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3.2. Communication
The shift from wired to wireless links for communication in sensor networks is the first revolutionary design element facilitating an unobtrusive introduction in the environment of sensory and smart embedded systems enabling ambient intelligence. The networking capability of WSNs is built up in layers. The link corresponds to a physical level. Radio-frequency (RF), acoustic, optical and infrared links are possible. Each has advantages and limitations.
Optical systems can be based on laser light, LEDs or IrDA interfaces. Generally, they are not energy hungry but require a free line of sight between the transmitter and the receiver. Both LEDs and IrDA [35] enables short-range communication, while laser light can cover longer distances. Furthermore, it enables transmission at rates up to around 1 GHz without the need of an antenna.
Another possible physical link can be based on ultrasonic carriers, but communication ranges are too short for WSN applications. However, acoustic communication of sensor data are employed heavily underwater where typical limitations and challenges are: acoustic frequencies limit the bit rates that can be transmitted; carrier frequencies from a few kilohertz to a few tens of kilohertz are commonly employed underwater; transmission rates are less than 10 kbits per second; multi-path propagation of signals is frequently verified.
RF is the most common channel used in sensor network systems although it uses the often-limited energy in a sensor node at a relatively high rate. Therefore, a consistent effort is made on designing ultra-low power transceivers [36]. Several aspects affect the power consumption of a radio, including the type of modulation scheme used, data rate and transmit power. Dynamic power management techniques are possible switching among operation modes: transmit, receive, idle and sleep. Many prototypes and commercial solutions adopted very simple radio transceivers (such as the RFM TR1001 [37] or the Chipcon CC2420 [38] http://www.chipcon.com.[38]) with ad hoc and low-power network protocols for medium access control (MAC) and routing when necessary.
Communication is not only about the physical link, but it regards also the protocol level. Fig. 2 shows different technologies representing the evolution of wireless communication in terms of bit rate, communication range and application mobility. General purpose systems such as ultra-portable devices (PDAs, cell phones) use radio communication based on WiFi, GSM, GPRS and Bluetooth standards. WiFi is particularly power hungry, thus being inadequate to WSNs. GSM and GPRS are controlled by telephone companies. Bluetooth is designed mainly for computer cable replacement, so it is not the best choice for a sensor network. Nevertheless, the relatively high data rate of Bluetooth suggests to use it for gateways between sensor nodes and infrastructure networks.
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Fig. 2. Wireless communication standards and their characteristics [41].
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Zigbee and IEEE802.15.4 [39] and [40] are standards developed for wireless sensor networking. IEEE802.15.4 is a low data rate solution from 20 to 250 kbps, depending on the frequency band used—compared to a nominal 1 Mbps for Bluetooth and 54 Mbps for Wi-Fi's 802.11g technology. For sending sensor readings, which are typically a few tens of bytes, high bandwidth is not necessary, and ZigBee's low bandwidth helps it fulfill its goals of low power, low cost, and robustness, thus augmenting sensors nodes lifetime to months and years. It is intended to operate in an unlicensed, international frequency band (868 MHz Europe, 915 MHz America, 2.4 GHz worldwide). IEEE802.15.4 defines the physical layer and the MAC. For these optimized short-range wireless solutions, the other key element above the physical and MAC layer is the network/security layers for sensor and control integration.
3.3. Sensors and interface electronics
The trend for sensors is evolving from remote sensors, far from the actual phenomenon, to local microsensors. Under the same conditions of accuracy, sensitivity is enhanced in one case by augmenting the sensitive area to distinguish the targets from environmental noise (e.g. using a matrix of sensors or augmenting their size), in the other through the increased vicinity to the physical phenomenon allowed by the small form-factor.
Microelectro-mechanical systems (MEMS) integrate mechanical elements, sensors, actuators, and electronics on a common silicon substrate through microfabrication technology. Putting together silicon-based microelectronics with micromachining, MEMS technology opens the way to complete systems-on-a-chip, integrating a complete sensor node with transmission, computational, storage and sensing capabilities onto the same die.
Actual MEMS sensors already integrate some processing capabilities. In many cases output is already converted in a digital form and some signal conditioning is available. Inertial sensors from ST microelectronics for example integrate features such as I2C/SPI digital output interfaces, can be programmed to provide interrupt at certain threshold, embed self-testing capabilities. This is very attractive because the final sensor node can spare on many additional interface components, with evident benefits in terms of size, power consumption and reliability (although some effort in designing low-power conditioning circuits such as ADC has been recently done [42]).
In conclusion, MEMS sensors and actuators are at present the best choice in terms of size, power consumptions and cost. They augment the decision-making capability of microelectronic integrated circuits with “eyes” and “arms”, to allow microsystems to sense and control the environment.
