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How would you……describe the overall significance of this paper?This work describes four ap-proaches d simulations on large 3-D microstructural datasets. Long computation times and large resource requirements may prohibit the accurate calculation of some materials properties, and the strategies presented here allow for statistically significant calculations with reduced time and resource requirements.

…describe this work to a ma-terials science and engineering professional with no experience in your technical specialty?Four approaches to the reduction of computation time and memory requirements were applied to 3-D datasets collected from polycrys-talline and composite materials. Parallelization of image processing codes allowed for faster data analy-sis, reduction of model parameters allowed for accurate simulation of mechanical behavior at reduced computational cost, data sampling was employed for observation of structure-property correlations, and lean, efficient meshing techniques were used to reduce FE mesh sizes.

…describe this work to a layper-son?As materials characterization moves from two-dimensional images to three-dimensional volumes, the data representing these materials be-comes quite large and requires vast amounts of computational power for processing and analysis. In this work, we describe some approaches one might use to make 3-D datasets, and the simulation of their behavior, more manageable.

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OverviewCorrosion in Biological Environments

Biological Interactions at Metal Surfaces

Douglas C. Hansen

How would you……describe the overall significance of this paper?This paper is a general review of the interactions and variables govern-ing the adsorption behavior of bio-molecules onto surfaces, especially metal substrates. The adsorption of biomolecules to a substrate is the first step in a sequence of events that can lead to adsorption of larger biomolecules, or macromolecules, and subsequent adsorption or for-mation of biofilms and micro-organ-isms. The variables that play a role in the primary interactions between biomolecules and a metal substrate are described and discussed. An example of a laboratory study of a particular biomolecule and its adsorption behavior and effect on the electrochemical behavior of the substrate metal is described as well.

…describe this work to a ma-terials science and engineering professional with no experience in your technical specialty?Adsorption of biomolecules, wheth-er it is a consequence of a material being put into service in a biologi-cal medium such as a biomedical implant device or a heat exchanger tube, are governed by fundamental interactions and surface conditions that are well characterized (and in some cases not so well). This work reviews those fundamental interac-tions and also describes a study of the adsorption of a naturally occurring biological polymer onto a stainless steel alloy.

…describe this work to a layper-son?A review of what governs how proteins stick to surfaces and how it can impact various areas of applications like food processing, chemical treatment plants, biomedi-cal implant devices, heat exchanger tubes, pipelines, and water treat-ment systems, to name a few.

Adsorption of biomolecules, whether it is a consequence of a material being put into service in a biological medium such as a biomedical implant device or a heat exchanger tube, are governed by fundamental interactions and surface conditions that are well characterized (and in some cases not so well). This work reviews those fundamental inter-actions and also describes a study of the adsorption of a naturally occurring biological polymer onto a stainless steel alloy.

IntrOduCtIOn

The interaction of biological mol-ecules with various metal surfaces, the concomitant circumstances under which they adsorb, the resulting effect on the substrate, and hence the forma-tion and adhesion of biofilms on metal surfaces is a way of life, both literally and figuratively. A biological molecule (or biomolecule) is any organic mole-cule that is produced by a living organ-ism including large polymeric mole-cules such as proteins, polysaccharides and nucleic acids as well as small mol-ecules such as primary metabolites, secondary metabolites, and natural products. When looking at this interac-tion, the most simplistic and straight-forward approach is to consider a sur-face (for the purposes of this discussion we will focus solely on metal surfaces) immersed in an aqueous environment and the sequential adsorption of ions, biomolecules, biopolymers, and later adhesion of biofilms and micro/macro-organisms. Therefore, the understand-ing and control of these interactions is important in a variety of fields, rang-ing from medical implant devices to power plant heat exchangers.1,2 Table I provides a summary of various fields and applications where these consid-

erations are deemed to be important. While this table is not comprehensive, it gives the reader an appreciation for the breadth of applications where the interaction of biological components (be they molecular or organismal) can play an important role in the deteriora-tion and degradation of the substrate or the surrounding environment. The specific recognition between biomolecules and a surface at the so-lution-substrate interface occurs under the influence of many factors such as: electrostatic interactions, pH, negative-ly charged surfaces, co-adsorption of low molecular weight ions, hydropho-bicity and isoelectric point.1,3–5 Other factors that may influence adsorp-tion of higher organized biomolecules (such as proteins and polymers) and adhesion of biofilms are intermolecu-lar forces between adsorbed molecules, solvent-solvent interactions, strength of functional bond groups, chemistry, and/or the topology and morphology of the solid surface.6

