i n t e rnetscale sens i n g Dermot Diamond Dublin City University (Ireland)
Incredible advances in digital communications and computer power have profoundly changed our lives. One chemist shares his vision of the role of analytical science in the next communications revolution.
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igital communications networks are at the heart of modern society. The digitization of communications, the development of the Internet, and the availability of relatively inexpensive but powerful mobile computing technologies have established a global communications network capable of linking billions of people, places, and objects. Email can instantly transmit complex documents to multiple remote locations, and websites provide a platform for instantaneous notification, dissemination, and exchange of information globally. This technology is now pervasive, and those in research and business have multiple interactions with this digital world every day. However, this technology might simply be the foundation for the next wave of development that will provide a seamless interface between the real and digital worlds. The crucial missing part in this scenario is the gateway through which these worlds will communicate: How can the digital world sense and respond to changes in the real world? Analytical scientists—particularly those working on chemical sensors, biosensors, and compact, autonomous instruments—are © 2004 AMERICAN CHEMICAL SOCIETY
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in a powerful position to provide this gateway. How will science and technology converge to catalyze the next stage in societal change? Some believe that cheap sensors linked to microprocessors and lasers will drive the next revolution (1). The move from traditional analog landline to digital mobile phones has been an important part of this revolution. Inexpensive mobile phones and other wireless technologies, coupled with palmtop PCs and personal digital assistants (PDAs), provide individuals with communications capabilities unimaginable a decade ago. The exchange of files containing text, graphics, and embedded audio and video through mobile communications platforms is now standard practice. The fact that these technologies are now commonplace indicates that the market is mature, and investors are now looking for the next disruptive technology. (Disruptive technologies render existing ones obsolete, for example, in the way digital imaging has displaced conventional film cameras.) In a sense, the communications industry is an infrastructure waiting for the next big idea.
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FIGURE 1. Establish the chain. All analytical measurements must be linked to realize the concept of Internet-scale sensing. Localized control of important parameters is maintained, but the information is shared with external users via the Internet.
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Molecular world meets digital world In recent years, research into wireless networking has been dominated by the need for high-bandwidth access to data-intensive files with extensive graphic, video, and audio content. Bluetooth has been heralded as the low-power wireless standard of the future, but it is very much a high-bandwidth technology, designed to integrate portable devices into this communications infrastructure. Similarly, wireless local area networks (e.g., IEEE 802.11 standard) and mobile phones can provide gateways between portable computing and digital communications. Analytical scientists are sitting in the exact “knowledge domain” that will trigger the next global technological revolution. Sensor research is driven by the need to generate a selective response to a particular analyte, for example, by a selective binding event that occurs in host–guest complexation, enzyme– substrate reactions, antibody–antigen interactions, or other forms of biomolecular recognition. Much research focuses on developing a fuller understanding of the molecular basis for intramolecular recognition, which may ultimately lead to more selective devices that may find use in futuristic applications. Coupled with this attention to selectivity is the need to provide a transduction mechanism, so that the binding event can be observed via an electronic signal. Researchers typically will look to electrochemistry (potentiometry, voltammetry, amperometry) or spectroscopy (visible absorbance, fluorescence) for this signal, often generated by appropriate redox-active sites or chromophores or fluorophores, either as part of the molecular sensor itself or as part of a sensing cocktail. Success depends on the molecular binding event triggering transduction of the signaling moiety without adversely affecting the overall selectivity of the binding process. In parallel with this sensor research, tremendous advances have been made in the development of compact, portable analytical instruments. For example, lab-on-a-chip devices enable complex bench processes (sampling, reagent addition, temperature control, analysis of reaction products) to be incorporated into a compact, low-power format to provide reliable analytical information within a controlled internal environment. The analytical literature is awash with ideas, concepts, demonstrator devices, and real examples of how to generate analytical information in real-time—exactly the information needed by global digital industries. Let us not underestimate what analytical devices can provide—information related to the molecular status of the device’s environment.
Internet-scale sensing and control A conventional control loop consisting of one or more sensors and actuators becomes part of a global information exchange when the basic principle of Internet enabling is implemented (Figure 1). Internet enabling immediately allows external browsing of the sensor’s status, provides external programming of control parameters, and facilitates feedback of information to individuals and other devices. Widespread adoption of this principle leads to Internet-scale sensing and control systems, in which millions of sensing devices and actuators are seamlessly linked with a wide variety of users, ranging from individuals to government
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FIGURE 2. Widespread implementation of Internet-enabled analytical measurements will link many application sectors and a wide variety of users.
