Nanotechnology-Enabled Wireless Sensor Networks: From a Device Perspective

IEEE SENSORS JOURNAL, VOL. 6, NO. 5, OCTOBER 2006 1331

Nanotechnology-Enabled Wireless Sensor Networks:From a Device Perspective

James Pei M. She and John T. W. Yeow, Member, IEEE

Abstract—The advancement of wireless communications andintegrated circuit technology has enabled the development of low-cost sensor networks. The sensor networks can be used for variousapplication areas (disaster recovery, health, military, homelandsecurity, environment, home, etc.). For each application area,there are different technical issues that researchers are currentlyresolving. However, many of them are trying to tackle the limi-tations of this field from a network perspective. Sometimes, theeffectiveness of some proposed approaches must be complementedby the supports of hardware design. This article points out thepossibilities of overcoming the same problem set from a deviceperspective by taking advantage of the merits of nanotechnologies.At the same time, open research issues and challenges are identi-fied to spark new interests and developments in this field.

Index Terms—Microelectromechanical systems (MEMS),nanotechnology, sensor network, system design, wireless.

I. INTRODUCTION

IN RECENT years, advances in miniaturization, low-powercircuit design, simple and reasonably efficient wireless

communication equipment, and improved small-scale energysupplies have combined with reduced manufacturing costs tomake a new technological vision possible, i.e., wireless sen-sor networks. These networks combine simple wireless com-munication infrastructure, minimal computation facilities, andminimally invasive sensors to form a network that can be deeplyembedded in our physical environment to create an informationworld. Typical sensing tasks for such a device could be tem-perature, light, vibration, sound, and radiation. The desired sizewould be a few cubic millimeters or even smaller. The targetprice should be less than US$ 1, including radio front end,microcontroller, power supply, and the actual sensor. All thesecomponents are integrated together in a single device to forma “sensor node.” While these networks of sensor nodes sharemany commonalities with existing ad hoc network concepts,there are also a number of fundamental differences and specificchallenges [1].

While designing and deploying a wireless sensor network,we found several major limitations that must be addressed.Most of the recent works in this field attempt to resolve theseissues by making compromises among the limitations from a

Manuscript received June 6, 2005; revised October 6, 2005. The associateeditor coordinating the review of this paper and approving it for publicationwas Dr. Giorgio Sberveglieri.

The authors are with the Advanced Micro- and Nano-Devices Laboratory,University of Waterloo, Waterloo, ON N2L 3G1, Canada (e-mail: [email protected]; [email protected]).

Color versions of Figs. 1 and 3–5 are available at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/JSEN.2006.881362

network perspective but hardly address all of them. This paperpresents how these limitations could be potentially overcomethrough nanotechnologies.

The remainder of this paper is organized as follows: In thenext section, we will describe the background of wireless sensornetwork and their applications. Section III presents the require-ments, limitations, and existing solutions from the networkperspective. Section IV describes how existing development ofnanotechnologies can overcome the limitations of sensor net-works. Section V presents the future trend of sensors and sensornetworks. Section VI introduces new research challenges in thefuture wireless sensor network. Finally, Section VII containsour concluding remarks.

II. BACKGROUND AND APPLICATIONS

Wireless sensor networks have benefited from advances inboth microelectromechanical systems (MEMS) [2] and net-working technologies. Such environments may have many in-expensive wireless nodes, each capable of collecting, storing,and processing environmental information, and communicatingwith neighboring nodes.

A sensor node is made up of four basic components, as shownin Fig. 1, namely 1) a “sensing unit,” 2) a “processing unit,”3) a “transceiver unit,” and 4) a “power unit.” They may alsohave additional application-dependent components such as a“location finding system,” “power generator,” and “mobilizer.”Sensing units are usually composed of two subunits, namely1) sensors and 2) analog-to-digital converters (ADCs). The ana-log signals produced by the sensors in response to the observedphenomenon are converted to digital signals by ADC and thenfed into the processing unit. The processing unit, which isgenerally associated with a small storage unit, manages theprocedures that make the sensor node collaborate with the othernodes to carry out the assigned sensing tasks. A transceiver unitconnects the node to the network. One of the most importantcomponents of a sensor node is the power unit. Power unitsmay be supported by power scavenging units such as solar cells.There are also other subunits that are application dependent.Most of the sensor network routing techniques and sensingtasks require knowledge of location with high accuracy. Thus,it is common that a sensor node has a location finding system.A mobilizer may sometimes be needed to move sensor nodeswhen it is required to carry out their assigned tasks. All of thesesubunits may need to fit into a matchbox-sized module or evensmaller.

