Friday, September 22, 2017

Artifact and Fatigue

The photograph below shows the A3030E lamp leads with ground spring, strain relief, and collars. Each lamp lead we wrap around one coil of the spring. When cemented in a head fixture, the spring will be rigid and the wrap of the lamp leads will provide strain relief for the two 3-mm black head shrink collars. The collars are glued to the lamp leads.

Figure: A3030E Batch E157.3-14 Lamp Leads. We have two leads wrapped around spring coils for strain relief.

We test the above arrangement in a mock dental cement fixture, shown below. From previous mock fixtures, we learn the dental cement must be allowed to cure for fifteen minutes while stationary so as to ensure that the cement will harden while tight around the collars, leads, and spring. In order for the grounding spring to be effective at stabilizing the potential of the animal body, here represented by the water in the perti dish, at least one coil of the spring must protrude from the head fixture to make contact with the body fluids.

Figure: Mock Head Fixture for A3030E Lamp Leads and Grounding Spring.

We cover the head fixture with water and place the lamp leads in the water with bare ends. We pull on the lamp leads with a 1-N force two hundred times. We flash the lamp 50 ms pulses 10 Hz full power. Total EEG noise is 8.7 μV rms. In the EEG spectrum we see a 7-μV harmonic at 10 Hz. We pull with a 2-N force on the leads another one hundred times. The total noise increases to 31 μV rms. We have broken at least one of the collar seals, and we see a 100-μVpp triangle wave on the EEG signal. Another hundred 2-N tugs on the lamp leads and the triangle wave is 140 μVpp. We vary pulse length and obtain the plot of noise amplitude versus pulse length.

Figure: Lamp Artifact Amplitude versus Pulse Length for 10-Hz Stimulus and Head Fixture with Breached Collars. Water: EEG leads and head fixture in water. Saline: EEG leads and head fixture in 1.2% saline. Ground: EEG leads out of water, ground spring absorbing lamp current.

We observe four stages of lamp artifact. In the Stage 1, lamp artifact is <10 μV rms. The collar seals and lamp leads are intact. In Stage 2, lamp artifact is <50 μV rms. The collar seals have been breached, but they are still tight. In Stage 3, the artifact can be as large as 200 μV rms. The collar seals are loose, with a thick layer of fluid to conduct lamp current into the ground lead. In Stage 4, the artifact can be as large as 2000 μV, reception can drop as low as 50%, and the lamp flashing is intermittent. One of the lamp leads is broken or its insulation has ripped. The current entering the ground lead is great enough to disrupt data transmission and lamp flashing.

For Stages 1 to 3, lamp artifact for 10 ms flashes at 10 Hz is <50 μV. In Stage 4, the lamp may not flashing and data transmission is failing. We reach Stage 4 only if we subject the lamp leads to so much fatigue that they break. Given that our helical leads have a long record of surviving implants of many months, we are hopeful that we will not reach Stage 4 during an ISL implantation.

Friday, September 1, 2017

Performance of A3030E Circuit

We have two prototypes of our A3030E Stage 8 ISL circuit. The photograph below shows circuit E8.1 with all leads loaded, including grounding spring and a temporary white LED for our pre-production tests.

Figure: Un-Encapsulated A3030E Circuit with Programming Extension.

We test all the new features of the A3030E. After a couple of minor modifications, we find these features all work. In particular, the grounding spring and differential amplifier together reduce our simulated lamp artifact to 20 μV, compared to at least 30 mV for the single-ended input of the A3030D. We claim the A3030E with its additional grounding lead will not require a collar seal around the lamp leads where they pass into the head fixture cement.

Antenna Switch: The A3030E shares a single antenna for data and commands. The crystal radio uses the antenna except when the logic chip is powered up and requests the antenna for transmission. This sharing appears to work perfectly, with no loss of either data transmission or reception compared to the two-antenna devices.

Crystal Radio: The new crystal radio layout is more compact. It provides more gain than the earlier 900-MHz crystal radios and extends the reliable command reception range from 50 cm to 80 cm.

