a concise review - SAGE Journals

DOI: 10.5301/ijao.5000040

Int J Artif Organs 2012; 35 ( 9 ): 629-641

REVIEW

Titanium oxide modeling and design for innovative biomedical surfaces: a concise review Luigi De Nardo1, Giuseppina Raffaini1, Edward Ebramzadeh2, Fabio Ganazzoli1 1 2

Politecnico di Milano, Department of Chemistry, Materials, and Chemical Engineering “G. Natta”, Milan - Italy UCLA / Orthopaedic, Hospital Department of Orthopaedic Surgery, David Geffen School of Medicine, JVL Orthopaedic Research Center, Los Angeles, CA - USA

Politecnico di Milano, Department of Chemistry, Materials, and Chemical Engineering “G. Natta”, Milan - Italy Politecnico di Milano, Department of Chemistry, Materials, and Chemical Engineering “G. Natta”, Milan - Italy UCLA / Orthopaedic Hospital Department of Orthopaedic Surgery, David Geffen School of Medicine, JVL Orthopaedic Research Center Los Angeles, CA - USA Politecnico di Milano, Department of Chemistry, Materials, and Chemical Engineering “G. Natta”, Milan - Italy

ABSTRACT The natural oxide layer on implantable alloys insulates the reactive underlying metal from the physiological environment, preventing substrate corrosion and device failure. This type of oxide film has had a major role in the minimization of functional failure and toxic response after implantation in the first generation biomaterials. Recent advances in theoretical, computational, and experimental surface engineering tools provide the foundation for the design of novel devices with improved performances in this regard based on conventional implantable metal alloys. An increasing number of technologies provide the possibility of tailoring chemico-physical and morphological parameters of the surface oxide layers. For some applications, such as dental implants, surface modifications result in substantial innovation and economic success. However, the selection of novel surfaces is in general based on experimental studies and has a limited theoretical and computational foundation. In this review, we offer a perspective analysis of the correlation between theoretical studies and chemical surface modification technologies, with a special emphasis on titanium oxide on Ti alloys. Theoretical approaches for the surface behavior at an atomistic level of description are presented, together with some adsorption studies on a rutile surface. The role of chemical and electrochemical surface modification technologies in modifying the TiO2 structure, morphology, and chemistry to tailor in vivo biological response is then briefly reviewed. Finally, we discuss the role of surface modeling as a powerful design tool for a new generation of implantable devices in which metal oxide surface can be tuned to yield specific biological response. KEY WORDS: Biomaterial surface modifications, Theoretical modeling, Metal oxide hydration, Surface adsorption, Implantable prostheses Accepted: September 2, 2011

The role of tio2 in the evolution of bone-contact prostheses Metal oxide as a passive protective layer Metal alloys are widely used for the fabrication of long-term implantable medical devices, with the aim of substituting or supporting biological functions (1-5). After implantation, the response to the physiological environment of metal devices is generally mediated by a thin oxide layer: these

sub-micrometric films that naturally cover alloy surfaces (6) have distinctive chemico-physical properties that depend on the bulk composition and are influenced by a combination of metallurgical and manufacturing transformation processes. In reviewing the evolution of biomaterials during the last 60 years, it is evident that the first generation of metal alloys (5) shared the common feature of biological “inertness” (7). The ability of the passivation layer to prevent corrosion processes and maintain functional characteristics

© 2012 Wichtig Editore - ISSN 0391-3988

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was the original motivation for the application of austenitic stainless steel, Co-, and Ti-based alloys in long-term implantable prostheses (2, 5). The in vivo rate of attack of the aforementioned alloys is, in general, very low (1): the compact oxide layer, acting as the main kinetic barrier to corrosion, impedes or prevents corrosion reactions from taking place (8), and results in the insulation of the reactive underlying metal (9). Metal alloys are also extensively used in partial or total joint replacement (10), which requires preservation of the mechanical stability coupled with reliable tribological performance (6). The surface features of metal oxides are thus of primary importance for friction, wear resistance, and lubrication in articulating metal surfaces. The key role of oxide layers is also demonstrated by efforts in increasing their thickness and homogeneity in order to improve corrosion resistance and tribological performance (11, 12). With the introduction of titanium alloys it became evident that the TiO2 film on Ti alloys preserves the properties of the bulk material and interacts with biological fluids and their constituents, resulting in a modulation of specific responses with the contact biological tissues (13-15). Understanding the specific details of the interactions between biological fluids and TiO2, by taking its structural specific into account, has thus become a primary goal in biomaterial interface science. Combining theoretical, computational, and experimental studies aimed at modifying and modeling TiO2 film at different length scales (Fig. 1), can help to explain the fundamentals of these processes. The ultimate goal is to design surfaces able to accelerate tissue-healing processes and improve the long-term in vivo performance of implanted prostheses.