In the past, sensors are connected by wire lines. Today, thisenvironment is combined with the novel ad hoc networking

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1332 IEEE SENSORS JOURNAL, VOL. 6, NO. 5, OCTOBER 2006

Fig. 1. Components of a sensor node (cited from [3]).

and wireless technologies to facilitate intersensor commu-nication [4], which greatly improves the flexibility of installingand configuring a sensor network. Sensor nodes coordinateamong themselves to produce high-quality information aboutthe physical environment. A base station (the sink) may be afixed node or a mobile node capable of connecting the sensornetwork to an existing communications infrastructure or to theInternet where a user can have access to the reported data.

Networking unattended sensor nodes may have profoundeffects on the efficiency of many military and civil applica-tions such as target field imaging, intrusion detection, weathermonitoring, security, tactical surveillance, and distributed com-puting; on detecting ambient conditions such as temperature,movement, sound, and light; or the presence of certain objects,inventory control, and disaster management. Deployment of asensor network in these applications can be in random fashion(e.g., dropped from an airplane) or can be planted manually(e.g., fire alarm sensors in a facility). For example, in a disastermanagement application, a large number of sensors can bedropped from a helicopter. These networked sensors can assistrescue operations by locating survivors, identifying risky areas,and making the rescue team more aware of the overall situationin the disaster area, such as tsunami, earthquake, etc. Sensornodes are densely deployed in close proximity or embeddedwithin the medium to be observed. Therefore, they usually workunattended in remote geographic areas. They may be workingin the interior of large machinery, at the bottom of an oceancontinuously [11], in a biologically or chemically contaminatedfield, in a battlefield beyond enemy lines, and in a home or largebuilding [6], [7].

Driven by all these exciting and demanding applicationsof wireless sensor network, several critical requirements havebeen addressed actively from the network prospective for sup-porting viable deployments, as follows:

• long longevity;• noninvasive form factor;• optimal sensing coverage and connectivity;• high sensing resolution.

This paper is not intended to provide a full overview of allthe limitations of the wireless sensor network. The focus is toevaluate the possibilities of overcoming the same problem setsfrom a device perspective through existing nanotechnologies

and to identify challenges that require further research anddevelopment in sensor and sensor network technologies.

III. LIMITATIONS AND SOLUTIONS FROM THE

NETWORK PROSPECTIVE

A. Longevity

The wireless sensor node, being a microelectronic device,can only be equipped with a limited power source (e.g.,< 0.5 Ah, 1.2 V). In many application scenarios, regular andfrequent recharging of the power unit is almost impossible, suchas medical sensor body area networks [40], habitat monitoringnetworks [4], and sensor networks in hostile environments [41].Sensor networks that can run for several months, a year, or10–15 years from nonrechargeable power sources would havesignificant demand today and future practical deployments.Prolonging the longevity of a sensor network can be achievedby power management, redundancy, and in-network processingthrough different networking approaches proposed recently.

Power Management: Energy-efficient media access control(MAC) scheduling protocol, routing, application, or data-centric content distribution protocols are the most active topicsin sensor networks lately. However, many previous works pro-posed, such as protocols THEEM [23], LEACH [24], PEGASIS[25], etc., are trying very hard to balance the tradeoffs betweendelay, throughput, quality-of-service (QoS), coverage, energydissipation equilibrium, and network lifetime. Especially, theenergy-aware routing is a limited but rapidly growing set of re-search results in the field. A very detailed study of the differentroutings in the wireless sensor network has been presented in[50]. Although a few optimal solutions [26] have comprisedmost of the factors, they still pose many limits in deployingsuch routing schemes for many practical applications. Funda-mentally, the sensor node lifetime shows a strong dependencyon the power source lifetime. Any breakthrough in the “powerunit” can dramatically relieve all stringent constraints related topower management.

