Gamma-irradiated Ultrahigh Molecular Weight Research Articles

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 (μa) and scattering coefficients (μs). 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 (μa & μs) 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 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.

Fabrication and Study of Hardness Behavior on Gamma Irradiated Ultra-High Molecular Weight Polyethylene (UHMWPE) Plate

Fabrication and Study of Hardness Behavior on Gamma Irradiated Ultra-High Molecular Weight Polyethylene (UHMWPE) Plate

Effect of radiation dose on depth-dependent oxidation and wear of shelf-aged gamma-irradiated ultra-high molecular weight polyethylene (UHMWPE)

The effect of radiation dose on the depth-dependent oxidation and wear of shelf-aged gamma-irradiated UHMWPE was investigated in this paper. FTIR, micro-indentation, pin-on-plate wear tests and SEM imaging were carried out at three representative regions (surface, subsurface and center) for each sample. The experimental results show that when the oxidation index (OI) <1, the wear rate is clearly affected by the radiation dose (crosslinking density). When 1<OI<3, the wear rates are mainly controlled by the OI. When OI>3 – except for the 1000kGy specimen – the wear resistance is severely deteriorated and the relationship with the radiation dose is difficult to predict. Results suggest that higher irradiation (above 200kGy) is capable of lowering the oxidative degradation of UHMWPE.

A novel approach to the chemical stabilization of gamma-irradiated ultrahigh molecular weight polyethylene using arc-discharge multi-walled carbon nanotubes

A complete study was made of the stabilization of gamma-irradiated ultrahigh molecular weight polyethylene (UHMWPE) using arc-discharge multi-walled carbon nanotubes (MWCNTs) as inhibitors of the oxidative process. MWCNTs were efficiently incorporated into the polymer matrix by ball milling and thermo-compression processes at concentrations up to 5 wt% and subsequently gamma irradiated at 90 kGy. Raman spectroscopy demonstrated the generation of radicals on the walls of the MWCNTs and that the G/D ratio was altered by their generation. The same spectra showed interactions between the polymer chains as a series of shifts are observed in the UHMWPE bands. The effect of the MWCNTs as inhibitors for the oxidative process of the UHMWPE was evaluated by means of Electron Spin Resonance (ESR) and Fourier Transformed Infrared Spectroscopy (FTIR). ESR detection of the radiation-induced radicals proved the radical scavenger behaviour of MWCNTs. FTIR measurements were performed to ascertain the influence of the irradiation and of the accelerated ageing protocol in the oxidation index of the polymer and the composites. The results pointed to the positive contribution of the MWCNTs in increasing the oxidative stability of the composite when compared to pure UHMWPE. A comparison is made between composites obtained using MWCNTs produced by the carbon vapour deposition and arc-discharge methods.

Positron Lifetimes and Mechanical Properties of Gamma-Irradiated Ultra High Molecular Weight Polyethylene

Positron annihilation lifetime spectroscopy (PALS) has widely been used for probing open volume defects in various materials. PALS is in principle non-destructive, yet conventional PALS is not strictly non-destructive because cutting out of two specimens from the material is required. Recently we developed a novel method of PALS, which is potentially applicable to non-destructive, onsite material inspection. In order to explore the possibility of onsite monitoring of polymer degradation by this novel method of PALS, we studied variations of positron lifetime and mechanical properties of ultra high molecular weight polyethylene (UHMWPE) subjected to γ-irradiation. Correlations were found between the mechanical properties and ortho-positronium lifetimes, suggesting the feasibility of non-destructive, onsite monitoring of polymer degradation by PALS. The effect of γ-irradiation on positronium formation is discussed.

Multi-walled carbon nanotubes acting as free radical scavengers in gamma-irradiated ultrahigh molecular weight polyethylene composites

Multi-walled carbon nanotubes (MWCNTs) were incorporated in ultrahigh molecular weight polyethylene (UHMWPE), which is a polymer used in industrial and orthopedic applications. The composites were prepared by ball milling and thermo-compression processes at concentrations up to 3wt.% and subsequently gamma irradiated at 90kGy. Electrical conductivity measurements showed a low percolation threshold of 0.5wt.%. Electron spin resonance detection of the radiation-induced radicals proved the radical scavenger behavior of MWCNTs: when the nanotube concentration increased, the number of radicals generated by the gamma irradiation process decreased. Allyl radicals seem to be the radicals most affected by the presence of nanotubes in this polymeric matrix. Fourier transformed infrared spectroscopy measurements and an accelerated ageing protocol were performed to ascertain the influence of the irradiation on the oxidation index. The results pointed to the positive contribution of the MWCNTs in increasing the oxidative stability of the composite compared to pure UHMWPE. Crosslinking density induced by gamma irradiation was obtained by swelling measurements. The findings showed that, despite the radical scavenger performance, MWCNTs are capable of maintaining the efficiency of the crosslinking density, unlike the other antioxidants, which inhibit radiation crosslinking.

