Effects of laser shock peening on the corrosion behavior and biocompatibility of a nickel-titanium alloy.

Nickel-titanium (NiTi) alloy is an attractive material for biomedical implant applications. In this study, the effects of laser shock peening (LSP) on the biocompatibility, corrosion resistance, ion release rate and hardness of NiTi were characterized. The cell culture study indicated that the LSP-treated NiTi samples had lower cytotoxicity and higher cell survival rate than the untreated samples. Specifically, the cell survival rate increased from 88 ± 1.3% to 93 ± 1.1% due to LSP treatment. LSP treatment was shown to significantly decrease the initial Ni ion release rate compared with that of the untreated samples. Electrochemical tests indicated that LSP improved the corrosion resistance of the NiTi alloy in simulated body fluid, with a decrease in the corrosion current density from 1.41 ± 0.20 µA/cm2 to 0.67 ± 0.24 µA/cm2 . Immersion tests showed that calcium deposition was significantly enhanced by LSP. In addition, the hardness of NiTi alloy increased from 226 ± 3 HV before LSP to 261 ± 3 HV after LSP. These results demonstrated that LSP is a promising surface modification method that can be used to improve the mechanical properties, corrosion resistance and biocompatibility of NiTi alloy for biomedical applications. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 1854-1863, 2019.

Hierarchical structures on nickel-titanium fabricated by ultrasonic nanocrystal surface modification.

Hierarchical structures on metallic implants can enhance the interaction between cells and implants and thus increase their biocompatibility. However, it is difficult to directly fabricate hierarchical structures on metallic implants. In this study, we used a simple one-step method, ultrasonic nanocrystal surface modification (UNSM), to fabricate hierarchical surface structures on a nickel-titanium (NiTi) alloy. During UNSM, a tungsten carbide ball hits metal surfaces at ultrasonic frequency. The overlapping of the ultrasonic strikes generates hierarchical structures with microscale grooves and embedded nanoscale wrinkles. Cell culture experiments showed that cells adhere better and grow more prolifically on the UNSM-treated samples. Compared with the untreated samples, the UNSM-treated samples have higher corrosion resistance. In addition, the surface hardness increased from 243 Hv to 296 Hv and the scratch hardness increased by 22%. Overall, the improved biocompatibility, higher corrosion resistance, and enhanced mechanical properties demonstrate that UNSM is a simple and effective method to process metallic implant materials.

A systematic study of mechanical properties, corrosion behavior and biocompatibility of AZ31B Mg alloy after ultrasonic nanocrystal surface modification.

Magnesium alloys have tremendous potential for biomedical applications due to their good biocompatibility, osteoconductivity, and degradability, but can be limited by their poor mechanical properties and fast corrosion in the physiological environment. In this study, ultrasonic nanocrystal surface modification (UNSM), a recently developed surface processing technique that utilizes ultrasonic impacts to induce plastic strain on metal surfaces, was applied to an AZ31B magnesium (Mg) alloy. The mechanical properties, corrosion resistance, and biocompatibility of the alloy after UNSM treatment were studied systematically. Significant improvement in hardness, yield stress and wear resistance was achieved after the UNSM treatment. In addition, the corrosion behavior of UNSM-treated AZ31B was not compromised compared with the untreated samples, as demonstrated by the weight loss and released element concentrations of Mg and Al after immersion in alpha-minimum essential medium (α-MEM) for 24h. The in vitro biocompatibility of the AZ31B Mg alloys toward adipose-derived stem cells (ADSCs) before and after UNSM processing was also evaluated using a cell culture study. Comparable cell attachments were achieved between the two groups. These studies showed that UNSM could significantly improve the mechanical properties of Mg alloys without compromising their corrosion rate and biocompatibility in vitro. These findings suggest that UNSM is a promising method to treat biodegradable Mg alloys for orthopaedic applications.

Surface amorphization of NiTi alloy induced by Ultrasonic Nanocrystal Surface Modification for improved mechanical properties.

