Corrosion of Lamp Circuit

Upon a visit to ION in July, we observed that lamp circuit No2.6 (with blue light) did not flash at all, and No2.1 (with green light) responds only with the antenna 10 cm from the animal. This was nine weeks after implantation. The head fixtures were still attached, the circuits were implanted, and the animals were healthy and active.

Now we have circuits No2.1 and No2.6 after removal from the animals, cleaning, and shipping to our office, and backing for 24 hours at 60°C. Circuit No2.6 does not respond to stimulus at all, but we poke through the encapsulation and measure 3.4 V on its battery, which is more than sufficient to power the circuit. Meanwhile, No2.1 responds well, with full operating range.

We encapsulated our A3024B-M and A3024B-R with clear, five-minute epoxy and then four coats of silicone. The silicone is about 1 mm thick all around. The epoxy coats the circuit and battery, making a better surface for the silicone to adhere to. We did not apply an acrylic coating to the circuit board, nor did we place the epoxy in a vacuum to remove cavities. There would have been little point in trying to remove cavities when the setting time of the epoxy was only a few minutes. After nine weeks implanted in large, healthy rats, the external silicone encapsulation of both devices is intact, and none of the leads show any sign of fatigue. But both circuits, within the unbroken encapsulation, are corroded. The photograph below shows corrosion on pins and pads carrying VBAT in No2.6.


Figure: Corrosion of Pins and Pads Carrying VBAT After Nine Weeks Implanted, A3024B-R No2.6. Click for higher resolution.

The battery voltage in the above circuit remains 3.4 V, but we cannot get lamp power out of the leads. The part on the left is U2, the boost regulator, and the part on the right is U1, the comparator. We see no trace of metal left in U2-6 or U1-5, the power supply pins for the two components. Meanwhile, circuit No2.1 now responds perfectly, with good operating range. But it, too, shows corrosion in the same location.



Figure: Corrosion of Pins and Pads Carrying VBAT After Nine Weeks Implanted, A3024B-R No2.1. Click for higher resolution.

In No2.1, we do see metal remaining in U2-6 and U1-5. The corrosion is a certain sign of condensation within the encapsulation, which we are familiar with from our subcutaneous transmitters. Condensation across R1, a 10-MΩ resistor, would reduce the resistance of R1, raising the device's power threshold, thus shortening its operating range.

UPDATE: [21-NOV-14] We encapsulated our Stage Four Implantable Lamps, of which No2.1 and No2.6 are two examples, in DP100 fast-curing epoxy. Its data sheet does not say that this epoxy corrodes copper, but other epoxies, such as DP270 state that they do not corrode copper, and so are recommended for encapsulation of electronic components.

ISL Prototype in Development

We are assembling and testing our prototype Implantable Sensor with Lamp (A3030). For details of development, see the manual. For circuit diagram, see S3030_1. The device combines an Implantable Lamp (A3024) with a single-channel Subcutaneous Transmitter (A3028B).


The A3030's logic chip contains enough programmable logic to implement a microprocessor, enough working memory to record several seconds of EEG, and enough non-volatile memory to retain its operating instructions. Our first implementation of the A3030 will provide only basic logic functions, such as decoding commands and generating trains of optical pulses. But in the long run, we plan to build a microprocessor in the logic chip. The A3030 provides a TQFP-100 footprint for the logic chip, which is 14 mm square. Future ISL devices will use the WLCSP-25, which is 2.5 mm square. Being a prototype circuit, we wanted to be able to make the A3030 by hand. The WLCSP-25 package is a 0.4-mm pitch BGA, which would be very difficult for us to load by hand.

The A3030's lamp power supply is almost identical to that of the A3024, except that it includes a switch to modulate the LED current at 5 MHz so as to dim the average power output without losing efficiency, and without producing any electromagnetic noise that would be picked up by our EEG monitor.

The command receiver uses the same split-capacitor tuning network as the A3024, except we now have a dual tuning diode to give us double the received signal amplitude. We hope this additional diode will increase the operating range of command transmission.

The EEG monitor is similar to the one in the A3028. It provides one channel with gain ×100 and cut-off at 160 Hz. To isolate the amplifier input radio-frequency noise and LED switching noise, we have a low-pass filter in front of the input, as we proposed earlier.

The A3030 has no magnetic sensor with which to turn on and off. We will switch it on and off with 146-MHz RF power through its Command Receiver. Each command transmission will activate, at least temporarily, all devices within range. After the command, some devices may remain awake, others may go to sleep, depending upon the commands transmitted. We will activate the EEG transmission through the Command Receiver also. The active current consumption of the A3030 with no EEG transmission and the lamp turned off is around 60 μA, which is much higher than the A3028's 20 μA and the A3024's 2 μA. This higher consumption is due to the higher-capacity logic chip.

The A3030 will be powered by a 382030 160 mA-hr lithium polymer battery. This battery is rechargeable, and we will be able to recharge it up until the time we encapsulate the entire device for implantation. After that, its value lies in its ability to delivery over 200 mA with minimal drop in output voltage.