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.