Head-Mounting Transmitter (A3040)

Contents

Description

[17-SEP-24] The Head-Mounting Transmitter (HMT, A3040) is a telemetry sensor that connects to an Electrode Interface Fixture (EIF) cemented to the skull of a subject animal. The HMT provides high-fidelity amplification and recording of up to four biopotentials. The EIF provides leads, electrodes, and a connector that mates with the HMT. Once cemented in place, the EIF provides not only electrical connection to the HMT, but is also the means by which the HMT is secured to the subject animal. The HMT transmits its signals using its flex-circuit antenna, which hovers over the back of the animal. The HMT uses the same wireless communication system as our Subcutaneous Transmitters (SCT).

Figure: The A3040AV1 Circuit with Battery Loaded. Battery is CR1225, 2000 μA-dy. Visible are the flexible antenna on the left, electrode interface connector in the center, and the battery holder on the right. This battery is a CR1225.

The HMT needs to be splash-proof and dirt-resistant, but it need not be water-proof or corrosion-resistant. We wrap the circuit in teflon tape, so as to protect the electronics from debris and water. We add a final layer of transparent tape to protect the teflon from scratching. When we deploy an HMT, we hold the subject animal securely, press the HMT's connector onto the EIF, and secure the two connectors together with a drop of silicone sealant such as KwikCast. The sealant makes it harder, but not impossible, for the animal to remove the HMT. If we make it impossible for the animal to remove the HMT, we also make it impossible for ourselves to remove the HMT without injuring the animal, and yet we must remove the HMT to replace its battery. We must accept that there is always a chance the animal will be able to remove the HMT, and once removed, destroy it by chewing on it, which they like to do. When we ourselves want to remove the HMT, we hold the animal securely, peel the sealant off with forceps, and slowly work the two connectors apart. Once the HMT is detached, we immediately load another HMT with the same telemetry channel numbers and amplifier configuration onto the EIF, secure with silicone sealant, and restore the animal to its habitat. We can now take the original HMT away, unwrap it, clea it, dry it, and set it aside, ready to act as the next replacement. By this means, a pair of HMTs will provide continuous recordings of indefinite length on the same telemetry channels, so long as we can tolerate five-minute interruptions for replacement.

The HMT has no on-off switch. When we slide the battery into the retainer, it starts transmitting. It keeps transmitting until the battery runs down or we remove the battery. We can deduce the state of the battery from the average value of the telemetry signals, so we will always have a day or two warning before we need to replace the battery if we want to maintain a continuous recording. The HMT provides four amplifiers with a common reference potential. It is designed for recording EEG from one to four locations on the brain, sharing a common reference potential over the cerabellum.

Figure: Head-Mounting Transmitter (A3040D) Wrapped in Teflon Tape. We recommend an additional layer of transparent tape to hold the teflon in place, see here. Battery is CR1225, 2000 μA-dy, total mass 2.2 g. A flexible antenna hovers over animal's back.

Current consumption of the HMT is linear with total sample rate. We use intecept ≤25 μA and slope ≤0.12 μA/SPS to calculate the maximum current consumption of an HMT, and divide the nominal battery capacity by this current to obtain our minimum operating life.

Ordering

[17-SEP-24] There are many possible configurations of the HMT. Each configuration that we ship graduates from being a mere "configuration" to a "version", and will be listed in the version table below. The minimum order quantity for any particular version is ten pieces, unless we have that particular version ready-assembled on the shelf. If we have what you want on the shelf, we will sell you one or more at the same per-piece price. We sell HMTs in pairs, each pair sharing the same channel numbers, sample rates, and amplifier configuration. Each pair will record from one animal. One HMT sits idle, waiting to receive a fresh battery and tape while the other sits on the animal transmitting signals. When you exchange HMTs, you remove one and load its partner in a matter of minutes. If one of your animals pulls off and destroys its HMT, you can load its partner and continue recording. You then have roughly a week to order a replacement partner to complete your pair. We will be able to supply this replacement quickly, because we always make more HMTs of each version than are required to fulfill an order of ten or more pieces. We can program one of the spares to match the channel numbers of the HMT you lost, and ship it to you promptly. The replacement will be with you within a week of receiving your replacement order.

