Wirelessly Powered and Bi-Directional Data Communication System With Adaptive Conversion Chain for Multisite Biomedical Implants Over Single Inductive Link.

A wirelessly powered and data communication system is presented which is implemented as a full system, designed for multisite implanted biomedical applications. The system is capable of receiving wireless power and data communication for each implant separately, using inductive links with different resonance frequencies. To achieve this, dual-band coils are presented in the system. In addition, the system supports bi-directional half-duplex data communication, utilizing amplitude and load shift keying (ASK and LSK) modulation schemes over a single inductive link. The system employs a digitally assisted active rectifier and an automatic resonance tuning system, to improve the power transfer efficiency (PTE) through various coupling coefficients, while minimizing the reverse current and power dissipation. The power control unit enables closed-loop monitoring to prevent high or low power delivery, and it can detect inefficient or excessive wireless power transmission or prevent temperature elevation by limiting the voltage to a safe level. A new structure of self-sampling separated- Vb ASK demodulator is proposed in the paper which is utilized within the data conversion chain, serving both the external and implanted units. The whole system is fabricated using a standard 180-nm 1.8/3.3 V CMOS process with a core area of 0.82 mm[Formula: see text]. The system is tested with coupled multisite inductive links and offers the maximum overall PTE of 31.2%, from the Tx coil to the implant load.

An Implantable Wireless System for Remote Hemodynamic Monitoring of Heart Failure Patients.

This article presents an implantable wireless system for remote hemodynamic monitoring, which enables direct, continuous (24/7), and simultaneous measurement of pulmonary arterial pressure (PAP) and cross-sectional area (CSA) of the artery. The implantable device, which measures 3.2 mm × 2 mm × 10 mm, comprises a piezoresistive pressure sensor, an ASIC implemented in 180-nm CMOS, a piezoelectric ultrasound (US) transducer, and a nitinol anchoring loop. An energy-efficient pressure monitoring system, which employs duty-cycling and spinning excitation technique, achieves 0.44 mmHg resolution in a pressure range from -135 mmHg to +135 mmHg and consumes 1.1 nJ conversion energy. The artery diameter monitoring system utilizes the inductive characteristic of the implant's anchoring loop and achieves 0.24 mm resolution within a diameter range of 20 mm to 30 mm, four times higher than echocardiography lateral resolution. The wireless US power and data platform enables simultaneous power and data transfer employing a single piezoelectric transducer in the implant. The system is characterized with an 8.5 cm tissue phantom and achieves a US link efficiency of 1.8%. The uplink data is transmitted by using an ASK modulation scheme parallel to the power transfer and achieves a modulation index of 26%. The implantable system is tested in an in-vitro experimental setup, which emulates the arterial blood flow, and accurately detects fast pressure peaks for systolic and diastolic pressure changes at both 1.28 MHz and 1.6 MHz US powering frequencies, with corresponding uplink data rates of 40 kbps and 50 kbps.

An Energy-Efficient Bridge-to-Digital Converter for Implantable Pressure Monitoring Systems.

This paper presents an energy-efficient, duty-cycled, and spinning excitation bridge-to-digital converter (BDC) designed for implantable pressure sensing systems. The circuit provides the measure of the pulmonary artery pressure that is particularly relevant for the monitoring of heart failure and pulmonary hypertension patients. The BDC is made of a piezoresistive pressure sensor and a readout integrated circuit (IC) that comprises an instrumentation amplifier (IA) followed by an analog-to-digital converter (ADC). The proposed design spins both the bridge excitation and the ADC's sampling input voltages simultaneously and exploits duty cycling to reduce the static power consumption of the bridge sensor and IA while cancelling the IA's offset and 1/f noise at the same time. The readout IC has been designed and fabricated in a standard 180-nm CMOS process and achieves 8.4 effective number of bits (ENOB) at 1 kHz sampling rate while drawing 0.53 µA current from a 1.2 V supply. The BDC, built with the readout IC and a differential pressure sensor having 5 k Ω bridge resistances, achieves 0.44 mmHg resolution in a 270 mmHg pressure range at 1 ms conversion time. The current consumption of the bridge sensor by employing duty cycling is reduced by 99.8% thus becoming 0.39 µA from a 1.2 V supply. The total conversion energy of the pressure sensing system is 1.1 nJ, and achieves a figure-of-merit (FoM) of 3.3 pJ/conversion, which both represent the state of the art.

