Improved wear performance of ultra high molecular weight polyethylene coated with hydrogenated diamond like carbon

Hydrogenated diamond like carbon (DLCH) thin films were deposited on medical grade ultra high molecular weight polyethylene (UHMWPE) by radio frequency plasma enhanced chemical vapor deposition. The DLCH coating thicknesses ranged from 250 to 700. nm. The substrates were disks made of UHMWPEs typically used for soft components in artificial joints, namely virgin GUR 1050 and highly crosslinked (gamma irradiated in air to 100. kGy) UHMWPEs. Mechanical and tribological properties under bovine serum lubrication at body temperature were assessed on coated and uncoated polyethylenes by means of nano-hardness and ball-on-disk tests, respectively. Morphological features of the worn surfaces were obtained by confocal microscopy and scanning electron microscopy. This study confirms an increase in surface hardness and good wear resistance for coated materials after 24. h of sliding test compared to uncoated polyethylene. These results point out that to coat UHMWPE with DLCH films could be a potential method to reduce backside wear in total hip and knee arthroplasties.Ministerio de Ciencia y Educación MAT2006-12603- C02-01, CSD2008-0002

Estabilización oxidativa del polietileno de ultra alto peso molecular en prótesis articulares

En la tesis "estabilización oxidativa del polietileno de ultra alto peso molecular en prótesis articulares" se ha desarrollado un trabajo de investigación original sobre la consecución y evaluación de la estabilidad oxidativa de este biopolimero con la incorporación de vitamina como antioxidante natural. Asimismo, se ha estudiado la validez de técnicas termo gravimétricas para discriminar el diferente comportamiento oxidativo de polietileno de ultra alto peso molecular irradiados y dopados con vitamina e, confirmando la utilidad de esta técnica en este objetivo. También se han estudiado los métodos de incorporación de vitamina e, mezclado y difusión, en el polímero, así como las técnicas apropiadas para la detección del antioxidante y medida de su concentración, destacando entre estas últimas las técnicas de colorimetría, de ángulo de contacto y dmta. Por último, la tesis aborda el efecto reforzante de nanotubos de carbono en el polietileno de ultra alto peso molecular, y por otra parte la actividad de los mismos como eliminadores de radicales libres.Peer reviewe

On the assessment of oxidative stability of post-irradiation stabilized highly crosslinked UHMWPEs by thermogravimetry

The characterization of the oxidative resistance of contemporary UHMWPE formulations currently relies upon time-consuming accelerated aging protocols and subsequent FTIR assessment. The potential of thermogravimetric analysis (TGA) for this purpose, however, remains mostly unknown. To study radiation and stabilization-induced changes in the decomposition process, TGA curves corresponding to virgin, e-beam irradiated, and post-irradiation stabilized UHMWPE materials were registered and analyzed. TGA traces of most materials exhibited similar features, with two weight increases before an initially non-linear decomposition and a portion of linear volatilization starting near 400 °C. After irradiation, the onset of the second weight increase shifted to lower temperatures and the linear volatilization stage began at higher temperatures. While post-irradiation annealing did not introduce significant changes in TGA curves, remelting delayed the onset of the second weight gain. Furthermore, this weight increase disappeared after vitamin E diffusion. Energies of activation of the initially predominant Thermoxidation and the subsequent thermodegradation processes were also calculated from fraction of conversion and reciprocal of temperature plots. As opposed to TGA, FTIR assessment of oxidation following accelerated aging at 120 °C for 36 h appeared to be less sensitive to discriminate between the different materials. This study confirms TGA as a promising technique to characterize the oxidative resistance of medical grade UHMWPEs.Research funded by Comisión Interministerial de Ciencia y Tecnología (CICYT), Spain. Project: MAT 2006-12603- C02-01.Peer Reviewe

Thermal and dynamic mechanical properties of vitamin E infused and blended ultra-high molecular weight polyethylenes