Versions

[05-AUG-24] All versions of the A3040 have the same profile: 14 mm × 14 mm × 8 mm with 14-mm rear flex antenna. But they may be programmed to enable one to four input channels at various sample rates and bandwidths. We equip the A3040 with a CR1225 battery, which provides capacity 2000 μA-dy (two thousand microampere days). All versions have mass 2.0 g without any wrapping. The table below gives defined versions with their telemetry channels, signal bandwidths, and operating life. Each HMT has five inputs: X1, X2, X3, X4, and GND. Each HMT has four amplifiers. Each HMT can transmit on up to four channels. These will channels will have consecutive channel numbers, offset from a base channel number. The GND connection is always used as a reference for at least one input. The other connections act either as an input to an amplifier or as a reference for an amplifier.

In the table above, we specify mass with no wrapping. A teflon tape wrap with two pieces of cello-tape adds 0.2 g to the mass. When one channel uses another as its reference potential, we can use this channel to monitor a potential elsewhere in the body, such as EMG. The A3040C2Z, for example provides two channels of EEG down to DC, and one channel of EMG 2-200 Hz.

Connector

[06-NOV-24] The A3040A provides an eight-way, dual-row, 0.025" pitch, hermaphroditic surface-mount connector that mates with a connector of the same type on the subject animal's head. We use Omnetics PZN-08-VV (A79612) on the circuit board, where we solder the connector's gull-wing leads to its footprint. We use either PZN-08-VV (A79612) or PZN-08-DD (A79614) in the Electrode Interface Fixture (EIF). In the HMT, we use the pin numbering given by the manufacturer, see S3040B_1.

In the HMT circuit diagram, we refer to the reference potential as "VC", the "common voltage". Once we connect this potential to an animal body, we say the HMT is "grounded". Our assumption is the one of the two GND pins on the EIF will be used for a low-impedance connection to the animal body. On the Electrode Interface Fixture (EIF), the numbers we assign to the pins do not match the manufacturer's data sheet, but they do match the pin numbers on the HMT circuit.

Figure: Head-Mounting Transmitter's Eight-Way Connector. Connector is PZN-08-VV. Pinout mates with the pintout on the HMT connector. Drawing taken from the Omnetics Corporation data sheet.

The EIF pin numbers in the sketch match the pin numbers of the mating connector on the HMT circuit. So we have Pin 1 and 2 are GND (VC in schematic, the reference voltages for the amplifiers). We have X1, X2, X3, and X4 on pins 7, 4, 3, and 8 respectively. When we construct the EIF, we will cut off all unused pins. We bend the remaining pins and solder up to five leads. One will be GND. The others can be X1-X4. Each electrode lead can be terminated with bare wire, a pin, or a depth electrode.

Figure: Electrode Interface Fixture (EIF8) Connector, Bottom-Left View. This connector is the through-hole PZN-08-DD before we bend and cut its legs. The pinout mates with that of the HMT connector. This is the view we have when assembling an EIF. For top view see EIF8 Connector.

The Electrode Interface Fixture may be equipped with up to five leads. One lead must always be present: the low-impedance ground, which will be connected to Pin 1 or Pin 2 or both. For historical reasons, the ground lead is always blue. The leads for X1, X2, X3 and X4 are optional, but they will be red, yellow, green, and cream-colored if present. The leads are 20 mm long by default. They are 0.5 mm in diameter, silicone-insulated springs. On the far end of each lead, we may solder a pin, a depth electrode, or leave a bare wire for securing in place with a screw, as requested by our customer.

