Ultra-high molecular weight polyethylene (UHMWPE) is the most common bearing material in total joint arthroplasty due to its unique combination of superior mechanical properties and wear resistance ...over other polymers. A great deal of research in recent decades has focused on further improving its performances, in order to provide durable implants in young and active patients. From "historical", gamma-air sterilized polyethylenes, to the so-called first and second generation of highly crosslinked materials, a variety of different formulations have progressively appeared in the market. This paper reviews the structure-properties relationship of these materials, with a particular emphasis on the in vitro and in vivo wear performances, through an analysis of the existing literature.
Background
Osteolysis due to wear of UHMWPE limits the longevity of joint arthroplasty. Oxidative degradation of UHMWPE gamma-sterilized in air increases its wear while decreasing mechanical ...strength. Vitamin E stabilization of UHMWPE was proposed to improve oxidation resistance while maintaining wear resistance and fatigue strength.
Questions/purposes
We reviewed the preclinical research on the development and testing of vitamin E-stabilized UHMWPE with the following questions in mind: (1) What is the rationale behind protecting irradiated UHMWPE against oxidation by vitamin E? (2) What are the effects of vitamin E on the microstructure, tribologic, and mechanical properties of irradiated UHMWPE? (3) Is vitamin E expected to affect the periprosthetic tissue negatively?
Methods
We performed searches in PubMed, Scopus, and Science Citation Index to review the development of vitamin E-stabilized UHMWPEs and their feasibility as clinical implants.
Results
The rationale for using vitamin E in UHMWPE was twofold: improving oxidation resistance of irradiated UHMWPEs and fatigue strength of irradiated UHMWPEs with an alternative to postirradiation melting. Vitamin E-stabilized UHMWPE showed oxidation resistance superior to that of irradiated UHMWPEs with detectable residual free radicals. It showed equivalent wear and improved mechanical strength compared to irradiated and melted UHMWPE. The biocompatibility was confirmed by simulating elution, if any, of the antioxidant from implants.
Conclusions
Vitamin E-stabilized UHMWPE offers a joint arthroplasty technology with good mechanical, wear, and oxidation properties.
Clinical Relevance
Vitamin E-stabilized, irradiated UHMWPEs were recently introduced clinically. The rationale behind using vitamin E and in vitro tests comparing its performance to older materials are of great interest for improving longevity of joint arthroplasties.
This paper reviews current understanding of the role of free radicals in the oxidation of polyethylene induced by high-energy irradiation (gamma or e-beam). To evaluate the reactivity, stability and ...mobility of different macroradicals, their reactions after gamma irradiation of different polyethylenes (LDPE, LLDPE, VLLDPE, HDPE, UHMWPE) are considered. Macroradicals are formed in all phases of PE (crystalline, amorphous and interphase). Their overall determination is possible only if both irradiation and detection are carried out at 100 K, or below. At this temperature, the most abundant macroradical formed is the secondary alkyl macroradical (R’’°). On gradually raising the temperature, the macroradicals decay with formation of vinylene double bonds and molecular hydrogen. At room temperature, the macroradical concentration is about 4% of the original quantity at 100 K for LDPE and around 15% for HDPE. In HDPE and UHMWPE; the macroradicals are mainly present in the crystalline phase and in short times (hours) they migrate to the amorphous phase. In LDPE and LLDPE macroradicals are mainly allylic, present in minimal amounts at the crystalline-amorphous interphase. In the nearly fully amorphous VLDPE no residual macroradicals can be detected at room temperature.
The mobility of R’’° is variable and is a function of the mobility of the polymer backbone, slow in the crystalline phase, relatively fast in the amorphous phase. Kinetic stability, or persistence, is often more important than thermodynamic stability in determining radical lifetimes, in particular for radical processes in the solid phase. Alkyl macroradicals react with chain imperfections, additives and oxygen, in a cyclic process involving initiation, propagation and termination reactions. The initiation reactions form macroradicals by cleavage of the C-H bonds of PE, induced by irradiation or by photo- or thermal-scission of peroxides formed during processing of the polymer. Propagation involves the reaction of R’’° with vinyl or vinylidene double bonds, with the formation of a new radical, in competition with their reaction with oxygen to form various oxidation products (ketones, hydroperoxides acids, alcohols and esters). It is notable that the formation of ketones does not necessarily require decomposition of the hydroperoxides. In the presence of stabilizing additives, radicals react with the additive (ADH), with the formation of a more kinetically stable radical (AD°), which considerably decreases the propagation rate, but a termination reaction between R’’° and AD° may also be envisaged.