Microstructural Investigation of Gamma-Irradiated Ultra High Molecular Weight Polyethylene in Nitrogen Atmosphere

In this paper the results of structural changes in ultra high molecular polyethylene (UHMWPE) upon gamma irradiation and storage in nitrogen environment are reported. Based on differential scanning calorimetry (DSC), wide angle x-ray scattering (WAXS), gel content, fourier transform infrared spectroscopy (FTIR), and scanning electron microscope (SEM), we show that upon gamma irradiation crystallinity percentage increases by 20% at high doses due to aging, at low doses up to 50 kGy there is no significant changes. Lamellar thickness increases with the dose, possibly due to chain scission and the addition of new molecular segments to the lamellae. Upon storage in nitrogen DSC data confirmed the appearance of new smaller lamellae. Crosslinking was achieved by irradiation but the ratio of scission to crosslinking is higher after aging. This was confirmed by the increase in the oxidation index and the high brittle behavior of the polymer after aging where fibrous surface was noticed.

The effect of gamma irradiation and shelf aging in air on the oxidation of ultra-high molecular weight polyethylene

This study has investigated the effect of shelf aging, for up to one year in air, on the properties of gamma-irradiated ultra-high molecular weight polyethylene (UHMWPE). A variety of techniques were used to characterize the properties of treated samples. Differential scanning calorimetery (DSC) was used to characterize the morphology. The extent of cross-linking in a polymer network was detected by swelling measurements. The durometer hardness test was used to measure the relative hardness of this material, and changes in density were also measured. Results from all these measurements were combined to explain the changes in the microstructure of the aged, irradiated UHMWPE. This study shows that crystallinity is increased with radiation dose and with aging due to chain scission, which leads to a reduction in the molecular weight of the material. This allows the chains to rearrange to form crystalline regions. Positron annihilation lifetime spectroscopy confirms these conclusions. Fractional free volumes have been deduced from lifetime parameters, which correlate with the data obtained by the other techniques.

Correlation of Mechanical and Chemical Changes in Gamma-Irradiated Ultra-High Molecular Weight Polyethylene

Article Correlation of Mechanical and Chemical Changes in Gamma-Irradiated Ultra-High Molecular Weight Polyethylene was published on June 1, 2005 in the journal Journal of Polymer Engineering (volume 25, issue 4).

Wear Mechanisms of Untreated and Gamma Irradiated Ultra-High Molecular Weight Polyethylene for Total Joint Replacements

Modification of the surface microstructure of ultra-high molecular weight polyethylene (UHMWPE) is essential for improving the wear resistance of orthopedic implants. A common approach is to cross-link the polymer by gamma irradiation. The objective of this study was to examine the tribological behaviors of untreated and gamma irradiated UHMWPE under physiologically relevant contact conditions. Emphasis was placed on the identification of the dominant wear mechanisms in the early stage of polymer wear. The irradiation dose exhibited a strong effect on the tribological properties of UHMWPE sliding against Co–Cr alloy in a bath of bovine serum. Transmission electron microscopy (TEM) and environmental scanning electron microscopy (ESEM) were used to examine the microstructure and morphology of the worn surfaces. Regularly spaced folds with average spacing depending on the irradiation dose (i.e., cross-link density) formed on the wear tracks. Surface folding was related to plastic flow and the degree of mobility of the crystalline lamellae. SEM and TEM results elucidated the roles of the cross-link density and crystalline lamellae in the wear process. Based on the experimental evidence, a deformation model was obtained that provides explanation for the dependence of surface folding on the cross-link density and lamellae reorientation during sliding.

Gel fraction measurements in gamma-irradiated ultra high molecular weight polyethylene

Ultra high molecular weight polyethylene (UHMWPE) was irradiated at room temperature in air using an industrial 60Co γ-source. Samples were exposed to integrated doses of 50, 250, 500, 750, 1000, 1500 and 1750 kGy and the soluble part of each irradiated specimen was extracted by exposure to refluxing boiling xylene solvent for 10 h using the Soxhlet method. From such extractions, the swelling ratio and the gel fractions were determined. It was found out that an increase in radiation dose leads to an increase in the gel fraction and a simultaneous decrease in the swelling ratio. These tendencies are attributed to the formation of a crosslinked network.

