report
9.12.3 Short description

The advancements in wireless communications and integrated circuit have made the development of sensory networks possible. A network of sensors to monitoring environmental conditions and presence of threatening agents could be used in the protection of infrastructure. These sensors could detect temperature, light, vibration, sound, chemical species, trace vapours and radiation. The concept of a sensor web has been reported in the literature. This sensor web has been proposed to comprise of three layers, sensing, communication and information. The sensor web provides functional characteristics such as interoperability between platforms, low cost, reliability, scale up, and high resolution. The multimodal information obtained from the earth observatory system is expected to considerable enhance data processing capabilities for accurate analysis and decision making [296].

A sensor node comprises of a sensing unit, a processing unit, a power unit and a transceiver unit. The sensor unit further is comprised of the sensor and an analog to digital converter. The processing unit serves the function of working with other units on assigned sensing tasks. The transceiver unit serves the purpose of connecting the node to the network. The power unit supported by power scavengers such as solar cells serve the purpose of providing functioning power to the sensor node. Networking of such sensor nodes is an important aspect of these sensor nodes, has important implication for civilian security such as target imaging, instruction detection in sensitive locations and surveillance by detecting ambient conditions or presence of objects [297].

Figure CM.1 - A schematic diagram of a sensor node [298]

Figure CM. 1 - A schematic diagram of sensor node [298]

Sensing - The use of carbon nanotubes as a sensing element has been mentioned in literature. A number of examples of such sensors are mentioned in the Detection section of civil security. The use of electrical and mechanical properties has been made in creating the smallest ‘balance’ that by mounting a single particle at the end of carbon nanotubes. On application of an electrical charge the carbon nanotubes vibrates like a spring and mass of the particle is calculated using changes in the resonant vibration frequency [299]. This remains an example of physical sensing. Nanobarcodes comprising of 50nm cross section cylindrical rods can be coated with analyte specific entities, for detection of complex molecules such as DNA [ 300, 301]. Chemical sensors based on carbon nanotubes have been demonstrated in the detection of NO2 or NH3 by signalling a change in the electrical resistance of the carbon nanotubes [302]. Titania nanotubes
sensors have been reported in the literature to have been incorporated in a wireless sensor
network for the detection of hydrogen [303].

Processing - Carbon nanotubes based transistors are expected to become an important component of the processing unit in sensor nodes. IBM has developed and successfully demonstrated using multiwalled carbon nanotubes or single walled carbon nanotubes, as channels for field effect transistors. The current flowing through the nanotubes can be changes by a factor of 100, 000 by changing the voltage [304]. NEC has reported developing a method of positioning the CNT which allow electrons to flow 100 times fasters than silicon and reduce the
power consumption by a factor of 20 [305].

Communication and Data transmission – The transfer of information maybe obtained
Microwave amplifiers are used in wireless communication are expected to form an important part of the transceiver unit. CNT cathode developed thin film fabrication techniques have demonstrated stable current densities in excess of 100 mA/cm2, which is the current density necessary for amplifiers [306]. Excellent RF properties of nanotubes and nanowires have been successfully demonstrated to have been integrated with wireless communication. The advantage
of this approach of using carbon nanotubes as an antenna is that it can communicate with the
sensing unit without have to use interconnects fabricated using lithography [307, 308]. Another advantage of the nanotubes antenna is that it can serve as an excellent impedance matching circuit to receive signals. Optoelectronic communication of information can be achieved through the use of nanooptoelectronics, diffractive optical elements, optoelectronic transducers and
photonic components. Opotoelectronic components are composed of quantum wells, quantum dots, and photonic crystals. A more detailed view on the research and development in the optoelectronic communication maybe obtained from the ICT Technology sector of the Observatory Nano project.

Storage - The storage unit of such a sensory network is expected to benefit from the improved storage capacity. The millipede technology demonstrated by IBM can hold up to a trillion bits per square inch, 20 times more information than the most dense information storage available. The millipede works by punching thousands of indentation using nanosharp tips, which represents
bits. Ultra fine tipped V shaped silicon cantilever produced by surface silicon micromachining. The millipede technology is expected to pack 10-15 GB of data [309]. CNT based memories
developed by Nantero have reported a 10GB memory. The NRAM offers advantages such as
non-volatility, high speed and price of a dynamic memory is expected to be a better candidate than other memories. The nanotubes are sprinkled on silicon wafer, where the nanotubes are balanced on the ridges of silicon. A change in the electrical charge can swing the nanotubes in
one of two positions representing a zero or one state. Another advantage offered by this memory is the low power consumption required to change the state due to the small size of the nanotubes [310]. A number of memory storage technologies are being developed that may have a potential
application in sensory network such as Molecular memory, Ferroelectric RAM, Magnetic RAM, Phase Change Bridge RAM. A more detailed view of these developments and other storage developments can be obtained from the Memory sub-sector of ICT Technology Sector in the Observatory Nano project.

