Implantable Stimulator-Transponder (A3036)

© 2019 Kevan Hashemi, Open Source Instruments Inc.




[22-AUG-19] The Implantable Stimulator-Transponder (A3036) is a wireless electrical stimulator that receives commands and responds with acknowledgements using a single antenna. The electrical stimulus is delivered through two silicone-insulated helical wires terminated with miniature pins. When combined with a light source like the Implantable Lamp (A3024HF), the A3036 provides optogenetic stimulus to the brain. When combined with bipolar depth electrode, the IST provides direct electrical stimulus to the brain. TWhen equipped with a 19-mAhr lithium-polymer battery, the IST is less than 1.0 ml. We first proposed the development of the IST in our IST Technical Proposal.

Figure: IST Circuit Board, Top Side. The circuit is 10 mm high, 10 mm wide at the base, and 7.6 mm wide at the ttop. We clip the head of the circuit board from its programming extension after epoxy encapsulation.

The IST is managed by a field-programmable gate array (FPGA) in a 2.5-mm square package, the XO2-1200. This device provides both volatile and non-volatile memory as well as thousands of programmable logic gates. It is capable of implementing arbitrarily-complex stimuli in response to a single command. The A3036A uses the same firmware as the Implantable Sensor with Lamp (A3030E), in which a single stimulus consists of a set number of pulses, each of fixed length, generated at regular intervales, or at random intervals with a known average value.

Figure: IST Circuit Board, Bottom Side.

The IST command receiver is a crystal radio that consumes less than 8 μA when the stimulator on standby. The commands arrive from a transmitter such as the 915-MHz Command Transmitter (A3029C), which we control with the same software we developed for the Implantable Sensor with Lamp (ISL). This software operates with our LWDAQ data acquisition hardware. The IST transmits its acknowledgements and metadata at 915 MHz also, for detection by a data receiver such as the Octal Data Receiver A3027E. The IST transmissions use the same protocol as our Subcutaneous Transmitter System (SCT). The IST command transmissions take tens of milliseconds, and during command transmission, SCT samples will be lost.

Version Battery
A3030A 19 0.8 33
Table: Versions of the A3033.

The IST is designed to be implanted together with an SCT so that stimuli can be generated in response to EEG activity in real time, with the help of the Event Classifier running on the data acquisition computer. Our earlier ISL device combined stimulator and sensor in the same device, powered by the same battery, and suffered from lamp artifact. When we separate the stimulator and the sensor into two circuits each powered by their own battery, we eliminate lamp artifact in optogenetic applications, and greatly reduce it for electrical stimulation applications.


S3030A_1: IST Schematic.
Code: Logic Programs and Test Scripts.
LCMXO2-1200ZE: The programmable logic chip data sheet.
WLCS-25: The 2.5 mm square BGA logic chip package. PCB for A3036A, Gerber files and drawing.
A3036A.ods: Bill of Materials and Pick and Place for A3036A.
Controller Tool: Software to control the IST.


[23-AUG-19] Schematic complete. Arrange components on 10-mm × 10 mm circuit board, narrowed at the top to fit into the terminal end of our 19-mAhr LiPo battery. The A3036 crystal radio uses the SMS7621079LF detector diode in 1.7-mm long SC-79 package. The threshold comparator is the same MCP6541 but in the smaller SC-70-5 package. The stimulus and battery-check switches are two N-channel enhancement mode mosfets provided by a DMG1024UV in a 1.7 mm × 1.0 mm SOT-563 package. The OR gate is is the same SN74AUP1G32, but in the much smaller UDFN-6 1.0 mm × 1.5 mm package, which we use on our A3028GV1 circuit boards. This package fails to load properly in roughly 1% of assemblies and is impractical to replace by hand, but it is small. The logic chip is the same LCMXO2-1200ZE, but in a 2.5 mm square WLCS-25 package with balls on a 0.4-mm pitch. We are not yet sure how to make a footprint for so fine a ball pitch.

[04-SEP-19] The IST uses the LCMXO2-1200ZE in a WLCS-25 package. The WLCS-25 has twenty-five balls on a 0.4-mm (15.7-mil) pitch. With 10-mil diameter pads the clearance between pads is only 6 mil, which is barely enough to run a 2-mil track, let alone the 5-mil tracks of our usual fabrication process. We consult with Epectec for a solution to the layout problem. They propose that we use 12-mil pads with 10-mil soldermask opening and a 4-mil laser-drilled microvia from the top copper (L1) to the first middle copper layer (L2).

Figure: BGA Footprint on A303601A Printed Circuit Board. Shown are the top copper (L1), top soldermask, and the drill holes.

The second layer of copper (L2) has 10-mil pads to receive the 4-mil microvias, as shown below.

Figure: The Middle Copper (L2) Beneath the BGA. Shown are the middle copper tracks with 10-mil pads to receive the microvias, and the 4-mil microvia holes themselves.

There are no pads for the 4-mil holes on the remaining four copper layers: ground plane (L3), power plane (L4), middle copper (L5) and bottom copper (L6). But our drill file specifies 4-mil holes, and we see these rendered in the plot below.

Figure: The L3-L6 Copper Layers. There are no pads for the 4-mil holes, because the 4-mil holes are indented to pass only between the L1 and L2 layers. These holes are, nevertheless, shown in this plot, because our drill file cannot specify partial drill holes.

We could have routed tracks directly beneath the microvia holes, but our layout software is incapable of understanding drill holes that pass between only two layers. So we kept the bottom four layers clear of copper around the drill holes. In order to generate the gerber files for the bottom four layers, we deleted the 4-mil vias temporarily.

[06-SEP-19] Completed draft version of P3036A01 firmware, including battery voltage measurement by timing how long it takes to charge up C4. When we request a battery measurement, the firmware will assert BT, which closes the battery test switch U3-6 to U3-1. Capacitor C4 starts to charge through R1 and R2. These resistors present a voltage source 80.5% of VB through a resistance of 6.43 kΩ. Capacitor C4 is 1.0 μF. The charging time constant is 6.4 ms. The BTV signal connects to an input on U8, a 3.0-V logic input with logic threshold around 1.5 V. When VB = 3.6 V, voltage BTV will take around 3 ms to reach 1.5 V, or 100 cycles of our on-board 32.768 kHz clock. To the first approximation, time taken is inversely proportional to the battery voltage, so we expect 86 periods when VB = 4.2 V and 106 periods when VB = 3.4 V. Submit A303601A Rev 1 printed circuit board for fabriction on a ten-day turn.