a screw. We can supply you with any of the screws listed in our Catalog along with your bare-wire leads. These screws will be 304 stainless steel and free of solder. Electrodes that are free of solder, such as the bare wire secured with a steel screw, generate far less low-frequency chemical artifact than electrodes in which we have soldered the lead to a screw.

Figure: A-Coil Implanted with Anchor Screw. Here we have a hole drilled in bone, a bare wire electrode in the hole, and a 3.2-mm long 1.6-mm diameter screw threaded into the hole to fasten the wire in place.

To create an angled-wire electrode for a skull hole, we stretch the exposed steel coil until it is nearly straight. We cut it to a length that will reach through a skull hole and penetrate the brain to the correct depth, or to rest upon the surface of the brain, depending upon our requirements. A penetration of 1 mm gives a more powerful EEG signal than a wire tip on the dura. In rats, we recommend a length of 5 mm. In mice, 2.5 mm. We bend the wire by ninety degrees half-way along its length. We place the wire in a skull hole and lay the insulated lead along the surface of the skull. We thread a screw into the hole. The exposed wire on top of the skull allows the screw to avoid interfering with the silicone insulation of the lead, and will allow dental cement to bond to the wire directly. In rats, the 2.5 mm wire passing through the skull will penetrate roughly 1 mm into the brain. In mice, the penetration of the 1.2-mm wire tip will be similar, because the skull is thinner. With the wire held in place by a screw, we cover the screw with dental cement. The cement anchors and insulates the head of the screw and bonds to the wire itself. The coil will flex between the transmitter and the anchor screw, but the bare wire and the screw will be fixed with respect to the skull. Movement of the brain with respect to the skull will generate movement artifact, but we will not see sudden steps due to intermittent metal-on-metal contact. Nor will we see EMG artifact from muscles above the screw, because we have insulated the top of the screw with cement. When we retrieve the transmitter from the animal, we tend to cut the leads where they emerge from the skull cement. It is possible to dissolve dental cement with vinegar, but usually the animal's brain is needed intact for examination. Having cut the leads, we remove silicone from the tips to expose more wire, and so we can re-create the bare wire electrode for a second implantation.

Another use of the bare wire is to measure electromyography (EMG). We straighten the wire, cut it back to one or two millimetrs, insert it into the muscle, and hold in place with cyanoacrylate. We insert two such wires into a pair of muscles or into one muscle, and record EMG with a a pair of inputs on one of our subcutaneous transmitters. Another method is to stretch the wire, put a 180° bend in the wire near the end of the silicone insulation, and cut off the excess wire to make a hook. We insert the hook in the muscle we want to monitor. The hook holds the lead in place sufficiently well to measure the EMG amplitude, provided we filter out movement artifact below 20 Hz.

Solid Wires

[10-APR-24] We make straight, silicone-insulated, solid-wire electrode by soldering a solid wire into the spring before we coat with silicone. In order to hold a reliable coat of silicone, the wire must be at least 125 μm in diameter. Our M-Wire is 125 μm bare silver wire we have soldered into a spring and insulated.

Figure: M-Wire. Fifteen millimeters of silver wire extend from the lead, insulated in silicone.

We can use the tip of the M-Wire as an electrode of diameter 125 μm, or we can remove some of the silicone insulation to make a longer electrode. We remove silicone from the end by cutting around the wire with a sharp scalpel and a gentle hand, then removing the detatched silicone segment.

Figure: Three M-Wires, Red, Blue, Yellow. The malleable silver wire tips remain bent after being gripped by test clips during quality control.

There is little point in attaching a solid stainless steel wire to the end of one of our electrode leads. The steel wire inside the lead is perfectly adequate whenever we want a stainless steel contact. Platinum or platinum-iridium might be useful as a solid wire, but we have not yet been asked to make them.

Fastening Screws

[20-FEB-24] Stainless steel screws are useful as electrodes for electroencephalogram (EEG) and electrocorticogram (ECoG) because they are easy to anchor to the skull. We drill a hole slightly smaller than the screw and thread the screw into the hole. There are two ways to connect our electrode leads to screws. One is to solder the lead to the head of the screw. The other is to use the screw to hold the end of the lead in the skull hole. Solder joints can simplify surgery, but they to generate chemical artifact. When we solder a lead to the screw, the electrode is the tip of the screw, and our recording is EEG. When we feed a wire through the hole, we can make the wire long enough to penetrate the brain, and keep the screw short enough so that it does not protrude beneath the skull. Our recording will be ECoG. Once our lead and screw are in place, we will fix the screw to the skull hole with cyanoacrylate. We cover with dental cement. The dental cement insulates the screws from body fluids and nearby muscles.

Figure: Bare Fastening Screws. For dimensions, see table below.

We stock four sizes of screw for implantation. We assign each a letter code for use when ordering transmitters. The table below gives the dimensions of each type, and provides a link to a drawing.

When we solder a lead to a screw, we solder the tip of the lead to the head of the screw. If the screw is large enough, we can keep the joint to one side of the slot in the screw, allowing the surgeon to turn the screw with a screwdriver. During surgery, we have a pick-up lead connected to the screw, so the lead rotates along with the screw. In order to make sure the pick-up lead relaxed and untwisted when the screw is in its final orientation, we must pre-twist the screw by the number of turns required to thread it into its hole. We are able to solder the lead to one side of the slot only for our B and K-Screws. For the smaller L and C-Screws, we fill the top of the screw with solder. These screws are so small that there is no need to thread them into a skull hole. We simply press them in.

Figure: Fastening Screws with Soldered Leads. Left: K-Screw. Right: B-Screw.

When we fasten a wire into a hole with a screw, we have no solder joint. The head of the screw is always free of solder, so we can instert a screwdriver into the head. But the C-Screw has no slot for a screwdriver, so we must always press it into place. The L-Screw provides a socket for a Torx T1 bit, which is not a standard piece of laboratory equipment. We will provide a Torx T1 bit with your first order of L-Screws. The K and B-Screws have slotted heads.