Redundancy: In a multihop sensor network, each node playsthe dual role of data originator and data router. The malfunc-tioning of a few nodes can cause significant topological changesthat might require rerouting of packets and reorganization ofthe network. Redundant sensors are required to maintain the

SHE AND YEOW: NANOTECHNOLOGY-ENABLED WIRELESS SENSOR NETWORKS: FROM A DEVICE PERSPECTIVE 1333

sensing coverage as well as network lifetime. Several redun-dancy strategies are recently published to achieve high networklifetime with minimal number of sensor nodes [34]. The largercoverage a sensor can provide can significantly facilitate theredundancy for longer network longevity. The improvement ofthe “transceiver unit” will take an important role on this issue.

In-Network Processing: There is a strong motivation thattypically, computation is much cheaper in terms of energy ex-penditure than communication. Therefore, in-network process-ing, such as compression and aggregation [32], [43], is anotheractive research area in a sensor network. The idea is to modifydata as it flows through the network in order to reduce theamount of transmission. This requires sophisticated designs ofthe “processor” such as the local signal search engine [42] andhigher capacity of “storage” to be available in the “processingunit.” The tradeoff between data resolution, delay, and size ofdata delivery applied a challenging balance on the researcheffort of this aspect.

B. Noninvasive Form Factor

The size of the sensor itself is definitely a fundamental factor.Miniaturization of “all components” in a sensor is definitelyan on-going effective approach for achieving the noninvasivedeployment. The smaller a sensor we can get, the less disruptionwill be generated to the natural processes or behaviors understudy. In many application scenarios, the sensor and sensornetwork themselves must be inconspicuous. They have to min-imize any effect it has to the normal biological functioningof the body, such as implanted biomedical sensors [36]. Thismotivates the use of wireless communication links with externalcontrol or computer systems to eliminate interconnections tothe sensor chip. This brings a challenge in the design of the“transceiver unit” for a longer range, so that less sensor andinterconnections can be used in a network.

C. Optimal Sensing Coverage and Connectivity

Loosely speaking, coverage determines how well sensors inthe network can monitor a geographical region in question.Since sensors are arbitrarily distributed, if some nodes sharethe common sensing region and task, then we can turn offsome of them to conserve energy and thus extend the lifetimeof the network. This is feasible if turning off some nodes stillprovide the same “coverage” (i.e., the provided coverage is notaffected). It has been proposed that a heuristic be used to selectmutually exclusive sets of sensor nodes such that each set ofsensors can provide a complete coverage of the monitored area[44]. Also targeted at turning off some redundant nodes, anothergroup proposed a probe-based density control algorithm to putsome nodes in a sensor-dense area to a doze mode to ensure along-lived robust sensing coverage [45]. A coverage preservingnode scheduling scheme is presented in [46] to determine whena node can be turned off and when it should be rescheduled tobecome active again.

Connectivity, on the other hand, simply describes the con-nectivity properties of the underlying network. The primarytask of wireless sensor networks is to have the sensors relay

information back to one or more base stations. The sensor nodesuse broadcasting techniques in a multihop manner to deliver in-formation. The connectivity between two sensors is determinedby whether they are within the communication range withineach other by themselves or through other sensors as the relays.Different routing and communication schemes as well as powerand redundancy constraints must be considered for a particularintended sensing task. The study on the connectivity propertiesof the sensor network has been studied in [14] and [28].

Improving the sensitivity of “the sensing unit” as well asthe communication range of “the transceiver unit” can facili-tate the coverage and connectivity by reducing the number ofsensors needed for a given area. Nanotechnologies have hugepotentials to play a role in these improvements. Examples ofhow nanotechnologies could make an impact are discussed inSection IV.

D. High Sensing Resolution

Some applications or missions of sensor networks will re-quire a very high sensing resolution of the data to be collected,such as for military, space, and environmental applications.In this case, multiple sensors are required to detect an event.For example, the triangulation-based positioning protocols[47]–[49] require at least three sensors at any moment tomonitor a moving object. Developing a more sensitive andselective sensing unit is probably the more essential approach.

IV. MERITS OF NANOTECHNOLOGIES

Nanotechnology will enable us to design sensors that aremuch smaller, less power hungry, and more sensitive than thecurrent micro- or macrosensors. Sensing applications will thusenjoy benefits far beyond those offered by MEMS and othermicrosensors. As illustrated before, those limitations of thesensor network are fundamentally affected by the componentunits themselves, which are probably solved or can be solvedeasily with the nanotechnologies available today. On the otherhand, some networking strategies can also be facilitated withmore powerful components unit enabled by nanotechnologies.Here, we will see how these limitations can be addressed fromthe device approach with the available nanotechnologies.

