Nanomedicine, Volume IIA: Biocompatibility
© 2003 Robert A. Freitas Jr. All Rights Reserved.
Robert A. Freitas Jr., Nanomedicine, Volume IIA: Biocompatibility, Landes Bioscience, Georgetown, TX, 2003
15.3.7 Biocompatibility of Nanorobot Effluents and Leachates
The biocompatibility of both purposeful and accidental effluents that might be released by medical nanorobots must also be examined. By and large, such effluents should have relatively low molecular weight (although chemical byproducts of energy generation or proteins broken down by nanomotors might be larger). For example, glucose engine (Section 220.127.116.11) effluents such as CO2 and H2O present few problems, and most low-molecular-weight chemicals (including many 20th century drugs and antibiotics) must be coupled with other substances such as proteins or conjugates before they can be recognized by the immune system . Such chemicals are called haptens . (Of course, entirely aside from their immunoreactivity, these low-molecular-weight chemicals could be directly toxic, especially if not efficiently cleared by the liver, and it has been proposed that small molecules originating from microbes might underlie nonspecific pyoinflammatory diseases .)
There is a distinction often made between antigens and immunogens that may be useful to emphasize here . An antigen is an agent that can bind specifically to components of the immune response, whereas an immunogen is an agent that can induce an immune response. Thus all immunogens are antigens, but not all antigens are immunogens. In general, compounds with molecular weight less than 1000-2000 daltons (e.g., penicillin, progesterone, aspirin, carbon dioxide, or kerosene ) are not immunogenic [1760, 2332]. Compounds with molecular weight between 2000-6000 daltons may or may not be immunogenic, but compounds over 6000 daltons generally are immunogenic .
A compound also needs a certain minimum chemical complexity to be immunogenic. For instance, amino acid homopolymers (e.g., a 30,000 dalton pure lysine polymer) are rarely good immunogens, and a 50,000 dalton homopolymer of poly-gamma-D-glutamic acid (the capsular material of Bacillus anthracis ) is not immunogenic at all. Large copolymers of several different amino acids tend to be highly immunogenic , albeit due to T cell processing and not size per se (a substance cannot be an antigen if there is no T cell epitope). Lipids and nucleic acids are poor immunogens [1760, 2332] (though antibodies have been raised to them), but become immunogenic when conjugated to protein carriers [1760, 2332]. Many carbohydrates and virtually all proteins are immunogenic [1760, 2332]. Most polysaccharides, fibrilar proteins (e.g., silk fibroin), and single-stranded nucleic acid polymers have sequence-specific antigenic determinants or “epitopes.” On the other hand, native double-stranded nucleic acids and most globular proteins have conformation-dependent epitopes  – antibodies can recognize primary, secondary, tertiary or even quarternary protein structures . A molecule that is “foreign” will also be immunogenic. For example, the release into the bloodstream of animal-derived synthetic proteins that have not been properly humanized [2276-2278, 5593] might induce a strong immune response. Most protein toxins are strongly immunogenic , while small chemical toxins are not.
Besides size and complexity, one final requirement for immunogenicity of possible nanorobot effluent molecules is degradability. In order for most antigens to stimulate T-cell-mediated immune responses, interactions must occur between antigen-presenting cells (APC) and helper T cells . (Most effective B cell responses are dependent on T cell help, but B cells per se do not need degradation to recognize and respond to antigens.) APCs must enzymatically degrade a protein antigen into fragments that can be bound to MHC proteins and then be presented at the APC surface to T cell receptors, activating the response. Thus proteins composed entirely of D-amino acids, which are resistant to enzymatic degradation, have low immunogenicity [2337, 2338], whereas peptides composed of L-isomers can be broken down and have normal immunogenicity – though counterexamples exist . (Bacteria employ D-isomer amino acids in their coats  for this very reason.) Carbohydrates are not processed or presented and thus are unable to activate T cells, although they can sometimes directly activate B cells . Biological sugars are typically monoisomeric, so polysaccharide molecules comprised of isomerically unusual sugar monomers that are unrecognizable to natural enzymatic degradation processes  (e.g., L-glucose or D-tagatose ) might be relatively nonimmunogenic and cytotolerant , though as yet there appear to be no experimental tests of this possibility. As noted earlier, pure diamond is expected to be nonimmunogenic, but fullerene and sapphire surfaces might be immunogenic in some circumstances (Section 18.104.22.168) and other structures such as exposed sorting rotor binding pockets or detached protein-based presentation semaphores might also be immunogenic – more research is needed to reach definitive answers. (Even if nanoparticulate forms of these substances are nonimmunogenic, there is a small possibility that they could act as inert irritants capable of activating nonspecific inflammatory responses (Section 15.2.4); the biocompatibility of larger nanorobot fragments is briefly discussed in Section 15.4.4.)
Nanorobot effluents might also collect as gas bubbles or solute crystals adhering to the nanodevice exterior, which material could then be recognized by the immune or inflammatory systems. This difficulty should largely be preventable by good design. In one experiment by Ward et al , eliminating trapped air microbubbles from materials having low surface tension significantly reduced complement activation by these materials, in rabbits. A related issue is that most surfaces exposed to ambient air acquire an adsorbed layer of hydrocarbons and other small molecules that is at least a few angstroms thick, and larger particles may also be present in the form of adherent dust or other debris . The inflammatory potential of these adherent materials should also be investigated.
In physiological environments, leaching of undesired moieties from intact nanorobots or chemically pure nanorobotic materials is unlikely with the possible exception of aluminum ions from sapphire (Section 22.214.171.124). There are no reports of such leaching even from comparatively impure contemporary diamond-like carbon (DLC) or CVD diamond surfaces (Section 126.96.36.199), although some elution of biomolecules (e.g., heparin) from organic coatings on diamond, graphene (e.g., fullerene), or fluorocarbon surfaces might be expected in some circumstances [5782, 5783]. Antibody-targeted chelated-radioisotope therapeutic agents can be chemically unstable under physiological conditions and can allow some radioisotope atoms to leach out into unintended tissues, but radioatoms trapped endohedrally inside fullerenes such as C60 (Section 188.8.131.52) cannot leach out and thus are inherently safer. No leaching has been observed even from dye-impregnated ceramic coatings on glassy-carbon electrodes , though silicon additives often found in pyrolytic carbon (Section 184.108.40.206) might possibly increase susceptibility to leaching of some components of those additives and ion leaching from graphite has been reported in specialized industrial applications . Traditional fluorocarbon applications in medicine often relate not to implantation but to inertness and purity – e.g., Teflon tubing  delivers biosolutions without altering them significantly by leaching organics or by chemically reacting with the solutions, Teflon surfaces support cell cultures without emitting toxic leachates [1190, 1357, 5782], Teflon coatings prevent toxic leaching from underlying materials , and Teflon is often used as a negative control in cytotoxicity studies of leachates . Teflon composites containing non-fluorocarbon components can produce (often nontoxic ) leachates.
Last updated on 30 April 2004