**An Efficient Adaptive Antenna-Impedance Tuning Unit Designed for Wireless Pacemaker Telemetry**

Francis Chan Wai Po1, Emeric de Foucauld2, Jean-Baptiste David2, Christophe Delavaud2 and Pascal Ciais2 *1Institut Supérieur d'Electronique de Paris, 2CEA LETI MINATEC, France* 

### **1. Introduction**

222 Modern Telemetry

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Since its first implantation into human body in 1958, pacemaker has known several evolutions [1-2] to become nowadays a vital cardiovascular device frequently prescribed to improve the quality of life for hearth failure patients. To increase the quality of service, pacemaker industry tends to integrate wireless telemetry technology into the medical device to allow home monitoring of the patient. Home monitoring technology challenges to analyse and to diagnose while the patient is sitting or sleeping at home.

The pacemaker radio communication module, designed to exchange data with an external base station, features new technologies including, but not limited, new architecture, low power design technique, acoustic wave filter co-integration, miniaturized antenna design, etc. The miniaturized antenna embedded on the pacemaker device is typically a narrow bandwidth high-Q antenna [3] easily detuned by unpredictable near field environmental factors [4-6]. The input impedance of the implanted antenna can vary due to the tissue (muscle, fat, skin, etc.) properties, thickness, and also the individual properties which differ from one person to another. In addition, the patient position and more generally the nearby objects may cause also change in the antenna input impedance. Mismatch of the antenna impedance significantly degrades the transmitter radiated output power, the receiver sensitivity, and therefore the power efficiency of the radio transceiver.

To highlight the possible random variability in the antenna input impedance that contributes to generate more or less important mismatch losses, precise characterization of the pacemaker antenna using different realistic human models is needed. In this way, electromagnetic simulations and measurements of the input impedance of the antenna immersed into homogeneous and heterogeneous human model were performed.

To guarantee the success of the wireless communication even in the presence of mismatch losses, traditional solution over specifies the design of the RF power modules consuming more energy at the expense of the battery lifetime. This solution is obviously not mandatory where a targeting lifetime at least greater than seven years is required for such implantable medical device. More suitable solution is focused on the addition of an adaptive antennaimpedance tuning unit to automatically match the antenna input impedance to the optimal impedance of the RF front-end radio communication module.

An Efficient Adaptive Antenna-Impedance

**2.1 Proposed wireless medical telemetry** 

external unit.

microsystem.

**2.1.1 BAW Filter design and integration** 

as analogue RF circuits.

Tuning Unit Designed for Wireless Pacemaker Telemetry 225

To allow home monitoring of the patient, it is necessary to integrate a wireless telemetry system into the medical device. Implantable pacemaker telemetry system provides a means for receiving downlink information from an external base station to the implanted medical device, and for transmitting uplink signals from the implanted device to the

Pacemaker microsystem typically embeds a controller, electrocardiogram (ECG) sensors and few analogue electronics blocs. In addition, the new generation device includes a wireless telemetry functionality which simplified bloc diagram is illustrated in Fig. 2. As the size of the final product should not be larger than its predecessor, it is necessary to tend towards a large scale integration of memory, controller, RF functionality including RF MEMS as well

The proposed wireless telemetry system integrates transceivers operating respectively at the Medical Implant Communication Service (MICS) frequency band and at the 2.4 GHz Industrial Medical Scientist (ISM) frequency band. The MICS 402-405 MHz frequency band is used to transmit short range secured data and for emergency link because only the exclusive MICS band is acknowledge as safe for medical devices by Food and Drug Administration (FDA). The ISM 2.4 GHz transceiver is dedicated for the implementation of a needed ultra low power wake up system and for transmitting data under higher equivalent isotropically radiated power (EIRP) to achieve the demanded increased communication range. In addition to the bi-band transceiver, a Bulk Acoustic Wave (BAW) filter and naturally a miniaturized loop antenna are embedded into the medical

The filter was implemented to address the high level risk of electromagnetic interferences in the widely used ISM 2.4 GHz frequency band using Solid Mounted Resonators (SMR). The resonators in SMR structures are realized on the top of an acoustic mirror structure based on the Bragg reflector principle [16]. The resonators layers were composed of classical couple AlN-Mo. In contrast to [17], the Bragg reflector was implemented using an exclusive dielectric stack composed of SiOC:H and SixNy. The acoustical performance of the fully dielectric stack is comparable to traditional SiO2-W reflectors. However, this fully dielectric configuration strongly reduces the electrical coupling between resonators, and ensures high

Fig. 2. Simplified bloc diagram of the pacemaker's wireless telemetry

Most of existing antenna-impedance tuning units [7-15] operates iteratively to successfully adapt source and load impedances. However, iterative methods generally spend several hundred milliseconds to calibrate the system and are not well suited to low power pacemaker applications where energy is lost during the calibration as the proper state configuration of the system is not yet obtained.

The design of an energy efficient antenna-impedance tuning units based on a single step calibration method is proposed in this chapter to achieve a low power automatic matching process. The proposed method aims to extract the antenna complex impedance value in order to calculate the parameters of the network that match the extracted antenna impedance to the impedance of the RF power module at a selected frequency.

In this chapter, we describe briefly the pacemaker telemetry system, the design constraints and the limitations in section II. Since the pacemaker antenna is easily detuned by tissues, we challenge to characterize the impedance of the antenna immersed into different realistic human model in section III. In section IV, we propose a novel antenna impedance tuning method based on a single step process calibration to adapt automatically the antenna impedance to the optimal impedance of the front-end radio.
