r = 4 mm (radius of middle ring, outer side)

Punch CRE (Tripolar, 16mm).f3d
.png)
To size it down from 16mm to 8mm and maintain the same proportions and spacing, we used 0.5 mm wide rings and wires (instead of the 1mm width on the 16mm design).
CRE (Tripolar, 8mm, constant ring width).f3d
.png)
_v1.png)
Concentric Ring Electrodes (Bipolar, 8 mm).f3d
_Concentric_Ring_Electrodes_(Bipolar_8_mm).png)
.png)