agencies, industrial users, or public service providers, across many application sectors (Figure 2). However, the real value lies in the realization that large-scale sensor networks can provide much more information than was originally predicted from simple linkages among localized collections of individual sensors. The potential is tremendous for the discovery and use of entirely new types of relationships between information extracted from this data continuum, giving rise to new business opportunities and completely new markets. Ambrosio has identified these relationships as one of the key factors that will fuel rapid developments in the information and communications technology sector in the coming years (2). David Culler, director of the recently established Intel– Berkeley Research Lab, believes that wireless sensor networks are currently at the same developmental stage as the Internet was in the 1970s, with the same or greater potential. The Intel– Berkeley team recently demonstrated a low-power wireless network (LPWN) consisting of 800 motes. Each mote included a light sensor on a compact electronic substructure that inte-
grates data acquisition and wireless communications functions, and a stripped-down operating system called TinyOS (the latest TinyOS source code is available free at http://sourceforge.net/ projects/tinyos). In network terminology, each point at which information can be introduced, extracted, or simply passed along is known as a node. In LPWN research, the Intel–Berkeley group has championed the term “mote” to mean a node that incorporates sensing and communications capabilities in a single platform. Creating field demonstrators that are scientifically interesting in terms of scale and at an appropriate cost remains a significant challenge, and there are few examples in the literature. The Berkeley team has assembled a 50-node sensor network to monitor seismic activity across the campus, and a 32-node sensor network linked by satellite communications via a base station has been used to study the microclimates associated with the nesting sites of storm petrels on Great Duck Island, Maine. Each node included a habitat monitoring kit that could measure light levels, heat, temperature, barometric pressure, and humidity (3). A U G U S T 1 , 2 0 0 4 / A N A LY T I C A L C H E M I S T R Y
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Labs-on-chips Complex sample handling, multiple analytes (nutrients, heavy metals)
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FIGURE 3. Instrument hierarchy. Analytical instruments can be arranged into a hierarchy in terms of sophistication, capabilities, operational costs, and degree of autonomy. A significant correlation will form between these factors and the density of distribution throughout the networked world. Providing effective communications between these layers provides routes to validate data from low-cost devices by using more reliable data obtained from sophisticated devices.
Investment in LPWN sensor research is increasing dramatically. For example, at the University of California, Los Angeles, Deborah Estrin heads the newly launched Center for Embedded Networked Sensors—a $40 million, 10-year-funded National Science Foundation facility (http://cens.ucla.edu). The European Union is investing €111 million in wireless research under the recently launched 6th Framework program (4). A forum for the exchange of information between interested parties from academia, industry, and government conducts regular meetings with the goal of developing a common vision for the future wireless world (www.wireless-world-research.org).
Device hierarchy Analytical devices can be layered into a hierarchy in terms of their complexity, degree of autonomy, and need for external services (Figure 3). Laboratory instruments are already integrated into conventional, site-based digital networks. In principle, information from these instruments is readily available, but in practice, it tends to be restricted to the site and typically requires sophisticated work-up and interpretation. Autonomous field-based instruments should be integrated into digital networks, but because they are placed at remote and “less serviced” locations, conventional networking strategies are not as feasible. As the number of remotely located autonomous instruments increases, the need for low-power operation becomes the determining factor. Clearly, the least sophisticated of these devices will be distributed in the largest numbers and will essentially operate at almost zero bandwidth (e.g., they may send only a few bits of information to indicate a threshold has been crossed). Of course, low bandwidth is very unattractive to the communications industry because the business models tend to be based on the volume of data transferred, hence the attractiveness of large audio and video files. In contrast to data quantity, the attractiveness of wireless sensor networks lies in the importance and value of the information they can provide. 282 A
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Research into the most densely distributed layer (very low cost, autonomous devices) is dominated by the use of physical transducers, such as pressure and temperature sensors that do not have to make intimate contact with the sample or environment (i.e., they can be totally encased within a protective cladding and still function). Robust chemical-sensing capabilities can be provided by old reliable pH and dissolved oxygen devices. Sensors that depend on polymer membranes or surface films for a response are affected by exposure to the sample over time, so more sophisticated devices that incorporate calibration routines are typical, which drives up the cost and makes dense distribution economically unviable. Labs-on-chips, which perform measurements in a sheltered microfluidic environment, are an attractive option, but they are also too costly for large-scale deployment at present. The challenge is to move devices toward the more densely distributed layers by driving down the cost while simultaneously maintaining reliability and quality of the analytical data. At the same time, physical transducers and reliable chemical sensors will be used more frequently in networked systems. For example, relatively simple measurements such as turbidity, color, pH, and conductivity can provide important general information about drinking water quality throughout a complex distribution network, which enables contamination to be detected at an early stage. Corrective action can be taken before contaminants spread throughout the entire system, which is unfortunately still an all-too-common occurrence. The success of these sensors will in turn drive demand for more complex measurements to be integrated into distributed networks.
Networking options It is now standard practice to network analytical laboratories, and special services such as laboratory information management systems have been developed specifically to integrate data with conventional administrative networks in large organizations. Every
instrument, down to the humble pH meter, has a PC interface, and increasingly, instruments cannot be operated except through a computer. However, cable-based networking is expensive to install, and this cost, coupled with the demand for mobile access to communications, has driven the rapid development of wireless networks. Examples include “wireless hot spots” (based on 802.11 standards) that are appearing at airports, hotels, cafes, and universities, offering high-bandwidth access to laptop and palmtop users seeking email or Web access. Bluetooth is another wireless network technology that could be incorporated into analytical devices but at present is targeted at connectivity between peripherals such as mice, keyboards, headsets, printers, mobile phones, palmtops, PDAs, and home appliances in a personal area network (www.bluetooth.com). However, both 802.11 and Bluetooth are designed to facilitate connectivity with conventional network communications, evidenced by the high-bandwidth specification and relatively high power consumption (Figure 4). Therefore, although these technologies will undoubtedly make it much easier to connect analytical devices within a building, in their present form, they are unlikely to be suitable for long-term autonomous operation in remote locations. In contrast, the ZigBee Alliance, which includes Honeywell, Mitsubishi Electric, Phillips, and Motorola, is developing hardware and software communications standards focused on lowbandwidth, low-power-consumption applications, which are more
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