Microstructure changes of extruded ultra high molecular weight polyethylene after gamma irradiation and shelf-aging

Study on the Aging Behavior of an Ultra-High Molecular Weight Polyethylene Fiber Barrier Net in a Marine Environment

In the present work, the performance of ultra-high molecular weight polyethylene (UHMWPE) barrier nets in marine environments is investigated by Fourier transform infrared spectroscopy, thermogravimetry, scanning electron microscopy, and tensile experiments. The chemical, morphological, thermal stability, and strength changes after aging in salt spray, hygrothermal, and ultraviolet (UV) environments are characterized. An environmental spectrum is designed to simulate a real service environment and predict the service life of UHMWPE. The results show that UV energy can activate UHMWPE molecules and lead to chain breaking, which lowers the breaking strength more efficiently than salt spray. In a hygrothermal environment, the UHMPE fibers bond into clumps, which causes a slight increase in breaking strength after the initial rapid decrease with aging time. The acceleration ratio of the environmental spectrum increases with increasing aging time, which may be caused by the cross-linking and degradation of macromolecular chains in the material. The environmental spectrum given by this work can be used to evaluate performance and predict the service life of UHMWPE barrier nets in marine environments.

Mapping the scientific research on the ionizing radiation impacts on polymers (1975–2019)

A bibliometric approach allowed us to study the global research trend on the impact of ionizing irradiations on polymers from 1975 to 2019. The investigation revealed 1,015 publications with growing interest since 1990. The research is split into three main categories: polymer science, nuclear science technology, and chemistry physical. The three main ionizing irradiations studied in this research are gamma, electron beam, and X-ray irradiations. The impact of ionizing irradiations on polymers under gamma irradiation is the most commonly studied field with 578 publications among the 1,015 publications. Electron beam irradiation is the second most studied field followed by X-ray irradiation. Whatever the irradiation modalities, publications focus on material degradation and material improvement studies.

Stabilization Effects of Natural Compounds and Polyhedral Oligomeric Silsesquioxane Nanoparticles on the Accelerated Degradation of Ethylene-Propylene-Diene Monomer

In this work the analysis on the stabilization activities of some natural antioxidants (rosemary extract, capsaicin, quercetin or oleanolic acid) is presented. A similar contribution of an inorganic structure-polyhedral oligomeric silsesquioxane (POSS) nanoparticles-is also evaluated. The stabilization effects on the oxidation protection were investigated for several formulations based on ethylene-propylene-diene-terpolymer (EPDM). The samples were examined in pristine state or after γ-irradiation, when the accelerated degradation scission of polymer macromolecules followed by the mitigation of oxidation. Three evaluation procedures: chemiluminescence, FTIR spectroscopy and thermal analysis were applied for the characterization of stability efficiency. The delaying effect of oxidative aging in EPDM matrix is illustrated by the values of activation energy, which are correlated with the type and concentration of embedded compounds. The durability of studied EPDM formulations is discussed for the assessment of material life. The improved behavior of structured hybrids useful for the optimization application regimes is essentially based on the antioxidant properties of polyphenolic components in the cases of natural antioxidants or on the penetration of free radical intermediates into the free volumes of POSS.

Gamma-irradiation-induced molecular-weight distribution and complexation affinity of humic acid with Cs+, Sr2+, and Eu3

Solutions of humic acid (HA) were irradiated with 0, 1, 5, 10, 50, and 100 kGy of gamma irradiation using a ⁶⁰Co source. The non-irradiated and irradiated HA molecules were fractionated by ultrafiltration into four categories: > 100, 50-100, 10-50, and < 10 kDa. Total organic carbon measurements and potentiometric titration analysis suggested that (1) some gamma-irradiated HA molecules were degraded into smaller molecules and (2) radiolytic degradation caused phenolic -OH became the predominant functional group in the small molecular-weight fractions of HA. The effect of absorbed dose of gamma rays on the distributions of Cs⁺, Sr²⁺, and Eu³⁺ ions in the molecular-weight fractions of the metal-HA systems was examined to discuss the complexation affinity. The metal ions were distributed in the smaller molecular-weight fractions at different doses, which corresponded to the degradation of HA molecules. For a predetermined absorbed dose, Cs⁺ ions did not change the molecular-weight distribution of the total organic carbon content of the degraded HA molecules. Conversely, the Sr²⁺ and Eu³⁺ ions redistributed organic carbon toward the larger molecular-weight fractions.