Contact Pins

[05-FEB-16] Gold-plated pins allow us to connect the end of our EEG lead to depth electrodes. The depth electrode is too cumbersome to pass easily beneath the skin of an animal from the transmitter to the head. The pin, on the other hand, is easy to move up under the skin. Our contact pins are made by Mill-Max, and are summarized in their data sheet. The photograph below shows our D Electrode, which mates with our H and J Electrodes.

Figure: D-Electrode. Diameter 0.30 mm, length 3.1 mm, Mill-Max 4353-0-00-15-00-00-33-0. The pin is soldered at right-angles to the tip of an EEG lead. Note the solder has wicked its way part-way down the pin, but the tip of the pin is still exposed gold plate.

The pins are easy to solder to a steel wire. They provide a gold-plated finish, which mates well with the gold-plated finish of a socket, to provide a corrosion-resistant electrical contact within dental cement saturated with water vapor.

Figure: F-Electrode. Pin, diameter 0.64 mm, length 4.1 mm, Mill-Max 5035-0-00-15-00-00-33-0. Mates with Plastics One socket E363/0.

Our F and G eletrodes are pins that mate with electrodes made by Plastics One.

Figure: G-Electrode. Pin, diameter 0.51 mm, length 6.0 mm, Mill-Max 5063-0-00-15-00-00-33-0. Mates with Plastics One socket MS303/6.

When you choose an electrode with a socket, we will look at the socket drawing and choose a pin that will fit sungly and take up as little vertical height as possible, to keep your implantation compact.

Crimp Contacts

[12-FEB-24] A crimp contact is one where we bring together two wires we want to join, surround them with a steel tube, squeeze the tube, and so hold the two wires together. We call the steel tube the crimp ferrule. No solder is involved. We eliminate the chemical artifact that is generated by the galvanic potential between solder and steel. Crimp contacts are reliable when at least one of the two conductors acts as a compression spring. In our crimp contacts, the helical electrode lead acts as the compression sring, pushing outwards on the ferrule and the straight, solid wire provided by a depth electrode. We insert the stright wire into the core of the helix, slide the ferrule over the helix, and squash the ferrule with pliers.

Video: The Q-Ferrule. Combines the helical lead and the ferrule, simplifying the crimping procedure. The helix is the MDC13867A stainless steel spring.

We combine the helix with the ferrule so as to simplify the crimping process, creating the Q-Ferrule. We slide the 2-mm ferrule onto a 3-mm P-Coil. We squeeze the ferrule at the base of the P-Coil so as to hold it in place. We trim the end of the P-Coil so that only half a millimeter of helix protrudes. When it comes to making the crimp contact, we slid the 2-mm bare end of a 125-μm diameter stainless steel wire into the helix. We push it in as far as it will go. We squeeze with pliers to complete the contact.

Video: Crimping an X-Electrode to a Q-Ferrule. Creates a solder-free joint. (Speaker says "C-Ferrule" at one point, but that's an error.)

The crimp contact requires that the bare steel wire enter the central cavity of the helical lead. The spring in our B-Leads has outer diameter 450 μm and inner diameter 250 μ. A bare 125-μm diameter steel wire fits easily into the helix of the spring. The ferrule is 2-mm long with inner diameter 510 μm and 50-μm walls. The ferrule squashes the helix and wire together, creating a press-contact that is maintained despite movement and stress by the compression of the spring. To perform the crimp, we recommend modified forceps, like the ones shown below. We cut off the forcep clamps, cut short the forcep jaws, and grind the jaws down until they are 2 mm wide at the tip, and 4 mm high. During surgery, we hold the Q-Ferrule in the jaws of these pliers, and slide it over the bare steel wire to which we want to make a crimp contact, and squeeze as tightly as we can to squash the ferrule.

Figure: Q-Ferrule Crimp Tool. Jaw width 2 mm, jaw height 4 mm, jaw tip to fulcrum 23 mm. Your institute's machine shop can make these out of forceps. Or we can make one and send it to you.

When you want to make a crimp contact, specify the Q-Ferrule with the letter "Q" in your part sensor part number's electrode list. When re-using an SCT, remove insulation from the end of the lead with the help of a scalpel, as shown in this video.

[20-JAN-20] We have three gauges of hypodermic 304SS tube: 304H23XX (20 mil inner diameter, 2 mil wall, Gauge 23XX), 304H22XX (22 mil inner diameter, 3 mil wall, Gauge 22XX), and 304H21XX (29 mil inner diameter, 2 mil wall, Gauge 21XX). We cut 3-mm sections of each by scribing with a triangular file until we break through the wall, then breaking by bending with pliers. Run a 0.4-mm diameter steel probe through the interior to round the ends. The result is 3-mm crimp ferrules. The 23XX is barely larger than our 18-mil helical lead. We screw the helix through the ferrule, put our 5-mil diameter stainless steel wire inside from the other end, and crimp. The 22XX is wide enough for the helix to slide through easily, but we can crimp the helix in place by squeezing one end of the ferrule. The 21XX is fragile, but with care we make the ferrules. When compressed, they are wider, but they work just as well. We would prefer 316SS for our ferrules, but 316SS tubes with 2-mil wall thickness are not available in 20-mil diameter. Our previous ferrules were 304SS, and we like the ease with which the thin-walled tube can be crimped. We order from MicroGroup 1000 of 2-mm ferrules made with the 304H23XX.

