ISL Stage 7 Delivery

We ship to ION the devices shown below. There are ten Implantable Sensors with Lamp (A3030D) and fifteen Head Fixtures (A3024HF) of various types.


Figure: Batch ISL7 Deliverables. For a close-up of the A3030D see here.

Each ISL provides a 50-mm 915-MHz command antenna and a separate 30-mm 915-MHz data and acknowledgement antenna. The A3030D introduces support for longer interval lengths as well as random pulse generation. We need Version 4 of the ISL Controller Tool to command the A3030D. The new controller tool provides a version menu button to select between the A3030C and the A3030D. In other respects, the use and operation of the A3030D is identical to that of the A3030C we delivered in .


Figure: Output Power, Color, and Cannula Type for ISL7 Head Fixtures. We measure optical power with a photodiode and 40 mA LED current. All head fixtures are A3024HFC, except 7.1, which is A3024HFD.

The photograph below shows an example of the ISL7 head fixture with inclined silica cannula guide and cannula dummy installed.


Figure: Silica Guide Cannulas.

We ran out of cannula dummies and then silica guide cannulas during production of our fifteen head fixtures. Some we are shipping without cannula dummies. We run out of silica cannula guides and have no time to wait for more, so we use 10-mm steel cannula guides instead.


Figure: Steel Guide Cannulas.

We have been using silica guides because we believed they would transport less electrical noise into the brain. But our study of the sources of lamp activation noise suggests that a steel guide will cause no increase in lamp activation noise at the tip of the fiber. The guide will, however, affect the flow of the EEG signal in its neighborhood. We would like to learn from these three steel guides whether the silica guide is necessary or redundant.

We made twelve A3030Ds and all twelve are fully functional after encapsulation. The figure below shows all of them turning on at once in response to a multi-device XON command from our Command Transmitter.


Figure: Twelve A3030Ds Turning on Simultaneously.

We forgot to bind the X and L leads into pairs before we soldered them to our circuit boards. We bound the leads afterward encapsulation, which produced an unattractive finish on the previously smooth and rounded silicone-coated springs, as shown below.


Figure: Imperfect Silicone Binding in X and L Lead Pairs.

We believe there will be no problem implanting these lead pairs, not any compromise of their fatigue resistance due to the cosmetic problems with the finish.

Lamp Power Control

The Implantable Sensor with Lamp (A3030D) provides 0-100% control of the lamp power in steps of 20% by means of modulation of the lamp current, as we describe elsewhere. The 1-MHz modulation frequency is so high as to have no effect upon the activity of neurons nor our EEG amplifier. We connected an EZ500C470 470-nm blue LED to the lamp leads of an A3030D. At 100% power, the LED current is 35 mA. We varied the output power from 1-100% and measured battery current and optical output power of the LED at each step.


Figure: Optical Output Power and Battery Current versus Lamp Modulation Duty Cycle. Switching transistor is the NTR4003N. Compare to this measurement of modulation current in ISL5/6, which used the NDS355AN switching transistor.

The average ISL7 head fixture couples 44% of the LED light to the tip of its fiber. At 100% power, the LED produces 30 mW and we expect 13 mW at the fiber tip. The battery current and light intensity scale almost linearly with power setting. If anything, the 40% power setting is slightly more efficient in its conversion of battery energy into light energy than the 100% setting.

Prompt Acknowledgment

The Implantable Sensor with Lamp (A3030C) for ISL6 provided acknowledgment messages in response to acknowledgement request instructions from the Command Transmitter (A3029B). The transmission of these acknowledgements was, however, delayed by up to 2 ms after the A3030C processed the acknowledgment request. This meant that the best synchronization we could obtain between lamp pulses and the recorded X signal would be ±1 ms. One of our objectives for ISL7 is to improve the precision of the lamp and recording synchronization.

The Implantable Sensor with Lamp (A3030D) for ISL7 transmits an acknowledgment a fixed 60 μs after it processes a request. The ISL Controller V4 requests an acknowledgment immediately before it initiates a stimulus cycle, so the delay between the reception of the acknowledgment and the start of the first light pulse of the stimulus is deterministic. The figure below shows the delay for 100-Hz pulses.


Figure: Prompt Acknowledgment Timing with Respect to First Lamp Pulse. Top trace has a 60-μs pulse marking the moment the acknowledgement is transmitted. Bottom trace has a longer HI interval that marks the start of the first light pulse.

For a 10-ms interval (100-Hz pulses), the delay is 750 μs. When we increase the interval length to 100 ms (10-Hz pulses), the delay is 6.3 ms. The delay is a fixed 150 μs plus 2/33 of the interval length. The A3030D's prompt acknowledgments should allow us to synchronize stimulus and recording within a fraction of a millisecond.