The unique features of TiO2 surfaces in modulating bone response The modulation of surface properties via chemical composition and structure modifications represents a fundamental “playground” for biomaterial scientists. Surface engineering advancements offer the tools for improving current materials with a wide variety of chemical and physical patterns as well as allowing in vivo control of cell size, shape, spatial organization, and proliferation (16). A number of TiO2 surfaces intended for osseointegration have been developed during the last 30 years and are currently commercially available (17). For some applications, such as dental implants (18), the study of new surfaces offers a way to improve perfor630

Fig. 1 – A schematic drawing of the thickness of the surface TiO2 film (upper abscissa) affected by different surface modification techniques (above) and the length scales (lower abscissa) explicitly described by the modeling methodologies (below). On the left we show a rutile TiO2 surface film above the bulk Ti alloy, two proteins (sizes in the range of a few nm), and a cell (size in the range of a few μm) close to the surface for illustrative purposes. The surface modification techniques reported in the upper part are chemical and electrochemical surface modification technologies (ELD = Electrochemical deposition; EPD = Electrophoretic deposition). According to the box scales and positions, they can result in surface etching or coating deposition. The modeling methodologies reported in the lower part are: quantum Density Functional Methods (DFT); Molecular Mechanics (MM) and Molecular Dynamics (MD) methods; Coarse-grained methods (typically using MD or Monte Carlo techniques); Finite Element Methods (FEM), where each component of the system is treated in continuous space. Note that the atomistic (DFT, or MM & MD) and coarse-grained models often use periodic boundary conditions to model a crystalline material, either in the bulk or to describe a surface, and therefore can in practice describe macroscopic systems formed by periodically repeating microscopic units.

mance and produce devices that are clearly distinguished from “standard” products (19), allowing each manufacturer a distinctive marketing tool. Table I briefly summarizes some of these surfaces for dental implants, from major international dental manufacturers. Most of these surfaces have proven clinical efficacy (20), including the first osseointegrated surfaces produced by machining the bulk titanium implants (15).


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TABLE I - COMMON TiO2 SURFACES IN COMMERCIAL DENTAL IMPLANTS Trade Name

Company

Treatment

Topography

Chemistry

Osseotite™

BIOMET 3i

Chemical etching

Sub-micrometric roughness

TiO2

(15, 56)

Nanotite™

BIOMET 3i

Chemical etching + hydroxyapatite nanoparticles in silane layer

Sub-micrometric roughness

nanometric layer cristalline Ca/P

(15, 20, 52, 66)

SLA

ITI-Strauman

Sand Blasting Large Grit Acide etching

Macro-roughness (sand blasting) Sub-micrometric roughness (acid etching)

TiO2

(18, 20, 56, 52)

OsseoSpeed™

AstraTech

TiO2 grits blasting Acid etching

Macro-roughness (sand blasting) Sub-micrometric roughness (acid etching)

TiO2

(15, 17, 20)

TiUnite™

Nobel Biocare

ASD (MAO)

Micro-porous surface

Thick TiO2 layer

Although several surfaces designed for osseointegration have been proposed and are currently available for clinical applications (Fig. 2), their role in the early events of osseointegration processes still remain poorly understood. The reasons for this are many, but it is mainly due to the complexity of biological events along with testing procedures that have not been standardized. In fact, in several cases, in vivo animal studies and randomized controlled trials (RCTs) have not demonstrated any significant difference between different morphologies and/or surface chemical compositions (21). When dealing with complex systems like implants, it is difficult to draw conclusions about the behavior of only one part based on the overall systemic response. A basic question is whether there is a way of predicting the actual in vivo response of an implanted surface in order to effectively design novel devices able to promote an early healing process. New surface modification technologies are available, offering endless opportunities for fabricating TiO2-modified surfaces (see Fig. 1). However, in order to design a new generation of medical devices with advanced functionalities, able to promote specific biological responses, a robust design instrument is mandatory. To develop a new generation of optimal surfaces in a timeand cost-effective way, in silico approaches are emerging as powerful tools to be coupled with in vitro and in vivo evaluation. In the following, we will offer a brief introduc-

Ref.

