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 tug on the lamp leads some more. The lamp turns off. We adjust the leads and it turns on again. The lamp turns off. We adjust the leads and it turns on again. The lamp lead itself is broken. We now find that data transmission is being disturbed by lamp flashes. The lamp lead silicone insulation is broken. We cannot obtain consistent measurements of lamp artifact for any given pulse length, but the artifact can be as large as 2 mV when the broken lamp lead makes direct contact with the ground spring.

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.