The influence of gamma irradiation and aging on degradation mechanisms of ultra-high molecular weight polyethylene

Thermal conversion of irradiated LLDPE waste into sustainable sponge-like compounds: a novel approach for efficient trace-level oil-water removal

The newest method for recycling waste linear low-density polyethylene (LLDPE) is the thermo-catalytic degradation technique known as catalytic pyrolysis. Typically, it is limited by 500-800 °C high temperatures. Catalytic pyrolysis releases toxins and forms harmful carbonized char. The current study is based on exposing wasted LLDPE to different gamma irradiation doses and then pyrolysis in castor oil (150-300 °C). The output product of Ir-(rLLDPE) is turned into another compound with a new structural architecture (sponge-like). SEM analysis confirms conversion, showing sponge-like spicules and layers. Ir-(rLLDPE) is sponge-like with a soft, malleable, absorbent texture. The DSC demonstrates altered thermal properties, with a melting point at 121 °C splitting into two peaks (endothermic at 117 °C and exothermic at 160 °C). The exothermic peaks signify the curing process of the sponge-like material. Ir-(rLLDPE) is assessed as an adsorbent for aqueous oils and solvents. The study examines irradiation doses, pyrolysis temperature, and time on adsorbent capacity. The oil removal obeys the Langmuir isotherm with monolayer adsorption, with a maximum adsorption capacity of 24.75 g/g of waste oil and 43 g/g of 1,1,2,2-tetrachloroethane. Squashing maintains adsorption after 20 reuses. Data shows sponges effectively clean marine oil spills and solvents.

Simulation of Light Distribution in Gamma Irradiated UHMWPE Using Monte Carlo Model for Light (MCML) Transport in Turbid Media: Analysis for Industrial Scale Biomaterial Modifications

(1) Background: This study investigated the miscibility of carbon-based fillers within industrial scale polymers for the preparation of superior quality polymer composites. It focuses on finding the light distribution in gamma irradiated ultra-high molecular weight polyethylene (UHMWPE). (2) Methods: The Kubleka–Munk model (KMM) was used to extract the optical properties, i.e., absorption coefficients (μ_a) and scattering coefficients (μ_s). Samples amounting to 30 kGy and 100 kGy of irradiated (in the open air) UHMWPE from 630 nm to 800 nm were used for this purpose. Moreover, theoretical validation of experimental results was performed while using extracted optical properties as inputs for the Monte Carlo model of light transport (MCML) code. (3) Conclusions: The investigations revealed that there was a significant decrease in absorption and scattering coefficient (μ_a & μ_s) values with irradiation, and 30 kGy irradiated samples suffered more compared to 100 kGy irradiated samples. Furthermore, the simulation of light transport for 800 nm showed an increase in penetration depth for UHMWPE after gamma irradiation. The decrease in dimensionless transport albedo  μs(μa+μs) from 0.95 to 0.93 was considered responsible for this increase in photon absorption per unit area with irradiation. The report results are of particular importance when considering the light radiation (from 600 nm to 899 nm) for polyethylene modification and/or stabilization via enhancing the polyethylene chain mobility.

Ultra-High-Molecular-Weight-Polyethylene (UHMWPE) as a Promising Polymer Material for Biomedical Applications: A Concise Review

Ultra-High Molecular Weight Polyethylene (UHMWPE) is used in biomedical applications due to its high wear-resistance, ductility, and biocompatibility. A great deal of research in recent decades has focused on further improving its mechanical and tribological performances in order to provide durable implants in patients. Several methods, including irradiation, surface modifications, and reinforcements have been employed to improve the tribological and mechanical performance of UHMWPE. The effect of these modifications on tribological and mechanical performance was discussed in this review.

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.

The impact of storage conditions upon gentamicin coated antimicrobial implants

A systematic approach was developed to investigate the stability of gentamicin sulfate (GS) and GS/poly (lactic-co-glycolic acid) (PLGA) coatings on hydroxyapatite surfaces. The influence of environmental factors (light, humidity, oxidation and heat) upon degradation of the drug in the coatings was investigated using liquid chromatography with evaporative light scattering detection and mass spectrometry. GS coated rods were found to be stable across the range of environments assessed, with only an oxidizing atmosphere resulting in significant changes to the gentamicin composition. In contrast, rods coated with GS/PLGA were more sensitive to storage conditions with compositional changes being detected after storage at 60 °C, 75% relative humidity or exposure to light. The effect of γ-irradiation on the coated rods was also investigated and found to have no significant effect. Finally, liquid chromatography-mass spectrometry analysis revealed that known gentamines C1, C1a and C2 were the major degradants formed. Forced degradation of gentamicin coatings did not produce any unexpected degradants or impurities.

