Influence of processing conditions on microstructural, mechanical and tribological properties of graphene nanoplatelet reinforced UHMWPE.

Ultra-high molecular weight polyethylene (UHMWPE) is a relevant thermoplastic in industry and a well-proven standard biomaterial in joint replacements. To enhance its tribological properties while preserving its bulk ones, composite coatings on a UHMWPE substrate were prepared using non-functionalised graphene nanoplatelet (GNP) at reinforcement concentration of 0.1-5 wt% and two mechanical mixing techniques (ball mill or blade mixer) with different consolidation temperatures of 175-240 °C. Changes in morphology and size of the UHMWPE particles before hot-pressing were observed in function of the mechanical mixing techniques applied. Wear rate was affected by graphene content, reaching a minimum at 0.5 wt% GNP, with a reduction of 20 and 15%, for ball milling and blade mixer, respectively. However, blade mixer increased the wear rate by around twice respect the ball milling results, for all the studied materials. The coefficient of friction decreased notably, by ~25%, below 3 wt% GNP content, and hardness increased by 24%, regardless of the mechanical mixing process used. Finally, consolidation temperature had a positive influence on wear rate at temperatures of around 195 °C, which could be related to the free radical scavenger effect of the GNP.

A methodology for the customized design of colonic stents based on a parametric model.

The choice of necessary stent properties depends mainly on the length of the stenosis and degree of occlusion. So a stent design with variable radial stiffness along its longitudinal axis would be a good option. The design proposed corresponds to a tube-based stent with closed diamond-shaped cells made from a NiTi alloy. By acting independently on different geometric factors, variable geometries can be obtained with different radial force reactions. A design adjustment according to specific requirements, in order to get a better fit to ill-duct and reduces complications, is possible. A parametric analysis using finite element has been conducted to determine the influence of slot length, number of circumferential slots, tube thickness and shape-factor on stent mechanical behavior, which allow eliminating the need for extensive experimental work and knowing and quantifying the influence of those factors. The results of finite element simulations have been used, by means of least-squares fit techniques, to obtain analytical expressions for the main mechanical characteristics of the stent (Chronic Expansive Radial Force and Radial Compression Resistance) in terms of the different geometrical factors. This allows the stent geometry to be customized without launching an iterative and costly process of modeling and simulation for each case.

Evaluation of carbon nanotubes and graphene as reinforcements for UHMWPE-based composites in arthroplastic applications: A review.

In this review we have evaluated the state of the art of CNT/UHMWPE and graphene/UHMWPE composites from four different points of view: mechanical properties, chemical stability, wear resistance and biocompatibility. The performance of these new carbonaceous composites allows us to conclude that these materials have overcome a first step on the way to developing into an alternative to the current HXLPEs used in orthopedic applications.

Impact resistance and fractography in ultra high molecular weight polyethylenes.

Highly crosslinked ultra high molecular weight polyethylenes (UHMWPE) stabilized by a remelting process or by the addition of an antioxidant are highly wear resistant and chemically stable. However, these polyethylenes currently used in total joint replacements suffer a loss of mechanical properties, especially in terms of fracture toughness. In this study we analyze the impact behavior of different polyethylenes using an instrumented double notch Izod test. The materials studied are three resins: GUR1050, GUR1020 with 0.1wt% of vitamin E, and MG003 with 0.1wt% of vitamin E. These resins were gamma irradiated at 90kGy, and pre and post-irradiation remelting processes were applied to GUR1050 for two different time periods. Microstructural data were determined by means of differential scanning calorimetry and transmission electron microscopy. Fractography carried out on the impact fracture surfaces and images obtained by scanning electron microscopy after etching indicated the existence of a fringe structure formed by consecutive ductile-brittle and brittle-ductile transitions, which is related to the appearance of discontinuities in the load-deflection curves. A correlation has been made of the macroscopic impact strength results and the molecular chain and microstructural characteristics of these aforementioned materials, with a view to designing future resins with improved impact resistance. The use of UHMWPE resins with low molecular weight or the application of a remelting treatment could contribute to obtain a better impact strength behavior.

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.

Probabilistic assessment of fatigue initiation data on highly crosslinked ultrahigh molecular weight polyethylenes.

