CSEM Scientific and Technical Report 2008

Distributed Electronics for Wearable Physiological Monitoring System
M. Correvon, J.-M. Koller, R. Gentsch, J.-A. Porchet, G. Dudnik, P. Pilloud, A. De Sousa, R. Rusconi
The majority of projects developed in the field of biomedical engineering concern the ambulatory vital sign monitoring. The configuration of
distributed sensors is dependent on the type of activity. When monitoring athletes, firefighters, rescuers or out-patients continuously innovative
solutions to solve the problem are required. The key issues of a robust and reliable patient monitoring system are the interconnection between the
distributed embedded electronics and the sources of energy.
Ambulatory monitoring of the human vital signs for different
applications such as sports, out-patients monitoring or the
supervision of professionals working in harsh environment
require a high level of comfort, reliability, accuracy, data
access, etc. Today, CSEM research activities in the life sign
monitoring domain focus on the sensor placement at body
such as the manubrium sterni (for SpO2, core body
temperature and activity monitoring), the thorax (ECG, activity
and respiration measurement), upper arm (blood pressure),
the ear cartilage (heart rate and activity monitoring) and the
wrist (activity monitoring). There is a common denominator for
all sensors at different body sites regarding the required
energy in order to acquire process and record the data. In
addition, for real-time monitoring of human vital signs the
connection to a wireless personal array network in order to
increase the ease of the system and to lower the maintenance
efforts.
Centralized architecture
In the configuration of a centralized architecture, the sensors
are either a simple electrical contact with the skin (in the case
of thorax sensor with ECG and respiration measurement) or
an electronic frontend integrating several sensor components
(in the case of the multi-parameter sensor at the manubrium
sterni with SpO2, activity and core body temperature, see
Figure 1). All the sensors are connected to a common
datalogger, which is made up of signal conditioning chains,
A/D converter, digital signal processor and user interface. The
datalogger also contains the energy source supplying the
overall monitoring system, in particular the electronics
frontend placed in the sensors. This latter is used either to
record the raw sensor data or to perform the data processing
if the application requires a real-time user feedback on a
display - some demanding applications require the data
recording and data processing at the same time. However, the
centralized architecture has some important drawbacks:
• Multiple wires between sensors and datalogger and
therefore difficulties to have an easy-to-use wearable
monitoring system with low reliability, lack of comfort,
difficulties to set up the system and wear it during
activities and an important SNR reduction.
• Distance between the measurement points (sensors) and
the signal conditioning chain (datalogger).
Partially distributed electronics
In the case of a partially distributed architecture, part of the
datalogger function is already integrated in the sensor
electronics. Such a configuration improves the signal quality
and as a consequence the robustness and reliability of the
signal processing. However, the higher signal quality comes
along with the following disadvantages:
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• Energy source in sensors (increase of weight and size).
• Wires between sensors are necessary, even though the
number of wires can be reduced.
Fully distributed electronics
The fully distributed electronics architecture targets the
development of intelligent sensors/active electrodes, able to
inter-communicate wirelessly. In this case it is much easier to
integrate the sensors in the garment. Biopotential (e.g. ECG)
or impedance measurements require a galvanic link between
sensors. However, thanks to modern manufacturing
techniques (knitting, weaving or coating of conductive
material), it is possible to distribute a unique reference
potential to each sensor. Analog signals are referred to this
reference potential, fixed in the middle of the supply voltage
range of each sensor. The data exchange between the
distributed sensors and the datalogger are performed via a
short-range wireless body sensor network. The datalogger
records either the raw data or the local processed data of the
sensors. For such an electronics architecture the energy
supply remains an extremely delicate issue. In fact, the
integration of sensors with radio and battery typically results
into systems with large size and unsuitable form factors with
sensor dimensions of several tens of cm3 and sensors
weighing several tens of grams.
Figure 1: SpO2 sensor to be placed on the manubrium sterni
developed at CSEM
Energy optimization
In order to tackle the problem of the energy supply, CSEM is
active along the following two research axes:
• Reduction of the power consumption of the short-range
wireless data transfer by optimizing the communication
protocol and the data compression.
• Improvement of the capabilities of the energy sources
(batteries, fuel cells).