[05-APR-23] Our crimp contacts have become easier to use and easier to make over the years. We began by wrapping the straight 125-μm diameter solid wire of the X-Electrode in a 3-mm length of 450-μm steel helix. We demonstrate crimping a 250-μm helix to a 125-μm wire with this original procedure in this tutorial video. Later, we eliminated the extra piece of helix and crimped the wire and lead together immediately with a laser-cut ferrule, as we show in this tutorial video. Our current solution is to partially-crimp one end of the ferrule to the lead helix, so that we can grab the ferrule and lead with forceps, slide onto the electrode wire, and crimp with one hand.

Depth Electrodes

[14-MAR-24] To reach farther into brain, we need a stiff, insulated wire. We assemble depth electrodes for the convenience of customers, but we also provide a Depth Electrode Assembly Guide. When implanting a depth electrode, we run the pick-up lead along the surface of the skull to the electrode, where we attach the lead to the electrode either with a crimp contact or a socket contact. During surgery, the depth electrode must be held firmly while we make this contact. Afterwards the electrode must be maneuvered into the correct location on the skull and lowered to the correct depth.

Figure: X-Electrode. Wire is teflon-insulated, full-hardened 316 stainless steel. Wire diameter 125 μm, insulation 200 μm. We trim electrode wire to the desired length during surgery.

Each electrode provides a mounting fixture by which we can hold the electrode during surgery. In the case of the X and W-Electrodes, the mounting fixture is a hypodermic tube. In the case of the R and J-Electrodes, the mounting fixture is a threaded plastic cannula guide. To secure an electrode to the skull, we place an anchor screw through the skull nearby and enclose the depth electrode and the anchor screw with cement. Depending upon the electrode, we remove the mounting fixture and seal with another layer of cement, or we leave the mounting fixture intact to act as a cannula guide.

Figure: X-Electrode Drawing.

The X-Electrode, part number SDE-X, provides a 450-μm diameter stainless steel tube as a mounting fixture. To hold the tube, we recommand a clamp like the RWD 68201, which attaches to your stereotactic mount. The tube has a slot cut into it, out of which we run the electrode wire for connection to an electrode lead with a crimp contact. Because the crimp contact does not contain any solder, the signal recorded by the electrode will contain far less low-frequency chemical artifacts. Crimp contacts allow us to record slow signals like cortical spreading depressions (CSDs). For use with the crimp contact, we must use one of our B-Leads with a crimp ferrule on the end, which we call a "Q-Ferrule". The video below shows how to crimp the X-Electrode to a bare helical wire.

Video: Crimping a Q-Ferrule to an X-Electrode.

Any wire we bury in body tissue will cause damage and scarring. The photograph below shows damage to the brain of a mouse, as seen in subsequent brain slices. The Y-Electrode, part number SDE-Y, is similar to the X-Electrode, but is equipped with a thinner wire, which in theory will cause less trauma to brain tissue. The Y-Electrode wire is 140-μm teflon insulation around a 75-μm solid 316SS wire. For details see photograph and drawing.

Figure: Brain Slices in Neighborhood of Syringe and Electrode Implantation. S: Path of syringe needle. X: Path of X-Electrode wire. (Source: Anonymous)

The R, W, J, and H-Electrodes all provide a socket into which we can plug a gold-plated pin. The pin that fits the socket is the D-Pin termination that we can provide at the end of any subcutaneous lead. The W-Electrode, part number SDE-W, is an X-Electrode equipped with a socket. The W-Electrode introduces two solder joints into the electrode: one between a helical lead and the pin, and the other between the electrode wire and the socket. Do not use W-Electrodes to record cortical spreading depressions. The W-Electrode is easier to implant than the X-Electrode because sliding a pin into a socket is easier than crimping a ferrule onto a bare wire.

Figure: W-Electrode. Wire is teflon-insulated, full-hardened 316 stainless steel. Wire diameter 125 μm, insulation 200 μm. We trim electrode wire to the desired length during surgery.

Our drawing of the W-Electrode is identical to that of our X-Electrode, except that the bare wire of the X-Electrode has a socket soldered to it.

Figure: W-Electrode Drawing.

The R-Electrode, part number SDE-R, provides a cannula through which we can inject a substance into subject animal's brain, and subsequently record from the neighborhood of the injection.

Figure: R-Electrode. Wire is teflon-insulated, 125-μm diameter, full-hardened 316 stainless steel. For sketch, see here

The R-Electrode wire is parallel to the axis of the cannula, 150±150 μm from the cannula protrusion. The socket is 3±1 mm from the plastic thread. The plastic thread is 5.7 mm long. The wire is 10 mm long, allowing the user to cut it back to the length they want. The cannula tube is stainless steel, inner diameter 290-330 μm, outer diameter 470 μm.

Figure: R-Electrode Drawing.

The J-Electrode, part number SDE-J, provides a straight stainless-steel wire mounted to a plastic cannula guide. With a clamp that holds the cannula guide, we can lower the electrode into position. The wire is insulated below the socket, but bare above the socket. By the time we are done with implantation, all the bare wire above the socket will be cut away.

Figure: J-Electrodes. The electrode wire is longer than necessary so we can cut it back during implantation. The guide cannula allows us to hold the electrode while the head fixture cement cures. We remove the guide entirely to complete implantation.

Once the electrode is in position, we cover the socket and anchor screw with dental cement and let it cure. We take care not to bury the tip of the guide cannula in the cement. Once the cement is cured, we cut the bare steel wire where it emerges from the top of the guide cannula. We raise the guide cannula off the wire. We cut the wire where it emerges from the cement. We cover the exposed tip of the wire with more cement. Now the depth electrode is fastened to the anchor screw by dental cement, and there are no exposed metal contacts.

Figure: J-Electrode Drawing. For details of the cannula guide, see the R-Electrode drawing.

Here are detailed instructions for implanting J-Electrodes. Our collaborators at ION/UCL took these instructions and composed these Pictoral Instructions, which you may find more useful than our written list.

