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

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

15.5.5.2.2 Disruption of Platelet Aggregation

Platelet aggregation is necessary for clotting to occur. Detailed studies of two-body collision hydrodynamics and platelet activation [4110] suggest that platelet aggregation takes place at a rate of ~192.5 fibrinogen bonds/micron²-sec, with ~50,000 GPIIb/IIIa fibrinogen cross-bridging receptors per platelet plasma membrane [4110] and ~46,000 plasminogen binding sites per platelet (the platelet surface serves as a site of assembly for plasminogen and tissue plasminogen activator and facilitates plasminogen activation) [4111].

Could the presence of medical nanorobots in human blood inadvertently – or purposely – interfere with this process? The force required to separate two platelets that have adhered via platelet membrane receptors cross-linked (i.e., covalent bonds) by fibrinogen was tested experimentally with receptor coated latex microspheres [4112]. Doublets were subjected to hydrodynamic shear stress of 0.6-2.9 N/m² and doublet breakup occurred as follows: 15.6-17.0% broke up within the force range (70-150 pN) to (230-310 pN), and all of these breakups occurred within the first 10 rotations of the doublet. The rest (~83%) of the doublets did not de-aggregate, up to ~310 pN. Another experiment [3988] found that the adhesion between a neutrophil and an activated platelet is broken in 130-630 millisec as the shear rate is decreased to 100 sec^-1 (F_bond = 86 pN) from 250 sec^-1 (F_bond = 172 pN).

These results suggest that forces of many hundreds of pN must be applied in order to mechanically separate a covalently attached platelet-platelet pair. Such forces are within the abilities of an individual nanorobot designed for the purpose but are unlikely to occur casually during in sanguo operations of the typical manipulator-equipped nanorobot. Of course, nanoaggregates should have no difficulty applying the requisite forces if that is their intended function: the elastic moduli of human platelets measured with AFM range from 1-50 kPa in the frequency range of 1-50 Hz [4113].

Last updated on 30 April 2004