(15, 17, 20)

tion to the main computational studies and the early results aimed at understanding the surface phenomena on TiO2 at the molecular level. We then discuss their correlation with surface modifications and experimental characterization.

TiO2 modeling as a predictive instrument for biomedical surface design One of the primary challenges to design new, improved biomedical devices with specific functionalities is to fully understand biomaterial surface properties and interface behavior at the atomic scale. Accordingly, “third-generation” (5) biomaterials are designed to stimulate specific responses at the molecular level. It is therefore crucial to study and model their interactions with adsorbed proteins in terms of the local surface chemistry and topology, because these proteins mediate the biological response and in particular the adhesion, spreading, differentiation, and proliferation of cells. Recent studies suggest that atomistic simulations are the most useful tools to design surfaces with specific properties and interface adsorption phenomena (22-24). However, understanding and predicting biological events taking place on specific TiO2 surfaces (see Fig. 1), requires detailed structural description at the atomistic level and a thorough modeling of the surface hydration. The latter


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Fig. 3 - The most stable TiO2 surface, rutile (1 1 0). Oxygen and titanium atoms are shown in dark and light gray spheres, respectively.

of larger molecules and then proteins (14). In the following, we briefly review some theoretical results about the initial surface events that may help to design TiO2 surfaces tailored for specific purposes.

Theoretical description of TiO2 surfaces and their hydration

Fig. 2 - Scanning Electron Microscopy micrographs showing different morphologies in commercial dental implants: a) Machined surface; b) Chemical Etched surface; c) Micro-Arc Oxidation (Technology similar to TiUnite™).

event may involve simple coordination of the water molecules (physisorption) or their reactive association at undercoordinated sites (chemisorption), both of which can affect the surface incorporation of small ions and the adsorption 632

In the last few decades, much theoretical work employing quantum methods has aimed to understand the structure and possible reconstruction of TiO2 surfaces in vacuo in rutile and anatase phases (25, 26) because of their importance in photocatalysis, for instance. The surface most extensively studied and thoroughly experimentally characterized is low index rutile (1 1 0) shown in Figure 3. It is the most stable among the TiO2 polymorphs, which undergoes some reconstruction compared to the bulk geometry in order to minimize the surface energy (25, 27-31). Other theoretical studies of bare TiO2 surfaces included additional low-index rutile (32, 33) and anatase surfaces (28), in good agreement with the available experimental results. The structure of some TiO2 surfaces was also studied at a finite temperature by classical molecular dynamic (MD) simulations, allowing studies of common surfaces of rutile, anatase, and brookite that are consistent with more accurate quantum results (25, 34–36). These surfaces may expose bridging (2-fold coordinated) O atoms and 5-fold coordinated Ti atoms, showing a vacancy when compared to the bulk 6-fold coordinated atoms. This is the case of rutile (1 1 0), where the exposed titanium is formally Ti3+, instead of the usual Ti4+ ion of bulk TiO2 (28). Theoretical studies have been compared to experimental results of the surface structure through low energy electron diffraction (37) and surface


