Negative influence of biofilm on CoCrMo corrosion.

Minimal studies exist investigating biofilm-induced corrosion of orthopaedic implants. This study investigates potential contributions of Pseudomonas aeruginosa and Staphylococcus aureus biofilms on corrosion resistance of CoCrMo under static and fretting conditions. Biofilms were cultured on CoCrMo coupons for either 4 weeks (static culture) or 6 days (fretting culture; pin-on-disk with a Ti6Al4V hemispherical tip pin). Morphology of biofilms and corrosion of coupon surfaces were analyzed via SEM. Open circuit potential and electrochemical impedance spectroscopy measurements were collected for corrosion performance evaluation. Results showed no visible corrosion on coupon surfaces in static culture, which suggests these biofilms alone do not induce severe corrosion under the conditions of this study. However, electrochemical data showed biofilm presence lowered coupon electrochemical impedance in static and fretting cultures, suggesting resistive and capacitive characteristics of the metal oxide-biofilm-media interface were altered. Under fretting, the P. aeruginosa group exhibited a distinct damage morphology and Co:Cr:Mo ratio within the wear scar when compared with S. aureus and the bacteria-free control. These differences suggest the presence of P. aeruginosa biofilms may negatively impact corrosion resistance at the fretting interface. Taken together these results demonstrate biofilms can contribute to implant corrosion by influencing the electrochemical impedance of implant metal surfaces.

Effects of seating load magnitude and load orientation on seating mechanics in 5°40' mixed-alloy modular taper junctions.

BACKGROUND: Mechanically-assisted crevice corrosion of modular tapers continues to be a concern in total joint replacements. Surgical factors that may affect taper seating mechanics include seating load magnitude and load orientation. Seating mechanics is defined as the seating load versus displacement behavior. In this study, mixed-alloy (CoCrMo/Ti-6Al-4V) modular head-neck 5°40' taper junctions were seated over a range of axially-oriented loads and off-axis orientations, capturing load-displacement during seating. The goals of the study were to assess the effects of seating load magnitude and load orientation on seating mechanics and correlate those findings with the taper pull-off load. METHODS: A testing fixture measured head-neck seating displacement as the load was quasistatically applied. Motion was captured using two non-contact differential variable reluctance transducers which were mounted to the neck targeting the head. Seating experiments ranged from 1000 N to 8000 N. Load orientation ranged from 0° to 20° at 4000 N. RESULTS: Seating load-displacement behavior at different seating loads showed a consistent characteristic behavior. Testing demonstrated increased seating displacement with seating load. Pull-off loads increased with seating load and were approximately 44% of the seating load across the range of seating loads investigated. Seating load orientation up to 20° had no significant effect on seating displacement and taper pull-off load. CONCLUSION: Increased seating load magnitude increased seating displacement, work of seating and pull-off loads in mixed-alloy 5°40' head-neck tapers. Altering load orientation up to 20° off-axis had no significant effect. Direct measurements of seating mechanics provides insights into the locking of taper junctions.

Effect of the support systems' compliance on total hip modular taper seating stability.

Assembly of a femoral head onto the stem remains non-standardized. The literature shows altering mechanical conditions during seating affects taper strength and lower assembly load may increase fretting corrosion during cyclic tests. This suggests overall performance may be affected by head assembly method. The purpose of this test was to perform bench-top studies to determine influence of peak force magnitude, load rate, and compliance of the system's support structure on initial stability of the taper. Custom manufactured CoCrMo femoral heads and Ti-6Al-4V taper analog samples were assembled with varying peak force magnitudes (2-10.1 kN), load rates (quasi-static vs impaction), and system compliance (rigid vs compliant). A clinically-relevant system compliance design was based off of force data collected during a cadaver impaction study. Tensile loads were then applied to disassemble the taper and quantify initial taper stability. Results indicated that taper stability (assessed by disassembly forces) increased linearly with assembly force and load rate did not have a significant effect on taper stability. When considering system compliance, a 42%-50% larger input energy, dependent on assembly force, was required in the compliant group to achieve a comparable impaction force to the rigid group. Even when this impaction force was achieved, the correlation between the coefficient, defined as distraction force divided by assembly load, was significantly reduced for the compliant test group. The compliant setup was intended to simulate a surgical scenario where patient and surgical factors may influence the resulting compliance. Based on results, surgical procedure and patient variables may have a significant effect on initial taper stability.

CD24 is required for regulating gene expression, but not glucose uptake, during adipogenesis.

