Flexible Wires[06-JAN-25] Our implantable, wireless devices demand flexible leads and antennas. Our Subcutaneous Leads (SCL) are silicone-insulated, stainless steel springs. They stretch and they bend. They are fatigue-resistant and corrosion resistant. We use them as electrode leads in Subcutaneous Transmitters (SCT) and as stimulus leads in Implantable Stimulator-Transponders (IST). Our Subcutaneous Antennas (SCA) are silicone-insulated, stranded, stainless steel cables. These implantable, wireless devices are coated in silicone, which joins with the silicone of our SCLs to make an unbroken water barrier around the leads, battery, and electronic circuit of the device. We sell SCLs separately for our customers to use in their own imlantable devices.
Both our leads and antennas must survive months of implantation in freely-moving animals. They will be immersed in saline at 37°C. They will be bent hundreds of thousands of times per day. We present our development of long-lasting implantable leads and antennas in Development of Flexible Wires. We present auxilliary work developing a water-proof encapsulation in Development of Encapsulation. In the passages below, we present details of the wires and cables we use in our leads and antennas. We provide instruction on removing silicone and tinnning in preparation for soldering.
The table below lists the springs and cables we currently use for leads and antennas respectively. We refer to the springs as a "helix" of wire, on the grounds that "spring" could be some other shape.
| Type | Description | Source | Uses |
|---|---|---|---|
| Helix | Pitch 175 μm, OD 450 μm, ID 250 μm, Wire 100 μm 316SS | MDC13867A | B Lead |
| Helix | Pitch 100 μm, OD 250 μm, ID 150 μm, Wire 50 μm 316SS | MDC2014B | C Lead |
| Helix | Pitch 300 μm, OD 500 μm, ID 200 μm, Wire 150-μm 316SS | MDC26398 | D Lead |
| Stranded | Strands 7×7, OD 360-μm, Wire 302SS | Sava 2014 | A/B Antenna |
| Stranded | Strands 7×7, OD 250-μm, Wire 302SS | Sava 2010 | D/E Antenna |
The figure below shows the three types of helix next to one another.
[06-JAN-25] Our subcutaneous leads are 316SS helical springs insulated with silicone. They stretch along their length and bend easily. They survive the repetitive stress of implantation indefinitely, so long as they are not bent sharply at the point where they emerge from the dental cement of a head fixture. All leads are coated with unrestricted medical-grade silicone, MED-6607. The inner silicone is SS-5001 with a dye added to give the lead a bright color.
| Lead Code |
Lead Diameter (mm) |
Spring Outer Diameter (μm) |
Spring Inner Diameter (μm) |
Wire Diameter (μm) |
Min Insulation Thickness (μm) |
Resistance (Ω/cm) |
Common Application |
|---|---|---|---|---|---|---|---|
| B | 0.7±0.1 | 450 | 200 | 75 | 75 | 6.3 | 45-mm leads for mice 100-mm leads for rat pups 200-mm leads for adult rats |
| C | 0.5±0.1 | 250 | 150 | 50 | 50 | 25 | 35-mm leads for mouse pups 45-mm leads for mice 50-mm leads for rat pups |
| D | 0.8±0.1 | 500 | 200 | 150 | 75 | 1.6 | 130-mm lamp leads for adult rats |
We recommend our 0.7-mm leads for recording in mice and rats. We recommend our 0.5-mm leads for recording in mouse and rat pups. When we are recording EEG or EMG, we are not concerned about lead resistance, because the input resistance of our amplifiers is of order 10 MΩ, which is very much greater than the lead resistance. When we want to tens of milliamps through a 150-mm lead for stimulation, however, we recommend our 0.8-mm leads, which present a resistance of only 1.6 Ω/cm.
Our 0.5-mm leads are eight times more flexible than our 0.7-mm leads, but the breaking strain of their steel wire is four times lower. In rat and mouse pups, the greater flexibility of the 0.5-mm leads justifies their greater fragility. But in larger animals, we recommend the more rugged 0.7-mm lead. When we remove silicone from the tip of a 0.5-mm lead, we must cut the silicone around the circumference of the lead with a scalpel and unscrew the silicone from the tip of the lead. There are two schools of thought on whether the scalpel blade should be sharp or blunt. In the sharp-blade school, we press lightly to cut through the silicone, and take care to avoid scratching the steel. In the blunt-blade school, we press more firmly to cut throught the silicone, but not so firmly that we bend the steel wire. When the steel wire is bent, it becomes difficult to unscrew the silicone from the helix of steel.
We sell insulated leads separately if you need them for your own purposes. The part number for a given lead is SCL-diameter-length. Thus SCL-0.7-130-Blue is a 0.7-mm diameter lead 130 mm long and dyed blue, SCL-0.5-130-Red is a 0.5-mm diameter lead 100 mm long, dyed red. See our Price List the price of our leads. Available lengths are 130 mm and 280 mm. Colors are blue, red, orange, purple, yellow, green, pink, or brown.
[06-JAN-25] We use strain-relieved, bare, 7×7 stranded, 304SS cable for sensor antennas. We insulate the antennas in clear, unrestricted, medical-grade silicone. In our A and B antennas we use 350-μm diameter cable, for our D and E antennas, we use 250 μm cable.