10 páginas, 10 figuras.Vitamin E (or α-tocopherol) is an alternative via to thermal treatments to achieve oxidative stability of gamma or electron beam irradiated ultra-high molecular weight polyethylenes (UHMWPE) used in total joint replacements. Our aim was to study the effects of vitamin E on the molecular dynamics and microstructural properties of UHMWPE. We hypothesized that the antioxidant would plasticize UHMWPE. Vitamin E was incorporated into UHMWPE at different concentrations by diffusion and blending and detected by ultraviolet and infrared spectroscopies from 500 ppm and 4000 ppm, respectively. Dynamic mechanical thermal analysis was used to characterize the influence of this antioxidant in the relaxations of the raw material. Differential scanning calorimetry and transmission electron microscopy served to characterize thermal and microstructure properties, respectively. Vitamin E concentrations above 3% by weight significantly reduced the degree of crystallinity and increased the melting transition temperature of raw UHMWPE. The presence of increasing concentrations of α-tocopherol introduced and/or strengthened the beta relaxation, which was also shifted toward gradually lower temperatures and had rising activation energies up to 188 kJ/mol. In addition, the gamma relaxation remained unaltered on vitamin E addition. Therefore, no plasticizing effects of vitamin E on the molecular dynamics of UHMWPE could be confirmed from mechanical spectroscopy data. However, the α relaxation was modified in intensity and location due to the changes in the degree of crystallinity introduced by the incorporation of vitamin E.Contract grant sponsor: Comisión Interministerial de Ciencia y Tecnología (CICYT), Spain. Contract grant sponsor: Ministerio de Ciencia e Innovación; contract grant number: MAT 2006-12603-C02- 01 and Consolider-Ingenio CDS2008-0023.Peer reviewe

Improved wear performance of ultra high molecular weight polyethylene coated with hydrogenated diamond like carbon

Hydrogenated diamond like carbon (DLCH) thin films were deposited on medical grade ultra high molecular weight polyethylene (UHMWPE) by radio frequency plasma enhanced chemical vapor deposition. The DLCH coating thicknesses ranged from 250 to 700. nm. The substrates were disks made of UHMWPEs typically used for soft components in artificial joints, namely virgin GUR 1050 and highly crosslinked (gamma irradiated in air to 100. kGy) UHMWPEs. Mechanical and tribological properties under bovine serum lubrication at body temperature were assessed on coated and uncoated polyethylenes by means of nano-hardness and ball-on-disk tests, respectively. Morphological features of the worn surfaces were obtained by confocal microscopy and scanning electron microscopy. This study confirms an increase in surface hardness and good wear resistance for coated materials after 24. h of sliding test compared to uncoated polyethylene. These results point out that to coat UHMWPE with DLCH films could be a potential method to reduce backside wear in total hip and knee arthroplasties. © 2010 Elsevier B.V.We thank the Ministry of Science and Education of Spain (projects MAT2006-12603-C02-01 and CONSOLIDER project CSD2008-00023 (FUNCOAT)) for financial support.Peer Reviewe

Efecto de la radiación gamma sobre PEUAPM/MWNTS nanocomposites

Trabajo presentado al XI Congreso Nacional de Materiales celebrado en Zaragoza del 23 al 25 de junio de 2010.El presente trabajo muestra una alternativa a los polietilenos altamente reticulados utilizados como material protésico. Se termoconformaron materiales compuestos de polietileno de ultra alto peso molecular (PEUAPM) con diferentes concentraciones de nanotubos de carbono multicapa (1%%, 3% y 5%peso MWNT). Posteriormente, los composites se irradiaron con radiación gamma a una dosis de 90 kGy. Las propiedades térmicas y microestructurales se determinaron con DSC, SEM y TEM. Los resultados de DSC mostraron que la incorporación de MWNTs reduce la cristalinidad del polímero. Los ensayos de tracción revelaron un aumento significativo en el módulo de Young, así como una compensación de la pérdida de tenacidad que la radiación produce sobre el polietileno, mostrando un efecto sinérgico entre la radiación y los nanotubos de carbono. Este efecto confirma la tendencia a atrapar radicales de los nanotubos de carbono.El presente trabajo ha sido financiado por el proyecto MAT2006-12603-C02-01 del Ministerio de Ciencia e Innovación.Peer reviewe

Microstructure, thermooxidation and mechanical behavior of a novel highly linear, vitamin E stabilized, UHMWPE