A. Power Unit

Probably one of the most established areas of nanotechnol-ogy is the use of nanomaterials in power generation and storage.Instead of being very conscious of energy consumption, a fewnanotechnology-based achievements have dramatically reducedthe size of energy storage device and increase the energydensity of the same device.

Nanobattery: Nanocrystalline materials and nanotubes havebeen demonstrated to significantly improve both power den-sity, lifetime, and charge/discharge rates [37]. Nanotubes areused to replace the normal graphite of lithium–graphite elec-trodes. Because of the nanostructure and the correspondinghigh surface area, nanotubes can incorporate more lithium thangraphite. With open single-walled nanotubes, capacities of up to


Fig. 2. Mass of a carbon sphere shifts the resonance frequency of the CNT towhich it is attached (cited from [21]).

640 Ah/kg have been reached in the laboratory. Nanocrystallinematerials synthesized by sol–gel techniques are candidatesfor separator plates in batteries because of their foam-like(aerogel) structure, which can hold considerably more energythan conventional ones. Nickel–metal hydride batteries madeof nanocrystalline nickel and metal hydrides are envisioned torequire less frequent recharging and to last longer because oftheir large grain boundary (surface) area.

Supercapacitor: Electrochemical double-layer capacitors,also known as supercapacitors or ultracapacitors, are electricalstorage devices that have a relatively high energy storage den-sity and a high power density. Recent developments in basictechnology, materials, and manufacturability have made su-percapacitors an imperative tool for short-term energy storagein power electronics. With much higher energy density thantoday’s capacitors and none of the problems associated withconventional battery technology, supercapacitors give accessto new power electronic and industrial storage applications.A standard capacitor can be transformed into a supercapaci-tor by increasing the surface area exponentially using carbonnanotubes (CNTs). The most cutting edge device can produce30 kW/kg, whereas the top-end capacitors available todayproduce about 4 kW/kg [5]. CNTs have a narrow distributionof mesopore sizes, highly accessible surface area, low resistiv-ity, and high stability, attracting great interest worldwide forbuilding a supercapacitor [12].

B. Sensing Unit

Nanotechnology can also enable very selective and sensitivedetection of a broad range of measurand. Many nanomaterialsusually pose both electrical properties and chemical or bio-logical properties at the same time. These properties enablebreakthroughs in creating very small and high-resolution sens-ing units that may not be explored in a sensor network before.

Physical Sensor: Researchers at the Georgia Institute ofTechnology led by W. de Heer devised the world’s smallest“balance” (see Fig. 2) by taking advantage of the unique elec-trical and mechanical properties of CNTs [12]. They mounteda single particle on the end of a CNT and applied an electricalcharge to it. Acting much like a strong flexible spring, the CNToscillated, without breaking, and the mass of the particle wascalculated from changes in the resonance vibrational frequencywith and without the particle. This approach may allow themass of individual biomolecules to be measured.

Biosensor: By using the sequential electrochemical reduc-tion of the metal ions onto an alumina template, we can nowcreate cylindrical rods made up of metal sections 50 nm to5 µm long [18]. These particles, trademarked Nanobarcodes

Fig. 3. DNA and other biomaterials can be sensed using encoded antibodieson nanobarcode particles (cited from [10]).

Fig. 4. Vertical CNTs are grown on a silicon chip. DNA molecules attachedat the ends of the tubes detect specific types of DNA in an analyte (citedfrom [10]).

(see Fig. 3), can be coated with analyte-specific entities such asantibodies for selective detection of complex molecules. DNAdetection with these nanoscale-coded particles has also beendemonstrated [19].

Researchers at the NASA Ames Research Center have takena different route [21]. They cover the surface of a chip withmillions of vertically mounted CNTs 30–50 nm in diameter.(see Fig. 4). When DNA molecules attached to the ends of thenanotubes are placed in a liquid containing DNA molecules ofinterest, the DNA on the chip attaches to the target and increasesits electrical conductivity. This technique, expected to reach the


Fig. 5. Schematic cross section of IBM’s CNT FET (cited from [8]).

sensitivity of fluorescence-based detection systems, may findapplication in the development of a portable sensor.

