Wednesday, June 22, 2016

Differential Input and Dual-Bore Collar

When we add collars to our ISL lamp leads, the collar seals last no more than a week before they break and a >30 mV lamp artifact appears. In our most recent implantation, we buried the lamp lead tips in dental cement with their collars, but without a lamp. Immediately after surgery, we observed full-scale lamp artifact, which suggests one or both seals broke as soon as the animal started to move.

We have difficulty adding collars to existing lamp leads because the leads are too thick to pass into the collar bore. We must stretch the leads by their steel wires, which compromises the elasticity of the insulation and causes the end of the lead to curl up. We now start with new lamp leads, with tapering insulation at one end, and thread these into dual-bore collars. We cut off the existing lamp leads and solder the new leads in place.

The ISL's EEG amplifier has a single-ended input. It measures the potential difference between the X+ and X− electrodes, but the X− electrode is a direct connection to the 1.2-V power supply in the ISL circuit. When the lamp lead collar seal breaks, fluid around the leads connects the surface of the skull to 5 V. The point connected to X− remains at 1.2 V. Current flows from 5 V to 1.2 V. The point connected to X+ is somewhere in the potential field generated by this current, and so we have the lamp artifact. If our ISL amplfier were truly differential, and presented a 10-MΩ impedance at both X+ and X−, the current flow to the X inputs would be greatly reduced. We would, however, still need a reference connection somewhere in the body to anchor our differential amplifier's input range, and this reference connection would re-create the problem we started with. But if we use a floating power supply for our differential amplifier, such as is provided by an indepenent SCT device, we get the near-perfect differential input without any need for a reference connection. In this way, we may be able to reduce the artifact arising from broken seals dramatically.

Our plan is to test the dual-bore collar and the floating differential EEG input in a single experiment. We have A3030Ds D7.9 and D7.11 modified as shown below, and ready to ship with (A3028E) numbers E116.7 and E116.12. We attached new lamp leads and insulated the joints with silicone. The lamp leads pass through a dual-bore collar, into which they are glued with Ultragel. We have a blue LED soldered to the ends of the lamp leads. We cut off the EEG leads and covered with silicone. The A3028Es have a pin and a bare wire for electrodes, but the choise of electrode is unimportant.


Figure: ISL and SCT Lamp Artifact Devices.

The circular A3028E has 10-MΩ input impedance and will measure lamp artifact with a power supply independent of the ISL. We have not joined the EEG leads along their length, thus reducing their sensitivity to magnetic fields, because at this point we know the >1 mV lamp artifact is generated by electrical conduction through water around the lamp leads, not by the lamp lead's magnetic field. The ISL itself measures only the internal electronic lamp artifact of the ISL circuit. Its EEG leads are cut off and sealed.

We put our two ISLs in water and start them producing 10-ms pulses at 10 Hz. We record their EEG input with the LEDs out of the water and with the LED in the water.


Figure: Two ISLs Flashing, Insulated EEG Leads. Left: lamp out of water. Right: lamp in water.

We put the two SCTs in the water and measure artifact with the LEDs out of the water and in the water.


Figure: One ISLs Flashing, Two SCTs in Water. Left: lamp out of water. Right: lamp in water.

With the lamp in the water, we have the No7 (salmon) EEG pick-up electrodes 10 mm from the exposed terminals of the LED, an the No6 (brown) electrodes are 40 mm away. In the 40-mm case, if we were to use the ISL's own amplifier, we would see artifact of >27 mV. With our floating EEG input we see roughly 3 mV. We ship these components to ION. If the collar seals are effective, we should see less than 100 μV artifact on the SCT amplifiers. If the collar seals break, we should see something between 100 μV and 3 mV artifact, depending upon the resistance of the fluid that penetrates the collar.

UPDATE: [09-AUG-16] We implant ISL D7.9 and SCT E116.7 at ION/UCL. The ISL's EEG leads are cut short and sealed. The SCT EEG electrodes are on the skull. The ISL's LED is embedded in a dental cement head fixture, with the dual-bore collar within the cement. Six days after implantation we see the following artifact during lamp stimulation.


