Vasiliki-Maria Archodoulaki

MALDI-TOF mass spectrometry imaging reveals molecular level changes in ultrahigh molecular weight polyethylene joint implants in correlation with lipid adsorption.

Ultrahigh molecular weight polyethylene (PE-UHMW), a material with high biocompatibility and excellent mechanical properties, is among the most commonly used materials for acetabular cup replacement in artificial joint systems. It is assumed that the interaction with synovial fluid in the biocompartment leads to significant changes relevant to material failure. In addition to hyaluronic acid, lipids are particularly relevant for lubrication in an articulating process. This study investigates synovial lipid adsorption on two different PE-UHMW materials (GUR-1050 and vitamin E-doped) in an in vitro model system by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry imaging (MSI). Lipids were identified by high performance thin layer chromatography (HP-TLC) and tandem mass spectrometry (MS/MS) analysis, with an analytical focus on phospholipids and cholesterol, both being species of high importance for lubrication. Scanning electron microscopy (SEM) analysis was applied in the study to correlate molecular information with PE-UHMW material qualities. It is demonstrated that lipid adsorption preferentially occurs in rough or oxidized polymer regions. Polymer modifications were colocalized with adsorbed lipids and found with high density in regions identified by SEM. Explanted, the in vivo polymer material showed comparable and even more obvious polymer damage and lipid adsorption when compared with the static in vitro model. A three-dimensional reconstruction of MSI data from consecutive PE-UHMW slices reveals detailed information about the diffusion process of lipids in the acetabular cup and provides, for the first time, a promising starting point for future studies correlating molecular information with commonly used techniques for material analysis (e.g., Fourier-transform infrared spectroscopy, nanoindentation).

Developing a Sealing Material: Effect of Epoxy Modification on Specific Physical and Mechanical Properties

To develop a matched sealing material for socket rehabilitation of grey cast iron pipes, an epoxy resin is modified by the addition of different components to improve the flexibility. Three different modifications are made by adding ethylene-propylene diene monomer (EPDM) rubber powder, reactive liquid polymer (ATBN) and epoxidized modifier. In this paper the effect of the modification method as well as the influence of absorption of water on the mechanical and physical properties are analyzed in terms of: tensile strength, modulus of elasticity, adhesion performance, pressure resistance, glass transition temperature and water content. A comparison with neat epoxy shows for all materials that the modulus of elasticity and strength decrease. Unlike other tested modification methods, the modification with rubber powder did not enhance the flexibility. All materials absorb water and a plasticization effect arises with further changes of mechanical and physical properties. The application of the sealant on the grey cast iron leads to a reduction of the strain at break (in comparison to the common tensile test of the pure materials) and has to be evaluated. The main requirement of pressure resistance up to 1 MPa was tested on two chosen materials. Both materials fulfill this requirement.

Failure analysis of retrieved PE-UHMW acetabular liners.

Ultra-high molecular weight polyethylene (PE-UHMW) acetabular liners have a limited lifespan in a patients body. There are many factors affecting the performance of the implant and furthermore the properties of the polymeric material are changing after implantation. In this work material changes according to structure and morphology and their implication on mechanical properties are in focus. The physical and mechanical properties of ten crosslinked (xL) PE-UHMW and nine conventional (conv) gamma-sterilized PE-UHMW hip components, used as sliding surface in total hip joint replacement, with different in-vivo times are compared. The evaluation of the retrieved acetabular liners is performed in view of crosslinking and conventional gamma-sterilization but also in terms of the influence of gender concerning alteration in properties. The oxidative degradation in the PE-UHMW is investigated by means of Fourier Transformed Infrared Spectroscopy (FTIR). The characterization of the morphology is carried out via differential scanning calorimetry (DSC). A depth profile of the micro-hardness and elastic modulus is taken over the cross-section of the components in order to find the influence of chemical constitution and morphology on the micro-mechanical properties. It could be shown that crosslinking and oxidative degradation influence the degree of crystallinity of the polymer. Oxidation occurs for both types of the material due to in-vivo time. Higher degree of crystallinity can be correlated to higher hardness and indentation modulus. No unequivocal superiority of crosslinked over conventional liners can be observed. The influence of sex concerning alteration of the evaluated properties matters but need to be further investigated.

