As suggested by this tabulation, close control of current density is of primary importance in the relatively long-term etching during the first three stages. When etching is complete, any residual sodium chloride is removed by sonication with distilled water.
The etched and cleaned iridium wire is then carefully cut into segments of about one centimeter (depending on the desired length of the final electrode) in length. Each segment is then seated in another pin-type connector 11 as already described, and soldered in place. The cleaning procedure described above is performed after completion of soldering.
The next step is to form a conical taper on the free end of the iridium wire segment. Supporting connector 11 is secured to a brass block of generally the same style as block 12, and which is again mounted in stand 30 as shown in FIG. 2 to position the wire tip just above the surface of etching solution 16. The precision scissors jack is then slowly elevated until the very end of the wire tip contacts solution 16 as signaled by an indication on ammeter 25 of a small current flow. Switch 26 is then opened, and dial indicator 34 is zeroed.
The scissors jack is then elevated about 350 microns to immerse the lower end of the wire in the etching solution, and switch 26 is closed to apply about 20 volts across the wire and carbon electrode 18. Etching is continued until current flow drops to zero, indicting that the lower end of the wire has been etched away. The purpose of this step is to eliminate residual stress in the wire as caused by cutting the wire into segments.
With clamp 33 stopped against further downward travel (but free to be raised by arm 32), the precision scissors jack is again elevated to immerse about 350 microns of the lower end of the wire in the etching solution. Switch 26 is closed to apply a potential of about 20 volts between the wire and electrode 18, and the wire lower end is dipped in and out of the etching solution by raising and lowering clamp 33 for about 7 seconds at a rate of 4 dips per second. Microscopic inspection will show that the lower end is now conically shaped (the core typically has an included angle of about 7 degrees) with a sharp tip.
In some applications, it is desirable to round the tip of the conical wire end. This is easily done while the brass block and wire are still mounted on stand 30. A potential of about 12 volts is applied between the wire and electrode 18, and about 100 microns of the sharp lower end of the wire is immersed in the etching solution. Switch 26 is then activated through ten on-off cycles of one second on and one second off. This procedure will round the end of the conical tip to a radius of curvature of about 2 microns.
The next step is to weld a platinum lead wire to a now formed and conically tipped electrode 37 (FIG. 4). Preferably, commercially available pure platinum wire of 0.002-inch diameter is used for the lead wire. Referring to FIG. 3, a deinsulated end of such lead wire 38 is coiled around a mandrel 39 having a diameter (e.g., 35 microns) corresponding to the shank diameter of the electrode. The shank of mandrel 39 is secured to a brass block 40 for ease of handling. When a lead-wire coil 41 is so formed, a protruding end 42 is snipped off.
Referring to FIG. 4, electrode 37 and connector 11 are clamped in a brass holding block 43, and preferably two
4
electrodes are so mounted as shown in the drawing. Grooves 44 are formed in opposite side edges of the block to receive the free end of each lead wire. Lead-wire coils 41 are then slipped over the electrode shank. The block is then secured
5 to a micromanipulator 46 as shown in FIG. 5, and the electrode and coil positioned between welding tips 47 of a precision spot welder 48.
The welding tips are over the approximate center of coil 41, and the welder is actuated at about 2.4 watt-seconds with a tip pressure of about 0.25 kilograms. After welding, the
10 unwelded end of the coil is cut and removed, and the weld junction is sonicated in detergent (Micro) for about one minute to remove weld residue, followed by three 30-second cycles of deionized-water sonification to remove any residual detergent.
15 Most applications require insulation of the electrode shank so only the end of the conical electrode tip is conductive, the electrodes remain seated on block 43, and a high-temperature baking varnish (Epoxylite 6001-50 is suitable) is placed in a sonicating bath. The electrodes are
20 then immersed in the agitated varnish, and slowly withdrawn during sonication to prevent formation of varnish bubbles.
The coated electrodes are then placed in a vacuum oven which is evacuated to -84 kPa, and maintained at that low
25 pressure for degassing for about 15 minutes, followed by slowly increasing the pressure to ambient. The electrodes are then placed in an oven which is initially heated to about 110° C. for 30 minutes for further degassing, with temperature thereafter increased to 165° C. for another 30 minutes for
30 baking of the varnish film.
Because the resulting insulating film is very thin, the coating procedure is repeated two times. The second coating cycle is identical to the first as already described, and the third cycle differs only in that sonification is terminated as
35 the electrodes are withdrawn, and the resulting thicker coating is based at 165° C. for 60 minutes instead of 30 minutes.
Because the tips as well as the shanks of the electrodes are now varnish coated, it is necessary to deinsulate the tips, and
40 this is most conveniently done by laser ablation. An ErYAG laser 51 of 2.97 micron wave length is suitable (and available from Premier Laser Company), and the equipment setup is shown in FIG. 6. Electrode 37 as still mounted on block 43 is secured to a three-axis stereotaxic microman
45 ipulator 52 with a precision X-axis depth micrometer 53. A stainless-steel shield 54 covers the electrode except for the tip to be ablated.
The laser is then set at an energy level of 50 mJ, and a pulse rate of 10 pps. The laser is actuated for 5 seconds, and
50 then the electrode is rotated 180 degrees so the back surface can be similarly ablated. Preferably, the laser ablation is performed in a helium environment.
FIG. 7 illustrates a now-deinsulated rounded conical tip 56 of electrode 37, with insulation 57 remaining on the
55 remainder of the electrode. The now-completed electrodes can be used individually, or in arrays, and can also be activated by forming a layer of iridium oxide on the uninsulated tip.
What is claimed is:
60 1. A method for reducing the diameter of iridium wire to less than about 50 microns for making neurological electrodes, comprising the step of electrolytically etching the wire in a saturated solution of sodium chloride by an electrical current flowing through the solution and wire.
65 2. The method defined in claim 1 in which the electrical current is in the range of 95 to 110 milliamperes per square centimeter of wire exposed to the etching solution.