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X-ray diffraction (SXRD) experiments (38) and they are in fair-to-good agreement, unlike earlier SXRD experiments (39), which were affected by a poorer surface quality and by potential experimental artefacts. Incidentally, this observation brings to light the problems that may arise with surface preparations and analyses, and their reproducibility in different laboratories. In water or in physiological fluids, the exposed TiO2 surfaces may show hydroxyl groups at the bridging O atoms (denoted as BOH groups) or as terminal groups on the 5-fold coordinated Ti atoms (TOH groups, or protonated TOH2 for undissociated water). The surface vacancies at the exposed Ti atoms affect its electronic and catalytic properties (31, 40), in fact, and may drive a reactive behavior on the part of many molecules—particularly water molecules—giving rise to surface hydroxyls. TiO2 hydration may also enhance the stability of anatase nanocrystals compared to rutile (41). Experimental studies of water interaction with TiO2 were recently reported by Henderson (26) and by Diebold (25), to mention only more recent theoretical work. Interestingly, most calculations predicted significant water dissociation on rutile (1 1 0), whereas experiments suggest molecular adsorption only, except at defect sites (25). The latter discrepancy has been recently resolved (42). A rather comprehensive theoretical and experimental study of the hydration of rutile (1 1 0) was recently reported (43) using both quantum methods and classical MD simulations, which allowed estimates of the water density near the surfaces that were consistent with the X-ray data (43). Later, a more thorough theoretical study of the hydration and protonation of rutile (1 1 0) was also carried out (40) through classical MD simulations for hydroxylated and non-hydroxylated surfaces, and showed strong hydrogen bonds of water with the BOH and in particular with TOH hydroxyls, in agreement with more recent quantum simulations (42). There have been few studies of the hydration of other TiO2 surfaces, including those of the various polymorphs. One notable exception is a comparative study by quantum methods (41) of the most common anatase and rutile surfaces, aimed at assessing the relative stability of their nanocrystals. While the water adsorption was found to be either associative or dissociative depending on the specific crystal face, the methodology also provided the surface energy and the surface tension. These suggested a somewhat larger stability for anatase nanocrystals in water than in vacuo compared to rutile.

In general, these studies mostly focused on the surface stability in water, but did not carry out a comparative analysis regarding the effect of the surface topology in the same or in different polymorphs on the distribution of the water molecules, their hydration shells, and surface mobility, which may in turn affect the adsorption of larger molecules. A preliminary account of such an ongoing study carried out by Raffaini and Ganazzoli with classical MD simulations on neutral, non-hydroxylated surfaces of rutile, anatase, and brookite has been recently published (44).

Adsorption of larger molecules on TiO2 by molecular dynamics simulations In the last few years, the physisorption of larger molecules, including mono-, di-, or tri-peptides (45-47) and lipids (48) on rutile (1 1 0) in explicit water has been addressed by classical MD simulations, mainly focusing on the coordination geometry. The adsorption of mono- and di-peptides was modeled on non-hydroxylated rutile (1 1 0) (45, 46). Quantum calculations in vacuo and classical MD simulations in water determined the preferred interaction geometry of a single aminoacid, which involved a bidentate interaction of the COO- anion with two adjacent Ti3+ atoms. Weaker interactions were obtained with chemically neutral moieties, involving only ion-dipole interactions. Further simulations were also carried out for two dipeptides with zwitterionic ends, namely Ala-Glu, with a net negative charge due to a COO- side group, and Ala-Lys, with a net positive charge due to an NH3+ side group. In both cases, the dipeptides showed strong bidentate coordination geometry onto adjacent Ti atoms through the terminal COOgroup of alanine (45). More recent studies considered different starting orientations to model the adsorption of the same dipeptides on rutile (1 1 0) in water (46). With neutral molecules, the interactions were due to either dipolar interactions of the amide groups with three neighboring Ti atoms or to intermolecular H-bonds with bridging O atoms, while the COOH group was seldom found close to the surface. On the other hand, the zwitterionic dipeptides provided a bidentate COO- coordination to two adjacent Ti atoms, consistent with the X-ray photoelectron spectroscopy data (46), while the terminal NH3+ group formed intermolecular hydrogen bonds with bridging O atoms, emphasizing the importance of charged groups. Thus, the MD


