Objective . Trauma induced by the insertion of microelectrodes into cortical neural tissue is a significant problem. Further, micromotion and mechanical mismatch between microelectrode probes and neural tissue is implicated in an adverse foreign body response (FBR). Hence, intracortical ultra-microelectrode probes have been proposed as alternatives that minimize this FBR. However, significant challenges in implanting these flexible probes remain. We investigated the insertion mechanics of amorphous silicon carbide (a-SiC) probes with a view to defining probe geometries that can be inserted into cortex without buckling. Approach . We determined the critical buckling force of a-SiC probes as a function of probe geometry and then characterized the buckling behavior of these probes by measuring force–displacement responses during insertion into agarose gel and rat cortex. Main results. Insertion forces for a range of probe geometries were determined and compared with critical buckling forces to establish geometries that should avoid buckling during implantation into brain. The studies show that slower insertion speeds reduce the maximum insertion force for single-shank probes but increase cortical dimpling during insertion of multi-shank probes. Significance. Our results provide a guide for selecting probe geometries and insertion speeds that allow unaided implantation of probes into rat cortex. The design approach is applicable to other animal models where insertion of intracortical probes to a depth of 2 mm is required.

Objective . Chronically implanted microelectrode arrays (MEAs) are used for stimulating and recording neural activity in research and clinical settings. However, their reliability can be compromised by insufficient encapsulation stability. Amorphous silicon carbide (a-SiC), a chemically stable and biocompatible material, has emerged as a potential thin-film encapsulation for MEAs. We aimed to evaluate thin-film a-SiC encapsulation using electrical-accelerated aging (EAA) and to demonstrate a methodology for obtaining acceleration factors for EAA by Weibull analysis. Approach . Interdigitated electrodes (IDEs) encapsulated with a-SiC were subjected to voltage cycling and stepped-voltage protocols to measure leakage currents in buffered saline at 37 °C. EAA employed incrementally increasing voltage biases over time to induce degradation and reveal failure mechanisms. Main results . IDEs exhibited a significant change in electrical behavior on exposure to saline, with failure initiating at specific voltages and accompanied by gas evolution at defect sites. Incremental voltage biasing revealed a capacitive-to-faradaic transition in leakage current response that was used as a failure criterion. Significance . Acceleration factors for voltage-driven accelerated aging of a-SiC thin-film encapsulation can be obtained by Weibull analysis using a mechanistic failure criterion. Breakdown occurs at processing-related defects in the a-SiC. This study demonstrates the use of EAA for evaluating failure in a-SiC thin-film encapsulation used in implantable MEAs. EAA is broadly applicable to thin-film MEAs and provides a highly relevant method of predicting implanted lifetimes of bioelectronics.

This study presents methods for evaluating the long-term electrochemical stability of Utah electrode arrays (UEAs) encapsulated with amorphous silicon carbide (a-SiC) or Parylene-C for intracortical microstimulation (ICMS). UEAs were implanted in rat motor cortex and monitored for 25 weeks using electrochemical impedance spectroscopy (EIS) to observe device impedance, cyclic voltammetry (CV) to obtain cathodic charge-storage capacity ( Q _stor,c ), and voltage transient (VT) measurements to derive maximum charge-injection capacity ( Q _inj ). Observed shifts in the open-circuit potential of the return electrode during stimulation, from in vitro to in vivo conditions, highlight the need to adjust potential limits when using a quasi-reference electrode. Both encapsulation materials exhibited stable impedance and maximum ( Q _inj ) trends in vivo , becoming similar after approximately 16 weeks post-implant. High-scan rate CV (up to 500,000 mV/s) was used to assess similarities between Q _stor,c and maximum Q _inj and determine if it can be used to estimate the maximum Q _inj . These methods can be used to evaluate the encapsulation and stimulation properties of chronically implanted neural electrodes.

Insertion of microelectrode arrays (MEAs) into brain tissue remains a mechanical challenge, especially for long, thin probes designed to access deep structures. This study investigates the mechanical properties of 5 mm long amorphous silicon carbide (a-SiC) probes with different geometries and the effect of vibration-assisted insertion on penetration into rat brain. Methods: Two planar a-SiC probe designs were fabricated with identical lengths and thicknesses but differing width geometries: one with a uniform width (175 µm) and the other with a tapered shape (tapering from 175 to 75 µm). Critical buckling forces (FCs) were estimated by finite element modeling (FEM) and validated experimentally. Insertion mechanics were assessed in a brain mimic of 1.2% agarose gel at varying insertion speeds (20–1000 µm/s) and in vivo by implantation in rat cortex. Insertion metrics included penetration force (F_P), cortical dimpling depth (D_d), maximum insertion force (F_max), and success rate of insertion, all evaluated with and without vibrational assistance. Results: The tapered design exhibited lower penetration force and higher insertion success compared to the uniform-width probe, despite having a lower critical buckling force. An optimal insertion rate of 100 µm/s was identified, balancing insertion time with low F_max and high insertion success across designs. Higher F_P and D_d with a lower success rate were observed for uniform probes compared with tapered probes in rat brain. Vibration-assisted insertion was then investigated with tapered probes. Applying vibration significantly reduced F_P, whereas D_d and F_max remained unchanged. Notably, in 40% of actuated insertions in rat, no detectable F_P peak was observed, suggesting unimpeded pial penetration. Conclusions: A tapered probe geometry and vibration-assisted insertion can reduce F_max and F_P while enhancing the insertion success rate for probe penetration in rat brain. These strategies are generally applicable to long-shank MEA insertions in brain and may inform design and insertion strategies.

Microelectrode arrays are used for neural stimulation and recording; however, their performance is limited by the high impedance and low charge-injection capacity of conventional metallic interfaces. To overcome these challenges, a variety of coatings, including metal oxides, carbon nanotubes, and conductive polymers, are applied to improve the electrochemical performance of microelectrode arrays. Among these materials, poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) has gained significant attention due to its low impedance, high charge-injection capacity, and compatibility with microfabrication processes. Despite these advantages, the long-term electrochemical stability of PEDOT:PSS at high levels of stimulation charge injection remains insufficiently understood. In this study, we investigated the stability of PEDOT:PSS films electrodeposited on gold microelectrodes for prolonged electrical stimulation. Voltage transient (VT) analysis revealed that PEDOT:PSS degradation occurs when the maximum cathodic excursion (E_mc) exceeds –0.6 V versus Ag|AgCl. The degradation is accompanied by the appearance of asymmetric access voltages and a voltage inversion in the anodal phase of the VT. Electrochemical impedance spectroscopy (EIS) revealed a sharp increase in PEDOT:PSS impedance at negative DC bias potentials, beginning at −0.5 V and increasing as the potential became more negative. These results identified –0.6 V as an electrochemical limit for PEDOT:PSS stability on gold and implicate the reduction and de-doping of PEDOT:PSS, and sequalae, in the observed PEDOT:PSS degradation. Nonetheless, PEDOT:PSS electrodes had stable charge-injection capacities of 3.6 mC/cm² (200 µs pulse width) over the course of 200 million pulses.