© 2003 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Nanomedicine, Volume IIA: Biocompatibility, Landes Bioscience, Georgetown, TX, 2003

15.5.6.1 Electrical Interactions with Cells

The gross effects of electrical interaction with cells are well known. For example, a macroscopic intravascular electrode with constant current intensity 1 mA induces thrombosis and injury of the vascular wall, ranging from minimal lesion of endothelium to almost total necrosis of the vascular wall [4146]. Schaldach [4147] reports that current densities in excess of 1-100 picoamp/micron² alter the thermodynamic equilibrium and cause “changes in pH, PO₂, irreversible reactions, and perhaps cell damage.” Obviously, nanorobots must avoid unintentional electrocution, electrocautery, or cytotoxic electroporation [4148] of the patient’s healthy cells and tissues. But electroporation may be used to temporarily permeabilize cell membranes to permit the insertion of foreign genes [4149], and lipid bilayer membrane demixing can be induced by applying tangential electric fields on the order of 4000 V/m [4150]. As noted in Section 9.4.7.4, electric fields can be used to drive leukocyte motion (Section 4.9.3.1). For example, a mild electric current induces lymphocytes to travel in the same direction as the current (“electrotaxis”) at speeds up to ~0.3 microns/sec [6170]. Small electric gradients have been shown experimentally [6171] to stimulate leukocyte diapedesis (Section 4.9.3.1), though multiple nanorobots would probably be required to generate the necessary currents.

Biological stimulation can alter the electrical characteristics of cells. For instance, the stimulation of thymocytes and B lymphocytes with specific mitogens causes the cells to increase in diameter from 5.6 to 8.8 microns, with membrane capacitance increasing from 7.6 to 12.4-14.6 mF/m² and from 9.3 to 16-17 mF/m², respectively [4151]. T cell membrane conductivity also increases from 50 to 210 S/m² [4151]. Various immune (and other) cell responses to externally-imposed oscillating electric/magnetic fields have been reported [4152-4157] although there are negative results as well [4158-4160]. In one experiment [4819], osteoblasts cultured on the surfaces of a polylactic acid-carbon nanotube composite and exposed to electric stimulation (10 µamps at 10 Hz) for 6 hours/day exhibited an upregulation of mRNA expression for collagen type-I after 1 day, a 46% increase in cell proliferation after 2 days, and a 307% increase in the concentration of extracellular calcium after 21 days. Electric fields can be therapeutic in some cases [4161, 4162]. Cellular galvanotaxis (electric field-induced cell migration) has been demonstrated in algae [4163], bacteria [4164], chondrocytes [4165], endothelial cells [4166], epidermal cells [4167], epithelial cells [4168, 4170], fibroblasts [4169-4171], granulocytes [4169, 4172], keratinocytes [4173], myoblasts [4174], neural crest cells [4175], neurons [4176], osteoblasts and osteoclasts [4177], protozoa [4178], and spermatozoa [4179], and has been modeled mathematically [4180]. Field-emitting nanorobotic systems might induce similar effects. Finally, a major hurdle in developing electronic implants is the design of devices that can withstand long-term exposure to the body’s warm, salty fluids without mechanical failure. The corrosion electrochemistries of potential nanorobot building materials are briefly discussed in Sections 15.3.1.5, 15.3.3.6, and 15.3.5.6.

Perhaps more subtle are the effects of cell-cell electrostatics which have been under investigation since the 1920s [4181]. As an example, the negative surface charge of red cells provides an electrostatic repulsive force tending to cause disaggregation [4092, 4182]. It has been proposed that the ~15 nm gap frequently observed between the surfaces of aggregated red cells in rouleaux [4183] represents the position of the potential energy minimum where the forces of electrostatic repulsion between negatively charged red cells and the van der Waals attractive forces are equal [1554]. Taking H = 30 zJ, r_red = 3 microns and z_sep = 15 nm in Eqn. 9.7, the net attractive force between red cells aggregated in rouleaux is F_vdW ~ 70 pN. (This lies well within the range of mechanical forces potentially accessible to medical nanorobots; Section 9.3.) Cationic particles strongly bind to human erythrocytes [6146]. The streptococcal bacterial coat has negatively-charged termini, creating a mild electrostatic repulsion of phagocytes [4184, 4185] (see below). The low Hamaker constants (Section 9.2.1) of cell plasma membranes gives rise to an appreciable mutual electrostatic repulsion between virtually all bloodborne cells [4184]. In general, at neutral pH there is a net negative charge for prokaryotic and eukaryotic cells and for DNA, although proteins may be either positive or negative [4187]. In an oscillating nonuniform electric field, Gram-positive bacteria experience positive dielectrophoresis because they appear more conductive than Gram-negative bacteria which experience negative dielectrophoresis, hence the two cell types are readily separated [4186, 4187]. Red cell membranes carry electric charge and are readily deformed in a high-frequency oscillating electric field [4188]. Similar fields can induce cell vesicle budding or fission [4189] or cell fusion [4190]. These and other useful electrical forces and influences – which may include systemic mechanoelectric transduction (Section 4.9.3.3) and mechanoelectric feedback throughout all human tissues [4191] – may be exploited by medical nanorobots or by macroscale nanoaggregates.