A Self-Powered and Battery-Free Vibrational Energy to Time Converter for Wireless Vibration Monitoring.

Wireless sensor nodes (WSNs) are the fundamental part of an Internet of Things (IoT) system for detecting and transmitting data to a master node for processing. Several research studies reveal that one of the disadvantages of conventional, battery-powered WSNs, however, is that they typically require periodic maintenance. This paper aims to contribute to existing research studies on this issue by exploring a new energy-autonomous and battery-free WSN concept for monitor vibrations. The node is self-powered from the conversion of ambient mechanical vibration energy into electrical energy through a piezoelectric transducer implemented with lead-free lithium niobate piezoelectric material to also explore solutions that go towards a greener and more sustainable IoT. Instead of implementing any particular sensors, the vibration measurement system exploits the proportionality between the mechanical power generated by a piezoelectric transducer and the time taken to store it as electrical energy in a capacitor. This helps reduce the component count with respect to conventional WSNs, as well as energy consumption and production costs, while optimizing the overall node size and weight. The readout is therefore a function of the time it takes for the energy storage capacitor to charge between two constant voltage levels. The result of this work is a system that includes a specially designed lead-free piezoelectric vibrational transducer and a battery-less sensor platform with Bluetooth low energy (BLE) connectivity. The system can harvest energy in the acceleration range [0.5 g-1.2 g] and measure vibrations with a limit of detection (LoD) of 0.6 g.

Advanced Monitoring Systems Based on Battery-Less Asset Tracking Modules Energized through RF Wireless Power Transfer.

Asset tracking involving accurate location and transportation data is highly suited to wireless sensor networks (WSNs) featuring battery-less nodes that can be deployed in virtually any environment and require little or no maintenance. In response to the growing demand for advanced battery-less sensor tag solutions, this article presents a system for identifying and monitoring the speeds of assets in a WSN with battery-less tags that receive all their operating energy through radio frequency (RF) wireless power transfer (WPT) architecture, and a unique measurement approach to generate time-domain speed readouts. The assessment includes performance characteristics and key features of a system on chip (SoC) purposely designed to power a node through RF WPT. The result is an innovative solution for RF to DC conversion able to address the principal difficulties associated with maximum power conversion efficiency (PCE) with sensitivity and vice versa, a strategy, and a design optimization model to indicate the number of readers required for reliable asset identification and speed measurement. Model validation is performed through specific tests. Experimental results demonstrating the viability of the proposed advanced monitoring system are provided.

3-Stage Miller Cross-Coupled Load based Photodiode Readout for Glucose Monitoring.

An innovative 3-stage miller compensted, cross-coupled load, based photodiode front end readout is designed for glucose monitoring. The Near Infrared Spectroscopy (NIRS) technique is used for the optical sensing of glucose non-invasively. The stable 3-stage open loop amplifier is designed for a gain of 68.3 dB, phase margin of 65°, GBW of 12.6 MHz and has a power consumption of 0.26 mW. The transimpedance amplifier achieves a gain of 84.46 dB, phase margin of 65°, has an input referred noise of 20.4 pAHz and consumes 0.55 mW of power from a 3.3 V supply using a 0.18 $\mu \mathrm{m}$ CMOS technology node.

A 30 $\mu\text{W}$ Remotely Powered Local Temperature Monitoring Implantable System.

An implantable local temperature monitoring system for a laboratory mouse is presented. Magnetic coupling is used to remotely power the passive implant. The temperatures of two local points are monitored by thermistors. A low-power readout circuit is implemented by directly amplifying and resolving the sensor response in the time domain. A free-running oscillator operating at 868 MHz transmits the sensor data to the base station. The average power dissipation of the implant is decreased by time interleaving between the sensor readout and the data communication. The power transfer to the implant is also time interleaved with other operations to avoid interference with data communication. A voltage regulation loop for the implant based on controlling the duration of powering the base station power amplifier is also described. A prototype chip is implemented in 0.18 [Formula: see text] CMOS. The implant requires average RF power of 29.5 [Formula: see text] for operation and is capable of measuring two thermistors with accuracy of ±0.05 °C.

In-Vivo Validation of Fully Implantable Multi-Panel Devices for Remote Monitoring of Metabolism.

