Electrodes and Terminations

Contents

Introduction

[03-JAN-25] Here we present the terminations and depth electrodes we can supply with our Subcutaneous Transmitters (SCT). 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 to act itself as an electrode. The "D-Pin" termination is a gold-plated pin soldered to the end of an electrode lead, and it's express purpose is to mate with the "E-Socket" mounted of a depth electrode. But the "A-Coil" termination is 1 mm of bare steel helix at the end of an electrode lead, and it's express purpose is to be used as an electrode. We can deploy it as an EEG electrode by straightening, bending, cutting short, and fastening in place with a skull screw. Or we can deploy it as an EMG electrode by making a cut in a musle, inserting into the cut, being sure that the silicone insulation enters the cut, and securing the wire with cyanoacrylate. Whenever a termination mates with an electrode, the electrode is always a "depth electrode": one that penetrates into the brain, and by virtue of its mounting fixture and straight wire cannot be attached to the electrode lead before implantation, because we cannot tunnel the electrode under the skin to the skull during surgery without bending the depth electrode wire.

Figure: Depth Electrodes. From left to right: X, W, J, and R-Electrodes.

For a separate discussion of 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), see The Source of EEG. For calculation and estimates of electrode impedance, see Electrode Impedance. For a presentation of the behavior of various electrode metals in response to DC signals, see Step Response of Metal Electrodes.

Catalog

[06-JAN-25] Each lead of the a Subcutaneous Transmitter (SCT) or Implantable Stimulator (IST) has its own termination, and each type of termination has a name.

A lead termination may itself be used as an electrode, or it might connect to a depth electrode during surgery. The A-Coil termination is almost always deployed as an electrode. But D-Pin always mates with an E-Socket on a depth electrode. The table below presents our depth electrodes.

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.

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 an SCT, we connect the depth electrode to the lead during surgery using 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 Coiled 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

[11-JUL-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.

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 can be re-created at the end of a freshly-cut lead with the help of a scalpel. 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 5 mm 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.

Electrodes that are free of solder, such as the bare wire secured with a steel screw, generate less low-frequency chemical artifact than electrodes that contain an exposed solder joint.

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.

For instructions on how to use the A-Coil to record electrocorticography (ECoG) and electromyography (EMG), and electrogastrogram (EGG), see our Implantation guide.

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

[18-OCT-24] To reach farther into brain, we need a stiff, insulated wire. The wire will come supplied longer than you need. You cut the wire back to the length you require. You can square-cut the end, or you can cut it at an angle so that it has a bevel to better penetrate the brain. When implanting with a subcutaneous transmitter (SCT), we run the SCT lead along the surface of the skull to the electrode and 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.

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, 26-gauge, inner diameter 250 μm, outer diameter 470 μm.

Figure: R-Electrode Drawing. The cannula guide is the a modified C315GS-5SP. We have ground down the guide to length 2 mm.

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 with cement. We take care not to bury the tip of the cannula in the cement. Once the cement is cured, we cut the bare steel wire where it emerges from the top of the cannula pedestal. We raise the cannula off the wire, cut the wire where it emerges from the cement, and cover the exposed tip of the wire with more cement.

Figure: J-Electrode Drawing. The cannula guide is the C313GS-4/SP-5MM from Protech International.

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

[02-JAN-25] For a chronological account of our development of our electrode catalog, see our separate Development and Production page.