© 1999 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999

9.4.3.6 Tank-Tread Rolling

When an injured cell "calls for help" from leukocytes, the cell secretes cytokines such as IL-1 and TNF, causing nearby vascular endothelial cells to express P-selectin and E-selectin on their luminal surfaces.⁴¹⁵ Passing white cells adhere to these protruding molecules because their carbohydrate coat contains complementary structures. When a leukocyte touches a venule wall, its rate of movement slows because of the stickiness between the cell and the wall, but the force of the circulating blood keeps the cell moving with a tank-tread motion in what is called a rolling interaction (Fig. 9.28). The average white cell rolling velocity is only <4% of the mean blood flow velocity in the venule,³⁶¹ typically ~10-40 microns/sec.^{1027,1484,1507}

This form of locomotion is easily implemented by rigid medical nanorobots. A primitive example would be a spherical device with numerous flexible knobs projecting radially from its surface, located a distance Dy apart. A mechanical gripper or binding pad located at the tip of each knob is switched on or off, depending on the direction the nanorobot desires to go. Anchored with one knob, Brownian or hydrodynamic forces cause the device to bounce around until one of the active knobs contacts the surface, whereupon that knob binds, and the previous anchor knob releases. The device has now progressed a distance Dy in the desired direction. A micron-size nanorobot could achieve v_nano ~ 1 micron/sec in a quiet environment or up to v_nano ~ 1 mm/sec in blood vessels, at a near-negligible power cost (<< 1 pW) because only footpads must be operated. In the limit of a flexible knob-dense surface, a nanorobot with ~1 micron² of continuous contact surface could, in theory, tow up to 3,000,000 pN of load before reaching the ~3 x 10⁶ N/m² membranolytic limit.¹⁴²²

Last updated on 21 February 2003