Ultrahigh molecular weight polyethylenes (UHMWPE) showing wear resistance, oxidative stability and good mechanical performance go on being a relevant research area in biomaterials for total joint replacements, where fatigue happens to be a recurrent damage mode that needs to be investigated. While crack propagation lifetime has been extensively studied, fatigue initiation data are scarcely offered in the literature, often due to the higher costs implied in the experimental programs. Moreover, their analysis is not always suitable to obtain reliable guidance. Different deterministic and probabilistic methods, generally resting on empirical bases have been previously used to analyze the fatigue initiation data. In this work, the probabilistic Weibull regression model of Castillo et al., based on both physical and statistical conditions, such as weakest link principle and the necessary compatibility between life-time and stress range distributions, is applied for the first time in the assessment of fatigue results of polymers, particularly to highly crosslinked UHMWPEs (HXLPEs). Accordingly, different published experimental data corresponding to HXLPE stabilized by thermal treatments and with α-tocopherol (vitamin E) are re-analyzed. Additional data are incorporated to assess the influence of notched HXLPE on fatigue performance. New conclusions are drawn from this revision.

DLC coatings for UHMWPE: relationship between bacterial adherence and surface properties.

Development of intrinsically antibacterial surfaces is of key importance in the context of prostheses used in orthopedic surgery. This work presents a thorough study of several plasma-based coatings that may be used with this functionality: diamond-like carbon (DLC), fluorine-doped DLC (F-DLC), and a high-fluorine-content-carbon-fluor polymer (CF(X)). The coatings were obtained by a radio-frequency plasma-assisted deposition on ultra high molecular weight polyethylene (UHMWPE) samples and physicochemical properties of the coated surfaces were correlated with their antibacterial performance against collection and clinical Staphylococcus aureus and Staphylococcus epidermidis strains. The fluorine content and the relative amount of C-C and C-F bonds were controlled by X-ray photoelectron spectroscopy, and hydrophobicity and surface tension by contact angle measurements. Surface roughness was studied by Atomic Force Microscopy. Additional nanoidentation studies were performed for DLC and F-DLC coatings. Unpaired t test and regression linear models evaluated the adherence of S. aureus and S. epidermidis on raw and coated UHMWPE samples. Comparing with UHMWPE, DLC/UHMWPE was the least adherent surface with independence of the bacterial species, finding significant reductions (p ≤ 0.001) for nine staphylococci strains. Bacterial adherence was also significantly reduced in F-DLC/ UHMWPE and CFx/UHMWPE for six strains.

Mullins effect behaviour under compression in micelle-templated silica and micelle-templated silica/agarose systems.

The mechanical properties of bioceramic conformed pieces based on micelle-templated silica (MTS) such as SBA15, MCM41 and MCM48 as well as MTS/agarose systems have been evaluated under static and cyclic compressive tests. The MTS pieces exhibited a brittle behaviour. Agarose, a biocompatible and biodegradable hydrogel, has been used to shape ceramic-agarose pieces following a low temperature shaping method. Agarose conferred toughness, ductility and a rubbery consistency up to a 60% strain in ceramic MTS/agarose systems leading to a maximum strength of 10-50 MPa, without losing their initial cylindrical structure. This combination of ceramic and organic matrix contributes to avoiding the inherent brittleness of the bioceramic and enhances the compression resistance of hydrogel. The presence of mechanical hysteresis, permanent deformation after the first cycle and recovery of the master monotonous curve of MTS/agarose systems indicate a Mullins-like effect similar to that found in carbon-filled rubber systems. We report this type of mechanical behaviour, the Mullins effect, for the first time in MTS bioceramics and MTS bioceramic/agarose systems.

Bacterial adherence on UHMWPE with vitamin E: an in vitro study.

Orthopaedic materials may improve its capacity to resist bacterial adherence, and subsequent infection. Our aim was to test the bacterial adherence to alpha-tocopherol (frequently named vitamin E, VE) doped or blended UHMWPE with S. aureus and S. epidermidis, compared to virgin material. Collection strains and clinical strains isolated from patients with orthopaedic infections were used, with the biofilm-developing ability as a covariable. While collection strains showed significantly less adherence to VE-UHMWPE, some clinical strains failed to confirm this effect, leading to the conclusion that VE doped or blended UHMWPE affects the adherence of some S. epidermidis and S. aureus strains, independently of the concentration in use, but the results showed important intraspecies differences and cannot be generalized.

Compression behaviour of biphasic calcium phosphate and biphasic calcium phosphate-agarose scaffolds for bone regeneration.

