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
8.6.1 Epidermal Navigation
Nanorobots traversing the epidermal regime (Section 9.5.2) must ascertain their position on the surface of the skin or in the hair. The epidermis averages ~100 microns thick but ranges up to 1000-1500 microns thick on the palms, soles, and other areas regularly subjected to rubbing or pressure. There are no blood vessels in the epidermis, so nanorobots may not be able to rely upon an internal microtransponder network or related navigational facilities.
The simplest solution to epidermal navigation is basic map-following. For example, the adult body has 2-4 million sweat glands (100-200/cm2, mean separation ~800 microns). Most of these are saline- and urea-emitting eccrine glands ~20 microns in diameter, prevalent on the back, chest, forehead, palms, and soles. A smaller number of scent-secreting apocrine glands ~200 microns in diameter are concentrated at the underarms, nipples, genitals, and anus. Apocrine secretions are promptly broken down by skin bacteria to make odoriferous androstenone ("stale urine"), androstenol ("musky"), and isovaleric acid ("sweaty goatlike").873 Sweat glands are persistent dermal landmarks that are readily mapped, along with their distinctive chemical secretions (including 0.028-0.4 mg/cm3 glucose).585 A positional or chemonavigational sweat gland map of the ~2 m2 human skin surface to ~800 micron resolution requires ~3 million pixels or 24 megabits of data storage assuming 8-bit pixels.
An alternative or supplementary system is a follicular or crinal map of 50-100 micron diameter hair shafts that are irregularly distributed over the surface of the skin. Follicular map elements are reasonably stable due to subdermal anchoring. Hair follicles normally are most numerous on the scalp (~125,000 hairs or ~200/cm2, with a ~700 micron mean separation between shafts and a mean natural life of 0.4-4 years), on the axillary (underarm), perineal and pubic regions, on the eyebrows and eyelids of both genders (each eyebrow has ~600 hairs with an average life of 112 days), and in a highly variable number density on the face, chest, arms and legs, most notably in males. An epidermal follicular map recording the locations of ~200,000 individual hair shafts using (700 micron)2 8-bit pixels requires 4 million pixels and 32 megabits of data storage. Crinal navigation is readily accommodated as the nanorobot uses dead reckoning to track its position while traversing individually addressed hair shafts.
Epidermal map reliability is enhanced by noting numerous landmarks that may be regarded as "permanent" features on the timescale of nanorobotic actions. Such features might include scars, birthmarks, tattoos, warts, corns, cysts, nevi, small skin flaps, or dematofibromas. Stretch marks are found over the abdomen and breasts during pregnancy, over the abdomen in cases of obesity or fluid retention, on the necks of goiter patients, over the loins of patients suffering from disorders affecting the sacroiliac region, and over any distended organ. Epidermal maps are further enhanced using dermatoglyphics,3425 the study of the patterns of ridges and lines on the skin of the fingers, palms, wrists, toes, soles, and neck. Many of these patterns are unique to a given individual (thus may be used to verify patient identity, e.g., fingerprints) and are persistent enough over time to serve as the basis for stable epidermal maps. Dermatoglyphics already plays a role in traditional diagnosis -- Down's syndrome produces a characteristic mid-palm crease, and infant palm prints can reveal telltale signs of congenital heart defect.3426-3428 Dermal ridges are also a sexually dimorphic trait -- males have more ridges than females, and both sexes usually have more ridges on the right hand than on the left hand.1052 In 1998, mole (nevi) photographic surveillance was a preventative skin mapping procedure to detect the onset of melanomas.
Less permanent but still useful epidermal landmarks include wounds, blisters, sunburns, eczematous or psoriatic regions, or chemical stains. Epidermal microbiotagraphic maps (Section 8.4.4) are also possible, but microbial populations usually cluster near apocrine glands and thus are unlikely to provide significant additional navigational information.
Skin temperature varies greatly across the epidermis (Section 8.4.1). The nonuniform number density of sweat glands in the skin produces highly localized periglandular temperature, moisture, and chemical compositional variations. However, these variations probably add little new information and are more difficult to interpret because readings may be strongly influenced by numerous complicating factors such as clothing and cosmetics, general physical activity level, localized physical activity (e.g., clapping hands), nonuniform sunlight-mediated heating, airflow and winds, immersion of body parts in water, and so forth. Nanorobots attempting to use this kind of information will need a very sophisticated global knowledge of somatic status.
Epidermal maps more accurate than ~1 mm might not prove particularly useful because natural movement of highly elastic dermal tissues may inject measurement errors of this magnitude into relative positional data used to locate adjacent glands, follicles or other epidermal landmarks.
Last updated on 20 February 2003