Irradiated Ultra-high Molecular Weight Research Articles

The use of vitamin E as an anti-adhesive coating for cells and bacteria for temporary bone implants

In recent years, vitamin E (or more specifically alpha tocopherol) has been widely discussed in the literature due to its strong anti-oxidant, -inflammatory, -cancer, and -bacterial properties and successfully applied in hip and knee arthroplasty to confer oxidation resistance to irradiated ultra-high molecular weight polyethylene (UHMWPE). In this study, vitamin E has been used, characterized, and in vitro tested as a coating on a chemically treated titanium alloy. The target final application is in temporary trauma fixation devices. The results of the physico-chemical characterization from FTIR-ATR and reflectance spectroscopy, z-potential titration curves, contact angle measurements, and tape test revealed a continuous coating, with hydrophobic behaviour, low surface energy (39 mN/m), and high adhesion to the substrate. Interestingly, the biological characterization revealed a strong anti-adhesion ability of the vitamin E coating as >90 % of the human mesenchymal stem cells (hMSC) seeded directly onto specimens' surface failed to attach, resulting nevertheless viable as demonstrated by the live/dead assay. Similarly, when vitamin E coated specimens were infected by the pathogens Staphylococcus aureus and Escherichia coli, a strong antifouling effect was observed as the colony forming units (CFU) count showed that >95 % bacteria were viable, but unable to adhere and colonize the Ti specimens (>2 logs reduction in comparison to controls).This work highlights a novel promising application of this biomolecule as a coating preventing unintended osseointegration on removable temporary devices, as well as preventing the adhesion of bacterial pathogens thus reducing the risk of implant-associated infections.

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.

Enhanced oxidation stability of highly cross-linked ultrahigh molecular weight polyethylene by tea polyphenols for total joint implants

Despite being currently state-of-the-art to prevent the oxidation of irradiated ultrahigh molecular weight polyethylene (UHMWPE) bearings, vitamin E (VE) poses concerns in the loss of cross-linking efficiency and is limited to be used at very low concentrations. It thus emphasizes the urgent demand for more efficient stabilizers. In this study, oxidation stability of highly cross-linked UHMWPE was demonstrated to be enhanced by tea polyphenols, such as lipid-soluble tea polyphenols (lsPPT), epigallocatechin gallate (EGCG), and lipid-soluble epigallocatechin gallate (lsEGCG). These antioxidants were blended with UHMWPE granules and consolidated by compression molding prior to E-beam irradiation. The presence of tea polyphenols substantially prolonged oxidation induction time of the irradiated UHMWPE before and after accelerated aging. Especially, lsEGCG was significantly superior to VE in terms of stabilizing capacity. Explained by the hydrogen donation mechanism, tea polyphenols with multiple phenolic hydroxyls could scavenge more radiation-induced free radicals than VE with only one phenolic hydroxyl, which was verified by the electron spin resonance spectra. Intriguingly, tea polyphenols showed less inhibitive effect on the cross-link density of irradiated UHMWPE than VE. Besides, there is no significant difference in crystallinity, mechanical performance as well as in vitro biocompatibility between the irradiated UHMWPE stabilized by tea polyphenols and VE. These findings highlight tea polyphenols, especially lsEGCG, are promising alternatives to extend the life span of UHMWPE implants.

The Influence of SBF on Surface Properties of Irradiated GO/UHMWPE Nanocomposites

In this study, the irradiated ultra-high molecular weight polyethylene (UHMWPE) components doping with graphene oxide (GO) were immersed in simulated body fluid (SBF) environment at 37°C for 6 months, and their surface properties were studied by ball indentation, scratch, and wetting tests. The results show that both the irradiation cross-linking treatment and the addition of 0.5 wt % GO can increase the indentation hardness and scratching coefficient of the surface of UHMWPE composites, and wettability of it becomes better. After soaking in SBF, the ball indentation and scratch coefficient of irradiated GO/UHMWPE nanocomposites decreased by 12.4 and 10.0%, respectively. Which may due to a swelling action and the oxidative degradation occurred on the surface of GO/UHMWPE nanocomposites. However, irradiation cross-linking and filling with GO can prevent the macromolecules from penetrating into UHMWPE base material and materials, thereby reducing the rate of swelling and oxidative degradation.

