Ti6Al4V Discs Research Articles

Friction and Wear Properties of WC-Co Cemented Carbide Sliding against Ti6Al4V Alloy in Nitrogen Gas

Titanium alloys are known for their strong chemical reactivity with surrounding gas due to their high chemical affinity, especially in dry machining. But it is very difficult to study the influence of surrounding gas on the tool-workpiece interface because of the machining processes’ complexity. In this paper, rotating pin-on-disc friction tests have been carried out at room temperature in ambient air and nitrogen gas to investigate the friction and wear behavior of WC-Co cemented carbide sliding against Ti6Al4V alloy. Scanning electron microscope (SEM) and Energy dispersive x-ray spectroscopy (EDX) have been used to examine the worn surface of the WC-Co pin and Ti6Al4V disc. The result shows that, compared to air, nitrogen gas brings a slight decrease in coefficient of friction, but a significant deduction in wear of the pin and disc. The SEM observation and EDX analysis indicate a distinct difference in wear mechanism between the pin and disc. Severe grooved wear, squeezing, adhering and tearing interactions are the main mechanisms causing the extensive wear of Ti6Al4V disc. Abrasion, adhesion and “pulling out” are the main mechanisms resulting in the wear of WC-Co pin.

Biological response of laser macrostructured and oxidized titanium alloy: An in vitro and in vivo study

To assess both the in vitro and in vivo biological response of a laser modified surface in an integrated manner. A combined innovative approach applies lasers to macrostructure as well as to oxidize the surface of titanium alloy implants. A Nd:YAG marking and ArF excimer lasers were used for macrostructuring and UV-oxidizing the surface of Ti6Al4V discs, respectively. Human fetal osteoblastic cell culture and a sheep tibia model were used to assess the cell response and the osseogeneration capability of as-machined, laser macrostructured and laser macrostructured and oxidized surfaces. In vitro: Laser macrostructuration alone did not promote cell response. Cellular proliferation was enhanced by the additional UV laser oxidation. In vivo: A greater significant percentage of bone-implant contact was obtained for both laser treated surfaces compared to machine-turned control samples, three months after implantation, in spite of the low cellular response for macrostructured samples. The use of sheep model for six months appears to be less adequate for a comparison because of the high level of bone integration in all samples. In spite of the often reported positive effect of titanium oxidation on the triggering of faster osseointegration, in this experiment the additional UV laser oxidation did not lead to a significant in vivo improvement. Laser macrostructuration of titanium alloy surfaces appears to promote bone apposition and may therefore constitute a promising surface modification strategy. In animal models, the natural process of titanium surface oxidation, because of physiologic fluids, alters properties observed in vitro with cells.

Zinc and silver glass polyalkenoate cements: An evaluation of their antibacterial nature

A biofilm is an accumulation of micro-organisms and their extracellular products forming a structured community on a surface. Biofilm formation on medical devices has severe health consequences as bacteria growing in this lifestyle are tolerant to both host defence mechanisms and antibiotic therapies. However, silver and zinc ions inhibit the attachment and proliferation of immature biofilms. The objective of this study is to evaluate whether silver and zinc ions eluted from novel glass polyalkenoate cement (GPC) coatings have the ability to inhibit Methicillin-resistant Staphylococcus aureus (MRSA) in vivo. A silver and zinc-containing GPC coating was synthesised, deposited onto Ti6Al4V discs and placed in a specified amount of analytical water for 1, 7 and 30 days. The resulting elutes were collected and Atomic absorption spectroscopy was used to measure ion release. The elutes were injected into Galleria mellonella larvae infected with MRSA and the antibacterial properties of these elutes were evaluated in vivo. The majority of the zinc and silver ions were released within the first 24 h; this corresponded with the greatest degree of protection observed in infected larvae. Results were compared to a conventional in vitro model where identical elutes were incubated with MRSA on nutrient agar. These results were consistent with those observed in the larval model, demonstrating a reduction in bacterial viability when co-cultured with elutes for 2 h. This work confirms the promise of the Galleria mellonella as a model for the assessment of antimicrobial agents and demonstrates the capacity of novel silver and zinc-containing GPCs to retard the colonisation of MRSA.

