Recent years have seen a plethora of health monitoring devices coming to the market. While it is possible to record single signals (e.g. one impedance for breathing rate measurement) with a relatively simple architecture (e.g. a patch on the chest), a more complete assessment of a subject’s health status (e.g. an EIT image of the lungs) typically requires the placement of sensors on several distant spots on the body. In the classical approach, this is done by connecting each sensor to a central box with an individual wire, see Figure 1. This architecture has a few disadvantages, namely: (1) The miniaturization of the sensors and of the central box is limited by the size of the cable connectors. (2) Manufacturing of a wearable with this architecture is relatively complex. (3) Each added cable makes the wearable stiffer and thus less comfortable to wear. This is especially true for shielded cables which are necessary for best measuring results.
About 15 years ago, CSEM has started to tackle the cabling issue of sensors in wearables and has developed an architecture, where all sensors are connected in a bus-like manner with one or two wires (not cables!), see Figure 2. This concept – so-called cooperative sensors – resolves all of the three problems mentioned above since there is no central box and since each sensor is only connected to one or two wires which may also be replaced by a conductive tissue. In addition, the concept is suitable for a high number of sensors (which may be placed anywhere on the body) and to measure a variety of body signals with different sensors. The main tasks of the bus-wires are (1) to give a reference potential to with respect to which each sensor measures a body surface potential; (2) to transmit the measured signals from the sensors to a central node; (3) to synchronize the sensor.
The first generation of cooperative sensors brought to the market was Sense, a two-sensor system which allows to measure a 1-lead ECG, the breathing rate, the body temperature and the activity of the wearer.
In the EU-funded project WELCOME, the cooperative sensors proved for the first time their full potential in a wearable multi-sensor device. The WELCOME vest is equipped with more than 20 cooperative sensors which measure in parallel EIT, ECG, the body activity and the oxygen saturation of the blood (SpO2), see Figure 4. The integration of such a high number of sensors has been made possible thanks to the advantages of cooperative sensors, namely the simple wiring which made the manufacturing of the vest feasible and allowed to keep the sensors reasonably small.
In WELMO, we are now developing the third generation of cooperative sensors. The big ambition of WELMO is to further reduce the size of the sensors. This is on one hand done by the development of an ASIC which integrates the sensor electronics on a chip smaller than a fingernail. The volume of each sensor is reduced even more by removing the batteries from each sensor: Where in the previous generations of cooperative sensors, a battery in each sensor was needed, now, in the WELMO design, the batteries are no longer present in each sensor. Instead, we design WELMO so that the sensors are powered from a central battery, which has the additional advantage that only this one central battery has to be recharged and not all individual sensors. One of the challenges in WELMO is therefore to design the sensor system in a way that allows to use the bus wires for power transmission, in addition to the data transfer, sensor synchronization and serving as a reference potential.
In summary, the centralized powering makes WELMO a very unique design which is adaptable to the number and type of sensors. Also, the miniaturization of the sensors increases a lot the wearing comfort of the system. Finally, this new powering strategy reduces greatly the complexity associated with the charging of such system, improving its usability.
Centre Suisse d’Electronique et de Microtechnique (CSEM)