Chemical Sensor: Various nanotube-based gas sensors havebeen described in the past few years. Chemical sensors basedon individual single-walled CNTs (SWNT) are demonstrated[29]. Upon exposure to gaseous molecules such as NO2 or NH3,the electrical resistance of a semi-conducting SWNT is foundto dramatically increase or decrease. This serves as the basisfor nanotube molecular sensors. Modi et al. have developed aminiaturized gas ionization detector based on CNTs [15]. Thesensor could be used for gas chromatography. Titania nanotubehydrogen sensors have been incorporated in a wireless sensornetwork to detect hydrogen concentrations in the atmosphere[16]. Kong et al. have also developed a chemical sensor forgaseous molecules such as NO2 and NH3 that is based onnanotube molecular wires [17]. Philip et al. proposed to useCNT/PMMA composite thin films for sensing organic gases byleveraging the chemical and physical properties of CNT [39].

C. Processing Unit

IBM researchers have successfully created transistors fromCNTs since 1998 [9]. Ten carbon atoms formed into a cylin-der are 500 times smaller than today’s smallest silicon-basedtransistors, and IBM’s new process for creating the nanotubescould mean “a new class of smaller, faster and lower powercomputer chips” that will keep Moore’s law in effect beyond its10–20 year remaining life span computer scientists have beenpredicting. They have fabricated and tested nanotube transistorsusing individual multiwall nanotubes (MWNTs) or SWNTs asthe channel of a field-effect transistor (FET). The change ofcurrent flowing through the nanotube can be changed by a factorof 100 000 by changing the voltage applied to a gate. Theirnew CNT FETs reported in 2002 (see Fig. 5) work even betterin terms of the switching rate and the amount of current theycan carry per width of conductor [8], which can outperform theleading silicon transistor prototypes available today.

NEC has also developed a method of positioning CNTs ina way that it has reported will make circuits run faster andconsume less power than the fastest and most powerful siliconchips. Electrons can flow through CNTs ten times faster than

they can in circuits made using silicon, and CNTs can carry100 times the current and dissipate 20 times less the heat ofcircuits made with silicon. CNTs in transistors can also amplifyabout 20 times more current than conventional silicon-basedtransistors [22].

D. Transceiver Unit

The microwave amplifier is a telecommunication device usedin the base stations of wireless communication networks. Oneof the key components of the amplifier is an electron gun.The current system uses heated thermionic cathodes. Due tothe high operating temperature (∼ 1000 ◦C), it has a rel-atively large size (approximately 1 m3) and short lifetime.Although considerable research efforts have been devoted inthe past to develop amplifiers using cold cathodes, the conven-tional emission materials could not deliver the current density(100 mA/cm2) required by the microwave amplifiers. Dr. Zhouand Lucent Technologies have developed CNT cathodes usingthe proprietary film fabrication method [33]. They show stablecurrent densities far exceeding that required by the microwaveamplifiers. Comparing with the current technology, this newtechnology has reduced the size of the microwave amplifierfrom its current size of 1 m3 to 3 × 3 × 1 in. It consumes lesspower and is operable at room temperature without the needs of“always switched on” and, therefore, lengthens the lifetime ofthe microwave amplifier.

Dr. P. J. Burke and his research team have successfullydemonstrated integration of wireless technology with nan-otechnology and have shown the outstanding RF properties ofnanowire and nanotube antenna [51], [52]. A nanotube couldbe used as an antenna to couple to nanosensors by wirelessinterconnects without the need for lithographically fabricatedelectronics. Simply speaking, one cannot think of a nanotubeantenna in the same way as a thin-wire antenna because of thelarge inductance on the order of 104 times the inductance ofa thin-wire antenna. This translates into performance predic-tions that are substantially different than thin-wire antennas,essentially because the wavelength of the current excitation is100 times smaller than the wavelength of the far-field radiation,which presents a unique situation. An advantage of nanotubeantennas is that the nanotube can serve as an excellent im-pedance matching circuit to receive signals from free-spacepropagation to high-impedance devices.

E. Storage Unit

IBM scientists have demonstrated a data storage densityof a trillion bits per square inch—20 times higher than thedensest magnetic storage available today [20]. IBM achievedthis remarkable density—enough to store 25 million printedtextbook pages on a surface the size of a postage stamp—ina research project code named “Millipede.” Rather than usingtraditional magnetic or electronic means to store data, Milli-pede uses thousands of nanosharp tips to punch indentationsrepresenting individual bits into a thin plastic film. The core ofthe Millipede project is a two-dimensional array of v-shapedsilicon cantilevers that are 0.5 µm thick and 70 µm long.


