Professor The University of Texas at Dallas, United States
Introduction:: Intracortical microelectrode arrays (MEAs) are implantable devices used for neural recording and stimulation. Following MEA implantation into the cortex, the neural recording performance decreases over time. The decline in performance is caused, at least in part, by glial scarring, initiated by microglia migration and astrocyte activation. The use of ultra-thin microelectrode arrays (UMEA) has been proposed as a means to minimize the glial scar. UMEAs have small cross-sectional area, and consequently high flexibility that reduces mechanical mismatch between brain tissue and the electrode, which is expected to minimize persistent foreign body response associated with micromotion after implantation. However, unaided implantation of UMEAs can be challenging because they tend to buckle prior to penetration into the brain. Therefore, this study investigates amorphous silicon carbide (a-SiC) UMEA designs with varying cross-sectional areas and lengths, using to determine those geometries that facilitate unaided implantation into the cortex of rat, pig, and macaque.
Materials and Methods:: First, we evaluated the critical buckling force (Fc) of UMEAs as a function of geometry. We measured UMEAs with 200, and 300 µm2 cross-sectional area and lengths of 1.3, 1.5, and 1.7 mm. We used a precision 20 g S-beam load cell and voltage digitizer to measure the dynamic forces during a-SiC UMEAs buckling against a rigid silicon wafer. The a-SiC UMEAs and load cell were mounted to a hydraulically controlled micro drive to control displacement rate. The UMEA tip was brought into contact with the wafer and the resultant axial force sampled at a rate of 100 Hz while driving the UMEA into the wafer at a rate of 10 µm s−1. We then characterized the penetration force (Fp) into the cortex of rat (n = 3), pig (n = 4), and macaque (n = 1) after exposing the brain through surgical craniotomy and durotomy procedures. Subsequently, the UMEAs were inserted in cortex at a controlled speed of 100 µm s−1 and the axial force measured.
Results, Conclusions, and Discussions:: As expected, increasing the length, and decreasing the cross-sectional area of the UMEAs decreased Fc. Moreover, through the comparison of the Fp and Fc of UMEA, it was observed that buckling is avoided during insertion as long as Fp is lower in magnitude than Fc. We found that a UMEA design with a length of 1.7 mm and cross-sectional area of 200 µm² required the least Fp (0.55 ± 0.304 mN) into the macaque brain. In contrast, in the pig and rat brains, it was the UMEA with a length of 1.5 mm and cross-sectional area of 200 µm² (Fp = 0.7 ± 0.25 mN and, Fp = 0.13 ± 0.14 mN, respectively).
The success rate of penetration of UMEAs was 100% in rat brain; however, the success rate of penetration in pig brain for UMEA with lengths of 1.3, 1.5, and 1.7 mm was 90%, 78% and, 33% for 300 µm2 cross sectional area, and 71%, 60% and, 0% for 200 µm2 cross sectional areas, respectively. We observed 100% success rate in macaque brain penetration for 1.5 and, 1.7 mm length UMEA and 300 µm2 cross sectional area. The success rate for penetration of 200 µm2 cross-sectional-area UMEAs was 50%, and 83% respectively.
This study aimed to investigate the mechanics and buckling behavior of a-SiC UMEAs inserted intracortically through the pia of rat, pig, and macaque animal models. By measuring the critical buckling force and using simple Eulerian mechanical modeling, the study determined an insertion-depth-dependent critical buckling force that could predict whether insertion would occur without buckling based on the UMEAs geometry. Currently, this approach is being used, in conjunction with finite element modeling, to evaluate UMEA designs in silico, prior to experimentation in animal models.
