Bipolar Electrodes

A bipolar electrode configuration is less complicated to implement than a tripolar electrode. It may be better suited for clinical use than the monopolar configurations because it can be useful at shorter electrode lengths.
An experimental asymmetric electrode with two pairs of contacts is shown in the figure. A.D. is the internal cuff diameter of the primary Anodes (1 and 4) and C.D. is the internal cuff diameter of the primary Cathodes (2 and 3). L-s is the length of insulation housing the Anodes. Red arrows show the current paths from anode to cathode, internal and external to the insulating cuff.
Electrode was placed around nerve to the Medial Gastrocnemius. EMG and muscle force were recorded during electrical stimulation. Orthodromic conduction block was tested with contact 3 as cathode and 4 as the anode (‘Arrest End’). Antidromic escape was tested with contacts 1 and 2 (‘escape end’).

Example of twitch force, (normalized to maximum elicited by supramaximal rectangular pulse) recorded from Medial Gastrocnemius is shown in the figure, plotted against the stimulus current amplitude. The pulse width of the plateau phase was 300 µsec. In addition, the exponential decay phase was 500 µsec. Filled circles are the results for the ‘arrest end’, with contacts 3 and 4. Open circles are the results for the ‘escape end’, with contacts 1 and 2.
For the ‘arrest end’, force started to decline at 0.28 mA, and was down to 10% at 0.48 mA. Conduction arrest at the ‘escape end’ resulted in a fall of 90% of maximum force at 0.69 mA. The ‘block window’ is defined by the 90% maximal current at the ‘escape end’ minus the 10% maximal current at the ‘arrest end’.

Sweeney, J. D. and J. T. Mortimer (1986). “An asymmetric two electrode cuff for generation of unidirectionally propagated action potentials.” IEEE Trans Biomed Eng. 33(6): 541-9.