Wear of '100 Mrad' cross-linked polyethylene: effects of packaging after 30 years real-time shelf-aging

Studies have shown that gamma-irradiation of polyethylene (PE) generally results in degradation by surface oxidation. However, from 1970 to 1978 Oonishi et al. used ultra-high-molecular-weight polyethylene (UHMWPE) cross-linked and sterilized by 100 Mrad of gamma-irradiation in air (100 Mrad PE) for total hip prostheses, and obtained excellent clinical results extending for 30 years. In the present study, we used a hip joint simulator to investigate the wear characteristics of 100 Mrad PE cups which had been shelf-aged for an extremely long period (30 years). The PE cups, aged in an air-containing triple polyethylene package for 30 years (packaged 100 Mrad PE), showed low wear with 3.4 mg of weight loss, even after 5 x 10(6) cycles. In contrast, non-packaged 100 Mrad PE showed considerable wear: 47.0 mg at run-in ((0-0.25) x 10(6) cycles) and 114.1 mg at the end of 5 x 10(6) cycles. The substantially, lower wear even in the presence of an oxidized surface layer for the packaged 100 Mrad PE, was comparable to the low wear seen on retrieved 100 Mrad PE after 30 years of clinical use. The long-term shelf-storage conditions, which affect the surface oxidative degradation of PE, are assumed to be the key factor in the wear-resistance of gamma-irradiated UHMWPE.

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.

gamma-irradiation of PEGd,lPLA and PEG-PLGA multiblock copolymers: II. effect of oxygen and EPR investigation

The purpose of this research was to evaluate how the presence of oxygen can affect irradiation-induced degradation reactions of PEGd,lPLA and PEG-PLGA multiblock copolymers submitted to gamma irradiation and to investigate the radiolytic behavior of the polymers. PEGd,lPLA, PEG-PLGA, PLA, and PLGA were irradiated by using a (60)Co irradiation source in air and under vacuum at 25 kGy total dose. Mw and Mn were evaluated by gel permeation chromatography. The stability study was carried out on three samples sets: (a) polymer samples irradiated and stored in air, (b) polymer samples irradiated and stored under vacuum, and (c) polymer samples irradiated under vacuum and stored in air. The thermal and radiolytic behavior was investigated by differential scanning calorimetry and electron paramagnetic resonance (EPR), respectively. Samples irradiated in air showed remarkable Mw and Mn reduction and Tg value reduction due to radiation-induced chain scission reactions. Higher stability was observed for samples irradiated and stored under vacuum. EPR spectra showed that the presence of PEG units in multiblock copolymer chains leads to: (a) decrease of the radiolytic yield of radicals and (b) decrease of the radical trapping efficiency and faster radical decay rates. It can be concluded that the presence of oxygen during the irradiation process and the storage phase significantly increases the entity of irradiation-induced damage.

Oxidation and wear of 100-Mrad cross-linked polyethylene shelf-aged for 30 years

Some previous studies suggest that aging influences wear and oxidatively degraded nonsterilized ultra-high-molecular-weight polyethylene (UHMWPE) exhibits decreased wear resistance. We therefore asked whether shelf-aging storage conditions influenced degradation and wear resistance of gamma-irradiated UHMWPE. We examined oxidation and wear of 100-Mrad gamma-irradiated UHMWPE (100-Mrad polyethylene) cups shelf-aged for 30 years without (n=2) or with (n=2) packages. The oxidation index of the unpackaged 100-Mrad polyethylene surface (4) was higher than that of the packaged one (2.7). The packaged 100-Mrad polyethylene cup exhibited a high wear resistance with a steady wear rate of 0.5 mg/10(6) cycles. In contrast, the unpackaged 100-Mrad polyethylene exhibited an extremely high initial wear rate of 187.9 mg/10(6) cycles over the first 0.25 x 10(6) cycles with a subsequently reduced wear rate of 5 mg/10(6) cycles after 5 x 10(6) cycles. Packaging over long periods inhibits surface oxidation and maintains the wear resistance of gamma-irradiated UHMWPE cups.