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simulations indicate that the surface interaction takes place through the COO- groups and to a lesser extent through the CONH groups at the exposed 5-fold coordinated Ti atom, with possible intermolecular H-bonds with bridging O atoms. On the other hand, the MD simulation of the adsorption of the tripeptide Arg-Gly-Asp (RGD) on rutile (1 1 0) in water (47) showed a different interaction pattern. RGD has a zero net charge due to the compensation among two positive and two negative charges, but it was found to interact with the surface mainly through the H-bonds of the guanidinium group with the bridging O atoms. The reason for this different interaction geometry of the peptides compared to the results discussed above is unclear. One possibility is that the interaction geometry on short time and space scales is affected by the presence of water molecules at vacant sites on Ti in the simulations. This effect of the surface water should be tested more thoroughly. The adsorption of lipids on TiO2 in water on rutile (1 1 0) has also been recently investigated by MD simulations (48), assuming a hydroxylated surface showing both TOH and BOH hydroxyls. The modeled lipids included a zwitterionic, an anionic, and a cationic lipidic derivative assumed to interact with TiO2 through the charged head groups. On a non-hydroxylated surface, the MD simulations indicated a strong interaction between the anionic groups and the exposed Ti atom(s), whereas a bulkier cationic group loosely interacted with the bridging O atoms. Conversely, in fully hydroxylated surfaces the Ti atoms are shielded by the terminal hydroxyls, and therefore the lipid molecules can diffuse almost freely on the surface. In addition, the MD simulations also provided information about the fluctuations of the anchored molecules and their surface diffusivity, showing the array of information that can be potentially achieved by these methods. We recently reported some further results about protein adsorption on the TiO2 polymorphs (44). An important finding from this study that relates to the present discussion is that, to some extent, the surface topology of the exposed polymorph surfaces affect protein adsorption, despite having the same surface chemistry. Though preliminary, these results show the relevance of the nanoscale surface topology to the behavior of metal oxides in a physiological environment, as implicitly pointed out in Figure 1. Accordingly, we suggest that theoretical modeling will increasingly provide a valuable tool for the design of tailored surfaces. 634

Technologies for tailoring titanium oxide on titanium alloys Although TiO2 is chemically stable (19, 49), when in contact with a physiological environment, it may drive several interface reactions. The performance of biomaterial devices depends on both their bulk and surface properties. The chemical groups exposed by biomaterials together with their surface roughness in contact with the physiological medium of interest drive the cell adhesion process. The ensuing spreading, shape, and cytoskeletal organization of cells (which affect their differentiation and proliferation) are mediated by protein adsorption. Therefore, in order to understand the biological response and to design improved biomedical devices with specific functionalities, it is increasingly important to accurately know the atomistic behavior at the interface between the biomaterial and biological environments. The modification of the surface, taking place at the titanium surface and producing a film of varying thicknesses (Fig. 1)—together with the simulations at the atomistic level discussed in the previous sections—are powerful tools to design biomedical surfaces with specific properties when in stable contact with biological tissues. Modulating the properties of TiO2 on Ti alloys can be exploited to create more effective implantable devices (50): in this sense, chemical and electrochemical surface technologies represent the most interesting tools for an easy industrial scale-up of implantable prosthesis surface modifications. This class of surface modifications has been recently reviewed in some works (15, 51, 52) and an interesting classification has been proposed for osseointegrated implant surfaces by Ehrenfest et al. (15). In the following, current chemical and electrochemical surface modification procedures related to TiO2 structuring on medical prostheses will be reviewed and correlated to the main ongoing computational activities.

Chemical modifications of structure and composition of native TiO2 Chemical surface modifications are based on chemical reactions between Ti and suitable reagent media and represent one of the most exploited technologies (53), ranging from simple decontamination procedures (14) to more complex coatings. Cleaning procedure. This is a class of treatments generally performed on Ti implants not intended for TiO2 struc-


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ture modifications. Chemisorption and physisorption of hydrocarbons generally take place when TiO2 is exposed to air, resulting in surface wettability modifications (31, 54) due to a significant reactivity with organic materials (14). These contaminants strongly affect the overall response of the implants in vivo, both on cell differentiation and growth factor production (55), and should be also taken into account when evaluating in vitro the chemico-physical response of the oxide to biomolecules. In fact, although several solvent-based decontamination processes have been described, carbon contamination always characterizes TiO2 surfaces, as shown by a significant C signal on spectroscopic analyses (53). Wet chemical etching. This is a standard procedure aimed at surface decontamination and morphology modification. The chemical stability of TiO2 limits the number of solutions that can be used for this purpose: nevertheless, this is the most popular and efficient way to modify surfaces and is attractive for large-scale manufacturing processes (50). Selected acid and alkaline solutions react with the oxide layer, mainly resulting in specific morphological and chemical modifications on several scales (50, 53, 56). Acid etching operated in proper solutions can be effectively and economically used to dissolve the protective oxide layer and to etch the metal substrate. An example of typical morphology is shown in Figure 2B. Ti alloys can be effectively etched in fluoride solutions and different acid mixtures, such as HF (57), HCl, H2SO4, malonic acid, and selected mixtures of these acids with HNO3 and H2O2. Many etching surface treatments were developed to improve osseointegration of implantable titanium devices, especially for dental applications (see, for instance, Tab. 1). The oxide and metal dissolution generally results in an increased surface roughness, contributing to improved hard tissue fixation through mechanical interlocking of the device to the bone. After the etching process, a new protective oxide film can be easily reconstituted in air or in a suitable oxidizing environment. Wet Chemical oxidation. Oxidation of TiO2 surfaces has been accomplished by using strong oxidants, with H2O2 and HNO3 used most commonly. The treatment of prostheses in HNO3 solutions is a standard passivation procedure for Ti alloys, often representing the last step in a surface preparation process. HNO3 passivation does not increase the thickness of the oxide film, but it can improve its homogeneity (58). H2O2 treatments have similar effects and represent the subject of some recent studies on in vitro experimental protein