Probing the Influence of γ-Sterilization on the Oxidation, Crystallization, Sliding Wear Resistance, and Cytocompatibility of Chemically Modified Graphene-Oxide-Reinforced HDPE/UHMWPE Nanocomposites and Wear Debris

Osteolysis and aseptic loosening due to wear at the articulating interfaces of prosthetic joints are considered to be the key concerns for implant failure in load-bearing orthopedic applications. In an effort to reduce the wear and processing difficulties of ultrahigh-molecular-weight polyethylene (UHMWPE), our research group recently developed high-density polyethylene (HDPE)/UHMWPE nanocomposites with chemically modified graphene oxide (mGO). Considering the importance of sterilization, this work explores the influence of γ-ray dosage of 25 kGy on the clinically relevant performance-limiting properties of these newly developed hybrid nanocomposites in vitro. Importantly, this work also probes into the cytotoxic effects of the wear debris of different compositions and sizes on MC3T3 murine osteoblasts and human mesenchymal stem cells (hMSCs). In particular, γ-ray-sterilized 1 wt % mGO-reinforced HDPE/UHMWPE nanocomposites exhibit an improvement in the oxidation index (16%), free energy of immersion (-12.1 mN/m), surface polarity (5.0%), and hardness (42%). Consequently, such enhancements result in better tribological properties, especially coefficient of friction (+13%) and wear resistance, when compared with UHMWPE. A spectrum of analyses using transmission electron microscopy (TEM) and in vitro cytocompatibility assessment demonstrate that phagocytosable (0.5-4.5 μm) sterilized 1 mGO wear particles, when present in culture media at 5 mg/mL concentration, induce neither significant reduction in MC3T3 murine osteoblast and hMSC growth nor cell morphology phenotype, during 24, 48, and 72 h of incubation. Taken together, this study suggests that γ-ray-sterilized HDPE/UHMWPE/mGO nanocomposites can be utilized as promising articulating surfaces for total joint replacements.

Raman spectroscopy of biomedical polyethylenes

With the development of three-dimensional Raman algorithms for local mapping of oxidation and plastic strain, and the ability to resolve molecular orientation patterns with microscopic spatial resolution, there is an opportunity to re-examine many of the foundations on which our understanding of biomedical grade ultra-high molecular weight polyethylenes (UHMWPEs) are based. By implementing polarized Raman spectroscopy into an automatized tool with an improved precision in non-destructively resolving Euler angles, oxidation levels, and microscopic strain, we become capable to make accurate and traceable measurements of the in vitro and in vivo tribological responses of a variety of commercially available UHMWPE bearings for artificial hip and knee joints. In this paper, we first review the foundations and the main algorithms for Raman analyses of oxidation and strain of biomedical polyethylene. Then, we critically re-examine a large body of Raman data previously collected on different polyethylene joint components after in vitro testing or in vivo service, in order to shed new light on an area of particular importance to joint orthopedics: the microscopic nature of UHMWPE surface degradation in the human body. A complex scenario of physical chemistry appears from the Raman analyses, which highlights the importance of molecular-scale phenomena besides mere microstructural changes. The availability of the Raman microscopic probe for visualizing oxidation patterns unveiled striking findings related to the chemical contribution to wear degradation: chain-breaking and subsequent formation of carboxylic acid sites preferentially occur in correspondence of third-phase regions, and they are triggered by emission of dehydroxylated oxygen from ceramic oxide counterparts. These findings profoundly differ from more popular (and simplistic) notions of mechanistic tribology adopted in analyzing joint simulator data. Statement of Significance This review was dedicated to the theoretical and experimental evaluation of the commercially available biomedical polyethylene samples by Raman spectroscopy with regard to their molecular textures, oxidative patterns, and plastic strain at the microscopic level in the three dimensions of the Euclidean space. The main achievements could be listed, as follow: (i) visualization of molecular patterns at the surface of UHMWPE bearings operating against metallic components; (ii) differentiation between wear and creep deformation in retrievals; (iii) non-destructive mapping of oxidative patterns; and, (iv) the clarification of chemical interactions between oxide/non-oxide ceramic heads and advanced UHMWPE liners.

Comparison of oxidation resistance of UHMWPE and POM in H2O2 solution from ReaxFF reactive molecular dynamics simulations

The oxidation mechanism of ultra-high-molecular-weight polyethylene (UHMWPE) and polyoxymethylene (POM) in hydrogen peroxide solution was investigated by molecular dynamics (MD) simulations via reactive force field (ReaxFF) method. MD results from ReaxFF suggested that UHMWPE provided better antioxidation activity at high temperature (>373 K) than its POM counterpart in the same concentration of hydrogen peroxide solution. Furthermore, POM was relatively more susceptible to erosion and swelling because of the infiltration of H2O2 solution. Calculations of the diffusion coefficient at different temperatures permit further understanding of the chemical phenomena involved in the level of oxidation in the course of MD simulations. Results of the simulations are generally consistent with the previous experimental available in literature. The simulations also provide new insights into understanding the mechanism resulting oxidation products among the interested polymers.

