Nanomedicine, Volume I: Basic Capabilities
© 1999 Robert A. Freitas Jr. All Rights Reserved.
Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999
188.8.131.52 Chemomessenger Cables
Unlike the chemical messaging described in Section 7.2.1, which was dominated by diffusive effects, information transfer using chemical messenger molecules that are confined within sealed pipes is controlled by the laminar bulk flow rate of the rapidly-moving carrier fluid.
Consider a chemomessenger cable of length lcable and radius rcable carrying a fluid of viscosity h at a volumetric flow rate of V maintained by a pressure differential of pcable between the two ends of the cable. Assuming the fluid is a 10% suspension of messenger molecules of information density Dmessage ~ 26 bits/nm3 (Section 184.108.40.206) with viscosity similar to human plasma, then net fluid information density Dfluid ~ 2.6 x 1027 bits/m3; from the Hagen-Poiseuille law (Section 9.2.5) the maximum information transfer rate is
Assuming a safe, ultraconservative pcable = 1 atm and taking rcable = 0.5 micron and h = 1.1 x 10-3 kg/m-sec, then 'Ichemo = 1014 bits/sec through a cable of length lcable = 50 microns, with power consumption Pchemo = p rtube4 pcable2 / 8 h lcable = 4500 pW (~0.04 zJ/bit). For a cable 0.5 meter in length, 'Ichemo = 1010 bits/sec and Pchemo = 0.4 pW (~0.04 zJ/bit). If pcable is more liberally raised to 1000 atm, a cable 0.5 meter in length can transfer 'Ichemo = 1013 bits/sec requiring Pchemo = 450 nanowatts (~40 zJ/bit). While these are phenomenal information transport rates compared to other methods, two important caveats are in order.
First, such high transfer rates are purchased at the price of significantly increased receiver complexity and message processing time, since the message molecules must be captured, oriented, unspooled, fed past a read head at relatively slow speed, then stored, recycled, or disposed of properly. Data carrier fluid must be returned to the transmitter using a second cable; a double-cable pair establishes a complete fluidic circuit. The additional transmitter complexity and extra power required for chemical modifications of message carriers may be confined to external facilities and hence do not significantly constrain in vivo operations.
Second, the message travel speed from one end of the cable to the other is limited to the fluid flow velocity vfluid = rtube2 pcable / 8 h lcable. Thus a given message requires a travel time tmessage = lcable / vfluid = 8 h lcable2 / rcable2 pcable ~ 0.001 sec to pass through a 50-micron-long 1-micron-diameter cable; a 109 bit message molecule measuring ~0.4 micron in diameter (Eqn. 7.2) travels at ~5 cm/sec and therefore only transfers the message information at ~1012 bits/sec, or ~5 zJ/bit, near the theoretical minimum. Similarly, tmessage ~ 4 days through a 1-meter-long micron-wide cable at 1 atm driving pressure (~6 minutes if driving pressure is raised to 1000 atm).
R. Merkle points out that these two restrictions may be ameliorated by employing a cable transporting monomeric units that can adopt one of two or more distinct physical conformations. In particular, if the monomeric units are flat (e.g., small bits of graphite), are held together by short sections of polyyne (carbyne rods), and if the cable is formed in a flat housing, then one bit of information can be transmitted by rotating each monomeric unit into one of two positions which are separated by 180°. Minimal energy should be required to rotate a monomer entering the cable input, or subsequently to read the rotational state of a transported monomer exiting the cable output. A monomer transport speed of v = 1 m/sec and a 1-nm separation between successively arriving bit-carrying monomeric units allows a ncable ~ GHz data transfer rate. From Eqn. 3.19, Pdrag ~ 10-16 watts per monomeric unit giving a total cable power dissipation of Pcable = Pdrag lcable ncable / v ~ 105 pW (105 zJ/bit) taking lcable = 1 meter, ncable = 1 GHz and v = 1 m/sec.
Last updated on 19 February 2003