Before discussing these topics any further, it is important to review the se-quence of events that leads to a surface exposed to an aqueous environment, having biomolecules attached to it with the inevitable formation of an adhered biofilm. Since 1861 when Quincke first de-scribed the concept of the electrical double layer, numerous models have been put forth to describe the organiza-tion of charged particles and dipoles at a solution-substrate interface.7 Recently, a review of the DLVO (Derjaguin, Lan-dau, Verwey, Overbeek) theory has in-vestigated the application of this theory to describe the adsorption of biomole-cules and the adhesion of bacteria onto a substrate.8 While the DLVO theory was originally developed to describe

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interactions between non-biological colloid particles through long-range in-teractions (Lifshitz-van der Waals and electrostatic interactions), it was found that biological molecules, biological polymers (i.e., proteins), and microbial organisms are somewhat more com-plex and heterogeneous than colloid particles, thereby making the applica-tion of the DLVO theory to the adsorp-tion and adhesion of biological compo-nents inappropriate. This is primarily due to the fact that even “simple” bio-logical molecules (such as carbohy-drates or amino acids) require specific binding or coordination sites between the substrate surface (adsorbent) and the molecule (adsorbate). This delicate specificity, often referred to as a “lock and key” mechanism, is rarely dem-onstrated between non-biological or

synthetic materials.9 Therefore, when considering the adsorption and adhe-sion of biomolecules, biopolymers, and micro-organisms to a substrate at the substrate-solution interface, it is nec-essary to appreciate the fundamental variables that play a role in the adsorp-tion/adhesion process. Table II presents the various solution conditions and the biomolecule’s properties important for attachment to a substrate. Figure 1 is a schematic view of a biomolecule inter-acting with a well characterized surface at the substrate-solution interface. As shown in the diagram, both the biomol-ecule and the surface of the substrate have well defined domains of hydro-phobic, charged, and polar character. The following summary discusses the major interactions or forces involved in the adsorption of biomolecules onto a

solid substrate-solution interface.

FOrCES and IntEraCtIOnS GOvErnInG BIOMOLECuLE

adSOrptIOn

Hydrogen Bonding

Hydrogen, due to its small size and its electropositive character, is easily attracted to electronegative atoms. Be-cause of this characteristic, their bonds can be described primarily as electro-static interactions although due to the unique strength of these interactions they are classified by themselves and given their eponymous moniker—hy-drogen bonds. Hydrogen bonds are very common in proteins being found in α-helix and β-sheet molecular struc-tures. Hydrogen bonds are generally linear in nature and can be viewed as a bridge between two electronegative atoms.10 While hydrogen bonds are in-credibly strong, they generally do not have a strong stabilizing effect in an aqueous environment where the fold-ing or association process of proteins and biomolecules are concerned. That is due to the fact that amino acids and peptides exchange hydrogen bonds with water molecules, resulting in no significant change in the overall bond energy of the system.11

Ionic or Electrostatic Interactions

These are forces due to the attraction or repulsion of two or more surfaces carrying a net charge (also known as van der Waals forces). In the case of biomolecules, this can be the result of the ionized amino acid side chains that make up a protein or polypeptide, or functional groups of a biomolecule that are available for solvation by the surrounding aqueous environment. The net charge of the biomolecule will re-sult in its electrophoretic migration in an electric field and the electrostatic interactions of the biomolecule are relatively short-range due to the high dielectric constant of water.1 The high dielectric constant of water means that the electrostatic interaction in water is roughly two orders of magnitude less than it would be in air; the end result being that only when the unlike charg-es are close enough that very few water molecules are in between is it that the electrostatic interactions by their na-

Table I. Areas or Fields where Interaction of Biological Molecules with Metal Surfaces is Important

Area or Field Issue

Medical Devices Fouling of surfaces of medical devices and implants by interaction with surrounding serum proteins; formation of biofilms and infections; enhanced corrosion rates

Freshwater Piping Systems Growth of biofilms decreases carrying capacity of conduits, fire suppression systems, and heat exchanger surfaces

