Mohamad Sawan

Wireless smart implants dedicated to multichannel monitoring and microstimulation

This invited paper covers several techniques and methods employed to build high reliability circuits and systems dedicated to implement advanced implantable and wirelessly controlled smart medical devices such as sensors and microstimulateurs. A global view of typical micro-devices such us neural signals monitoring device, cortical multichannel stimulator, as well as peripheral nerve interfaces to recuperate bladder functions, is given. In addition, case studies related to peripheral and cortical neural systems are reported. In all these devices, special attention is paid to low-power management of involved systems and to the design of corresponding circuits of such typical implantable multi-disciplinary microsystems.

A Mixed-Signal Multichip Neural Recording Interface With Bandwidth Reduction

We present a multichip structure assembled with a medical-grade stainless-steel microelectrode array intended for neural recordings from multiple channels. The design features a mixed-signal integrated circuit (IC) that handles conditioning, digitization, and time-division multiplexing of neural signals, and a digital IC that provides control, bandwidth reduction, and data communications for telemetry toward a remote host. Bandwidth reduction is achieved through action potential detection and complete capture of waveforms by means of onchip data buffering. The adopted architecture uses high parallelism and low-power building blocks for safety and long-term implantability. Both ICs are fabricated in a CMOS 0.18-mum process and are subsequently mounted on the base of the microelectrode array. The chips are stacked according to a vertical integration approach for better compactness. The presented device integrates 16 channels, and is scalable to hundreds of recording channels. Its performance was validated on a testbench with synthetic neural signals. The proposed interface presents a power consumption of 138 muW per channel, a size of 2.30 mm2, and achieves a bandwidth reduction factor of up to 48 with typical recordings.

A fully integrated low-power BPSK demodulator for implantable medical devices

During the past decades, research has progressed on the biomedical implantable electronic devices that require power and data communication through wireless inductive links. In this paper, we present a fully integrated binary phase-shift keying (BPSK) demodulator, which is based on a hard-limited COSTAS loop topology, dedicated to such implantable medical devices. The experimental results of the proposed demodulator show a data transmission rate of 1.12 Mbps, less than 0.7 mW consumption under a supply voltage of 1.8 V, and silicon area of 0.2 mm/sup 2/ in the Taiwan Semiconductor Manufacturing Company (TSMC) CMOS 0.18-/spl mu/m technology. The transmitter satisfies the requirement of applications relative to high forward-transferring data rate, such as cortical stimulation. Moreover, the employment of BPSK demodulation along with a passive modulation method allows full-duplex data communication between an external controller and the implantable device, which may improve the controllability and observability of the overall implanted system.

A Low-Power Integrated Bioamplifier With Active Low-Frequency Suppression

We present in this paper a low-power bioamplifier suitable for massive integration in dense multichannel recording devices. This bioamplifier achieves reduced-size compared to previous designs by means of active low-frequency suppression. An active integrator located in the feedback path of a low-noise amplifier is employed for placing a highpass cutoff frequency within the transfer function. A very long integrating time constant is achieved using a small integrated capacitor and a MOS-bipolar equivalent resistor. This configuration rejects unwanted low-frequency contents without the need for input RC networks or large feedback capacitors. Therefore, the bioamplifier high-input impedance and small size are preserved. The bioamplifier, implemented in a 0.18-mum CMOS process, has been designed for neural recording of action potentials, and optimised through a transconductance-ef-ficiency design methodology for micropower operation. Measured performance and results obtained from in vivo recordings are presented. The integrated bioamplifier provides a midband gain of 50 dB, and achieves an input-referred noise of 5.6 muVrms. It occupies less than 0.050 mm2 of chip area and dissipates 8.6 muW.

A Highly Flexible System for Microstimulation of the Visual Cortex: Design and Implementation

This paper presents the design of a system intended to be used as a prosthesis allowing profoundly visually impaired patients to recover partial vision by means of microstimulation in the primary visual cortex area. The main component of the system is a bio-electronic device to be implanted inside the skull of the user, composed of a plurality of stimulation modules, whose actions are controlled via an interface module. Power and data are transmitted to the implant wirelessly through a bidirectional inductive link, allowing diagnosis of the stimulating device and its environment after implantation, as well as power delivery optimization. A high level of flexibility is supported in terms of stimulation parameters, but a configurable communication protocol allows the device to be used with maximum efficiency. The core of an external controller implemented in a system on a programmable chip is also presented, performing data conversion and timing management such that phosphene intensity can be modulated by any parameter defining stimulation, either at the pulse level or in the time domain. Measured performances achieved with a prototype using two types of custom ASICs implemented in a 0.18-mum CMOS process and commercial components fulfill the requirements for a complete visual prosthesis for humans. When on/off activation is used with predefined parameters, stimuli measured on an electronic test bench could attain a rate in excess of 500 k pulses/s.

