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.