The H-Electrode is now obsolete. It provided a platinum-iridium wire, but was otherwise identical to the J-Electrode. Now that we have a reliable source of straightened, full-hardened stainless steel wire, we no longer make electrodes with platinum-iridium wire. The stainless steel is much stiffer, and when it comes to depth electrodes, stiffer is better.

Figure: Close-Up of H-Electrode. Note the 1-mm separation between the tip of the guide cannula and the socket. This separation allows dental cement to secure the socket without adhering to the guide cannula.

[26-SEP-15] Edinburgh University implants the prototype H-Electrode in transgenic mice. The figure below shows depth and surface field potentials recorded simultaneously with a dual-channel subcutaneous transmitter. The surface and reference electrodes are 0.5-mm screws.

Figure: Depth (Pink) and Surface (Blue) Potentials.Recorded at Univerity of Edinburgh, (M1443256453.ndf) with a Subcutaneous Transmitter (A3028A-HCC).

When implanting the H-Electrode, the Edinburgh group monitored EEG from the H-Electrode while lowering the tip of the electrode into the hippocampus. They adjusted the depth of the electrode until they obtained the largest possible EEG amplitude. The 125-μm tip of the depth electrode has impedance roughly 10 kΩ, compared to 2 kΩ for the screw. The depth recording has baseline amplitude 400 μV compared to 80 μV for the surface recording. There we see one or two step artifacts per hour in these dual-channel recordings. But the artifacts are shared between the depth and surface recordings, like this, so they cannot be the result of movement of the H-Electrode alone.

[08-AUG-16] We receive archive M1470389836.ndf from ION/UCL. Two A3028H-DDC transmitters are implanted in two conscious, freely-moving mice with two J-Electrodes each. Amplitude is roughly 80 μV from all channels. We look through the hour-long recording and see none of the step artifacts that motivated the development of the H and J-Electrodes.

Figure: Overview of One Hour of J-Electrode Recordings. Archive M1470389836.ndf, two A3028H-DDC transmitters equipped with four J-Electrodes. Range is normalized for each channel.

We note that ION/UCL did not adjust the depth of the J-Electrode to obtain the maximum amplitude of EEG, but rather set the depth to a certain value during implantation without monitoring EEG.

Conductive Epoxy

[05-FEB-16] Conductive epoxy might allow us to cement a bare wire directly onto the skull in two places so as to provide EEG recording without drilling any holes. We tried silver epoxy, as we describe below. The recordings pick up muscular activity from the scalp of the host rat, which we see also in un-insulated screw electrodes. Such activity we can detect and classify in order to distinguish it from EEG. The recordings showed continuous drift in the baseline voltage, not by transient steps as we see in poorly-secured wire electrodes, but as a drift of several millivolts over one-second intervals. Such drifts we can overcome with our event classification. The conductive epoxy recordings are, we believe, adequate for event detection. But the electrodes themselves are not as reliable as screws, in that they tend to come loose. More work with silver epoxy might increase reliability, but such work has not yet occurred.

[12-AUG-10] CHB implanted transmitter No10 and glued the ends of the analog wires to the animal's skull using silver epoxy, which provides an electrically conducting contact. With these contacts, we observed seizure-like activity in a non-epileptic rat, as shown here in archive M1281121460. We are convinced that the epoxy contacts are picking up EEG. Later, in archive M1281455686, we can compare No10's contacts with No8's steel screw contacts. When we say baseline signal we mean the signal low-pass filtered to 1 Hz or so, which is a measure of the average value of the signal over a fraction of a second.

Figure: Baseline Swings from Epoxy Contacts. Archive M1281455686 at time 224 s. The gray plot is No10 with epoxy contacts. Blue plot is No8 with screw contacts.

From this one experiment, we see the baseline swinging up and down compared to the stable baseline we obtain from the screws. But the silver epoxy contact sensitivity compares well to that of the bare wires, and exceeds that of the screw contacts.

Figure: Simultaneous Seizures Recorded from Two Rats. The No10 (gray) trace is from silver epoxy contacts. The No8 (blue) trace is from screw contacts.

The following plot shows power in the 2−20 Hz band for the two transmitters during three hours of recording.

Figure: Seizure Power. The graph covers three hour-long archives M1281455686, M1281460570, and M1281464170 recorded at CHB. Both animals receive kainic acid between time 300 s and 500 s. The No10 (gray) trace is from silver epoxy contacts. The No8 (blue) trace is from screw contacts.

The peak power from No10 corresponds to vigorous seizures like this one, not any transient spikes or level shifts. In addition to seizure detection, the silver epoxy contacts are also sensitive to wave bursts in the 40−160 Hz band. The plot below shows simultaneous wave-bursts from No8 (screws) and No10 (epoxy).

Figure: Wave Bursts in Epileptic EEG. Both animals have been injected with kainic acid an hour before. Here we see wave bursts in both signals, in the absence of a seizure.

It appears that the silver epoxy has only one disadvantage compared to screws: a moving baseline. But the moving baseline does not appear to interfere with seizure or wave burst detection, and the silver epoxy method does not require holes in the skull.

Movement Artifact

[30-MAY-17] The movement of electrodes with respect to the surrounding animal tissue will add step-changes in potential to the EEG signal. By default, we equip the amplifiers in our SCTs with a high-pass filter with cut-off frequency 0.3 Hz. When a step-change passes through this high-pass filter, it come out looking like a pulse with a sharp front edge, followed by a relaxation half a second long. The theoretical step response of the A3028B, for example, is the blue graph shown here. If we solder the two leads of a transmitter together, drop the transmitter in water, and stir it around, the standard deviation of the signal is less than 20 μV and we see no step artifact. If we separate the leads and allow them to measure the variation in electric potential in the water as we stir, we see 1-mV rumble, but not sharp steps. When we allow the electrodes to make intermittent contact with one another as we stir that we start to see 10-mV steps in the signal. We call these steps "movement artifact". The longer-term changes in potential that we observe with amplifiers that have no high-pass filter we call "chemical artifact", and we discuss those artifacts separately.