A novel, vitamin E-stabilized, medical grade ultra-high molecular polyethylene, MG003 (DSM Biomedical; The Netherlands), has been very recently introduced for use in total joint replacements. This homopolymer resin features average molecular weight similar to that of conventional GUR 1050 resin (5.5-6*10 6 g/mol), but a higher degree of linearity. The aim of this study was to characterize the microstructure, thermal and thermooxidation properties as well as the mechanical behavior of this novel MG003 resin before and after gamma irradiation in air to 90 kGy. For this purpose, a combination of experimental techniques were performed including differential scanning calorimetry (DSC), thermogravimetry (TG), transmission electron microscopy (TEM), X-Ray Diffraction, electron paramagnetic resonance (EPR), and uniaxial tensile tests. As-consolidated MG003 materials exhibited higher crystalline contents (~ 62%), transition temperatures (~ 140 °C), crystal thickness (~ 36 nm), yield stress (~ 25 MPa) and elastic modulus (~ 400 MPa) than GUR 1050 controls (55%, 136 °C, 27 nm, 19 MPa, and 353 MPa, respectively). Irradiation produced similar changes in both MG003 and GUR 1050 materials, specifically increased crystallinity (63% and 60%, respectively), crystal thickness (39 nm and 30 nm), yield stress (27 MPa and 21 MPa), but, above of all, loss of elongation to breakage (down to 442 and 469%, respectively). Thermogravimetric and EPR results suggest comparable susceptibilities to oxidation for both MG003 and GUR 1050 polyethylenes. Based on the present findings, MG003 appears as a promising alternative medical grade polyethylene and it may satisfactorily contribute to the performance of total joint replacements. © 2012 Elsevier B.V.Research funded by the Comisión Interministerial de Ciencia y Tecnología (CICYT), Spain. Project: MAT 2010-16175 and Consolider-Ingenio CDS2008-0023 from the Ministerio de Ciencia e Innovación.Peer Reviewe

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 3 wt.% and subsequently gamma irradiated at 90 kGy. Electrical conductivity measurements showed a low percolation threshold of 0.5 wt.%. 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. © 2012 Elsevier Ltd. All rights reserved.Research funded by the Comisión Interministerial de Ciencia y Tecnología (CICYT), Spain. Project: MAT 2010-16175.Peer Reviewe

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. © 2013 Springer Science+Business Media New York.The research was funded by the Comisión Interministerial de Ciencia y Tecnología (CICYT), Spain, Project: MAT 2010-16175.Peer Reviewe

Effects of gamma-irradiation on UHMWPE/MWNT nanocomposites

Ultra high molecular weight polyethylene (UHMWPE) is a polymer that is widely used in industrial and orthopaedic applications. In this work, pristine multiwalled carbon nanotubes (MWCNTs) were incorporated into UHMWPE in different concentrations (1, 3 and 5. wt.%) using a ball milling process. UHMWPE/MWCNT nanocomposites were gamma irradiated at 90. kGy to improve the interaction between MWCNTs and the polymer matrix. Structural, thermal and mechanical characterizations were conducted by means of transmission electron microscopy (TEM), Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA) and uniaxial tensile techniques. Gamma irradiation produced an increase in the melting temperature, crystallinity and temperature of maximum decomposition rate. The irradiation produced a 38% decrease in the toughness of neat UHMWPE. The incorporation of MWCNTs did not significantly affect the melting point of the neat UHMWPE but decreased the degree of crystallinity of the raw UHMWPE, which was related to a reduction in the UHMWPE lamellar density. An increase in thermal stability was also observed for the nanocomposites compared to neat UHMWPE. The tensile tests showed a 38% increase in the Young's modulus in the reinforced nanocomposites and a small decrease in toughness (5%). Gamma irradiation of the nanocomposites increased crystallinity, which was related to an increased lamellar thickness, and also improved their thermal stability. The Young's modulus increased by up to 71% for irradiated nanocomposites and their toughness showed no significant changes in comparison with the non-irradiated nanocomposites. The incorporation of MWCNTs reduced the negative effects of irradiation and compensated for the reduction in toughness. This fact might be attributed to the radical scavenger behaviour of the MWNT as was proved by Electron Spin Resonance (ESR) detection of the radiation-induced radicals. © 2010 Elsevier Ltd.Research funded by the Comisión Interministerial de Ciencia y Tecnología (CICYT), Spain. Project: MAT 2010-16175.Peer Reviewe