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

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

5.3.6 Presentation Semaphores

It will often be necessary to modify nanorobot surface biochemical characteristics, as for example to present self-antigen to ensure biocompatibility (Sections 15.2.3.3-15.2.3.6), to present targeted non-self-antigens to facilitate cytocarriage (Section 9.4.7) or elimination from the body (Chapter 16), or to create a traveling solvation wave (e.g., hydrophilic, lipophilic) on the nanorobot exterior surface to facilitate cytopenetration (Section 9.4.5.3). At concentrations of 1-10 nanomolar, there are 10⁴-10⁵ MHC Class I plasma membrane molecules per typical tissue cell, or ~10 MHC proteins/micron² at the cell surface (Section 8.5.2.1).*

* As few as 210-340 MHC Class II/peptide complexes per antigen-presenting cell (~0.1 molecules/micron²) are needed to stimulate T cell interleukin-2 production³⁴⁵⁶ and a minimum of ~0.2 molecules/micron² of agonist MHC/peptide complex can trigger proliferation and immunological synapse formation,³⁴⁵³ although a threshold density of >~60 molecules/micron² of accumulated MHC-peptide complexes are required for full T cell activation.³⁴⁵³

The MHC Class I molecule (Section 8.5.2.1) consists of a ~45,000 dalton glycosylated polypeptide chain crudely shaped like a "hand" which is noncovalently grasping a 12,000 dalton nonglycosylated peptide microglobulin (Fig. 8.33). Eliminating anchor and attachment components which do not participate in recognition leaves an active antigen "semaphore" component measuring roughly 3 nm x 6 nm x 8 nm that must protrude from the cell surface.

To present this semaphore to the external environment at a nanorobot surface, a device roughly analogous to the manipulator arm described in Section 9.3.1.4 may be used. The manipulator arm includes a 7-nm diameter internal tool transport channel which can allow presentation and exchange of MHC-like molecules mounted on diamondoid jigs to the ~700 nm² device tip, where they may be locked into position for temporary display. Total presentation mechanism volume is ~10⁵ nm³, plus ~200 nm³ per antigen stored on board. Staggered arrays of presentation ligands mounted on pistons can create nanorobot surfaces with controlled spatially structured chemical characteristics (Fig. 5.18).

An alternative method involves a two-roller mill-type device using a continuous conveyor belt 80 nm in length with 10 antigen-binding jigs mounted on the belt spaced ~5 nm apart. An opening at one end allows exposure of one antigen at a time to the external environment. Each of the 10 antigens, all of which may be different, are rotated into place as required, under computer control. As with the previous system, existing bound antigens may be exchanged internally using mill-like mechanisms. The core of the device measures 10 nm x 30 nm x 50 nm ~ 15,000 nm³, or ~50,000 nm³ including drive and control mechanisms and housing, with a ~300 nm² presentation face. Assuming a ~10⁵ watt/m³ power requirement (Section 6.5.6), power dissipation is ~10^-17 watts per semaphore or ~0.03 pW/micron².

Either semaphore device has an external presentation surface area <1000 nm². Providing the requisite ~10 antigens/micron² requires at most ~1% of nanorobot surface committed to semaphores, or ~0.1% of the volume of a 1 micron³ nanorobot for all semaphore devices, excluding antigen storage.

Last updated on 16 April 2004