Movement artifact arises in EEG recordings when the electrodes are not secured with respect to the skull. As implanters have learned to secure wires and screws more effectively to the animal skull, the frequency of these artifacts has decreased. In mice, these artifacts are particularly difficult to eliminate. Here are some examples of movement artifact from mouse recordings.

Figure: Modest Movement Artifact from Skull Screws in a Mouse, A3028B (Edinburgh).

Here we see pulses caused by steps of order 10 mV, which are shaped into pulses by the transmitter's high-pass filter. In the figure below we see far larger artifact from the same M0.5 skull screws in another mouse.

Figure: Large Movement Artifact from Skull Screws in a Mouse, A3028C (MRC Harwell).

Here we see large artifact with more complex progression, also from skull screws in a mouse. The amplitude is 15 mVpp.

Figure: Large Movement Artifact from Skull Screws in a Mouse, A3028B (Edinburgh). Two separate animals, one showing artifact and the other not.

We are not certain of the origin of the following features. The two traces are from a dual-channel transmitter implanted in a mouse. If the electrode potential of the common electrode changes, we expect the effect upon the X and Y channels to be shared. If the electrode potential of either X or Y changes, we expect to see the effect only on X or Y. Here we see opposite changes in X and Y.

Figure: Modest Movement Artifact in Dual-Channel Mouse EEG. Scale is 400 μV/div.

The number of movement artifacts in a recording varies greatly from one implant to the next, and is a strong function of who performs the surgery. Recordings from rats tend to have fewer than one artifact per hour for all experienced implanters. Movement artifact in mice varies from fewer than one per hour to hundreds of artifacts per hour. Those implanters whose recordings are almost free of artifact have reduced the artifact rate by working on better anchoring of their electrodes on the mouse skull. We are currently working on developing a guide to successful implantation in mice.

Chemical Artifact

[12-FEB-24] Most of our SCTs provide a 0.3-Hz high-pass filter to remove electrode potential and changes in electrode potential from the recorded signal. If we want to look for phenomena such as cortical spreading depressions (CSDs), we must use an SCT without a high-pass filter. Now we become concerned with changes in electrode potential that take place over tens of seconds. We call such changes chemical artifact. Solder joints combined with stainless steel will generate a sustained galvanic potential when immersed in saline, and variations in this galvanic potential appear in our recordings as chemical artifact. No matter how hard we try to keep water off a solder joint in a head fixture, it makes its way to the solder joints and generates chemical artifact. Here is an example of chemical artifact recorded from a mouse with depth electrodes that use soldered pins and sockets.

Figure: Chemical Artifact Recorded in a Mouse with W-Electrodes. Recorded with A3028U, dual-channel, DC-160 Hz. The X and Y electrodes are D-Pins soldered plugged into W-Electrodes. The reference, C, is a bare wire held in place with a screw. By Rob Wykes, ION/UCL.

To reduce chemical artifact, we eliminate solder joints. One way to eliminate solder is to use a crimp contact between our lead and depth electrode. Another way is to hold a bare steel wire in place on the skull with a screw. The crimp contact provides better immunity to movement.

Figure: Electrodes in Saline. Left: Crimped electrodes, one stainless steel wire crimped to steel spring, one crimp on steel spring only. Right: Bare wire and screw electrodes in dental cement.

One device we equip with two crimps, one leading nowhere and one connecting to a bare steel wire. The other device we equip with two screws and a mock animal skull made out of dental cement. We record continuously for two days within a faraday enclosure that is disturbed only be changes in ventilation and lighting.

Figure: First Hour Recording from Crimps (Pink) and Screws (Blue).

During the night, when the lights are off in the OSI office, nobody is moving around, and nobody is touching the table upon which our experiment is running, the recordings are like the example shown below.

Figure: Night-Time Hour-Long Recording from Crimps (Pink) and Screws (Blue).

During the day, we see step changes in potential followed by relaxation on both channels. We call these "pulse artifacts". In forty-eight hours of recording we see 31 pulse artifacts from the screw electrodes and 9 from the crimp electrodes. The recording below shows several pulses from the screws.

Figure: Pulse Artifacts from Screws (Blue) With Crimps (Pink) For Comparison. For pulses from crimps see M1547602637.

We sometimes see changes in both signals occuring at the same time. In M1547588237 we see a pulse from the crimps occurring at the same time as a linear decrease in the voltage recorded by the screws. The screws suffer from more pulses, but the crimps show more short-term drift, as in M1547599037, and occasionaly larger drift as in M1547602637. When we return to the petri dishes after two days, the electrodes are still covered with water, but barely so. We perform a series of tests.

Figure: Agitated Recording from Crimps (Pink) and Screws (Blue). (1) Rock enclosure for 60 s, leave for 140 s. (2) Rock for 10 s, no action for 80 s. (3) Open enclosure and add water to both petri dishes, followed by 425 s of no action. (4) Open enclosure and remove excess water, which takes 70 s and involves loss of signal. (5) Transmitters restored to enclosure and no action for 170 s. (6) Rock for 10 s, followed by no action for 210 s. (7) Sprinkle a pinch of salt into each petri dish, followed by 2350 s of no action. Raw data M1547821946.ndf.

After this hour, we leave the dishes alone. The salt is dissolving. The lights and heating are on. For the next three hours, we see only one pulse. Raw data M1547825546. Other than this pulse, the signal remains stable to ±100 μV.

[12-FEB-24] We record for ten days from this arrangement of screws in dental cement, immersed in stationary 1% saline. Two of the signals recorded by the A3047A1B-A transmitter extend to 0.0 Hz, so we use these to look for chemical artifact.

