Small autonomous wireless sensors, linked into a network, can be used in a variety of applications ranging from health and lifestyle, automotive, smart building, and predictive maintenance to smart packaging. The miniature sensor nodes have their own energy supply consisting of energy harvesting and energy storage devices; a low-power wireless connection to the other sensor nodes within the network; and some built-in intelligence to carry out basic data-processing tasks. This article discusses the benefits and technological challenges of using such wireless sensor nodes as a body area network (BAN) that may enhance existing health monitoring systems and enable new personal health applications.
Sensor node basics
Wireless sensor networks (WSNs) consisting of small nodes with sensing, processing, and wireless communications capabilities are becoming widely used in our society. They are being used for therapeutic and diagnostic purposes, as well as for monitoring industrial processes, in active RFID tags, and in automotive applications. At present, the majority of WSN nodes rely on batteries for operation. The nodes we'll discuss are autonomous - relying on energy harvesting for power - and thus require low-power electronics and sophisticated energy management.
Figure 1 shows the major components of a typical node. Developing such a node typically requires a combined expertise in wireless ultra-low-power communication, packaging and 3D integration, sensors and actuators, low-power design, and energy harvesting technologies. The latter are needed to make the products truly autonomous.
A number of energy harvesting principles are currently under development and the first commercial systems are entering the market. The main technologies are based on vibrational, thermal, photovoltaic, or RF harvesting and it is expected that they can supply energy in the 10 µW to 1 mW range. Another requirement for general use and interoperability of WSNs is the availability of a standard protocol for communication. Emerging communications standards such as IEEE 802.15.4 are becoming available for WSNs.
The specific requirements and technology challenges for WSNs obviously depend on the application they will serve, on the effect to be sensed, and on the data rate of the transmitted data. Consider power consumption as an example. On the one hand, 90 µW seems enough to power a pulse oximeter, to process data, and to transmit them at intervals of 15 s. On the other hand, 10 µW turns out to be sufficient to measure and transmit temperature readings every 5 s. In general, 100 µW is considered to be sufficient for relatively complex autonomous WSN nodes operating at relatively high data rates. If one considers MEMS-based energy harvesters, 100 µW/cm2 is considered to be a value that is attainable but challenging. Importantly, MEMS technology offers a route towards cost-effective harvester fabrication.
BANs for personal health
The use of wireless sensor nodes is technologically most advanced in the healthcare and lifestyle sector. It is expected that WSN technology will soon enable people to carry their personal BAN that provides medical, lifestyle, wellness, assisted living, sports, and entertainment functions for the user.
The network would comprise a series of miniature sensor/actuator nodes implanted or located at the body surface. Each node will have its own energy supply consisting of energy storage and energy harvesting devices. Each node has enough intelligence to carry out its own tasks and is able to communicate with other sensor nodes or with a gateway node worn on the body. The gateway node communicates with the outside world using a standard telecommunication infrastructure such as a wireless local area or cellular phone network, allowing experts to provide services to the individual wearing the BAN.
Early adopters of BAN technology are seen in the sports and lifestyle area, using a wireless sensor node to measure one or a few body parameters - e.g., heart rate or physical activity - and communicate the data wirelessly to a cell phone, usually via Bluetooth.
These examples have important limitations, mainly in terms of power consumption. The current state-of-the-art in low-power electronics enables significant increases in the lifetime of these systems while reducing their size - ultimately these may become autonomous systems. Early prototypes based on this technology are now becoming available.
Wireless ECG patches
The availability of such BANs will play an important role in future healthcare, which will evolve from disease-centric to patient-centric care, transitioning the point of care from the hospital to the home, and shifting the emphasis to prevention rather than cure. BAN technology will benefit some existing applications, reducing the monitoring burden and enhancing the patient's comfort, e.g., a wireless ECG patch for ambulatory monitoring of cardiac activity. Moreover, BANs also have the potential to enable applications that were impossible before this technology was introduced.
The wireless ECG patch has been tested in ambulatory settings, to evaluate how physical activity affects the quality of ECG recordings. Results have shown that the patch provides signals with excellent quality in resting conditions but the quality of the signal degrades as the level of activity increases. For activity levels up to those that correspond to running on a treadmill at 7.5 km/h, the quality of the signal is maintained at a usable level for further analysis.
Key challenges
Technology evaluation in various application environments has led to the identification of key technology challenges that need to be addressed to enable a widespread deployment of BANs and to meet the ultimate target of developing wireless body sensor nodes consuming 100 µW power on average. The challenges are related to the following.
Ultra-low-power technologies. Recent advances in the design of ultra-low-power front ends can already enable drastic reductions in the node's power budget. To meet the target of 100 µW per body sensor node, further research is needed on ultra-low-power analog interfaces, sensors, DSP, and radios.
Autonomous systems. Today's prototypes can run for a few days at full functionality. Breakthroughs in ultra-low-power technologies will enable months or years of autonomy. Harvesting energy from the environment during the operation of the system will eventually allow the system to run perpetually with a small rechargeable battery or a supercapacitor acting as a temporary energy buffer.
Multi-parameter sensors. Extending the range of functionality to include new sensing modalities will be crucial in fostering research in personal health applications, leading to new discoveries.
Increasing functionality. Low-complexity and real-time algorithms are required to enable intelligent autonomous systems.
Dry electrodes. These are required to enable simple setup of the system by the user.
Integration technology. Electronic integration in bidimensional flexible and stretchable foils will enable unobtrusive body sensor nodes that are integrated in patches, clothes, or even fashion accessories.
Wireless sensor nodes are emerging in diverse application fields, where they all face some common challenges when it comes to a widespread acceptance: autonomous operation, functionality, intelligence, miniaturization, and manufacturing cost. The specific requirements depend on the target application. In healthcare and lifestyle, the benefits of using WSNs have already been demonstrated, and an early deployment of BAN technology has led to the identification of key technological challenges for further improvement.
Ruud Vullers is principal researcher and program manager of the micropower program at Holst Centre/Imec
Julien Penders is program manager at Holst Centre/Imec
Mihai Patrascu, PhD., MSc has been a researcher at the Holst Centre since 2006
This article originally appeared in Sensors