Effect of Micrometer-Scale Roughness of the Surface of Ti6Al4V Pedicle Screws in Vitro and in Vivo

Titanium implants that have been grit-blasted and acid-etched to produce a rough microtopography support more bone integration than do smooth-surfaced implants. In vitro studies have suggested that this is due to a stimulatory effect on osteoblasts. It is not known if grit-blasted and acid-etched Ti6Al4V implants also stimulate osteoblasts and increase bone formation clinically. In this study, we examined the effects of micrometer-scale-structured Ti6Al4V surfaces on cell responses in vitro and on tissue responses in vivo. Ti6Al4V disks were either machined to produce smooth surfaces with an average roughness (Ra) of 0.2 microm or grit-blasted, resulting in an Ra of 2.0, 3.0, or 3.3 microm. Human osteoblast-like cells were cultured on the disks and on tissue culture polystyrene. The cell number, markers of osteoblast differentiation, and levels of local factors in the conditioned media were determined at confluence. In addition, Ti6Al4V pedicle screws with smooth or rough surfaces were implanted into the L4 and L5 vertebrae of fifteen two-year-old sheep. Osteointegration was evaluated at twelve weeks with histomorphometry and on the basis of removal torque. The cell numbers on the Ti6Al4V surfaces were lower than those on the tissue culture polystyrene; the effect was greatest on the roughest surface. The alkaline-phosphatase-specific activity of cell lysates was decreased in a surface-dependent manner, whereas osteocalcin, prostaglandin E(2), transforming growth factor-beta1, and osteoprotegerin levels were higher on the rough surfaces. Bone-implant contact was greater around the rough-surfaced Ti6Al4V screws, and the torque needed to remove the rough screws from the bone was more than twice that required to remove the smooth screws. Increased micrometer-scale surface roughness increases osteoblast differentiation and local factor production in vitro, which may contribute to increased bone formation and osteointegration in vivo. There was a correlation between in vitro and in vivo observations, indicating that the use of screws with rough surfaces will result in better bone-implant contact and implant stability.

Antibacterial coatings for medical devices based on glass polyalkenoate cement chemistry

A biofilm is an accumulation of micro-organisms and their extracellular products forming a structured community on a surface. Biofilm formation on medical devices has severe health consequences as bacteria growing in this lifestyle are tolerant to both host defense mechanisms and antibiotic therapies. However, silver and zinc ions inhibit the attachment and proliferation of immature biofilms. The objective of this study is to evaluate whether it is possible to produce silver and zinc-containing glass polyalkenoate cement (GPC) coatings for medical devices that have antibacterial activity and which may therefore inhibit biofilm formation on a material surface. Two silver and zinc-containing GPC coatings (A and B) were synthesised and coated onto Ti6Al4V discs. Their handling properties were characterised and atomic absorption spectrometery was employed to determine zinc and silver ion release with coating maturation up to 30 days. The antibacterial properties of the coatings were also evaluated against Staphylococcus aureus and a clinical isolate of Pseudomonas aeruginosa using an agar diffusion assay method. The majority of the zinc and silver ions were released within the first 24 h; both coatings exhibited antibacterial effect against the two bacterial strains, but the effect was more intense for B which contained more silver and less zinc than A. Both coatings produced clear zones of inhibition with each of the two organisms tested. In this assay, Ps. aeruginosa was more sensitive than S. aureus. The diameters of these zones were reduced after the coating had been immersed in water for varying periods due to the resultant effect on ion release.

Patterns of gene expression in rat bone marrow stromal cells cultured on titanium alloy discs of different roughness.

