| AVS 61st International Symposium & Exhibition | |
| Thin Film | Thursday Sessions |
| Session TF-ThA |
| Session: | Thin Film for Permeation Barriers and Membranes |
| Presenter: | Loren Rieth, University of Utah |
| Authors: | L.W. Rieth, University of Utah R. Caldwell, University of Utah X. Xie, University of Utah F. Solzbacher, University of Utah |
| Correspondent: | Click to Email |
Encapsulation of penetrating neural interfaces with complex geometries is one of the greatest challenges to achieve long-term functionality and stability need for therapeutic systems. We present results from testing a novel encapsulation scheme that combines atomic layer deposited (ALD) Al2O3 and Parylene-C for biomedical implants with integrated electronics. The ALD alumina is utilized for its very low water vapor permeation rate, and the Parylene acts as a kinetic barrier to prevent dissolution of the alumina and as a well-regarded biocompatible coating.
Our devices were first coated with a combination of 52 nm of Al2O3 deposited by PA-ALD at 120 °C. The organo-silane adhesion promoter A-174 was applied, followed by a 6-µm film of Parylene-C deposited using a CVD process. Acceleration conditions included temperature and/or voltage bias with interdigitated electrodes (IDEs) and Utah Arrays. IDE testing focused on the effects of additional topography on the encapsulation lifetime at acceleration temperatures up to 80 °C. Topography was added by attachment of 0402 surface mount capacitors and custom 5.5 mm wire-wound Au coils, both used in our fully wireless neural interfaces. A >50% decrease in lifetime from > 280 days to 140 days was measured with the addition topography. We are investigating the mechanism for the decreased lifetime to determine if it is associated with chemical degradation, contamination, or mechanical forces through a comparison with thermal cycling measurements.
In-vitro measurements of the impedance stability for passive UEAs have been used to test the encapsulation performance of these devices while soaking in phosphate buffered saline (PBS). The impedances of these arrays are widely reported to decrease during soak testing due to water ingress. Contrary to this trend, we saw an increase in the impedance of arrays from median impedances of 60 kΩ to 160 kΩ during soaking for 960 days of equivalent time at 37°C. The mechanism for the increase impedance is likely the loss of tip metal.
The impact of voltage bias was also investigated using IDEs and fully-integrated wireless neural interface systems. These devices were maintained 37 °C or higher temperatures for acceleration, and a continuous 5 V bias. For the fully integrated wireless devices, the bilayer encapsulated devices continued to function for 140 days (37 °C equivalent) compared to > 1860 days of equivalent soaking for unpowered devices, indicating ~10 times shorter lifetimes. The lifetime is also more than 10x longer than devices only encapsulated with Parylene-C, indicating the bilayer encapsulation is a significant improvement for these very challenging conditions.