Selecting the Optimum Electrode Configuration

The following graph and metal selection guide will allow you to select one of four available tip profiles and the metal type most appropriate for your application. 

TIP PROFILE - What tip profile is best for my application?  
The tip profile is a subtle specification which will determine the ultimate success of your recording or stimulation paradigm. It is however, a microprobe parameter for which there is some debate. Many first time users will want to experiment with different tip profiles in order to see which one works best for their recording or stimulation protocols.

A - Standard tip profile (25:1 Taper)
Tungsten - Platinum/Iridium - Pure Iridium -
Stainless Steel

H - Heat treated tapered tip profile 
Tungsten - Platinum/Iridium - Pure Iridium -
Stainless Steel

 F - Extra fine tip profile (40:1 Taper)
Tungsten only


B - Blunted tip profile (25:1 Taper)
Tungsten - Platinum/Iridium - Pure Iridium -
Stainless Steel

A  Standard tip microprobes are the most widely used tip profile. It works best for most recording situations and is recommended for stimulation. The insulation at the tip of the microprobe is abruptly sheared using a proprietary high voltage arcing technique, leaving a clean exposed metal surface.

H  Heat treated tapered configuration should be used mainly by those investigators who must penetrate their microprobes through tough membranes such as the dura mater of larger mammals. By applying a small local heat source near the electrode tip under a microscope we are able to provide a microprobe that has a more gradual tip profile than the "A" type. The heat treatment also will have the effect of toughening the insulation near the tip making it less likely to push back when penetrating tougher membranes. This profile is not recommended for chronic studies because the heated portion of the Parylene-C at the tip maybe unstable in the biological environment and degrade after several days or weeks of implantation causing the impedance to drop.

F  Extra fine tip profile is only available in the (3" X 0.005") tungsten microprobe, WE3001X.XF series, intended for generally shallow penetrations where one requires extremely good isolation. For penetrations greater than 3 microns in which the tip impedance is greater than 1.5 megohms the addition of polymide tubing (WE3PT1X.XF) series is recommended to reduce capacitive shunting.

B  Blunted is a recent addition to our inventory as a result in feedback from many of our users. By making the tip bullet shaped the electrode's exposed length is less than it would be if it was sharp for the same impedance. Isolation can therefore be improved in some situations since the exposed tip looks more like a point source. Many investigators feel that this provides greater selectivity than the conventional sharper tip profiles and that they hold up better for heavy stimulation protocols. Some investigators have also reported that they feel there is less puncturing of cells with the blunted tips.

METAL TYPE  -   What metal type is best for my application?  

WE  Tungsten is the most versatile and widely used probe material because of it's stiffness, biocompatibility and cost. It is ideal for demanding recording situations and most stimulation protocols. Tungsten comes in a variety of lengths and widths including the very popular 127mm (5") electrode which is used when recording from deeper structures in the brain. It is also used extensively for "Deep Brain" studies in Parkinson disease and other studies requiring an electrode configuration longer than 203mm (8").

PI   Platinum/Iridium is extremely inert and is much more resistant to corrosion than either tungsten or stainless steel when used in extensive stimulation protocols. Platinum/Iridium also has a lower concomitant tip impedance than tungsten or stainless steel and therefore has a lower tip impedance for the same amount of metal exposure. This normally equates to a higher signal to noise ratio when recording. Because it is extremely biocompatible it is an excellent selection for chronic implants.

SS   Stainless Steel (Elgiloy) is widely used in doing "Vibrating Probe Studies" because of its stiffness and it can be easily electrochemically coated with other metals used in these type of studies. Our stainless steel is very unique in that it is an alloy, Elgiloy, which is used for dental implants, so it is also very corrosion resistant and biocompatible. It has also been used extensively for Prussian blue staining by passing very low dc anodal current through it for histological marking.

IR   Pure Iridium is the newest addition to our line of microelectrodes. Iridium has recently found its way into many journal articles especially for chronic implants and devices being tested for potential neural prosthesis. Iridium has by far the lowest concomitant tip impedance of any of the noble metals. It is extremely inert and very resistant to corrosion. Electrochemical cyclic voltametry has been used to "activate" the surface of electrodes to increase charge storage capabilities up to an order of magnitude over bright iridium making it an excellent stimulating electrode. A similar decrease in impedance is also effected by the activation techniques

ELECTRODE STYLE - What Electrode configuration do I need?

