Edinburgh Napier University

  With the advancements in material sciences, the sensor research field is heading towards the implementation of an ultimate “nerves system” that can be utilized to sense various, if not all, physical, chemical and biological aspects not only of living beings but also of natural and/or man-made surroundings. This is evident from current trends towards the “smart city” concept, where the implementation of sensor systems to monitor the physical conditions of the civil structures of a city plays a vital role in addressing the economic benefits and ethical need for safe and sustainable infrastructures.

One main stakeholder of the said smart concept is the employment of appropriate structural health monitoring (SHM) schemes that can “sense” the physical conditions of structures to ensure their safe and efficient operation. Conventional techniques used for SHM are limited to electrical means i.e., the use of strain gauges for strain measurement, for instance, where a pool of wires carrying a current not only poses a safety threat, but also is tedious to install and is not resource-efficient.

Following the introduction and use of fibre optics during the telecommunications boom in the 1990s, both research and industry confidence had gathered momentum in the use of fibre optic technology not only for telecommunications purposes but also for sensing applications. Recent research has also looked into the possibility of embedding sensors in smartphones, paving way for the wider application and use of fibre optic technology. In terms of SHM applications, fibre optic sensors (FOSs) have considerable advantages over their conventional electrical counterpart, i.e., being small in size, light weight, electrically passive and hence immune to electromagnetic interference.

Since optical fibre are made of silica (glass), they are robust and can withstand harsh environments, i.e., FOSs can be used in high-temperature, radiation-hard and corrosive environments where electrical sensors cannot be used. Due to the low light attenuation of optical glass fibre, they can be multiplexed and interrogated over several kilometres. The multiplexing and wider bandwidth capabilities render FOSs most suitable, therefore, when extracting data from a vast amount of sensing elements such as those required for SHM of larger areas, i.e., SHM of sewerage tunnels, dams and bridges.

Conventional approaches to SHM using FOSs have been generally focused on wavelength detection-based sensors that are mounted to the structure afterwards, i.e., using epoxy for instance, so that effective measurement of physical parameters, such as strain, can be obtained. While these techniques are a step forward from electrical strain gauges, the need for specialist personnel to install them on the structure and the relative tediousness of having to glue them on the surface makes these schemes less attractive and flexible for civil engineers. If the sensory elements can somehow be embedded with the strengthening mechanism of a structure, it would not only address the issue of having to install the sensors separately, but it would also pave way to a relatively more accurate system that would not require an intermediate transfer agent, e.g., the epoxy. Thus, it is of great interest to design a sensor system that could be “embedded” into the strengthening mechanism of a structure, providing the dual purposes of sensing and strengthening, saving a considerable amount of time and money Advances in material fabrication and processing engineering have rendered a step towards the possibility of “smart textiles” incorporating embedded sensors inherently in structures/textiles in order to not only mitigate the risk of failure due to an overload or unwanted inhomogeneity resulting from the fabrication process, but also for the identification, assessment and location detection of physical parameters, i.e., such as strain, experienced by the composite containing the sensors.

In light of the discussion above, the paper intends to present a review of latest smart sensors, their operating mechanisms and applications. In doing so, the aforementioned work by Edinburgh Napier University on the smart textile incorporating embedded sensors, will also be highlighted. The intention of the paper is to introduce and “enthuse” the readers with this latest technology, which will be a major player towards “smart cities” concept. It will also highlight the importance of Optical Fibre Sensors field, in not only monitoring civil infrastructure, but also in healthcare systems, where patients will have “smart sensors” which would alert relevant authorities when an urgent action is needed.