On the Reliability of Chronically Implanted Neural Stimulation Electrodes in Peripheral Nerves

Micro-fabricated polyimide-based peripheral nerve interfaces (PNIs) with their key advantages like chemical inertness, mechanical flexibility and low water uptake are increasing in importance within translational research and treatment of phantom limb pain (PLP) of amputees. Amongst other PNIs, the transverse intrafascicular multichannel electrode (TIME) presents the most promising features. Higher stimulation selectivity can be achieved at the expenses of higher invasiveness.

The generation TIME-3H was implanted percutaneously in a first-in-human clinical trial for 30 days in the median and ulnar nerves of an upper limb amputee and showed outstanding clinical outcomes. In depth evaluation of the electrical performance in vivo during the clinical trial and analysis of device integrity post explantation was conducted. Measures, including the introduction of adhesion promoting layers to the thin-film compound, design changes for intrinsic stress reduction and optimization of the implant assembly were introduced to increase the long-term stability for chronic application of up to six months.

The next generation of TIMEs was implanted in the median and ulnar nerves of three trans-radial amputees. The implants exhibited reliable performance in vivo in terms of eliciting sensation and staying within the electrochemical safe limits. In depth analysis after termination of the clinical trials and after explantation of the devices showed no corrosion or morphological changes of the thin-film metallization. Mechanical failures could be traced back exclusively to mechanical manipulation during explantation or handling afterwards.

From a clinical perspective, the TIME implants restored sensory feedback in patients allowing performances of a new range of tasks in combination with a sensorized myoelectric hand prosthesis while reducing PLP. The TIME implants proved their readiness for a fully implantable system, which would empower patients with a higher level of independence.