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Sensing

Sensors fall into two realms based on the frequency of data coming through; these are a continuous feature stream and a discrete state description. An example of a continuous feature stream is the data coming from real-time monitoring of an ECG signal; whereas a discrete state stream might be the data coming from a motion sensor. In general, sensors can be also classified in terms of physical, chemical and biosensors. Sensors for WBSN generally fall into the physical sensing classification.

Physical Transducers

These sensors do not have to be directly exposed to the sample and are therefore more rugged and reliable in long term deployments. They are generally low power and often low cost. They are the most suitable for scale up and for real-time data generation. Although they are not specific sensors for chemical or biological species, they can provide general information about the environment and personal health. Examples include thermistors, vibration sensors, accelerometers, photodetectors, pressure sensors, acoustic sensors, and non-contact conductivity/impedance. Low-cost spectral information can be generated by combining selective light sources like LEDs (narrow band emitters) with photodetectors so that the spectral region is associated with the particular absorbance band of a specific target [1]. An example is the use of red LEDs in pulse oximetry to determine the varying concentration of oxygenated haemoglobin in real time. Important developments in this area include the emergence of new materials such as soft polymer sensors and actuators (e.g. that are biocompatible and can mimic the function of muscles) [2]. These are increasingly being integrated into "lab on a chip" devices to provide low-cost, low power methods for moving samples and reagents around microfluidic manifolds, and perform relatively complex analytical measurements in a compact device [3]. They are also being integrated into textiles to generate wearable systems capable of sensing movement and breathing. Related devices in this class also include ECG/EKG/EMG electrodes that provide real-time data on aspects of heart function [4]. A particular focus for research in this field is on how to obtain good quality data from dry contact electrodes.

References

1. King-Tong, L., S. Baldwin, M. O'Toole, R. Shepherd, W. J. Yerazunis, S. Izuo, S. Ueyama and D. Diamond, "A low-cost optical sensing device based on paired emitter-detector light emitting diodes", Anal. Chim. Acta., Vol. 557, No. 1-2, 2006, pp. 111-116.
2. Brady, S., K.T. Lau, W. Megill, G.G. Wallace and D. Diamond, "The Development and Characterisation of Conducting Polymeric-based Sensing Devices", Synthetic Metals, Vol. 154, No. 1-3, 2005, pp. 25-28.
3. Causley, J., S. Stitzel, S. Brady, D. Diamond and G. Wallace, " Electrochemically-induced fluid movement using polypyrrole ", Synthetic Metals, Vol. 151, No. 1, 2005, pp. 60-64.
4. Linz, T., C. Kallmeyer, R. Aschenbrenner, and H. Reichl, "Embroidering Electrical Interconnects with Conductive Yarn for the Integration of Flexible Electronic Modules into Fabric", Proc 9th IEEE International Symposium on Wearable Computers (ISWC'05), Osaka, Japan, Oct 18-21, 2005, pp 86-91.