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Floating Microelectrode Arrays
| FMA: Frequently Asked Questions |
Arrays
Connectors
Implantation
Arrays
Can you make the microelectrodes to have different lengths within an individual array?
Yes, electrodes can be made to be any length between 0.5 mm to 10 mm long within an array. Use wiring nomenclature to specify individual lengths by position in the array. Back to top
What metals can I select for the electrodes?
At this time we are offering either platinum/iridium, 70%/30%, or pure iridium as the electrode's solid core conductors. Although tungesten is slightly stiffer, it is not considered by most investigators to be as inert or biocompatible. Back to top
What are the impedance values of the electrodes?
The impedance values may be selected anywhere between 10 kilohm to 2.5 megohm depending on your application. Impedance values may also be different for different electrodes within an array. You may also choose to include an electrode with virtually zero impedance for ground or common, which is an unisulated electrode. Back to top
How large are the electrode tips?
Typically the microelectrode tips are between 2 to 3 microns for most Pt/Ir electrodes. However, this value can be specified for special applications. As an example, we are currently supplying pure iridium electrodes for a visual prosthesis study that has a diameter on the order of 5 to 6 microns. Back to top
What is the insulation over the electrodes?
The microelectrodes are insulated with Parylene-C, a biocompatible polymer that has been successfully used for many years as the primary insulator for many implantable devices and electrodes. Back to top
Why do you show 2 electrodes with much longer lengths?
For FMAs requriing microelectrode lengths less than 2 mm we recommend that at least 2 electrodes have lengths of 2.5 mm to act as anchoring pins. the anchoring pins provide needed support to insure that the array remains securely anchored in the brain or spinal cord. These electrodes are typically chosen to be the reference and/or ground electrodes. Back to top
Can I use theses arrays for either acute or chronic studies?
Yes, although the FMAs ere primarily designed for long-term chronic studies, they may also be used for acute studes where one needs to insure that the array will float with the brain during recording or stimulation protocols. If brain movement is not an issue, investigators typically use MPI's Multi-Electrode Arrays. Back to top
Can I select to have less than 18 electrodes?
Yes, 4 is the minimum number of electrodes that may be ordered within a single FMA. Back to top
What is the tolerance of the specified electrode length?
At this time we can guarantee tha specified electrode length to be within 200 microns. Back to top
Can I reuse the arrays?
You can reuse an array used for acute studies. It is recommended you clean the array in 50% bleach and distilled water. Chronically used FMAs may typically not be reused because of tightly bound connective tissue around the FMA cable. Back to top
Can I specify an array with less or more electodes having a substrate gerometry different from the 1.95 x 2.45 mm size that is offered?
Yes, you can specify differ layout geometries that also require a different number of electrodes. We have provided custom designed ceramic substrates for other users. There is a one-tim nonreoccuring-engineering, NRE, charge of $3,800.00 (typical) to have your own layout an implant geometry designed for you particular application. Inner electrode spacing must be within 250 to 1000 microns constraints. Please allow 2 to 3 months for delivery of FMAs with custom designed ceramic substrates. Back to top
Connectors
Can you make the FMAs with different connectors?
No, at this time MPI uses custom nano connectors made by Omnetics Crop., designed for sonic bonding techniques needed to attach the fine gold wires form the FMA. MPI may provide interface cables that can be attached between the OMnetics A8141-001 type design of the FMA and your headstage amplifier connector. Back to top
What are the available cable lengths?
The cable lengths can be specified from 1.5 cm to 12 cm.
How can I minimize the cable tethering forces?
Putting a slight kink in the cable between where the cable exits the bone edge and the implanted array will help provide necessary slack during potential brain to skull movements. Also, many investigators have found that by appplying a very small aount of highly viscous biocompatible cyanoacrylate glue over the cable about 2 to 3 mm from the implanted array and securing the cable to the pail surface is very effective in eliminating potential movement. Keeping the animal relatively immobile during the first few days after the implant is also recommended. This assists in the natural encapusulation of the array at the surface of the brain due to normal tissue adhesions that occur around any foreign body implant. Back to top
What type of wire is used from the array to the connector?
We use 0.001" (25 micron) gold wire from the ceramic substrate array to the connector. The cable is over-coated with MDX4-4210 silicone elastomer for flexibility and strength. Back to top
Implantation
What is the best way to implant the array?
The primay concern when handling the FMA prior and during implantation is to insure that the microelectrodes do not touch anything other than soft tissue. The microelectrode tips are very fragile and may bend if the electrode tips are allowed to come in contact with bone or any other hard surface. It is imperative the the microelectrodes be implanted perpendicular to the surface of the brain to insure minimal tissue disruption during insertion. MPI offers a vacuum insterter system that is described in the User Instructions, which is the preferred method of inserting the arrays. Back to top
How fast should I insert the array into the brain?
It is recommended that the FMAs be inserted very slowly. The FMA should be lowered until there is only a small amount of dimpling of the the brain. Allow the electrodes to penetrate the pial membrance before advancing the FMA further. Slowly lower the FMA into the brain stopping the advancement every few hundred microns to allow the pial membrane to relace after insertion. The concept of this procedure is to advance the FMA at a slow enough speed so as not to depress the pial membrane, which will compress the brain, during insertion of the electrodes. Back to top
Is their tissue damage resulting from the insertion?
Any tissue damage can be almost completely avoided by insureing the following precautions. First, try to avoid placing the electrodes over any blood vessels when initially positioning the array for implantation. It is important to ensure that the electrodes are as close to perpendicular to the brain's surface as possible before lowering the array into the brain. Lastly, lower the array as slow as possible into the brain. Back to top
What parts of the inserter tool can be sterilized?
The tubing including the inserter wand can all be sterilized although in most instances it usually is only necessary to sterilize the tubing and leading form the wand. Back to top
Can I implant the array through dura?
It is possible to implant the array through dura for smaller animals, although this is not advised for chronic studies. One reason for this is that there maybe lateral realative movement between the pial and sura membranes, which may cause the elctrodes tips to move causing local adhesions near the tip. Back to top
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