Oxidation kinetics of titanium thin films in model physiologic environments

Effectiveness of Direct Laser Interference Patterning and Peptide Immobilization on Endothelial Cell Migration for Cardio-Vascular Applications: An In Vitro Study

Endothelial coverage of an exposed cardiovascular stent surface leads to the occurrence of restenosis and late-stent thrombosis several months after implantation. To overcome this difficulty, modification of stent surfaces with topographical or biochemical features may be performed to increase endothelial cells’ (ECs) adhesion and/or migration. This work combines both strategies on cobalt-chromium (CoCr) alloy and studies the potential synergistic effect of linear patterned surfaces that are obtained by direct laser interference patterning (DLIP), coupled with the use of Arg-Gly-Asp (RGD) and Tyr-Ile-Gly-Ser-Arg (YIGSR) peptides. An extensive characterization of the modified surfaces was performed by using AFM, XPS, surface charge, electrochemical analysis and fluorescent methods. The biological response was studied in terms of EC adhesion, migration and proliferation assays. CoCr surfaces were successfully patterned with a periodicity of 10 µm and two different depths, D (≈79 and 762 nm). RGD and YIGSR were immobilized on the surfaces by CPTES silanization. Early EC adhesion was increased on the peptide-functionalized surfaces, especially for YIGSR compared to RGD. High-depth patterns generated 80% of ECs’ alignment within the topographical lines and enhanced EC migration. It is noteworthy that the combined use of the two strategies synergistically accelerated the ECs’ migration and proliferation, proving the potential of this strategy to enhance stent endothelialization.

Enhancement of bioactivity on medical polymer surface using high power impulse magnetron sputtered titanium dioxide film

This study utilizes a novel technique, high power impulse magnetron sputtering (HIPIMS), which provides a higher ionization rate and ion bombardment energy than direct current magnetron sputtering (DCMS), to deposit high osteoblast compatible titanium dioxide (TiO2) coatings with anatase (A-TiO2) and rutile (R-TiO2) phases onto the biomedical polyetheretherketone (PEEK) polymer substrates at low temperature. The adhesions of TiO2 coatings that were fabricated using HIPIMS and DCMS were compared. The in vitro biocompatibility of these coatings was confirmed. The results reveal that HIPIMS can be used to prepare crystallinic columnar A-TiO2 and R-TiO2 coatings on PEEK substrate if the ratio of oxygen to argon is properly controlled. According to a tape adhesion test, the HIPIMS-TiO2 coatings had an adhesion grade of 5B even after they were immersed in simulated body fluid (SBF) environments for 28days. Scratch tests proved that HIPIMS-TiO2 coatings undergo cohesive failure. These results demonstrate that the adhesive force between HIPIMS-TiO2 coating/PEEK is stronger than that between DCMS-TiO2 coating/PEEK. After a long period (28days) of immersion in SBF, a bone-like crystallinic hydroxyapatite layer with a corresponding Ca/P stoichiometry was formed on both HIPIMS-TiO2. The osteoblast compatibility of HIPIMS-TiO2 exceeded that of the bare PEEK substrate. It is also noticeable that the R-TiO2 performed better in vitro than the A-TiO2 due to the formation of many negatively charged hydroxyl groups (-OH(-)) groups on R-TiO2 (110) surface. In summary, the HIPIMS-TiO2 coatings satisfied the requirements for osseointegration, suggesting the possibility of using HIPIMS to modify the PEEK surface with TiO2 for spinal implants.

In situ scanning tunneling microscopy study of Ca-modified rutile TiO2(110) in bulk water

Despite the rising technological interest in the use of calcium-modified TiO2 surfaces in biomedical implants, the Ca/TiO2 interface has not been studied in an aqueous environment. This investigation is the first report on the use of in situ scanning tunneling microscopy (STM) to study calcium-modified rutile TiO2(110) surfaces immersed in high purity water. The TiO2 surface was prepared under ultrahigh vacuum (UHV) with repeated sputtering/annealing cycles. Low energy electron diffraction (LEED) analysis shows a pattern typical for the surface segregation of calcium, which is present as an impurity on the TiO2 bulk. In situ STM images of the surface in bulk water exhibit one-dimensional rows of segregated calcium regularly aligned with the [001] crystal direction. The in situ-characterized morphology and structure of this Ca-modified TiO2 surface are discussed and compared with UHV-STM results from the literature. Prolonged immersion (two days) in the liquid leads to degradation of the overlayer, resulting in a disordered surface. X-ray photoelectron spectroscopy, performed after immersion in water, confirms the presence of calcium.

