Nanopatterned Black Silicon Electrodes for High-Charge-Density Intraspinal Microstimulation

Restoring motor and sensory function after spinal cord injury (SCI) through intraspinal microstimulation (ISMS) has primarily focused on gray matter targets, while the longitudinally organized axon tracts of spinal cord white matter remain an underexplored route for activating ascending sensory and descending motor pathways. Tract-selective stimulation requires small electrode sites to confine the stimulation field, but miniaturization increases impedance and electrode polarization, limiting the charge that can be delivered from a fixed geometric footprint. To address this challenge, we developed platinum-coated black silicon (BSi-Pt) microelectrodes, in which cryogenically etched black silicon (BSi) nanostructures are conformally coated with platinum to increase electrochemical surface area within a fixed 30×30 μm² footprint. Relative to thickness-matched, optimized flat Pt controls, BSi-Pt electrodes exhibited a ∼2.2-fold reduction in 1 kHz impedance. Fast- and slow-scan cathodal charge storage capacity (CSC_c) values for BSi-Pt were 3.12 mC/cm² at 250 V/s and 22.3 mC/cm² at 50 mV/s, corresponding to ∼3.8-and ∼2.3-fold increases, respectively. BSi-Pt electrodes achieved a water-window-based charge injection capacity (CIC) of 0.9 mC/cm² and a charge injection limit (CIL) of ∼2.2 mC/cm², defined by the onset of nonlinearity in the input-output response. Finite element analysis (FEA) identified a ground-sheet return configuration that maintained field confinement comparable to a concentric-return configuration while doubling electrode density. Acute proof-of-concept experiments in rat spinal cord (n = 4) evoked propagating activity in the fasciculus gracilis and single-muscle EMG responses from dorsal corticospinal tract stimulation, while continuous pulsing at elevated charge density produced no measurable platinum dissolution. These findings establish BSi-Pt as a manufacturable, CMOS/MEMS-compatible platform for high-density tract-selective ISMS.