Propagation of Fatigue Cracks in Friction of Brittle Hydrogels

In order to understand fatigue crack propagation behavior in the friction of brittle hydrogels, we conducted reciprocating friction experiments between a hemi-cylindrical indenter and an agarose hydrogel block. We found that the fatigue life is greatly affected by the applied normal load as well as adhesion strength at the bottom of the gel–substrate interface. On the basis of in situ visualizations of the contact areas and observations of the fracture surfaces after the friction experiments, we suggest that the mechanical condition altered by the delamination of the hydrogel from the bottom substrate plays an essential role in determining the fatigue life of the hydrogel

Investigation of Wear and Wear Particles from Shelf-Aged Crosslinked UHMWPE under Different Contact Pressures

The wear particles generated from ultra-high molecular weight polyethylene (UHMWPE) are considered as the main reason to cause the osteolysis and aseptic loosening related to the failure of the artificial joint. This paper investigated the effect of radiation dose and contact pressure on the wear rate and wear particles of shelf-aged crosslinked UHMWPE using a multi-directional pin-on-plate wear tester. Scanning electron microscopy (SEM) and necessary software were used for quantitative analysis of wear particles. Results showed that for the shelf-aged crosslinked UHMWPE, the surface region where exhibited a low oxidation level, still maintained very good wear resistance. The specific wear rate was decreased with the increase of radiation dose and contact pressure. Quantitative analysis results of wear particles from crosslinked UHMWPE showed that after the wear tests, the percentage volume of smaller particles, complexity, specific biological activity (SBA) index of wear particles and the functional biological activity (FBA) index were reduced with increasing radiation dose. The contact pressure did not obviously affect the complexity of wear particles from crosslinked UHMWPE but had a clear influence on wear rate, SBA index and FBA index, and the effect on the particle size distribution was getting weaker as the radiation dose increases

Investigation on Oxidation of Shelf-Aged Crosslinked Ultra-High Molecular Weight Polyethylene (UHMWPE) and Its Effects on Wear Characteristics

Oxidation degradation of gamma-irradiated UHMWPE (Ultra-high Molecular Weight Polyethylene) is a well-known problem related to the failure of total joint replacement. The purpose of this study is to investigate the effects of oxidation on the mechanical and wear properties and to identify the main causes of the change of wear mechanisms of crosslinked UHMWPE (50 kGy and 100 kGy) after long-term shelf-ageing. The techniques used were FTIR mapping and micro-indentation test, which were performed on the same cross-section of UHMWPE specimen. Three distinct regions (the surface region, the more oxidized subsurface region, and the less oxidized center region) were prepared to carry out the differential scanning calorimetry (DSC) and multi-directional wear tests. The worn surfaces were checked by optical microscopy and scanning electron microscopy to reveal the wear mechanisms. The experimental results showed that the micro-hardness, elastic modulus and crystallinity increased with the increase of the oxidation index. The wear resistance deteriorated when the oxidation index increased. The slope of specific wear rate against oxidation index of the 100 kGy sample is higher than that of the 50 kGy sample. Our results may indicate that, when the oxidation index is lower than the critical threshold, the wear mechanism is mainly dependent on crosslinking density; when the oxidation index is higher than the critical threshold, the oxidation behavior plays an important role in the change of wear mechanisms