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
126.96.36.199 No-Fly Zones
Most circumcorporeal aerial nanorobots should take care to avoid certain well-defined zones near the human body. Perhaps the most important medical issue is that particles under 5 microns in diameter are dangerous to inhale. Concentrations of >108 m-3 of free silica particles <5 microns in size are usually considered hazardous.1572 Airborne nanorobots may be deployed in number densities of ~1012 m-3 or higher (Section 7.4.8), so anti-inhalation, inhale-safe, and post-inhalation extraction protocols are essential in this application. (Particles larger than 2-5 microns that are inspired through the nose become trapped on the nasal mucus membrane and do not reach the lower airway; Section 8.2.2.)
At rest, inhalation velocity at the trachea is vinhale rest ~ 0.3 m/sec. During the heaviest exercise, turbulent airflow into the trachea peaks near ~3.3 liters/sec at a maximum inhalation airspeed of vinhale max ~ 5 m/sec (Section 8.2.2 and Eqn. 9.30). Nanorobots capable of flying at vnano >~ vinhale max can outrun even the fastest inspired air by simply reversing course. Nanorobots restricted to slower speeds can also avoid inhalation by promptly initiating lateral motion immediately upon entering either of two hemiellipsoidal no-fly zones which are very conservatively estimated to be of major-axial radius:
centered on the vestibule of the mouth and on the anterior nares of the nose, where Roral ~ 2.5 cm is the radius of the oral orifice. Thus a nanorobot with maximum airspeed vnano = 1 m/sec must observe a perifacial no-fly major-axial radius of Rclear ~ 6 cm.
The second most important no-fly zone is the surface of the skin, including (in special circumstances) the oral mucous (tissue) membrane, tongue, soft palate, larynx, trachea, and nasal passages. It is theoretically possible for diamondoid nanorobots with rigid protrusions that accidentally impact the epidermis at high speeds to irritate or even tear the skin. Consider a nanorobot of density r and dimension L with a rigid appendage of dimension q L, that impacts an epidermal surface of tearing strength stear at a velocity v, coming to a halt in a distance Xscratch. If the impact energy is transferred exclusively through the appendage, then the skin tears if:
Taking r = 2000 kg/m3, L = 100 microns, v = 10 m/sec, q/L = 0.1, and stear = 107 N/m2, then Xscratch <~ 100 microns -- potentially leaving up to a 100-micron long, 10-micron deep scratch on the skin. Long scalp hair, which often waves randomly in the wind, should also be avoided (or actively managed; Chapter 28) by the aerial nanorobot cloud. Nanorobots with 10-100 micron-long wings operated at 0.1-1 MHz (avoiding subharmonics to ensure buzzless performance) can have transverse wingtip velocities up to 1-10 m/sec, which could damage soft tissues upon impact.
Another possible no-fly zone is the interior of the auditory canal, especially immediately adjacent to the tympanic membrane. In theory (Section 7.4.8), a very dense nanorobot cloud using acoustic communications at high bandwidth could generate a pressure intensity at the eardrum that exceeds the threshold of pain, although this is unlikely in practice. (More interestingly, such a cloud could enter the ear canal and emit coordinated audible subharmonics in the 100-1000 Hz range, thus "speaking" directly to the user; Section 188.8.131.52.)
Optical communication intensities are equally unlikely to exceed safe limits in the vicinity of the human eye (Section 7.4.8). However, entirely aside from the epidermal no-fly zone (above), general-purpose aerial nanorobots should especially avoid all contact with the surface of the cornea, the conjunctiva, and the inner surfaces of the eyelid, in order to prevent irritation, or serious scoring and gouging (e.g., corneal ulcer, conjunctivitis, or superficial punctate keratitis), or embedment in these much softer, exposed mucosal tissues.
Last updated on 22 February 2003