3.4. The WiMoCA system
As a case study of WSN node design, we describe the WIMOCA dice-size node, shown in Fig. 3, designed at the University of Bologna [43]. It is deployed in a wireless body area sensor network for a wireless/wearable distributed gesture recognition system for HCI and AmI applications, with nodes mounted on many parts of the human body. WiMoCA (wireless motion capture with accelerometers) in its basic setup is equipped with a tri-axial MEMS accelerometer, but many other sensors have been added (e.g. gyroscopes, bend sensors). It has a modular architecture to ease fast replacement and update of each component. It is composed by three sections namely microcontroller/sensors, RF and power supply. In the present prototype, the microcontroller is an ATMEL ATMEGA8 and the transceiver an RFM TR1001. We also designed a special node that takes care of interfacing the BAN with an external processing unit, e.g. a PC or workstation. This node, namely a gateway, uses the same microcontroller and RF transceiver but it is not equipped with a sensing unit. The gateway interfaces with the external processing unit (PU) through a Bluetooth wireless link. Network support has been developed. Our system has been designed for real-time interactive applications with low-power requirements and for this reason we focused on minimizing software overhead by implementing our own component drivers and communication layer.
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Fig. 3. WIMOCA sensor node.
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The goal of WIMOCA is enabling natural interaction and context-aware services to inhabitants of a smart environment. WIMOCA nodes are worn to form a body area network (BAN), which captures and processes behavioral information about the user, enabling the environment to react consistently and contextually to the user's personal needs and activities. This is achieved transparently from the user and proactively if possible, or through natural interaction if an explicit request is issued by the user. For this purpose, WIMOCA BAN is mainly based on inertial sensors for user activity detection and gesture recognition.
4. Looking forward: symbiotic and bio-inspired sensor nodes
Some of the most exciting and challenging fields of application of future, wireless sensor networks are deeply related to areas as bioscience, biotechnology and nanoscience. We can mention for instance: (i) point-of-care portable or in-body, easy-to-use, stand-alone systems to perform medical analysis out of clinical laboratories [44] and [45]; (ii) in vivo controlled drug release systems, which can be ingested or injected into a human body and which must act to deliver appropriate quantity of drugs or other caring means in a pre-determined, self-regulating or real-time-controllable way [46] and [47].
In order to be adapted to these applications, systems should be autonomous in terms of avoiding as much as possible the direct contribution of a human operator to their functions—which can be achieved by integrating different devices and circuitry for data processing—and in terms of power supply. In addition, a key factor for the development will be the possibility to connect them to obtain information and to control them from remote. The hardware design of nodes for medical and biological applications has to address bio-compatibility issues and the high specific interaction properties at the molecular and atomic level. This can be achieved through development of several technologies, as: (i) advanced synthetic—or not—molecular receptors [48]; (ii) innovative three-dimensional membranes for highly-controlled molecular release [49]; (iii) bio-hybrid systems as bio-coated nanoparticles to vehicle drugs or transfection agents inside cells [50]; (iv) bio-inspired systems, to mimic peculiar characteristics of bio-systems as adaptability and self-regulation [51]; (v) smart organic surfaces to transduce electronically specific mechanical or optical stimuli [52]. From this incomplete list, it is clear that the high bio-nanoscience content involved in the design of new sensor systems will fuel innovation in many fields as molecular sensing, new transduction techniques of physical signals, new smart, multi-functional materials.
The conceivable evolution of sensor systems could be depicted as developed along two phases. The first one is the application of conventional (micro) and non-conventional (nano) technology to the bio-physical world and, in particular, by patterning and disposing biological matter and by handling fluids in a microscopic, high-controlled way to separate and process small volumes of sample or selectively interact with single cells (see Section 4.1). The second phase will be dominated by the exploitation of bioscience for the creation of micro and nanoscale systems and materials with unique properties and functions. These systems will be characterized by properties of self-assembly, reversibility, adaptability, self-replication, possibility of interaction at the atomic scale [53] (see Section 4.2). Examples of existing systems and promising elements for the deployment in WSNs will be given for both phases focusing on the cited bio-monitoring applications (Sections 4.1.1 and 4.2.1).
4.1. Symbiotic sensor nodes
The first phase consists in the development of lab-on-a-chip, BioMEMS, micro total analysis systems (μTAS). These systems are characterized by deep integration and miniaturization.
Integration will exploit microtechnology to create structures using both standard microelectronics materials and non-conventional materials. Silicon, glass and metals will be employed to integrate data and signal processing, semiconductor sensors, microchannels, electro-mechanical microcomponents, thermal actuators. New-concept materials of polymeric nature (e.g., polydimethylsiloxane) will be preferred for their bio-compatibility, and their suitability to be patterned to create channels for handling and processing of biological samples. Aggressive integration will lead to (i) self-contained, easy-to-use, reliable analysis systems, by integrating on a single chip microfluidics and heating-control elements for the pretreatment of the samples (processing, separation); (ii) high-throughput and flexible systems, by integrating sensors of different nature on the same device; (iii) improved speed and reliability by allowing the same sample to be tested at the same time with different probes patterned on microsites into a unique substrate. Integration of batteries and communication modules (as outlined in the previous section) will lead to stand-alone, self-contained controllable-programmable on-field systems.
