Long-term reliability of Al2O3 and Parylene C bilayer encapsulated Utah electrode array based neural interfaces for chronic implantation

Abstract

Objective. We focus on improving the long-term stability and functionality of neural interfaces for chronic implantation by using bilayer encapsulation. Approach. We evaluated the long-term reliability of Utah electrode array (UEA) based neural interfaces encapsulated by 52 nm of atomic layer deposited Al2O3 and 6 µm of Parylene C bilayer, and compared these to devices with the baseline Parylene-only encapsulation. Three variants of arrays including wired, wireless, and active UEAs were used to evaluate this bilayer encapsulation scheme, and were immersed in phosphate buffered saline (PBS) at 57 °C for accelerated lifetime testing. Main results. The median tip impedance of the bilayer encapsulated wired UEAs increased from 60 to 160 kΩ during the 960 days of equivalent soak testing at 37 °C, the opposite trend to that typically observed for Parylene encapsulated devices. The loss of the iridium oxide tip metallization and etching of the silicon tip in PBS solution contributed to the increase of impedance. The lifetime of fully integrated wireless UEAs was also tested using accelerated lifetime measurement techniques. The bilayer coated devices had stable power-up frequencies at ∼910 MHz and constant radio-frequency signal strength of −50 dBm during up to 1044 days (still under testing) of equivalent soaking time at 37 °C. This is a significant improvement over the lifetime of ∼100 days achieved with Parylene-only encapsulation at 37 °C. The preliminary samples of bilayer coated active UEAs with a flip-chip bonded ASIC chip had a steady current draw of ∼3 mA during 228 days of soak testing at 37 °C. An increase in the current draw has been consistently correlated to device failures, so is a sensitive metric for their lifetime. Significance. The trends of increasing electrode impedance of wired devices and performance stability of wireless and active devices support the significantly greater encapsulation performance of this bilayer encapsulation compared with Parylene-only encapsulation. The bilayer encapsulation should significantly improve the in vivo lifetime of neural interfaces for chronic implantation.