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

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

15.5.5.1.3 Disruption of Erythrocyte Aggregation

At low shear rates, red cells aggregate into rouleaux (i.e., the stack-of-coins configuration) and migrate inward, forming a network of linear and branched chain aggregates in the core of the vascular tube (Section 9.4.1.3). Individual rouleaux may incorporate 10-20 red cells, or more, creating by far the largest cellular elements normally present in the blood. At the highest shear rates, the rouleaux break up entirely into single red cells, and the red cells then distribute themselves more uniformly in the radial direction (Section 9.4.1.3). Red cell disaggregation is essentially complete when the shear stress of the cell suspension is raised above 0.2 N/m² [4092].

Could medical nanorobots similarly disaggregate erythrocyte rouleaux? Collisions between free-floating nanorobots and rouleaux should produce shear stresses <0.1 N/m² (Section 9.4.2.2). The energy required to disaggregate individual red cells has been estimated as ~10⁴ ergs/cm² (~10 pJ/micron²) [4093] or a force of ~70 pN (Section 15.5.6.1). A specialized nanomanipulator driven by a ~10 pW power source plausibly could purposely pry apart two aggregated red cells with a mutual contact area of ~10 micron² in a time on the order of ~10 sec. But random disaggregation is unlikely to occur during simple elastic impacts between free-floating nanorobots and red cell rouleaux because the kinetic energy of a ~1-micron³ diamondoid nanorobot even traveling at 1 m/sec is only ~0.001 pJ.

During tube flow, rouleaux migrate inward forming a network of aggregates in the core of the tube surrounded by a peripheral cell-depleted layer consisting of single cells, occasional small rouleaux, white cells, platelets, and, potentially, nanorobots. This results in a two-phase flow of a relatively higher shear rate peripheral zone surrounding a lower shear rate, high cell concentration, central zone [4094]. Even at a maximum 10% Nct blood concentration, nanorobots represent at most 20% of total red cell mass, so collisional dispersion of the high-shear peripheral zone should be modest and effective diffusion rates should remain high. Moreover, free-floating medical nanorobots should exhibit no axial preference [4094] and should be maximally marginated towards the vessel walls even under high-shear conditions (Section 9.4.1.3). This, along with their biochemically inactive diamondoid surfaces, suggests that the mere presence of medical nanorobots in the blood should not interfere with adhesive processes involved in axial rouleaux formation far from vessel walls, where shear rates are lower.

Last updated on 30 April 2004