Stranded wire offers the best compromise between transmission efficiency and fatigue resistance. Stranded wires cannot endure repetetive stretching fatigue for months, but they can endure repetetive flexing, which is all that is required of an antenna. We prefer to use stress-relieved cable because it is easier to work with when we cut it and tin the end for soldering.
| Antenna Code |
Length (mm) |
Description |
|---|---|---|
| A | 50 | Stranded steel loop antenna, 360-μm diameter 7×7 304SS wire, coated with clear MED-6607 silicone, for transmitters in rats. |
| B | 30 | Stranded steel loop antenna,
360-μm diameter 7×7 304SS wire, coated with clear MED-6607 silicone, for transmitters in mice. |
| D | 30 | Stranded steel loop antenna, 250-μm diameter 7×7 304SS wire, coated with clear MED-6607 silicone, for transmitters in small mice. |
| E | 50 | Stranded steel loop antenna, 250-μm diameter 7×7 304SS wire, coated with clear MED-6607 silicone, for radio-controlled implants. |
There is no "C-Antenna", because that one we discontinued. It was an antenna made our of a straight helical lead, and it was not efficient. The A-Antenna emits the most power. The A and B-Antennas are the toughest. The D and E-Antennas are for implantation in mice, where we do not expect the implantation to last more than a few months.
[04-APR-21] Silicone adheres well to our helical wires. It cannot be burned off. It cannot be scraped off. we want only a millimeter of straightened wire, we can grab the tip of the wire at the end of the lead and pull. Now we can bend and cut to length in preparation for securing with a screw or cyanoacrylate. But if we want to solder the wire to a screw, or attach it to a crimp ferrule, we need to expose at least a millimeter of the helix. To remove the insulation, we use a scalpel and a flat surface. The tutorial movie below shows how to remove silicone from one of our 0.7-mm diameter leads.
The procedure for our 0.5-mm diameter leads is similar, but more delicate. We have an additional movie to explain the extra precautions one must take to remove the silicone from these smaller leads.
[06-JAN-25] The video below shows how to tin a stainless steel helix with acid flux, and solder it to a pin. In the video we claim that you can use hydrochloric acid as flux for soldering stainless steel, but we recommend you use zinc chloride acid flux. Later comparisons between hydrochloric acid and zinc chloride flux showed that the former was greatly inferior to the latter.
Our soldering iron is temperature-controlled. Acid flux gives the best performance with the iron at 400°C. Once the wire tip has been tinned with the help of acid flux, it can be soldered at any time. The flux we use in the video is the No75 zinc chloride acid flux from Superior Flux Company, which we purchase from McMaster. We have also made our own zinc chloride flux using the following recipe.
The following video shows how one can extend helical leads that have been cut short, perhaps after removal from an animal. Lead extension is of particular importance in work with mice, where we begin with 45 mm leads and require at least 40 mm to make the journey from the transmitter to the brain. If we start with more than 45 mm, the leads bend and overcrowd the mouse.
Instead of extending your leads, we recommend you order long leads to begin with, allowing for 5 mm to be cut off every time you explant the device. Lead extension is something we can do for you if you accidentally order transmitters with leads that are too short. You can send them back to us and we will extend the leads for small fee.
The particular batch shown above we shipped with 45-mm long leads, when the customer really wanted 90-mm leads. Each lead is now extended to 90 mm. The solder joint at the center is covered by three layers of silicone.
[13-MAR-24] We encapsulate our transmitter bodies in black epoxy and then coat them with silicone. Our antennas we make out of stranded stainless steel wire coated with silicone. Our EEG leads we make out of 316 stainless steel springs coated with silicone. The outer coat of silicone on leads, antennas, and transmitter bodies is always an unrestricted medical grade silicone such as MED-6607. The first coat of silicone on a transmitter body is usually SS-5001 with a dye, but the outer coating is always MED-6607.
Our customers often use cyanoacrylate adhesives to fasten electrodes in place. Cyanoacrylate dissolves slowly in acetic acid. We placed two fully-encapsulated A3028B transmitters in acetic acid at 60°C for a week. They still functioned perfectly afterwards. Silicone resists acetic acid very well. We have heard no reports of animals reacting badly to silicone that has been soaked in acetic acid.
Our customers often use ethanol to sterilize their transmitters before implantation. Soaking in hot ethanol for three days causes visible stress to our silicone encapsulation. But soaking in ethanol at room temperature causes no damage. You may use room-temperature ethanol freely as a disinfectant on our transmitters and leads.
Most of our customers use cold-cure dental cement to construct head fixtures to hold screws, pins, optical fibers, and guide cannulas in place on the skulls of their laboratory animals. Dental cement dissolves over-night at room temperature in acetone, so we would like to use acetone to remove dental cement from explanted transmitters and their leads. Acetone at 60°C damages our silicone coatings within twenty-four hours. At 60°C, silicone swells up as it dissolves one quarter of its mass of acetone. Room temperature acetone acts far more slowy. We recommend soaking in 20°C acetone for six hours in order to dissolve dental cement. Place the transmitter with head fixture in a jar of acetone. After six hours, shake vigorously. Wash the transmitter in fresh, clean acetone twice, so as to remove all traces of dissolved cement. Leave the transmitter in air for a day to allow any acetone dissolved in the silicone to evaporate. For further details of our tests with silicone and acetone see Development of Flexible Wires.
[06-JAN-25] For a chronicle of our work on flexible leads for implantation, including our studies of fatigue and corrosion, see Development of Flexible Wires.