adsorption in correlation to theoretical calculations. Sousa et al (59) have shown the kinetic behavior of Fibronectin (Fn) adsorption on TiO2 on commercially pure titanium after immersion in H2O2 (TiO2 cp in the original text) and TiO2 sputtered on Si (TiO2 sp in the original text). Fn adsorbed more, and more strongly, on TiO2 cp surfaces compared to TiO2 sp. The aggregate structure had an intermediate feature shared by some protein fibrillar assemblies at interfaces, which is believed to promote cell adhesion and cytoskeleton organization. Moreover, in a previous study (60) the kinetics of adsorption of albumin on the same surfaces demonstrated that TiO2 cp adsorbs less albumin than TiO2 sp, but the adherent molecules are more strongly attached to the TiO2 cp surface. Furthermore, competitive adsorption measurements (61) indicate that a significantly larger amount of fibronectin than human serum albumin is adsorbed on TiO2 cp surface. Sol-gel coatings. Sol-gel is a wet technology for the synthesis of inorganic materials (62) via a sequence of chemical and physical steps similar to an organic polymerization. It involves: i) inorganic hydrolysis and polycondensation in solvent (usually alcohol); ii) gelation; iii) aging; iv) drying; and v) densification and crystallization. In biomedical applications, sol-gel processes have been utilized to produce silica-derived bioactive materials in thin film and monolith preparation (63), binary (SiO2–CaO) and ternary (SiO2–CaO– P2O5) systems (64), and hydroxyapatite (HA) nanoparticles in a 3-aminopropyltriethoxysilane matrix (65). With regard to titanium oxide, thin coating preparation on different substrates, including Ti alloys, is possible. TiO2 sol-gel coatings, based on either conventional (66) or Stepwise Surface processes (67) have been the subject of several studies aimed at understanding their effects on water interaction, calcium phosphate (Ca/P) crystallization, and protein adsorption (see references (67-69) and citations therein). Novel TiO2 organic-inorganic hybrid materials were synthesized by the sol-gel method from a multicomponent solution. The bioactivity of the synthesized hybrid materials was demonstrated by the formation of a layer of hydroxyapatite on the surface of the Poly (Ether-Imide)/TiO2 samples in simulated body fluid (SBF) (70). The evidence that sol-gel TiO2 coatings show the ability to bond to bone after implantation (69) spurred subsequent studies aimed at understanding the bioreactivity of such surfaces. For instance, Advincula et al (67) showed that TiO2 sol-gel coatings demonstrate an increased capacity to initiate Ca/P nucleation. This behavior has been explained by


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the presence of Ti-OH species on sol-gel surfaces, which promote electrostatic attraction of Ca++ or hydrogen bonding with phosphates. Moreover, the presence of hydroxyl groups and a specific crystallographic surface (see the previous section on TiO2 modeling ) influences the deposition of proteins on TiO2 sol-gel coatings and their presence delays the growth of Ca/P on the surface of TiO2 coating by decreasing the re-crystallization rate of the initially formed amorphous calcium phosphate (71). Theoretical modeling of these systems, involving the competitive adsorption of proteins from a solution in the presence of different electrolytes, is challenging and has been addressed to some extent only very recently. We can confidently expect, however, that in the near future computer simulations will offer new insights about these phenomena, complementing the experimental observations and providing deeper insights.