Rat bone marrow stromal cells were cultured on either Ra (0.14 microm) or Ra (5.8 microm) Ti6Al4V discs for 24 or 48 h. Cells on the Ra (0.14 microm) surface showed typical fibroblastic morphology, whereas cells on the Ra (5.8 microm) surface were in clusters with a more epithelial appearance. RNA was extracted from the cells at both time points, and gene expression was analyzed by using a rat gene microarray. At 24 and 48 h, a similar number of genes were both up- and down-regulated at least twofold on the Ra (5.8 microm) surface compared to the Ra (0.14 microm) surface. We analyzed the relative level of specific groups of genes related to bone and cartilage development, cell adhesion and extracellular matrix proteins, transcription factors, bone morphogenetic proteins, phospholipases, and protein kinases. Roughness did not appear to be a specific stimulator of osteogenesis because genes of both the bone and cartilage lineage were up-regulated on the Ra (5.8 microm) surface. The most prominent change among transcription factors was up-regulation of Hox 1.4 on the Ra (5.8 microm) surface. Up-regulation of phospholipase A2 and SMAD 4 indicate these genes are also involved in the response of cells to an Ra (5.8 microm) surface. Our data show surface roughness alters the expression of a large number of genes in marrow stromal cells, which are related to multiple pathways of mesenchymal cell differentiation.

Collagen Type I Prevents Glyoxal-Induced Apoptosis in Osteoblastic Cells Cultured on Titanium Alloy

Advanced glycation end products (AGEs) irreversibly cross-link proteins with sugars and accumulate at a higher age and in diabetes, processes which can interfere with the integration of implants into the tissue. Glyoxal is a highly reactive glycating agent involved in the formation of AGEs and is known to induce apoptosis, as revealed by the upregulation of caspase-3 and fractin (caspase-3 being a key enzyme activated during the late stage of apoptosis and fractin being a caspase-cleaved actin fragment). In this study, we investigated the influence of collagen type I coating on the cytotoxic effect of glyoxal on rat calvarial osteoblastic cells and on human osteosarcoma cells (Saos-2) grown on titanium alloy, Ti6Al4V. Activation of caspase-3 and fractin was measured by counting immunohistochemically stained cells and by flow cytometry with propidium iodide (detection of the apoptosis indicating a sub-G₁ peak). Our results showed an increased number of apoptotic osteoblasts after incubation with glyoxal on Ti6Al4V discs. However, the number of apoptotic cells on collagen-coated titanium was significantly smaller than on uncoated titanium after the same treatment. The present findings demonstrate that osteoblasts treated with glyoxal undergo apoptosis, whereas collagen type I coating of titanium alloys (used for implants) has an antiapoptotic function.

Properties of nanocrystalline diamond thin films grown by MPCVD for biomedical implant purposes

Nanocrystalline diamond (NCD) thin films have been grown by microwave plasma chemical vapor deposition (MPCVD) and investigated to determine their suitability for biomedical applications. Growth conditions were chosen to produce very uniform films over the surface of curved temporomandibular joint implants. These parameters include methane flow rates exceeding 20% of the hydrogen gas flow rate, and chamber pressure and microwave power were maintained at 30 Torr and 0.73 kW, respectively, in a Wavemat 6 kW MPCVD device. Films (3 μm thick) that completely coated 2.54-cm-diameter Ti–6Al–4V disks under these conditions exhibit mean grain size of 30.4 nm as determined by XRD peak broadening, hardness of 80 GPa as determined by nanoindentation, RMS mean roughness of 15.3±5.3 nm as determined by stylus profilometry, and film adhesion toughness ( Γ C) of 158 J/m 2 as determined by a Rockwell indentation method. Similar deposition performed on small Ti–6Al–4V hemispheres produce films with smaller mean grain size of 21.1 nm and correspondingly lower hardness and roughness. Overall, these films exhibit properties well suited for use in biomedical applications.