Novel peptide-based platform for the dual presentation of biologically active peptide motifs on biomaterials

Biofunctionalization of metallic materials with cell adhesive molecules derived from the extracellular matrix is a feasible approach to improve cell-material interactions and enhance the biointegration of implant materials (e.g., osseointegration of bone implants). However, classical biomimetic strategies may prove insufficient to elicit complex and multiple biological signals required in the processes of tissue regeneration. Thus, newer strategies are focusing on installing multifunctionality on biomaterials. In this work, we introduce a novel peptide-based divalent platform with the capacity to simultaneously present distinct bioactive peptide motifs in a chemically controlled fashion. As a proof of concept, the integrin-binding sequences RGD and PHSRN were selected and introduced in the platform. The biofunctionalization of titanium with this platform showed a positive trend towards increased numbers of cell attachment, and statistically higher values of spreading and proliferation of osteoblast-like cells compared to control noncoated samples. Moreover, it displayed statistically comparable or improved cell responses compared to samples coated with the single peptides or with an equimolar mixture of the two motifs. Osteoblast-like cells produced higher levels of alkaline phosphatase on surfaces functionalized with the platform than on control titanium; however, these values were not statistically significant. This study demonstrates that these peptidic structures are versatile tools to convey multiple biofunctionality to biomaterials in a chemically defined manner.

Bone response to surface-modified titanium implants: studies on the early tissue response to implants with different surface characteristics

In a series of experimental studies, the bone formation around systematically modified titanium implants is analyzed. In the present study, three different surface modifications were prepared and evaluated. Glow-discharge cleaning and oxidizing resulted in a highly stoichiometric TiO₂ surface, while a glow-discharge treatment in nitrogen gas resulted in implants with essentially a surface of titanium nitride, covered with a very thin titanium oxide. Finally, hydrogen peroxide treatment of implants resulted in an almost stoichiometric TiO₂, rich in hydroxyl groups on the surface. Machined commercially pure titanium implants served as controls. Scanning Auger Electron Spectroscopy, Scanning Electron Microscopy, and Atomic Force Microscopy revealed no significant differences in oxide thickness or surface roughness parameters, but differences in the surface chemical composition and apparent topography were observed. After surface preparation, the implants were inserted in cortical bone of rabbits and evaluated after 1, 3, and 6 weeks. Light microscopic evaluation of the tissue response showed that all implants were in contact with bone and had a large proportion of newly formed bone within the threads after 6 weeks. There were no morphological differences between the four groups. Our study shows that a high degree of bone contact and bone formation can be achieved with titanium implants of different surface composition and topography.

In vivo evaluation of titanium oxide and hydroxyapatite as an artificial cornea skirt

Keratoprosthetic devices are subject to chronic inflammatory, pathological processes and the external environment that affect their stability and biocompatibility with the ocular surface and adjacent ocular tissues. We compared the corrosion resistance property and tissue-implant reaction of titanium oxide (TiO(2)) with hydroxyapatite (HA) in artificial tear fluid and a rabbit skin implantation model. The dissolution properties of the implant surfaces were evaluated with scanning electronic microscope (SEM) and atomic force microscope (AFM). Tissue inflammatory reactions were evaluated by Hematoxylin & Eosin staining, avidin biotin peroxidase complex (ABC) immunoassay and immunofluorescence. SEM and AFM images showed that there was less pitting corrosion on the surface of TiO(2) implants compared with HA. TiO(2) and HA exhibited a similar pattern of foreign body capsule formation and inflammatory cellular responses. The Collagen I/Collagen III ratio of the TiO(2) capsule was higher than that of the HA capsule. TiO(2) implants possess a high corrosion resistance property both in vitro and in vivo and the inflammatory cellular response to TiO(2) is similar to HA. With regards to corrosion resistance and inflammatory tissue responses, TiO(2) appears to be a promising material for keratoprosthetic skirt devices.