High-Speed OQPSK and Efficient Power Transfer Through Inductive Link for Biomedical Implants

Biomedical implants require wireless power and bidirectional data transfer. We pursue our previous work on a novel topology for a multiple carrier inductive link by presenting the fabricated coils. We show that the coplanar geometry approach is better suited for displacement tolerance. We provide a theoretical analysis of the efficiency of power transfer and phase-shift-keying communications through an inductive link. An efficiency of up to 61% has been achieved experimentally for power transfer and a data rate of 4.16 Mb/s with a bit-error rate of less than 2 × 10-6 has been obtained with our fabricated offset quadrature phase-shift keying modules due to the inductive link optimization presented in this paper.

Delayed Stochastic Decoding of LDPC Codes

A new stochastic decoding algorithm, called Delayed Stochastic (DS) decoding, is introduced to implement low-density-parity-check (LDPC) decoders. The delayed stochastic decoding uses an alternative method to track probability values, which results in reduction of hardware complexity and memory requirement of the stochastic decoders. It is therefore suitable for fully-parallel implementation of long LDPC codes with applications in optical communications. Two decoders are implemented using the DS algorithm for medium (2048, 1723) and long (32768, 26624) LDPC codes. The decoders occupy 3.93- mm2 and 56.5- mm2 silicon area using 90-nm CMOS technology and provide maximum core throughputs of 172.4 and 477.7 Gb/s at [(Eb)/(No)]=5.5 and 4.8 dB, respectively.

An Ultra Low-Power CMOS Automatic Action Potential Detector

We present a low-power complementary metal-oxide semiconductor (CMOS) analog integrated biopotential detector intended for neural recording in wireless multichannel implants. The proposed detector can achieve accurate automatic discrimination of action potential (APs) from the background activity by means of an energy-based preprocessor and a linear delay element. This strategy improves detected waveforms integrity and prompts for better performance in neural prostheses. The delay element is implemented with a low-power continuous-time filter using a ninth-order equiripple allpass transfer function. All circuit building blocks use subthreshold OTAs employing dedicated circuit techniques for achieving ultra low-power and high dynamic range. The proposed circuit function in the submicrowatt range as the implemented CMOS 0.18-mum chip dissipates 780 nW, and it features a size of 0.07 mm2. So it is suitable for massive integration in a multichannel device with modest overhead. The fabricated detector succeeds to automatically detect APs from underlying background activity. Testbench validation results obtained with synthetic neural waveforms are presented.

Implantable selective stimulator to improve bladder voiding: design and chronic experiments in dogs

Among the treatments to enhance the bladder voiding, the sacral roots neurostimulation is one of the most promising techniques. The electrostimulation of sacral nerves provokes a simultaneous contraction of the detrusor muscle as well as the external urethral sphincter (EUS). A new simplified-architecture implantable stimulator with its wireless controller have been designed to investigate high-frequency inhibition stimulation strategies. This innovative technique based on high-frequency inhibition reduces sphincter activity during stimulation. Low-frequency current pulses also applied to the sacral roots induces contraction of the detrusor muscle resulting in low pressure voiding. Chronic experiments were carried out on ten male mongrel paraplegic dogs. One cuff electrode was implanted along with each stimulator for eight months. The animals were stimulated twice a day using the prototypes of our implantable selective stimulator while voided and residual urine volume were measured during the procedure. These experiments revealed that the proposed stimulation strategy enhances bladder voiding by more than 50% in comparison with low-frequency only stimulation. The residual urine volume was reduced to an average of 9% and low pressure micturition was achieved as shown by weekly cystourethrogram.

EM radiation behavior upon biological tissues in a radio-frequency power transfer link for a cortical visual implant

This paper presents a method to evaluate the interactions between a radio-frequency power transfer link and biological tissues. First the authors analyze the propagation of a plane wave model at the skin interface to evaluate the frequency that optimize the transfer ratio. They then use the Finite-Difference Time-Domain method to calculate the exact electromagnetic fields propagation in a discrete 3D space and time for a box brain model with an inductive emitting antenna. The results exhibit the electric field pattern in the antenna axis plane and in the back of the head. They show for an optimized frequency carrier (20 MHz) that the attenuation of electric field strength is 15 dB and that most of the energy is reflected at the air-tissue interface.