Adipose tissue dysfunction in obesity and lipodystrophy results in major health complications such as heart disease and stroke, and is associated with an increased risk of some cancers. We have previously found that the cell surface receptor CD24 regulates adipogenesis as measured by lipid accumulation and gene expression in mature adipocytes. How CD24 regulates these processes remains unknown. To begin answering this question, we first determined that CD24 does not affect glucose uptake in differentiating adipocytes in vitro. We then examined changes in global gene expression via DNA microarray in 3T3-L1 adipocytes with siRNA-mediated knock-down of CD24 expression. We found that CD24 expression is necessary for upregulation of up to 134 genes. We validated the CD24-mediated regulation of 4 of these genes during in vitro adipogenesis of 3T3-L1 and primary cells isolated from the inguinal white adipose tissue depots of CD24 knockout mice. Surprisingly, we found that only 1 of these genes was also regulated by CD24 in cells from the epididymal depot. Overall, these data suggest that CD24 is necessary for select gene expression in a depot-specific manner during adipogenesis in vitro. These findings could help elucidate the mechanisms regulating lipid accumulation in adipocytes thereby aiding in the development of novel treatment strategies for obesity and lipodystophy.

Effects of Seating Load Magnitude on Incremental Cyclic Fretting Corrosion in 5°40' Mixed Alloy Modular Taper Junctions.

BACKGROUND: Mechanically assisted crevice corrosion of modular tapers continues to be a concern in total joint arthroplasties. A surgical factor that may affect taper fretting corrosion during cyclic loading is seating load magnitude. In this study, modular head-neck taper junctions were seated, capturing load-displacement, over a range of axially oriented loads, and electrochemical and micromotion data were captured during short-term incremental cyclic fretting corrosion (ICFC) tests. The hypothesis is low seating loads result in greater motion and fretting corrosion in ICFC tests. The effect of assembly load on pull-off force post-ICFC testing was also evaluated. METHODS: The study employed custom-built test fixtures which measured head-neck micromotion and an electrochemical chamber to monitor electrochemical reactions. Head-neck motion measurements were captured using 2 noncontact differential variable reluctance transducers mounted to the head. Seating experiments ranged from 1000 to 8000 N. RESULTS: Significant differences due to seating loads were reported in seating displacement, ICFC subsidence, and fretting current at 4000 N cyclic load. Seating load decreased but did not eliminate currents. Fretting onset load remained fixed (approximately 1200 N) for tapers seated above 2000 N. Fretting subsidence was negligible for seating loads of 4000 N or higher, and increased subsidence was observed below 4000 N. CONCLUSION: This short-term test method evaluated the acute performance of modular implants which were assembled under various loads and demonstrated the link between seating loads, fretting motions, and electrochemical reactions. While increased seating loads reduced fretting corrosion and taper subsidence, it did not prevent fretting corrosion even at 8 kN seating.

Novel microwave assisted chemical synthesis of Nd2Fe14B hard magnetic nanoparticles.

The high coercivity and excellent energy product of Nd2Fe14B hard magnets have led to a large number of high value added industrial applications. Chemical synthesis of Nd2Fe14B nanoparticles is challenging due to the large reduction potential of Nd(3+) and the high tendency for Nd2Fe14B oxidation. We report the novel synthesis of Nd2Fe14B nanoparticles by a microwave assisted combustion process. The process consisted of Nd-Fe-B mixed oxide preparation by microwave assisted combustion, followed by the reduction of the mixed oxide by CaH2. This combustion process is fast, energy efficient and offers facile elemental substitution. The coercivity of the resulting powders was ∼8.0 kOe and the saturation magnetization was ∼40 emu g(-1). After removal of CaO by washing, saturation magnetization increased and an energy product of 3.57 MGOe was obtained. A range of magnetic properties was obtained by varying the microwave power, reduction temperature and Nd to Fe ratio. A transition from soft to exchange coupled to hard magnetic properties was obtained by varying the composition of NdxFe1-xB8 (x varies from 7% to 40%). This synthesis procedure offers an inexpensive and facile platform to produce exchange coupled hard magnets.

Potential and frequency effects on fretting corrosion of Ti6Al4V and CoCrMo surfaces.

Fretting corrosion has been reported at the metal-metal interfaces of a wide range of medical devices, including total joint replacements, spinal devices, and overlapping cardiovascular stents. Currently, the fretting corrosion phenomenon associated with metal-on-metal contacts is not fully understood. This study investigated the effect of potential and fretting frequency on the fretting corrosion performance of Ti6Al4V/Ti6Al4V, Ti6Al4V/CoCrMo, and CoCrMo/CoCrMo alloy combinations at fixed normal load and displacement conditions using a custom built fretting corrosion test system. The results showed that the fretting current densities increased with increases in potential and were highest for Ti6Al4V/Ti6Al4V couple (1.5 mA/cm(2) at 0 V vs. Ag/AgCl). The coefficient of friction varied with potential and was about two times higher for Ti6Al4V/Ti6Al4V (0.71 V at 0 V vs. Ag/AgCl). In most of the potential range tested, the fretting corrosion behavior of CoCrMo/Ti6Al4V and CoCrMo/CoCrMo was similar and dominated by the CoCrMo surface. Increase in applied fretting frequency linearly increased the fretting current densities in the regions where the passive film is stable. Also, the model-based fretting current densities were in excellent agreement with the experimental results. Overall, Ti6Al4V/Ti6Al4V couple was more susceptible to fretting corrosion compared with other couples. However, the effects of these processes on the biological system were not assessed.