Fabrication of hydroxyapatite ultra-thin layer on gold surface and its application for quartz crystal microbalance technique

We present a method for coating gold quartz crystal microbalance with dissipation (QCM-D) sensor with ultra-thin layer of hydroxyapatite nanocrystals evenly covering and tightly bound to the surface. The hydroxyapatite layer shows a plate-like morphology and less than 20 nm in thickness. The hydroxyapatite sensor operated in liquid with high stability and sensitivity. The in-situ adsorption mechanism and conformational change of fibrinogen on gold, titanium and hydroxyapatite surfaces were investigated by QCM-D technique and Fourier-transform infrared spectroscopy. The change of secondary structures of fibrinogen adsorbed on the surfaces depended on the adsorbed amounts of protein. The secondary structure of fibrinogen adsorbed on the surfaces changes with increasing coverage. This is explained by repulsion among fibrinogens, affecting water structure and thus the strength of fibrinogen interactions on the surface. The study indicates that the hydroxyapatite sensor is applicable for qualitative and conformational analysis of protein adsorption.

In situ imaging and impedance measurements of titanium surfaces using AFM and SPIS

Surfaces of commercially pure titanium and titanium, 6-aluminum, 4-vanadium were subjected to simultaneous polarization/impedance testing and in situ electrochemical atomic force microscopy imaging to evaluate how the structure and properties of the passive oxide film is affected by varying potential and hydration. Current transients were acquired via a step polarization impedance spectroscopy technique: the voltage was stepped between -1 and 1 V in 50 mV increments, while current transients and surface morphology were digitally recorded. Numerical Laplace transformation applied to the current transient data provided frequency-dependent admittance (impedance(-1)). Simultaneous AFM imaging of dry surfaces, initially hydrated surfaces, and surfaces immersed and changing with potential revealed that all sample surfaces were covered with protective titanium oxide domes that grew in area and coalesced due to hydration and as a function of increasing applied voltage and time. Reversal of dome growth did not occur upon voltage reduction, while impedance behavior was quasi-reversible, suggesting independence between structural and electrical properties. Oxide growth appeared to occur in part by lateral spreading and overgrowth of domes at the oxide-solution interface. Interfacial impedance data reflect oxide passivity and n-type semiconductor behavior. Non-linear Mott-Schottky fits specified multi-layer donor concentrations between 10(18) and 10(19)cm(-3), depending on the surface.

Effect of hydrogen peroxide on titanium surfaces:In situ imaging and step-polarization impedance spectroscopy of commercially pure titanium and titanium, 6-aluminum, 4-vanadium

To analyze titanium's response to representative surgical wound environments, a study was conducted on commercially pure titanium (CPTi) and titanium, 6-aluminum, 4-vanadium (Ti-6Al-4V) exposed to phosphate-buffered saline (PBS) with 30 mM of hydrogen peroxide (H(2)O(2)) added. The study was characterized by simultaneous electrochemical atomic force microscopy (EC AFM) and step-polarization impedance spectroscopy (SPIS). Surfaces were covered with protective oxide domes that indicated topography changes with potential and time of immersion. Less oxide dome coarsening was noted on surfaces treated with PBS containing H(2)O(2) than on surfaces exposed to pure PBS. Electrical data deduced from current transients collected while stepping voltage between 0 V and 1 V indicated that charge transfer in hydrogen peroxide solutions was an order of magnitude larger than it was in pure PBS. Oxide (early) resistances of CPTi samples were higher than were Ti-6Al-4V oxide resistances in both types of solutions, but CPTi oxide resistance was lower in the hydrogen peroxide solution compared to pure PBS. Capacitance data suggest that CPTi oxide films thicken in hydrogen peroxide solution more than they do in pure PBS. Differences in electrical properties between CPTi and Ti-6Al-4V surfaces suggest that CPTi, but not Ti-6Al-4V, has catalytic activity on H(2)O(2) and that the catalytic activity of CPTi oxide affects its ability to grow TiO(2). Differences in electrical properties are related to catalytic and oxidative mechanisms that take place directly on the titanium oxide surface and in wound environments. The study provides a foundation and theoretic basis for the porous oxide model on commercially pure titanium exposed to hydrogen peroxide.

