ELECTRODES FOR MEASURING OXYGEN TENSION 



Open-ended electrodes can be made with platinum, gold or iridium- 

 platinum wire of any desired size down to 25 fi (iridium/platinum) but 

 gold appears to give the most consistent results although it is too soft for 

 easy insertion into tough organs. Insulation with the epoxy resin Araldite 



Capillary tube 

 Platinum wire 



Open ended electrodes 

 Araldite insulation 



Tip ground flat and smooth 

 to known surface area 



Recess of known 

 volume and length 



Figure 36.4 Oxygen tension electrodes 



985 E (Aero Research Ltd., Duxford) is the most satisfactory as this resin 

 has very good insulating properties, is extremely tough and is shghtly 

 flexible. Several coats are necessary and each must be baked for about 

 10 minutes at 200°C so that it is quite hard before the next layer is applied. 

 The insulated wire is placed in a Tufnol clamp and ground flat at the end 

 by hand, using a very fine hone. Mechanical grinding, e.g. with a jeweller's 

 wheel, is not satisfactory as it tends to spread a thin layer of metal over the 



Suture 



Bare back end 

 of electrode. 



Skin^ 



Skin fold 



Suture 



Electrode 



y//////////y////////7//////////////y/y 



Figure 36.5 A method of fixing an intradermal or subcutaneous electrode 



end of the insulation, thereby increasing the effective area of the exposed 

 end of the wire. It is essential that the electrode tip should be ground quite 

 flat and have a known area if approximately absolute levels of oxygen 

 tension are being measured at the same time as changes. These electrodes 

 are relatively robust and may be inserted into the tissues by hand or 

 mechanically. If changes over several days are to be measured they may be 

 left imbedded in the animal with only a small part projecting through the skin. 

 This may be useful in following the development of inflammatory reactions 

 {Figure 36.5). 



Recessed electrodes are made with platinum wire which is first degassed 

 by heating to white heat to prevent bubble formation in the next stage. 



577 



