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.

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.