Robert A. Fleming

Diamond-like carbon coatings with zirconium-containing interlayers for orthopedic implants

Six types of diamond-like carbon (DLC) coatings with zirconium (Zr)-containing interlayers on titanium alloy (Ti-6Al-4V) were investigated for improving the biotribological performance of orthopedic implants. The coatings consist of three layers: above the substrate a layer stack of 32 alternating Zr and ZrN sublayers (Zr:ZrN), followed by a layer comprised of Zr and DLC (Zr:DLC), and finally a N-doped DLC layer. The Zr:ZrN layer is designed for increasing load carrying capacity and corrosion resistance; the Zr:DLC layer is for gradual transition of stress, thus enhancing layer adhesion; and the N-doped DLC layer is for decreasing friction, squeaking noises and wear. Biotribological experiments were performed in simulated body fluid employing a ball-on-disc contact with a Si3N4 ball and a rotational oscillating motion to mimic hip motion in terms of gait angle, dynamic contact pressures, speed and body temperature. The results showed that the Zr:DLC layer has a substantial influence on eliminating delamination of the DLC from the substrates. The DLC/Si3N4 pairs significantly reduced friction coefficient, squeaking noise and wear of both the Si3N4 balls and the discs compared to those of the Ti-6Al-4V/Si3N4 pair after testing for a duration that is equivalent to one year of hip motion in vivo.

Fabrication of stable superhydrophilic surfaces on titanium substrates

We report a method of producing superhydrophilic surfaces on titanium substrates via sandblasting and dip-coating with colloidal silica nanoparticles. The surface exhibits a high level of hydrophilic stability, as it stays superhydrophilic for an excess of 40 days and through multiple wetting–dewetting cycles. The combination of microscale roughness from the sandblasting and nanoscale roughness from the silica particles results in a micro-nano binary structure, which greatly enhances the hydrophilicity of the titanium samples. Due to the simplicity and ease of implementation of this method, such a surface is suitable for potential use in a variety of applications, such as prosthetic dentistry and other biomedical fields.

Biogeography and organic matter removal shape long-term effects of timber harvesting on forest soil microbial communities

The growing demand for renewable, carbon-neutral materials and energy is leading to intensified forest land-use. The long-term ecological challenges associated with maintaining soil fertility in managed forests are not yet known, in part due to the complexity of soil microbial communities and the heterogeneity of forest soils. This study determined the long-term effects of timber harvesting, accompanied by varied organic matter (OM) removal, on bacterial and fungal soil populations in 11- to 17-year-old reforested coniferous plantations at 18 sites across North America. Analysis of highly replicated 16 S rRNA gene and ITS region pyrotag libraries and shotgun metagenomes demonstrated consistent changes in microbial communities in harvested plots that included the expansion of desiccation- and heat-tolerant organisms and decline in diversity of ectomycorrhizal fungi. However, the majority of taxa, including the most abundant and cosmopolitan groups, were unaffected by harvesting. Shifts in microbial populations that corresponded to increased temperature and soil dryness were moderated by OM retention, which also selected for sub-populations of fungal decomposers. Biogeographical differences in the distribution of taxa as well as local edaphic and environmental conditions produced substantial variation in the effects of harvesting. This extensive molecular-based investigation of forest soil advances our understanding of forest disturbance and lays the foundation for monitoring long-term impacts of timber harvesting.

Mechanical Wear and Oxidative Degradation Analysis of Retrieved Ultra High Molecular Weight Polyethylene Acetabular Cups

The number of revision joint replacements has been increasing substantially over the last few years. Understanding their failure mechanism is extremely important for improving the design and material selection of current implants. This study includes ten retrieved and four new mildly cross-linked ultra-high molecular weight polyethylene (UHMWPE) acetabular liners. Among them, most of the prostheses (n = 5) were reported to be revised and replaced due to aseptic loosening, followed by painful joint (n = 2), dislocation (n = 1), intra articular ossification (n = 1), combination of wear (liner) and osteolysis (stem) (n=1). Surface deviations (wear, material inflation and roughness), oxidative degradation and change of material properties were measured using micro-computed tomography (micro-CT) scan, 3D laser scanning microscopy, raman spectroscopy and nanoindentation, respectively. Prostheses having eccentric worn areas had much higher linear wear rates (228.01 ± 35.51µm/year) compared to that of centrically worn prostheses (96.71 ± 10.83µm/year). Oxidation index (OI) showed similar trends to the surface penetration depth. Among them, sample 10 exhibited the highest OI across the contact area and the rim of the cup liner. It also had the lowest hardness/elasticity ratio. Overall, wear and creep, oxidative degradation and reduced hardness/elasticity ratio all contributed to the premature failure of the UHMWPE acetabular cup liners.

Nanostructure-Textured Surfaces with Low Friction and High Deformation Resistance

ABSTRACT Nanotextured surfaces can effectively reduce friction and adhesion, especially in applications with micro- and nanoscale contact interactions. However, for these surfaces, a common weakness is a lack of structural integrity of the individual nanotextures when subjected to contact loading, resulting in permanent deformation at even the moderate contact forces encountered in microscale systems. Nanostructure-textured surfaces (NSTSs), composed of arrays of novel Al/a-Si core–shell nanostructures (CSNs), have been developed with a desirable combination of low friction and high deformation resistance. When subjected to nanoscratch testing, these surfaces are shown to have extremely low coefficients of friction (as low as ∼0.015), as well as no detectable nanostructure deformation at contact forces up to 8,000 μN (estimated contact pressure greater than 1 GPa). In addition, the NSTSs have low adhesion (pull-off) forces on the order of less than 1 μN. The unique properties of these NSTSs provide avenues for designing low-friction, deformation-resistant surfaces that could benefit a variety of fields, including micro/nanoelectromechanical systems (MEMS/NEMS), microelectronics, magnetic recording, or any other application where the mechanical integrity of nanostructures is important.