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

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

3.4.3 Internal Transport Streams

After pump mechanisms described above have reliably sorted externally-encountered target molecules into reservoirs filled with species of a single type, a medical nanodevice may require these molecules to be transported to specific internal locations for further processing. Bulk fluid flow or fluidized (solvated or suspended) transport through nanopipes may suffice for some purposes (Section 9.2.5). However, in many cases it will be necessary to present reagent molecules to other subsystems as a well-ordered stream of precisely positioned moieties transported in vacuo, especially for mechanochemical operations (Chapter 19).

For this purpose, Drexler¹⁰ proposes molecular mills -- eutactic systems of nanoscale belts moving over rollers, with reagent-binding devices mounted on the belt surface (Fig. 3.9). This class of device can be assembled into complex molecular transportation networks using conditional switching, crossed-axis belting, and transit speed/frequency multipliers,¹⁰ and may also be employed to drive mechanosynthetic chemical reactions. The benchmark mechanism uses 10-nm diameter rollers to carry closely packed reagent devices measuring 4 nm x 4 nm x 2 nm, or 32 nm³. A 20-roller mill mechanism 1 micron long has a ~2 micron long belt with 500 reagent devices and delivers 10⁶ molecules/sec at a belt speed of 4 mm/sec. Total power dissipation is ~1.4 x 10^-18 watts, a rate of ~0.001 zJ per moiety (or per reagent device) delivered or ~10^-6 zJ/nm traveled per reagent device. Total mill mechanism mass is ~6 x 10^-20 kg.

An alternative to roller/belt mill mechanisms is a nonconnected stream of pallets pushed along tracks, also in vacuo. Such tracks may include merging junctions, distribution junctions, multiplane crossings and switching stations, as well as straight and curved sections. Assuming each pallet is a 32 nm³ reagent device held to the track by pins in grooves resembling cam followers, energy dissipation by phonon scattering¹⁰ is given approximately by:

{Eqn. 3.19}

where e_p = 2 x 10⁸ joules/m³ (phonon energy density), s_therm (a thermally-weighted scattering cross section) ~10^-20 m² for reagent devices of mass m = 10^-22 kg assuming a sliding contact of stiffness ~30 N/m in a moderately stiff medium, v = 4 mm/sec sliding speed, and v_sound = 10⁴ m/sec (~speed of sound in diamond), giving P_drag ~4 x 10^-21 watts per reagent device, or P_drag / v ~ 10^-6 zJ/nm traveled per reagent device (pallet). Note that volume containerization of pallet-transported molecules is least efficient at the smallest scales, where surface area per unit enclosed volume is highest, since energy usage is proportional to the surface area of the carrier. Containerization (Section 9.2.7.7) of n >> 1 molecules for large-pallet transport is more efficient.

A less energy-efficient, but far more versatile, internal molecular transport device is the 100-nm telescoping manipulator arm¹⁰ described in Section 9.3.1.4. This flexible ~10^-19 kg device employs a binding tip to pick and place small and large molecules alike, moving them at ~1 cm/sec with repeatable placement accuracy of 0.04 nm. Multiple devices can be used to establish an internal ciliary transport system (Section 9.3.4); standardized volume containerization of molecules permits rapid stereotypical handoff motions and efficient parcel routing. Conveyance through a 100-nm arc takes 10^-5 sec consuming 0.1 pW while the arm is in motion, or ~10 zJ/nm traveled per reagent molecule or per container transported (vs. ~1000 zJ per typical covalent bond).

In molecular cytobiology, vesicles and organelles are transported throughout the interior of a cell by riding on microtubular cables crisscrossing the cytosol (Section 8.5.3.11). For example, neural vesicles show transport speeds up to 2-4 microns/sec.⁹³⁸

Last updated on 7 February 2003