Analog Inputs

[25-OCT-24] The A3040D3 is equipped with four amplifiers with gain ×100 and frequency range 0.3-160 Hz. The A3040D3 samples all four inputs with sample rate 512 SPS. Each sample is a sixteen-bit number 0..65535, where the number zero corresponds to the bottom of the input dynamic range, and 65535 corresponds to the top of the range. The plot below shows their response to a 15-mVpp sinusoidal input with 10-MΩ source resistance as we increase the sinusoidal frequency from 0.25 Hz to 1 kHz. Because the input resistance of the amplifiers is 10 MΩ, we see 7.5 mVpp on each input.

Figure: Response of A3040D3Z Transmitters to 60-mVpp Sweep. The D3Z is a 0.0-160 Hz DC transmitter. For response of A3040D3 and A3040D2 AC transmitters to 20-mV sweep see here.

The dynamic range of the inputs is equal to the battery voltage divided by the amplifier gain. For the A3040D3 at 10 Hz, the gain is ×100. The nominal output voltage of the CR-series batteries we use with the HMT is 3.0 V. So the dynamic range is 30 mV. All inputs are referred to a single reference voltage, VC = 1.8 V. The A3040D3 provides a high-pass filter at its inputs. The average value of any of its input is zero. The amplifiers use VC = 1.8 V to represent an input of zero. With battery voltage 3.0 V, an input of zero will be converted to 1.8 V / 3.0 V × 65535 = 39321 cnt (ADC counts). Count zero corresponds to −18 mV at the input, and count 65535 corresponds to +9 mV. The A3040 amplifiers may be configured to provide low-pass cut-off frequencies 40 Hz, 80 Hz, 160 Hz, 320 Hz, and 740 Hz. These cut-off frequencies accompany sample rates 128 SPS, 256 SPS, 512 SPS, 1024 SPS, and 2048 SPS respectively. The low-pass filter is a three-pole Chebyshev filter with some ripple in the pass-band to allow for a sharper cut-off at the top of the pass band. The default high-pass filter is a single-pole RC filter with cut-off at 0.3 Hz. We can remove the high-pass filter to provide response down to 0.0 Hz.

Figure: Spectrum of A3040D3 Input Noise, All Four Channels. Vertical 0.41 μV/div. Horizontal 10 Hz/div.

Noise on the A3040D3 inputs in a Faraday enclosure with leads ends in water is 20 cnt rms. Full scale is 30 mV or 65536 counts, so each count is 0.41 μV. We multiply 20 cnt rms by 0.41 μV/cnt to get 8 μV rms. The plot below shows the spectrum of this noise.

Figure: Four Channels of EEG Recorded with A3040D2. Scale: 100 ms/div horizontal, 200 μV/div vertical. Taken from M1725630494.ndf, courtesy of Niamh McLaughlin, University of Edinburgh.

The analog inputs share a single reference potential. We recommend placing this reference potential over the cerabellum and distributing the other four as you see fit. The other four can record electroencephalogram (EEG) or electrocortogram (ECoG) with wire ends, or local field potential (LFP) with depth electrodes. In the example above we have one second of EEG from four electrodes during an ictal event. In order to be sure that the EEG we record on one channel is due only to the potential between that channel's electrode and its reference, we must be assured that the cross-talk between amplifiers is far smaller than the amplitude of the signals we are observing. The plot below shows the amplitude of cross-talk verses frequency when we connect a sinusoid to one input and observe the amplitude recorded by all three channels of a three-channel transmitter.

Figure: Amplitude of X1, X2, and X3 in A3040C2 with 20-mVpp Sinusoidal Sweep Applied to X3. Yellow: X3. Blue: X1. Orange: X2. 20 kcnt amplitude is 20 mVpp.

With X3 reporting amplitude 20 mVpp, X1 reports 100 μVpp, X2 reports 50 μVpp. In electrical engineering terms, the crosstalk is −46 dB and −52 dB for X1 and X2.

Battery Life

The HMT current consumption from a 3-V Lithium Primary cell will be no greater than:

We divide the nominal battery capacity by the maximum active current to obtain our minimum operating life. The typical operating life is 10% higher, because the typical active current is 10% lower than that given above.

