Pt/Ir electrodes have been extensively used in neurophysiology research in recent years as they provide a more inert recording surface as compared to tungsten or stainless steel. While floating microelectrode arrays (FMA) consisting of Pt/Ir electrodes are an option for neuroprosthetic applications, long-term in vivo functional performance characterization of these FMAs is lacking. In this study, we have performed comprehensive abiotic-biotic characterization of Pt/Ir arrays in 12 rats with implant periods ranging from 1 week up to 6 months. Each of the FMAs consisted of 16-channel, 1.5 mm long, and 75 μm diameter microwires with tapered tips that were implanted into the somatosensory cortex. Abiotic characterization included (1) pre-implant and post-explant scanning electron microscopy (SEM) to study recording site changes, insulation delamination and cracking, and (2) chronic in vivo electrode impedance spectroscopy. Biotic characterization included study of microglial responses using a panel of antibodies, such as Iba1, ED1, and anti-ferritin, the latter being indicative of blood-brain barrier (BBB) disruption. Significant structural variation was observed pre-implantation among the arrays in the form of irregular insulation, cracks in insulation/recording surface, and insulation delamination. We observed delamination and cracking of insulation in almost all electrodes post-implantation. These changes altered the electrochemical surface area of the electrodes and resulted in declining impedance over the long-term due to formation of electrical leakage pathways. In general, the decline in impedance corresponded with poor electrode functional performance, which was quantified via electrode yield. Our abiotic results suggest that manufacturing variability and insulation material as an important factor contributing to electrode failure. Biotic results show that electrode performance was not correlated with microglial activation (neuroinflammation) as we were able to observe poor performance in the absence of neuroinflammation, as well as good performance in the presence of neuroinflammation. One biotic change that correlated well with poor electrode performance was intraparenchymal bleeding, which was evident macroscopically in some rats and presented microscopically by intense ferritin immunoreactivity in microglia/macrophages. Thus, we currently consider intraparenchymal bleeding, suboptimal electrode fabrication, and insulation delamination as the major factors contributing toward electrode failure.

Changes in biotic and abiotic factors can be reflected in the complex impedance spectrum of the microelectrodes chronically implanted into the neural tissue. The recording surface of the tungsten electrode in vivo undergoes abiotic changes due to recording site corrosion and insulation delamination as well as biotic changes due to tissue encapsulation as a result of the foreign body immune response. We reported earlier that large changes in electrode impedance measured at 1 kHz were correlated with poor electrode functional performance, quantified through electrophysiological recordings during the chronic lifetime of the electrode. There is a need to identity the factors that contribute to the chronic impedance variation. In this work, we use numerical simulation and regression to equivalent circuit models to evaluate both the abiotic and biotic contributions to the impedance response over chronic implant duration. COMSOL® simulation of abiotic electrode morphology changes provide a possible explanation for the decrease in the electrode impedance at long implant duration while biotic changes play an important role in the large increase in impedance observed initially.

This paper provides evidence for electrical 1 / f noise as the dominant source of excess noise in piezoresistive microelectromechanical systems ͑MEMS͒ microphones. In piezoresistors, the fundamental noise sources may be divided into frequency independent thermal noise and frequency dependent 1 / f excess noise dominating at low frequencies. Noise power spectra are presented for both commercial and research-prototype MEMS piezoresistive microphones as a function of applied voltage bias for both free and blocked membranes. The contributions of various mechanical and electrical noise sources are compared using a lumped noise equivalent circuit of the piezoresistive microphone. The bias dependence and membrane independence of the output noise indicate that the primary source of the excess noise is electrical in origin.

The reduction of acoustic microphone size using microelectromechanical systems (MEMS) technology enables increased spatial and temporal resolution. Whether the small size can be effectively utilized depends on the signal-to-noise ratio and minimum detectable signal (MDS) that are a function of the structural geometry, material properties, and transduction method. The optimal MDS depends on both electronic and thermomechanical noise sources. Fundamental noise sources may be divided into frequency independent thermal noise and frequency dependent excess noise dominating at low frequencies. There have been some questions regarding the dominance of electrical or mechanical sources of the excess noise in piezoresistive microphones [A. Zuckerwar et al., J. Acoust. Soc. Am. 113, p. 3179–3187 (2003)]. Noise power spectra have been measured for various piezoresistive microphones. We present results on the bias dependence of the excess noise that indicate that the primary source of excess noise is electrical. The relative contributions of mechanical and electrical noise sources will be discussed.

The process dependence of 1/f noise in p-type piezoresistors was investigated in this work using both electrical and materials characterization approaches. P-type piezoresistors were fabricated with 20 keV and 40 keV boron implants with and without implant oxide and varying isochronal 900 °C inert anneals. The devices were characterized electrically using I-V, Hall Effect, and power spectral density (PSD) noise measurements. The defects were visualized using cross-section transmission electron microscopy and plane view TEM The measured 1/f noise PSDs in the p-type piezoresistors are systematically compared to the number and dimension of bulk defect densities measured with TEM after each annealing condition of the piezoresistors. The 1/f noise PSDs of the piezoresistors implanted with 20 keV boron track the TEM defect number densities while those implanted with 40 keV boron through SiO2 with inert and oxidizing anneals track the faulted loop areas.