Influence of Experimental Parameters on Fatigue Crack Growth and Heat Build-Up in Rubber

Loading parameters (frequency, amplitude ratio and waveform) are varied to determine their influence on fatigue crack growth in rubber. Up to three different rubber blends are investigated: one actual engineering material and two model materials. Fatigue crack growth curves and strain distributions of pure shear and faint waist pure shear samples are compared for a model material. Fatigue behavior is studied for three different frequencies (1 Hz, 3 Hz and 5 Hz). Amplitude ratio appears to be another important influence factor concerning fatigue crack growth in rubber. The beneficial effect of positive amplitude ratios (tensional loading conditions) is shown for different materials. However, fatigue crack growth is considerably increased for negative amplitude ratios (tensional-compressional loading conditions). Furthermore, the influence of the waveform is determined for three different waveform shapes. One is sinusoidal, and two have a square shape, including dwell periods and sinusoidal slopes. Special focus lies on heat build-up, which is substantial, especially for large loads, high frequencies and/or highly filled rubber blends. Plateau temperatures are determined for various loading conditions and rubber blends. A very simple linear relationship with dissipated energy per time and unit area is obtained. Results gathered with dynamic mechanical analyses show, likewise, a linear trend, but the heat build-up is very small, due to different sample geometries.

Influence of the Molar Mass on Long-Chain Branching of Polypropylene

Long-chain branching (LCB) with peroxydicarbonates (PODIC) is known as a suitable post-reactor process to introduce strain-hardening behaviour and an increase of melt strength to a linear polypropylene (PP). This opens up new possibilities for processing and therefore application. Especially in the case of adding value to PP post-consumer waste, LCB is a promising approach. LCB takes place by a combination of chain scission and recombination after radical activation of the PP macromolecule. However, chemical modification of post-consumer waste is challenging because of the inhomogeneous composition and the manifold number of PP grades. The influence of the molar mass of the linear PP precursor on this reaction was studied with different PP grades ranging from extrusion grade to injection moulding grade. To exclude side effects, all PP grades had similar polydispersity indices. A PP with higher molar mass undergoes significant chain scission during the LCB process compared to a PP with low molar mass for injection moulding. Therefore, the two grades differ significantly in their branching number, which influences their behaviour in elongational flow.

Epoxy Modifications—A Novel Sealing Material for Rehabilitation of Pipe Joints

The main reason for the flexibilisation of this epoxy sealing material is the necessity of the materials ability to compensate relative movements between the pipe segments. Epoxy (EP) modified with ethylene–propylene–diene rubber (EPDM) powder, reactive liquid polymer (ATBN—amine-terminated butadiene–acrylonitrile), and epoxidised modifiers as well as two customised epoxies were analysed. Concerning the dynamic-mechanical analysis, the formulations with reactive liquid polymer (ATBN) or EPDM showed nearly constant mechanical properties in the application temperature range (7–17 ℃). The formulations with epoxidised modifier and the customised proprietary epoxy showed a slope in this temperature range. Generally the glass transition temperature decreased and thus a change of the materials operating temperature range occurred. Thermogravimetric analysis showed that all epoxy modifications absorb water after immersion. Due to the water absorption a plasticisation effect was observed. Further changes in the materials operating temperature ranges have to be considered. A comparison with neat epoxy shows for all formulations a reduction of the mechanical properties like tensile strength and modulus of elasticity. However, no enhancement of strain at break could be achieved by modifying with EPDM powder. The modified epoxy EP/ATBN 2 as well as the customised epoxy C-EP 1 fulfilled the requirement of pressure resistance up to 1 MPa. The measured major strain was negligible. Results of the pressure and inclination test at the fibre concrete test rig show, that the tightness as well as the flexibility (inclination) of the repaired socket can be guaranteed up to a pressure of 1.3 MPa. Finally, a pressure test in a first field application confirmed the suitability of the epoxy based material C-EP 1 for use as a robot processed sealing material.

Studies on tribological behavior of PEEK and PE-UHMW

The tribological properties of Poly-ether-ether-ketone (PEEK) and Ultra-high-molecular-weight-polyethylene (PE-UHMW) were studied in a 2-body sliding system using a ball on plate (BOP) tribometer. The surface’s roughness (Ra) was changed for both PEEK and PE-UHMW to observe the influence of Ra on wear response. During the experiments, coefficient of friction (COF) was measured and correlated with the wear mechanisms. The frictional behavior of PEEK seems to develop in 3 stages, whereas PE-UHMW seem to have a stable frictional behavior throughout the entire testing period. Furthermore, changing the surface’s roughness allowed to observe that PEEK shifts its wear rate behavior when roughness is reversed. The wear rate seem to decrease with the increase of testing time for the samples with low Ra values whereas for high Ra values the samples registered an increase in wear rate with the increase of testing time. In case of PE-UHMW the change in roughness did not change the trend of the wear rate.