Long‐Duration Transmission of Information with Infofuses

This paper describes a new system for non-electronic communication that can transmit alphanumeric information encoded as pulses of light, over intervals of hours. The objective of this research was to design “infofuses” that improved the previously described systems in three ways: 1) they could transmit information for hours instead of seconds, 2) they could transmit long messages, and 3) they resisted accidental extinction. These characteristics improve the functionality and potential for practical use of infofuses, and make themmore convenient to use as a test bed for a new approach to fusing chemistry and information—“infochemistry”. We consider the elements of a system for manipulating information that comprises seven steps: 1) generating the message (either by writing it, or by collecting it from a sensor), 2) encoding the information in a form that a device can transmit, 3) transmitting the information, 4) receiving the information, 5) decoding the information, 6) interpreting the decoded information, 7) acting on this information. Here, we focus on steps (2) and (3). To simplify the problem, we assume that reception and decoding will be accomplished optically and electronically, and that there are no constraints (that we must consider) on the complexity, cost, or performance of the systems that accomplish these functions. We also assume that generating the message involves a separate set of issues which may or may not be primarily chemical. Systems based on infochemistry combine the storage and transmission of encoded information with four attractive features of chemistry: 1) high energy density; 2) autonomous generation of power that can be used for both sensing and for transmission; 3) no requirement for batteries; 4) facile coupling with certain kinds of chemical sensing. We have described two infochemical systems that do not require external electrical power (they use only chemical interactions or reactions) to transmit alphanumeric information. The first system—which we call an “infofuse”—is based on a strip of flammable polymer (nitrocellulose). In this system, patterns of spots of thermally emissive salts encode information. The second—which we call a “droplet shutter”—is a microfluidic device that capitalizes on the high stability of operation of a flow-focusing nozzle to generate bubbles and droplets. In this system, windows in an opaque mask encode information. The combination of optically transparent droplets and windows serve as optical shutters. A third system—a frequency-agile microdroplet-based laser—requires electrical power to operate the pump laser, and is intended for different applications. In our previous design of infofuses, information was encoded as patterns of ions (Li, Rb, Cs) on a strip of nitrocellulose. Ignition of one end of a nitrocellulose strip initiated propagation of a flame-front (at T 1000 8C) at 2– 3 cms . As this moving hot zone reached each spot containing added metal ions, it caused the emission of light at wavelengths characteristic of the thermal emission of the corresponding atomic species. The pattern of emissive salts, ordered in space, therefore, became a sequence of pulses of light at characteristic wavelengths, ordered in time. We have described infofuses that used three thermal emitters (the perchlorate or nitrate salts of Li, Rb, and Cs, with Na as an internal standard for intensity) to encode and transmit alphanumeric messages. Because each of these three ions can either be present or absent in a particular spot, there are seven unique combinations (2 1; we have chosen not to use 0,0,0 to avoid ambiguities between a pulse and a space between pulses). Seven unique pulses allow an encoding scheme that assigned alphanumeric characters to combinations of two (7= 49) sequential optical pulses. The infofuses fabricated according to this design demonstrated the principle of a successful strategy for coupling chemistry with the encoding and transmitting of chemical information, but suffered from a number of weaknesses. Two were of primary concern to us: 1) While burning, the flame-front had to remain far (> 1– 2 mm) from surfaces, so that the heat transfer from the flame to the surface did not cool and extinguish the flame. When the flame-front of nitrocellulose infofuses with dimensions we used (1–3 mm wide, ca. 100 mm thick) came close (< 1–2 mm) to any solid surface, their rate of propagation decreased, and the flame frequently extinguished. This characteristic required that the burning infofuses have a vertical orientation, and be out of contact with solid surfaces; these restrictions limited the practical applicability of these infofuses. 2) The rapid propagation of the flame-front (ca. 3 cms ) precluded times of transmission longer than about 1 min (an infofuse on which the flame propagated at this rate would require a length of 2.6 km to transmit continuous or repetitive messages for 24 h). Here we describe a new experimental platform for infofuses that addresses these weaknesses by using a “dualspeed” arrangement (Figure 1), in which a fuse that burns slowly and continuously (a “SlowFuse”) and that does not transmit information, intermittently ignites fuses (strips of nitrocellulose) that burn quickly and transmit information [*] Dr. C. Kim, Dr. S. W. Thomas, III, Prof. G. M. Whitesides Department of Chemistry and Chemical Biology Harvard University 12 Oxford Street, Cambridge, MA 02138 (USA) E-mail: gwhitesides@gmwgroup.harvard.edu