There is an acknowledged need for shaping 3-D scaffolds with adequate porosity and mechanical properties for biomedical applications. The mechanical properties under static and cyclic compressive testing of dense and designed porous architecture bioceramic scaffolds based on the biphasic calcium phosphate (BCP) systems and BCP-agarose systems have been evaluated. The dense and designed porous architecture scaffolds in BCP systems exhibited a brittle behaviour. Agarose, a biocompatible and biodegradable hydrogel, has been used to shape designed architecture ceramic-agarose scaffolds following a low-temperature shaping method. Agarose conferred toughness, ductility and a rubbery consistency for strains of up to 60% of in ceramic BCP-agarose systems. This combination of ceramic and organic matrix helps to avoid the inherent brittleness of the bioceramic and enhances the compression resistance of hydrogel. The presence of mechanical hysteresis, permanent deformation after the first cycle and recovery of the master monotonous curve indicate a Mullins-like effect such as that observed in carbon-filled rubber systems. We report this type of mechanical behaviour, the Mullins effect, for the first time in bioceramics and bioceramic-agarose systems.

Effect of surface roughness and sterilization on bacterial adherence to ultra-high molecular weight polyethylene.

Sterilization with ethylene oxide (EO) and gas plasma (GP) are well-known methods applied to ultra-high molecular weight polyethylene (UHMWPE) surfaces in the belief that they prevent major material changes caused by gamma irradiation. However, the influence of these surface sterilization methods on bacterial adherence to UHMWPE is unknown. UHMWPE samples with various degrees of roughness (0.3, 0.8 and 2.0 µm) were sterilized with either GP or EO. The variations in hydrophobicity, surface free energy and surface functional groups were investigated before and after sterilization. Sterilized samples were incubated with either Staphylococcus aureus or Staphylococcus epidermidis in order to study bacterial adherence to these materials. Fewer bacteria adhered to UHMWPE after sterilization with EO than after sterilization with GP, especially to the smoothest surfaces. No changes in chemical composition of the UHMWPE surface due to sterilization were observed using X-ray photoemission spectroscopy analysis. The decreased bacterial adherence to UHMWPE found at the smoothest surfaces after sterilization with EO was not directly related to changes in chemical composition. Increased bacterial adherence to rougher surfaces was associated with increased polar surface energy of EO-sterilized surfaces.

Influence of gamma irradiation on carbon nanotube-reinforced polypropylene.

Single walled carbon nanotubes (SWNT) have been incorporated into a polypropylene (PP) matrix in different concentrations (range: 0.25-2.5 wt%). The nanotubes were blended with PP particles (approximately 500 microm in size) before mixing in an extruder. Finally, rectangular plates were obtained by compression moulding. PP-SWNT composites were gamma irradiated at different doses, 10 and 20 kGy, to promote crosslinking in the matrix and potentially enhance the interaction between nanotubes and PP. Extensive thermal, structural and mechanical characterization was conducted by means of DSC, X-ray diffraction, Raman spectroscopy, uniaxial tensile tests and dynamic mechanical thermal (DMTA) techniques. DSC thermograms reflected higher crystallinity with increasing nanotube concentration. XRD analysis confirmed the only presence of a monoclinic crystals and proved unambiguously that CNTs generated a preferred orientation. Raman spectroscopy confirmed that the intercalation of the polymer between bundles is favored at low CNTs contents. Elastic modulus results confirmed the reinforcement of the polypropylene matrix with increasing SWNT concentration, although stiffness saturation was observed at the highest concentration. Loss tangent DMTA curves showed three transitions for raw polypropylene. While gamma relaxation remained practically unchanged in all the samples, beta relaxation temperatures showed an increase with increasing CNT content due to the reduced mobility of the system. Gamma-irradiated PP exhibited an increase in the beta relaxation temperature, associated with changes in glass transition due to radiation-induced crosslinking. On the contrary, gamma-irradiated nanocomposites did not show this effect probably due to the reaction of radiative free radicals with CNTs.

Comparison between DEXA and finite element studies in the long-term bone remodeling of an anatomical femoral stem.

The implantation of a cemented or cementless femoral stem changes the physiological load transfer on the femur producing an effect on the bone called adaptative remodeling. The patterns of this remodeling are attributed to mechanical and biological factors, and those changes in bone mineral density have been determined in long-term densitometry studies. This technique has proved to be a useful tool able to quantify small changes in bone density in different femoral areas, and it is considered to be ideal for long-term studies. On the other hand, the finite element (FE) simulation allows the study of the biomechanical changes produced in the femur after the implantation of a femoral stem. The aim of this study was to contrast the findings obtained from a 5 year follow-up densitometry study that used a newly designed femoral stem (73 patients were included in this study), with the results obtained using a finite element simulation that reproduces the pattern of load transfer that this stem causes on the femur. In this study we have obtained a good comparison between the results of stress of FE simulation and the bone mass values of the densitometry study establishing a ratio between the increases in stress (%) versus the increases in bone density (%). Hence, the changes in bone density in the long term, compared with the healthy femur, are due to different load transfers after stem implantation. It has been checked that in the Gruen zone 7 at 5 years, the most important reduction in stress (7.85%) is produced, which coincides with the highest loss of bone mass (23.89%). Furthermore, the simulation model can be used with different stems with several load conditions and at different time periods to carry out the study of biomechanical behavior in the interaction between the stem and the femur, explaining the evolution of bone density in accordance to Wolff's law, which validates the simulation model.