Fretting corrosion of CoCrMo and Ti6Al4V interfaces.

Mechanically assisted corrosion (fretting corrosion, tribocorrosion etc.,) of metallic biomaterials is a primary concern for numerous implant applications, particularly in the performance of highly-loaded medical devices. While the basic underlying concepts of fretting corrosion or tribocorrosion and fretting crevice corrosion are well known, there remains a need to develop an integrated systematic method for the analysis of fretting corrosion involving metal-on-metal contacts. Such a method can provide detailed and quantitative information on the processes present and explore variations in surfaces, alloys, voltages, loadings, motion and solution conditions. This study reports on development of a fretting corrosion test system and presents elements of an in-depth theoretical fretting corrosion model that incorporates both the mechanical and the electrochemical aspects of fretting corrosion. To demonstrate the capabilities of the new system and validate the proposed model, experiments were performed to understand the effect of applied normal load on fretting corrosion performance of Ti6Al4V/Ti6Al4V, CoCrMo/Ti6Al4V, and CoCrMo/CoCrMo material couples under potentiostatic conditions with a fixed starting surface roughness. The results of this study show that fretting corrosion is affected by material couples, normal load and the motion conditions at the interface. In particular, fretting currents and coefficient of friction (COF) vary with load and are higher for Ti6Al4V/Ti6Al4V couple reaching 3 mA/cm(2) and 0.63 at about 73 MPa nominal contact stress, respectively. Ti6Al4V coupled with CoCrMo displayed lower currents (0.6 mA/cm(2)) and COF (0.3), and the fretting corrosion behavior was comparable to CoCrMo/CoCrMo couple (1.2 mA/cm(2) and 0.3, respectively). Information on the mechanical energy dissipated at the interface, the sticking behavior, and the load dependence of the inter-asperity distance calculated using the model elucidated the influence of mechanical factors on the experimental results. It was observed that the lowest amount of work was required to generate some of the highest fretting corrosion currents in Ti6Al4V/Ti6Al4V couples compared to the other combinations. The elements of the model presented here provide an excellent basis to explain many of the observed behaviors of these interfaces.

Photoelectrochemical studies of DNA-tagged biomolecules on Au and Au/Ni/Au multilayer nanowires.

The use of nanowires (NWs) for labeling, sensing, and sorting is the basis of detecting biomolecules attached on NWs by optical and magnetic properties. In spite of many advantages, the use of biomolecules-attached NWs sensing by photoelectrochemical (PEC) study is almost non-existent. In this article, the PEC study of dye-attached single-stranded DNA on Au NWs and Au-Ni-Au multilayer NWs prepared by pulse electrodeposition are investigated. Owing to quantum-quenching effect, the multilayer Au NWs exhibit low optical absorbance when compared with Au NWs. The tagged Au NWs show good fluorescence (emission) at 570 nm, indicating significant improvement in the reflectivity. Optimum results obtained for tagged Au NWs attached on functionalized carbon electrodes and its PEC behavior is also presented. A twofold enhancement in photocurrent is observed with an average dark current of 10 µA for Au NWs coated on functionalized sensing electrode. The importance of these PEC and optical studies provides an inexpensive and facile processing platform for Au NWs that may be suitable for biolabeling applications.

Electrochemical investigation of chromium oxide-coated Ti-6Al-4V and Co-Cr-Mo alloy substrates.

Hard coatings for articulating surfaces of total joint replacements may improve the overall wear resistance. However, any coating approach must take account of changes in corrosion behavior. This preliminary assessment analyzes the corrosion kinetics, impedance and mechanical-electrochemical stability of 100 µm thick plasma sprayed chromium oxide (Cr2O3) coatings on bearing surfaces in comparison to the native alloy oxide films on Co-Cr-Mo and Ti-6Al-6V. Cyclic potentiodynamic polarization, electrochemical impedance spectroscopy, and mechanical abrasion under potentiostatic conditions were performed on coated and substrate surfaces in physiological saline. SEM analysis characterized the coating morphology. The results showed that the corrosion current density values of chromium oxide coatings (0.4-1.2 µA/cm²) were of the same order of magnitude as Ti-6Al-4V alloy. Mechanical abrasion did not increase corrosion rates of chromium oxide coatings but did for uncoated Co-Cr-Mo and Ti-6Al-4V. The impedance response of chromium oxide coatings was very different than Co-Cr-Mo and Ti-6Al-4V native oxides characterized by a defected coating model. More of a frequency-independent purely resistive response was seen in mid-frequency range for the coatings (CPE(coat) : 40-280 nF/cm² (rad/s)(1-α) , α: 0.67-0.83) whereas a more capacitive character is seen for Co-Cr-Mo and Ti-6Al-4V (CPE(ox) around 20 µF/cm² (rad/s)(1-α) , α around 0.9). Pores, interparticle gaps and incomplete fusion typical for thermal spray coatings were present in these oxides which could have influenced corrosion resistance. The coating microstructure could have allowed some fluid penetration. Overall, these coatings appear to have suitable corrosion properties for wear surfaces.