Peptide-modified p(AAm-co-EG/AAc) IPNs grafted to bulk titanium modulate osteoblast behavior in vitro

Interpenetrating polymer networks (IPNs) of poly(acrylamide-co-ethylene glycol/acrylic acid) (p(AAm-co-EG/AAc) applied to model surfaces prevent protein adsorption and cell adhesion. Subsequently, IPN surfaces functionalized with the RGD cell-binding domain from rat bone sialoprotein (BSP) modulated bone cell adhesion, proliferation, and matrix mineralization. The objective of this study was to utilize the same biomimetic modification strategy to produce functionally similar p(AAm-co-EG/AAc) IPNs on clinically relevant titanium surfaces. Contact angle goniometry and X-ray photoelectron spectroscopy (XPS) data were consistent with the presence of the intended surface modifications. Cellular response was gauged by challenging the surfaces with primary rat calvarial osteoblast (RCO) surfaces in serum-containing media. IPN modified titanium and negative control (RGE-IPN) surfaces inhibit cell adhesion and proliferation, while RGD-modified IPNs on titanium supported osteoblast attachment and spreading. Furthermore, the latter surfaces supported significant mineralization despite exhibiting lower levels of proliferation than positive control surfaces. These results suggest that with the appropriate optimization, this approach may be practical for surface engineering of osseous implants.

Effect of surface chemistries and characteristics of Ti6Al4V on the Ca and P adsorption and ion dissolution in Hank's ethylene diamine tetra-acetic acid solution

This study examined the influence of chemistries and surface characteristics of Ti6Al4V on the adsorption of Ca and P species and ion dissolution behavior of the material exposed in Hank's solution with 8.0 mM ethylene diamine tetra-acetic acid at 37 degrees C. The variation of chemistries of the alloy and nano-surface characteristics (chemistries of nano-surface oxides, amphoteric OH group adsorbed on oxides, and oxide thickness) was effected by surface modification and three passivation methods (34% nitric acid passivation. 400 degrees C heated in air, and aged in 100 degrees C water). X-ray photoelectron spectroscopy and Auger electron spectroscopy were used for surface analyses. The chemistries of nano-surface oxides in a range studied should not change the capability of Ca and P adsorption. Nor is the capability affected significantly by amphoteric OH group and oxide thickness. However, passivations influence the surface oxide thickness and the early stage ion dissolution rate of the alloy. The rate-limiting step of the rate can be best explained by metal-ion transport through the oxide film, rather than hydrolysis of the film. Variation of the chemistries of titanium alloy alters the electromotive force potential of the metal, thereby affecting the corrosion and ion dissolution rate.

In vitro bioactivity of aerosol-gel deposited TiO(2) thin coatings

Bioactive, pure, and Ca- or P-doped TiO(2) thin coatings on Ti metal and Si wafers were prepared by the aerosol-gel technique. The coatings were characterized by X-ray photoelectron spectroscopy, atomic force microscopy, Fourier transform infrared spectroscopy, and scanning electron microscopy. Bioactivity was determined in vitro in a simulated body fluid and was shown to be fully comparable to sol gel-derived TiO(2) coatings prepared by dip-coating. However, the formation rate of carbonate containing apatite decreased with increasing dopant concentration, which was related to changes in chemical composition and topology of the coatings.

Laminin-5 coating enhances epithelial cell attachment, spreading, and hemidesmosome assembly on Ti-6A1-4V implant material in vitro

Enhancement of epithelial cell attachment to laminin-5-coated titanium alloy (Ti-6Al-4V) implant material was evaluated in vitro. Protein analysis showed that Ti-6Al-4V has a high affinity for laminin-5 and adsorbed significantly more laminin-5 than laminin-1. DNA analysis showed that laminin-5 enhanced attachment of normal human epidermal keratinocytes (NHEK) to Ti-6Al-4V significantly more than did laminin-1 or uncoated controls. The effect of passivation on laminin-5 adsorption and activity on Ti-6Al-4V also was evaluated. Passivation had no significant effect on the amount of protein adsorbed; however, AFM, ESCA, and ToF-SIMS analyses suggested that passivation affects the conformation of adsorbed laminin-5. Although laminin-5 coating significantly enhanced rapid attachment of epithelial cells to both passivated and unpassivated Ti-6Al-4V, surface area measurements showed that cells spread on laminin-5-coated passivated Ti-6Al-4V covered a significantly larger surface area than cells spread on laminin-5-coated unpassivated samples. TEM analysis showed that cells formed significantly more hemidesmosomes on the surface of laminin-5 coated passivated than on the surface of laminin-5 coated unpassivated titanium alloy. The enhancement of rapid cell attachment, spreading, and hemidesmosome assembly on laminin-5-coated passivated samples may reflect better integration between epithelial cells and titanium alloy and thus may be predictive of long-term implant stability.

