Apparatus to investigate the insulation impedance and accelerated life-testing of neural interfaces

Abstract

Objective. Neural interfaces and other implantable micro-devices that use polymerencapsulated integrated circuits will only be allowed in medical devices when their lifetimes can be estimated from experimental data. An apparatus has been developed and tested that allows hundreds of insulated samples (interdigitated combs) to be aged under accelerated conditions of high temperature and voltage stress. Occasionally, aging is paused while the sample's impedance is measured; the impedance spectrogram may show degradation as it progresses before failure. Approach. The design was based on practical considerations which are reviewed. A Solartron Modulab provides the frequency response analyser and the femtoammeter. The apparatus can accommodate batches of samples at several temperatures and with different aging voltage waveforms. It is important to understand features of the spectra that are not due to comb-comb leakage, but come from other places (for example substrate-solution leakage); some have been observed and investigated using SPICE. Main results. The design is described in detail and test results show that it is capable of making measurements over long periods, at least up to 67 °C. Despite the size of the apparatus, background capacitance is about 1 pF and comb-comb capacitances of about 30 pF can be measured down to 10 mHz, an impedance of about 100 GΩ. An important discovery was the advantage of grounding the bathing solution, primarily in that it raises the measurement ceiling. Observation and SPICE simulation shows that leakage from the substrate to the bathing solution can give phase lags >90°, in contrast to comb-comb leakage which reduces phase lag to <90°. Significance. The value of this paper is that it will facilitate research into the endurance of small implanted devices because, given a description of a proven apparatus, researchers can start building their own apparatus relatively quickly and with confidence.

Silicone encapsulation of thin-film SiO x , SiO x N y and SiC for modern electronic medical implants: a comparative long-term ageing study

“…Manufactured samples are shown in figure 1. Screen printed ceramic adapters were used to facilitate electrical connections between the IDCs and the pins in the test tube bung (more details on the bung manufacturing process have been previously reported [27]). PtAu paste (ESL 5837) was screen printed onto 0.5 mm thick scribed alumina ceramic substrates (96% Coorstek grade ADS96R) and fired at 850 • C for approximately 1 h. A secondary glass sealing layer (ESL 4026A) was then printed over the PtAu tracks and fired at 500 • C for 5 h, leaving only the pads for wire bonding and soldering exposed.…”

“…After curing, the stainless steel wires from the adapters were soldered to pins in test tube bungs (figure 1(a)). Two lengths of stainless steel wire were soldered to pins in the bung to make the PBS depth measurement electrodes (DE), which monitored evaporation of PBS in the test tubes [27]. The pins and base of the bungs were then encapsulated in Dow Corning 3140 RTV silicone.…”

Long-term Encapsulation of Platinum Metallization Using a HfO2 ALD - PDMS Bilayer for Non-hermetic Active Implants

“…Soaking was conducted in room temperature (23 °C) with proper sealing to avoid any evaporation and deterioration of the PBS. More details on the set-up specifications and preparation can be found in [12]. In this work, electrochemical impedance spectrometry (EIS) was carried out to evaluate the insulating performance of the ALD and ALD-PDMS bilayer over time.…”

Effect of Signals on the Encapsulation Performance of Parylene Coated Platinum Tracks for Active Medical Implants

“…Proper care on the vial sealing was taken to minimize water evaporation. More details on the set-up specifications and preparation can be found in [7]. Three samples were used for passive testing where no signals were applied to the samples except for the measurement signal, applied once every two weeks.…”