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

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

9.4.5.6 Breach Sealing and Intrusiveness

Some cytopenetration techniques involve tearing the lipid bilayer. This breach must be resealed after transit. A fractured lipid membrane presents two "greasy" surfaces to water; it is thermodynamically favorable for these two cut surfaces to fuse together so as to eliminate the unfavorable water-fat interface.^27,2316 Such fusion may be encouraged mechanically by the passing nanorobot (Section 9.4.4.3), although the edges of the tear will attempt to round off and self-seal, without rejoining.

Nevertheless, it is not uncommon to observe rapid natural resealing of plasma membranes with little loss of intracellular contents, a useful property given the many transient plasma membrane disruptions that commonly occur in cells that experience significant mechanical stress (Section 15.5.7.2), such as gut, skin, endothelium, and muscle.^1534,3665 In one experiment, tissue cell plasma membranes were punctured using 2-3 micron diameter micropipettes and a 300 millisec transit (wounding) time, and the torn plasma membrane spontaneously resealed in 10-30 sec with relatively little visible loss of injected dye.¹⁵³⁴ Exocytosis-based resealing^3666-3668 of a microneedle puncture through the fibroblast plasma membrane occurs in 5-10 sec,³⁶⁶⁷ but a second puncture at the same site heals faster than the initial wound³⁶⁶⁸ -- at first wounding, the cell uses existing endocytotic compartment to add membrane necessary for resealing, but Ca⁺⁺ entry at the first wound stimulates vesicle formation from the Golgi apparatus, resulting in more rapid resealing of the second membrane disruption.³⁶⁶⁸

Watertight breach sealing might even be possible for nanorobots trailing narrow, untensioned tethers with lipophilic coatings, although such tethers may cause other problems (Sections 6.4.3.6 and 7.3.3); 0.1-micron optical fiber tips have been poked through a cellular plasma membrane to measure the pH of the cytoplasm inside, in single cells and in single rat embryos, without ill effect on these large cells.⁵⁷⁷ In small cells (2-15 micron diameter), "stab" microinjection at high pressure (0.1-0.2 atm) is problematic because the nucleus-to-cytoplasm ratio is higher so the nucleus is more likely to be damaged during the stab. In one experiment, less than 5% of neutrophils survived the high-pressure stab intact, but a low-pressure (~0.01 atm) injection through a lipid bridge produced a ~100% survival rate.²³⁴⁶

How many nanorobots (or other pieces of exogenous matter) can safely squeeze into a living cell? This question is addressed further in Section 15.6.3, but an extremely conservative volumetric injection limit is ~50-100 micron³ per tissue cell (0.5-1% of typical cell volume) without any significant observed effect on cell viability¹¹⁹² -- equivalent to ~3-100 bloodborne nanorobots, depending upon object size. Individual lymphocytes (~200 micron³)⁷⁷⁷ have also been observed circulating for hours inside large living cells, with no evident ill effect (Section 8.5.3.12).

In some nanorobotic system configurations and applications, it may be useful to deploy smaller intracellular probes tethered to larger extracellular command centers. For example, axonal "cleanout" could be an important anti-aging activity, given the evidence that axonal transport slows with age presumably due to debris accumulation or a decline in energy supplies; yet full-size nanorobots might not conveniently fit inside some neural axons. Transmembrane tethers (tipped with relatively small unfurlable mechanisms) may be passed into the cell's interior by threading them through an anchored transmembrane sleeve device. Fluid leakage between tether and inner sleeve wall, either into or out of the cytosol, can be minimized if both surfaces are hydrophobic. There are, of course, many important drawbacks to the use of tethered systems generally (Sections 6.4.3.6 and 7.3.3).

Last updated on 16 April 2004