Bacterial cell surfaces possess net negative electrostatic charge by virtue of ionized phosphoryl and carboxylate substituents on outer cell envelope macromolecules which are exposed to the extracellular environment [4665]. For example, Gram-negative bacteria have an outer layer of lipopolysaccharide (LPS) and protein which forms a highly charged surface that is stabilized by cation binding [4666]. Variations in the structure and chemical composition of the LPS have been shown to affect bacterial surface charge and the ability of bacteria to adhere to both glass and polystyrene surfaces [4669]. LPS can occur in two general forms, a hydrophobic (A form) and a charged hydrophilic (B form) [4667-4669].* Most protozoan [4670] and bacterial cells [4671-4674] are negatively charged to varying degrees, though there are a few rare instances of positively-charged bacteria such as S. maltophilia [4672]. Bacterial negative charge can be reduced by antibiotics [4675, 4676], and complete bacterial charge reversal, from negative to positive, has been observed in the presence of certain metals and high pH [4677, 4678]. Note that the internal bacterial proton gradient does not affect the external charge.**

* There is also a bacterial form which has no LPS coat – the “L form” [5980] – in which the bacterium exists, in essence, as a liposome (i.e., no protein/polysaccharide surrounding coat).

** A proton gradient builds up in the space between the outer face of the bacterial cell membrane and the innermost face of the bacterial cell wall outer coat, as a result of NADH-mediated translocation of H⁺ from cytoplasm to the periplasmic space and the resulting accumulation of cytoplasmic OH^-. Once the charge gradient is large enough, the periplasmic protons enter back into the bacterial cytoplasm through channels in transmembrane ATP synthase enzyme complexes which drives the production of bacterial ATP from ADP. These charges do not escape through the bacterial outer coat.

As mentioned earlier (Section 15.4.3.6.2), the phagocytosis of polystyrene beads (as measured by cellular oxygen consumption) appears strongly dependent on local surface potential and thus upon fixed surface charge [3327]. Surface charge heterogeneity across domains as small as 1-4 microns can greatly affect phagocytic ability [3328]. For instance, bacteria and epithelial cells, both of which possess a negative surface charge, should repel one another, but do not. Investigations [4674] by atomic force microscopy of the structures involved in the attachment of Moraxella catarrhalis bacteria (which have a net negative surface charge) and pharyngeal epithelial cells found that the cell surface microplicae have a positive charge of +30.1 mV whereas the depressions between the microplicae have a negative surface charge of -43.5 mV. Thus there are both positively and negatively charged domains on the surface of human pharyngeal epithelial cells, and M. catarrhalis evidently attaches to the positively charged domains.

Bacteria adhere more readily to positively charged surfaces [4679], and enzymes with a large global positive charge more easily penetrate bacteria cell walls [4680]. The effects of surface charge on adhesion [4681] and absorption [4682, 4691] by phagocytes has been studied with variable results due to inconsistent experimental conditions. Some studies [4683-4686] indicate no effect of bacterial surface charge on phagocytosis, e.g., no significant difference in phagocytosis between cationic or anionic surfaces when compared at a zeta potential of the same absolute value [2865]. Other studies find increasing phagocytosis with increasing negative charge [4687, 4688] or reduced phagocytosis with reduced negative charge [4689]. Still others [4690] show increased phagocytosis of microcapsules by a leukocyte only for targets of different charge from itself. The most recent results are that phagocytosis appears somewhat inhibited for negatively charged particles [2336, 2880, 4691], and somewhat increased for less-negatively charged [4675] or for positively charged particles [2880, 4691, 4692]. Brodbeck et al [5507] found that phagocyte adherence is minimized for hydrophilic surfaces and for anionic surfaces containing negatively-charged groups, such as polymers made from organic acids. In another experiment [2865], the least phagocytosis was observed for cellulose microspheres with non-ionic hydrophilic surfaces. But a reduction of phagocyte membrane negative surface charge has also been shown to decrease macrophage phagocytosis [4693]. These studies should be evaluated for quality and reliability, and further research may be required before we can make a definitive statement regarding the relationship between the surface charge of bacteria and phagocytosis.

The surface electrical characteristics of possible medical nanorobot building materials are only beginning to be explored. For instance, Donaldson [898] notes that the alumina (sapphire) surface is amphoteric. In a sufficiently acidic environment, the sapphire surface equilibrates with that environment by adsorbing hydrogen ions, acquiring a positive charge. In an alkaline environment, the sapphire surface acquires hydroxyls and a negative charge. At some intermediate pH near human physiologic (i.e., ~7.4), the sapphire surface is neither positively nor negatively charged – that is, it is isoelectric. The isoelectric point for pure alumina has been measured as a pH of 6.6 for anhydrous Al₂O₃ [1101], 9.2 for alumina submerged in water for a week [1101], or 8.0 using an ISFET [1102]. Similarly, the electrical conductivity of DNA-based structures has been investigated (and controversial) for many years [5770-5776], but until very recently [5777] could not readily be studied in a physiological environment.

The role of surface charge in the possible pathogenicity of microparticles [4192] and the influence of microparticle shape on electrocompatibility has only been lightly studied. For example, sharp edges and corners may produce higher local fields or create current density hotspots [4193] – the positively charged edges of kaolinite particles contribute slightly to particle cytotoxicity [4194].

Last updated on 30 April 2004