Abstract
A major challenge in biomaterials research is the regulation of protein adsorption at metallic implant surfaces. Recently, a number of studies have shown that protein adsorption can be influenced by metallic nanotopographies, which are discussed to increase electric field strengths near sharp edges and spikes. Since many metallic biomaterials form a native passivation layer with semiconducting properties, we have analyzed the influence of this layer on the near-surface electric field distribution of a nanostructure using finite element simulations. The Poisson-Boltzmann equation was solved for a titanium nanostructure covered by a TiO2 passivation layer in contact with a physiological NaCl solution (bulk concentration 0.137 mol/L). In contrast to a purely metallic nanostructure, the electric field strengths near sharp edges and spikes can be lower than in planar regions if a passivation layer is considered. Our results demonstrate that the passivation layer has a significant influence on the near-surface electric field distribution and must be considered for theoretical treatments of protein adsorption on passivated metals like titanium.
1. Introduction
Protein adsorption at the solid-liquid interface is one of the first steps of biomaterial-biosystem interaction after an implant is brought into contact with body fluid. In a number of studies it is concluded that the initially formed protein layer on a biomaterials surface is a key factor in influencing the subsequent cell attachment and the function of adherent cells [1–3]. Therefore, a detailed understanding of the adsorption process is of crucial importance. It is well established that various interaction types such as hydrophobic and entropic forces and electrostatic and dispersion interactions affect protein adsorption. However, it still remains to be elucidated to what extent each of these factors contributes to the overall process. In recent years, several studies have suggested that, besides various chemical functionalization methods, topographically nanostructured biomaterial surfaces are a promising approach for the regulation of protein adsorption. It was demonstrated that topographical nanostructures significantly affect the amount [3–6] and the biological function of adsorbed proteins [6–9]. Increased electric field strengths at sharp edges and spikes of the surface, geometric effects, and locally varying dispersion interactions are often discussed as possible explanations for this effect [10–12]. However, many medically relevant metallic implant materials form a passivation layer which separates the bulk metal from the wet environment of the biosystem. The physicochemical properties of these layers must therefore be considered, when discussing protein adsorption at most metallic implant surfaces. This is particularly important for titanium, one of the most commonly used materials for biomedical devices, whose excellent biocompatibility is attributed to its air-formed passivation layer [13]. According to the literature, the amorphous film of stoichiometric TiO2 [14] shows semiconducting properties [13, 15, 16]. Consequently, compared to a metal-electrolyte interface, an additional space charge in the semiconductor must be considered in the discussion of the interface electrostatics [17]. The aim of this study was to investigate the influence of the passivation layer on the electric field distribution near a topographically nanostructured titanium-electrolyte interface in an FEM simulation.
2. Material and Methods
2.1. The Electrolyte Solution Domain
Immediately after a metal is immersed into an electrolyte solution, an electrical double layer is formed; that is, the surface charge of the metal is compensated by an ionic counter charge [18]. For the description of the ion concentration distribution in diluted solutions, the Boltzmann distribution can be applied in the form with the ion concentration in the electrolyte bulk , the ion charge , the electric potential , the Boltzmann constant , and the absolute temperature . Inserting expression (1) in Poisson’s equation commonly with a constant permittivity , results in the classical nonlinear Poisson-Boltzmann equation, which adequately describes the electrical double layer for low electric surface potentials and low electrolyte concentrations. Figure 1 shows the course of the resulting electric potential and the distribution of ions in a diluted symmetric 1 : 1 electrolyte near a planar interface. The counter ions in the electrolyte screen the electric surface potential , leading to an exponential decrease to zero in the solution bulk (Debye length ≈ 1 nm for a symmetric 0.1 M 1 : 1 electrolyte [18]). However, the treatment of ions as point-like charges can lead to unrealistically high ion densities near the surface, when exceeding a certain electric surface potential [19]. Therefore, we use a modified ion concentration distribution proposed by Kilic et al. [19] which considers steric effects: where is the packing parameter with as the effective ion size. We consider as the Stokes diameter of the hydrated electrolyte ions , which leads to the ionic space charge density of a symmetric electrolyte solution:
2.2. Influence of the TiO2 Passivation Layer on the Interface Electrostatics
In a number of studies, it has been concluded that the passivation layer of titanium behaves like an n-type semiconductor with oxygen vacancies as main donor species [13, 15, 16, 20]. In contact with an electrolyte solution, a potential difference is formed over the interface, just as in the case of a metal-electrolyte system. However, the conductivity of semiconductors is usually below that of an electrolyte, so that the potential drop takes place predominantly in the solid and leads to band bending. Therefore, when dealing with the semiconductor-electrolyte interface, an additional diffuse space charge layer in the solid must be considered [17].