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

Wear study of cross linked UHMWPE hybrid composite-TiN interface

The effects of influencing factors at the dry sliding interface are essential in bio tribology, as the study on consequences of wear mechanism under wet condition. The gamma (γ) irradiated Ultra High Molecular Weight Poly Ethylene (UHMWPE) hybrid polymer bio composite for orthopaedic implants has been considered for analysing the complex wear mechanism against the bio ceramic TiN surface. The SEM morphology and the surface profile of the worn surface had confirmed that miscellaneous wear mechanisms occurred at the interface. The wear maps clearly indicate the combined effects of these factors and their relative influences on the wear behaviour of the tribopair. The correlation and distinction between the surface roughness (Ra) and the wear depth to the Coefficient Of Friction (COF) and the specific wear rate in the dry sliding wear phenomenon have been demonstrated.

Spectroscopic and sub optical band gap properties of e-beam irradiated ultra-high molecular weight polyethylene

Abstract Muller matrix spectro-polarimeter has been used to study the absorption behavior of pristine and e-beam irradiated (30, 65,100 kGy) ultra-high molecular weight polyethylene (UHMWPE) over the visible spectral range i.e. 400–800 nm. As a result, significant changes occur in the absorption behavior of irradiated samples due to radiation induced physical and chemical changes. To analyze these (radiation induced) changes in polymer matrix, Urbach edge method is employed for the calculation of optical activation energy. In addition to this, direct and indirect energy band gaps along the number of carbon atoms in C=C unsaturation have been determined by using modified Urbach formula and Tauc's equation, respectively. The results obtained during study reveal that Urbach energy decreases with radiation treatment and has a lower value for 100 kGy sample i.e. E u =71.63 meV. The values of direct and indirect energy band gaps are also following the decreasing trend with e-beam irradiation. Moreover, indirect energy gaps are found to have lower values as compared to direct energy gaps. The number of carbon atoms in clusters (as estimated from modified Tauc’s equation) has been found to vary from ∼6 to 8 for direct energy band gaps and from ∼9 to 11 for indirect energy band gaps.

Wear behavior of γ-irradiated ultra high-molecular weight polyethylene in a hip joint simulator

In this work, the wear behavior of cross-linked ultra-high-molecular weight polyethylene (UHMWPE) is discussed. The UHMWPE specimens are molded through injection molding techniques by varying the parameters of melting temperature (MT). The cross linking of UHMWPE was carried out by iridium 192 isotopes, where the specimen received 25 kGy energy. The wear tests were conducted on a hip joint simulator that was designed and fabricated in the laboratory. The contact loads are varied from 50 N to 100 N. The study revealed that MT has influenced the hardness and wear properties such as coefficient of friction and wear rate, respectively. γ-Irradiated UHMWPE has better wear and friction resistances than unirradiated UHMWPE. The worn out surfaces were examined with the help of scanning electron microscope, and it revealed the presence of wear mechanisms such as ironing, scratching, ploughing, plastic deformation, and fatigue wear.

The optical band gap and surface free energy of polyethylene modified by electron beam irradiations

In this study, investigations have been carried out on electron beam irradiated ultra high molecular weight polyethylene (UHMWPE). Polyethylene samples were irradiated with 1.5MeV electron beam at doses ranging from 50 to 500kGy. Modifications in optical properties and photoluminescence behavior of the polymer were evaluated by UV–vis and photoluminescence techniques. Changes of surface layer composition of UHMWPE produced by electron irradiations were studied by Rutherford back scattering spectrometry (RBS). The change in wettability and surface free energy induced by irradiations was also investigated. The optical absorption studies reveal that both optical band gap and Urbach’s energy decreases with increasing electron dose. A correlation between energy gap and the number of carbon atoms in clusters is discussed. Photoluminescence spectra were reveal remarkable decrease in the integrated luminescence intensity with increasing irradiation dose. Contact angle measurements showed that wettability and surface free energy increases with increasing the irradiation dose.