Measurements of free radicals over a period of 4.5 years in gamma-irradiated ultra-high molecular weight polyethylene

Gamma irradiation-induced free radicals have been investigated for 4.5 years in medical grade ultra-high molecular weight polyethylene resins, GUR 4120, GUR 4150 and Himont 1900, at room temperature (23 °C) in absence (vacuum) or presence of oxygen (air), and at 75 °C in vacuum. In vacuum, the primary radicals (predominantly allyl) decayed following Y= Y R+ ae− bt and, in 30 days at 75 °C or 100 days at 23 °C, their concentration was reduced from ∼10 17 radical/g (initial) to a minimum Y R∼10 15 radical/g and remained at this level as long as the samples were kept in vacuum. When the samples were exposed to air, at any time during this study, these residual radicals were found to decay to a very stable, polyenyle-type terminal radical.

Use of radiation in biomaterials science

Radiation is widely used in the biomaterials science for surface modification, sterilization and to improve bulk properties. Radiation is also used to design of biochips, and in situ photopolymerizable of bioadhesives. The energy sources most commonly used in the irradiation of biomaterials are high-energy electrons, gamma radiation, ultraviolet (UV) and visible light. Surface modification involves placement of selective chemical moieties on the surface of a material by chemical reactions to improve biointeraction for cell adhesion and proliferation, hemocompatibility and water absorption. The exposure of a polymer to radiation, especially ionizing radiation, can lead to chain scission or crosslinking with changes in bulk and surface properties. Sterilization by irradiation is designed to inactivate most pathogens from the surface of biomedical devices. An overview of the use of gamma and UV radiation to improve surface tissue compatibility, bulk properties and surface properties for wear resistance, formation of hydrogels and curing dental sealants and bone adhesives is presented. Gamma and vacuum ultraviolet (VUV) irradiated ultrahigh molecular weight polyethylene (UHMWPE) exhibit improvement in surface modulus and hardness. The surface modulus and hardness of UHMWPE showed a dependence on type of radiation, dosage and processing. VUV surface modified e-PTFE vascular grafts exhibit increases in hydrophilicity and improvement towards adhesion of fibrin glue.

Gamma‐irradiated cross‐linked polyethylene in total hip replacements — analysis of retrieved sockets after long‐term implantation

The purpose of this study was to evaluate the mechanical and chemical characteristics of gamma-irradiated, cross-linked polyethylene after long-term service in vivo. Two gamma-irradiated ultra high molecular weight polyethylene (RCH 1000, molecular weight: 106) total hip replacement sockets were retrieved at 15 and 16 years after implantation. Mechanical and chemical characteristics of the sockets were evaluated in comparison with nonirradiated sockets. Significant surface oxidation occurred in the nonirradiated sockets; up to 75% of that seen in the irradiated ones. The mechanical properties of the irradiated sockets were not subject to increased deterioration in the presence of high free radical content. The cross-link was stable and was retained for a long period both in vivo and in ambient air. These data indicate that gamma-irradiated polyethylene was not subject to increased oxidative degradation during long-term service in vivo and confirmed the usefulness of this material as an articulating surface in total hip replacement.© 2001 John Wiley & Sons, Inc. J Biomed Mater Res (Appl Biomater) 58: 167–171, 2001

Gamma-irradiated cross-linked polyethylene in total hip replacements ? analysis of retrieved sockets after long-term implantation

The purpose of this study was to evaluate the mechanical and chemical characteristics of gamma-irradiated, cross-linked polyethylene after long-term service in vivo. Two gamma-irradiated ultra high molecular weight polyethylene (RCH 1000, molecular weight: 10(6)) total hip replacement sockets were retrieved at 15 and 16 years after implantation. Mechanical and chemical characteristics of the sockets were evaluated in comparison with nonirradiated sockets. Significant surface oxidation occurred in the nonirradiated sockets; up to 75% of that seen in the irradiated ones. The mechanical properties of the irradiated sockets were not subject to increased deterioration in the presence of high free radical content. The cross-link was stable and was retained for a long period both in vivo and in ambient air. These data indicate that gamma-irradiated polyethylene was not subject to increased oxidative degradation during long-term service in vivo and confirmed the usefulness of this material as an articulating surface in total hip replacement.

Relationship between gravimetric wear and particle generation in hip simulators: conventional compared with cross-linked polyethylene.

Hip-simulator studies have shown reduced gravimetric wear rates for inert-gas gamma-irradiated ultra-high molecular weight polyethylene when compared with conventional ethylene-oxide-sterilized ultra-high molecular weight polyethylene. Analysis shows a greater number of particles generated from inert-gas gamma-irradiated ultra-high molecular weight polyethylene. This study was undertaken to examine particle-generation rates of polyethylene with different levels of cross-linking and to correlate them with gravimetric wear data. Particle-generation rates did not correlate with gravimetric wear rates. Particle analysis should be performed to predict the in vivo behavior of bearing surface materials. Cross-linked ultra-high molecular weight polyethylene subjected to 10 Mrad (100,000 Gy) of gamma irradiation generated significantly fewer particles than ethylene-oxide-sterilized ultra-high molecular weight polyethylene; it also demonstrated a 96% reduction in the volume of particles.