For reasons of simplification we introduce a number of assumptions for our semiconductor model: (i) the TiO2 has an ideal electronic band structure, (ii) the system is in kinetic equilibrium; that is, the Fermi energies are aligned, (iii) surface states at the solid-liquid boundary of the TiO2 layer are neglected, (iv) the semiconductor is regarded as nondegenerate, (v) the transition from the titanium to the TiO2 layer is abrupt, and (vi) the formation of an inversion layer, that is, the accumulation of minority charge carriers, is excluded because of the considered low band bending.
The investigated physicochemical situation is illustrated by means of a planar titanium NaCl solution interface. The liquid is separated from the metal by the TiO2 passivation layer with a thickness of approximately 3 to 10 nm [21] (Figure 2). In kinetic equilibrium the potential drop over the interface is expressed by where is the work function of titanium and is the reference electron energy level in the bulk electrolyte, which is equal to the normal redox potential of the body fluid under physiological conditions [16, 22]. The course of the electric potential in the passivation layer corresponds to the band bending and is related to the energy of the bottom conduction band edge in the semiconductor bulk (Figure 2).
When only considering the spatially varying electron density , the space charge density in the semiconductor is connected to the electric potential by the expression [18] with the maximum density of oxygen vacancies , and the electric potential in the flat band case . , relative to the electric potential at the titanium surface , is given by the built-in potential difference : where is the theoretical work function of TiO2. is the sum of the TiO2 electron affinity and the energy gap between the lower conduction band edge and the Fermi energy which is equal to with the effective density of states of electrons in the conduction band , Planck’s constant , and the effective mass of electrons in the TiO2 layer . Finally, inserting (7) in (6) yields the space charge density in the TiO2 passivation layer:
Another important point in the discussion of the titanium-electrolyte interface is the pH-dependent surface charge of TiO2. Titanium hydroxides are amphoteric; that is, they show both acidic and basic properties because of their two different types of coordination to the surface titanium cations [13]. In contact with an aqueous electrolyte solution, this amphoteric behavior results in the pH-dependent formation of negatively and positively charged hydroxides that are considered in the literature as potential determining ions [23]. We assume that the point of zero charge of TiO2, that is, the pH value at which the positive and negative charges at the surface cancel each other, is close to neutrality [13]. According to Ridley et al. [24], the physiologically relevant pH range from 5 to 7.5 [25] is therefore equivalent to a surface charge density ranging from 0.05 to −0.05 C/m2.
2.3. Finite Element Model
FEM-based electrostatics simulations were conducted with COMSOL Multiphysics (v4.2, COMSOL AG) using triangular elements and the multifrontal massively parallel sparse direct solver (MUMPS). The mesh was refined until the relative error (a posteriori) of the electric field strength in the electrolyte near the planar area of the surface was below 1% resulting in approximately triangular elements after four mesh refinements (see Supplementary Material available online at http://dx.doi.org/10.1155/2013/820914). The iteration was stopped when the relative error estimate for each nonlinear iteration step was below .
Figure 3 shows the 2D simulation box of the topographically nanostructured titanium NaCl electrolyte interface. A grooved nanostructure was chosen as model geometry, because it represents the simplest topographical variation of a surface which, however, contains the majority of geometric features: concave corners, convex edges, and planar structure elements. Furthermore, the complexity of the problem and the computational effort can be reduced by treating a 2D cross-section of a groove. The model geometry consists of three planar regions, a 90° concave corner, and a 90° convex edge with the radius of curvature of the solid-liquid phase boundary . We considered a passivation layer with a constant thickness , which is in contact with a physiological NaCl electrolyte solution with a bulk concentration of 0.137 mol/L. The growth of the TiO2 layer is assumed to occur only in direction of the titanium bulk because of the unique oxide mechanism of this metal [15, 26, 27]. The growth of a film with thickness therefore results in an edge radius of at the concave corner of the TiO2-titanium boundary. Depending on the ratio of and , two cases must be differentiated for the convex edge of the TiO2-titanium boundary: (i) : the radius of the convexly curved boundary is and (ii) : a sharp convex edge is formed. In the latter case, a radius of 0.2 nm (approximate atom radius of titanium) was subsequently inserted to avoid numerical problems.
was applied at the titanium-TiO2 interface using Dirichlet boundary conditions. In the bulk solution the electric potential decreases to the body potential (top boundary: ). The geometry allows applying periodic boundaries conditions at the left and the right boundary. Neumann boundary conditions were used for the pH-dependent surface charge at the TiO2-electrolyte phase boundary. The space charge densities in the electrolyte and the TiO2 domain are described by (4) and (8), respectively.