Figure: Active Current Consumption of A3040AV1 versus Total Sample Rate. Straight line fit gives intercept 17 μA and slope 0.110 μA/SPS.

Surgery

[15-MAY-24] Installation of the HMT is not in itself a surgery: we are connecting the HMT to an Electrode Interface Fixture (EIF) that has already been installed on the head of the subject animal. It is the installation of the EIF that involves surgery. For details of the surgery, see the surgical protocol provided by Kate Hills of University of Manchester. This protocol includes a description of how one can attach the HMT in a secure but reversible fasion with the help of Kwik-Cast silicone sealant.

Wrapping

[06-MAR-24] The A3040A has no permanent encapsulation, other than epoxy we apply around its connector to secure the connector in place, and epoxy we apply to the radio-frequency components to protect them from electrostsatic discharge. Before loading the transmitter on the animal, we fold the circuit as shown in our sketch. We wrap the circuit in teflon tape, building up two or three layers, keeping the connector free, and finally adhering the tape to itself. We complete the wrapping with one partial layer of transparent tape, such as Scotch Tape or Sellotape. The teflon wrap stops debri and water getting to the HMT circuit. The transparent tape makes it harder for the mouse to scratch off the teflon tape.

Figure: A3040D2 Wrapped in Teflon and Transparent Tape. Coutesy of Henry Martin, ION/UCL.

When we replace the battery, we remove the wrapping with our fingers. We do not use scissors because these are likely to touch the circuit inside while it is powered by the battery, and so cause peramanent damage. We remove the battery by pushing it out of its holder with a wooden applicator. We now we have the opportunity to wash the circuit if we like. We rinse in hot water while scrubbing gently with a tooth brush. We blow dry with compressed air. We soak the circuit in ethanol for a few minutes, blow dry, load battery, and wrap with sterile gloves.

Any water-proof tape that adheres well to itself is a likely candidate for wrapping the HMT during use. Self-fusing silicone tape is rugged and water-proof, but adds 0.5 g to the transmitter mass, see here. Plumber's teflon tape is thinner, and adds only 0.2 g to the mass. The British clingfilm adds only 0.2 g, see here. Parafilm adds 0.2 g see here. Gardener's tape adds only 0.2 g as well, see here.

Our customers have obtained best results with a two-tape wrap. Begin with teflon plumber's tape, then hold the plumber's tape in place with a layer of transparent paper tape, also called "cellotape" and "scotch tape".

Mount and Unmount

[20-MAY-24] To mount a Head-Mounting Transmitter (HMT) on its Electrode Interface Fixture (EIF), hold the animal's head in one hand. Looking from the side, push the HMT onto the EIF. Once the connectors are mated, with no gap between their housings, apply Kwik-Cast silicone sealant around the joint between the connectors. The sealant will make sure the mouse is unable to wiggle the HMT off its connector.

To unmount the HMT, hold the animal's head in one hand between thumb and forefinger. Remove the Kwik-Cast with tweezers from all around the connector. Hold the HMT in your other hand and rock it slightly back and forth along perpendicular to its length. The two connectors will slowly work themselves apart until they separate. Do not attempt to pull the HMT off directly, because doing so generates so much force that the EIF may tear off the animal's skull. During this process, some of our customers prefer to sedate the mouse so as to facilitate the removal and avoid frightening the animal. Other customers prefer to avoid the use of sedative because the sedative might interfere with their study.

Antenna

[08-FEB-23] The antenna on the A3040 is a flexible circuit with a rounded end. Counting the neck at its base, the antenna is 14 mm long. It is 4 mm wide. It carries a zig-zag conductor of total length 70 mm, which is slightly less than one quarter of the wavelength of our telemetry transmissions. We transmit in 902-928 MHz, for which the wavelength is around 330 mm.

Figure: Transmit Antenna. Ruler marks are millimeters.

The A3040AV1 provides footprints for a series-parallel matching network between the circuit's radio-frequency (RF) oscillator and the antenna. The AV1 does not attempt to match the antenna to the oscillator: the two are coupled with a single 1 nF capacitor. But the AV2 loads 27 nH for L1 and 0.2 pF for C26 to increase power output by a factor of ten.

