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
10.4.2.3 Gross Cellular Disruption
A number of crude cytocidal techniques have been proposed which for the most part are inadvisable. For instance, simple lancing or "harpooning" of cells (e.g., Feynman's "stab the paramecium"355) would likely be followed by self-sealing or a messy necrosis, depending on the duration and severity of the penetration.* Acoustic shock waves would cause random mechanical damage, ending in necrosis. Bulk pressurization (see Section 10.3.3) can be lethal to biology -- high pressure treatment to kill bacteria was first described in 1895 by Royer,3102 and by Hite and colleagues3105 in 1899 in connection with the high pressure preservation of milk. Cells are generally inactivated between 2000-5000 atm, bacteria and viruses above 5000 atm, antibodies and enzymes over 10,000-20,000 atm (Table 10.3). Bacterial spores are killed at ~6000 atm and 45°C - 60°C.3103,3104 A pressure-initiated apoptotic response might be difficult to control because the apoptosis-inducing and necrosis-inducing pressure thresholds are not widely separated and may vary among cell types, physiological states, external conditions, and even among cells of apparently similar characteristics. Bulk pressurization followed by release would almost certainly permanently inactivate viruses (Table 10.3), but could lead to inflammation if large populations of deactivated viruses are set adrift in the tissues.
* Self-sealing of ruptured neurons is inhibited by administering cysteine protease inhibitors or calmodulin inhibitors.3664
Some fragmentation of DNA molecules in aqueous solution by ~1 MHz ultrasound irradiation has been reported at intensities as low as ~2000 watts/m2 for >15-min exposures, but 10-min exposures to 50,000 watts/m2 completely fragment the DNA, due to shearing forces acting on the large molecules rather than cavitation (which requires higher intensities).730 Plant cell chromosomal damage occurs at 104-105 watts/m2, and red blood cell suspensions suffer increased membrane permeability but no membrane disruption up to 30,000 watts/m2, although platelets in aqueous suspension may be disrupted by >2000 watts/m2 at 1 MHz for >5 min.730
Local hyperthermia can selectively destroy cancer cells in vivo,505 presumably with an apoptotic outcome. Temperatures of 42.5°C or higher for 20-30 minutes appear necessary for tumoricidal effects, and several sessions are needed for significant tumor regression. Focused 1 MHz ultrasound beams produce brain lesions, with in situ spatial-peak intensities at 2 megawatts/m2 for 10 sec causing purely thermal damage and higher intensities up to 0.2 gigawatts/m2 for 300 microsec adding a contribution from direct mechanical effect, possibly cavitation.505 Large numbers of nanorobots in a small tissue volume could produce significant localized or cell-wide heating for extended durations that might be difficult or impossible for individual nanorobots to achieve.
Very high intensity laser light (>1011 watts/m2) can necrotically "optocute" biological cells;1630,1631 bioparticle optical trapping pioneer A. Ashkin notes that during his experiments, "if the power got too high, the bacteria would just explode."2145 Threshold laser ablation rates are ~1 nJ/micron2 for a 248-nm KrF excimer laser photoablating organic material2146 and 0.5 nJ/micron2 for ablative etching of corneal tissue at 193 nm (probably as a result of photodecomposition of the peptide bonds).645 At 193 nm, a single 20-nanosec, 20 nJ/micron2 (~1012 watt/m2) pulse ablates 2.4 microns of bile duct tissue;645 hard biomaterials like tooth dentine and dental enamel ablate 0.5-1.9 microns at 248 nm and a fluence of ~1013 watts/m2. Microlasers are an established technology,497 but such methods produce necrosis and require far too much power to be practical for in vivo cytocide.
However, ultraviolet UVC band photons (190-290 nm) induce cell destruction645 much more selectively and energy-efficiently. For example, nucleic acids have an optical absorption maximum near ~260 nm508,997 that is characteristic for each base;997 the absorption of DNA itself is ~40% less than would be displayed by a mixture of free nucleotides of the same composition, known as the hypochromic effect.997 The primary mode of UV damage is the formation of thymine dimers in the DNA, which block both transcription and replication until repaired. In sufficient numbers, these defects probably activate the DNA damage response (repair or apoptotic) pathways. At higher-energy shorter wavelengths (e.g., 193 nm), the UV photons don't penetrate the cell far enough to reach the nucleus.645 Lower-energy longer-wavelength UV photons can still damage DNA by producing oxidizing chromophores and decreasing enzyme synthesis, which has the effect of reducing the repair and regrowth properties of DNA;645 protein containing the aromatic (chromophoric) amino acids tryptophan and tyrosine have maximum absorption near 275 nm508,996 and phenylalanine more weakly near 260 nm,996 and peptide bonds absorb strongly at wavelengths under 240 nm.508 Thus photons of a precise wavelength can cause very selective damage. Suspensions of Streptococcus, Lactobacilli and Actinomyces bacteria stained with toluidene blue experienced millionfold kill rates after 60-sec exposure to helium-neon laser photons at an intensity of only 5600 watts/m2.2147 Viral genetic material is also strongly susceptible to UV damage.328
Particulate radiation is another poor choice for inflicting selective damage to individual cells in a controlled manner. It is true that Co60 irradiation is used to sterilize sutures,359 and antitumor radiation treatments have been commonplace in 20th century medicine. However, the range of natural-emission a-rays in protoplasm is >20 microns, raising the likelihood of some collateral damage, and an accelerator capable of generating artificial lower-energy alpha beams would be energetically and geometrically prohibitive in most in vivo nanomedical contexts.
Last updated on 24 February 2003