Miniaturization is a key factor for the development of in-body devices as it reduces dramatically obtrusiveness. At the same time, it leads to some key advantages. Miniaturization of sensing element, sample pretreatment, actuators and delivery chambers reduce the amounts of reagents needed and, thus, costs necessary to conduct a chemical process: miniaturized handling volumes are in the nanoliter to picoliter range rather than the microliter range or larger in conventional experiments; moreover, small volumes lead to higher effective concentrations [54], thus reducing time to result and improving performance. It should be noted that the ability to handle molecular receptors and therapeutic agents which are at the scale of the target molecules is needed to provide the necessary sensitivity and selectivity.
4.1.1. Examples and applications
Point-of-care devices for the implementation of outpatient clinics to enable high level of care outside laboratories and hospitals is extremely desirable for reasons related to cost, comfort and efficiency. Microfabricated systems for in-body molecular detection and bio-parameters monitoring would be a breakthrough in this domain [55]. Many attempts have been done in the direction of implementing on-chip common analytical techniques. The most successful are gas chromatography [56], capillary electrophoresis [57] and polymerase chain reaction [58]. The latter is one of the most interesting approaches of microfabricated implementation which relies on the good properties of thermal conductivity of silicon and the possibility to easily integrate thermal resistances. Existing devices integrate only a few of the basic steps involved in molecular detection which is the most promising, challenging and complex analysis demand. The steps can be subdivided according to their function: (i) pre-sensing steps: extraction, separation, amplification; (ii) sensing steps: sensing, transduction for the generation of electrical signals; (iii) read-out steps: signal conditioning and data processing.
Handling of microfluidic samples for the integration of pre-sensing steps can be achieved by assembling basic fluidic components like electrokinetic and chemical separation channels, valves, mixing structures and chemical reactors. Even nanoliter-scale structures have been used for continuous flow, stopped flow, and thermal cycling reactions [59] and [60].
The integration and miniaturization of the sensing steps involves patterning and immobilization of molecular receptors on surfaces in an array format [61], the implementation of matrices of sensors, transducers and actuators on active substrates which can contain circuits for signal processing. Some examples report the use of array of microfabricated electrodes used as substrates of molecular-receptor sites for active receptor immobilization [62], target manipulation [63], electronic transduction of target recognition [64]. The latter demonstrates the electrical detection of chemical-labeled-DNA molecules on a silicon chip developed with standard CMOS process. A section of the die which exhibits surface post-processing to expose sensing gold sites is shown in Fig. 4. Recently, the feasibility of a label-free fully-electrical DNA detection technique easily integrable on chip has been also demonstrated [65].
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Fig. 4. Infineon technologies has designed a silicon chip for chemical-mediated electronic detection of DNA sequences. Gold electrodes, which can be modified with DNA sensing elements have been exposed on the chip surface and connected to the internal circuitry of the chip. Courtesy of Infineon Technologies.
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Some very interesting applications of integrated semiconductor sensors for DNA and protein detection include the use of field-effect transistors where the gate has been substituted with an electrolyte conductive solution [66], [67] and [68]. The use of surface and bulk acoustic wave sensors [69], nanowire potentiometric sensors based on field effect [70] and microcantilever employed as surface-stress and gravimetric transducers [71] have also been recently investigated.
The possibility to release drugs from in-body controlled microsystems could have a tremendous impact on medical procedures, eliminating the injection pain and infections due to frequent injections and offering a mean to reduce side effects and drug volumes by allowing a more precise and efficient delivery. The use of conventional micromachined devices, equipped with wells, microfluidics and circuitry has been tested. Santini et al. presented a multi-well silicon chip. The release of drugs from the compartments is controlled by an electrochemical stimulus which selectively dissolve covering gold membranes. This signal could be pre-programmed or controlled real-time by sensors coupled with the device [72].
Microfabricated devices are usually built on 0.5 mm-thick wafers. This size allow the delivery of the device into the human body by ingestion but it is too large for injection , inhalation and releasing into circulation . An off-wafer technique has been proposed which release a micromachined part of the wafer with a small well containing drugs [73] (Fig. 5). In vivo programmable chemical synthesisers which could provide drug synthesis on demand are very promising tools [74]. Coupling of microdelivery systems with sensing devices and processing will be crucial for pre-programmed or remote-stimulated controlled therapeutic treatment. Sensing functions based on wireless energy scavenging from remote sources have been demonstrated. RF [75] and [76] and ultrasounds have been employed [77] to design pressure sensors and bio-sensors.