This paper presents the in-vivo tests on a Fully Implantable Multi-Panel Devices for Remote Monitoring of endogenous and exogenous analytes. To investigate issues on biocompatibility, three different covers have been designed, realized and tested in mice for 30 days. ATP and neutrophil concentrations have been measured, at the implant site after the device was explanted, to assess the level of biocompatibility of the device. Finally, fully working prototypes of the device were implanted in mice and tested. The implanted devices were used to detect variations in the physiological concentrations of glucose and paracetamol. Data trends on these analytes have been successfully acquired and transmitted to the external base station. Glucose and paracetamol (also named acetaminophen) have been proposed in this research as model molecules for applications to personalized and translational medicine.

A remotely powered implantable biomedical system with location detector.

An universal remote powering and communication system is presented for the implantable medical devices. The system be interfaced with different sensors or actuators. A mobile external unit controls the operation of the implantable chip and reads the sensor's data. A locator system is proposed to align the mobile unit with the implant unit for the efficient magnetic power transfer. The location of the implant is detected with 6 mm resolution from the rectified voltage level at the implanted side. The rectified voltage level is fedback to the mobile unit to adjust the magnetic field strength and maximize the efficiency of the remote powering system. The sensor's data are transmitted by using a free running oscillator modulated with on-off key scheme. To tolerate large data carrier drifts, a custom designed receiver is implemented for the mobile unit. The circuits have been fabricated in 0.18 um CMOS technology. The remote powering link is optimized to deliver power at 13.56 MHz. On chip voltage regulator creates 1.8 V from a 0.9 V reference voltage to supply the sensor/actuator blocks. The implantable chip dissipates 595 µW and requires 1.48 V for start up.

A 10.5 cm ultrasound link for deep implanted medical devices.

A study on ultrasound link for wireless energy transmission dedicated to deeply implanted medical devices is presented. The selection of the frequency to avoid biological side effects (e.g., cavitations), the choice of the power amplifier to drive the external transducers and the design of the rectifier to maximize the energy extraction from the implanted transducer are described in details. The link efficiency is characterized in water using a phantom material for a transmitter-receiver distance of 105 mm, transducers active area of 30 mm × 96 mm and 5 mm × 10 mm, respectively, and a system efficiency of 1.6% is measured.

Full fabrication and packaging of an implantable multi-panel device for monitoring of metabolites in small animals.

In this work, we show the realization of a fully-implantable device for monitoring free-moving small animals. The device integrates a microfabricated sensing platform, a coil for power and data transmission and two custom designed integrated circuits. The device is intended to be implanted in mice, free to move in a cage, to monitor the concentration of metabolites. We show the system level design of each block of the device, and we present the fabrication of the passive sensing platform and its employment for the electrochemical detection of endogenous and exogenous metabolites. Moreover, we describe the assembly of the device to test the biocompatibility of the materials used for the microfabrication. To ensure biocompatibility, an epoxy enhanced polyurethane membrane was used to cover the device. We proved through an in-vitro characterization that the membrane was capable to retain enzyme activity up to 35 days. After 30 days of implant in mice, in-vivo experiments proved that the membrane promotes the integration of the sensor with the surrounding tissue, as demonstrated by the low inflammation level at the implant site.

Remote powering platform for implantable sensor systems at 2.45 GHz.

Far-field remotely powered sensor systems enable long distance operation for low-power sensor systems. In this work, we demonstrate a remote powering platform with a miniaturized antenna and remote powering base station operating at 2.45 GHz. The rectenna, which is the energy receiving and conversion element of the sensor system, is designed and measured. The measurements for the tag are performed within 15 cm distance from the remote powering base station. The realized gain of the tag antenna is measured as -3.3 dB, which is 0.5 dB close to the simulations, where simulated realized gain is -2.8 dB.

Fully integrated biochip platforms for advanced healthcare.

Recent advances in microelectronics and biosensors are enabling developments of innovative biochips for advanced healthcare by providing fully integrated platforms for continuous monitoring of a large set of human disease biomarkers. Continuous monitoring of several human metabolites can be addressed by using fully integrated and minimally invasive devices located in the sub-cutis, typically in the peritoneal region. This extends the techniques of continuous monitoring of glucose currently being pursued with diabetic patients. However, several issues have to be considered in order to succeed in developing fully integrated and minimally invasive implantable devices. These innovative devices require a high-degree of integration, minimal invasive surgery, long-term biocompatibility, security and privacy in data transmission, high reliability, high reproducibility, high specificity, low detection limit and high sensitivity. Recent advances in the field have already proposed possible solutions for several of these issues. The aim of the present paper is to present a broad spectrum of recent results and to propose future directions of development in order to obtain fully implantable systems for the continuous monitoring of the human metabolism in advanced healthcare applications.