We presently offer three different electrode configurations at this time, although we have fabricated many custom designs for customers in the past. As you observe what our part numbers look like for our probes, as seen under our Product section, you will notice they have a part number like WE30031.0A5. The 00 portion of the part number specifies the microprobe configuration.

 

ST specifies our bipolar or stereotrodes. These electrodes when ordered with impedances less than 0.5 megohms are excellent for localizing stimulation current fields. Higher impedance stereotrodes are excellent for enhancing the isolation of single neural elements by simultaneous recording of multiple units on two closely spaced microelectrodes. The tip spacing is typically equal to the shaft diameter of one of the electrodes used in making the stereotrode. Different tip spacing is available upon request.  

INSULATION THICKNESS - What is the insulation thickness?  
All except the 76mm (3") Extra Fine-F profile tungsten microprobe, which has a 1 micron coat of Parylene-C insulation, have 3 microns of Parylene-C. It has been proven that this thickness works best for most all electrode tip profiles we offer. MPI selected 3 microns to provide a sufficiently small tip profile for getting close to neural elements, ease of electrode insertion and to minimize attenuation for higher impedance electrodes. Attenuation of the signal can occur as a result of capacitive shunting when recording with higher impedance microprobes in deep structures, so additional insulation maybe required in the form of our PT, polymide microprobes. The F-Extra Fine profile for the 3 inch tungsten electrodes provides an extremely fine microprobe tip which is excellent when recording from small densely packed cell structures.

TIP IMPEDANCE - What tip impedance or exposure do I need?  
Because of MPI´s unique fabrication process and the special properties of Parylene-C MPI is able to expose any microprobe with microscopic precision and reproducibility. Each microprobe is individually exposed under a high power microscope, inspected and electrically characterized. MPI´s microprobes have a lower impedance value for the same tip exposure as other commercially available electrodes. It is therefore recommended that those who have not used MPI´s electrodes before specify a range of impedance in order to select the best impedance value for their application. There is no additional charge for specifying a range of impedance values for any box of microprobes.

SHAFT DIAMETER - What Shaft diameter is best for my application?
MPI offers three different microprobe diameters, 76µm (0.003"), 127µm (0.005") and 254µm (0.01"). Whereas the tungsten electrodes are offered in all three diameters, Platinum/Iridium is offered in 254µm diameter due to stiffness requirements, Stainless Steel (Elgiloy) is actually 229µm dia. and the Pure Iridium microprobes use 76µm dia. iridium rod. The 76µm diameter tungsten microprobe is fairly new and was developed in response from researchers who wanted a microprobe which produced minimal tissue damage especially with regard to chronic implantation. Also many users wanted to implant a large number of microprobes within a small area where the overall size of the electrode is critical. The 127µm microprobe is the most common size for tungsten users although many users require the heavier 254µm dia. microprobe for penetrating tougher tissue and are viewed as being slightly less fragile.  

What type of connectors are used with MPI's electrodes? The M201 pin connectors are attached to the distal end of MPI´s electrodes. You can purchase these connectors as well as the mating connector  M202. Many users prefer to use MPI´s electrodes without any connector, which is fine. It is simple to remove the connectors for you if requested. There is no discount for this since the connectors are attached at the beginning of fabrication process.

Concentric  Bipolar Electrodes

Concentric  bipolar electrodes are excelent for shielded macro recording as wellas evoken potentials. They rae especially well suited for bipolar stimlation paradigms.

Concentric  bipolar electrode with exposed metal shield

Insulated stainless steel shield with exposed concentric surface

Concentric  bipolar electrodes can be ordered with 76mm (3") or 127mm (5") long Tungsten or 51mm (2") Platinum/Iridium core conductors. The electrode is etched to a sharp point. the outer stainless steel conductor can be specified exposed or insulated with Polimide tubing. 'W' is the overall outside diameter, 'X' is the outer diameter of the inner Polimide sleeve, and 'Y' is the length of the exposed core conductor.
 

EMG "Patch" Electrodes

The most selective and non-damaging implantable EMG electrodes available. 

Easily, securely affixed patch holds contacts in optimal bipolar configuration against desired muscle while “shielding” out cross-talk source on the opposite side.

EP105  single sided,    x = 5mm                 EP205            double sided,    x = 5mm

Special designs quoted on request  - send sketch.
Custom options include multiple contacts on one or both sides, special orientations and intervals.

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