Figure: Electrode Potential (mV from bottom of range, averaged over 5 min) versus Time (days). Orange: Y, 0-40 Hz, 108 mV. Yellow: Z, 0-160 Hz, 108 mV. Green: Temperature, uncalibrated.

The downward spike at time 6.5 days occurs only in the Y-input, see here, which suggests it is generated by a chemical reaction at either the Y+ or Y− electrodes, not by movement, light, or temperature, which would affect all electrodes.

[09-MAY-19] Our collaborators at ION/UCL implanted an A3028SZ2-AA equipped with two X-Electrodes crimped to the bare ends of the leads. There are no solder joints. The frequency response of this device is 0.0-80 Hz with dynamic range ±100 mV. The plot below shows the first fifteen hours of recording after implantation, overlayed upon one another.

Figure: Recording from X-Electrodes with Crimp Contacts and 0.0-80 Hz Bandwidth.

The drift of several millivolts in the first two hours is consistent with the transmitter battery voltage settling after it is first turned on. In the subsequent 26 hours we find no sign of chemical artifact. The animal dies an hour before the end of the recording, and after death, we see the potential rise by 5 mV in 20 minutes.

[01-MAR-21] Crimp contacts used with a recording bandwidth of 0.0-160 Hz record cortical spreading depression (CSD), as shown below.

Figure: Cortical Spreading Depression Recorded in Two Locations with Two X-Electrodes. Bandwidth 0.0-160 Hz, A3028U-AA, crimp contacts to X-Electrodes. (Rob Wykes, ION/UCL).

Here is an example of a spontaneous seizure recorded with 0.0-160 Hz showing changes in EEG baseline during a spontaneous seizure.

Figure: Spontaneous Seizure Recorded with X-Electrode. Bandwidth 0.0-160, A3028U-AA, crimp contact to X-Electrode. (Rob Wykes, ION/UCL).

For crimp contacts, we must select our B-Type leads, which provide a 450-μm steel helix that is well-suited for crimping to electrode wires.

Development

[04-AUG-10] We obtain samples of a screw, 00-96X1-16 from Plastics One, from CHB. With the help of acid flux, we solder screws to the tips of the wires of an A3013A, as shown here. The wires are soldered to the top sides of the heads of the screws. We tried soldering the wires to the underside of the screw heads, but we found that solder spread all down the length of the screw, obscuring the threads. Even when we solder the wire to the top of the head, we must take care not to allow solder to spread to the threads, and we do this by threading the screw into a small hole in a piece of box cardboard. The cardboard also provides us with a means of holding the screw while we make the joint.

[12-AUG-10] Archive M1281455686 contains EEG recorded by screws in No8 compared to EEG recorded by epoxy contacts in No10. The screws provide a stable baseline. Both contacts provide wave burst and seizure detection. The screws appear to be slighly less sensitive to seizure activity than silver epoxy or the bare wire electrodes. Peak seizure power in the 2−10 Hz range is around 5000 k counts-sq compared to 10,000 k counts-sq for the other electrodes.

[15-AUG-10] CHB implants No6 with screws. The wires of this transmitter are only 100 mm long, a bit short. We see large transient jumps on the signal. Sameer reports, "I think the unstable baseline from transmitter#6 can be attributed to the use of epoxy to secure one of the screws instead of a drilled hole. Due to some reason, I think size or location of leads, YingPeng couldn't drill it in. However, the other screw was fastened into the skull."

[25-AUG-10] We receive two sizes of 303 stainless steel screws from SmallParts (now Amazon Supply). Both have what is called the binding head, which is flatter than the screws we started with, as you can see in the photograph below.

Figure: Three Stainless Steel Screws. All three are 1.6 mm long. Left: 000-120 thread, binding head, B000FN0J58 from Amazon Supply. Center: 00-90 thread, binding head, B002SG89QQ. Right: 00-96 thread, round head, 00-96X1-16 from Plastics One.

All these screws are 1.6 mm long (1/16"). The 000-120 has diameter 0.86 mm with thread pitch 0.21 mm. The 00-90 has diameter 1.2 mm with thread pitch 0.28 mm. the 00-96 has diameter 1.2 mm and thread pitch 0.26 mm.

[03-APR-11] We have settled upon the 00-90, 1.6-mm long binding head screw as the standard for the A3019D transmitter for implanting in rats. For mice, Louise Upton has chosen an M0.5-0.125 screw, the B0038QOYFA, which has a thread 1.1 mm long and 0.5 mm in diameter, with pitch 0.125 mm. We soldered this screw to the 70-mm leads of an A3019A. She drilled holes in the skull of a dead mouse and declared the screw to be the perfect size. She did not thread the screw into the hole, but instead pushed it in. The threads held the screw in place.

[03-APR-11] We study 15 hours of recordings from ION/UCL, from eight control animals. The transmitters are A3019Ds with 00-90 binding head screws 1.6 mm long. The leads are insulated right up to the solder joints on the screw head. The screws are set directly into the skull with dental cement to hold them in place and insulate them. For a typical recording from eight sets of screw electrodes, see here.

We see few artifacts in the EEG. The amplitude of each trace is around 40 μV. Another similar period of recordings from eight different animals with the same electrodes shows 40 μV baseline EEG also. The consistency between the baseline amplitudes suggests that the screws are giving a repeatable contact with the brain.

[04-APR-11] ION/UCL suspects that the 00-90 screws are coming loose from the skull after a few weeks. "Yes, I also had the impression that the power per frequency band was quite consistent between animals, at least during the first week of recording. After that power tends to decrease across all bands in quite a lot of animals. This is probably caused by the skull getting thicker and slowly pushing the screws out. This ends in rats loosing their electrodes after 2 to 4 weeks. I've now soldered larger screws on a couple of transmitters and will also add a few more screws just to anchor the headpiece to see if this will enable recordings with stable band power for two or three months."