It is observed that radiation-induced oxidation has a constant rate during irradiation. Post-irradiation, the oxidation occurs via transfer of the macroradicals from the crystalline phase and the interphase to the amorphous phase, where oxygen is available, and the rate decreases, approaching zero. The occurrence of termination is apparent. Termination must occur through the reaction of two macroradicals. The reaction between two R’’° is difficult, due to steric hindrance. The most probable reaction is that between peroxy macroradicals (ROO°), fixed in position on the polymer chain and R’’°, which migrate through the polymer bulk. This reaction is difficult to confirm experimentally because of the lack of reliable analytical methods for ROOR species in the presence of ROOH.
The formation reactions of the different oxidation products are reported and the branching reactions occurring in thermal and photo-oxidation are also discussed.
•All the oxidation products are formed by directly reaction between R’’° and d oxygen.•The formation of ketones does not necessarily require decomposition of the hydroperoxides.•The reaction between two R’’° is difficult, due to steric hindrance.•The most probable termination reaction is that between peroxy macroradicals (ROO°), and R’’.
Wear is among the main factors affecting the performance of artificial couplings in joint arthroplasty. In the attempt of investigating the applicability of PEEK and CFR PEEK in such applications, ...wear behavior of three medical grade PEEK formulations (unfilled PEEK, 30% wt. pitch carbon fiber reinforced PEEK and 30% wt. PAN carbon fiber reinforced PEEK) have been tested under dry and bovine serum lubricated conditions in a pin on flat test against Al2O3 1/8″ spheres, replicating physiological conditions whereas possible. The crystallinity and the resulting mechanical properties of the three selected material were differentiated by annealing treatments performed at 200, 250, and 300 °C. All the specimens were tested under both lubrication regimes.
The observed coefficient of friction variations, as well as the calculated volumetric wear rate, displayed an improved wear behavior of the reinforced formulations in both dry and bovine serum. Correlation among the modified material properties by annealing and the wear behavior was possible. The formation of a lubricating film was identified as responsible for an improved wear performance of PEEK and CFR PEEK formulations.
•Biotribology of unreinforced and carbon fiber reinforced PEEK is assessed.•Influence of annealing on the wear behavior of the tested material is demonstrated.•Material properties, reinforcement type, lubrication regime impact wear performance.•Paper contributes to the investigation on suitable PEEK formulations for orthopaedics.
Maltodextrins are products of starch hydrolysis that can be processed into dry fibres through electrospinning and subsequently cured via mild thermal treatment to obtain nonwoven cross-linked ...polysaccharide-based mats. The sustainability of the process and the bioderived nature make this class of materials suitable candidates to be studied as renewable sorbents for the removal of contaminants from water. In this work, electrospinning of water solutions containing 50% wt. of commercial maltodextrin (Glucidex 2
) and 16.6% wt. of citric acid was carried out at 1.2 mL/h flow and 30 kV applied voltage, followed by thermal curing at 180 °C of the dry fibres produced to obtain cross-linked mats. Well-defined fibres with a mean diameter of 1.64 ± 0.35 µm were successfully obtained and characterised by scanning electron microscopy, thermogravimetric analysis, and attenuated total reflectance Fourier transform infrared spectroscopy. Afterwards, a series of sorption tests were conducted to evaluate the effectiveness of the mats in removing atenolol from water. The results of the batch tests followed by HPLC-UV/Vis showed high sorption rates, with over 90% of the atenolol removed, and a maximum removal capacity of 7 mg/g. Furthermore, continuous fixed-bed sorption tests proved the positive interaction between the polymers and atenolol.
Carbon-fiber reinforced (CFR) PEEK implants are used in orthopedic applications ranging from fracture fixation plates to spinal fusion cages. Documented implant failures and increasing volume and ...variety of CFR PEEK implants warrant a clearer understanding of material behavior under monotonic and cyclic loading. To address this issue, we conducted monotonic and fatigue crack propagation (FCP) experiments on orthopedic grade unfilled PEEK and two formulations of CFR PEEK (PAN- and pitch-based carbon fibers). The effect of annealing on FCP behavior was also studied. Under monotonic loading, fiber type had a statistically significant effect on elastic modulus (12.5 ± 1.3 versus 18.5 ± 2.3 GPa, pitch versus PAN CFR PEEK, AVG ± SD) and on ultimate tensile strength (145 ± 9 versus 192 ± 17 MPa, pitch versus PAN CFR PEEK, AVG ± SD). Fiber type did not have a significant effect on failure strain. Under cyclic loading, PAN CFR PEEK demonstrated an increased resistance to FCP compared with unfilled and pitch CFR PEEK, and this improvement was enhanced following annealing. Pitch CFR PEEK exhibited FCP behavior similar to unfilled PEEK, and neither material was appreciably affected by annealing. The improvements in monotonic and FCP behavior of PAN CFR PEEK is attributed to a compound effect of inherent fiber properties, increased fiber number for an equivalent wt% reinforcement, and fiber aspect ratio. FCP was shown to proceed via cyclic modes during stable crack growth, which transitioned to static modes (more akin to monotonic fracture) at longer crack lengths. The mechanisms of fatigue crack propagation appear similar between carbon-fiber types.