To facilitate a comparison to the metal-semiconductor-electrolyte system, we also introduce a simple metal-electrolyte model. In this case the position of the solid-liquid interface was fixed and the passivation layer thickness was set to zero. In this reference model a constant electric potential was applied at the metal-electrolyte phase boundary using Dirichlet boundary conditions.
2.4. Model Parameters
The parameters used in this study are listed in Table 1. For some parameters multiple values are given in the cited literature . The surface charge density was varied according to the physiologically relevant pH range. Mean values of the other parameters were used for the results shown below. Furthermore, sensitivity studies were conducted for , and in order to estimate the influence of the respective parameter on the results (see supplementary material). The electric potential at the titanium-TiO2 boundary and the potential in the TiO2 layer in the flat band case are calculated using (5) and (7), respectively, and amount to V and V.
3. Results and Discussion
3.1. The Metal-Electrolyte Interface
Figure 4(a) shows a schematic representation of the topographically nanostructured metal-electrolyte interface. The position-dependent electric field variation in the NaCl electrolyte is compensated within a few nanometers distance from the surface (Debye length < 1 nm) because of strong ionic shielding. Therefore, the field strength was determined at the concave corner, the planar region, and the convex edge in 0.3 nm distance from the solid-liquid phase boundary (Figure 4(b)). Compared to the position near the planar surface (II), the electric field strength is increased at the concave corner (I) and decreased at the convex edge (III) by approximately 5%. This relatively small variation can also be attributed to the ionic space charge in the electrolyte. A comparative simulation with a 0.01 M NaCl electrolyte solution showed a 15% increase and a 20% decrease compared to the planar region at the convexly and the concavely curved boundaries, respectively. The consideration of a grooved nanostructure is another reason for the slight electric field variation, because the surface curvature only occurs in two dimensions. We expect a more strongly pronounced influence of topography on the local electric field distribution at 3D structures, for instance, pits and spikes.
(a)
(b)
3.2. The Titanium-TiO2-Electrolyte Interface
Figure 5(a) shows the simulation box of the topographically nanostructured titanium-electrolyte interface. The system consists of the bulk titanium which is covered by a 5 nm thick TiO2 passivation layer and the adjacent NaCl electrolyte solution. The corresponding distribution of the electric field strength allowing for the semiconducting passivation layer is depicted in Figure 5(b). The range of the electric field in the semiconductor is considerably larger than in the electrolyte because of the comparatively low charge density. Similar to the metal-electrolyte interface, the strong decrease of the electric field strength in the electrolyte occurs within a few nanometers distance from the solid-liquid phase because of the ionic shielding. Therefore, the concavely and the convexly curved area of the interface are depicted greatly magnified in Figures 5(c)-5(d) and Figures 5(e)-5(f), respectively. It is apparent from a comparison of Figures 5(d) and 5(f) that the electric field distribution in close proximity to the TiO2-electrolyte phase boundary is inverted in comparison to the pure metal-electrolyte system (Figure 4(b)): the electric field at the concave corner is significantly stronger than at the convex edge.
Values of the electric field strength near the curved and the planar boundary elements in 0.3 nm distance from the solid-liquid phase boundary are given in Figure 6 for three pH values. At pH 5 the electric field strength at the concavely and the convexly curved boundaries is approximately increased by 10% and decreased by 20%, respectively, compared to the planar region. The inversion is maintained over the entire considered pH range and it is most strongly pronounced near the physiological pH value, where an increase of 20% and a decrease of 60% are observed at the concave corner and the convex edge, respectively.
In order to understand the behavior of the electric field described above, the surface equipotential lines at the convex edges of a metal without a passivation layer and of passivated titanium are schematically depicted in Figure 7. The equipotential line at the metal surface strictly follows the geometry (Figures 7(a)-7(b)); that is, the surface potential is constant. This potential distribution results in increased electric field strengths at convex edges because of greater field line densities. Regarding the semiconducting passivation layer of titanium, this typically metallic characteristic changes: the equipotential line of the TiO2 surface cannot follow the geometry. The surface potential of the TiO2 film therefore depends on the curvature of the interface (Figures 7(c)-7(d)) and in case of the convex edge this leads to a locally decreased electric potential.
Figure 8 illustrates how this altered electric potential distribution affects the electric field strengths near the topographically nanostructured titanium-electrolyte interface by means of cut lines at the curved and planar regions of the interface (Figure 8(a)). The courses of the potential and the corresponding field strength along these cut lines are depicted in Figures 8(b) and 8(c), respectively. In the concave corner (black cut line) the potential in the TiO2 decreases less strongly compared to the planar region (red cut line), whereas it decreases more strongly near the convex edge (green cut line). The electric field strengths along the cut lines according to the courses of the electric potential are depicted in Figure 8(c). As mentioned above, considerable differences of the local electric field strength in the electrolyte only occur very close to the interface. The locally varying electric potential induces varyingly strong ionic shielding, and differences are compensated in approximately 2 nm distance from the solid surface.