At the end of each cantilever is a downward-pointing tip lessthan 2 µm long. The current experimental setup contains a3 mm × 3 mm array of 1024 (32 × 32) cantilevers, whichare created by silicon surface micromachining. A sophisticateddesign ensures accurate leveling of the tip array with respectto the storage medium and dampens vibrations and externalimpulses. Time-multiplexed electronics, similar to that used inDRAM chips, address each tip individually for parallel oper-ation. Electromagnetic actuation precisely moves the storagemedium beneath the array in both the x- and y-directions, en-abling each tip to read and write within its own storage field of100 µm on a side. The short distances to be covered help ensurelow power consumption. The result is akin to a nanotech versionof the venerable data processing “punch card” developed morethan 110 years ago but with two crucial differences, namely1) the “Millipede” technology is rewriteable (meaning it can beused over and over again) and 2) may be able to store more thanthree billion bits of data in the space occupied by just one holein a standard punch card. While Flash memory is not expectedto surpass 1–2 GB of capacity in the near term, the Millipedetechnology could pack 10–15 GB of data into the same tinyformat without requiring more power for device operation.

On the other hand, the first 10-GB nanotechnology mem-ory (NRAM) device has been built in the laboratories of aUS company, Nantero. Using CNTs sprinkled onto a siliconwafer, the device has been made using mostly standard chipproduction techniques. Nantero [31] claims that the technologycan combine the speed and price of dynamic memory withthe nonvolatility of Flash, making it a strong candidate forthe eagerly awaited universal memory devices that the industryhopes will replace all other types—DRAM, Flash memory, andhard disks in a wide variety of digital devices. NRAM works bybalancing the nanotubes on ridges of silicon. Under differingelectrical charges, the tubes can be physically swung into oneof two positions representing one and zero. Because the tubesare so small, this movement is very fast and needs very littlepower, and because the tubes are a thousand times as conductiveas copper, it is very easy to sense their position to read backthe data. Once in position, the tubes stay there until a signalresets them.

V. FUTURE TRENDS OF SENSOR AND SENSOR NETWORKS

As a cubic millimeter mote with today’s sensor technologyhas been successfully implemented [27], nanotechnology willconceivably push the limits of system integration size downeven further. The four components necessary in a sensor (sens-ing, processing, transceiver, and power) are all demonstratedwith CNTs as well as other nanomaterials. Beside the sizeof the sensor itself, both the sensor and the sensor networkswill able to provide more capacities and more sophisticatedfunctionalities as well as operations in the future because oftechnology push of nanotechnology and increasing social pullsof practical applications. In the future, a sensor network withbidirectional communication function is not limited by justpicking up data and transferring it back to a centralized sitefor processing but also allow the central location to transmitinformation back to the remote sensors and the ability for

sensing devices to react and respond. Emerging nanotechnologycan be a key way to make this happen, which enables low-power actions. Coupling sensor networks to nanotechnology-enabled devices thus provides the ability to feel and move at aremote location—thus dramatically extending the applicationspotential of wireless sensor network. Therefore, it is not diffi-cult to believe that the future sensor and sensor network will beequipped with the following features:

• programmable sensors with multisensing functions;• wireless sensors with mobility;• sensors with devices to react and respond by actions;• sensor with both short and long range or multiple commu-

nications capability;• in-sensor database;• and more.Singh [53] has also presented a new Boston Consulting

Group matrix analysis for identifying the trends in the sensortechnology, in which miniaturization, distributed processingand communication, and real-time intelligence are the majorfoci in the global trends. All these new features are further push-ing the envelope on the expectations of all component units,especially the processor and storage, which have the potentialto make a sensor node closer to a miniaturized computer.

VI. NEW RESEARCH CHALLENGES IN FUTURE

WIRELESS SENSOR NETWORKS

With such a promising trend of wireless sensor networkenabled by nanotechnology, tremendous potential applicationsand many new challenging research opportunities have beencreated. It will be impossible to outline all new research chal-lenges here, but we would like to sort out several critical aspectsbelow due to the impact of nanotechnologies, which wouldrequire earlier research attentions today from both networkingand devices communities.