Fouling (Freshwater and Marine) Vessel Surfaces Growth of biofilms and subsequent fouling of surfaces by macro-organisms increases drag and hydrody-namic performance suffers

Food Processing Bacterial attachment and biofilm formation leading to contamination of food products resulting in food spoilage or transmission of diseases

Soils and Agriculture Bioremediation of soils, biodegradation of hydro-carbons and heavy metals; protein adsorption and enzyme activity on mineral surfaces for microbial growth, nutrient uptake and plant root system viability

Water Treatment/Sanitation Biofilm formation, thickness and activity for the degra-dation and breakdown of waste products

Biological/Chemical Sensors Sensors for monitoring biological functions in the hu-man body such as glucose, urea, lactate, cholesterol, immunosensors, DNA biosensors, biochips, biosen-sors for environmental monitoring, and biosensors for the food industry and fiber optic probes.

Microbiologically Influenced Corrosion Accelerated or inhibited corrosion of metal substrates due to the formation of biofilms and bacterial consortia in both terrestrial and aquatic habitats

Table II. Biomolecule and Solution Variables Important in Adhesion and Adsorption

Biomolecule Variable Solution Variable

Isoelectric Point pH

Net Charge and Charge Distribution Ionic strength, ionic composition

Location and Nature of Hydrophobic Domains Types and concentration of biomolecules in solution

Charge-transfer Domains Temperature and/or pressure

Bound Water Hydrodynamics

Impurities or Surface Contaminants —

Conformational Rigidity or Steric Hindrance —

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ture would be responsible for directing a biomolecule to a charged interface. Once the biomolecule is close to the charged surface does the electrostatic coupling that can occur become signifi-cant.

Hydrophobic Interactions

The stability and interactions of a biomolecule are dependent on the over-all free energy of the system, which means the entropy of the system needs to be considered. Hydrophobic interac-tions are entropic interactions mostly due to the phenomena of order-disorder in an aqueous solvent. The theory most consistent with experimental observa-tion is that the attractive force results from the increased dynamic structur-ing of water in the vicinity of nonpolar species incapable of hydrogen bonding with water.12 This increased structuring of the solvent in the interfacial region leads to a large interfacial energy and a thermodynamic driving force to re-duce the amount of structured water. This is accomplished by bringing the nonpolar surfaces in contact and elimi-nating the water-nonpolar interfaces. This attractive force remains in place over a considerable range.13 However, since water becomes ordered at the air or apolar interfaces (which is entropi-cally undesirable), the tendency is for the minimization of these hydrophobic areas on the surfaces of biomolecules; this tendency is the driving force re-sponsible for the minimization of the surface area of a water droplet and air, or oil droplets in water. Since the con-cept of surface tension is equivalent to the concept of surface free energy on a molecular scale, one can consider the effect that the critical surface tension of a substrate has on the adsorption of a molecule from a solution.14 The critical surface tension for spreading was first demonstrated by Zisman and has since been used to characterize the surface properties at the solution-substrate in-terface.15 A diagram summarizing sub-strate composition and the concomitant critical surface tension of the various substrates are presented in Figure 2.16

In the case of a protein for example, its surface hydrophobicity is dependent upon the number and distribution of nonpolar amino acid residues on the molecular surface, which in turn will

influence the adsorption behavior at the substrate-solution interface. In the case of a folded, globular protein hav-ing many nonpolar surface regions, the biomolecule is considered to have a high surface hydrophobicity and there-fore a high affinity for amphiphilic (po-lar/nonpolar) interfaces.17 In contrast, a folded, globular protein having few nonpolar surface regions is considered to have a low effective surface hydro-phobicity and therefore a low affinity for amphiphilic interfaces.

