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
22.214.171.124 Adhesive Nanorobot Surfaces
In many nanomedical applications, it will be important that the nanorobot exhibit a strong affinity for the specific biological tissue with which it is designed to interact. For example, diamondoid bone implant should show good osseointegration, preferably with bone tissue infiltrating some portion of the foreign diamondoid structure and with cells tightly adherent to the implant, locking it firmly in place in the manner of bioactive materials previously discussed (Section 126.96.36.199). Entry into the body by free nanorobots traversing the gut might be assisted using mucosal-binding attachments . It may be desirable for the surfaces of artificial nanorobotic organs to encourage attachment, migration and coating by certain types of cells. This could facilitate biochemical transfers between blood or tissues and the mechanisms within, help avoid immune system rejection, or establish good mechanical stability within the peri-implant space. In general, cells attach to synthetic surfaces via adsorbed adhesive proteins such as fibronectin. By controlling the chemistry of the surface, it is possible to modulate the adsorption of the proteins, which then govern cell attachment and spreading. Cell spreading has been correlated with fibronectin adsorption to a variety of surfaces [1486-1488]. Rates of cell migration have been shown to depend on the concentration of adhesive proteins preadsorbed on polymer surface [1488-1490]. These rates of migration are optimal at intermediate substrate adhesiveness , as expected from mathematical models of cell migration .
So far, no general principles have been identified that allow precise prediction of the extent of attachment [1493, 1494], spreading, or growth of cultured cells on various artificial surfaces such as polymers [1491, 5729, 5730]. Certain chemical groups present on a material surface can alter cell response . Interesting correlations have been found in vitro for specific cell types with parameters such as the density of surface hydroxyl [1496, 1497] or sulfonic  groups, surface C-O functionalities , surface free energy [1500-1502] or surface wettability [756, 1503, 1504], hydrogenated amorphous “unsaturated” carbon phases , fibronectin adsorption , and equilibrium water content , but there are exceptions in all cases . The ability of macrophages to form multinucleated giant cells at a hydrogel surface has been correlated with the presence of certain chemical groups at the interface: macrophage fusion decreases in the order (CH3)2N- > -OH = -CO-NH- > -SO3H > -COOH(-COONa) [1509, 1510]. Cell attachment and growth on surfaces with grafted functional groups decreases in the order -CH2NH2 > -CH2OH > -CONH2 > -COOH .
In vivo, cell attachment to the surrounding environment may be mediated by various small, biologically active functional groups such as oligopeptides , saccharides [1512-1514], or glycolipids (patterns of glucose residues attached to membrane lipids) [1517-1519] via specific peptide sequences within proteins . Peptides or peptide sequences may act as signaling molecules, attachment sites, or growth factors that mediate the conversation between cells and the surrounding extracellular matrix in an information-rich dynamic structure. The dynamics of these processes can be seen both in development and in wound healing, where fibroblasts lay down a matrix that guides regeneration or development in a specific controlled manner. Appropriately functionalized nanorobot surfaces may be able to strongly influence such processes.
Perhaps the best-known example is the peptide sequence arginine-glycine-aspartic acid (RGD)* [1520-1522] which was first identified in the cell-binding domain of fibronectin , an adhesion-related glycoprotein that provides attachment sites for many cells through cell surface receptors called integrins (Section 188.8.131.52). RGD is also present in many other proteins such as collagen, entactin, laminin, tenascin, thrombospondin, and vitronectin without losing receptor specificity, so its interactions are probably conformation dependent . The YIGSR and IKVAV sequences in laminin [1491, 1528] and the FHRRIKA sequence  also show cell binding activity and mediate adhesion in certain cells.
* Amino acids are customarily identified by one-letter abbreviations: A = alanine, B = asparagine (N) or aspartic acid (D), C = cysteine, D = aspartic acid, E = glutamic acid, F = phenylalanine, G = glycine, H = histidine, I = isoleucine, K = lysine, L = leucine, M = methionine, N = asparagine, P = proline, Q = glutamine, R = arginine, S = serine, T = threonine, V = valine, W = tryptophan, Y = tyrosine, and Z = glutamine (Q) or glutamic acid (E).