A. Heterogeneous Sensor Deployment

Most research has discussed problems in the deployment of alarge number of low-cost homogeneous devices. It will becomevery common to consider the deployment of heterogeneous de-vices with different capabilities due to the applications, reusesof deployed sensors, and interoperability between sensor net-works. In such a situation, devices with high capability can beconsidered to increase the quality and quantity of data process-ing within the network and extend network lifetime throughhigh energy capacity. Experiences in grid computing can beleveraged to inspire an insight of handling such heterogeneity inthe case of a wireless sensor network. All strategies of routing,in-network processing, coverage, connectivity, redundancy, andpower management should consider this heterogeneous con-dition rather than just simply factoring the on–off status of asensor.

B. Overhearing

The sensing unit becomes more and more sensitive with alarger coverage, which can even sense something at the nano


level or even with the concentration of a few atoms/molecules.A promising potential can also be seen that the transceiver unitwill offer a larger communication range as well. This couldpotentially create an ongoing dilemma along the way due tointerference in between the improved coverage, sensitivity, andcommunications range. Eliminating unnecessary noises such asunwanted atoms or molecules for an accurate sensing eventin a micro/macroscale device can place a huge challenge ondevice design, but clustering and configuring a network as wellas finding the optimal strategy of coverage can be complicatedunder such a situation.

C. Multiple-Attributes Sensing

Most of the sensor networks today are application specificwith a particular sensing mechanism. For certain situations,we may need sensors with multiple sensing attributes andfunctions within the same area in order to meet the applica-tion needs. The “sensor unit” itself in this case will requirefurther research and development. A multiple-attributes sensorhas been demonstrated for offering multiple sensing functionssimultaneously [35]. A sensor may detect multiple measurandsat the same time while also processing the data collected locallyfor in-network processing. The integration of multiple types ofsensors such as seismic, acoustic, and optical in one networkplatform will make the global and collaborative sensing taskmore informative and powerful but, at the same time, morecomplicated. Data fusion strategies will not be that simple, likethose based on homogenous sensing condition, since differenttypes of data collected may share the same routing tree ororganized in different routing paths according to their priorityand fusion properties. Cross layers and attributes considerationsare needed to achieve an optimal power management among therouting, coverage, and redundant strategies.

D. Interoperability of Smart Sensors and Sensor Networks

There are more than 3000 global sensor manufacturers. Moreand more sensor networks are to be deployed. A sensor caneasily interact and operate with another sensor network, or asensor network can interoperate with another sensor network.The future sensor networks are expected to support those ap-plications that need mobile monitoring. A new IEEE standard(IEEE 1451 [13]) is being pushed hardly to be finalized inorder to standardize the wireless interfacing standard that canmake all complying sensors, instruments, and systems worktogether with relative ease. IEEE 1451.1 is still in the draft state.With latest nanotechnologies, we believe that the transceiverunit in all sensors should support this standard for enabling theinteroperability between sensors. Beside the communicationhardware, some standard protocol stacks should be proposedin order to enable the interoperability of the applications overdifferent sensors, sensor networks, and applications. Recentdevelopments of pervasive/ubiquitous networking [56] or ex-tending existing TCP/IP standard [57] could be the possibleapproach to address such considerations in future wirelesssensor networks.

E. Reliability and Security

Sensors can be deployed in an uncertain or hostile environ-ment. Sensor nodes may disconnect from the network due tophysical damages, communication blockings by unpredictableobstacles or intended jamming, sudden electronic failures, ormechanical fatigues of sensor components. The sensor networkitself should able to react to any kind of these failures and pro-vide reliable data delivery for maintaining the network lifetimeto those mission or data critical applications. Early initiativesfrom the network community such as delay tolerant network[30], tetherless computing [38], or common multipath approach[38] are proposed to deal with reliable data delivery in dis-connected or extreme wireless network environments. On theother hand, it will not be difficult to imagine that another majorchallenge in the sensor and the sensor network is the systemand data security. A similar issue has drawn huge attention aswell as many concerns, efforts, and remedies in today’s Internetsuch as computer virus outbreaks, digital frauds, and Denialsof Services (DoS) attacks. These concerns should be rigor-ously addressed to minimize economical, ecological, and socialcosts. However, the benefits and advantages of applying sensorsand sensor networks in applications areas such as medicalsand health care, homeland security and terrorism detection, andenvironmental and disasters monitoring will far outweigh theperils. The physical security and data hijacking of unattendedwireless sensor nodes could potentially put a big question markon the success of future wireless sensor network. Many existingnetworking approaches are working hard to address both relia-bility and security issues by normally trading off the QoS orreal-time performance of data delivery. This dilemma may berelieved easier with the early considerations of these two issuesfrom device design through nanotechnologies, especially dataand system security.

