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
22.214.171.124 Flexible Fabric Model
Two properties of metamorphic surfaces are most useful: flexibility and extensibility. The graphene sheet (of which fullerene tubes and ellipsoids are made; Section 2.3.2) is probably the ideal example of a flexible but nonextensible isoareal nanofabric that could be used to wrap a nanorobot exterior with any continuous shape having turning radii as small as 1.1 nm. The graphene sheet is a one-atom thick purely carbon surface (no hydrogen passivation yet chemically inert) arranged in a mostly hexagonal array of atoms with an occasional pentagon or heptagon for curvature (Section 126.96.36.199). This sheet has the highest tensile strength of any known 2-dimensional network, about 50 times the strength of high-carbon steel and 1.5 times the strength of crystalline diamond (Table 9.3). It has a higher packing density (atoms/nm2) than any other network made of any other atoms in the periodic table -- even a 2-D slice of diamond. The graphene sheet is effectively impermeable under normal chemical conditions -- naked carbon atoms or small carbon cluster radicals will not readily bond with it, and even helium atoms up to 5 eV (~40,000 K) just bounce off. The material is probably extremely bioinactive (Section 15.3.2). Graphene is also an excellent conductor of heat (as good as diamond) and electricity (better than copper if appropriately doped) along the plane of the sheet, but is a poor cross-plane conductor.
The ultimate flexible and extensible volume in common experience is the rubber balloon, which readily distends its volume by several orders of magnitude with an areal extensibility of ~15,000%. Rubber is a linear polymer of isoprene (C5H8)n with individual molecules of n = 1000-5000 heavily cross-linked during vulcanization with 12% sulfur, somewhat resembling a network of coiled springs that is easily stretched. Although isoprene surfaces are porous and bioactive, hence inappropriate for nanorobot exteriors, Drexler10 has proposed an analogous coiled pleat configuration (Fig. 5.15) that could probably be implemented using graphene sheets embedded with reciprocal nonzero-curvature elements to provide the needed countertension. The triple-pleat configuration shown has areal extensibility earea ~ 8.00(800%). As with the Accordion Model, a drawback of this design is a propensity for surface fouling in vivo due to the presence of numerous trapping pockets.
Other more dynamic configurations of flexible fabrics may be imagined. Graphene sheets may be unfurled from interior storage, like a spring-roller window shade, a rolled-up sleeping bag, or a serpentin or pito (coiled "party blowouts"). For example, R.C. Merkle2281 notes that the minimum gas pressure differential needed to uncoil and inflate a collapsed, tightly-coiled graphene tube of radius R is given by:
where Ev is the energy of two graphite sheets held together by the van der Waals forces between them. Merkle estimates Ev ~ 0.25 J/m2 from computer simulations, in line with Kelly's experimental value2280 of Ev = 0.234 J/m2. Thus a coiled tube with R = 100 nm requires at least pinflate >~ 50 atm to inflate; larger diameter tubes require less pressure differential to uncoil.
Fabrics may be comprised of tightly-woven 1.1-nm fullerene tubes with the length of each such "thread" individually controlled by winding or unwinding from numerous internal spools. In biology, various species of worms achieve linear extensibilities of up to 800% by changing the relative pitch angles of helically wound inextensible collagen fibers.529 A dynamically reconfigurable support trusswork sheathed in flexible surface materials also permits ready shape-changing; the octet truss geometry is one of the strongest known [P. Salsbury, personal communication, 1997].
Last updated on 16 April 2004