High density polyethylene/graphite nano-composites for total hip joint replacements: processing and in vitro characterization

The main objective of the present study is to investigate how the thermal, rheological, mechanical and cytotoxicity behavior of High Density Polyethylene (HDPE) can be changed by the addition of graphite nano particles (GNPs) at different contents. The HDPE/GNPs composites were prepared using melt blending in a co-rotating intermeshing twin screw extruder. The in vitro tests results showed that the original material (HDPE) and all HDPE/GNPs composites do not exhibit any cytotoxicity to the WISH cell line. The microscopic examination of the nano-composite tensile-fractured surface found a good distribution of GNPs in the HDPE matrix. The Differential Scanning Calorimetry (DSC) results indicated that the crystallization percentage increased by adding GNPs to HDPE up to 4%. The XRD patterns of the HDPE/GNPs composites showed an increase in peak intensity compared to neat HDPE. This increase echoed the crystallinity results obtained from DSC. The rheological tests showed that the complex viscosity of the HDPE increased as the percentage of GNPs increased due to the restriction of the molecular mobility. The tensile test results showed that with increasing the GNPs content, Young's modulus and the yield strength of the HDPE/GNPs composite increased while the strain at fracture decreased. Finally, the preliminary results of the abrasion test indicated that the abrasion rate decreased by increasing the GNPs ratio up to 4% content. The prepared HDPE/GNPs composites appear to have fairly good comprehensive properties that make them a good candidate as a bearing material for the total joint replacement.

Macrophage Response to UHMWPE Submitted to Accelerated Ageing in Hydrogen Peroxide

Ultra-high molecular weight polyethylene (UHMWPE) has been the most commonly used bearing material in total joint arthroplasty. Wear and oxidation fatigue resistance of UHMWPE are regarded as two important properties to extend the longevity of knee prostheses. The present study investigated the accelerated ageing of UHMWPE in hydrogen peroxide highly oxidative chemical environment. The sliced samples of UHMWPE were oxidized in a hydrogen peroxide solution for 120 days with their total level of oxidation (Iox) characterized by Fourier Transformed Infrared Spectroscopy (FTIR). The potential inflammatory response, cell viability and biocompatibility of such oxidized UHMWPE systems were assessed by a novel biological in vitro assay based on the secretion of nitric oxide (NO) by activated murine macrophages with gamma interferon (IFN-γ) cytokine and lipopolysaccharide (LPS). Furthermore, macrophage morphologies in contact with UHMWPE oxidized surfaces were analyzed by cell spreading-adhesion procedure using scanning electron microscopy (SEM). The results have given significant evidence that the longer the period of accelerated aging of UHMWPE the higher was the macrophage inflammatory equivalent response based on NO secretion analysis.

Polyethylene oxidation in total hip arthroplasty: evolution and new advances

Ultra-high molecular weight polyethylene (UHMWPE) remains the gold standard acetabular bearing material for hip arthroplasty. Its successful performance has shown consistent results and survivorship in total hip replacement (THR) above 85% after 15 years, with different patients, surgeons, or designs. As THR results have been challenged by wear, oxidation, and liner fracture, relevant research on the material properties in the past decade has led to the development and clinical introduction of highly crosslinked polyethylenes (HXLPE). More stress on the bearing (more active, overweighted, younger patients), and more variability in the implantation technique in different small and large Hospitals may further compromise the clinical performance for many patients. The long-term in vivo performance of these materials remains to be proven. Clinical and retrieval studies after more than 5 years of in vivo use with HXLPE in THR are reviewed and consistently show a substantial decrease in wear rate. Moreover, a second generation of improved polyethylenes is backed by in vitro data and awaits more clinical experience to confirm the experimental improvements. Also, new antioxidant, free radical scavengers, candidates and the reinforcement of polyethylene through composites are currently under basic research.Oxidation of polyethylene is today significantly reduced by present formulations, and this forgiving, affordable, and wellknown material is still reliable to meet today's higher requirements in total hip replacement.

