© 1999 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999

7.2.1.8 Assessment of Chemical Broadcast Messaging

Chemical broadcast messaging in theory permits information transfers up to 10⁹ bits/sec but appears to be extremely energy-inefficient. The assembly of (CHX)_n messenger molecule units may require 1-1000 zJ/bit (Section 6.5.6(B)). For instance, replacing CH (642 zJ) with CF (876 zJ) costs ~234 zJ, but recycling CH₃F molecules into CCH₂F chains actually produces 97 zJ of energy. A feedstock of CH₃F and CH₄ will release energy when the tape is first assembled, but will cost energy to tear apart into its fundamental subunits. R. Bradbury believes that the net cost of tape production and recycling can be reduced to 10-50 zJ/bit, given sufficient space to store lots of tape subunits: "If the assembly process is efficient at harvesting the energy produced, and the molecules don't get lost to cellular recycling machinery, then you could produce a system with very high overall efficiency, though you will have to worry about distribution of harvested energy and feedstock."

However, since ~10⁶ molecules must be released to ensure receipt of at least one message molecule by an intended recipient ~50 microns away (e.g., Section 7.2.1.3), the energy cost per received bit rises sharply to ~10⁶-10⁹ zJ/bit, which is >> 3 zJ/bit, the theoretical minimum (Eqn. 7.1). Thus a 1-pW communications power budget limits steady-state bandwidth to ~1 bit/sec across ~50-micron transmission ranges. Additional disadvantages of chemical communications are slowness of transmission (except in nonstationary media or across short distances) and the ability to be delayed or blocked by semipermeable or impermeable barriers, or by absorptive nanorobotic agents active in the environment. Noise on chemical channels may almost be eliminated by using artificial messenger molecules, reducing the natural background nearly to zero, though noise may still be present due to the presence of old signals, interference or crosstalk from other manmade devices, or even from intentional medical or military jamming. The encoding mechanism of "genome packets" would act less like email, and more like FTP, because the amount of information transferred per messenger molecule is so large.¹⁶⁵² Finally, used messenger molecules must be degraded by nanomachines (or other means) after the "destroy by" date has been passed; the number of messenger molecules to be captured and degraded or recycled by the messaging nanorobot population must approximate the number of messenger molecules released.

Nanorobot-to-nanorobot chemical broadcasts may be useful in the case of nanorobots working in close quarters where low emission rates will suffice, as in bacterial quorum sensing.³²³⁶ For example, achieving the minimum detectable concentration c_min ~ 10^-9 molecules/nm³ for 100-bit message molecules inside a (20 micron)³ human tissue cell requires emission of only ~10⁴ molecules (Section 7.2.1.3), costing ~1 pJ to manufacture and representing just 10^-7 % of tissue cell volume. Chemical communications may also be useful in various special applications such as "silent alarms" using premanufactured messenger molecules vented from onboard storage tanks, thus moving the manufacturing energy cost ex vivo. Assuming 0.1 micron³ tankage per nanorobot, each device may store ~10⁸ 100-bit message molecules available for ready release, enough to signal ~10⁴ in cyto alarm events. Each such alarm signal may permeate the entire cytosol in ~1 sec.

Chemical broadcast messaging may be most useful when the high energy cost of creating the message molecules is borne by some external or macroscale agency, thereby requiring individual nanorobots only to receive the chemical signal but not to send it. For example, messenger molecules manufactured in the laboratory, treatment clinic, or implanted dedicated communication organ (Section 7.3.4) may be injected into the bloodstream in sufficient concentration to ensure receipt. Given c_min ~ 1 x 10^-9 molecules/nm³ (I_message = 100 bits/molecule) or c_min ~ 9 x 10^-11 molecules/nm³ (I_message = 10⁹ bits/molecule), the minimum number of messenger molecules needed to promptly raise the entire bloodstream concentration to c_min (assuming no absorption by the body) is >7 x 10¹⁵ 100-bit messenger molecules (a ~0.006 mm³ injection) or >5 x 10¹⁴ 10⁹-bit messenger molecules (a 4 cm³ injection producing ~0.9 gm/liter concentration, somewhat exceeding the ~0.1 gm/liter threshold that occasionally produces anaphylactoid-type reactions). By comparison, a single nanorobot with a 1 pW power budget can only manufacture ~10⁴-10⁷ 100-bit message molecules/sec.

In 1998, chemical messaging was only starting to be applied in robotics. For example, in one experiment simple pheromone-guided macroscale mobile robots could follow a scent plume using silkworm moth antennae wired into an electronic neural network.²¹⁶³

Last updated on 18 February 2003