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
7.4.8 Transvenue Outmessaging
Can nanorobots stationed inside one patient communicate with nanorobots stationed inside another patient? Other than simple linkages of previously described standard inmessaging and outmessaging modalities, remote communication between two fully physically disjoint sets of intracorporeal nanorobots can probably be ruled out. However, in vivo and epidermal nanorobots can in theory be programmed to take advantage of serendipitous opportunities using mobile communicytes. Such opportunities for extracorporeal contacts may include patient-to-patient communicyte exchange by direct physical contact (e.g., handshaking or sexual activity), by indiscriminate broadcast transfers (e.g., sneezing, bleeding, desquamation, or sharing tools or utensils), by serendipitous anonymous contacts (e.g., doorknobs, public toilet seats, library books), or by deliberate airborne nanorobot migrations (Section 9.5.3). Security protocols (Chapter 12) are especially important in these applications.
Data is also readily transferred purposely between patients, as for example via matching dermal communications patches embedded in the palms of each patient's hands which are briefly pressed together while shaking hands. In 1998, IBM's Personal Area Network (PAN) used near-field oscillating galvanic potentials with a low carrier frequency (~330 KHz), inducing ~50 picoamp currents from a ~1.5 milliwatt power source, to exchange a few hundred bytes of personal data between users during a handshake.2700
Communication among large numbers of airborne exodermal nanorobots (Section 9.5.3) is a particularly interesting and useful application (e.g., Chapter 21). Such communication may be accomplished by chemical means (Section 8.6.2), but here we will review only optical and acoustic methods.
Consider a cloud of aerial nanorobots with number density nbot (nanorobots/m3) and mean separation Xpath = nbot-1/3 (meters) that is stationkeeping around a human user (Section 9.5.3). Each nanorobot has a communications energy budget of Pcomm (watts) and broadcasts photons (or sound waves) of frequency n from a circular transmitter of radius rantenna to a receiver of similar size located a mean distance Xpath away.
In the case of optical communication, the information transfer rate 'I may be regarded as the number of photons received per second from a neighboring device, or
where e% is the ergophotonic efficiency of photon generation and h = 6.63 x 10-34 joule-sec (Planck's constant). Conservatively taking e% = 0.01 (1%), Pcomm = 10 pW, rantenna = 1 micron, and assuming red photons with n = 4.3 x 1014 Hz (l = 700 nm, ~280 zJ/photon), then inter-nanorobot 'I ~ 10 bits/sec for Xpath = 100 microns, or nbot = 1012 nanorobots/m3. A nanorobot cloud of this density should appear visually dense, possibly nearly opaque, since 1012 nanorobots each of cross-sectional area ~p rantenna2 ~ 3 micron2 may occult >1 m2 looking through a 1 m3 cubic cloud having a face area of 1 m2. The maximum broadcast optical intensity of a ~1 meter column uniformly distributed at this number density is ~0.1 watts/m2, which is safe for the human eye. Nanorobot cloud waste heat at this number density, due to communications activities, is ~10 watts/m3.
In the case of acoustic communication, the efficiency of ergoacoustic transmission efficiency e% = Pout/Pin, as defined by Eqn. 7.7, and the input power budget for communication Pcomm = Pin from Eqn. 7.9, except that in air the exponential attenuation must be replaced with exp(2 aair n2 Xpath) as described in Section 126.96.36.199 for pure fluids. Using variables as previously defined, then 'I ~ n fduty and
which is valid in pure fluids if rantenna/l << 1. We assume h = 0.018 x 10-3 kg/m-sec and vsound ~ 343 m/sec for air at 20oC,763 kT ~ 4.3 zJ, SNR = 2, kr = 2 for a piston radiator, r = 1.3 kg/m3, rantenna = 1 micron, and aair = 1.4 x 10-10 sec2/m (Table 4.2). For any given Xpath, there is an optimum transmission frequency n that minimizes Pcomm.* Taking Xpath = 100 microns (nbot = 1012 nanorobots/m3), internanorobot 'I ~ 40 bits/sec at an optimum frequency of 4.2 MHz and fduty = 0.001% duty cycle (Pcomm ~ 7 pW). Note that Eqn. 7.22 is extremely sensitive to antenna size. At rantenna = 2 microns, 'I ~ 2000 bits/sec at 4.2 MHz for Xpath = 100 microns.
* Airborne nanorobots traveling at a Dv ~ 1 m/sec relative velocity will incur a Doppler shift in the received frequency of n(1 + Dv / vsound) ~ (1.003)n, defining a +3 KHz per MHz band bracketing the intended frequency.
What is the maximum acoustic intensity at the ear? Measuring from the outermost boundary of the nanorobot cloud and moving inward, ultrasound power extinguishes rapidly, falling to an intensity e-1 (~37%) in a distance lx = (2 aair n2)-1 ~ 200 microns at n = 4.2 MHz. If the nearest boundary of the cloud is maintained a distance Xear from the tympanic membrane of the human ear, then the acoustic intensity reaching the eardrum surface is approximately:
If the cloud fills the auditory canal, then Xear = 0 and Icloud ~ 10-3 watts/m2 (~93 dB, "shouting"), which is probably safe enough for the human ear and is inaudible at 4.2 MHz. At Xear = 3 mm, Icloud ~ 10-9 watts/m2 (~30 dB, "whispering"); if the cloud remains entirely outside the auditory canal, then Xear ~ 2 cm and Icloud ~ 10-46 watts/m2.
Photonic or acoustic modalities may serve as the basis for ex vivo mobile communication networks (Section 7.3.2). Since bit rates are significantly enhanced by proximity (e.g., smaller Xpath), it may be useful to assign circumcorporeal aerial nanorobots to condensed geometric or traffic patterns, and to allow opportunistic conferencing among them as required, which might appear to the user as irregular or stellate aggregations, diaphanous tendrils, ordered rows or clump arrays at Cartesian grid vertices, evanescent toroidal cloudlets, or other floating geometries conducive to rapid and effective data sharing. In the earlier examples given above, reducing Xpath to 10 microns raises the optical transfer rate to 900 bits/sec and the acoustic transfer rate to 50,000 bits/sec at 13.4 MHz, for rantenna = 1 micron.
These modalities could also be used by a circumcorporeal nanorobot cloud to communicate visual and auditory messages directly to the user.
Last updated on 19 February 2003