In theory, a nanorobot surface functionalized with RGD should exhibit greatly enhanced adhesion to cells, because to the cells, the surface may appear much like ECM (extracellular matrix). As summarized by Saltzman , RGD has been experimentally attached to amine-functionalized quartz [1530, 1531], glass , and synthetic polymer surfaces including PEG , PET [1533, 1534], PEU [1535, 1546], PLA , polyacrylamide , poly(carbonate urethane) , poly(N-isopropylacrylamide-co-N-n-butyl-acrylamide) , PLGM films , PMMA/PEG latex , PTFE [1533, 1534], and PVA . Addition of RGD or RGDS  to these surfaces induced cell adhesion, cell spreading, and focal contact formation on otherwise nonadhesive or weakly-adhesive polymers in vitro [1543-1546]. In one experiment , glass surfaces functionalized with spatially-precise patterns of cell-adhesive regions and cell-repulsive regions were able to control the direction of neuron cell adhesion and neurite outgrowth across the surface. Another experiment  used a combination of adhesive (RGD) and nonadhesive (PEG) moieties to modulate cell spreading.
Since cells contain cell adhesion receptors that recognize only certain ECM molecules, surface functionalization with an appropriate cell-binding sequence can produce cell-selective surfaces  in which the population of cells adhering to the artificial surface is determined by the peptide structure . In vivo, the presence of serum proteins can attenuate the adhesion activity of peptide-grafted surfaces , but this problem can be overcome by attaching the peptide to a base surface that is itself biocompatible yet resistant to protein adsorption, such as PEG-rich foundations [1548-1551].
Besides cell-binding peptides, other biologically active molecules have been used to enhance cell adhesion to artificial surfaces. For certain cell types, adhesion can be improved by adsorption of homopolymers of basic amino acids such as polyornithine and polylysine. As an example, poly-L-lysine (MW ~ 21,400)  is a water-soluble polycation that can bind to anionic (negative) sites on glycoproteins and proteoglycans in the extracellular matrix, and on cell surfaces [1553-1555]. Covalently bound amine groups have influenced cell attachment and growth [1556, 1557]. Polymer-immobilized saccharides can also influence cell attachment and function. For instance, in vitro rat hepatocytes adhered (via asialoglycoprotein receptors) to surfaces derivatized with lactose [1514-1516] or N-acetyl glucosamine , and remained in a rounded morphology consistent with enhanced function. Finally, whole proteins such as collagen can be immobilized to artificial surfaces, providing adherent cells with a substrate that most closely resembles the natural ECM found in tissues .
Various other simple surface modifications can improve cell adhesion [1505-1508]. For example, negative silver ions implanted in hydrophobic polystyrene at doses from 1-600 x 1018 ions/m2 hydrophilize the surface and lead to enhanced growth of human vascular endothelial cells . Adhesion and proliferation of endothelial cells is likewise drastically improved when the cells are cultivated on an Ne+ or Na+ ion-implanted polyurethane surface with a ~1019 ions/m2 fluence, though cells did not proliferate on such surfaces exposed to 1018 ions/m2 or less . Endothelial cells are not capable of proliferating on polyurethane surfaces except in regions of carbon deposition; promotion of cell proliferation on a carbon-deposited surface is probably due to selective adhesion of adhesive proteins to the surface . Plasma ion-implantation is now routinely used to alter the top few atomic layers of medical polymers . This controls their wettability to allow adhesive bonding (1) for preparation of angioplasty balloons and catheters, (2) for treating blood filtration membranes, and (3) to manipulate surface conditions of in vitro structures to enhance or prohibit culture cell growth . “Smart” polymers with switchable hydrophobic/hydrophilic properties also are known , and various parameters of urinary bladder mucoadhesivity for microspheres have been investigated .
In their study of the systematic control of nonspecific protein adsorption on biocompatible materials, Satulovsky et al  suggest that in systems where it is necessary to control protein adsorption during in vivo missions of modest duration (i.e., hours to days), it is probably best to use relatively dense polymer layers with long polymers that are not attracted to the underlying surface, a strategy that should provide the best kinetic control. For materials that must remain in contact with the bloodstream for years, the ideal type of polymer may be one that is attracted to the underlying surface, which should provide the best thermodynamic control. Because very high surface coverage of grafted polymers is hard to obtain using conventional experimental techniques , alternative approaches prior to the development of machine-phase nanotechnology might include mixtures of polymers (perhaps designed using Satulovsky’s quantitative guidelines ) that allow optimal kinetic and thermodynamic control under conditions that are experimentally realizable as of 2002. Molecular manufacturing will allow the fabrication and bonding to nanorobot surfaces of grafted polymer coatings having far greater variety, maximum packing densities, and more precise positioning than is possible today.
Last updated on 30 April 2004