Electrochemical modifications of structure and composition of native TiO2 Electrochemical surface modifications are based on electric polarization of a Ti alloy device in a suitable bath in three different ways (Fig. 4): i. Anodic oxidation: in an aqueous solution, a TiO2 film grows as a consequence of metal oxidation and oxygen diffusion (Fig. 4A); ii. Cathodic polarization or Electrolytic Deposition (ELD): in an aqueous solution a coating is deposited over Ti cathodically-polarized as a consequence of local pH variation, ion transport and the reduction of species (Fig. 4A); iii. Electrophoretic deposition (EPD): in a solvent-based electrochemical cell, Ti is polarized either as cathode or anode and the coating is grown as a consequence of the deposition of suspended charged powder particles by applying an electric field (Fig. 4B). Anodic polarization and anodic spark deposition. Anodic polarization produces thick, conveniently structured Ti oxide films over the surface of Ti alloys: electrode reactions in combination with electrical field-driven metal and oxygen ion diffusion lead to the formation of a thick oxide film at the anodic surface. The explanation for the electrochemical processes involved in anodization can be found in several works (72-74), showing that both metal and oxygen migration generally contribute to the charge transport (75). During anodization, anodic oxide film growth carries on as long as the electrical field is strong enough to drive ap636

Fig. 4 - Schematic of (A) electrochemical anodic and cathodic surface modifications, showing the main electrochemical reactions and gas formation; and (B) electrophoretic deposition (EPD), showing electrophoretic motion of negatively charged ceramic particles followed by coagulation of the particles to form EPD deposits.

propriate chemical species through the film, resulting in a two-stage film growth. The first phase is characterized by linear growth of the TiO2 film thickness (up to applied voltage drop of 100-160 V) that varies depending on the process conditions (52). In


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the second stage, when anodization is carried out at higher voltages, the process leads to increased gas evolution and often to sparking. Under these conditions, Micro-arc oxidation (MAO) or Anodic Spark deposition occurs (ASD). Anodization is used to produce films of increased oxide thickness, porous coatings, and selected crystallographic forms. Subsequently, morphological and structural features of oxides can be tuned over a wide range via ion insertion. An example of such a surface is presented in Figure 2C. More recently, TiO2 nanotubes have also been proposed by several authors (76). Low voltage anodization is generally used to obtain thicker (up to few hundred nm) and compact TiO2 films that result in colorization and increased corrosion resistance (11). TiO2 ion doping at low voltage anodization is a controversial issue, however: according to some studies, anodic film composition is constituted mainly by TiO2 (77). However, other studies, based on XPS analysis or Rutherford backscattering spectrometry report the insertion of electrolyte elements in the oxide film and the presence of a certain amount of hydrogen in the very outer oxide layer (73, 74, 77-79). Ti6Al4V behaves very similarly to commercially pure Ti, although the concentration of Al relative to titanium in the oxide has been observed to be higher than in the metal and depletion of vanadium in the outermost layers has also been reported (51). Diffraction studies showed the oxide films to be either totally amorphous or in some cases partially crystalline. The most commonly observed phases are anatase and rutile (both tetragonal), but Ti3O5 and brookite (orthorhombic or tetragonal) have also been detected (78). This aspect is of particular interest because in a study conducted by our research group, the anatase phase has been demonstrated to play an important role in diminishing bacterial colonization on surfaces (80). Micro-Arc Oxidation occurs when anodization is carried on at higher voltages, promoting local melting and re-crystallization processes of the oxide film, generally resulting in the incorporation of ionic species with a concentration gradient along the oxide. The electrolytic bath composition affects the final chemical composition of the grown oxide (81); simultaneously, sparks generate a morphology with micro-porosity (52). Moreover, TiO2 significantly grows in thickness, improving the corrosion and ion- release resistance properties (81). This process can be effectively used for titanium modification and coating preparation for biomedical applications, taking advantage of both the mor-