The global plastic waste problem has created an urgent need for the development of more sustainable materials and recycling processes. Polyurethane (PU) plastics, which represent 5.5% of globally ...produced plastics, are particularly challenging to recycle owing to their crosslinked structure. Covalent adaptable networks (CANs) based on dynamic covalent bonds have emerged as a promising solution for recycling PU waste. CANs enable the production of thermoset polymers that can be recycled using methods that are traditionally reserved for thermoplastic polymers. Reprocessing using hot-pressing techniques, in particular, proved to be more suited for the class of polyurethanes, allowing for the efficient recycling of PU materials. This Review paper explores the potential of CANs for improving the sustainability of PU recycling processes by examining different types of PU-CANs, bond types, and fillers that can be used to optimise the recycling efficiency. The paper concludes that further research is needed to develop more cost-effective and industrial-friendly techniques for recycling PU-CANs, as they can significantly contribute to sustainable development by creating recyclable thermoset polymers.
Cyclodextrins and cyclodextrin derivatives were demonstrated to improve the antiviral potency of numerous drugs, but also to be endowed with intrinsic antiviral action. They are suitable building ...blocks for the synthesis of functionalized polymer structures with potential antiviral activity. Accordingly, four water-soluble hyper-branched beta cyclodextrin (βCD)-based anionic polymers were screened against herpes simplex virus (HSV-2), respiratory syncytial virus (RSV), rotavirus (HRoV), and influenza virus (FluVA). They were characterized by FTIR-ATR, TGA, elemental analyses, zeta-potential measurements, and potentiometric titrations, while the antiviral activity was investigated with specific in vitro assays. The polymer with the highest negative charge, pyromellitic dianhydride-linked polymer (P_PMDA), showed significant antiviral action against RSV and HSV-2, by inactivating RSV free particles and by altering HSV-2 binding to the cell. The polymer fraction with the highest molecular weight showed the strongest antiviral activity and both P_PMDA and its active fractions were not toxic for cells. Our results suggest that the polymer virucidal activity against RSV can be exploited to produce new antiviral materials to counteract the virus dissemination through the air or direct contact. Additionally, the strong HSV-2 binding inhibition along with the water solubility of P_PMDA and the acyclovir complexation potential of βCD are attractive features for developing new therapeutic topical options against genital HSV-2 infection.
Electrospun beta-cyclodextrin (βCD)-based polymers can combine a high surface-to-volume ratio and a high loading/controlled-release-system potential. In this work, pyromellitic dianhydride ...(PMDA)/βCD-based nanosponge microfibers were used to study the capability to host a common insect repellent (
,
-diethyl-3-toluamide (DEET)) and to monitor its release over time. Fibrous samples characterized by an average fibrous diameter of 2.8 ± 0.8 µm were obtained and subsequently loaded with DEET, starting from a 10 g/L diethyl ether (DEET) solution. The loading capacity of the system was assessed via HPLC/UV⁻Vis analysis and resulted in 130 mg/g. The releasing behavior was followed by leaving fibrous DEET-loaded nanosponge samples in air at room temperature for a period of between 24 h and 2 weeks. The releasing rate and the amount were calculated by thermogravimetric analysis (TGA), and the release of the repellent was found to last for over 2 weeks. Eventually, both the chemical composition and sample morphology were proven to play a key role for the high sample loading capacity, determining the microfibers' capability to be applied as an effective controlled-release system.
Until recently, recycling thermoset polyurethanes (PUs) was limited to degrading methods. The development of covalent adaptable networks (CANs), to which PUs can be assigned, has opened novel ...possibilities for actual recycling. Most efforts in this area have been directed toward inventing new materials that can benefit from CAN theory; presently, little or nothing has been applied to industrially producible materials. In this study, both an industrially available polyol (Sovermol780®) and isocyanate (Tolonate X FLO 100®) with percentages of bioderived components were employed, resulting in a potentially scalable and industrially producible material. The resultant network could be reworked up to three times, maintaining the crosslinked structure without significantly changing the thermal properties. Improvements in mechanical parameters were observed when comparing the pristine material to the material exposed to three rework processes, with gains of roughly 50% in elongation at break and 20% in tensile strength despite a 25% decrease in Young’s modulus and crosslink density. Thus, it was demonstrated that theory may be profitably applied even to materials that are not designed including additional bonds but instead rely just on the dynamic urethane bond that is naturally present in the network.
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