## 3 Materials and Methods  
Helical-wire hook electrodes were made from sixty-five strands of thin 316 LVM stainless-steel wires (0.001 inches in diameter each) insulated with Teflon (Med-wire 65, Cooper Wire Co, Chatsworth, CA, USA). A lead was formed into a helix near the stimulating tip by wrapping the lead 10 times around a 21-gauge needle with a tension of 2 to 3 kg (Figure 1A). The stimulating tip consisted of one and a half cm of exposed wire, which was also wrapped into a helix and bent back to form a hook. For comparison, the Permaloc(r) intramuscular electrode, which is used for phrenic nerve stimulation and respiratory management in SCI patients, is shown (Figure 1B). The insertion rod is used with both types of electrodes during the needle insertion. The rod is put into the needle and advanced to the helical area of the electrodes. During insertion, the rod ensures that the electrode tip stays where it is placed in the needle; after insertion, the rod can be held in place and the needle removed without the electrode tip moving. Two isolated electrical stimulators (Model SD-9, Grass Inc, Quincy, MA, USA) were used with 400 us pulse durations, as the 400 us duration was reported to be the most effective in a current-response study of DBWS; also, a high frequency of 40 Hz was used as previously reported [6; 14; 15; 16; 17; 18]. The stimulators were synchronized for simultaneous pulses, and they were connected to the implanted electrodes as two bilateral sets of bipolar electrodes. Stimulation tests consisted of either increasing the current every few seconds or using a single current for 10 to 20 s [18]. The protocol was approved by the Ethics Committee for Research on Animals of the Banat University of Agricultural Sciences and Veterinary Medicine, Timisoara, Romania (Ethics Committee Approval No. 73/30 January 2019, and Project Approval from Romanian Veterian Authority No. 5/5 February 2020). Three healthy Gottingen Minipigs (females, 8 months of age, 20-30 kg; Ellegaard Minipigs, Dalmose, Denmark) were used. Prior to surgery, the minipigs were acclimated for six months to remain in a study cage. General anesthesia was induced as previously described, and a Foley catheter (Ch10, Coloplast, Humlebeaek, Denmark was placed [19]. For creating a defined sacral spinal cord lesion, the pigs were placed prone, and after a 15 cm midline incision with the preparation of the dorsal spinal elements, a two-level-laminectomy (L\({}_{5}\), L\({}_{6}\) and partially S\({}_{1}\)) was performed, the dura was opened longitudinally and the spinal cord with the lumbosacral roots was exposed. The plan was to achieve a complete sacral spinal cord lesion by resecting the conus medullaris (Figure 2), but it was difficult to assess intraoperatively where the conus medullaris ended and the terminus plum (terminal thread) started. For placement of the bladder wall electrodes, a subumbilical midline abdominal incision was performed with the abdominal rectus muscle separated along the midline, and the peritoneum was incised. The ureters were identified in the posterior parietal peritoneum, just over the common iliac artery, and dissected down to their insertion into the bladder wall. Two previously sterilized bilateral sets of bipolar electrodes were implanted, using sites already described by other authors [14; 15; 16]. A caudal set was implanted adjacent to the bladder neurovascular bundle and 3 mm ventral to the ureters. The second bipolar set was implanted 1.5 cm directly rostral to the first set [14; 15; 16]. In the first animal, the  
Figure 1: Photograph comparing the helical-wire hook electrode used in this study to the commercial Permaloc® electrode used for respiratory pacing in SCI patients. (**A**) The lead of the helical-wire hook electrode is shown in a 14-gauge needle. The needle would be advanced up to the exposed hook of the stimulating tip prior to insertion. Needle insertion was used in the first minipig and implantation with imbrication on the bladder wall was used in the last two animals.