[13-SEP-12] By now we have supplied to ION over thirty transmitters with 00-80 screws, 1.6 mm in diameter and 3.2 mm long. These screws require a hole in the skull of diameter roughly 1.2 mm. They appear to work well for implants greater than four weeks, when the smaller screws lose sensitivity.

[12-JUN-13] Our M0.5-0.125-1.0 screw is $8 each in quantity 100, and it is often out of stock at the supplier. By now, Louise Upton and Sukhvir Wright of Oxford University have implanted roughly forty transmitters with these screws, and they found them to be too long. They use a 0.4-mm thick washer to back the screw away from the top of the skull. The bone is 0.4 mm thick, so the tip of the screw would be roughly 0.2 mm into the brain, pressing upon the surface. Today we ordered, from US Microscrew, quantity 1000 of a custom-made screw, diameter 0.5 mm, shaft length 0.6 mm, which we name M0.5-0.125-0.6.

[23-JUL-13] We receive our 1000 screws and find that their threads are fine, but the heads are mangled by the slot cut, as shown here (US Microscrew, 0.5 mm diameter, 0.6 mm length screw, first attempt).

[02-AUG-13] We receive from US Microscrew another 1000 screws to replace the first set. This lot don't have a slot cut in the head. We cover the entire head with solder, so we don't need the slot. Removing the slot makes the part easier to make.

Figure: US Microscrew, 0.5 mm diameter, 0.6±0.1 mm length screw, second attempt. Note the screw has no slot in the head.

[05-APR-23] These M0.5 threaded electrodes have provided hundreds of thousands of hours of reliable recordings.

[20-JUL-21] We find that failure to wash off acid flux from X-Electrode tubes causes corrosion in the laser-cut stainless steel surface of the tube's slot, as shown below. This corrosion will not affect the functioning of the electrode, but is unsightly and could, if allowed to continue, cause the tube to snap.

Figure: Corrosion of W-Electrode Tube When We Fail to Wash Off Acid Flux Promptly.

[03-NOV-22] We hear from one of our collaborators that crimping during surgery is a two-person process: we need more than two hands to hold everything in place for the crimp. Part of the problem is that the crimping pliers are so large. We order smaller pliers and grind down their jaws to produce a smaller tool. The shortened jaws are powerful enough to squash the crimp ferrule at their tips.

Figure: Custom-Made Crimp Tool.

[14-DEC-22] We are working on a new electrode at the request of Rob Wykes, which we are calling the R-Electrode. We take a small cannula designed for mice, PlasticsOne part "C200GS-5/SPC GUIDE 26GATW BLK (F/O)", and attach a steel wire and socket to it like so.

Figure: Sketch of Propsoed R-Electrode with Socket.

[06-APR-23] We alter our R-Electrode so that the wire is flush up against the cannula protrusion. We receive straightened wire from A-M Systems in 300-mm sections. The teflon insulation of the wire has been roughened by the straightening process, as we can see in the photograph below. When we roll the straightened wire in our fingers, we can feel that it is no longer round.

Figure: Top: Unstraightened Teflon-Insulated Stainless Steel Wire. Bottom: Same Wire, After Straightening.

We test the insulation of several sections for breaches by immersing in water and measuring resistance between the stripped end of the section and the water. We see no sign of any electrical contact through the insulation. When we push the straightened wire into our electrode post, the fit is tighter than for unstraightened wire. Nevertheless, we proceed to make a dozen electrodes with the straightened wire, and the result is W and X Electrodes with perfectly straight wires, and no contact between the wire and the tube.

[03-MAY-23] The S-Electrode is a bare stranded stainless steel wire. To make the S-Electrode, we solder the stranded steel wire to our helical lead and insulate them both. We cut the stranded lead and remove some insulation from the end to reveal the strands. These we can place on muscles beneath the skin.

Figure: Prototype S-Electrodes.

[13-DEC-23] We are having a hard time buying our existing, little-used L-Screw, a 000-120 thread, 0.063" slotted binding head. Small lots are out of stock. We can buy 1000 for $2.15 each from the manufacturer. But we can buy this screw, 000-120, 0.063" with torx-plus button head in packets of 100 for $14. We switch our L-Screw to this new part, and provide this drawing.

[08-JAN-24] We record for 72 hrs with an A3029WZ1, a DC-40 Hz transmitter with two channels and 270 mV input range. The X-input is a 316SS wire, Y-input and C-input are silver blackened by over-night soak in bleach, which we believe to be AgCl that has been blackened by exposure to sunlight. Each wire is 40 mm long. We fasten the SCT to the ouside of a jar full of 1% saline. The X, Y, and C electrodes bend over the top of the jar and down 20 mm into the saline. The saline is not covered. It evaporates a few millimeters per day. As a control we have an A3028E3 running with 316SS leads in air.

Figure: Electrode Potential Drift (mV) versus Time (hr). Blue: 316SS/AgCl in saline. Orange: AgCl/AgCl in saline. Yellow: 316SS in air.

[09-JAN-24] We repeat the above experiment but with Y and C wires silver soaked in HCl, which roughens the surface of the silver, so that it is dull gray. We call this "AgR".

Figure: Electrode Potential Drift (mV) versus Time (hr). Blue: 316SS/AgR (roughened silver). Orange: AgR/AgR. Yellow: 316SS/316SS in air.

We stop the experiment after three hours because of the excessive artiface on Y, which is AgR/AgR. We have helical steel leads with silver wire soldered to the end and insulated in silicone. We equip Y and C with these, and cut the insulated 125-μm silver wire off flush at the end, leaving a 125-μm diameter silver electrode. We equip X with a helical steel lead, flush cut, leaving a 100-μm diameter steel electrode. We immerse electrodes in saline and record for ten minutes while agitating the saline in various ways.

Figure: Electrode Potential Drift (mV) versus Time (s). Blue: 316SS/Ag in saline. Orange: Ag/Ag in saline.

