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SYSTEMS 8 response time (after receptor attachment) of no more than 1 second. The active detection area should be no more than 25 nm2. The research is being conducted at the Army Research Laboratory (ARL), Naval Research Laboratory (NRL), MITRE, and Northwestern, Harvard, and Yale Universities. Kwok speculated that with the new sensing capabilities of molecular-scale sensors, one would be able to differentiate dirty radiological bombs from nuclear weapons and locate hard-to-identify terrorists, though he did not explore how this would be done operationally. Kwok noted two concept targets that might result from this effort: a personal computer with the thickness of a piece of paper with a chem/bio detector attached; and a black box detector mounted on a watch battery. RFID TECHNOLOGY: OPPORTUNITIES AND CHALLENGES Rich Fletcher reviewed the basics of RFID tags, which were invented in 1975, noting that WalMart is requiring all vendors to tag their products by 2007, with DoD and the FDA also issuing requirements for vendor RFID tags to help in management of product inventories. Credit cards are also converting to RFID technology (e.g., ExxonMobil's smart tag system). The simplest type of RFID tags are chipless tags, which are stamped metal foils that can code on the order of 10 bits of information. At short range, where the tag and reader are in close proximity, the RFID tag looks like a resonant LC circuit, and information is encoded in the resonant frequency or the loss profile, or by modulation. At longer ranges, a chip (which may be powered by a battery) is used in the circuit to create an active tag, and information is coded in the modulated backscatter. Radio frequency is not the only available power source for these kinds of tags; tags can also be powered by light, sound, or by changes in the electric field (capacitively coupled). Chipless RFID sensors are tags that convey information not only about the object's ID but also about its state or historyâfor example, has it been broken or tampered with, or is it being squeezed? Commercial examples include simple pressure and temperature sensors. A major challenge with RFID tags is providing the power to turn them on; they must be held close to the reader to be activated, although the sensing function itself does not require much power. Fletcher favors capacitively coupled tags because they do not require large activation currentsâsmall gradients in the electric field are sufficient. Examples include (1) touch-based tags, where touch can be used to transfer information (e.g., an instrumented doorknob) and (2) tags that use the human body, both as a power source and as an antenna. An option for electronically displaying information from RFID tags is to use microencapsulated black and white ink particles that respond differently to dipole fields and can be read optically. Fletcher cited some key challenges for the future. One is reducing the cost of manufacturing and handling tiny chips, which may be 0.1 Âµm on a side. Fluidic self-assembly based on shaking the chips and allowing them to settle in precision-machined channels is now used. Another challenge is ensuring compatibility of different RFID systems. Options include making readers with multiple front-ends to read different tags or making more agile tags that can be read by multiple readers such as those using multifrequency power and readout. In the future, analog information from sensors may also be encoded in smart antennas. Polymer electronics with printed logic and batteries and solar cells and displays on flexible substrates are enabling a whole new range of smart tags for products. Also in the future, DNA could be tagged with gold nanoparticles attached to specific sites that could be read by irradiating with a specific frequency that would be absorbed, producing local heating that would change the local molecular conformation. In conclusion, Fletcher noted that RFID applications are already a large business that is growing very rapidly. Materials and manufacturing technologies are enabling an increasing market for wireless and cheap tags. Nanotechnology and new power sources can enable greater performance.