Power Supply - The supply of power to sensory nodes is of utmost importance for continuous condition monitoring of critical infrastructure. The use of nanocrystalline material and nanotubes has been mentioned in the literature to improve power density, lifetime, and power charge/discharge rates of batteries. Nanotubes as a replacement in the graphite-lithium electrode of portable batteries are expected to improve performance by increasing performance due to higher surface area. Nanocrystalline materials have been mentioned as potential material for
separator plates due to the foam like structure which can hold more energy [311]. Other materials are being considered are transition metal oxides of cobalt, zinc, iron and copper for electrodes in lithium ion batteries. The surface electrochemical reactivity is improved by nanoparticles thereby improving the performance of lithium ion batteries [312]. Experiments have reported a 600%
increase in reversible energy capacity. Material for cathode based on carbon nanotubes, titanium dioxide, and vanadium oxide have been mentioned in the literature [181]. Super capacitors have been suggested as power units for such the sensory node due to their high energy storage density and high power density. Carbon nanotubes increase the surface area of the capacitors exponentially and thus are an ideal candidate material. Low resistivity, high stability and narrow
distribution of mesoporous size also make carbon nanotubes ideal for super capacitors [313].

Solar cells based on quantum dots have been mentioned to be suitable material for energy scavenging applications for sensory nodes. CdS quantum dots have been demonstrated to self assemble on the surface of nanocrystalline titanium dioxide and light harvesting electrode has been fabricated from P25 particles using a pressing method. The method was mentioned to have caused some loss in performance of the CdS coating though it was considered to be a suitable option for low cost manufacturing [314]. A number of materials are being considered and developed in photovoltaic, such as silicon based, polycrystalline thin film (copper indium diselnide and cadmium telluride), single crystalline thin film, dye sensitised thin film, organic and polymer solar cells. A more detailed view on the developments is available from the technology sub-sector of photovoltaics in Technology Sector Energy of the Observatory Nano project.

A number of other methods have been suggested in the literature based on solar power, thermal gradients and fluid flow [315]. Mechanical vibrations have been harnessed through piezoelectric for transforming vibrations to electrical energy. Lead zirconate titanate (PZT) has been reported in the literature for harvesting energy from vibrations. A thin film lead zirconate titanate MEMS device has been developed. The device resonates at specific frequencies of an external vibration source. The cantilever has a PZT/Silicon nitride bimorph structure with a proof mass attached to its end. The platinum/titanium electrode was reported to be patterned on top of a sol-gel coated PZT thin film. A spiral design for the cantilever has been proposed and demonstrated for compactness, lower resonant frequency, and a minimum damping coefficient. The device was shown to demonstrate a continuous supply of 1 μW of continuous power [316].  An array of zinc oxide nanowires have been reported in the literature for harnessing energy from mechanical vibrations. The nanowire array is placed below zig-zag metal electrode with a small gap. The piezoelectric semi-conducting coupling converts mechanical energy to energy. The device has shown to produce high power density in relation to other microgenerators [317]. A more detailed view of the technology research and development of the power supply are available from the ‘Power' sub sector of the ICT Technology Sector in Observatory Nano project.

Role of Sensor Networks in Emergency Response - The role of sensors in emergency response has been reported in the literature. The protocol design, application development and security model has been mentioned to address the resource limitations of sensor nodes and network. Some of the problems the architecture named ‘Code Blue' aims to address are robustness of signals received from sensors and communicated to the device, routing of signal data to multiple nodes, and prioritisation of critical data flow over communication networks.  Security of wireless communication is expected to be enabled by providing enough computational power at nodes allowing them to use dynamic cryptography protocols. The ability to locate the node and sensory devices play an important role. In the event of serious causalities being brought in to medical facilities, the facilities are overwhelmed with decision making at demanding times. Sensory networks can be used for patient triage and tracking, temporary storage of patient information, simultaneous monitoring of physical variables and tracking first responders in providing trauma care to victims [318].