Battery Monitor: The A3030E measures its own battery voltage using its spare ADC input channel. We are able to monitor the immediate drop in battery voltage due to turning on the lamp, and the slow decline in battery voltage due to the lamp remaining on. The battery monitor will allow the Neuroarchiver to issue a warning before the battery runs down so far that it suffers permanent damage.

Battery Recharging: We connect the L− to +8 V and the L+ lead to 0 V and charge the battery at a rate of 40 mA through the 75-Ω resistance of the lamp leads. The lead resistance and the charging diode voltage drops complicate the charging process, but with a specially-designed charger, we could re-charge them in the field in less than ten hours. As it stands, we can re-charge them in twenty-four hours with our own power supplies.

Differential Amplifier: The A3030E EEG input is a differential amplifier. In water-filled Petri dishes, a 2-V lamp switching voltage applied to both EEG inputs produced no more than 100 μV lamp artifact. The differential amplifier is, however, vulnerable to 1 MHz switching noise from lamp modulation, which we use to reduce the average power of the lamp. In air, this modulation artifact can be as large as 200 μV, but within a conducting animal body we expect it to be negligible. At 100% brightness, however, this modulation artifact does not exist.

Grounding Spring: The 3030E grounding pads allow us to attach a fifth lead to act as a ground in the tunnel made by the lamp leads as they emerge from the cement of the head fixture. We simulate an animal body with head fixture in Petri dishes. We use a tunnel resistance 100 kΩ, this being the minimum we have observed in our collar seal tests. Without the grounding spring, the separated EEG electrodes pick up 500 μV of lamp artifact. The grounding spring reduces artifact to 20 μV.

We initiated production of more circuits today, and expect to have them next week. We have a few bugs in the logic program to figure out, and the new firmware should provide one battery voltage measurement per second as part of normal operation, using the meta-data channel number fifteen.

Friday, July 28, 2017

Grounding the Lamp Current

In Sources of Lamp Artifact we present our understanding of how lamp artifact of order 100-mVpp appears on the ISL's EEG signal when we flash the lamp. As we have reported before, this artifact is due to current flowing from the tips of the lamp leads, along the water-coated silicone insulation of the lamp leads, through the tunnels the leads make in the dental cement, and into the animal body. From there it flowed to the A3030C/D reference terminal for EEG monitoring.

Figure: Lamp Artifact Current Flowing From L+ to X− When Lamp Is On. Voltages with respect to the implanted A3030D's 0V supply. Head fixture made of dental cement. Current in the leads is not shown.

We later tried the A3030D-LO, lamp-only version, with a Subcutaneous Transmitter A3028E to monitor EEG. We saw no lamp artifact in the EEG we recorded, as the following diagram attempts to explain.

Figure: Separate SCT for EEG, ISL without EEG for Lamp. Head fixture made of dental cement.

One way to restore the ISL's own EEG measurement is to insulate the lamp leads with a ceramic collar, as we describe here. The ceramic collar makes is hard to explant and re-use an ISL because the solvent used to dissolve dental cement will also attack the adhesive we use to bind the silicone to the ceramic. The ceramic collar is arduous to assemble, and makes implantation of the head fixture more difficult. Furthermore, although we have tested the ceramic collar for fatigue resistance with machines and water here in our laboratory, we remain uncertain as to its reliability during a two-month experiment.

We propose to eliminate the ceramic collar. Instead of blocking the lamp current, we will absorb it with a coil of wire that enters the head fixture and wraps around the lamp leads. We will use a stainless steel compression spring for the purpose. For EEG detection we will use a differential amplifier in the A3030E rather than a single-ended amplifier. The current flow in the head fixture and animal we expect to be as shown below.

Figure: Grounded Seal Arrangement. Head fixture made of dental cement.

Our initial tests of this arrangement are promising. We await the arrival of the A3030E circuits, when we will be able to test the arrangement with a differential amplifier built into the ISL.

UPDATE: Here is a prototype grounding arrangement. A stainless steel spring is coiled around two lamp leads.

Figure: Grounding Spring Coiled Around Two Leads.

We can slide the spring up and down the lamp leads, but the lamp leads are pressing outwards against the coils. If the spring protrudes from the head fixture a few millimeters, it will provide a grounding point for our EEG measurement. Because it is wrapped around the lamp leads in the tunnel they make through the head fixture cement, the spring will absorb all lamp current that might flow through the tunnel.