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Fig. 5. Required dimensions of in vivo drug-delivery devices are indicated with respect to the different delivery approaches. Standard and innovative microfabrication techniques which can provide the adapted level of miniaturization are reported. Off-wafer technique is based on surface and bulk micromachining of silicon and polymeric materials to obtain micrometric structures (an example has been sketched [73]). Bottom-up techniques refer to the employment of self-assembly and the specific bio-modification of nanoparticles.
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4.2. Bio-inspired sensor nodes
The possibilities of sensing and actuation functions of bio-molecular-inspired devices relate on the use of bio-materials and on their unique properties. The core of these devices is made by bio-materials, bio-hybrids and two or three-dimensional assembled structures which are characterized by high bio-compatibility, adaptability, unique specificity toward targets and capability to change their state (often reversibly) by a switching stimulus (remote or in situ). Moreover, nanoscale systems are often needed to perform specific functions at an atomic and molecular level. Bio-materials which have been recently employed in innovative sensing or delivery tools are supramolecular systems, photo-chromic and thermo-chromic molecules, cell cultures, oligonucleotides (strands of DNA molecules) [52]. They are employed in association with inorganic molecules, deposited on solid substrates and entrapped in membranes.
4.2.1. Examples and applications
Recent advances in in vivo therapeutics concerned the employment of isolated microchambers containing colonies of genetically engineered cells. They are able to produce and release in vivo substances as insulin [78] and stimulate locally anti-cancer mechanisms [79]. Cells have also been employed on conductive substrate to sense their electrical reactions to toxins or viruses [54]. Metal or semiconductor nanoparticles coupled with bio-materials are extremely powerful tools. They have been widely employed in sensing devices, drug delivery and remote control of molecular reactions. A silicon bio-chip for DNA-sequence detection has been developed by employing bio-hybrids gold nanoparticles-oligonucleotide probes (the technique is shown in Fig. 6) [80]. Bio-hybrids bind to the target molecules which have been previously detected on an insulating substrate. Successively silver clusters can be grown on gold nanoparticles and provide a highly conductive electrical path between two near-deposited electrodes.
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Fig. 6. Bio-hybrid complex, gold nanoparticle/oligonucleotide for electronic detection of DNA sequences on a microchip [80]. The following steps: (i) target molecule detection (left); (ii) bio-hybrid binding and subsequent silver deposition (right) determine a 106 increase of the conductance between the two electrodes. Molecule detection (left); (ii) bio-hybrid binding and subsequent silver deposition (right) determine a 106 increase of the conductance between the two electrodes.
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Nanoparticles can incorporate drugs forming self-assembled structures [81]; they are among innovative synthetic methods of drug delivery which can overcome drawback related to the use of viral vectors in terms of costs, manufacturability and safety. The most interesting property concerns the fact that derivatives of functionalized nanoparticles are not detected by the immune systems, but they can penetrate the cell-membrane and targeting the nucleus. Once inside the cell, under low-pH conditions they dissolve and release their genetic content [82].
Nanoparticles can control molecular reactions of bio-materials immobilized on their surface by proving a local heating. Remote control of the hybridization of a DNA molecules by RF has been demonstrated. A 38-mer hairpin loop DNA molecule have been covalently linked to a 1.4 nm gold nanoparticle. The strand forms a duplex at room temperature, but experience a separation if heated. The reaction has been leaded by inductive coupling of a radio-frequency magnetic field to the gold nanoparticles and is completely reversible [83].
Electrically-controllable organic surfaces that can change reversibly their physical and chemical properties can be employed to release drugs with varying rate. A switchable surface obtained by an organic self-assembled monolayer of mercaptohexadecanoic acid on gold changes its hydrophobicity as a consequence of small alterations of the substrate potential [84].
5. Conclusion
In this paper, we surveyed the evolution of wireless sensor networks, starting from early shoebox-sized obtrusive devices and moving to forward-looking bio-hybrid devices based on nanoscale molecular engineering. A few common themes have emerged: the quest for a flexible and inexpensive micro-fabrication technology to reach the ultimate limits of integration and bio-compatibility, the strong push toward power reduction, from watts to microwatts (or less), the requirement for a holistic design approach, where all system components are jointly optimized. The opportunities are immense. WSNs can open huge markets in consumer applications, as well as in security and environmental monitoring. Looking forward, the evolution of sensor networks toward symbiotic and bio-inspired architectures could drastically improve the health conditions and lifetime expectation of a large number of people.
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Corresponding author.
Microelectronics Journal
Volume 37, Issue 12, December 2006, Pages 1639-1649 Result list | previous < 1 of 1 > next
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