Wear resistance of highly cross-linked and remelted polyethylenes after ion implantation and accelerated ageing.

Ion implantation may provide medical polyethylenes with excellent mechanical and tribological properties, helping to lower the risk of long-term osteolysis. Highly crosslinked and remelted polyethylenes, materials currently used as soft components in artificial joints, were implanted with N+ and He+ ions at different ion fluences. The mechanical and tribological properties under distilled water lubrication at body temperature were assessed after ion implantation by means of microhardness and pin-on-disc tests respectively. Thus, the influences of the ionic species and implantation dose on surface hardness, friction coefficient, and wear factor were fully characterized. Furthermore, the tribological behaviour was evaluated after an accelerated ageing protocol (120 degrees C for 36h). Ion implantation increased the surface hardness, as well as friction coefficients, and decreased the wear factors especially at the highest doses. Also, even though all artificially aged materials showed a worse wear behaviour, polyethylenes implanted with either N+ or He+ at the highest doses maintained a relatively good wear factor in comparison with the aged non-implanted material. The origins of these modifications are discussed according to the effects of ion implantation on the microstructure of the polymer.

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.

Mechanical comparative analysis of stents for colorectal obstruction.

The goal of this work is the mechanical comparison of different types of stents for colorectal obstructions. We consider self-expanding and balloon-expanding stents made of two different materials such as stainless steel and shape memory NiTi alloy. The mechanical parameters are expansion rate, shortening, radial compression resistance, longitudinal and perimetral adaptability, and buckling resistance. This analysis results in a better understanding of global mechanical behavior and also allows better design and device selection for colonic lesions.

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.

Design, manufacture and evaluation of a NiTi stent for colon obstruction.

We have designed, manufactured and evaluated a prototype of a new stent based on the superelasticity of the NiTi alloy for colon obstruction, which is the first clinical manifestation of colorectal cancer in up to 29% of cases. The stent is auto-expandable diamond cell type, manufactured from a NiTi tube with 4.5 mm in diameter, in which longitudinal grooves were performed by cutting laser technique. The expansion process to the final shape was made by deformation in the martensite state. The stent reaches a high free expansion rate of 7, with a longitudinal variable radial strength and a bell-shaped profile in the extremes in order to avoid migration. Finite elements models were used for simulating the mechanical behaviour. The prototype was evaluated by implantation in a colon stenosis model of 6 mm in diameter performed in a porcine speciment. The stent gave a good deployment, fixation and capability to open the gap up to 15 mm in diameter.

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.

Fractography evolution in accelerated aging of UHMWPE after gamma irradiation in air.

We studied the fracture surface evolution of ultra high molecular weight polyethylene (UHMWPE) specimens, manufactured from GUR 1050 compression moulded sheets, after gamma sterilisation in air followed by different aging times after thermal treatment at 120 degrees C. Degradation profiles were obtained by FTIR and DSC measurements after 0, 7, 14, 24 and 36h aging. We observed by SEM the morphology patterns at these aging times, in surface fractographies after uniaxial tensile test of standardised samples. The results pointed out clear differences between short and long aging times. At shorter times, 7h, the behaviour was similar to non-degraded UHMWPE, exhibiting ductile behaviour. At longer times, 24-36h, this thermal protocol provided a highly degraded zone in the subsurface, similar to the white band found after gamma irradiation in air followed by natural aging, although closer to the surface, at 150-200mum. The microstructure of this oxidation zone, similarly found in gamma irradiated samples shelf-aged for 6-7 years, although with different distribution of microvoids, was formed by fibrils, associated with embrittlement of the oxidised UHMWPE. In addition, the evolution of the oxidation index, the enthalpy content, the mechanical parameters, and the depth of the oxidation front deduced from the fractographies versus aging time showed that a changing behaviour in the degradation rate appeared at intermediate aging times.