Triple Helix Leads

Instead of a sealed ceramic collar to isolate EEG from lamp power, we propose for the A3030E to add a grounding spring around the lamp leads to absorb the lamp current that leaks out of the head fixture through the tunnels in the dental cement made by the lamp leads. Running to the head fixture we would have three leads: L+, L−, and GND. The GND is the ground connection. On the A3030E circuit, GND is connected directly to the 1.2-V power supply. We consider joining these three leads together in a triple-helix lead, as shown below.

Figure: Triple-Helix Leads. Top: 1 coat on individual leads followed by 2 coats on three together. Bottom: 2 coats on individuals, 1 coat on combination.

As we report here, these leads provide perfect electrical performance. But we find it impractical to prepare the ends for application of pins and a grounding spring. We must cut the wires apart and remove silicone from each tip without damaging the lead elsewhere. For now, we plan to stick with the well-tested individual wires, and hope that they do not take up too much space.

Lead Resistance Power Loss

So far, the ISL lamp leads for all versions of the A3030 device have been 100 mm, stretched MDC-13867A stainless steel springs with resistance ≈30 Ω. The buck converter on the A3030 supplies 5.0 V to one end of the lamp leads, the LED forward voltage is ≈3.0 V, leaving 2.0 V across the leads. Of the power delivered by the buck converter, 40% is lost in the lead resistance. In the past, we have tried to increase the maximum optical power emitted by the tip of the ISL fiber. We found that a 15 mW fiber-tip power could produce circling behavior with 2-ms pulses at 10 Hz. If we were to use 100-mm un-stretched leads, their resistance would be ≈60 Ω and fiber tip power would be 7.5 mW. We could then flash the light for 4 ms instead of 2 ms and emit the same number of photons, thus producing the same ontogenetic effect. The energy we lose in the lead resistance will be the same. In the A3030E, we propose to either increase the lead length to reduce strain on the head fixture, or to keep the lead length the same and use an un-stretched lead to achieve the same reduction of strain.

Monday, October 24, 2016

Lamp-Only ISL

In our experiments with implanted ISLs, the A3030D has been plagued by lamp artifact caused by water carrying the lamp voltage out of the head fixture and into the animal body, where it corrupts the EEG signal. Ceramic dual-bore collars around the lamp leads stop the water conduction, but the seal within the collar can still fail, and when it does, the EEG signal is compromised. The A3030D-LO is designed to remove the concern of seal failure from an optogenetics experiment by removing the EEG pick-up leads alltogether. We combine the A3030D-LO with a separate subcutaneous transmitter to monitor EEG. The EEG and lamp power circuits are now separated and their battery power supplies are floating with respect to the animal body. Failure of the collar seal will introduce some artifact into the EEG signal, but two orders of magnitude less than if the EEG X− lead is connected to the lamp power 0 V at the same time the seal breaks.

We take two A3030Ds, D7.11 and D7.6, and dismantle them both. We remove their batteries, antennas, lamp leads, and sensor leads. We load new sensor leads with collars and new antennas. We do not load EEG leads. The result is the A3030D-LO, the Lamp-Only version of the ISL. The EEG amplifier is still operational. The device will transmit data and acknowledgements.

Figure: Implantable Sensor with Lamp, Lamp-Only Version (A3030D-LO).

We take care to make sure the pins are soldered in the correct direction on the leads. Looking at this picture we see that the positive (orange) lead must be in the foreground when the pins are pointing to the left. Otherwise the implanter has to rotate the pins. The length of lead from the collar to the pin is 5 mm so the implanter will not need to fold the leads within the head fixture, but instead can build the head fixture directly over the pins and collar. The devices are encapsulated in black epoxy using our rotator procedure, and then coated in SS-5001 silicone, with extra silicone applied to the sharp corners. The result is not pretty, but it is resilient and flexible.

Figure: A3030D-LO Lamp Artifact, 10 ms pulses.

The figure above shows the artifact we see on the disconnected EEG signal when we flash a white LED with a 40-mA lamp current at 100% brightness. We will ship these two devices to ION/UCL this week.

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.