Implantation mechanics of tungsten microneedles into peripheral nerve trunks

50 microm tungsten microneedles have been used as a means to introduce longitudinal intra-fascicular electrodes (LIFE) into small peripheral nerve fascicles. However, recent attempts to implant LIFEs into larger, human sized nerves with the same needles resulted in buckling failure of the introducer needle. In the present study, the implantation mechanics (penetration forces and penetration dimple depth) of electrosharpened tungsten microneedles ranging in diameters from 50 to 200 microm into freshly excised porcine peripheral nerve trunks between 3 and 5 mm in thickness was characterized to understand the implantation mechanics and to find the minimum needle diameter that would result in successful penetration. The implant success rate was found to be highest with needles having diameters between 80 and 120 microm. The force of successful penetration ranged from 7.2 +/- 0.6 to 71.8 +/- 19.5 mN, and increased monotonically with needle diameter. It also had a tendency to increase with increasing tip angles. The dimple depth for successful penetrations varied between 1 and 1.5 mm, and also tended to increase with increasing tip angles, although it was generally not affected by increased needle diameter. Only the smallest penetration dimple depth was found to be different from the others and was associated with the smallest diameter needle (50 microm). Analysis based on the critical buckling force and the measured implantation forces indicated a 15 mm long needle of 80 microm diameter would be necessary and sufficient to penetrate medium to large sized nerves.