F. Integrating Nanoelectronic Devices WithMicro-/Macroscopic Sensor

One of the most important and unsolved problems in nan-otechnology is to make electrical contact from nanoelectronicdevices to the macroscopic world without giving up on thepotential circuit density achievable with nanoelectronics, suchas the use of nanotube antenna. The next logical question is“what does one connect to the terminals of the antenna?” Thisis related to the nascent field of nanoelectronics architecture,which has many issues of its own to be solved [54], [55].

Last but not least, future wireless sensor networks woulddefinitely trigger many interesting and challenging research op-portunities more than we could outline here. However, the mes-sage we want to covey here from those aspects described aboveis that the sensing power of future wireless sensor networkwould certainly be increased and improved dramatically, notjust in terms of capacity, but extended to different domains ofsensing application with multiple dimensions of deploymentsas well. Bidirectional and interoperable communications willnot just be limited as a local function over a sensor node onlybut also a global platform over the whole sensor network toallow the functionalities more than just sensing—acting and


reacting, collaborating and learning. With such diversifyingpossibilities of applications over a wireless sensor network, wecan foresee that the future problem set of the wireless sensornetwork will keep on dealing with the limitations by optimizingthe tradeoffs from different layers and multiple domains butalso balancing the constraints at various physical scales rangedfrom macro, mirco, to nano level. By simply tackling the lim-itations from only the networking prospective, it is likely thatthe deployment will be complicated or almost impractical toimplement. For example, in-network processing can potentiallyresult in a nonnegligible delay of information delivery due toaggregations of data from sources that are farther away. Inthe worst case, the delay will be proportional to the numberof hops between the sink and the farthest source, as indicatedin [58]. On the other hand, in-network processing makes end-to-end security mechanisms more difficult to deploy becauseintermediate nodes require direct access to the content of themessages [59], [60].

VII. CONCLUSION

The high sensing fidelity, low-cost, and rapid deploymentcharacteristics of sensor networks have created many new andexciting applications for remote sensing. In the future, this widerange of applications will make sensor networks an integralpart of our lives. However, the realization of sensor networksneeds to satisfy the critical constraints such as power consump-tion, environmental effects, hardware limitations, transmissionmedia, and communication protocols. Since these constraintsare highly stringent and specific for sensor networks, newwireless networking techniques are definitely required.

Given that several critical limitations of the wireless sensornetworks today that are being addressed actively by manycreative proposals from the network prospective, we have illus-trated the possibilities of overcoming the same problems froma device perspective by existing nanotechnologies, whereasthe trends of sensor and sensor network are presented, andseveral new challenges of future research directions are iden-tified to simulate thoughts and actions. Many researchers arecurrently engaged in developing the technologies needed fordifferent aspects of the sensor networks from the networkingapproach alone. Along with the tremendous efforts in develop-ing networking technology to implement sensor networks, it isapparent that collaboration with nanotechnology-based sensorcommunity is pertinent to leverage the merits of nanotechnolo-gies to develop a viable and large-scale sensor network.

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James Pei M. She received the B.Eng. degreein computer engineering and the M.Phil. degreein electrical and electronic engineering from theHong Kong University of Science and Technology,Kowloon, Hong Kong, in 1998 and 2000, respec-tively. He is currently working toward the Ph.D.degree in computer science at the University ofWaterloo, Waterloo, ON, Canada.

His research interests include novel nanomaterial-based sensor and sensor networking.

John T. W. Yeow (S’99–M’99) received the B.A.Sc.degree in electrical and computer engineering andthe M.A.Sc. and Ph.D. degrees in mechanical and in-dustrial engineering from the University of Toronto,Toronto, ON, Canada, in 1997, 1999, and 2003,respectively. He is currently an Assistant Professorwith the Department of Systems Design Engineering,University of Waterloo, Waterloo, ON, Canada.

His research interests include microsystem andnanodevice design.

Dr. Yeow was the recipient of the Micralyn Mi-crosystem Design Award, the NSERC Innovation Challenge Award andColton’s Medal of Research Excellence.

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Nanotechnology-Enabled Wireless Sensor Networks: From a Device Perspective