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

 

5.3.3 Metamorphic Power and Control

The power required to control a given section of metamorphic surface may be provided by two simple counterbalancing mechanisms. First, the space between an interior hard shell and the distensible outer surface is pressurized using a working fluid such as gas or water. Second, the pressurization force is resisted by a gridwork of independently controllable nanoscale cables each attached to specific sets of metamorphic surface units (e.g., Fig. 5.12). Applying tension to a cable causes the units to which it is attached to selectively retract; releasing tension allows those same units to distend. This system is crudely analogous to the system of circular and longitudinal muscle fibers found in the intestines, and to the hydrostatic skeleton found in small marine organisms such as earthworms, nematodes, and to a lesser extent in echinoderms, some molluscs, caterpillars and spiders.364 A counterbalanced surface reverts to its compact shape in the event of a physical breach with escape of working fluid, an important fail-safe design element.

Fully inflating one of the 3-micron long metamorphic fingers described in Section 5.3.1.3 requires filling its 0.015 micron3 interior volume with ~700,000 molecules of N2 gas at a 2-atm working pressure, accomplished in 70 millisec within a 0.1 pW power budget using a bank of 10 sorting rotors (Section 3.4.2). This gas may be stored onboard in a (41 nm)3 (~0.00007 micron3) pressure vessel at 1000 atm (Table 10.2). If energy efficiency is the primary concern, it should be possible to recover most of the compressive sorting power using a generator subsystem to charge an energy storage buffer as the gas passes from the high to the low pressure regime. If distension speed is the primary design goal, gas may be rapidly vented from the pressure vessel into the finger volume in milliseconds, bypassing the generator subsystem.

Of course, such rapid venting causes the working gas to cool. Ignoring the normally relatively small amount of work done on the external medium, the temperature change during free expansion of a van der Waals gas (Section 10.3.2) is simply:

{Eqn. 5.7}

where AvdW = van der Waals gas constant for intermolecular attraction, mgas is the moles of gas present, CVgas is volumetric heat capacity of the gas, and Vinit and Vfinal are the starting and ending gas volumes, respectively.1031 Taking AvdW = 1.390 x 10-6 m6-atm/mole2 (Table 10.1) and CVgas = 20.8 J/mole-K for N2 gas, and from the above example mgas = 1.2 x 10-18 moles (~700,000 molecules N2), Vinit = 6.9 x 10-23 m3, and Vfinal = 1.5 x 10-20 m3, then DTexpand = -120 K. However, this temperature change is rapidly conducted throughout a micron-scale diamondoid nanorobot structure in ~10-9 sec (Section 4.6.1), since diamond is ~3200 times more thermally conductive than water. The maximum instantaneous temperature decline in a Vnanorobot ~ 1 micron3 block of diamond with CVdiamond = 1.82 x 106 joules/m3-K is only DTcool = DTexpand CV mgas / Vnanorobot CVdiamond = 0.0016 K, below biologically detectable limits (Section 4.6.1).

Controlled power transmission may also be achieved by dividing the extensible surface into compartments, then pressurizing or decompressing adjacent compartments to produce differential movement, or triggering contractile gels in various compartments; by manipulating tension to tendon bundles affixed to the end of a lengthy protuberance to control tip position (Section 9.3.1.2); by mechanically rotating an internal ribcage consisting of noncoplanar ovate sections; by acoustically triggering a progression of locks and ratchets; or by using telescoping screw drives (Section 9.3.1.4) or electrostatic drives (Section 6.3.5) to extend or retract specific surface segments. In biology, butterflies exemplify the first of these methods. Upon emerging from its pupal skin, the insect pulls its abdominal segments inward to raise its blood pressure, which inflates the veins of the wings and expands their membranes.2022

The dexterity of a controlled protuberance will depend upon the fineness of the control mechanism, the stiffness of the metamorphic configuration, the degree and speed of extension, the applied tip load, special jointing and numerous other factors; ~0.1-1 nm positioning accuracy and application of nanonewton forces at the tip should be feasible. Pressure-driven ratchets may transduce acoustic power/control pulses transmitted internally to sensors or end effectors located at the tip.

 

Last updated on 18 February 2003