Dosimetric features and kinetic analysis of thermoluminescence from ultra-high molecular weight polyethylene

Thermoluminescence (TL) from beta irradiated ultra-high molecular weight polyethylene has been studied for measurements between 30 and 200 °C. An aliquot studied in this work produced TL glow curves consisting of two peaks, the main peak at 88 °C and a weaker intensity peak at 148 °C for heating at 1 °C s−1 following an excitation dose of 215 Gy. The position of the main peak is poorly reproducible for heating rates of 0.2 and 0.6 °C s−1 investigated with the peak position decreasing when the sample is freshly irradiated and the TL re-measured. The said change in peak position is however less of an effect for measurements made at 1 °C s−1 with the peak position being fairly reproducible in this case. Further measurements of the dosimetric properties of ultra-high molecular weight polyethylene showed that its dose response is linear from 26 Gy to about 161 Gy but exhibits slower growth in intensity with dose from about 860 Gy after regions of sub- and supra-linearity in between. If the TL is not measured immediately after irradiation, the signal fades with the delay approximately exponentially. In addition, a number of tests including phosphorescence analysis showed the possibility that the order of kinetics might not be unique but sensitive to several factors including measurement temperature. Thus for instance, the dependence of the peak position on the stop temperature in the partial heating procedure Tm − Tstop implied first-order kinetics but analysis of the geometrical factor μg for the same set of data gave μg = 0.46 ± 0.03 a value corresponding to characteristics somewhat intermediate between first and second order. In comparison, the results of analysis of the phosphorescence recorded at several temperatures on the rising edge of the main peak were only in agreement for measurements at 40 °C with general-order analysis suggesting second-order kinetics apply as did TL-like transformation of the monotonic phosphorescence decay. Both results were also consistent with an additional finding that the time dependence of the isothermal decay at 40 °C was consistent with second-order kinetics. Analysis of the TL for the activation energy using the initial rise method, variable heating rate procedure, TL-like transformation of phosphorescence and the peak shape estimates, produced a self-consistent set of values equal to 0.76 ±0.05 eV, 0.66±0.03 eV, 0.77±0.06 eV and values of the order of 0.8 eV, respectively. The kinetic parameters found in this study compare favourably with literature values.

HYSCORE and Davies ENDOR study of irradiated ultra high molecular weight polyethylene

Ultra high molecular weight polyethylene (UHMWPE) has been studied with different magnetic resonance techniques to elicit information on the nature and the location of radicals generated during high energy irradiation. Field swept electron paramagnetic resonance, pulsed Davies electron nuclear double resonance and hyperfine sublevel correlation spectroscopic measurements allowed extracting for the first time the full (1) H hyperfine coupling tensors of the most abundant radical, i.e. a secondary alkyl radical and to ascertain the formation of allyl radicals in the first stages of the irradiation process. The (1) H hyperfine coupling tensors are analogous to those reported for single crystal irradiated polyethylene, suggesting that radicals generated in UHMWPE are located in the crystalline region of the polymer.

Knee Simulator Wear of Vitamin E Stabilized Irradiated Ultrahigh Molecular Weight Polyethylene

Wear and damage of ultrahigh molecular weight polyethylene (UHMWPE) tibial inserts used in total knee arthroplasty are accelerated by oxidation. Radiation crosslinking reduces wear but produces residual free radicals adversely affecting stability. One alternative to stabilize radiation-crosslinked UHMWPE is to infuse the material with vitamin E (vit E). We investigated the properties of 100-kGy e-beam–irradiated UHMWPE that was subsequently doped with vitamin E in comparison with conventional UHMWPE. Both polymers were sterilized with gamma irradiation in vacuum packaging. Vitamin E–doped UHMWPE showed lower wear before and after aging (2.4 ± 0.5 and 2.5 ± 0.8 mg/million cycle, respectively, vs 26.9 ± 3.5 and 40.8 ± 3.0 mg/million cycle for conventional UHMWPE). Conventional UHMWPE showed oxidation after accelerated aging, and its mechanical properties were adversely affected, whereas vit E–doped UHMWPE showed no oxidation or changes in its mechanical properties. Vitamin E stabilization of radiation-crosslinked UHMWPE resulted in low wear and high oxidation resistance; it is an alternative load-bearing material for total knee applications.