We strip 5 mm of silicone off our two silver wires, Y and C. We remove 3 mm of silicone from our steel helix, X. We immerse the leads in saline and start recording within thirty seconds of immersion. The transmitter remains outside the jar.

[10-JAN-24] We have fourteen hours recording from our Z1 SCT with Ag/Ag and 316SS/Ag. As reference, we continue to use an A3028E3 with its electrodes in air.

Figure: Electrode Potential Drift (mV) versus Time (hr). Orange: Ag/Ag in saline. Blue: 316SS/Ag in saline. Yellow: 316SS/316SS in air.

Our Z1 transmitter has exhausted its CR1025 battery by the end of our experiment. The linear drift upwards in 316SS/Ag and downwards in Ag/Ag are consistent with a transmitter that is about to stop. We remove the battery and replace with a CR2032, which will last a long time.

We measure the potential between pairs of electrodes in 1% saline. What we call "AgCl" is what we get when we immerse silver wire in bleach overnight. We see a persistent, black coating, which is consistent with AgCl blackened by exposure to sunlight. When we expose silver soaked in bleach to ultraviolet light, it turns even darker. What we call "AgR" is what we get when we immerse silver wire in HCl overnight. We see a persistent, white coating that we believe is Ag that has been dissolved and then re-deposited on the wire. We name our electrodes Cathode and Anode and we measure the potential between with a 10-MΩ voltemeter.

Figure: Potentials Generated by Pairs of Electrodes. Ag: shiny silver wire. AgR: silver wire roughened by HCl. AgCl: silver wire blackened by soaking in bleach. (Calvin Dahlberg)

[12-JAN-21] We blackened the two silver leads of our WZ1 by immersing in bleach. We believe the black coating is AgCl. We immerse the leads in saline, along with our 316SS termination and start recording a few minutes later. For the first four hours of the recording, the AgCl/AgCl potential is saturating at the top of the 290-mV voltage input range of the WZ1's DC input. We end this experiment and try 316SS/316SS and Ag/316SS.

Figure: Electrode Potential (mV from bottom of range) versus Time (hr). Blue: 316SS/AgCl in saline. Orange: AgCl/AgCl in saline. Yellow: 316SS/316SS in air.

[16-JAN-24] We have four days recording from 316SS/316SS and Ag/316SS. The Ag/316SS is saturated on the negative side for the entire recording with the exception of the first few minutes. The 316SS/316SS drops from 10 mV in the first hour, then recovers during the next ten hours. For the next ninety hours, it varies by ±0.5 mV.

Figure: Electrode Potential (mV from Bottom of Range) versus Time (hr). Blue: 316SS/316SS in saline. Orange: Ag/316SS in saline, out of range at 1 mV. Yellow: 316SS/316SS in air.

The downward glitch 44 hrs into the experiment is a ten-minute, 30-mV downward pulse at 10:30 am on a Sunday morning. So far as we know, nobody was in the office at that time.

[26-JAN-24] We now have almost two weeks of recording from our electrodes in saline.

Figure: Electrode Potential (mV from Bottom of Range) versus Time (hr). Blue: 316SS/316SS in saline. Orange: Ag/316SS in saline. Yellow: 316SS/316SS in air.

The positive slope in the yellow line is the gradual exhaustion of the battery in the transmitter we are using as a control. The negative Ag/316SS potential is dropping erratically. The 316SS in saline shows 24-hour cycles.

Figure: Electrode Potential (mV from Bottom of Range) versus Time (hr). Blue: 316SS/316SS in saline.

The stainless steel electrode potential varies by less than ±1 mV. We see occasional spikes, but we are occasionaly disturbing the saline jar.

[02-FEB-24] We construct a dental cement platform with an ample number of 00-80 1/8" screws for fastening wires in place. We connect the six leads of an A3047A1B-A transmitter to six of the screws. We dissolve 5.8 g of salt in 52 ml of water and place in a jar along with our electrodes and the transmitter.

Figure: Six Electrodes in Dental Cement with A3047A1B-A Transmitter.

The A3047A1B provides three analog inputs: X 2-80 Hz 256 SPS 54 mV, Y 0.0-40 Hz 128 SPS 108 mV, and Z 0.0-160 HZ 108 mV. It also provides a 128 SPS temperature signal. See A3047 for details. We turn on the transmitter and start to record from an ALT in our Faraday canopy.

[12-FEB-24] We have ten days of recordings from our three-channel transmitter and control. In the plot below we give pass-band of amplifier and dynamic range of input to specify the analog amplifiers. The X, Y, and Z are all 316SS wires held by 304SS screws.

Figure: Electrode Potential (mV from Bottom of Range, averaged over 5 min) versus Time (days). Blue: X, 2-40 Hz, 54 mV. Orange: Y, 0-40 Hz, 108 mV. Yellow: Z, 0-160 Hz, 108 mV. Green: Temperature, uncalibrated. Brown: Control.

The DC channels fluctuate with temperature, as we can see more clearly here. We see a downward pulse on the 0-160/108 trace at time 6.5 days. We seek out this pulse in our recording and present it below.

Figure: Large Pulse At 6.5 Days on DC Input.

There is another smaller pulse at time 8.5 days on the same channel.

Figure: Small Pulse At 8.5 Days on DC Input.

The two pulses have the same time evolution, but dramatically different amplitudes. The onset of the pulse takes a few seconds, and the relaxation takes about a minute.

[22-MAR-24] We solder silver wire, A-M Systems 781500, 125 μm in diameter, to the end of one of the springs we use for B-Leads. The wire fits easily into the 250-μm inner diameter of the spring. We solder the wire to the final 1 mm of the spring. We coat with silicone. The result is an insulated silver wire that we can strip easily at the end if we want to, see M_Wire.jpg.