phological and chemical results of this kind of treatment. For instance, using MAO it is possible to incorporate Ca, P, and many other ions into the anodically grown oxide film, in order to promote the osseointegration process. The addition of silver ions could also be considered in order to provide antibacterial properties to the metal surface. These films show controlled porosity and are highly adherent to the substrate (81). MAO-based treatments have been studied recently and applied to dental prostheses to improve osseointegration and fixation of dental implants. The biological response of MAO-based treatments has shown very encouraging results (82) and clinical applications are currently in use. Electrophoretic deposition (EPD). Electrophoretic deposition allows the processing of bulk materials and the deposition of coating from almost any material class, including metals, polymers, and ceramics (83-85). The mechanism of electrophoretic deposition involves two steps (86): i. Electrophoresis in which charged particles in a suitable solvent move toward an electrode of opposite charge (87): any solid available in fine powder or colloidal suspension can thus be processed using this technique (88); ii. Deposition on the working electrode, due to the motion of charged particles and their local accumulation. Coating formation is achieved via particle coagulation (89). The basic difference between EPD and electrolytic deposition/polarization is that the former is based on the suspension of particles in a solvent whereas the latter is based on a solution of ionic species, which in general undergo redox reactions that lead to coating formation. EPD appears of particular interest in surface modifications of materials for biomedical applications (90), especially for Ti alloys, because it allows the deposition of either inorganic or hybrid coatings on complex geometries with demonstrated high purity. Although this type of deposition has not been completely developed for biomedical applications, promising results have been obtained (83-85). Moskalewicz et al (86), for instance, proposed the use of EPD as a tool for specific TiO2 crystal deposition. Starting from commercial TiO2 nanoparticles with anatase/rutile fraction of 0.2, an oxide coating with a ratio of approximately 0.1 was deposited. In terms of the treated geometries and the relative ratios of crystallographic species, this shows the potential of a simple tuning process of the TiO2 coating structure starting from specific particle compositions.


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OUTLOOK New applications for materials in life sciences are providing true challenges in terms of scope and complexity that have perhaps not yet been properly addressed by existing systems. Advances in cell biology, chemistry, and materials science have enabled a new generation of materials to be produced (91) with multiple and combined functions inspired by biology (16, 92, 93). Such new biomimetic materials have been specifically developed by applying modern methods of material science design (94). Atomistic computer simulations and/or theoretical modeling are increasingly providing powerful access keys to a fuller understanding of surface reactions and interface phenomena. This approach has already been applied at both the quantum and classical levels, typically by molecular dynamics simulations. Such theoretical approaches can be also successfully applied to model TiO2 interface phenomena. In this context, we can expect that increasingly realistic systems will be modeled, allowing the analysis of the explicit solvent in the presence of appropriate electrolytes. It will also enable increasingly sophisticated TiO2 surfaces, in terms of specific crystallographic planes and experimental surface charge densities at the pH of interest. Furthermore, the simultaneous presence of more crystalline phases (in particular rutile and anatase) poses further theoretical problems for the phenomena that may take place at the grain boundary and at their common interface with the outer environment. In this context, recent MD simulations (95) have begun to model the rutile-anatase interfaces formed by the common surface of these polymorphs (86). These simulations may provide a basis for understanding the influence of the interface on the mechanical and electronic properties of these polycrystalline materials, and on the surface adsorption phenomena at the grain boundary.

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At the same time, several surface modification technologies are emerging as powerful tools for tailoring TiO2 in terms of the chosen polymorphs and surfaces, possibly with ion inclusion. Among them, chemical and electrochemical techniques have been widely applied in TiO2 chemistry and crystallography modifications on titanium alloys, aimed at promoting specific responses when they are in contact with a biological environment. Several of these treatments are currently used in biomedical device industries (96), and increasing demand, sometimes commercially-driven, pushes their application (97). However, a better understanding of material/environment interaction fundamentals could help to exploit these technologies and the resulting surfaces in a more specific and appropriate way. In conclusion, we confidently expect that an increasingly wider use of theoretical and simulation methodologies will integrate and complement the experimental approaches. These methodologies will provide a theoretical foundation for the performance-related phenomena observed in native and modified surfaces, eventually constituting a major step in the design of suitably tailored biomaterials. Financial Support: L. De Nardo and G. Raffaini are grateful for funding from MIUR - FIRB Futuro in Ricerca (Surface-Associated Selective Transfection - SAST, RBFR08XH0H). L. De Nardo also thanks the Politecnico di Milano (Grant: 5 per Mille Junior) and the Italian Institute of Technology (IIT) for financial support. Conflict of Interest Statement: None.

Address for correspondence: Luigi De Nardo Politecnico di Milano Department of Chemistry, Materials and Chemical Engineering “G. Natta” Piazza L. da Vinci 32 20133, Milano, Italy e-mail: [email protected]

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