Theoretical study for safe and efficient energy transfer to deeply implanted devices using ultrasound.

The goal of this paper is to prove that a safe and efficient energy transfer is possible between an external transducer located on the patient's skin and a device deeply implanted in the abdomen. An ultrasound propagation model based on the Rayleigh-Sommerfeld diffraction integral is coupled with the data from the Visible Human Project to account for the geometry of the organs in the body. The model is able to predict the amount of acoustic power received by the device for different acoustic paths. The acoustic model is validated by comparison with measurements in water and in heterogeneous liquid phantoms. Care is taken to minimize adverse bioeffects-mainly temperature rise and cavitation in tissues. Simulations based on the bio-heat transfer equation are performed to check that thermal effects are indeed small.

In-vitro platform to study ultrasound as source for wireless energy transfer and communication for implanted medical devices.

A platform to study ultrasound as a source for wireless energy transfer and communication for implanted medical devices is described. A tank is used as a container for a pair of electroacoustic transducers, where a control unit is fixed to one wall of the tank and a transponder can be manually moved in three axes and rotate using a mechanical system. The tank is filled with water to allow acoustic energy and data transfer, and the system is optimized to avoid parasitic effects due to cables, reflection paths and cross talk problems. A printed circuit board is developed to test energy scavenging such that enough acoustic intensity is generated by the control unit to recharge a battery loaded to the transponder. In the same manner, a second printed circuit board is fabricated to study transmission of information through acoustic waves.

Suitable acoustic paths to transfer energy in depth using ultrasound.

The goal of this study is to optimize the transfer of acoustic energy deep in the body. In order to find suitable acoustic paths for the energy transfer to be efficient, we propose a model of ultrasound propagation that takes into account a realistic geometry of the tissue layers inside the human body. The data from the Visible Human Project is used to identify the tissues between the transducer array and the target point located on the epicard of the heart. The model is validated by comparison with measurements of the pressure field radiated by a focused 64-element array in water. Then, model results are presented in a realistic configuration, choosing a suitable input acoustic power so that adverse bio-effects are minimized. The model predicts the amount of acoustic energy that can be obtained at the target point, and is therefore useful to select the best acoustic path among several possibilities.

Numerical analysis of temperature elevation in the head due to power dissipation in a cortical implant.

This article presents the numerical analysis of temperature increase in the human head resulting from the power dissipation in a cortical implant. A 3-D head phantom with 22 tissue types and 0.2 mm x 0.2 mm x 2 mm resolution has been used in the simulations. The dependencies of the temperature increase on the power dissipation level, chip size, and location of the implant are investigated. Moreover, distributing power dissipation by using multiple integrated circuits in the implant is discussed. Maximum allowable total power dissipation in a cortical implant of size 2 x 2 mm2 is found to be 4.8 mW, whereas, it is 8.4 mW for an implant with two chips of same size placed 10 mm apart.

Gait assessment in Parkinson's disease: toward an ambulatory system for long-term monitoring.

An ambulatory gait analysis method using body-attached gyroscopes to estimate spatio-temporal parameters of gait has been proposed and validated against a reference system for normal and pathologic gait. Later, ten Parkinson's disease (PD) patients with subthalamic nucleus deep brain stimulation (STN-DBS) implantation participated in gait measurements using our device. They walked one to three times on a 20-m walkway. Patients did the test twice: once STN-DBS was ON and once 180 min after turning it OFF. A group of ten age-matched normal subjects were also measured as controls. For each gait cycle, spatio-temporal parameters such as stride length (SL), stride velocity (SV), stance (ST), double support (DS), and gait cycle time (GC) were calculated. We found that PD patients had significantly different gait parameters comparing to controls. They had 52% less SV, 60% less SL, and 40% longer GC. Also they had significantly longer ST and DS (11% and 59% more, respectively) than controls. STN-DBS significantly improved gait parameters. During the stim ON period, PD patients had 31% faster SV, 26% longer SL, 6% shorter ST, and 26% shorter DS. GC, however, was not significantly different. Some of the gait parameters had high correlation with Unified Parkinson's Disease Rating Scale (UPDRS) subscores including SL with a significant correlation (r = -0.90) with UPDRS gait subscore. We concluded that our method provides a simple yet effective way of ambulatory gait analysis in PD patients with results confirming those obtained from much more complex and expensive methods used in gait labs.