**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

**7.2.5.5 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 l_{cable}
and radius r_{cable} carrying a fluid of viscosity h
at a volumetric flow rate of V maintained by a pressure differential of p_{cable}
between the two ends of the cable. Assuming the fluid is a 10% suspension of
messenger molecules of information density D_{message} ~ 26 bits/nm^{3}
(Section 7.2.1.1) with viscosity similar to human
plasma, then net fluid information density D_{fluid} ~ 2.6 x 10^{27}
bits/m^{3}; from the Hagen-Poiseuille law (Section
9.2.5) the maximum information transfer rate is

_{}
{Eqn. 7.20}

Assuming a safe, ultraconservative p_{cable} = 1 atm
and taking r_{cable} = 0.5 micron and h =
1.1 x 10^{-3} kg/m-sec, then 'I_{chemo} = 10^{14} bits/sec
through a cable of length l_{cable} = 50 microns, with power consumption
P_{chemo} = p r_{tube}^{4}
p_{cable}^{2} / 8 h l_{cable}
= 4500 pW (~0.04 zJ/bit). For a cable 0.5 meter in length, 'I_{chemo}
= 10^{10} bits/sec and P_{chemo} = 0.4 pW (~0.04 zJ/bit). If
p_{cable} is more liberally raised to 1000 atm, a cable 0.5 meter in
length can transfer 'I_{chemo} = 10^{13} bits/sec requiring
P_{chemo} = 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 v_{fluid} = r_{tube}^{2}
p_{cable} / 8 h l_{cable}. Thus a
given message requires a travel time t_{message} = l_{cable}
/ v_{fluid }= 8 h l_{cable}^{2}
/ r_{cable}^{2} p_{cable} ~ 0.001 sec to pass through
a 50-micron-long 1-micron-diameter cable; a 10^{9} 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
~10^{12} bits/sec, or ~5 zJ/bit, near the theoretical minimum. Similarly,
t_{message} ~ 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 n_{cable} ~ GHz
data transfer rate. From Eqn. 3.19, P_{drag}
~ 10^{-16} watts per monomeric unit giving a total cable power dissipation
of P_{cable} = P_{drag} l_{cable} n_{cable}
/ v ~ 10^{5} pW (10^{5} zJ/bit) taking l_{cable} = 1
meter, n_{cable} = 1 GHz and v = 1 m/sec.

Last updated on 19 February 2003