Miniaturized wireless sensing system for real-time breath activity recording N. André 1 , S. Druart 1 , P. Gerard 2 , R. Pampin 1 , L. Moreno-Hagelsieb 1 , T. Kezai 2, L. Francis 1 , D. Flandre 1 and J.-P. Raskin 1 1 Research Center in Micro and Nanoscopic Materials and Electronic devices (CeRMiN) Université catholique de Louvain, Louvain-la-Neuve, BELGIUM E-mail: nicolas.andre@uclouvain.be 2 WINIT, Université catholique de Louvain, Louvain-la-Neuve, BELGIUM Abstract— A portable, non-invasive and easy to operate wireless system has been developed for monitoring the breathing activity of patient. The system is composed of a capacitive microsensor (airflow- humidity sensor) integrated on a silicon chip and of a Negative Temperature Coefficient thermistor; both are connected to a wireless network to allow efficient healthcare at home as well as in hospitals. The capacitive sensitive part of the microsensor is an array of interdigitated metallic electrodes covered by 100 nm- thick dense anodized aluminum oxide layer. The breath water vapor is adsorbed over the interdigitated electrodes and changes the sensor characteristic capacitance by up to 2 orders of magnitude. This modulated signal is then digitized and either stored in a memory or directly transmitted to a monitor through a short distance Radio Frequency (RF) link. Results show that the wireless platform can be powered by two AAA batteries and deployed in a mesh or star configuration as wireless sensor network. Full size of the microsensor is less than 1 cm² and is conveniently implemented in a classical adhesive bandage or in nasal prongs. This microsystem is proposed for monitoring sleep- disordered breathing as well as breathing rhythm of athletes during effort. Keywords — Breath monitoring, interdigitated sensor, temperature sensor, wireless set-up, SAHS. I. INTRODUCTION Building bridges between the medical and engineer worlds, to sense human life parameters, has been from always a constant care to help diagnoses. As breath is one of the vital parameters, several systems to monitor respiration have been developed: thermistors, thoraco-abdominal band, nasal prongs and pneumotachograph [1]. The gold standard pneumotachograph is very invasive and cannot be used for routine diagnosis. Indeed, in homecare and hospital applications, medical body-mounted sensors should not impede the patient movements and must take into account its social comfort. To satisfy these specifications, a wireless concept is an interesting way for designing a new kind of portable respiratory rate measurement system. Following this idea, airflow can be detected by change of temperature, humidity or CO 2 concentration in breathed air [2]. The thermistor is the most common sensor for oro- nasal flow. Nevertheless, it shows a long time-constant response and a certain inability to detect change in volume exhalation [3]-[4] expected in other phenomena than apneas. A pressure transducer located at the end of nasal prongs, assessing indirectly the ventilation, authorizes detection of the complete range of events appearing in sleep apnea-hypopnea syndrome (SAHS): apneas, hypopneas and flow limitations [4]. In this paper, the use of a thermistor coupled to a nasal humidity sensor provides, respectively, a reliable and fast breath activity tracking, allowing for sufficient humidity detection into nasal prongs even in case of oral respiration. To achieve at the best all the requirements – low hysteresis, high range, resistance against chemicals, short time response, no drift, no degradation - we chose to focus on miniaturized humidity sensors, and especially ceramic- based. Even if the trend is to use porous materials [4], we demonstrated a change in capacitance by 2 orders of magnitude for dense aluminum oxide sensing materials, described in Section II. Combined with a classical negative temperature coefficient (NTC) thermistor, the ambition of our system is to improve comfort and reliability of patient assessment during sleep. Transduction techniques for very small capacitance are multiple: capacitance conversion into frequency, phase, voltage, etc., possibly oriented towards monolithic co- integration [5], [6]. Enabling microcapacitive as well as resistive sensing with only three components, a modulated frequency signal provided thanks to a Schmitt trigger oscillator was chosen to record respiration rate on near real- time for further diagnosis of respiration behavior of patients. Hence, we focus on both parts of the wireless system, described in Section III: the interdigitated CMOS microsensor and the wireless platform based on a ZigBee compliant chip (CC2430) from Texas Instruments. The wireless platform is capable of transmitting data at 2.4 GHz (standard IEEE802.15.4) and can be easily integrated in an