© 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 n_bot (nanorobots/m³) and mean separation X_path = n_bot^-1/3 (meters) that is stationkeeping around a human user (Section 9.5.3). Each nanorobot has a communications energy budget of P_comm (watts) and broadcasts photons (or sound waves) of frequency n from a circular transmitter of radius r_antenna to a receiver of similar size located a mean distance X_path 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

{Eqn. 7.21}

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%), P_comm = 10 pW, r_antenna = 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 X_path = 100 microns, or n_bot = 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 r_antenna² ~ 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% = P_out/P_in, as defined by Eqn. 7.7, and the input power budget for communication P_comm = P_in from Eqn. 7.9, except that in air the exponential attenuation must be replaced with exp(2 a_air n² X_path) as described in Section 4.9.1.3 for pure fluids. Using variables as previously defined, then 'I ~ n f_duty and

which is valid in pure fluids if r_antenna/l << 1. We assume h = 0.018 x 10^-3 kg/m-sec and v_sound ~ 343 m/sec for air at 20^oC,⁷⁶³ kT ~ 4.3 zJ, SNR = 2, k_r = 2 for a piston radiator, r = 1.3 kg/m³, r_antenna = 1 micron, and a_air = 1.4 x 10^-10 sec²/m (Table 4.2). For any given X_path, there is an optimum transmission frequency n that minimizes P_comm.* Taking X_path = 100 microns (n_bot = 10¹² nanorobots/m³), internanorobot 'I ~ 40 bits/sec at an optimum frequency of 4.2 MHz and f_duty = 0.001% duty cycle (P_comm ~ 7 pW). Note that Eqn. 7.22 is extremely sensitive to antenna size. At r_antenna = 2 microns, 'I ~ 2000 bits/sec at 4.2 MHz for X_path = 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 / v_sound) ~ (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 l_x = (2 a_air n²)^-1 ~ 200 microns at n = 4.2 MHz. If the nearest boundary of the cloud is maintained a distance X_ear from the tympanic membrane of the human ear, then the acoustic intensity reaching the eardrum surface is approximately:

{Eqn. 7.23}

If the cloud fills the auditory canal, then X_ear = 0 and I_cloud ~ 10^-3 watts/m² (~93 dB, "shouting"), which is probably safe enough for the human ear and is inaudible at 4.2 MHz. At X_ear = 3 mm, I_cloud ~ 10^-9 watts/m² (~30 dB, "whispering"); if the cloud remains entirely outside the auditory canal, then X_ear ~ 2 cm and I_cloud ~ 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 X_path), 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 X_path 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 r_antenna = 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