Chronic nerve root entrapment: compression and degeneration

Electrode mounts are being developed to improve electrical stimulation and recording. Some are tight-fitting, or even re-shape the nervous structure they interact with, for a more selective, fascicular, access. If these are to be successfully used chronically with human nerve roots, we need to know more about the possible damage caused by the long-term entrapment and possible compression of the roots following electrode implantation. As there are, to date, no such data published, this paper presents a review of the relevant literature on alternative causes of nerve root compression, and a discussion of the degeneration mechanisms observed. A chronic compression below 40 mmHg would not compromise the functionality of the root as far as electrical stimulation and recording applications are concerned. Additionally, any temporary increase in pressure, due for example to post-operative swelling, should be limited to 20 mmHg below the patient's mean arterial pressure, with a maximum of 100 mmHg. Connective tissue growth may cause a slower, but sustained, pressure increase. Therefore, mounts large enough to accommodate the root initially without compressing it, or compliant, elastic, mounts, that may stretch to free a larger cross-sectional area in the weeks after implantation, are recommended

Corrosion of silicon integrated circuits and lifetime predictions in implantable electronic devices

Corrosion is a prime concern for active implantable devices. In this paper we review the principles underlying the concepts of hermetic packages and encapsulation, used to protect implanted electronics, some of which remain widely overlooked. We discuss how technological advances have created a need to update the way we evaluate the suitability of both protection methods. We demonstrate how lifetime predictability is lost for very small hermetic packages and introduce a single parameter to compare different packages, with an equation to calculate the minimum sensitivity required from a test method to guarantee a given lifetime. In the second part of this paper, we review the literature on the corrosion of encapsulated integrated circuits (ICs) and, following a new analysis of published data, we propose an equation for the pre-corrosion lifetime of implanted ICs, and discuss the influence of the temperature, relative humidity, encapsulation and field-strength. As any new protection will be tested under accelerated conditions, we demonstrate the sensitivity of acceleration factors to some inaccurately known parameters. These results are relevant for any application of electronics working in a moist environment. Our comparison of encapsulation and hermetic packages suggests that both concepts may be suitable for future implants

Unsafe Stimulation Correlates with Oxide Reduction Onset in Unbuffered Saline

Damage mechanisms in electrical stimulation must be better understood to address the demands of new electrode technologies. In this work, we studied the effect of pH on the charge injection mechanisms in a repeated pulsing experiment. We show that damage occurs when the electrode potential enters the oxide reduction region

An integrated circuit to enable electrodeposition and amperometric readout of sensing electrodes

This paper presents the design of an integrated circuit (IC) for (i) electrochemical deposition of sensor layers on the on-chip pad openings to form sensing electrodes, and (ii) amperometric readout of electrochemical sensors. The IC consists of two main circuit blocks: a Beta-multiplier based current reference for galvanostatic electrodeposition, and a switch-capacitor based amperometric readout circuit. The circuits are designed and simulated in a 180-nm CMOS process. The reference circuit generates a stable current of 99 nA with a temperature coefficient of 141 ppm/°C at best and 170 ppm/°C on average (across corners) over a supply voltage range of 1.2-2.4 V, and a line regulation of 0.7 %/V. The readout circuit measures current within pm 2 mu mathrmA with 99.9% linearity and a minimum integrated input-referred noise of 0.88 pA

CAPITEL: Design and Implementation of a wireless 6 channel EMG measurement system for permanent in vivo use: in vitro results

Introduction Surface Electromyography of partial limb amputees is used to control prostheses. Implantable EMG systems offer a higher Signal to Noise Ratio (SNR) as well as improved muscle specificity, and a more convenient daily use. Material and methods We present a design for an implantable device (“implant”) with 6 channels, each suitable for an electrode array with 3 electrodes. The implant uses an ADS1298 analog front end with ADG2188 multiplexers for versatile electrode array configuration. The analog input is filtered with a balanced analog bandpass filter with corner frequencies of 30 and 800 Hz. The ADC sample rate is 2 KHz per channel, with 9 bits resolution. The dimensions of the PCB implant are 17.2 x 14.1x2.15 mm. To measure the SNR a sinusoidal signal with a peak to peak amplitude of 7 mV and a frequency of 200 Hz was applied to each input. To simulate muscle impedance, an equivalent muscle impedance model (Figure 1) was placed between the generator and each channel of the implant [1]. We have implemented two data transmission methods: wired duplex communication and wireless inductive link. The wired link is used to transfer raw data, while only the 6 EMG envelopes, with an update rate of 20 Hz, are sent via the wireless link. Results Each analog input channel performed with a SNR better than 52 dB, both for wired and wireless operation. Wired data was received successfully at 115200 bps and wireless data at 1080 bps. Discussion Our design achieves a high SNR and data rate. These early results are promising and we are packaging the PCBs for in-vivo testing. Conclusion We have demonstrated a very compact design suitable for the monitoring of 6 EMG channels, with options for raw data or EMG envelope transmission. [1] Kalvoy, H, 2009, doi: 10.1088/0967-3334/30/2/002

Effect of pH and gel electrolyte on safe charge injection and electrode degradation of platinum electrodes

Platinum (Pt) is a widespread electrode material choice for neural interfaces and electrochemical biosensors, due to its supposed electrochemical inertness. However, faradaic reactions can take place at Pt electrodes, including Pt oxide formation and reduction. Repeated redox cycles of Pt can lead to Pt dissolution, which may harm the tissue and significantly reduce electrode lifetime. In this study, we investigated how the electrolyte may influence Pt dissolution mechanisms during current pulsing. Two electrolyte characteristics were considered: pH and gelation. We confirmed that empirically reported tissue damage thresholds correlate with Pt oxide formation and reduction. Varying electrolyte pH occasioned a shift in recorded potentials, however, damage thresholds correlated with the same mechanisms for all pH values. The similar behaviour observed for pH values in the central range (4 ≤ pH ≤ 10) can be explained by variations of local pH at the electrode surface. Gel electrolytes behaved comparably to solutions, which was confirmed by statistical similarity tests. This study extends the knowledge about platinum electrochemistry and shows the necessity to carefully choose the stimulation protocol and the electrolyte to avoid platinum dissolution and tissue damage

Reflection on boon and bane of Water Absorbing Components in Active Implants during Package Testing and Operation

For package testing of active implants internal humidity measurements are performed. Thereby, the water absorbency of the measurement components can affect the measurement result, leading to an erroneously prolonged predicted lifetime. Here, the influence of internal components is discussed using an exemplary setup. It is described how to model and consider the components’ absorbencies for lifetime estimation and how beneficial they are during implant operation

Implantable electronic devices technology challenges for long-term human implantation

Purpose - The purpose of this paper is to discuss the requirements for long-term implantation of electronic devices with a focus on packaging and encapsulation.Design/methodology/approach - Owing to their intended long-term use in the human body, implants for electrical stimulation present specific challenges to the engineers. The respective roles of packaging and encapsulation must be clearly understood to make the most of new materials and modern machining technologies. This paper offers an introduction to the current situation and highlights challenges for future developments.Findings - The innovative application of modern technologies may be useful to tackle key issues of encapsulation and sealing of small electrical devices for long-term implantation.Originality/value - Two examples of innovative application of alternative package manufacture and sealing method are described