Contact profilometry and correspondence analysis to correlate surface properties and cell adhesion in vitro of uncoated and coated Ti and Ti6Al4V disks

A fundamental goal in the field of implantology is the design of specific devices able to induce a controlled and rapid “osseointegration”. This result has been achieved by means of surface modifications aimed at optimizing implant-to-bone contact; furthermore, bone cell adhesion on implant surface has been directly improved by the application of biomolecules that stimulate new tissue formation, thus controlling interactions between biological environment and implanted materials. Actually, methods for biochemical factor delivery at the interface between implant surface and biological tissues are under investigation; a reliable technique is represented by the inclusion of biologically active molecules into biocompatible and biodegradable materials used for coating implant surface. This paper focuses the application of three polymeric materials already acknowledged in the clinical practice, i.e. poly- l-lactic acid (PLLA), poly- dl-lactic acid (PDLA), and sodium alginate hydrogel. They have been used to coat Ti (Ti2) and Ti6Al4V (Ti5) disks; their characteristics have been determined and their performances compared, with specific regard to the ability in allowing osteoblast adhesion in vitro. Moreover, profilometry data analysis permitted to identify a specific roughness parameter (peak density) which mainly controls the amount of osteoblast adhesion.

The effects of patient age on human osteoblasts’ response to Ti–6Al–4V implants in vitro

Osseointegrated implants are a common therapy for the elderly population as lifespan increases. Understanding the effects of age and sex on osseointegration is important for successful implant therapy. Therefore, the response of primary human osteoblasts (HOB) to implant materials was studied. HOBs were obtained by outgrowth of cells from bone from orthopaedic procedures and categorized as Young (Y), <15; Middle (M), 30–50; and Old (O), >60 years old. Initially the HOB phenotype was determined on tissue culture plastic. Alkaline phosphatase (ALP) staining and activity were significantly increased in HOBs from older patients. Message levels of type I collagen (COL), bone sialoprotein (BSP) and ALP were significantly higher (from 2.3- to 3.8-fold) in Y subjects compared to M and O patients at 2 weeks. Studies of the response of HOBs to implant materials were undertaken using Ti–6Al–4V disks prepared in a manner similar to orthopaedic implants. A 1.4-fold ( p<0.05) increase in cell attachment was found in HOBs from Y compared with O in female subjects but not in male subjects. Cell proliferation at 24 h was not significantly different by age or sex, nor was DNA content different at 2 and 4 weeks. Mineralization in HOB-implant cultures was 2.3-fold higher in Y than in O, and 1.7-fold higher in Y compared to M HOBs from female but not male subjects at 4 weeks. Northern blot and RT-PCR analysis at 2 weeks of culture showed significantly higher levels (1.6–2.3-fold) of COL, BSP, and osteocalcin (OC) mRNAs in Y HOBs compared to M and O HOBs from female subjects. We conclude that human osteoblasts from older female patients have a decreased ability to form bone on implants.

Tribological behaviour of alumina sliding against Ti6Al4V in unlubricated contact

Tribological behaviour of alumina balls (99.5%) sliding against a Ti6Al4V disc over a range of loads (5–80 N) and speeds (0.0625–1 m s −1) has been investigated using a pin-on-disc tribometer under unlubricated conditions. The maximum wear coefficient was observed to be several orders of magnitude higher than the reported value for alumina against alumina or alumina against steel counterfaces. When the load or speed increased, the wear rate of the alumina ball increased initially and then decreased, showing typical transition features. On the other hand, the friction coefficients for the Ti6Al4V/alumina tribosystem were found to increase inversely with the applied loads or the sliding speeds. The wear mechanisms and the wear transition were investigated based on examinations of worn surfaces as well as debris using SEM, XRD and XPS, and it was revealed that a tribochemical mechanism accounted for the observed high wear rate of alumina sliding against the titanium alloy.