Charge-Transfer Interactions

Charge-transfer interactions are also important in the adsorption and adhe-sion of biomolecules at the surface-solution interface. Hydrogen bonding can be thought of as a particularly strong and special case of a charge-transfer interaction, where a positively charged hydrogen proton is donated to a suitable acceptor. One can easily en-vision a case where an excess of elec-tron density is present which can then be donated to an electrophilic species. Electron density can also serve as an

acceptor for positive charge; in water, these solute interactions are primarily due to π-π electron effects. It is clear, then, that the solid sub-strate and the substrate-solution inter-face must be well characterized if one hopes to understand the attachment of biomolecules, and this becomes even more important when considering how much of the biomolecules are absorbed as a function of bulk solution concen-tration (adsorption isotherm) and as a function of time (adsorption kinetics). Finally, the importance of the confor-mation and orientation of the biologi-cal molecule as it is adsorbed to the substrate cannot be overstated, as this will determine the optimum interac-tion between the molecule and the substrate. If the molecule is adsorbed so that it’s three dimensional shape is changed, then the molecule has un-dergone a change in conformation. This change in conformation, which must result in a lowering of the over-all Gibbs free energy of the molecule-substrate system, is sometimes referred to as “denaturation” when considering

Hydrophobic Domains

Polar Moieties

Ionic Interactions

Substrate-Solution Interface

Biomolecule in SolutionSubstrate

Fluorination Chlorination

Zone of minimal cell spreading or bacterial attachment

Fluoropolymers NylonsMost common paint surfaces

dynescm

GlassPVC

Perflu

orolau

ric

acid

Teflo

n

Polye

thylen

ePo

lystyr

ene

Plexigl

ass,

Lucit

e and

PVC

Most m

etals

&

metal o

xides

0 10 20 30 40 50

Figure 1. Schematic view of a biomolecule in solu-tion interacting with a well defined substrate. The biomolecule and sub-strate have a number of domains with hydropho-bic, charged and polar character.

Figure 2. Critical surface tension scale for various common substrates (Compiled by S.C. Dexter. Used with permission).

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large biomolecules or polymers (such as proteins) since the “normal” charac-teristics of the adsorbed molecules or polymers have been altered.1

In this review the interaction of bio-molecules with metal surfaces will be discussed as it pertains to oxide films, corrosion and corrosion inhibition, bio-fouling, enoblement by biofilms, and microbiologically influenced corro-sion.

IntEraCtIOn OF BIOMOLECuLES WItH an OxIdE SurFaCE

When considering the interaction of biomolecules with metal surfaces, it is usually in the context of the adhe-sion and subsequent adsorption of the biomolecule(s) to the metal surface at the substrate-solution interface. Con-sideration of what forces (attractive and repulsive) act upon the biomole-cules resulting in their ultimate adsorp-tion or desorption is critical in defining whether a biofilm will form, thus lead-ing to biofouling of the surface. In general, it can be said that the in-teraction of biomolecules and biofilms with a metal oxide surface is much more complex than has been consid-ered, even as recently as the last de-cade.18 For example, it is well known that biofilms facilitate the attachment of bacteria to surfaces, allowing for the isolation of the surface from the overly-ing solution.19 These biofilms can have strong and specific chelating or binding properties resulting in the subsequent adsorption of cations, anions, and or-ganics depending on the properties of the film itself.20 The surface chem-istry of metal oxides and the dissolu-tion of the oxides are usually discussed in terms of surface complexes and ligand-exchange pathways; the par-ticular pathways for dissolution of the metal oxides/hydroxides depends upon whether adsorbed protons, ligands, de-protonations or electron exchanges are controlling the surface reactions.21,22 Dissolution rates of metal oxides in-crease with the equilibrium concen-tration of adsorbates (such as protons, hydroxyl groups, or ligands) on the surface, and rates of dissolution can increase with both negative and posi-tive charge reflecting the amphoteric nature of the surface.18 In terms of or-

ganic ligands present on biomolecules and the resultant binding or dissolution of metal oxides, the amount of metal in solution as a function of adsorbed biomolecule on stainless steel has been reported.23 In that study, adsorption measurements of an adhesive protein (MAP-1) isolated from the blue mussel (Mytilus edulis L) onto type 304L stain-less steel was compared to that of the protein bovine serum albumin (BSA) and two molecules: poly-L-lysine (po-ly-K) and 3,4-dihydroxybenzoic acid (DHBA), all in buffered solutions (pH = 7.5); adsorption measurements were also made on DHBA at a pH of 5.5. The two latter molecules were utilized due to the relatively high concentration of lysine and the presence of the novel catecholic amino acid 3,4-dihydroxy-phenylalanine (L-Dopa) in MAP-1, re-spectively. It was found that the mussel adhesive protein significantly inhibited the dissolution of iron and chromium metal from the stainless steel substrate whereas BSA, poly-K, and DHBA en-hanced it. A closer look at the results of this specific study may help in ex-plaining the observed behavior of the adsorbates onto a heterogeneous metal oxide surface, for example. First, a common method for describ-ing the adsorption of proteins, amino acids, or other small adsorbates onto a substrate is to apply the Langmuir the-ory for gas adsorption.24,25 In doing so, the following equation may be used23,26