On the assessment of oxidative and microstructural changes after in vivo degradation of historical UHMWPE knee components by means of vibrational spectroscopies and nanoindentation

This study reports on the suitability of different experimental techniques to evaluate chemical, microstructural, and mechanical changes associated with in vivo oxidation encountered in historical polyethylene components. To accomplish this aim, eight traceable tibial inserts were analyzed after revision surgery. The knee bearings were gamma sterilized in air and implanted for an average of 11.5 years after a shelf life of no longer than 1 year. Characterization of oxidation and transvinylene indexes, crystallinity, amorphous, and intermediate phase fractions, along with hardness and surface modulus, were performed in transverse sections of each bearing using Fourier transform infrared spectroscopy, Raman spectroscopy, and nanoindentation, respectively. Generally, subsurface maxima in the crystallinity, oxidation index, and hardness were observed at a depth of about 1 mm in all of the bearings. The superior surfaces and anterior-posterior faces of the inserts exhibited significantly higher oxidation and greater crystallinity than the inferior side. These observations suggest that the metallic tray may limit the access of molecular oxygen to the backside of the tibial inserts. We conclude that chemical, physical, and mechanical properties data confirm the occurrence of in vivo degradation in the long-term implanted knee components following gamma irradiation in air. Furthermore, infrared spectroscopy alone appeared to provide excellent insight into the oxidation and crystallization state of the in vivo oxidized polyethylene.

Mechanical performance of electron-beam-irradiated UHMWPE in vacuum and in air

Ultrahigh molecular weight polyethylene (UHMWPE) was modified by a 5-MeV energy electron beam at different temperatures before, during, and after irradiation, both in air and in high vacuum. Wear resistance, hardness, and tensile strength of irradiated polyethylene were compared with those of untreated one. Physical analyses (like infrared spectroscopy and calorimetric analysis) were carried out to investigate about the changes in the material induced by irradiation. Experimental results suggested that structural changes (double bonds, crosslinks, and oxidized species formation) occur in the polymer depending on the environmental conditions of the irradiation. Mechanical behavior is related to the structural modifications. A temperature of 110 degrees C before, during, and after the in vacuum irradiation of UHMWPE produces a high amount of crosslinks and improves polymeric tensile and wear resistance, compared to that of the untreated material.

Structural changes of UHMWPE after e-beam irradiation and thermal treatment

Ultra-high molecular weight polyethylene (UHMWPE) was irradiated with accelerated electrons (1 MeV in air) using high dose rates (> 25 kGy/min) and thin specimens (thickness 1 mm). Parts of the specimens were remelted (200 degrees C for 10 min; 150 degrees C for 0, 2, 10, 30, 60 min). All specimens were stored in nitrogen in the dark at 5 degrees C. Supermolecular structure, extent of crosslinking, oxidative degradation, and macroradical content were studied by a number of methods (SAXS, WAXS, SEM, DSC, FTIR, ESR, TGA, solubility experiments, image analysis). The results obtained with irradiated samples were compared with those obtained with irradiated and remelted samples. It was confirmed that crosslinking predominates over chain scission at very high dose rates, even if the irradiation is performed in air. Discrepancies concerning supermolecular structure changes in UHMWPE after irradiation and thermal treatment, found in various studies in the literature, are discussed. A simple model, which describes and explains all supermolecular structure changes, is introduced. An effective way of eliminating residual macroradicals in UHMWPE is proposed.

Comparative fatigue behavior and toughness of remelted and annealed highly crosslinked polyethylenes

Highly cross-linked polyethylenes (HXLPEs) have been incorporated into the hip replacement armamentarium based on their improved wear resistance. However, two different methods of thermal treatment separate the orthopedic community as strategies to control potential long-term oxidation, and controversy remains with problems in the long-term use of acetabular liners (long-term oxidation, rim fracture after impingement, etc.). Meanwhile, the mechanical properties of HXLPEs that may alleviate these problems are still unclear. On the other hand, HXLPEs are scarcely used in knee replacements, as there exists concern about the probably reduced fatigue and fracture performances of these materials. Thus, our aim was to compare the effects of both thermal treatment regimes on mechanical properties and to associate these findings with the material microstructure. The fatigue behavior of annealed and remelted HXLPEs was characterized using short-term cyclic stress-strain, long-term fatigue, and fatigue crack propagation tests. On the other hand, impact tests, tensile experiments, and the J-integral multispecimen method allowed us to assess toughness. Microstructure features such as crosslink density, crystallinity percentage, and lamellar thickness were investigated by swelling measurements, differential scanning calorimetry, and transmission electron microscopy, respectively. This study confirms that annealing preserves mechanical properties better than remelting from both fatigue and fracture resistance points of view, and it remarks that a suitable selection of irradiation and stabilization conditions is needed to achieve optimal mechanical performances of ultra high molecular weight polyethylenes for each specific total joint replacement.