Figure: Subcutaneous Transmitters (A3019A) Encapsulated in Epoxy with 120-mm Wires. The tips of the wires are equipped with stainless steel screw electrodes.
[04-AUG-10] The Subcutaneous Transmitter (A3019) and its predecessor the Subcutaneous Transmitter (A3013) are wireless sensors designed to monitor biometric signals within live animals.
We discuss the encapsulation of the transmitters in Encapsulation, and the design of their silicone-insulated, helical steel leads in Flexible Wires. Here we discuss the state of the tips of the leads, where they are supposed to pick up a biometric potential. In the figure above, we see un-tinned stainless steel coils at the tips. These we obtain by cutting around the circumference of the silicone insulation and unscrewing the liberated length of silicone so that it comes off the end of the lead. We can tin the stainless steel, making it easy solder to, with acid flux, as we describe elsewhere.
In our discussion, we use the term baseline to mean the average value of an EEG signal from one second to the next, which is another way of saying the frequency components below 1 Hz.
[12-AUG-10] So far, the ION group has used bare steel wire tips as their electrodes. They drill a hole in the skull, push the tip of the wire into the hole, and hold the wire in place with a screw. The second wire they attach to a bare wire ten or twenty millimeters long, and held down by two screws in the skull. This second wire is the ground wire. With these contacts, epileptic seizures generate roughly 5000 k counts-sq in the 3−30 Hz band (see Frequency Spectrum). We also see wave bursts in the 40−160 Hz band, as we describe here.
But the bare wires pick up a signal when we clap our hands near the rats. The rats startle and move the pick-up wires. The movement appears to be generating a signal. A few times an hour during recording, we observe sudden steps changes in the EEG potential, which we call transients (see Frequency Spectrum). These complicate the detection of seizures. It may be that the contact between the screw and the bare wire is generating a contact potential that varies as the contact shifts with movement.
[12-AUG-10] CHB implanted transmitter No10 and glued the ends of the analog wires to the animal's skull using silver epoxy, which provies an electrically conducting contact. With these contacts, we observed seizure-like activity in a non-epileptic rat, as shown here in archive M1281121460. We are convinced that the epoxy contacts are picking up EEG. Later, in archive M1281455686, we can compare No10's contacts with No8's steel screw contacts.
From this one experiment, we see the baseline swinging up and down compared to the stable baseline we obtain from the screws. But the silver epoxy contact sensitivity compares well to that of the bare wires, and exceeds that of the screw contacts.
The following plot shows power in the 2−20 Hz band for the two transmitters during three hours of recording.
The peak power from No10 corresponds to vigorous seizures like this one, not any transient spikes or level shifts. In addition to seizure detection, the silver epoxy contacts are also sensitive to wave bursts in the 40−160 Hz band. The plot below shows simultaneous wave-bursts from No8 (screws) and No10 (epoxy).
It appears that the silver epoxy has only one disadvantage compared to screws: a moving baseline. But the moving baseline does not appear to interfere with seizure or wave burst detection, and the silver epoxy method does not require holes in the skull.
[04-AUG-10] Screws are one way to make contact with an animal brain. We drill a hole in the skull and thread the screw into the hole. The tip of the screw is the contact. Our wires are flexible, so we can turn a screw a couple of times, winding the wire around, without generating too much tention in the wires. We obtained samples of some steel screws from Harvard Children's Hospital (part number 00-96X1-16 from Plastics One). With the help of acid flux, we soldered screws to the tips of the wires of an A3013A, as shown below.
The wires are soldered to the top sides of the heads of the screws. We tried soldering the wires to the underside of the screw heads, but we found that solder spread all down the length of the screw, obscuring the threads. Even when we solder the wire to the top of the head, we must take care not to allow solder to spread to the threads, and we do this by threading the screw into a small hole in a piece of box cardboard. The cardboard also provides us with a means of holding the screw while we make the joint.
CHB asks for the X+ wire to be 130 mm with a screw on the end, and the X− wire to be 120 mm with another screw on the end. We did as he asked on a transmitter No8.
[12-AUG-10] Archive M1281455686 contains EEG recorded by screws in No8 compared to EEG recorded by epoxy contacts in No10. The screws provide a stable baseline. Both contacts provide wave burst and seizure detection. The screws appear to be slighly less sensitive to seizure activity than silver epoxy or the bare wire electrodes. Peak seizure power in the 2−10 Hz range is around 5000 k counts-sq compared to 10,000 k counts-sq for the other electrodes.
[15-AUG-10] CHB implants No6 with screws. The wires of this transmitter are only 100 mm long, a bit short. We see large transient jumps on the signal. Sameer reports, "I think the unstable baseline from transmitter#6 can be attributed to the use of epoxy to secure one of the screws instead of a drilled hole. Due to some reason, I think size or location of leads, YingPeng couldn't drill it in. However, the other screw was fastened into the skull."
[25-AUG-10] We receive two sizes of 303 stainless steel screws from SmallParts. Both have what is called the binding head, which is flatter than the screws we started with, as you can see in the photograph below.
All these screws are 1.6 mm long (1/16"). The 000-120 has diameter 0.86 mm with thread pitch 0.21 mm. The 00-90 has diameter 1.2 mm with thread pitch 0.28 mm. the 00-96 has diameter 1.2 mm and thread pitch 0.26 mm.
[03-APR-11] We have settled upon the 00-90, 1.6-mm long binding head screw as the standard for the A3019D transmitter for implanting in rats.
For mice, Louise Upton has chosen a 1-mm long M0.5 screw, B0038QOYF from SmallParts. We soldered this screw to the 70-mm leads of an A3019A. She drilled holes in the skull of a dead mouse and declared the screw to be the perfect size.
[03-APR-11] We study 15 hours of recordings from Joost at ION, from eight control animals. The transmitters are A3019Ds with 00-90 binding head screws 1.6 mm long. The leads are insulated right up to the solder joints on the screw head. The screws are set directly into the skull with dental cement to hold them in place and insulate them. The following four-second display of the eight signals is typical.
We see few artifacts in the EEG. The amplitude of each trace is around 40 μV. Another similar period of recordings from eight different animals with the same electrodes shows 40 μV baseline EEG also. The consistency between the baseline amplitudes suggests that the screws are giving a repeatable contact with the brain.
[04-APR-11] Joost suspects that the 00-90 screws are coming loose from the skull after a few weeks. "Yes, I also had the impression that the power per frequency band was quite consistent between animals, at least during the first week of recording. After that power tends to decrease across all bands in quite a lot of animals. This is probably caused by the skull getting thicker and slowly pushing the screws out. This ends in rats loosing their electrodes after 2 to 4 weeks. I've now soldered larger screws on a couple of transmitters and will also add a few more screws just to anchor the headpiece to see if this will enable recordings with stable band power for two or three months."
[03-APR-11] Bare wires held in place by a screw produces sudden jumps in baseline potential, which produces power in the seizure and wave burst bands, thus complicating seizure detection. Given that the bare wires require holes in the skull, and yet produce these baseline jumps, they are currently our least favored electrode.
Wires held upon the skull surface by silver epoxy require no holes. But the method produces a moving baseline voltage in our existing experience, and we are not certain the contact resistance is consistent from one animal to the next. The skull must be scraped to expose the bare bone, and dried so that the epoxy can adhere properly.
Steel screws held in place by dental cement give a stable and consistent baseline without transients. Experiments with several dozen animals show 40 μV baseline EEG amplitude with no sign of movement artifacts or baseline swings. For a rat's skull we find that the 00-90 screw 1.6 mm long is the best size. For mice, we prefer an M0.5 screw 1 mm long.