We report herein the effects of Ultrasonic Nano-crystal Surface Modification (UNSM), a severe surface plastic deformation process, on the microstructure, mechanical (hardness, wear), wettability and biocompatibility properties of NiTi shape memory alloy. Complete surface amorphization of NiTi was achieved by this process, which was confirmed by X-ray diffraction and high-resolution transmission electron microscopy. The wear resistance of the samples after UNSM processing was significantly improved compared with the non-processed samples due to increased surface hardness of the alloy by this process. In addition, cell culture study demonstrated that the biocompatibility of the samples after UNSM processing has not been compromised compared to the non-processed sample. The combination of high wear resistance and good biocompatibility makes UNSM an appealing process for treating alloy-based biomedical devices.

Bioactive surface modification of metal oxides via catechol-bearing modular peptides: multivalent-binding, surface retention, and peptide bioactivity.

A series of multivalent dendrons containing a bioactive osteogenic growth peptide (OGP) domain and surface-binding catechol domains were obtained through solid phase synthesis, and their binding affinity to hydroxyapatite, TiO2, ZrO2, CeO2, Fe3O4 and gold was characterized using a quartz crystal microbalance with dissipation (QCM-d). Using the distinct difference in binding affinity of the bioconjugate to the metal oxides, TiO2-coated glass slides were selectively patterned with bioactive peptides. Cell culture studies demonstrated the bioavailability of the OGP and that OGP remained on the surface for at least 2 weeks under in vitro cell culture conditions. Bone sialoprotein (BSP) and osteocalcein (OCN) markers were upregulated 3-fold and 60-fold, respectively, relative to controls at 21 days. Similarly, 3-fold more calcium was deposited using the OGP tethered dendron compared to TiO2. These catechol-bearing dendrons provide a fast and efficient method to functionalize a wide range of inorganic materials with bioactive peptides and have the potential to be used in coating orthopaedic implants and fixation devices.

Effects of persistent luminescence decay on mechanoluminescence phenomena of SrAl2O4:Eu2+, Dy3+ materials.

This Letter reveals for the first time, to the best of our knowledge, the effects of stress-free persistent luminescence (PL) decay on the mechanoluminescence (ML) phenomena and the effects of stresses and strain rates on the PL decay of SrAl(2)O(4):Eu(2+), Dy(3+) (SAOED) materials. Previous research on ML phenomena in this material has focused on the effects of strain rates and stress variations on ML light intensity. However, experimental evidence provided herein shows that the ML light emission is also related to the PL decay time elapsed until the onset of stressing and the PL decay rate is dependent on the stress, strain rate, and the stress-free PL decay time interval. For quantitative stress measurements using SAOED materials, understanding of ML light sensitivity and its dependence on critical factors (strain rate, stress-free PL decay time interval, photoexcitation time, instantaneous PL decay rate, etc.) is crucially important. This Letter provides new and important perspectives that are essential for developing predictive models and/or calibration procedures for ML stress sensors.

Tribological advancements for reliable wind turbine performance.

Wind turbines have had various limitations to their mechanical system reliability owing to tribological problems over the past few decades. While several studies show that turbines are becoming more reliable, it is still not at an overall acceptable level to the operators based on their current business models. Data show that the electrical components are the most problematic; however, the parts are small, thus easy and inexpensive to replace in the nacelle, on top of the tower. It is the tribological issues that receive the most attention as they have higher costs associated with repair or replacement. These include the blade pitch systems, nacelle yaw systems, main shaft bearings, gearboxes and generator bearings, which are the focus of this review paper. The major tribological issues in wind turbines and the technological developments to understand and solve them are discussed within. The study starts with an overview of fretting corrosion, rolling contact fatigue, and frictional torque of the blade pitch and nacelle yaw bearings, and references to some of the recent design approaches applied to solve them. Also included is a brief overview into lubricant contamination issues in the gearbox and electric current discharge or arcing damage of the generator bearings. The primary focus of this review is the detailed examination of main shaft spherical roller bearing micropitting and gearbox bearing scuffing, micropitting and the newer phenomenon of white-etch area flaking. The main shaft and gearbox are integrally related and are the most commonly referred to items involving expensive repair costs and downtime. As such, the latest research and developments related to the cause of the wear and damage modes and the technologies used or proposed to solve them are presented.