METAL MICROELECTRODES 



with greater care as small as 1 //. These very small tips are not very satis- 

 factory as they are extremely brittle and usually of very high impedance. 



The method of preparation is simple but care is needed to obtain really 

 fine regular points (Bishop and Collier'^, Grundfest, Sengstaken, Oettinger 

 and Gurry^). A steel wire is connected in circuit with a copper plate 

 (negative) in a Petri dish of N hydrochloric acid to a 6 V accumulator. 

 The needle tip is dipped into the dish for 2 minutes and then withdrawn, 

 dipped again, withdrawn and so on. As the process continues the battery 

 voltage should be decreased to 2 V. When a fine enough point has been 

 obtained the needle is disconnected, washed in water and degreased in 

 acetone. When dry it is insulated with Synobel varnish (I.C.I. Ltd.) by 

 dipping the whole electrode into a deep pot of the varnish and withdrawing 

 it slowly. The needle is then turned with its point upwards and allowed 

 to drain, after which it is baked in an oven at 200°C. Several coats of 

 varnish are necessary but even so the extreme tip of the needle often remains 

 uninsulated so that the electrode is at once ready for use. If however the 

 tip is insulated it is only necessary to touch it with a piece of paper to break 

 off" the insulation. 



37 s.w.g. Staybrite steel wire (Johnson, Matthey and Co., London) may 

 be used if the electrodes are to remain in the tissues for more than a few 

 hours, and this necessitates a minimum of 4 V for electrolytic reduction 

 of the tip. 



A tungsten microelectrode has recently been described by Hubel^ which 

 is said to behave as well as microglass electrodes. The advantage of tungsten 

 over steel is that it is much stiffer and therefore the original wire can be 

 very thin but still have good mechanical properties. A tip less than 5 fj, in 

 diameter is suitable for picking up extracellular potentials but for intra- 

 cellular work the tip must be smaller than 1 /u. 



Tungsten wire 125 fx diameter is inserted into a 27 gauge hypodermic 

 needle which is then crimped to hold the wire tightly. The short end of 

 tungsten protruding from the point of the needle is then reduced by electro- 

 polishing to the required diameter. The electrolysis is carried out in a cell 

 containing saturated aqueous potassium nitrite using 2-6 V, a.c, and a 

 nearby carbon electrode. The wire should be raised and lowered as described 

 in the preparation of steel microelectrodes so that a taper is produced on 

 the wire. If raising and lowering is not carried out a very abrupt point 

 will be produced which will not penetrate tissues very readily. When a 

 suitable taper has been achieved the last part of the reducing process should 

 be completed without moving the electrode in the cell in order to make 

 a very fine uniform point. Tips as small as 0-5 to 0-05 fx diameter are said 

 to be obtainable without great practice or skill. The wire should be washed 

 in detergent and after drying coated with clear vinyl lacquer which has 

 been allowed to thicken slightly by exposure to air. The coating is carried 

 out by dipping the electrode into a deep pot of lacquer then withdrawing it 

 and allowing it to drain tip downwards, after which the tip only is continually 

 re-dipped until no further beads of lacquer form on it. It is then dried for 

 24 hours. 



The insulation can be tested for leaks by placing the electrode in 0-9 per 

 cent NaCl and making it the cathode in circuit with a 12 V accumulator. 



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