Long-term Insulation of Active Electronics Embedded in Compliant Neural Probes

Compliant neural probes suffer from poor long-term reliability due to the limited insulation capabilitiesof thin polymer films. Reliability is particularly critical for clinical applications wherelifetimes on the order of decades will be required. In addition, large scale recording and stimulationprobes to be used clinically require probes augmented with active circuits, which haveeven stronger requirements for insulation. Parylene has been considered a promising material forneural probes because of its biocompatibility and high barrier capabilities. However, neural probesinsulated with parylene are prone to failure over timescales of days to months. The multilayer insulation consisting of parylene and ALD deposited alumina/titania nanolaminate proposed in this thesis can provide the necessary long term insulation performance and enable active circuitembedding. This thesis explores the long-term insulation capability of parylene thin films with embeddedchips for the purpose of neural recording and stimulation. The different failure modes of parylene films in phosphate buffered saline (PBS) are investigated through simulation, experiments and modeling. Test structures representative of neural probe insulation are fabricated and theirimpedance measured over six months. Experiments designed to mimic the thermal (body temperatureof 37 C) and electrical conditions experienced by probes in recording (small signal voltage applied) and stimulation (1 Vpp applied) cases allow to assess the performance in more realistic conditions. In order to improve on the limited lifetime of parylene probes, additional thin film barrierlayers are evaluated: atomic layer deposited ceramics surrounding the metal and enclosed in theparylene can limit the rate of degradation of the insulation, without affecting the biocompatibility or mechanical properties of the probes. However because the adhesion of parylene to many materials is poor, the overall insulation might be degraded due to delamination between thelayers. Therefore additional adhesion layers also have to be used to enable the full benefits ofthe multilayer insulation. These adhesion layers can be additional ceramic layers, or conformallydeposited polymers. In practice, increasing the electrode count in compliant probes is limited by the wiring arearequired to connect all the electrodes to the external circuitry. In order to allow very large electrodecounts (>100 electrodes per probe), multiplexing circuitry is therefore required in the probe. For polymer probes, this requires embedding chips in the probe. A CMOS chip was designed that would allow multiplexing of 8 electrode channels. For recording electrodes, in order to limit theadditional noise generated by multiplexing, amplifiers are present at each electrode. A simple technique for integrating thin chips, explored in this thesis, uses anisotropic conductive film adhesives (ACF) to ensure mechanical and electrical connection to thin silicon chipsthat are be flip-chip bonded to the probe before the final parylene layer is deposited. A prototypeCMOS chip allows multiplexing of 8 electrode channels and cascading of multiplexers. For recording electrodes, in order to limit the additional noise generated by multiplexing, amplifiers are present at each electrode. In addition to higher electrode counts, the addition of integratedcircuitry opens the door for many possibilities for neural recordings, including better signal to noiseratio (amplification), higher throughput (spike sorting/digitization), and wireless capabilities.