Figure: Lamp Stimulation Artifact with Brightness, Simultaneous in SCT and ISL. Six days after surgery. Pulse length 20 ms, interval 50 ms, 200 pulses. Left: from SCT E116.7, right: from ISL D7.9.

The artifact in the ISL reaches a maximum at 40% brightness, which is a characteristic fo the electronic artifact generated within the ISL circuit. Its amplitude is of order 100 μV. There is no sign of artifact in the SCT recording. We look at the spectrum of the SCT signal over eight seconds during 100% brightness and see no sign of a peak at 20 Hz (50 ms period), which means the lamp artifact in the SCT recording is <10 μV.

We implant D7.11 and E116.12 in another rat. This time, the EEG leads of the ISL are cut short but not sealed, so that the wire tips are several millimeters apart in the abdomen and exposed to body fluid. Two hours after implantation we observe the following lamp artifact.


Figure: Lamp Stimulation Artifact, Simultaneous in SCT and ISL. Two hours after surgery. Ten pulses of 10 ms, interval 100 ms. Red: from SCT E116.12, Gray: from ISL D7.11.

The artifact in the SCT signal remains <10 μV. But the artifact in the ISL signal is of order 20 mV. If the collar seals were to break, current would flow from the head fixture to the ISL's X− input, which is connected to the ISL's 1.2-V power supply. A potential field will develop through the animal's body. The SCT electrodes on the skull, being of order 10 mm apart, will pick up this pulsing potential field, as will the ISL's X+ electrode, which is a few millimeters from X−. But we see no artifact in the SCT EEG, so the artifact cannot be caused by a break in the collar seal. Instead, we suspect that the seal around our lamp lead solder joints has failed after minimal fatigue or handling, and the result is a pulsing current between an exposed lamp power lead and the ISL's X−. The ISL's X+, being nearby, picks up this field, but the SCT's EEG electrodes, being far away on the skull, do not pick it up.

Tuesday, March 15, 2016

Lead Flexing Machine

We have four single-bore ceramic collars cemented with silicone to four leads. The two small collars are 1.0 mm ID (inner diameter), 2.3 mm OD (outer diameter), and 3.2 mm long. They will slide over leads with only three coats of silicone. The two large collars are 1.7 mm ID, 4.3 mm OD, and 4.8 mm long. They slide over any lead. Both beads have a concave side and a convex side. We slide the lead into the concave side. We coat the lead with silicone and slide in and out until we see silicone on the lead on both sides of the bead. The concave side holds a body of silicone around the lead. We hand to cure. The results are shown below.


Figure: Single-Bore Ceramic Collars. The gray marks on the collars show where we have scraped silicone off the outer surfaces.

Once cured, we note that silicone has crept half-way up the outside of the collars. We scrape the silicone off with a scalpel. Cut cut the short ends to the same length, remove 2 mm of silicone to expose the wires, and cement into a tub of dental cement all together. We place in water and set up a motor-driven wire-flexer that pulls on the leads while they sit in a beaker of water. We clamp the leads 50 mm from the collars and we streth them by 10 mm once per second.



Figure: Wire Flexing. We are measuring isolation of the orange lead while flexing all four leads by 20% once per second.

After thirty seconds, we measure isolation resistance of each lead with respect to the water in which the cement fixture is immersed. For the blue, red, and purple leads, the isolation is only 1 MΩ, indicating the seal between the silicone and ceramic has broken. We are seeing the resistance of the microscopic film of water between the lead and the collar. The orange lead's isolation is >100 GΩ. We put the cement fixture in our ove at 60°C. After half an hour, we place in water again and isolation resistance is >100 GΩ for all four leads. We begin flexing. Within thirty seconds, isolation resistance is 1 MΩ for blue, red, and purple. The isolation of the orange lead remains >100 GΩ after 3600 s of flexing.

