Effect of Position of the Contacts.

Minimum current to produce arrest in cuffs of 17 - 24 mm overall length occurred when the contact was 5 - 9 mm from the distal end of the cuff.
Current amplitudes for complete orthodromic arrest of propagation (at the distal end of the cuff) in relation to the contact position is shown in the figure. The position (mm) is the distance of the monopolar contact from the end of the cuff. Nerve cuffs of three different internal diameters (1.09, 1.27 and 1.65 mm ) were applied on a nerve of ~ 1 mm diameter. Stimulus pulse widths were between 300 to 400 µsec. Absolute current values and slope increased with larger cuff diameters.

The proximal end of the electrode was electrically isolated by raising it from the saline pool around the nerve. This allowed all the applied current to flow at the distal 'virtual anode'.
As all the current flows distally, they reach a plateau for varying contact distances. Current amplitudes show increase with increasing cuff diameter. These results suggest that the electrode contact for the monopolar cuff should be positioned about 6mm from the end of the cuff. The larger the asymmetry between the arrest end and the escape end the larger the “block window”. An asymmetry of 3 yields a marginally useful window where an asymmetry of 7 is more desirable. With an asymmetry of 7 and electrode would be of the order of 5 cm in length, which may be too long for some applications.
Stimulus current amplitude for complete arrest of evoked APs are shown in the Figure. Results are for two different preparations, with cuff electrodes of three different diameters (1.09, 1.27 and 1.65 mm ). Pulse widths were 350µsec with a minimum exponential decay phase.

Minimum charge injection was with pulse widths between 300 to 400 µsec with falling phase time constant held at a minimum. Electrodes with monopolar contacts closer to the distal ('arrest') end of the cuff required more charge injection for complete arrest.
Charge injected at distal end of cuff for electrodes with varying contact position and diameters shown in figure. The 6.5,11 (asterisk) is with the monopolar contact 6.5 mm from the distal cuff end and 11 mm from the proximal end. The 5,15 (open square) is with the monopolar contact 5 mm from the distal cuff end and 15 mm from the proximal end and 4, 16 (open circle) 4 mm from distal and 16 mm from proximal. Two cuff diameters of 1.2 and 1.8 mm were applied around a nerve of ~ 0.9 mm diameter.

Ungar, I. J., J. T. Mortimer, et al. (1986). “Generation of unidirectionally propagating action potentials using a monopolar electrode cuff.” Ann Biomed Eng 14(5): 437-50.