Oxidation of gamma-irradiated ultrahigh molecular weight polyethylene

Ultrahigh molecular weight polyethylene (UHMWPE), the current polymer of choice in orthopedic prosthetic devices, is typically sterilized by exposure to Co-60 gamma irradiation prior to packaging for long-term storage. However, the exposure to Co-60 irradiation generates free radicals along the polymer chain that can participate in a series of reactions commencing with the oxidation of the free radicals to form reactive peroxy radicals. This study was undertaken to identify the role of hydroperoxide species in shelf-aged and accelerated aged UHMWPE samples by using a nitric oxide derivatization technique. It is shown that the concentration of hydroperoxides did not change appreciably with shelf aging. However, during accelerated aging the hydroperoxide concentration increased to a plateau and then decreased, suggesting its role as an intermediate in the process. By contrast, the concentrations of carbonyl species continued to increase during shelf aging and accelerated aging. The effects of several packaging materials on the oxidation characteristics were also investigated. A vacuum foil package is shown to be effective in preventing oxidation to a significant extent during accelerated aging. However, accelerated aging after removal from the foil pack resulted in oxidative degradation. Extended vacuum to remove dissolved oxygen and a 5-week room-temperature healing process in the foil pack were shown to be ineffective in reducing oxidative degradation. It also was shown that increased moisture content in the aging environment did not affect the degradation process. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2525–2542, 2000

Prevention of fatigue cracks in ultrahigh molecular weight polyethylene joint components by the addition of vitamin E.

Flaking-type wear, so-called delamination, is often observed in polyethylene joint components. This is thought to occur partly due to crack formation and propagation at grain boundaries. This study examined the effect of vitamin E on the crack formation and/or propagation in UHMWPE by using 2-dimensional sliding fatigue testing and micro indenter testing. An in vitro sliding fatigue test was performed under two simplified articulating movements, and the cracks produced were observed by scanning acoustic tomography (SAT). Gamma-irradiated ultrahigh molecular weight polyethylene (UHMWPE) specimens demonstrated a smaller area of accumulated cracks as compared to virgin specimens, when the loading movement was reciprocated on a single linear locus. However, four out of five gamma-irradiated UHMWPE specimens exhibited severe flaking-like destruction under the complicated sliding condition, suggesting that gamma irradiation accelerated crack propagation under multidirectional loading. All the gamma-irradiated vitamin-E-containing specimens demonstrated no subsurface crack formation and no flaking-like destruction. Results using micro indenter testing showed that the dynamic hardness at grain boundary was higher than that in grain, and was increased by gamma irradiation. This hardening at grain boundary was reduced by adding vitamin E. It is possible that the presence of vitamin E prevents crack propagation partly due to reduced hardness at grain boundaries. The gamma-irradiated vitamin-E-containing UHMWPE is a promising material to prevent flaking-like destruction of polyethylene joint components.

Fatigue Strength of Polyethylene After Sterilization by Gamma Irradiation or Ethylene Oxide

The oxidation level of ultrahigh molecular weight polyethylene specimens sterilized by gamma irradiation in either air or Ar gas was compared with that of unsterilized and ethylene oxide sterilized ultrahigh molecular weight polyethylene. The fatigue strength of ultrahigh molecular weight polyethylene specimens sterilized by gamma irradiation in air was compared with that of unsterilized and ethylene oxide sterilized ultrahigh molecular weight polyethylene. At the specimen surface, oxidation was highest for ultrahigh molecular weight polyethylene gamma irradiated in air, lower for ultrahigh molecular weight polyethylene gamma irradiated in Ar gas, and absent in unsterilized and ethylene oxide sterilized ultrahigh molecular weight polyethylene. At a depth of 3.5 mm below the specimen surface, oxidation levels were equivalent for ultrahigh molecular weight polyethylene gamma irradiated in either air or Ar gas whereas unsterilized and ethylene oxide sterilized specimens were again unoxidized. Thus, even in an inert atmosphere, oxidative degradation of gamma irradiated ultrahigh molecular weight polyethylene occurs. The 10 million cycle fatigue strength was similar for unsterilized and ethylene oxide sterilized ultrahigh molecular weight polyethylene whereas the fatigue strength of gamma irradiated in air ultrahigh molecular weight polyethylene was lower. Results of this study show that ethylene oxide gas does not degrade ultrahigh molecular weight polyethylene whereas gamma radiation in air causes changes in the polymer that adversely affect its mechanical properties. Ethylene oxide gas is a viable alternative to gamma radiation in air that avoids oxidation and fatigue strength degradation known to accompany irradiation of ultrahigh molecular weight polyethylene polymer bearing surfaces in total joint implants.