(10) A Promising Unidirectional Lead for Neuromodulation

Introduction: The future of neuromodulation relies on neural interfaces that deliver precise stimulation. Spinal Cord Stimulation (SCS) is an example of these advances, in which innovative electrode technologies play a key role. Using Supersonic Cluster Beam Implantation (SCBI), a low-impedance electrode was previously developed for neuromonitoring, showing superior Signal-to-Noise Ratio (SNR) due to reduced interface noise [1]. Building on this success, a multicolumn unidirectional expandable lead was designed for SCS using SCBI [2]. Unlike Cylindrical Leads (CL) that deliver current both towards and away from the spinal cord, this new device was realized to generate unidirectional electric fields only towards the dorsal columns, giving it the characteristic of a Unidirectional Lead (UL). Therefore, due to its features, we wanted to demonstrate that UL improves stimulation performance compared to the CL. A computational model was developed to simulate the electrical field distribution when stimulating with either UL or CL leads. These results were then validated in an animal model.

Methods: Applying 1 mA of current, a computational model simulated the current distribution and electric field in the spinal cord, providing a comprehensive evaluation of UL performance compared with a standard CL. To validate the results from the computational model, an in-vivo experiment on sheep was conducted. In the animal experiments the performance of the two leads was evaluated by comparing the amplitude of Evoked Compound Action Potentials (ECAPs) elicited by the UL and CL with the same stimulation current. Stimulation thresholds and ECAPs amplitudes were measured.

Results: The computational model showed that the stimulating current from the UL propagates deeper in the spinal grey matter when compared to the CL stimulation (Figure 1). This suggests also more efficient stimulation of the white matter from the UL. These results were confirmed by those observed in the animal experiments, where the ECAPs evoked by the UL with 1 mA stimulus were greater than those evoked by the CL with the same stimulation intensity (mean + SD: 58.86 ± 3.39 µV, 35.49 ± 4.62 µV, respectively; Figure 2). Accordingly, the threshold for eliciting ECAPs was lower for the UL with respect to the CL ( < 0.7 mA vs. 1 mA, respectively).

Conclusion: The integration of SCBI technology into the UL increases its stimulation performance leading to superior SNR. This innovation sets a new benchmark for more energy-efficient neural interfaces in next-generation therapies. These results also suggest applications beyond SCS, including brain-computer interfaces and peripheral nerve stimulation.