The growth of cracks in Ti–6Al–4V plate under combined high and low cycle fatigue

The fatigue crack growth rates in cross-rolled Ti–6Al–4V plate subjected to combined major and minor stress cycles have been measured at room temperature. The concept of crack closure was used to model the data for a test sequence using 1000 minor cycles per major cycle, and the model validated by either the accurate or safe prediction of the crack growth rates for a second series of tests involving 10,000 minor cycles per major cycle. Fatigue threshold values for the minor cycles derived from the growth rate data for combined major and minor cycle loadings were lower than those determined by the conventional load shedding method. In comparison with the behaviour of Ti–6Al–4V disc material which had been forged, the cross-rolled plate material exhibited: first, a clearly defined bilinear growth rate curve under a separate major cycle loading; second, similar or lower derived threshold values with separate minor cycle loadings; and third, reduced crack propagation lives for loadings combining major and minor cycles.

Surface engineering to improve tribological performance of Ti–6Al–4V

A series of thermal oxidising treatments has been carried out on Ti–6Al–4V alloy at temperatures ranging from600 to 850°C to develop a new surface engineering technique for Ti–6Al–4V and other titanium alloys. Systematic characterisation of the thermal oxidation treated surface layers was performed using GDS, XRD, SEM, and nanoindentation testing. Ball on disc friction tests show that the coefficient offriction of a Ti–6Al–4V disc against an alumina ball was significantly reduced after oxidation treatment. The rolling–sliding wear resistance of Timetal 6–4 material against hardened 709M40 steel under lubricated conditions has been increased by thermal oxidation (TO) treatment to such an extent that it is even superior to that of the steel counterpart. The results also show that the electrochemical corrosion resistance in 3%NaCl solution isimproved by proper TO treatment.

Mechanical properties and structure of Ti—6A1—4V alloy implanted with different light ions

The effect of N + and C - implantation on the properties of Ti—6Al—4V alloy is widely documented. However, some authors claim that other light ions, such as O + or B +, also have an effect on this alloy, improving its mechanical properties. In this work, Ti—6Al—4V alloy samples have been implanted with C +, N + and O + light ions. Energies from 50 to 180 keV and doses of the order of 10 17 ion cm -2 have been used, keeping the substrate temperature below 500°C. Mechanical properties such as the hardness or elastic recovery have been evaluated by means of microindentation tests, with a loading-unloading cycle at loads up to 10 mN. An increase in surface hardness of more than 100% has been observed in most of the implanted samples. Pin-on-disc wear tests under lubricated conditions have been performed to evaluate and compare the tribological behaviour of implanted samples against ultrahigh molecular weight polyethylene. A decrease in the friction coefficient from 0.1 to 0.05, resulting from ion implantation, has been observed. Unlubricated wear tests using an alumina ball on a Ti—6Al—4V disc have also been carried out. Wear tracks on the Ti—6Al—4V, evaluated by means of optical profilometry and scanning electron microscopy, have shown that implantation can improve the abrasive wear resistance by two orders of magnitude. X-ray photoelectron spectroscopy analyses also were carried out on selected samples, showing the presence of hard phases, such as oxides or carbides, in the implanted samples.

Wear behaviour of the couple polyethylene Ti6Al4V: Effects of the metallic surface preparation and nitrogen implantation

The wear behaviour improvement of the tribological couple Ti6Al4V-UHMWPE is of great interest to the medical field. Wear tests were carried out in water on a reciprocating UHMWPE annulus on implanted Ti6Al4V disc tribotestcr, with loads and velocities simulating those of hip joints. A comparison of wear behaviours was also carried out between untreated Ti6Al4V samples and Ti6Al4V subjected to a special lapping procedure. UHMWPE worn against ion-implanted and especially lapped Ti alloy showed the lowest wear rate, while, the highest (about one order of magnitude) was shown by the UHMWPE against untreated Ti6Al4V samples. XPS and AES surface analyses were carried out on metallic discs to examine the chemical composition of the surface before wear tests. Moreover depth distribution of nitrogen in implanted samples was determined using the same techniques. SEM observations displayed a polyethylene transfer film on all metallic surfaces, particularly on untreated Ti6Al4V samples. A discussion about uselessness of more conventional surface treatments for the Ti alloy is also reported.