Surface analysis of human plasma fibronectin adsorbed to commercially pure titanium materials

Protein binding on metallic implant surfaces, such as titanium, is governed by the physico-chemical nature of the metallic surface. Human plasma fibronectin (HPF) is an important matrix glycoprotein that mediates cell and protein attachment to each other or to the extracellular matrix present during wound healing. The objective of this study was to investigate the adsorption of HPF onto polished commercially pure titanium (cpTi) by using atomic force microscopy (AFM) and electron spectroscopy for chemical analysis (ESCA) and to measure the resultant surface contact angle before and after HPF binding. Two types of cpTi disks, one highly polished in our laboratory (HSS) and one commercially prepared (31), were reacted with HPF solutions of varying concentrations (1 microg/mL-10 ng/mL). ESCA survey spectra of samples coated with 1 microg/mL of fibronectin showed an increase in organic nitrogen and carbon compared with uncoated controls. Contact angle measurements of HSS and 31 cpTi disks showed no significant difference in average contact angle (36.3 degrees +/- 3.5 and 39.1 degrees +/- 3.1) despite differences in local root mean square (RMS) surface roughness (4.45 +/- 0.46 nm and 22.37 +/- 4.17 nm) as measured by AFM. Images obtained by AFM showed that 31 specimens were more irregular, with large parallel polishing grooves. Adsorbed HPF appeared in a globular form with an average length of 16.5 +/- 1.0 nm, a height of 2.5 +/- 0.5 nm, and a width of 9.6 +/- 1.2 nm. Fibronectin coating on both HSS and 31 cpTi specimens resulted in a significant increase in hydrophobicity compared to uncoated specimens. These results indicate the significance of HPF on cpTi and may explain how cpTi implants function in situ.

Effect of surface treatment on unalloyed titanium implants: spectroscopic analyses

Surgical implant finishing and sterilization procedures were investigated to determine surface characteristics of unalloyed titanium (Ti). All specimens initially were cleaned with phosphoric acid and divided into five groups for comparisons of different surface treatments (C = cleaned as above, no further treatment; CP = C and passivated in nitric acid; CPS = CP and dry-heat sterilized; CPSS = CPS and resterilized; CS = C and dry-heat sterilized). Auger (AES), X-ray photoelectron (XPS), and Raman spectroscopic methods were used to examine surface compositions. The surface oxides formed by all treatments primarily were TiO2, with some Ti2O3 and possibly TiO. Significant concentrations of carbonaceous substances also were observed. The cleaning procedure alone resulted in residual phosphorus, primarily as phosphate groups along with some hydrogen phosphates. A higher percentage of physisorbed water appeared to be associated with the phosphorus. Passivation (with HNO3) alone removed phosphorus from the surface; specimens sterilized without prior passivation showed the thickest oxide and phosphorus profiles, suggesting that passivation alters the oxide characteristics either directly by altering the oxide structure or indirectly by removing moieties that alter the oxide. Raman spectroscopy showed no crystalline order in the oxide. Carbon, oxygen, phosphorus, and nitrogen presence were found to correlate with previously determined surface energy.

Crystal structure of the surface oxide layer on titanium and its changes arising from immersion

The passivating surface oxide on titanium is one of the elements considered in the explanation of the favorable biologic response of this metal in implant applications. In the present study, transmission electron microscopy was used to identify the crystal structure and morphology of the oxide film on commercially pure titanium specimens before and after immersion in simulated physiologic fluids. The results show that the oxide layer is composed mainly of anatase and rutile, both of which are tetragonal in structure. Although the simulated physiologic fluids did not induce an observable change in the crystal structure for the immersion times investigated, the results indicate an immersion-induced change in microstructure from a fine-grained to a coarser-grained structure. The grain growth observed could be attributed to the growth of the native oxide crystals; however, it most likely results from the formation of a new oxide layer. The results also support oxide thickening as one of the processes associated with passive dissolution of titanium.