In Vivo Biological Response to Vitamin E and Vitamin-E-Doped Polyethylene

Cross-linking has decreased the wear of ultra-high molecular weight polyethylene, a cause of osteolysis leading to total joint replacement failure. Compared with melting or annealing, doping cross-linked ultra-high molecular weight polyethylene with vitamin E stabilizes free radicals from irradiation while maintaining mechanical properties and wear resistance. This study was done to determine the local tissue effects of free vitamin E and vitamin E eluted from ultra-high molecular weight polyethylene implants in the joint space. Three studies were performed. First, pure vitamin E and solubilized vitamin E were injected into rabbit knees to simulate vitamin-E elution from radiation cross-linked ultra-high molecular weight polyethylene; second, vitamin-E-doped, irradiated ultra-high molecular weight polyethylene plugs were implanted into dorsal subcutaneous pouches of rabbits to determine the local effects of vitamin-E elution from radiation cross-linked ultra-high molecular weight polyethylene; and, third, two groups of vitamin-E-doped, irradiated acetabular liners (high surface and uniform vitamin-E concentration profiles) were compared with undoped, control ultra-high molecular weight polyethylene liners in a canine model of total hip replacement to determine the effect of possible vitamin-E elution on bone ingrowth and the local tissue response to it in a load-bearing environment. Injection of solubilized vitamin E resulted in histologically normal surrounding soft tissue at both two and twelve-week follow-up intervals, while injection of pure vitamin E resulted in acute and chronic inflammation at the time of the two-week follow-up. Both control and vitamin-E-doped subcutaneous plugs showed inflammation associated with surgery at two weeks of follow-up, but showed stable fibrous encapsulation without inflammation at twelve weeks of follow-up. In the canine total hip replacement model, there was no qualitative difference in local tissue appearance and no significant difference in the percent bone ingrowth and the percent bone density between the control and vitamin-E groups. These investigations showed that vitamin-E-doped ultra-high molecular weight polyethylene plugs and total hip replacement components are well tolerated in both a small and a large-animal model with no observed adverse effects on the surrounding tissues at twelve weeks of follow-up.

On the Mechanism of Polyenyl Photoconversion in Irradiated Ultrahigh Molecular Weight Polyethylene

The mechanism of polyenyl radical creation upon photoirradiation of ultrahigh molecular weight polyethylene (UHMWPE) containing free radicals is shown to be direct photoconversion from lower to higher order polyenyls. Therefore, the total radical concentration remains unchanged during this process. The rate of photoconversion is identical in the presence and absence of oxygen but is suppressed by hydrogen gas. This demonstrates that the conversion does not occur by a linear alkyl radical addition mechanism wherein alkyl radicals migrate to stable polyene unsaturations and polyenyl radicals, thereby increasing their order, as was previously suggested. Monochromatic photoirradiations show that dienyl radicals are created from diene unsaturations, when photoirradiated at 235 nm, and from allyl radicals, when photoirradiated at 275 nm. Additionally, it is shown that polyene unsaturations are not created during this process, and unlike allyl radicals, polyenyl radicals do not photoconvert to alkyl radicals.

On the morphology of some irradiated ultra high molecular weight polyethylenes

The morphology of various grades of ultra high molecular weight polyethylene (UHMWPE), prepared for use in orthopaedic implants, has been examined using differential scanning calorimetry (DSC), wide and small angle X-ray diffraction (WAX and SAX) and Raman spectroscopy. Preparation included gamma irradiation at various dose rates and mechanical annealing, and post-irradiation changes were of particular interest. The experimental results are interpreted in terms of previous proposals that UHMWPE is best considered as a three phase material, fully amorphous, all-trans amorphous and fully crystalline. The all-trans amorphous material is thought to be interfacial. The phase analysis shows that the age related increase in crystallinity occurs through conversion of all-trans material to fully crystalline, and there is little change in the total amorphous content of the polymers. SAX patterns show a change in the sharpness of the main diffraction peak and the emergence of a second diffraction peak at a higher q value, and this is considered to arise from crystallisation of all-trans amorphous material. Increasing the irradiation dose rate has a similar effect on the crystallography as does ageing the material. Mechanically annealed polymer also shows a similar trend towards a bimodal crystal population, accompanied by a reduction in interfacial material.

