ABSTRACT
The long-term electrical leakage performance of parylene-C/platinum/parylene-C (Px/Pt/Px) interconnect in saline is evaluated using electrochemical impedance spectroscopy (EIS). Three kinds of additional ceramic encapsulation layers between the metal and Px are characterized: 50 nm-thick alumina (Al2O3), 50 nm-thick titania (TiO2), and 80 nm-thick Al2O3-TiO2 nanolaminate (NL). The Al2O3 and TiO2 encapsulation layers worsen the overall insulation properties. The NL encapsulation layer improves the insulation when combined with a TiO2 outer layer to promote adhesion to the Px. Experiments are performed with various insulation promotion treatments: A-174 silane (A174) treatment before Px deposition (to promote adhesion); SF6 plasma treatment (F) after Px deposition (to increase hydrophobicity); and ion-milling descum (IM) after Px deposition (to prevent parylene oxidation). A174 and F treatments do not have a significant impact, while IM leads to worse insulation performance. A circuit model elucidates the insulation characteristics of Px-ceramic-Pt-ceramic-Px interconnect. These studies provide a foundation for processing ultra-compliant neural probes with long-term chronic utility.
Subject(s)
Ceramics/chemistry , Platinum/chemistry , Polymers/chemistry , Xylenes/chemistry , Aluminum Oxide/chemistry , Biofilms , Coated Materials, Biocompatible/chemistry , Dielectric Spectroscopy , Electric Impedance , Models, Theoretical , Saline Solution/chemistry , Surface Properties , Titanium/chemistryABSTRACT
Stable chronic functionality of intracortical probes is of utmost importance toward realizing clinical application of brain-machine interfaces. Sustained immune response from the brain tissue to the neural probes is one of the major challenges that hinder stable chronic functionality. There is a growing body of evidence in the literature that highly compliant neural probes with sub-cellular dimensions may significantly reduce the foreign-body response, thereby enhancing long term stability of intracortical recordings. Since the prevailing commercial probes are considerably larger than neurons and of high stiffness, new approaches are needed for developing miniature probes with high compliance. In this paper, we present design, fabrication, and in vitro evaluation of ultra-miniature (2.7 µm x 10 µm cross section), ultra-compliant (1.4 × 10-2 µN/µm in the axial direction, and 2.6 × 10-5 µN/µm and 1.8 × 10-6 µN/µm in the lateral directions) neural probes and associated probe-encasing biodissolvable delivery needles toward addressing the aforementioned challenges. The high compliance of the probes is obtained by micron-scale cross-section and meandered shape of the parylene-C insulated platinum wiring. Finite-element analysis is performed to compare the strains within the tissue during micromotion when using the ultra-compliant meandered probes with that when using stiff silicon probes. The standard batch microfabrication techniques are used for creating the probes. A dissolvable delivery needle that encases the probe facilitates failure-free insertion and precise placement of the ultra-compliant probes. Upon completion of implantation, the needle gradually dissolves, leaving behind the ultra-compliant neural probe. A spin-casting based micromolding approach is used for the fabrication of the needle. To demonstrate the versatility of the process, needles from different biodissolvable materials, as well as two-dimensional needle arrays with different geometries and dimensions, are fabricated. Further, needles incorporating anti-inflammatory drugs are created to show the co-delivery potential of the needles. An automated insertion device is developed for repeatable and precise implantation of needle-encased probes into brain tissue. Insertion of the needles without mechanical failure, and their subsequent dissolution are demonstrated. It is concluded that ultra-miniature, ultra-compliant probes and associated biodissolvable delivery needles can be successfully fabricated, and the use of the ultra-compliant meandered probes results in drastic reduction in strains imposed in the tissue as compared to stiff probes, thereby showing promise toward chronic applications.
Subject(s)
Electrodes, Implanted , Mechanical Phenomena , Microtechnology/instrumentation , Needles , Brain-Computer Interfaces , Elastic Modulus , Equipment Design , Models, BiologicalABSTRACT
Ultra-compliant neural probes implanted into tissue using a molded, biodissolvable sodium carboxymethyl cellulose (Na-CMC)-saccharide composite needle delivery vehicle are subjected to fluid-structure interactions that can displace the recording site of the probe with respect to its designed implant location. We applied particle velocimetry to analyze the behavior of ultra-compliant structures under different implantation conditions for a range of CMC-based materials and identified a fluid management protocol that resulted in the successful targeted depth placement of the recording sites.
