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

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

15.3.5.1 Protein Adsorption on Alumina and Sapphire

Alumina ceramic is a hydrophilic material with high wettability [973]. The rates of adsorption and desorption to the alumina surface of proteins [975, 1059, 4791, 4798] including albumin, glycoprotein, plasminogen, fibrinogen, fibronectin, IgA, IgG, and IgM, and of other substances such as sulfapyridine [976] (a sulfa drug component), various pesticides [4802] and surfactants [4807], and carbon monoxide [4808] have been investigated, though a comprehensive survey is beyond the scope of this text. For example, in one experiment the adsorption of blood proteins on alpha-alumina ceramic disks after 2 hours at 37 ^oC and pH 7.35 after removal of eluate was measured as 0.0147 mg/m² (130 molecules/micron²) for albumin but only 0.00198 mg/m² (4 molecules/micron²) for fibrinogen [977]. Another experiment found that attachment and spreading of human bone-derived cells cultured on Al₂O₃ ceramic surface during the first 90 minutes was reduced by 73-83% in vitronectin-free serum, with much less reduction in fibronectin-free serum, suggesting that initial attachment and spreading of cells to an alumina surface is a function of vitronectin adsorption [978].

As with aqueous solubility [980], protein adsorption to alumina is pH dependent. Thus at pH 7.6, for example, acetylated ovalbumin does not show any affinity for alumina surface while unmodified protein does [981]. Electrostatic interactions, surface unfolding of proteins, and surface hydrophobicity of protein also control the adsorption of ovalbumin onto alumina [981]. An extensive series of experiments by Sarkar and Chattoraj [982-987] have examined the competitive adsorption and desorption, relative affinities, and the molecular size, shape, orientation and folding of proteins (esp. bovine serum albumin, beta-lactoglobulin, gelatin, hemoglobin, lysozyme and myosin) at the alumina-water interface as a function of pH, ionic strength of various salts, temperature, and protein concentration. For example, at physiological human blood serum albumin concentrations (35-52 x 10^-3 gm/cm³; Appendix B) and 27 ^oC, albumin (BSA) adsorption onto ~20-nm alumina powder surface is 36,400 molecules/micron² at pH 6.4, 14,300 molecules/micron² at pH 5.0, and zero at pH 3.6 [985]. At the two higher pH values, adsorption reaches a plateau above an environmental albumin concentration of 0.6-1 x 10^-3 gm/cm³. (For ~50-nm graphite particles under similar conditions, albumin adsorption is 13,300 molecules/micron² at pH 6.4, 22,100 molecules/micron² at pH 5.0, and 41,800 molecules/micron² at pH 3.6, with adsorption plateaus at all pH values, for albumin concentrations between 0.3-1.5 gm/cm³ [985].)

Since anchorage and binding of protein to the alumina surface are enthalpy-controlled processes, whereas surface denaturation (including protein rearrangement and folding) is an entropy controlled process [986], the initial adsorption processes can be characterized by the standard free energy of transfer as measured at the state of monolayer saturation. For one mole of protein or protein mixture, under various physiochemical conditions, the standard free energy of transfer is observed to be ~38.5 KJ/mole [985, 987] or ~64 zJ/molecule. Let us provisionally take this figure as representative of the molecular binding energies required at a nanorobot sorboregulatory surface (Section 15.2.2.4) capable of providing desired specific proteophobic or proteophilic action upon exposure to physiological blood serum. Assuming noncovalent (van der Waals) adsorption forces only, then from Table 3.6 a sorboregulatory surface should incorporate physical binding features on the order of ~6 nm² in area or ~1.5 nm³ in volume. A number density of ~10⁴-10⁵ receptors/micron² would imply an areal surface coverage ranging from 6-60%. Sorboregulatory surfaces must bind tightly enough to forestall desorption of the preferred protein as environmental conditions change. For instance, it is known that when beta-lactoglobulin is added to the environment, previously adsorbed bovine serum albumin can be quantitatively desorbed from alumina surface back into solution [983].

Other useful simple surface modifications have been demonstrated. For instance, Yoshida et al [899] have fabricated an ultrasmooth single-crystal alpha-Al₂O₃ sapphire plate which was shown via AFM scanning to have terraces with atomic steps only 0.2 nm in height, comparable to the exterior surface roughness anticipated in medical nanorobotic devices. This material, as obtained by high-temperature annealing, had relatively hydrophobic surfaces (e.g., water-drop contact angle theta ~ 80^o) and thus could not be used for the AFM observation of plasmid DNA. When the material was treated with alkaline Na₃PO₄ aqueous solution, the surface became uniformly coated with a 0.3-nm-thick Na₂HPO₄ compound layer having a more hydrophilic character (e.g., water-drop contact angle theta ~ 20^o), allowing DNA molecules to adhere and be scanned by AFM [899]. Other polarized organic molecules such as the mucopolysaccharides [1037] may attach to the polarized alumina surface (in the wet milieu) [1023] by van der Waals forces. Specific enzymes have also been covalently immobilized onto polyethyleneimine-impregnated gamma-alumina surfaces [4772].

Interestingly, single-crystal sapphire that is exposed to 30 kilogray gamma-ray irradiation (as is common in gamma ray sterilization) produces oxygen vacancies in the sapphire structure accompanied by a deformation of the crystalline lattice resulting in a modification of electrical properties [4773]. At room temperature, irradiated alpha-Al₂O₃, unlike non-irradiated alpha-Al₂O₃, can trap electrons, from which it can be concluded that gamma-ray sterilization modifies the cohesive energy of alpha-Al₂O₃. This could lead to mechanical changes in surface charge, friction, wear, fracture strength, and the like [4773].

Last updated on 30 April 2004