© 1999 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999

8.6.2 Exodermal Navigation

Exodermal navigation encompasses an extremely broad range of operating environments. Most nanomedically relevant are nanorobots resident on or in droplets or boluses of sweat, saliva, mucus, epidermal flakes, hairs, ejaculate, urine or feces that are discharged from the body. Exodermal navigation also encompasses all medically relevant environments such as:

1. clothing or bedding;

2. the surfaces and interiors of furniture (upon which fingerprints typically stand ~5 microns tall), domiciles, vehicles, food, cosmetics, and refuse; and

3. all possible locales including land (e.g., sidewalks, roads, public buildings), water (e.g., drinking glasses containing beverage, bathtubs, swimming pools, rivers), air (e.g., exhaled nanorobots or airborne elements of nanorobotic personal defensive systems) and even vacuum (e.g., spaceborne nanorobots). Each environment poses unique navigational challenges, a complete treatment of which is quite beyond the scope of this book.

The airborne nanorobot (Section 9.5.3) is an interesting example of these unique challenges. Airborne nanorobots can identify their host patient by chemical signature, much like a bloodhound or mosquito following its quarry's scent.³³⁵² Such chemical signatures or "odortypes" may include:

1. naturally-produced "baseline" chemical scents;

2. behaviorally-related scents which may appear or intensify during specific events such as heavy exercise, fear reactions (e.g., emotional excitement alone can increase the sweat rate by ~50%), defecation or flatulence, sexual activity, intoxication, and the like;

3. artificial scents such as perfumes, colognes, cosmetics and deodorants; and

4. artificial molecular taggants specially designed to simplify the recognition task, as for instance an odorless, volatile, digitally-encoded messenger molecule emitted from an external facility that is controlled by the patient.

Airborne nanorobots can stationkeep in the vicinity of the host patient by acoustic homing on a coded ultrasonic beacon worn by the patient, all of whose emanations are inaudible to the human ear. From Eqn. 4.52 the amplitude of a n = 100 KHz acoustic wave passing through STP air over a range of x = 1 meter only attenuates to ~25% of the original amplitude. Operational broadcast information is readily passed from patient to circumcorporeal aerial nanorobots using other similar acoustic channels (Section 7.4.8). Ex vivo nanorobots can also detect normal conversational speech at a range of ~2 meters using >2.4 micron³ pressure sensors (Section 4.9.1.6).

Airborne nanorobots can navigate and avoid no-fly zones (Section 9.5.3.6) by various methods. For instance, a flying nanorobot approaching an infinite planar sheet of 308 K (35°C) human flesh would detect spatially anisotropic thermal emissions (peak wavelength l_max = 9.35 microns, n_max = 32 THz; Eqn. 6.20) in a response time of t_meas ~ 5 nanosec, assuming a photosensor area A_e = 1 micron², SNR = 2 (~7 photons per detection), and dermal radiative intensity⁹¹⁹ I_d ~ 30 W/m² (see Eqn. 4.59). (Mosquitos register a ~0.0006% temperature differential at a 1-cm distance from human skin in real time during flight; Section 4.6.1.)

As another example, all aerial nanorobots can continuously transmit relative skin-proximity data to their neighbors, allowing each device within a virtual "warning lattice" to estimate its rate of approach to the nearest prohibited surface. Consider a grid of 200 million nanorobotic acoustic buoys deposited on the ~2 m² dermal surface of a nude human body. The buoys are spaced 100 microns apart, and each device has a 10-micron-diameter acoustic radiator operating on a 16.7% duty cycle at 4.2 MHz, giving a ~10 microsec signal repeat time using 7-wave signal packets (SNR = 2). An aerial nanorobot with an acoustic receiver of equal size can detect a warning buoy signal at a distance of 100 microns from the skin-resident grid, assuming a continuous buoy broadcast power of ~35 pW/buoy through air at 298 K (25°C), giving a total grid emission power of 7 milliwatts. Note that a nanorobot flying at 10 m/sec travels 100 microns in 10 microsec. Monitoring Doppler shifts in received frequency allows only the largest changes in relative velocity to be measured over multiple packet cycles in a usefully short period of time. At 4.2 MHz, summing ~50 packets (~500 microsec receipt time) allows a relative velocity change of ~1 meter/sec to be detected (Section 4.3.2).

Direct acoustic air ranging by individual flying nanorobots (i.e., echolocation) is also feasible but should be combined with a "warning lattice" system to allow neighbors and prohibited surfaces to be efficiently distinguished.

Power requirements may be estimated using Eqn. 7.22 by doubling X_path in the exponential, multiplying P_comm by (X_path/r_antenna)² to account for the doubled path length, and dividing P_comm by the coefficient of reflection at the air/skin interface (0.9995; Section 4.9.1.6). A 23.1% duty cycle at 3 MHz (again giving a ~10 microsec repeat time at SNR = 2) allows aerial nanorobots to detect an approaching skin surface from a distance of 100 microns, while consuming a continuous power draw of ~29,000 pW for a 10-micron diameter receiver or ~230 pW for a 20-micron receiver, in air at 298 K.

Last updated on 20 February 2003