Electrode Development and Production
[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.
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.
[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.
[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.
[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.
[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.
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.
[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 A3028WZ1, a two-channel DC-40 Hz transmitter with two channels and 290 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.
[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".
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.
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.
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.
[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 present the voltage recorded by the WZ1 in millivolts from the bottom of its 290-mV dynamic range. When we apply a voltage of zero volts top the amplifiers, we expect a measurement of 180 mV. We end this experiment and try 316SS/316SS and Ag/316SS.
[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.
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.
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.
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.
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. For each input, we give the dynamic range and the nominal zero-potential.
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.
There is another smaller pulse at time 8.5 days on the same channel.
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.
[11-JUL-24] We are using an A3047A2C four-channel transmitter with all-DC inputs to measure the response of various electrodes to a 20-mVpp, 0.0025-Hz square wave applied to saline. With a delivery electrode we bring this signal into a 100-mm diameter petri dish of 1% saline. On the opposite side we have three test electrodes, all with much smaller area than the delivery electrode, and of the same metal. A fourth electrode connects in air directly to the 20 mVpp square wave as a control.
We begin with steel, which shows a 20-mV step on each edge, decaying to a 7-mV step with a 100-s time constant. In the very short term we see the full step, but in the long term we see only 33% of the step amplitude.
Silver provides near-100% amplitude response, sustained. Platinum-iridium wire produces a similar response to 316SS, with a faster time constant. We have an initial 20-mV step, then a decay to 7 mV with a time constant of around 20 s.
[12-JUL-24] We perform the same experiment with Copper sheet. All three copper electrodes show a sustained 100% response to the square wave, combined with offsets of five or ten millivolts. We offset the 20 mVpp square wave by 5 mV to make sure it stays in the 60-mV dynamic range of two of our inputs.
[17-JUL-24] We continue our electrode tests with new materials. We begin with tungston, which is a material we cannot solder to, has a high resistivity, and good corrosion resistance. We see full amplitude and no sign of a decay time constant. But the electrodes introduce offsets of ±10 mV.
Brass is an alloy of copper and zinc. It corrodes slowly in saline. We try brass sheet as electrodes and see full amplitude, no decary, with some drift.
Our M-Wire is a 125-μm silver electrode with silicone insulation. We try cutting the tip of an M-Wire to leave only the cross-section of the silver wire as our electrode. We try 1-mm and 2-mm lengths of bare wire as well. These are the silver electrodes we might recommend for recording CSDs. The 1-mm electrode introduces a +10 mV offset. They provide the same response as coiled silver wire, with the exception of the bare cut end, which shows a decay time constant of ten or twenty seconds, followed by detecting only 25% of our applied amplitude.
We coat coiled silver wire with silver chloride. The silver chloride electrodes show full amplitude, no attenuition, no drift, and shared offset of −4 mV. One of them, however, is noisy, which suggests that imperfections in the coating can generate noise.
[22-JUL-24] We take three M-Wires, remove 1 mm of silicone from the end of the insulation, and coat with silver chloride. We compare the signals they measure from saline to our control signal.
[25-JUL-24] Repeat the AgCl M-Wire experiment to see if we get the same result. This time we see one offset of 5 mV, but the same full-amplitude, sustained response to the square wave.
[07-OCT-24] We have 30 new R-Electrodes made with white plastic cannula. The guide tube is 26 Gauge. We are able to screw a dummy cap on all of them, but some allow only one turn of the cap because the wire is only 2 mm from the top of the cannula. We add a dimension to the R-Electrode drawing: the wire should be 3±0.5 mm from the top of the cannula thread.
[02-JAN-25] We are working on an improved J-Electrode with Baoluen Chang. According to Baoluen, the original J-Electrode suffered from weak solder joints. We inspect four old J-Electrodes we have on the shelf and one of the four joints is indeed faulty: only a dry joint. Our second iteration of the J-Electrode provided bare wire above the socket and according to Baoluen, the joints were strong. We put this down to improved quality control, practice making these joints, but also the fact that when we can strip the entire top of the wire, we can make a better joint. But the second iteration, shown here and drawn here does not provide enough space under the pedestal for the surgeon to see the socket. We propose an improved design that will look like Electrode_J.jpg and be built to the new Electrode_J.gif drawing.
[13-JAN-25] We hear from Dr. Robert Wykes that he found the Q-Ferrule with X-Electrode crimping to be straightforward, but he cut back the bare wire on the X-Electrode from 2 mm to 1 mm so that the entire bare wire would fit in the crimp coil. We update our X and Y-Electrode drawings to make the bare wire 1 mm.