The fit between the lead and the collar in the previous four examples was tight only in the case of one of the smaller collars, although we did not make note of whether it was the orange or the purple. The only way to make the fit tight is to start with a lead of tapering thickness. We have eight such leads. We make four tight-fitting collars with silicone as adhesive and leave to cure in warm humidity. Another possible source of failure in the collars we glue in place with silicone is failure of the seal outside the collar because of silicone residue on the ceramic surface. We make another four and glue with Loctite Ultragel. We allow the glue to cure in warm humidity for one hour. We assemble into a dental cement head fixture and load into the wire flexer. We connect all four free ends of the leads together to our isolation meter. All four isolation resistances in parallel are 3 GΩ. After 600 s, combined isolation is 4 GΩ. After 5000 s, it is >100 GΩ. We leave the flexer running.

UPDATE: [16-MAR-16] After 18 hrs of flexing, leads 1-3 of our set of 4 have broken by fatigue where they pass over the lip of the beaker in our wire-flexing apparatus. Lead 4 is intact, with isolation resistance >100 GΩ. We remove silicone from the free ends of the other leads, and isolation resistance is >100 GΩ for all three. In one case, we have 65,000 flexes and the collar seal is still intact. In the other three cases, we have failure of the spring by fatigue before the collar seal fails by fatigue.



Figure: Four Surviving Collars. Leads 1-3 broke by fatigue some time between 5k and 65k flexes. Lead 4 survived 65k flexes.

UPDATE: [17-MAR-16] We left the four collars in water overnight. Their combined isolation resistance is >100 GΩ. We also have four collars cemented to leads with silicone dispersion. All four have a layer of silicone roughly 50 μm thick on the outer surface at the ends of the collar. We pull on the collars and find that the cement in three of them breaks easily, and in one of them a good tug gets it free. Given the success of the Loctite Ultragel cement, we abandon the silicone cement.

Monday, February 29, 2016

Ceramic Collar

We have a double-bore ceramic tube. We break off 15 mm of it and run two fresh springs through the two bores. We coat the springs with silicone and move them in and out until the silicone penetrates all the way through the bore. We leave to cure. If silicone bonds well to ceramic, and ceramic bonds well to dental cement, we may have an effective insulating collar.

Our two springs are cemented securely in the ceramic tube by silicone. We tug with sufficient force to stretch the springs, but there is no yield in the bond between the ceramic and the steel. We put the ceramic tube in dental cement.


Figure: Two-Bore Ceramic Collar in Dental Cement.

We pull on the springs with pliers. They stretch all the way until nearly straight. The silicone bond within the ceramic tube remains intact. We pull harder and the steel tears out of the silicone. We grab the ceramic with pliers and try to break it out of the dental cement. We crack the ceramic, but the bond between the two materials does not break.

Friday, February 19, 2016

Collars Implanted

At ION on 16-FEB-16, we apply single 3-mm collars to all four leads of three A3030Ds, using the procedure described earlier. We implant D7.8 in a rat, making sure the collars are separated and no contact occurs between the leads at any point from the collar to the electrode. We cover the leads and collars with free-flowing dental cement. We apply a second coat of dental cement to build up the head fixture. We touch up around the guide cannula with a third coat. An hour after surgery we observe lamp artifact. At 0% we have a brief command-reception artifact at the beginning of the stimulation. The data transmission stops while the stimulation command is received. There is no other artifact for 0% power. As we increase the lamp power to 100%, we see 10-ms spikes of up to 200 μV.



Figure: Lamp Artifact in D7.8 One Hour after Implantation. From archive M1455643796. Pulses 10-ms at 10 Hz.

The direction of the spikes changes as we increase the lamp power. The spikes have magnetic and capacitive sources, and these sources vary in different directions as lamp power increases. The spectrum of the 100% brightness spikes is shown below.


Figure: Spectrum of Lamp Artifact. From archive M1455643796. Pulses 10-ms at 10 Hz, 100% power.

The first harmonic of the lamp artifact is a little over 10 Hz. The second harmonic is larger, which is consistent with the 10% duty cycle of the lamp stimulation. The ninth harmonic is at 93Hz, which suggests the pulse rate is 10.3 Hz rather than 10 Hz. In theory, the ISL clock should be accurate to 20 ppm, with a period resolution of 30 μs. We suspect a bug in the firmware is responsible for the 0.3-Hz error in the pulse period.