Influence of the remelting process on the fatigue behavior of electron beam irradiated UHMWPE

Electron beam irradiation at doses below 150 kGy is a widely used technique to obtain highly crosslinked ultra-high-molecular-weight polyethylene (UHMWPE). Its current use in total joint replacement components may improve wear resistance and decrease UHMWPE particle debris. However, currently used post-irradiation thermal treatments, which aim to decrease the free radicals within the material, introduce microstructural changes that affect UHMWPE mechanical properties, particularly the fatigue strength. This influence may be crucial in total knee replacements, where fatigue-related damage limits the lifespan of the prosthesis. Therefore, more studies are required to understand UHMWPE fatigue after current crosslinking protocols. This study was planned to evaluate the influence of UHMWPE remelting after irradiation on the material fatigue resistance. The remelting was achieved at 150 degrees C for 2 h on UHMWPE previously irradiated at 50, 100, and 150 kGy. Fatigue evaluation included short-term tests under cyclic tensile stress with zero load ratio, R = 0, and 1 Hz. In addition, stress-life testing was performed using 12% yield as the criterion for failure. Near-threshold fatigue crack propagation experiments were also performed at a frequency of 5 Hz, and crack length was measured in nonthermally treated and remelted irradiated UHMWPE. Crystallinity percentage was calculated from DSC measurements. The results pointed out that irradiation positively contributed to total life analysis, but the further remelting process decreased the flaw initiation resistance. On the other hand, both processes negatively affected the fatigue resistance of notched components. From a clinical point of view, the results suggest that the material fatigue behavior should be carefully studied in new UHMWPE to avoid changes related to material processing.

Comparative cyclic stress-strain and fatigue resistance behavior of electron-beam- and gamma-irradiated ultrahigh molecular weight polyethylene

Fatigue-related damage in UHMWPE is one of the main causes of long-term failure in total joint replacements. Crosslinking ultrahigh molecular weight polyethylene (UHMWPE) by gamma or electron-beam irradiation, in combination with prior or further thermal treatment, enhances its wear resistance against metallic components in total hip replacements, and eventually in knees. However, little information is available on the fatigue response of this modified UHMWPE. The objective of this study was to compare electron-beam-irradiated UHMWPE at 50, 100, and 150 kGy, with the well-known 25 kGy gamma-irradiated UHMWPE. Two different cyclic tests were performed under tensile stress, with a zero load ratio, R = 0. First, specimens were subjected to a sinusoidal load cycle at 1 Hz, which provided stress-life curves with the use of a failure criterion based on 12% yield strain. Second, specimens were tested under 50 load cycles at a displacement rate of 15 mm/min, which provided information about the evolution of secant modulus and plastic strain. The incubation period was also analyzed. DSC measurements were carried out to check the crystallization effect of irradiation. According to the results of fatigue resistance there was a crossover behavior between gamma- and electron-beam-irradiated UHMWPE regarding the applied stress. When the stress was higher than the crossover value, the fatigue resistance of gamma-irradiated samples was higher than electron-beam-irradiated ones. When the stress was lower, the fatigue behavior was the opposite. The crossover stress depended on the electron-beam-irradiation dose. The clinical relevance of this study lies in an improved knowledge of electron-beam-irradiated material under extreme mechanical circumstances, such as fatigue.

Oxidation of orthopaedic UHMWPE

Retrieved EtO sterilised acetabular cups usually show much less degradation than gamma-ray sterilised cups. Some of our retrieved EtO sterilised cups did, however, reveal unexpected bulk oxidation. It was observed that this oxidation was always accompanied by whitening of the material. This whitening was found to be due to a break-up of the compression moulded material into its original particles. It was noticed that there was no oxidation in all parts, where the break-up and whitening appeared. The oxidation did, however, occur exclusively in the parts where there was a badly consolidated material. Upon examining shelf aged, unsterilised samples, it was found that the degradation was also present here. This shows that the observed phenomenon is not due to the service in vivo and that it must originate from the processing step. Just as for the retrieved samples, the shelf aged cups only showed oxidation in the bulk and not at the surface. It was concluded that the material used for the cups had been badly fused together during the compression moulding and that the machining had created a bad stress situation in the cups leading to a break-up of the particles. The mechanism that initiates the oxidation is not known, but it is believed that the distribution depends on how the internal stresses have acted to break up the structure. In the areas where the particles have been separated, there is probably a higher availability of oxygen than what is normally observed in UHMWPE.