Warranty

[19-SEP-24] The HMT is designed to be used repeatedly. We replace the battery and wrap it again. Unlike our implantable transmitters, the HMT is not encapsulated in epoxy. When in use, it is not immersed in body fluid at 37°C. It is not vulnerable to corrosion. But it is vulnerable to scratching, abrasion, and possible accidents involving water and urine. If an HMT becomes detached during an experiment, it is likely to be chewed and destroyed by the subject animals. While cutting off our wrapping, we might cut the antenna, or crack one of the HMT's tiny, ceramic components. We classify all such damage to the HMT as unfortunate events. We offer no warranty against unfortunate events.

Figure: Two Unfortunate Events. Both HMTs pulled off their EIFs and chewed on by host mouse.

The vulnerability of the HMT to physical damage during its multi-use lifespan threatens to undermine any experiment we plan with the device. Therefore, we recommend that you purchase twice as many HMTs as you need for each experiment, arranged so that each animal has two identical HMTs that you can deploy to sustain continuous recording. The minimum order for HMTs is ten pieces, arranged as five pairs. Once you have your pairs established, you can order replacements from us in smaller quantities.

Figure: Chewed Antennas and Flex Connections.

With two identical HMTs for each animal, you can prepare the replacement HMT with a fresh battery, pick up the animal, remove the expiring HMT, load the fresh HMT, and put the animal back in the cage, all in one or two minutes. You do not have to take the expiring HMT, unwrap it, clean it, replace the battery, check it's running, wrap it, and go back to the animal room. You have ample time to clearn, dry, and prepare the new HMT before the replacement takes place.

Design

[28-AUG-23] Here is the HMT conceptual sketch from our technical proposal.

Figure: Head-Mounting Transmitter Sketch. The transmit antenna protrudes behind the body of the device. The mouse's nose will be to the left in the drawing, and its tail to the right. The connector axis is parallel to the mouse body axis.

Below are schematics, data sheets, and design files.

The A304001A is a rigid-flex circuit board that provides two rigid areas for transmitter components, one rigid area for programming and calibration, which we cut off once the transmitter is configured correctly, and a flexible antenna.

Figure: Rigid-Flex Printed Circuit Board for Prototype HMT, Top Side. Connector is on the bottom side.

The A304001A's battery holder supports the CR1220, CR1225, and CR1025 coin cells.

Modifications

[31-OCT-24] Modifications listed in reverse chronological order.

[31-OCT-24] To make the A304001C out of the A304001B we made the following modifications.

1. Connect the extension ground pad to 0V.
2. Move cross-over of tracks in flex region to mid-point of flex region.
3. Add a footprint to allow X4 to be connected to R23, so we can use X4 as the reference potential for X3.
4. Remove J1 from solder paste stencil so it is flat gold.
5. Remove unused ground pad next to analog multiplexer.

[11-JAN-23] When updating A304001A to A304001B we performed following modifications.

1. Replace existing J1 footprint for PZN-08-VV.
2. Add polarity to U5 silk screen and permanent footprint silk screen.
3. Connect SCK through A4 to B5, an output in Bank 1 that provides 3.0-V logic output.
4. Place a ground pad on the programming extension.
5. Place silk screen white area on programming extension.
6. Add ground pad to top layer next to analog multiplexer.
7. Add pull-down resistor on TCK on programming extension.

[11-JAN-22] To make the A3040AV1 from the A304001A PCB we must perform the following modifications:

1. SCK Reroute: Remove R7, connect U3-7 to D0, a logic output on Bank 0. This output is accessible by wire modification, but provides only 1.8-V logic. The 1.8-V HI level proves sufficient to drive the ADC when powered by a 3.0-V coin cell.
2. Antenna Detune: Remove L1 and replace with 1.0 nF. Remove C26.

Development

[25-OCT-24] For a chronological account of our development of the A3040 HMT, see our separate Development and Production page.