[23-FEB-16] After one week implanted, we see full-scale lamp power artifact on D7.8's EEG input. We assume that repetitive stress upon the bond between the collar and the silicone leads has caused them to fail by fatigue. We note that we did not allow sufficient time for full curing of the superglue gel before implantation, but it could be that even fully-cured superglue gel will fail in the same way. We will look into stronger and more resilient bonds between silicone and the plastic tube collar.

[25-FEB-16] Bought some silly putty, which sticks very well to silicone. But dental cement won't stick to silly putty, nor will our heat shrink tubing stick to silly putty.

[26-FEB-16] We filled the gap between a two shrink tube collars and two silicone leads with fresh silicone. The tube has not yet been shrunk. We place these under a petri dish with some water, and heat with a lamp. The silicone cures over-night in the humid air. When we tug on one of the collars, it comes off immediately, leaving cured silicone behind on the lead. We shrink the other collar. When we tug on it, the shrunk collar comes off easily as well.

We have ordered some ceramic tube to try out as a collar.

Friday, December 18, 2015

Insulating Collar

Our objective is to stop water propagating along the outside of our silicone lamp leads and reaching the metal of the electrodes. We propose to put a collar around each lamp leads that is non-conducting, but which the dental cement bonds to securely. With contact between the dement and the collar, and the collar and the silicone, we would divide the water film into two parts, one in contact with the lamp power pins, the other in contact with the rest of the body.



Figure: Cross-Section of Insulating Collar. The water film around the silicone leads is interrupted by the insulating collar. The dental cement binds directly to the collar.

We try heat shrink tubing as a collar. The smallest diameter we have is 2.5 mm on the outside before we shrink it, and roughly 0.7 mm on the inside after we shrink it. Our lamp leads are 0.9-1.2 mm in diameter. When we shrink the tubing onto one of these leads, we can slide the collar up and down the lead afterwards by stretching the lead so as to reduce its diameter. We need a narrower diameter tubing or we need a way to glue the tubing to the silicone. Setting that problem aside for now, we put some tubing in dental cement to see if we get good wetting of the plastic surface by the cement.


Figure: Adherence of Dental Cement (Acrylic) to Heat Shrink Tubing.

When we try to pull the tubing out of the cement, it breaks at the top surface of the glue, leaving the rest inside. We put collars on two silicone leads. The leads have bare spring terminations. We glue the collars into dental cement, as shown below. Because of the collars, the two leads become separated by a layer of dental cement, which surrounds the collars by capillary action of the acrylic. Resistance between the two leads is >40 MΩ.


Figure: Collars Glued Into Dental Cement.

When the cement has hardened, we put the cup and leads in water at 60°C for half an hour, to allow water to penetrate wherever it can by capillary action. Resistance is >40 MΩ. We put the beaker on our bench and measure the resistance as time goes by. Fifty minutes after first contact with water, the wires are still insulated from one another. We add salt. Resistance between two pins 1 cm apart is now less than 1 kΩ. With the cement still in salt water, we hold it in place and pull on the two leads one hundred times. Insulation is intact. After another ten minutes, insulation is still intact.

We pull the silicone off our epoxy-sealed head fixture. We cut back the lamp leads on D7.7 and attach another set of pins. We add a collar to each lead, connect the pins to the head fixture, and encapsulate in dental cement. Once the cement is hardened, we break the cement out of its red plastic cup. We immerse in hot water, see here. We pull on the lamp leads 100 times. Lamp artifact with 100% brightness is a few hundred microvolts, and consistent with no conduction between the lamp leads and the EEG leads. We put the ISL with head fixture, and the two-wire apparatus, in saltwater for the weekend.

UPDATE: [22-DEC-15] After four days in saltwater, we see 100-mV artifact when flashing the head fixture. The resistance between each wire and the surrounding water is around 5 MΩ. The resistance between the blue wire and the water is >40 MΩ, but between the red wire and the water is only 10 MΩ. We use Loctite Super-Glue Gel to glue single 3-mm long collars on two blue wires, and triple 1-mm long collars on two red wires.


Figure: Triple Collars. Each collar is glued to the silicone with super-glue gel. Modelled after the triple o-ring seals we see in pacemaker connections.

