Anne Vanhoestenberghe

Active Books: The Design of an Implantable Stimulator That Minimizes Cable Count Using Integrated Circuits Very Close to Electrodes

This paper presents an integrated stimulator that can be embedded in implantable electrode books for interfacing with nerve roots at the cauda equina. The Active Book overcomes the limitation of conventional nerve root stimulators which can only support a small number of stimulating electrodes due to cable count restriction through the dura. Instead, a distributed stimulation system with many tripole electrodes can be configured using several Active Books which are addressed sequentially. The stimulator was fabricated in a 0.6-μm high-voltage CMOS process and occupies a silicon area of 4.2 × 6.5 mm2. The circuit was designed to deliver up to 8 mA stimulus current to tripole electrodes from an 18 V power supply. Input pad count is limited to five (two power and three control lines) hence requiring a specific procedure for downloading stimulation commands to the chip and extracting information from it. Supported commands include adjusting the amplitude of stimulus current, varying the current ratio at the two anodes in each channel, and measuring relative humidity inside the chip package. In addition to stimulation mode, the chip supports quiescent mode, dissipating less than 100 nA current from the power supply. The performance of the stimulator chip was verified with bench tests including measurements using tripoles in saline.

The Limits of Hermeticity Test Methods for Micropackages

Hermeticity is crucial for the long-term implantation of electronic packages. Pushed by advances in micromachining, package volumes are decreasing and current leak detection methods are no longer sensitive enough. This article reviews the limits of the most common methods and exposes their inadequateness for medical electronic applications where the devices life is 50 years or longer.

Moisture Ingress Into Packages With Walls of Varying Thickness And/Or Properties: A Simple Calculation Method

For electronic devices that work at high humidity (such as implants), but for which only a moderate lifetime is required, it is possible to use polymer packages for protection. However, these materials are porous and allow moisture to diffuse through the package. Ficks laws applied to water diffusion have a solution which can either be solved numerically, or approximated accurately with the single exponential “quasi-steady-state (QSS) model.” This paper adapts the so-called QSS model to the case of moisture ingress into a package which has walls or elements of different thicknesses or properties. Using an electrical analogy, we propose a model which allows estimating the change in relative humidity with time inside the enclosure cavity, using a simple calculation method.

Integrated electrode and high density feedthrough system for chip-scale implantable devices.

High density feedthroughs have been developed which allow for the integration of chip-scale features and electrode arrays with up to 1141 stimulating sites to be located on a single implantable package. This layered technology displays hermetic properties and can be produced from as little as two laminated 200 μm thick alumina sheets. It can also be expanded to a greater number of layers to allow flexible routing to integrated electronics. The microelectrodes, which are produced from sintered platinum (Pt) particulate, have high charge injection capacity as a result of a porous surface morphology. Despite their inherent porosity the electrodes are electrically stable following more than 1.8 billion stimulation pulses delivered at clinically relevant levels. The ceramic-Pt constructs are also shown to have acceptable biological properties, causing no cell growth inhibition using standard leachant assays and support neural cell survival and differentiation under both passive conditions and active electrical stimulation.

Silicone rubber encapsulation for an endoscopically implantable gastrostimulator

Gastrointestinal stimulator implants have recently shown positive results in treating obesity. However, the implantation currently requires an invasive surgical procedure. Endoscopy could be used to place the gastric stimulator in the stomach, hence avoiding the riskier surgery. The implant then needs to go through the oesophagus and be located inside the stomach, which imposes new design constraints, such as miniaturization and protecting the electronic circuit against the highly acidic environment of the stomach. We propose to protect the implant by encapsulation with silicone rubber. This paper lists the advantages of this method compared to the more usual approach of a hermetic enclosure and then presents a method to evaluate the underwater adhesive stability of six adhesive/substrate couples, using repeated lap-shear tests and an elevated temperature to accelerate the ageing process. The results for different adhesive/substrate couples tested, presented on probability plots, show that FR4 and alumina substrates with MED4-4220 silicone rubber are suitable for a first implantable prototype. We then compare these with the predicted lifetimes of bonds between historical standard silicone rubber DC3140 and different substrates and describe the encapsulation of our gastrostimulator.

Realization of an active book for multichannel intrathecal root stimulation in spinal cord injury — Preliminary results

After spinal cord injury, electrical stimulation of the roots inside the spinal column at the level of the cauda equina is a safe and effective way to regain some degree of control over lower body function, e.g. bladder and bowel management and leg movement. The success of current systems used for so-called intrathecal stimulation is limited by the low number of stimulation channels, which are in consequence of the maximum acceptable number of transdural cables. In order to overcome this limitation, we developed an active electrode with integrated electronics, providing four individual stimulation channels that requires one cable only. This paper outlines the different elements of the so-called active book with the emphasis on its preliminary construction and assembly.

A dedicated electrode driving ASIC for epidural spinal cord stimulation in rats

This paper discusses the design of an application-specific integrated circuit (ASIC) suitable for mounting on a multi-electrode array for epidural spinal cord stimulation in rats. The ASIC acts as a demultiplexer, driving 12 electrodes on the array in any configuration. It is capable of routing biphasic constant current pulses of up to 1 mA to high impedance loads (with a maximum output voltage swing of approximately 25 V) and is small enough to be implanted into a rats spinal column. Communication with its driver is achieved via 3 wires to minimize the number of interconnections. The circuit was implemented in a 0.18-μm high-voltage CMOS technology occupying a core area of 0.36 mm2. Power dissipation is about 110 μW. Post-layout simulations are presented which show the correct operation of the system.

Towards an optimized wearable neuromodulation device for urinary incontinence

The subject of this paper is a wearable device (probe) for treating urinary incontinence. The device is dedicated to trans-rectal stimulation of the pudendal nerve which is triggered by the external sphincter EMG signal. In an effort to identify the optimum stimulation parameters, a family of strength-duration curves with varying pulse repetition frequencies was generated. The electrode-tissue interface was modeled as an RC circuit and a heuristic method for estimating the model parameters was used. A 2D model of the electric field around the probe was also constructed, using finite element methods, to investigate further possibilities for optimization.

A stimulator ASIC with capability of neural recording during inter-phase delay

This paper presents a single chip solution for a combined stimulation and recording system for functional electrical stimulation applications. The on-chip recording amplifier blanks large stimulation artifacts occurring in the cathodic (i.e., stimulation) and anodic (i.e., recuperation) phases of a stimulation pulse. By making the stimulator output stage float and recording during the delay between cathodic and anodic impulses, the recording start time can be greatly advanced from the end of a complete stimulation cycle to the end of the cathodic phase. The ASIC was fabricated in a 0.6 μm HV CMOS technology, occupies a core area of 5.3 mm2 and operates from a single 18 V power supply. It has 5 I/O pads for power and data communication and another 5 I/O pads for connecting to the electrodes. The operation of the ASIC has been verified both in-vitro and in-vivo.

In vivo validation of a less invasive gastrostimulator

Gastrointestinal stimulator implants have recently shown promising results in helping obese patients lose weight. However, to place the implant, the patient currently needs to undergo an invasive surgical procedure. We report a less invasive procedure to stimulate the stomach with a gastrostimulator. After attempting fully endoscopic implantation, we more recently focused on a single incision percutaneous procedure. In both cases, the challenges in electronic design of the implant are largely similar. This article covers the work achieved to meet these and details the in vivo validation of a gastrostimulator aimed to be endoscopically placed and anchored to the stomach.