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.