Once the gel has cured, we can no longer slide the collars along the silicone. We embed the collars and wire tips in dental cement. When cured, we immerse in saltwater and arrange the free ends of the wires so we can measure easily the resistance between each wire and the water. We stretch all four leads by 15% one hundred times each while they are wet with saltwater. This stretching is within the elastic limit of the springs. We leave them for one hour. We combine a 10-V power supply with our 10-MΩ millivoltmeter to make an instrument capable of measuring resistance up to 100 GΩ. The water connection resistance is 20 GΩ for the two triple-collar red wires and >100 GΩ for the single-collar blue wires.

UPDATE: [24-DEC-15] Water connection resistances today are 100 GΩ, >100 GΩ, >100 GΩ, and >100 GΩ for red left, red right, blue left, and blue right. We connect the two red wires to D7.7's lamp power, with a blue LED in parallel. The blue LED flashes. We lower the ISL into a beaker of water with the cement fixture. We see 500 μV artifact. We do the same with the two blue wires, but connect no LED. We obtain 500 μV artifact. We pull 100 times on each of the two blue leads, stretching them by 30% each time while they are wet with saltwater. This stretching is at the elastic limit of the springs. We put the cement fixtures back in the saltwater with the ISL and observe the full-scale artifact below.


Figure: Artifact After Breaking Single-Collar Seals with 5-N Tugging. This display is centered with 65536 range, when it should be simply displayed with the same range. In simple display we see the entire waveform, in this display we are missing the tops of the artifact.

We have broken the seal. We do the same with the pair of triple-collar red wires, applying a force sufficient to stretch them by 30%, tugging 100 times. When we are done, the wires curl up because we have deformed the springs. We put the fixture back in the water and observe the same full-scale artifact. We smash open the cement fixtures. The collars are held firmly in the cement. But the wires now slide freely through the collars. The super-glue seal is broken. The leads are narrower because they have been elongated. They are no longer in compression where they pass through the collar.

The triple-collar performed no better than the single-collar. Once the super-glue seal breaks, the seal cannot recover. We would rather make a seal by compressing the silicone so forcefully that the lead cannot slide through the collar, even if we pull on it with sufficient force to stretch it by 30%. We have ordered narrower tubing and will try it when it arrives.

In the meantime, we are curious about the resistance of the shrink tubing and super-glue seal to saltwater. We assemble two single-collar seals. We measure their water contact resistance to be >100 GΩ. We put them in saltwater and set them in our oven to poach at 60°C.

UPDATE: [30-DEC-15] The two leads we left in saltwater at 60°C still provide isolation >100 GΩ. On day four of the six-day poach, we took the leads out and pulled on them one hundred times, stretching them by 15% each time. Isolation remains >100 GΩ.

UPDATE: [05-JAN-15] We have heat shrink tubing SFTW-203 in 1/16" size. Outer diameter is 2.0 mm before heating, 1.5 mm after. Inner diameter 1.5 mm before heating, 0.50 mm after. A 10.8-mm length is 10.1 mm after heating. We take a 100-mm long, 1-mm diameter silicone-insulated helical steel lead. We cut a 3-mm length of tubing and slide it onto the lead. We stretch the lead by 20 mm so its diameter reduces to around 0.8 mm, but the spring does not deform. We shrink the tubing and allow it to cool for ten seconds. We can slide the collar along the wire easily if when the wire is stretched by 20%, but the collar grips the wire when we relax the spring. We hold the 3-mm tube in one hand and the far end of the lead in the other. We pull on the collar. When the lead has stretched by 17%, the collar starts to move. We slide the collar back into place and repeat several times, obtaining similar results. We try a 10-mm collar. The spring extends by 28% before the collar slips.

We glue a 3-mm collar to the silicone lead with 3M's Vetbond. The glue does not appear to bond the silicone to the collar. We try Loctite Ultragel, allowing ten minutes to cure. We pull on the collar until the lead has extended by 33%, by which point the lead has deformed. The collar is still fixed in place. We try MED10-6607 dispersion as an adhesive. We wait one hour for it to cure and find that the collar appears to be lubricated by the silicone, which has not yet cured.

