© 2003 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Nanomedicine, Volume IIA: Biocompatibility, Landes Bioscience, Georgetown, TX, 2003

15.4.2.5 Geometrical Trapping Elsewhere in the Circulation

Most capillary beds in body tissues will permit smooth-surface ~4-micron diameter nanorobots to pass easily. However, passively circulating nanorobots might become trapped, at least temporarily, at several other sites:

(1) Heart. Early therapeutic drug-containing microspheres 75-150 microns in diameter implanted directly in the heart during open heart surgery caused extensive myocardial necrosis [2797]. A particle trapping experiment in cat and rabbit hearts using 7.9-, 8.6-, and 14.6-micron microspheres found that only the largest spheres were completely trapped, whereas significant quantities (7-8%) of the smaller spheres were found in perfusate leaving the heart [2798]. Complete trapping of 14.4-micron spheres proved there were no leaks or arteriovenous shunts nearby [2798]. A few nanorobots also might become trapped in eddies immediately downstream of the four heart valves – specifically, in the mural cusps on the ventricular side of the tricuspid and mitral valves [2799-2802], and in the sinuses of Valsalva [2818, 2821] on the arterial side of the aortic and pulmonic valves [2803-2805], where flow can temporarily stagnate in hydrodynamic vortices, or in cases of cardiac regurgitation [2806-2809].

(2) Vein Valves. Nanorobots might become caught in the sinuses behind vein valves (Figure 8.3) where there may exist pressure traps [2810] or semi-stagnant pockets [2811].

(3) Hemodynamic Anomalies. Nanorobots could get caught in eddy vortices immediately downstream from vessel constrictions (Figure 9.18A), as might be caused, for example, by atherosclerotic partial occlusions, stenoses, or various vascular lesions. Other unusual flow reversal conditions caused by partial occlusions in particular locations may trap nanorobots in specific branches of the circulation for extended periods of time, as in the “steal syndrome” (tends to refer to subclavian steal syndrome and is significant because of the neurological effects seen due to vascular insufficiency to the vertebral artery and all the branches it supplies) [2812-2816] where blood reverses its flow direction, in some cases simultaneously in two anastomosed arteries (e.g., “double steal” near the circle of Willis [2815]) or even in three arteries (e.g., “triple steal” [2812]). Steal syndrome can cause cerebral ischemia and stroke.

(4) Vascular Aneurysms and Fistulas. Passively circulating nanorobots, like microbes [2817], could become trapped in concave vascular caverns such as cardiac aneurysms [2818-2824], arterial aneurysms [2821-2824, 4603-4606], infectious aneurysms [2825-2827], drug-induced aneurysms [2828], venous aneurysms [2829, 2830], and giant vascular fistulas [2831]. The fate of particles trapped in this manner could be phagocytosis (Sections 15.4.3.2 and 15.4.3.4) or foreign body reaction (Section 15.4.3.5).

Last updated on 30 April 2004