Q = KNC/(1+KC) (1)

which can be rewritten as C/Q = 1/NK + C/N (2)

where C is the equilibrium concentra-tion of the adsorbate in the bulk solu-tion, Q is the number of macromole-cules adsorbed per unit surface area of adsorbent (calculated as the difference

between the initial and final concentra-tion of macromolecule in the bulk solu-tion divided by the area of adsorbent), N is the maximum number of adsorption sites per unit surface area of adsorbent, and the parameter K reflects the affin-ity that the adsorbate molecules have for the adsorption sites. A plot of C/Q versus C should yield a straight line, and N and K can be derived from the slope and the intercept of that line, re-spectively. Once N and K have been de-termined for an adsorbate, a theoretical adsorption isotherm can be calculated for the adsorbates based upon Equation 1, where C is the bulk concentration of the adsorbate at equilibrium. When these calculated Q values are plotted versus the bulk solution concentrations at equilibrium, the resulting theoretical curve should agree with the measured Q values. Table III lists the adsorption parameters derived from the adsorption isotherm measurements of the adsor-bates onto Type 304L stainless steel. As can be seen from the correlation coefficient values in Table III for each adsorbate, the relationship is linear and therefore fits the Langmuir model for adsorption. However, for DHBA at pH 7.5, the linearization coefficient is not as high as that measured at pH 5.5, in-dicating that the maximum number of binding sites by this molecule with the catecholic moiety is not achieved at the more neutral pH. The calculation of the resulting N values for DHBA shows that the stainless steel exhibited the highest number of adsorption sites for this particular adsorbate at both of the pH ranges measured. In the case of po-ly-K, the calculated number of binding sites on the metal surface is the low-est of all the adsorbates tested. Assum-ing that the oxide layer is composed mostly of Fe(OH)3 and Cr(OH)3 in this case,27,28 and the pH zero point of

Table III. Adsorption Parameters Calculated from Adsorption Isotherms of Each Adsorbate onto 304L Stainless Steel

AdsorbateLinearization Correlation

Coefficient ®

Maximum Number of Adsorption Sites, N

(moles/m2)Affinity Constant, K

(mol–1)BSA 0.987 5.087 × 10–8 3.139 × 108

DHBA (pH 5.5) 0.989 1.366 × 10–5 3.526 × 105

DHBA (pH 7.5) 0.909 3.985 × 10–5 5.162 × 104

MAP-1 0.986 5.238 × 10–7 3.660 × 108

Poly-K 0.969 1.606 × 10–8 2.637 × 108

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charge (pHzpc) for Fe(OH)3 is 8.5,29 then at a pH range of 7.5–8.0 the stainless steel surface will have a net positive charge. At this same pH range, poly-K will exhibit an overall positive charge due to the ε-amino groups contained within the side chains of the poly-mer. Since the net charge of the oxide layer of the stainless steel is positive, the establishment of a diffuse double layer could facilitate the binding of the positively charged side chains of the poly-K polymer to the oxide ions on the surface. This can well explain the low value calculated for the number of binding sites for the poly-K to the stainless steel. The difference in the number of binding sites for the metal surface be-tween the MAP-1 and BSA proteins reflects the heterogeneity of the amino acid composition of MAP-1 regard-ing potential binding sites. The mus-sel protein has an isoelectric point (pI) greater than 10, which means the protein would have an overall positive charge at the pH range of 7.5–8.0.30 The BSA, with a pI of 5.2 would have an overall negative charge at the mea-sured pH range, yet MAP-1 still dis-played a higher binding affinity onto the positively charged stainless steel substrate than the BSA.31 Adsorption of BSA onto a hydrophilic surface, such as the stainless steel oxide sur-face, would be weaker than that onto a hydrophobic surface and therefore more sensitive to electrical factors.32 It has been determined that adsorption of BSA onto a hydrophilic metal surface could be due to electrostatic forces; this has been suggested since the amount of BSA adsorbed was higher on metal sur-faces with greater positive charge than metals with less noble standard poten-tials.33