We apply 3-mm collars to two leads. We glue them in place with Ultragel. We have 3 mm of silicone insulation between the collar and the 2-mm exposed helix of steel. We encapsulate with dental cement. Keeping the cement fixture in water, we measure the resistance between the leads as we flex the leads 100 times, extending them by roughly 15% with each flex. Resistance is still >100 GΩ between each lead and the water.

CONCLUSION: It looks like a 3-mm length of the 1/16" SFTW-203 tubing with Loctite Ultragel glue provides a reliable seal against water penetration. We recommend that all four ISL leads be equipped with these collars.

Friday, December 11, 2015

Sealing the Lamp Leads

Following our discovery of conduction through tunnels, we perform the following experiment to test if we can, during implantation, seal the ends of the lamp leads with silicone. We make a head fixture with no guide cannula or fiber. We seal the LED with clear epoxy. We hold the fixture with the LED downwards and connect the leads of our D7.7 ISL, like this. We apply MED10-6607 silicone dispersion until it almost drips off the underside, see here. We allow the silicone to cure. After ten minutes it looks like this. After twenty, as below.



Figure: Silicone Around Lamp Leads, Twenty Minutes After Application.

We apply dental cement, allowing it to run off the underside. We start with thin cement, then thicken and make sure we cover the top side. The result is shown below. We note an obvious tunnel into the cement between the two lamp leads.



Figure: First Coat of Dental Cement over Head Fixture.

We allow the cement to cure for twenty minutes. We note that the pads around the LED are still exposed on the bottom side. We place the ISL and head fixture in water and observe full-scale lamp artifact. We surround the entire fixture with cement.



Figure: Encapsulating Coat of Dental Cement.

We let the above cement cure for ten minutes, then we place the ISL and head fixture in water. We get the following lamp artifact after one minute.



Figure: Lamp Noise Just After Dropping in Water.

We put hot water in the beaker. We take out the ISL and flex its lamp leads twenty or thirty times. We put the ISL and head fixture back in the water and let it settle down. We see the artifact below.



Figure: Lamp Noise 20 Minutes After Dropping in Water.

We move the EEG leads out of the water and record noise. We get this, which is pretty much the same as with the leads in the water. We wrap the lamp leads around in a loop with the EEG leads and put everything in the hot water. No change to the noise. We wait one hour to give capillary action time to move water into the dental cement.

Figure: Lamp Noise One Hour After Dropping in Hot Water. Range 800 μV, Interval 1 s, Pulses 50 ms, Pulse Period 500 ms. From top-left to bottom right: Lamp stimulus off, stimulus with 0% brightness, 20%, 40%, 60%, 80%, 100%, 100% with EEG leads out of water.

We see the artifact reversing direction as we increase the lamp current. If we re-arrange the leads, the artifact changes.



Figure: Lamp Noise for 100% Brightness After Rearranging Leads. Range 800 μV, Interval 1 s, Pulses 50 ms, Pulse Period 500 ms.

We now encapsulate the EEG leads with dental cement. When we immerse in water, the artifact remains much the same. It appears that our silicone sealing of the lamp leads has increased the resistance between the lamp power terminals and the water in the beaker.

UPDATE [14-DEC-15]: After 72 hours soaking in water, we flash the lamp power artifact has increased by at least a factor of ten, perhaps a hundred.


Figure: Lamp Noise 72 Hours After Dropping in Water.

UPDATE [15-DEC-15]: After a one-hour dry bake at 60°C, lamp noise for 100% brightness in water has dropped back to normal amplitude. We prepare another head fixture and cover all electrical pads on its top and bottom surfaces with clear DB270 epoxy, see below.



Figure: Epoxy Sealing of Head Fixture Conducting Surfaces.

We coated both sides, but stayed clear of the two sockets. We used a thin wire to apply epoxy between the sockets and the LED footprint. We cut the existing head fixture and cemented pins off D7.7 and solder new pins to the lamp leads. We strip and tin the EEG leads.