Crystal ageing in irradiated ultra high molecular weight polyethylene

Medical grade ultra high molecular weight polyethylene (UHMWPE) of two molecular weights has been gamma irradiated in air to give received doses of 3.5 and 10 Mrad and aged in air for 25 months. Differential scanning calorimetry and wide and small angle X-ray diffraction (WAX and SAX) techniques and transmission electron microscopy have been used to characterize the materials. Polymer from an orthopaedic component, retrieved 10 years after implantation, has been subjected to the same analytical programme. The X-ray diffraction data shows that following irradiation two events occur with time, first a crystal refinement process, indicated by pronounced sharpening of the SAX peak, and secondly growth of a new crystal population of reduced lamellae thickness compared to the original crystal structures, shown by the development of a bimodal SAX pattern. Following irradiation crystallinity increases with time and this second crystal population makes a significant contribution to that increase. The retrieved component shows full development of these processes. It is considered that these crystallographic changes with time are responsible for the observed time dependent changes in the mechanical properties of air irradiated UHMWPE.

Measurements of gas desorption from polyethylene-UHMWPE irradiated by 5 MeV electrons

Gas desorption in vacuum from electron irradiated ultra high molecular weight polyethylene (PE) is measured with a high sensible mass quadrupole spectrometer. Measurements are performed in thick PE irradiated with 5 MeV electron beams at doses of the order of tens of kGy. The irradiation modifies the PE molecules producing dehydrogenation, emission of different C x –H y groups, C-enrichment and carbon cross-linking processes. Results indicate that the radiation damage depends on the dose and that a significant change of chemical and physical polymer properties is reached for a critical dose of 18 kGy.

Examination of the long-lived, oxygen-induced radicals in irradiated ultra-high molecular weight polyethylene

Electron spin resonance (ESR) measurements were conducted to determine the thermal stability and structure of the long-lived, oxygen-induced radicals in irradiated ultra-high molecular weight polyethylene (UHMWPE). Employing a microwave saturation technique in conjunction with low-temperature (−133 °C) ESR, it was revealed that two most likely radicals, polyenyl (R1) and oxygen-centred di- or tri-enyl (R2) radicals, constitute the so-called oxygen-induced radicals (OIR). The reported single line due to OIR has in fact two resonance structures, one can be detected as an isolated signal (due to polyenyl, R1) at low microwave power (near 0.01 mW) and the other (oxygen-centred di- or tri-enyl) is prominent at high microwave power (>10 mW). The g-values of the radicals R1 and R2 were found to be 2.0044 and 2.0056, respectively. While R1 is a well-defined singlet with Δ H pp = 5.5 G, R2 has weak hyperfine lines (6) with proton coupling constant a H = 4.8 G. Annealing tests near 130 °C and 140 °C further suggest that R1 resides in crystalline regions, and R2 near crystalline surface with a dangling bond or in the voids or imperfections within the crystalline regions favouring oxygen attachment.

Lifetime of alkyl macroradicals in irradiated ultra-high molecular weight polyethylene

The interaction of UHMWPE with an electron beam in vacuum and in the presence of oxygen has been investigated. UHMWPE irradiated to various doses was examined with Electron Paramagnetic Resonance (EPR) and Fourier Transform infra-red (FTIR) spectroscopies. EPR was used to explore the nature of radicals produced in UHMWPE upon irradiation and to follow their decay as a function of time. Hydroperoxides formation and distribution throughout the samples were studied with FTIR spectroscopy. A correlation between the rate of decay of macroradicals and that of hydroperoxide formation has been proposed. Accordingly, the lifetime of secondary alkyl macroradicals in the amorphous phase of the polymer was found to range approximately from 3 to 10 h.

Migration stability of α-tocopherol in irradiated UHMWPE

The oxidation resistance of irradiated ultra-high molecular weight polyethylene (UHMWPE) components used in total joint arthroplasty can be improved by adding α-tocopherol (vitamin E) through diffusion. To ensure long-term oxidative stability, a minimum α-tocopherol concentration needs to be maintained throughout these components. Migration of α-tocopherol out of the components is one mechanism that could compromise long-term oxidative stability. We hypothesized that α-tocopherol could elute out during standard implant fabrication steps such as cleaning as well as during in vivo use. We doped 85 kGy irradiated UHMWPE with α-tocopherol at 120 °C and homogenized at 120 °C. We determined the extent of elution of α-tocopherol or its effect on oxidative stability following cleaning in isopropyl alcohol (IPA) and following 5 million cycles (MC) of simulated normal gait in bovine serum. There was no significant elution of α-tocopherol in repeated and prolonged cleaning in IPA as measured by average surface and bulk α-tocopherol concentrations. There was no change in the oxidative stability following 5 MC of hip simulator testing, indicating minimal elution during simulated normal gait.