IMPLANTED ELECTRODES 



mechanically strong enough, and of suitable shape, to be pushed through 

 overlying tissues ; (b) they should be extremely well insulated except at the 

 sip and designed so that they can be held in the clamp of some form of 

 mechanical stereotaxic instrument in order that they can be placed accurately 

 in position in the depths of the tissue. 



Platinum or silver wire insulated with glass capillary tubing and shellac 

 or synthetic resin are commonly used for stimulating electrodes (Harris^), 

 the glass providing both the insulation and the rigidity. This type of 

 electrode will penetrate brain tissue readily but is easily damaged by 

 contact with bone. 



To avoid damage to the electrode which may occur if it encounters an 

 unexpected resistance the wire should be mounted on a sprung platform 

 which can be watched during insertion (Figure 36.1). Unusual stress on 



Sealing wax 



Carrier of stereotaxic instrument 



Sealing wax 

 Spring 



^y^-lead 



Insulation 



^^^Electrode tip 



Figure 36.1 Sprung electrode base 



the electrode will bend the spring before the electrode breaks and this is 

 particularly useful if the tip of the electrode is situated near the skull floor. 



When electrodes have to be implanted and left in situ for long periods, 

 for example during investigations of the effect of stimuli on conscious 

 animals, the reaction of the tissues to the presence of the electrode must 

 be considered. Any strong tissue reaction will cause the electrode to become 

 surrounded by inflammatory cells which will alter markedly the relationship 

 of the electrode to the normal tissues. Chronically implanted electrodes 

 should be as far as possible both chemically and mechanically inert. 

 Platinum and gold satisfy these requirements but gold is too soft for general 

 purposes. Most chronically imbedded electrodes are made of platinum or 

 platinum-iridium alloy, although stainless steel, tungsten and silver are 

 also employed. Silver may give rise to black deposits in the tissues after 

 several weeks (Harris^) which are probably silver sulphide. 



The surface of an implanted metal electrode should be carefully polished 

 before insertion to eliminate roughness, and the insulating material must be 

 non-toxic and as far as possible non-irritant to tissues. It is an added 



571 