UPDATE [18-DEC-15] We connect lamp leads to the head fixture shown above and flash the light. We cover the pins and sockets with silicone. We wait twenty minutes, with the silicone curing above a bath of warm water. We touch the silicone with tweezers and note that it is not yet tack-free. If we were to apply dental cement, we would disturb and compormise the silicone coating. After one hour, it is tack-free. We lower the entire assembly and ISL and EEG leads into water and observe lamp full-scale lamp noise. We put the lamp in our mouth and cannot taste the signal, which means it must be less than 500 mV. So we suspect we are seeing lamp noise on the EEG of order 100 mV. We feel sharp edges around the corners of the circuit board. The silicone is discolored, because it has not had sufficient time to cure. We conclude that applying one coat of silicone during surgery is not sufficient to seal the lamp power electrodes.

Tuesday, November 17, 2015

Conduction Through Tunnels

In Electrical Coupling Through Dental Cement we showed that the resistivity of dental cement, wet or dry, was too great to cause the lamp stimulation artifact we observed on X for the first few weeks after implanting D7.2. A few weeks after implanting D7.6, the lamp stimulation artifact is a full-scale square wave, as shown below.


Figure: Lamp Pulse Artifact Two Weeks After Implantation. D7.6, 0% brightness, 10-ms pulses, 100 ms period. See M1447342849.ndf.

At ION on 12-NOV-15, we implant D7.5 and note the insulated exterior of the L+ and X+ leads are touching as we are about to apply dental cement. We believe the previous two implants were done exactly the same way. This time, we move the leads apart. If they were to touch, there would be a passage through the dental cement for water to creep along and so connect X+ to L+. Half an hour after implantation, we generate 0% brightness, 10-ms long pulses, with 100 ms period and see the artifact below on the X input.




Figure: Lamp Pulse Artifact Immediately After Implantation. D7.5, 0% brightness, 10-ms pulses, 100 ms period. See M1447350049.ndf.

We see a step up on X as the lamp power turns on and a relaxation afterwards. Brightness is 0%, so L+ is connected to +5V. When we turn off lamp power, L+ relaxes as a 10-μF capacitor discharges. At no time is Q1 turned on, so at no time is the LED draining the capacitor. With 100% brightness, the artifact on X is a pulse, as shown below. With 100% brightness, L+ drops quickly at the end of a pulse because the LED is connected to L+ for 200 ns after the end of the pulse, and conduction through the LED drains the 10-μF capacitor.



Figure: Lamp Pulse Artifact Immediately After Implantation. D7.5, 100% brightness, 10-ms pulses, 100 ms period. See M1447350049.ndf.

The artifact on X is consistent with a resistive connection between L+ and X+ of around 250 kΩ (the L+ lead carries +5V pulses, the X input's two skull electrodes have resistance is of order 1 kΩ, and the pulses on X are around 20 mV). We take two silicone-insulated steel leads and solder them to the pins of a connector. We cover the connector with dental cement, but leave the solder joints exposed. We tie the two leads in a knot to make sure they are touching. We put saltwater on the joints. We cover the leads, joints, and connector pins with cement. The resistance between the two leads is >40 MΩ (the largest resistance we can measure with our voltmeter). We allow the cement to cure. The resistance is >40 MΩ. We cover with salt water. Resistance is >40 MΩ. We wait half an hour, resistance has dropped to 120 kΩ. We pour away the saltwater and dry the cement surface. Resistance is 100 kΩ. Fifteen minutes later, resistance is 150 kΩ. We bake in the oven at 60°C for half an hour and measure 130 kΩ. One hour later, the resistance is >40 MΩ.


Figure: Conduction Through Tunnels. Visible through the pink cement are two leads we wrapped together to create a passage from one lead tip to another. The pink cement is at this moment covered by clear saltwater.

We know from long experience that water creeps along passages by capillary action. In this case, we have two solder joints being connected along a 20-mm tunnel by saltwater that has crept along the tunnel walls. The resistance of this saltwater connection is 100 kΩ. When implanted, every lead emerges from the head fixture somewhere. Where it emerges, there will be body fluids. And every lead creates a tunnel along which these body fluids can creep to the exposed conductor at the end of the lead. We suggest that these water films are the source of our lamp stimulation artifact.