Nanomedicine, Volume IIA: Biocompatibility
© 2003 Robert A. Freitas Jr. All Rights Reserved.
Robert A. Freitas Jr., Nanomedicine, Volume IIA: Biocompatibility, Landes Bioscience, Georgetown, TX, 2003
Pruritus (itching) is an unpleasant cutaneous sensation [3672-3675] that usually (but not always ) evokes the urge to scratch. These poorly-localized primary sensory impulses are carried on unmyelinated C fibers through the spinothalamic tract to the thalamus and on to the sensory cortex [3674, 3676, 5595]. Scratching appears to interrupt the rhythm of afferent impulses to the spinal column and relieves the sensation of itching . Chemical mediators such as histamine and peptidases such as papain  (a plant enzyme) produce itching when injected, known as alloknesis , while several mediators (e.g., bradykinin, neurotensin, secretin, substance P) stimulate the release of histamine from histamine-containing cells . Kinins may be pruritic, but prostaglandins are only weak pruritogens . Subdermal bile acids are found in cases of pruritus associated with obstructive biliary disease . Some cases of generalized pruritus can be attributed to dry skin. But there are many other causes including parasites (e.g., scabies, pediculosis), physical obstruction of ducts (e.g., miliaria), and physical or chemical irritation of the skin (e.g., fiberglass dermatitis, contact dermatitis) . Histamine release can be elicited by activation of complement (Section 126.96.36.199); by immunoglobulin IgE (Section 188.8.131.52) which mediates some allergic responses (Section 184.108.40.206); or by nauseogenic stimuli (Section 220.127.116.11). This in turn produces itching, probably mediated by subdermal “itch units” consisting of unmyelinated nerve fibers .
Can the passage of nanorobots across the epidermis cause itching, either chemically or mechanically? Nanorobots should be designed with chemically nonpruritic external surfaces, and the mechanical forces produced by individual 1- to 100-micron nanorobots traversing human skin at <1 m/sec appear to be insensible (Section 9.5.2). However, >1000 large legged nanorobots simultaneously traversing the receptive field of a single dermal tactile receptor (10-100 mm2; Table 7.3) at >45 cm/sec might just be detectable at some highly sensitive locations such as tongue and fingertip (Section 9.5.2(F)). This implies a minimum threshold dermal-sensible number density of >103-104 nanorobots/cm2, which is ~10-100% coverage of the skin. For comparison, the scabies mite Sarcoptes scabiei, ~100-300 microns in diameter, is perhaps the smallest legged mite that causes itching . But this itching is caused by a local Type IV delayed hypersensitivity (allergic) reaction (Section 18.104.22.168) and not by mechanical irritation . Abundant dust mites of similar size live uneventfully on human epidermis and hair follicles, producing allergenic itching only rarely  and without producing mechanical itch – though the superficial dust mites are not an ideal comparison because they don’t burrow as deeply as scabies mites (or as deeply as nanorobots might).
Pruritus from immotile medical nanorobots cannot be completely ruled out because nanorobot-size pruritic fiberglass strands measuring 7-24 microns in diameter are known to produce a mechanical abrasive dermatitis [196, 197, 201, 3680-3685] whose severity increases with applied pressure . However, the author’s informal experiments with 0.25- to 250-micron diamond grit particles on his own skin (Section 15.1.1) suggest that diamondoid nanorobots with generally smooth surfaces should elicit few if any mechanical itching sensations. Obviously, more formal studies with quantifiable results are needed for confirmation.
Last updated on 30 April 2004