Tobias Volk

Efficient Wireless Powering of Biomedical Sensor Systems for Multichannel Brain Implants

This paper describes the complete mathematical optimization process of an inductive powering system suitable for the application within implanted biomedical systems. The optimization objectives are thereby size, energy efficiency, and tissue absorption. Within the first step, the influence of the operational frequency on the given quantities is computed by means of finite element simulations, yielding a compromise of power transfer efficiency of the wireless link and acceptable tissue heating in terms of the specific absorption rate. All simulations account for the layered structure of the human head, modeling the dielectric properties with Cole-Cole dispersion effects. In the second step, the relevant coupling and loss effects of the transmission coils are modeled as a function of the geometrical design parameters, enabling a noniterative and comprehensible mathematical derivation of the optimum coil geometry given an external size constraint. Further investigations of the optimum link design also consider high-permeability structures being applied to the primary coil, enhancing the efficiency by means of an increased mutual inductance. Thereby, a final link efficiency of 80% at a coil separation distance of 5 mm and 20% at 20 mm using a 10-mm planar receiving coil can be achieved, contributing to a higher integration density of multichannel brain implanted sensors. Moreover, the given procedure does not only give insight into the optimization of the coil design, but also provides a minimized set of mathematical expressions for designing a highly efficient primary side coil driver and for selecting the components of the secondary side impedance matching. All mathematical models and descriptions have been verified by simulation and concluding measurements.

RFID Technology for Continuous Monitoring of Physiological Signals in Small Animals

Telemetry systems enable researchers to continuously monitor physiological signals in unrestrained, freely moving small rodents. Drawbacks of common systems are limited operation time, the need to house the animals separately, and the necessity of a stable communication link. Furthermore, the costs of the typically proprietary telemetry systems reduce the acceptance. The aim of this paper is to introduce a low-cost telemetry system based on common radio frequency identification technology optimized for battery-independent operational time, good reusability, and flexibility. The presented implant is equipped with sensors to measure electrocardiogram, arterial blood pressure, and body temperature. The biological signals are transmitted as digital data streams. The device is able of monitoring several freely moving animals housed in groups with a single reader station. The modular concept of the system significantly reduces the costs to monitor multiple physiological functions and refining procedures in preclinical research.

Formal Description of Inductive Air Interfaces Using Thévenin's Theorem and Numerical Analysis

With the development of new integrated circuits to interface radio frequency identification protocols, inductive air interfaces have become more and more important. Near field communication is not only able to communicate, but also possible to transfer power wirelessly and to build up passive devices for logistical and medical applications. In this way, the power management on the transponder becomes more and more relevant. A designer has to optimize power consumption as well as energy harvesting from the magnetic field. This paper discusses a model with simple equations to improve transponder antenna matching. Furthermore, a new numerical analysis technique is presented to calculate the coupling factors, inductions, and magnetic fields of multiantenna systems.

Efficient inductive powering of brain implanted sensors

This paper describes a size and tissue absorption based comprehensive approach to optimize a pair of coils for the purpose of wireless powering of brain implanted sensors. In the first step, the optimum transmission frequency is determined by considering tolerable coil size, power transmission efficiency and tissue absorption effects. After modeling the important quantities at the frequency of interest, a numerical analysis is performed, revealing a set of coils suitable for efficient inductive powering. This numerical analysis was verified by both FEM simulation and concluding measurements. All simulations account for the layered structure of the human head, modeling the dielectric properties with Cole-Cole dispersion effects. Furthermore, a strategy of boosting power transmission efficiency is covered in simulation and measurement, particularly the application of a ferrite shielding to the transmission coil. In consequence, a link efficiency of 80% at a coil separation distance of 5mm and 20% at 20 mm using a 10mm planar receiving coil can be achieved, contributing to a higher integration density of multi-channel brain implanted sensors.