ASIDE: While working on the prompt acknowledgments, we discovered that the A3030's radio-frequency oscillator, which is the same oscillator we use in our subcutaneous transmitters, needs roughly 50 μs to stabilize when it has been inactive for more than 100 ms. When transmitting at 512 SPS, the oscillator is never inactive for more than 2 ms. But in the ISL, we can request acknowledgment even when the data transmission is disabled, and in that case, the A3030C of ISL6 did not allow sufficient time for the oscillator to stabilize, so that acknowledgments were unreliable when data transmission was turned off. In the ISL7 we turn on the oscillator 60 μs before any acknowledgment transmission, and we get 100% reception of acknowledgments under all circumstances in a faraday enclosure.

Optogenetics Growth

The National Center for Biotechnology Information (NCBI) provides a database of published neuroscience papers. We used the database to obtain a count of the number of optogenetics papers published each year for the past ten years and obtained the figure below.


Figure: Number of Optogenetics Papers Published versus Year. The final point is a projection based upon the number published so far in 2015.
In the first seven months of 2015, 402 optogenetics papers have been published, as you can see for yourself with this search link. Assuming a constant rate of publication for the next five months, we project the total number in 2015 to be 689, and we added this point to the end of our graph.

Random Pulses

Firmware A3030D01.vhd for ISL7 (A3030D) is debugged and running. It provides random lamp pulses for any interval and pulse length. For a view of the modulation noise generated by random pulses, see our previous post. The ISL7 stimulus length is the expected number of pulses. The interval length is the average pulse period in milliseconds. The pulse length as a fixed multiple 30.5 μs.

The A3030D divides each pulse interval into thirty-two sections. For regular pulses, the A3030D always initiates a pulse in the first section and only in the first section. For random pulses, the A3030D initiates a pulse in each section with probability 1/32. Thus the average period of pulse initiation is the same as the interval length. With pulses longer than one section length, the pulses overlap. Pulses that occur at the end of an interval can overlap pulses occurring at the beginning of the next interval.

With 2-s intervals and 100-ms pulses, we counted 100 pulses in 207 s. With 100-ms interval and 10-ms pulses, we used a recording of lamp modulation noise to count 559 pulses in 60 s. The figure below shows the distribution of the number of pulses we counted per one-second recording interval.


Figure: Distribution of Pulses per Second for Random 10-ms Pulses with 100-ms Interval. Average pulse rate is 9.3 pulses/s. For 32-s view of recording and spectrum, see here.
We appear to have a passable approximation to a Poisson distribution of pulse generation. When the pulse length is significant compared to the interval length, the average number of pulses drops because the pulses are overlapping. But we still get 9.3 pulses/s average for 10-ms pulses at 10 Hz.

NOTE: The ISL6 accepted the interval length in multiples of 30.5 μs rather than 1 ms, so the ISL Controller Tool 4.1 has a version selector that allows it to control both ISL6 and ISL7 devices.

Quiet Modulation

The figure below shows lamp modulation noise on the X input of an ISL6 (A3030C) for 20% brightness. This noise occurs even with no lamp attached and no X input leads.


Figure: Lamp Modulation Noise on X for A3030C with 20% Brightness, 50-ms Pulses at 10 Hz.
When we modulate the lamp, we apply a 1-MHz clock signal to the gate of Q1 so as to turn on and off the lamp current. During lamp modulation, the A3030C battery current increases by around 5 mA. We suspect that this 5 mA causes the 3V0 output of our TPS70930 to drop by 500 μV, which in turn causes X to step up by 2 mV.

In the A3030C, Q1 is the NDS355AN, with 200 pF gate capacitance. We load our A3030D with NTR4003N for Q1. The figure below shows A3030D modulation noise for random 10-ms pulses.


Figure: Lamp Modulation Noise on X for A3030D with 80% Brightness, 10-ms Random Pulses at 10 Hz. Vertical range is 140 μV.
The new transistor reduces modulation noise to 50 μVpp. The NTR4003N has resistance 1.5 Ω, which is much larger than the 0.2Ω of the NDS355AN, but still insignificant compared to our 60-Ω lamp lead resistance.

ISL7 Head Fixture

The photograph below shows our first ISL Head Fixture (A3024HFD-B). Version D of the head fixture has the guide cannula cemented parallel to the optical fiber.


Figure: ISL7 Head Fixture with Blue LED (A3024HFD-B).

This guide cannula may be used to hold the head fixture in place during implantation. Once the head fixture is covered by cement, the silica guide cannula tube may be crushed with scissors, or a thin pair of cutters, just below the thread. We now have a more compact head fixture, with no guide cannula thread protruding from it, and the hole in the animal's skull must accommodate only the optical fiber. With our previous head fixtures, the skull hole had to accommodate both the fiber and the guide cannula.

In the above example we have roughly 3 mm space between the circuit board and the guide cannula thread. We believe this is sufficient to allow cement over the circuit board and still have space enough to cut the guide tube. But we await comments from ION before deciding the final geometry.

UPDATE [24-JUN-15]: ION tells us they would rather have the older A3024HFC-B.