While it was clear that electrostatic interactions cannot be totally respon-sible for the adsorptive behavior of MAP-1 onto a stainless steel oxide sur-face, a consideration of the conforma-tion of MAP-1 and its high percentage of hydroxylated amino acids (3-Hy-droxyproline, 4-Hydroxyproline, Ser-ine, and Tyrosine) and catecholic func-tional groups (L-Dopa) is required; these factors may account for the high-er number of binding sites calculated for MAP-1. This is in stark contrast to

the BSA protein, which is globular and contains a small region for the binding of ligands.31

Iron and chromium are present at the surface of the stainless steel in the forms of Fe(OH)3 and Cr(OH)3, form-ing the passivating oxide layer. These metals would therefore be available for complexation by the catecholic moiety present either in the form of the DHBA molecule, or within the MAP-1 protein (as the amino acid, L-Dopa). Complex-ation by the MAP-1 protein of iron(III) to form catecholato iron(III) complexes has been demonstrated.34,35 Hansen et al. reported significant amounts of met-al in solution when DHBA was present and the amounts of iron and chromium in solution in the presence of BSA were significantly higher than when MAP-1 was present.23 Merritt et al. found that almost all of the dissolved metal from a stainless steel substrate was bound to the albumin fraction of whole blood, with the chromium and nickel repre-senting the most concentrated of the metals measured.36 Indeed, Woodman et al. determined that serum albumin bound chromium, cobalt, and nickel from 316L stainless steel in the form of corrosion product.35 The predominant form of the corrosion product being a metal-ligand complex comprised of the hydroxides of these metals complexing with albumin. In light of what is known about the number of potential binding sites on the MAP-1 protein, and the ob-servation that lower amounts of metal were measured in solution when the MAP-1 protein was present suggests that any metal complexed by the MAP-1 protein at the metal surface is being shielded from dissolution by the pro-tein remaining on the surface. This is also supported by the finding that the MAP-1 protein acts as an effective cor-rosion inhibitor on metals such as alu-minum and stainless steel.37

Finally, interaction of biomolecules with metal oxide surfaces in nature can be considered in terms of their adsorp-tion and interaction with the surface of a rock or mineral in a tide pool, near a benthic hydrothermal vent or even in a “pre-biotic” system. That is, it is well known that biomolecules such as ami-no acids can assemble and polymerize into protein-like chains on fine-grained clay particles.38–40 Minerals such as

clays and metal hydroxides are able to adsorb relevant biomolecules, includ-ing amino acids and nucleotides.41,42 Moreover, once two molecules are ad-sorbed close to each other, they have a tendency to polymerize.41 In nature, close packed metal surfaces including Pt, Ag, Au, and Cu flat surfaces can only exist for (100), (110), and (111). All other close-packed metal surfaces must include steps and kinks. These kink sites are found on all high-index surfaces and can direct behavior of adsorbed biomolecules on these sur-faces.43–47 Work by R.M. Hazen (and others) suggests that these interactions could possibly lead to the organization of biomolecules at the mineral-metal hydroxide surface and therefore be a possible mechanism for the higher level organization of “pre-biotic” mol-ecules such as amino acids, carboxylic acids, sugars and lipids leading to self-replicating molecular assemblies.48–53

COnCLuSIOnS

Adsorption of proteins and biomol-ecules on metal surfaces and their in-teraction with the metal substrate are of major concern in a variety of areas, some of which have been described in this article. While there exists in the lit-erature a number of reviews pertaining to the adsorption of biomolecules onto metal substrates, one has to be cogni-zant of the contributions that surface energies, the overall free energy of the metal-adsorbate system, pH, isoelec-tric point, point of zero charge and the conformation of the biomolecule(s) have on its interaction and adsorption behavior. It should be obvious that the study of adsorption of biomolecules onto metal substrates is a complex and fascinating phenomenon, and that much is still left to explore and under-stand. This is particularly true when it comes to applying that knowledge to seemingly unlimited practical con-siderations where a metal surface will come into contact with a biological me-dium.

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Douglas C. Hansen is senior research scientist and associate professor in the Chemical and Ma-terials Engineering Department, University of Day-ton Research Institute, 300 College Park, Dayton, OH 45469.

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