Implantable microelectrode arrays (MEAs) enable the recording of electrical activity of cortical neurons, allowing the development of brain-machine interfaces. However, MEAs show reduced recording capabilities under chronic conditions, prompting the development of novel MEAs that can improve long-term performance. Conventional planar, silicon-based devices and ultra-thin amorphous silicon carbide (a-SiC) MEAs were implanted in the motor cortex of female Sprague–Dawley rats, and weekly anesthetized recordings were made for 16 weeks after implantation. The spectral density and bandpower between 1 and 500 Hz of recordings were compared over the implantation period for both device types. Initially, the bandpower of the a-SiC devices and standard MEAs was comparable. However, the standard MEAs showed a consistent decline in both bandpower and power spectral density throughout the 16 weeks post-implantation, whereas the a-SiC MEAs showed substantially more stable performance. These differences in bandpower and spectral density between standard and a-SiC MEAs were statistically significant from week 6 post-implantation until the end of the study at 16 weeks. These results support the use of ultra-thin a-SiC MEAs to develop chronic, reliable brain-machine interfaces.

Intracortical microstimulation (ICMS) of the somatosensory cortex via penetrating microelectrode arrays (MEAs) can evoke cutaneous and proprioceptive sensations for restoration of perception in individuals with spinal cord injuries. However, ICMS current amplitudes needed to evoke these sensory percepts tend to change over time following implantation. Animal models have been used to investigate the mechanisms by which these changes occur and aid in the development of new engineering strategies to mitigate such changes. Non-human primates are commonly the animal of choice for investigating ICMS, but ethical concerns exist regarding their use. Rodents are a preferred animal model due to their availability, affordability, and ease of handling, but there are limited choices of behavioral tasks for investigating ICMS. In this study, we investigated the application of an innovative behavioral go/no-go paradigm capable of estimating ICMS-evoked sensory perception thresholds in freely moving rats. We divided animals into two groups, one receiving ICMS and a control group receiving auditory tones. Then, we trained the animals to nose-poke – a well-established behavioral task for rats – following either a suprathreshold ICMS current-controlled pulse train or frequency-controlled auditory tone. Animals received a sugar pellet reward when nose-poking correctly. When nose-poking incorrectly, animals received a mild air puff. After animals became proficient in this task, as defined by accuracy, precision, and other performance metrics, they continued to the next phase for perception threshold detection, where we varied the ICMS amplitude using a modified staircase method. Finally, we used non-linear regression to estimate perception thresholds. Results indicated that our behavioral protocol could estimate ICMS perception thresholds based on ~95% accuracy of rat nose-poke responses to the conditioned stimulus. This behavioral paradigm provides a robust methodology for evaluating stimulation-evoked somatosensory percepts in rats comparable to the evaluation of auditory percepts. In future studies, this validated methodology can be used to study the performance of novel MEA device technologies on ICMS-evoked perception threshold stability using freely moving rats or to investigate information processing principles in neural circuits related to sensory perception discrimination.

Objective. Intracortical microelectrode arrays (MEAs) are implantable devices used for neural recording and stimulation. However, their long-term performance is often compromised due, at least in part, to glial scar formation, initiated by microglial migration and astrocyte activation following implantation. To address this issue, ultra-thin MEAs (UMEAs) have been proposed as an alternative due to their reduced cross-sectional area (CSA) and enhanced flexibility that minimizes the mechanical mismatch between brain tissue and the electrode. These properties are expected to mitigate the persistent foreign-body response associated with micromotion. However, unaided implantation of UMEAs can be challenging, as their high flexibility may increase the likelihood of buckling and curtail the precise penetration into the brain. Approach. We investigated flexible amorphous silicon carbide (a-SiC) probe designs with varying CSAs and lengths to identify geometries that enable unsupported implantation into the cerebral cortices of rat, pig, and macaques. The critical buckling force of the a-SiC probes was experimentally determined as a function of geometry and formally described via finite element modeling, which predicted buckling behavior. Additionally, the penetration behavior of a-SiC probes was evaluated by measuring force–displacement responses during insertion into cortical tissues across species. Main results. Our findings demonstrated that the penetration force and cortical dimpling depth were not significantly influenced by the range of probe geometries tested. However, we observed that penetration force and dimpling depth were significantly lower in rat cortices than in larger species. Importantly, probe geometries with a higher ratio (⩾2) of critical buckling force to penetration force exhibited a 100% success rate for unaided insertion. Significance. This study provides a framework for designing and evaluating UMEA geometries to optimize unsupported implantation in both small and large animal brains.

Objective. Our objective was to perform a complete analysis of in-vitro impedance data for sputtered iridium oxide film (SIROF) micro-electrodes. The analysis included quantification of the stochastic and bias error structure and development of a process model that accounted for the chemistry and physics of the electrode–electrolyte interface. Approach. The measurement model program was used to analyze electrochemical impedance spectroscopy (EIS) data for SIROF micro-electrodes at potentials ranging from −0.4 to +0.6 V(Ag|AgCl). The frequency range used for the analysis was that determined to be consistent with the Kramers–Kronig relations. Interpretation of the data was enabled by truncating frequencies at which the ohmic impedance influenced the impedance. Main results. An interpretation model was developed that considered the impedance of the bare surface and the contribution of a porous component, based on the de Levie model of porous electrodes. The influence of iridium oxidation state on impedance was included. The proposed model fit all 36 EIS spectra well. The effective capacitance of the SIROF system ranged from 32 mF cm⁻² at −0.4 V(Ag|AgCl) to a maximum of 93 mF cm⁻² at 0.2 and 0.4 V(Ag|AgCl). Significance. The model developed to interpret the impedance response of neural stimulation electrodes in vitro guides model development for in-vivo studies.

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.