Recently, a number of studies concerning the adsorption behavior of biomolecules on both stochastically and regularly topographically nanostructured biomaterial surfaces have reported a significantly increased amount of adsorbed proteins in comparison to planar surfaces [3–6]. Moreover, spatially resolved measurements by means of scanning force-microscopic methods facilitated the identification of preferential adsorption sites at concave regions of the nanotopographies [3, 6]. Our simulations predict the highest electric field strengths near concave regions of a passivated titanium surface. This result suggests that the increased amount of adsorbed biomolecules in these regions, among others, may be caused by the increased electric field strengths.
4. Conclusion
In this study, the influence of the air-formed TiO2 passivation layer on the electric field distribution near a topographically nanostructured titanium-electrolyte interface was investigated in a finite element-based simulation. Compared to a planar region of the interface, increased and decreased electric field strengths were determined near the concave and convex regions, respectively, allowing for the semiconducting film. This constitutes an inversion of the electric field strength distribution near the reference metal-electrolyte interface, where the highest electric field strengths occur near the convexly curved region. Although the absolute values of the field strength are altered by the pH-dependent surface charge, particularly at the convex edge, the inversion is maintained over the entire considered pH range. Therefore, the band bending in the semiconducting TiO2 layer is regarded as the main factor determining the characteristic near-surface electric field strength distribution of the passivated titanium-electrolyte interface. Our results may provide a possible explanation for the increased accumulation of proteins at concave regions of the interface as it was observed in a number of experimental and computational studies on the influence of the electrostatic interaction on protein adsorption at topographical nanostructures.
Acknowledgment
Funding from the Deutsche Forschungsgemeinschaft DFG (Graduate School “Welisa” 1505/2 and Project BE 2362/2-2) is gratefully acknowledged.
Supplementary Materials
Supplementary Figure S1: Convergence of the relative error (a posteriori) of the electric field strength in the electrolyte at three different positions in 0.3 nm distance from the semiconductor-electrolyte boundary (d = 5 nm, r = 3 nm). The mesh was refined until the deviation of the electric field norm in the planar region was below 1% resulting in approximately 3.2·105 triangular mesh elements.
Supplementary Figure S2: (a) 2D mesh distribution of the passivated titanium nanotopography (d = 5 nm, r = 3 nm, no mesh refinement): 1259 triangular mesh elements, maximum element size 3.53·10-9 m, maximum element growth rate 1.3, resolution of curvature 0.3, resolution of narrow regions 1.0. Mesh quality: minimum element quality 0.682, average element quality 0.939. (b) Four refinements of the mesh result in 322304 triangular elements. In order to illustrate the mesh distribution, the concave and the convex area of the interface are depicted in detail in (c) and (d), respectively.
Supplementary Figure S3: Influence of the passivation layer permittivity εTiO2 on the electric field strengths in the electrolyte at three different positions in 0.3 nm distance from the semiconductor-electrolyte boundary (d = 5 nm, r = 3 nm, ND = 1.5·1020 cm-3, ΦBody = 450 mVSCE).
Supplementary Figure S4: Influence of the donor density in the passivation layer ND on the electric field strengths in the electrolyte at three different positions in 0.3 nm distance from the semiconductor-electrolyte boundary (d = 5 nm, r = 3 nm, εrTiO2 = 78, ΦBody = 450 mVSCE).
Supplementary Figure S5: Influence of the reference electric potential in the electrolyte solution Φbody on the electric field strengths in the electrolyte at three different positions in 0.3 nm distance from the semiconductor-electrolyte boundary (d = 5 nm, r = 3 nm, εrTiO2 = 78, ND = 1.5·1020 cm-3).
Supplementary Figure S6: Influence of the passivation layer thickness d on the electric field strengths in the electrolyte at three different positions in 0.3 nm distance from the semiconductor-electrolyte boundary (r = 3 nm, εrTiO2 = 78, ND = 1.5·1020 cm-3, ΦBody = 450 mVSCE).
Supplementary Figure S7: Influence of the interface radius of curvature r on the electric field strengths in the electrolyte at three different positions in 0.3 nm distance from the semiconductor-electrolyte boundary (d = 5 nm, εrTiO2 = 78, ND = 1.5·1020 cm-3, ΦBody = 450 mVSCE).