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
18.104.22.168 Rotations and Collisions in sanguo
From Eqn. 9.62 describing the axial fluid velocity vPois as a function of radial distance r in laminar tube flow, the velocity gradient, or shear rate 'g, is seen to increase linearly with r, as:
When a suspending liquid is subjected to laminar flow, the fluid stresses on the surface of suspended rigid bodies cause those bodies to rotate as they travel down the tube.1319 Rigid free-floating spherical nanorobots will rotate with uniform angular velocity, or, more specifically, with a rotational frequency of:
For example, a rigid free-floating spherical nanorobot traveling in an artery with rtube = 500 micron and vflow = 100 mm/sec (Table 8.2) rotates at nnano ~ 63 Hz at r = 495 microns (very near the vessel wall), which is much faster than the random ~0.1 Hz Brownian rotation typical for micron-scale nanorobots (Section 3.2.1). Non-spherical rigid particles1343 and red cells1344 display variable angular velocity but spend more time in each orbit aligned with the direction of flow. Platelets can be seen tumbling in arterioles, especially near the vessel wall,1340 but they tend to align with the flow, a tendency which is strongest nearest the wall.1341
Gyroscopic effects are present in every rotating body, with pitch, roll, yaw, and turning producing torques which cause precessions, giving periodic increases in bearing pressures and stresses. Fortunately, these stresses remain modest in most nanomechanical designs. For example, consider a rapidly spinning component inside the tumbling spherical nanorobot described in the previous paragraph. This internal component is a diamondoid disk of radius rdisk, thickness hdisk, and density rdisk, spinning at an angular velocity wdisk around an axis oriented in the plane of the nanorobot tumble (tumbling is at frequency nnano), and supported by two coaxial bearings located a distance xbearing apart. The gyroscopic reaction force on the bearings is given by:
Taking rdisk = 100 nm, hdisk = 20 nm, rdisk = 3510 kg/m3, wdisk ~ wmax ~ 1010 rad/sec (Eqn. 4.17), nnano = 63 Hz, and xbearing ~ hdisk, then Fbearing ~ 2 pN.
In general, if two micron-scale objects are translating at the same mean speed, more energy is dissipated by the one that produces the larger amount of vorticity -- that is, an axisymmetric object that rotates as it swims is less efficient than the nonrotating swimmer.1389 The tangential force required to neutralize the hydrodynamic torque acting on a rigid sphere of radius Rnano and tumble frequency nnano is torque divided by radius, or:1396
For Rnano = 1 micron, nnano = 63 Hz, and h = 1.1 x 10-3 kg/m-sec for plasma at 310 K, then Ftumble = 11 pN. Application of such small forces may be controlled using measurements of absolute orientation taken from an onboard nanogyroscope (Section 22.214.171.124). Given that Fbearing and Ftumble are of similar magnitude, direct anti-tumble gyrostabilization and gyroscopic rotational locomotion (gyrorotation) may be possible in some nanorobot applications.
The velocity gradient often brings suspended particles into close proximity. Kinetic theories of flowing suspensions have been developed from experiments on model systems involving two-body collisions between rigid and deformable spheres1345 and between rigid cylinders.1343 Goldsmith and Mason1312 estimate that the per-object collision frequency K for equal-sized rigid spheres of volume concentration c flowing at velocity v at a radial distance r from the center of a tube of radius rtube is:
However, no complete theory yet exists to describe all the interactions of multiple classes of rigid bodies and deformable cells in blood vessels.
As a crude approximation, consider the laminar flow of a fluid suspension of two types of spherical objects (i = 1, 2) of radius Ri, density ri (kg/m3), number density ni (m-3), and volumetric concentration ci = (4 p / 3) ni Ri3 expressed as a fraction. Each object occupies a cubic fluid volume of dimension Li ~ ni-1/3 = (4 p Ri /3 ci)1/3, and in laminar flow, two radially-adjacent boxes move past each other (a "collision") at a mean relative velocity of vij ~ 'g (Ri + Rj) when Ri, Rj << rtube, and interact for a time tij ~ Li / vij (j = 1, 2). From the viewpoint of object i, the probability that after Ncoll ij collisions at least one object j has been encountered is pj = 1 (1 cj)Ncoll ij. Taking pj ~ 0.9 (the final result is relatively insensitive to the exact threshold selected), and multiplying by 2 because there is a concentric layer on either side of object i, then the mean collision rate experienced by each object i with an object j also present in the suspension is Kij ~ 2 (Ncoll ij tij)-1, which, at a radial distance r in the tube, may be written as:
and the mean free path (Iij) between an object i and an object j in the suspension (neglecting all velocity profile and margination effects) is given approximately by:
For example, in a suspension consisting of Ri = 1 micron nanorobots with volume concentration ci = 0.10 (Nct = 10%), and Rj ~ 1.5 micron platelets with cj = 0.0035 (mean physiological plateletocrit), flowing through a small artery with rtube = 500 microns at mean velocity vflow = 100 mm/sec, and at a radial distance r = 495 microns (near the tube wall), then for nanorobot/platelet interactions, Kij ~ 2 collisions/sec, mean collision velocity vij ~ 2 mm/sec, and Iij = 9.7 microns. For nanorobot/nanorobot interactions, Kij ~ 40 collisions/sec (cf. K = 200 collisions/sec using Eqn. 9.70), vij ~ 1.6 mm/sec, and Iij = 3.5 microns. These figures are crude estimates at best, since they ignore many possible complicating factors such as aggregation and margination, collision inelasticities and n-body collisions (n > 2), nonspherical object shapes, the presence or absence of specific macromolecules, receptor interactions, pulsatile flow, ionic strength of the medium, and cell surface electrostatics.1325,3545
Free-floating nanorobots that collide with blood vessel walls produce negligible shear forces, given the no-slip condition at the wall. Powered nanorobots of radius Rnano that impact a vessel wall at velocity vnano may apply a maximum shear stress of pshear ~ rnano vnano2; taking rnano ~ 1000 kg/m3 and vnano ~ 1 cm/sec, then pshear ~ 0.1 N/m2. By comparison, the time-averaged shear stress for blood circulation in normal vessels362,1346,1347,1352 is 1-2 N/m2 (range 0.5-5.6 N/m2)386, reaching up to 10-40 N/m2 when small arteries and arterioles are partially occluded as by atherosclerosis or vascular spasm.1348,1349 This may also be compared to the threshold limit for shear stress-induced platelet aggregation of 6-9 N/m2,1346,1349-1351 the shearing stress of 5-100 N/m2 acting at the interface between a leukocyte and an endothelium when the leukocyte is adhering to or rolling on the endothelium of a venule,366 and the critical shear stress of 42 N/m2 known to initiate major changes in the endothelial cells in the arteries.365
Last updated on 21 February 2003