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
7.3.2 Mobile Networks
In a fiberless network, mobile communicytes may be deployed in tissue and blood, and serve as communications nodes. Devices enter a tissue and station themselves for optimum data transfer. All nodal and internodal traffic is acoustic. With a spacing of 100 microns between nodes, continuous internodal message transmission at 100 MHz with fduty = 1% using an r = 1 micron radiator requires ~600 pW (Eqn. 7.9). From relations given in Section 7.3.1, a whole-body installation with Vtiss ~ 0.1 m3 requires ~1011 communicytes uniformly distributed throughout the tissues, a maximum network dissipation of ~60 watts plus another 0.5 watts for node switching losses. Since broadcasts are necessarily omnidirectional for transmitters smaller than ~vsound / n = 15 microns at 100 MHz, the network is entirely uniform with no backbone and total long-distance message capacity is ~106 bits/sec in broadcast mode. Limits on broadcast power can minimize bandwidth overlap and crosstalk between nonadjacent nodes. At 100 MHz and fduty = 10%, a whole-body network capacity of 107 bits/sec generates an uncomfortable ~600 watts of waste heat; if these communicytes are installed in only <17% of the body volume, then local 107 bit/sec networks may be made available at <100 watts. There may be many conceptual similarities to telephonic cellular networks,1650,1651 including frequency reuse, handoff algorithms, and the likelihood of large numbers of point-to-point ~106 bit/sec communication sessions through different modes without straining the system. Such design details, while interesting, are beyond the scope of this book.
Noncommunicyte mobile nanorobots access the network via acoustic channels as in the fiber network. The n fduty = 1 MHz/node limit implies that long-distance communications will be sharply curtailed within the tissues. However, local-area communications may have quite satisfactory bit rates. For example, each of 100 nanorobots present in one (100 micron)3 communications voxel can send continuous ~100 bit/sec messages (using only ~100 Hz of available bandwidth) simultaneously to each of 100 nanorobots located in an adjacent voxel. Thus total mononodal traffic over all 1011 nodes can approach the ~1018 bits/sec of the optical fiber backbone throughout the entire installation volume. At a 0.01 MHz/channel nodal access rate for each of 100 nearby nanorobots, a simple 200-bit message takes 20 millisec to upload or download, plus ~200 microns / vsound ~10-7 sec to pass between adjacent communications voxels at the speed of sound (vsound ~ 1500 m/sec in soft tissues and whole blood; Table 6.7). Hence simple messages may be passed between any two specific nanorobots in nearby cells in ~40 millisec, though a complex 109-bit message requires an impractical ~105 sec.
Bloodborne communicytes provide a supplementary long-distance messaging capability. Deployment of ~1010 5-micron3 communicytes in a 5.4-liter blood volume (<0.001% concentration by volume) gives a mean interdevice separation of ~80 microns. Communicytes passing through a 1 mm capillary (averaging 10 embedded nodes along its length) in ~ 1 sec can upload or download ~105 bits at each node. Each bloodborne communicyte has a patrol volume of (80 microns)3 = 500,000 micron3, about ten times the volume of the average capillary, so a fresh communicyte enters each capillary about once every 10 seconds and can receive messages totalling 106 bits during each transit. The fixed patrol volume per bloodborne communicyte implies that these devices will not remain in continuous contact with their bloodborne neighbors while in transit through vessels much smaller than ~80 microns in diameter (e.g., terminal arterioles, metarterioles, capillaries, and the postcapillary and collecting venules; Section 8.2.1). For example, in a 40 micron-diameter terminal arteriole the mean interdevice spacing grows from ~80 microns to ~400 microns, resulting in a temporary communications blackout between bloodborne neighbors during transit through the microvasculature. The blackout may normally last 5-10 seconds (Section 220.127.116.11).
Once received by a bloodborne communicyte, simple long-distance messages are quickly rebroadcast (post-blackout) throughout the bloodstream communicyte fleet (which draws ~6 watts continuous), the signal propagating at near the speed of sound, finally reaching the one communicyte situated nearest to the intended recipient node in at most ~2 meters / vsound ~ 10-3 sec. Assuming a 10 microsec delay between rebroadcasts due to reading a 10-bit routing header at 1 MHz implies an additional rebroadcast delay of at most ~0.2 sec through a chain of ~20,000 communicytes over a ~2 meter path length. Allowing up to ~10 sec to enter and traverse the entire length of the relevant capillary, then long-distance point-to-point messaging requires at most ~11 sec and allows transfer rates of ~105 bits/sec or better systemwide. Total long-distance capacity is limited to acoustic bandwidth and the maximum transfer rate of each communicyte, say, at fduty = 1%, or ~106 bits/sec (Section 7.2.6) equivalent to ~0.01% of the entire Internet backbone in 1997. Bloodborne communicytes also provide a useful messaging facility for bloodborne noncommunicyte nanorobots.
Besides serving as mobile message repeater stations, bloodborne communicytes may further boost total system capacity by acting as physical message carriers. For example, a single communicyte bearing messages totalling 1010 bits (Section 7.2.6) circumnavigates the entire vascular circuit once every ~60 seconds. Thus, in theory, point-to-point messages may be carried throughout the entire operational volume at an effective ~108 bits/sec rate in this manner, most useful for nanorobots in peripheral tissues attempting to send messages to devices (or postal depots) located in the heart or lungs (organs which the entire blood volume reliably transits once every circuit). (Acoustic downloading at the destination is limited to 106 bits/sec at fduty = 1%, but modest locomotion skills and maneuverability would permit physical docking with nodes allowing up to ~1010 bit/sec mechanical downloading rates; purely statistical collision-mediated data transfer is not efficient at these low bloodstream concentrations.) A total of 1020 (message-carrier) bits may be in transit at any one time in the blood. Except for this message-carrier function and the "blackout" effect in the microvasculature, communicyte network performance is not significantly affected by conditions of bradycardia or cardiostasis.
As in the fiber network, communicyte nodes may store considerable quantities of data. Assuming up to ~1010 bits for each of 1011 nodal devices, either network has a maximum total storage capacity of ~1021 bits. A single dedicated (4 mm)3 library nodule (Section 7.3.4) implanted anywhere in the body can also contain ~1021 bits.
Last updated on 19 February 2003