Wireless power distribution system for brain implants

Implants like brain pacemakers or brain computer interfaces (BCI) fundamentally requires an improved and efficient wireless power distribution system. This work therefore presents a novel concept based on an intermediate resonator, which provides the opportunity to power multiple implants and to minimize furthermore the dimensions of the external power transmitter. Numerical computations specify requirements to the antenna configuration and a model show the electrical behavior. Finally, a prototype system presents an initial implementation, allowing the evaluation of the concept.

Motion capture sensor to monitor movement patterns in animal models of disease

In this paper we present two new motion capture sensors to monitor motoric dysfunction in laboratory animals. The parameters that are recorded by our system correspond to neurological deficits that are typical for multiple sclerosis (MS)-like symptoms in animals. Normally, quantification of motor impairment requires neurological examination and complex behavioral testing. However, to perform these tests is an error-prone and time consuming process. Therefore, a strong interest exists in the automation and objective analysis of motoric behavior. Our presented small, accurate, and lightweight motion sensors provides an optimal solution for this problem. The developed motion sensors have the smallest volume and weight requirements available at the moment to monitor motoric dysfunction of animals. The collected data from the sensor is more representative since the subjective human factor is minimized and the animal can stay in its usual environment rather than being placed in a separate observation cage. We present two wireless motion sensors. An active sensor is powered with battery and saving the data on a SD-Card. The second sensor works completely passive and is powered via electromagnetic field. The sensors provide full control over the data of a three-axis accelerometer, a three-axis gyroscope, and a three-axis magnetometer. The two principles were successfully tested in an initial animal experiment.

Novel concept for a wireless and batteryless brain implant array

In case of a neuro-degenerative disorder, closed-loop brain stimulation is able to reduce tremor, rigidity, bradykinesia, as well as postural instability and consequently to improve the patients quality of life. Therefore, we investigate a human machine interface based on a multi-implant approach, which avoids drawbacks of conventional systems. The following paper presents concept, theory, as well as an initial prototype of a novel wireless power distribution / communication system together with the used signal processing. Finally, the publication shows a prospect to the future implant.

A programmable and self-adjusting class E amplifier for efficient wireless powering of biomedical implants.

In this paper, an enhanced approach of a class E amplifier being insensitive to coil impedance variations is presented. While state of the art class E amplifiers widely being used to supply implanted systems show a strong degradation of efficiency when powering distance, coil orientation or the implant current consumption deviate from the nominal design, the presented concept is able to detect these deviations on-line and to reconfigure the amplifier automatically. The concept is facilitated by a new approach of sensing the load impedance without interruption of the power supply to the implant, while the main components of the class E amplifier are programmable by software. Therefore, the device is able to perform dynamic impedance matching. Besides presenting the operational principle and the design equations, we show an adaptive prototype reader system which achieves a drain efficiency of up to 92% for a wide range of reflected coil impedances from 1 to 40 Ω. The integrated communication concept allows downlink data rates of up to 500 kBit/s, while the load modulation based uplink from implant to reader was verified of providing up to 1.35 MBit/s.

Semi-passive RFID sensor implant

Based on common RFID technology the University of Applied Sciences Offenburg is developing a new bio-telemetrical system called μTrans. Semi-passive RF transponders implanted in small animals measure ECG, pressure, temperature, oxygen saturation and activity. Using modern cloud technology, it is possible to build up a sensor network of a nearly unlimited size.

A disposable passive temperature sensor with RFID ISO15693 interface

The idea behind the ThermoTag is the need to develop a complete passive tag with a simple circuit for measuring temperature. This paper focuses on the development of the tag which includes the proper designing of the antenna, which is very important for the passive development and also the measuring principle and the technique used for measurement. As a result a small and thin transponder (45mm in diameter and 3.5mm in thickness) is developed which is used for measuring temperature over a wide range. The transponder is one of its kinds because of its size, simplicity and by using a NTC helps to produce a “disposable passive sensor device”.