Irradiation of UHMWPE

The effects of irradiation on the mechanical properties of UHMWPE from selected literature are summarized in Table 7.3. 

A study on irradiated UHMWPE/MWCNTs composite has been a topic of recent interest. The performance of UHMWPE/MWCNTs composites 

Dosage Young s modulus Fracture stress Yield stress % Strain at fracture Researcher 

Sreekanth et al. [127] reported a more detailed study on the effect of MWCNTs in restricting the degradation of mechanical properties by considering the effect of ageing at different time periods. UHMWPE was reinforced by MWCNTs up to 2 wt.% and then subjected to gamma irradiation doses of 25, 50,75 and 100 kGy. The irradiated samples were shelf aged at different period times, such as 10,60 and 120 days after irradiation, and its mechanical properties, and relative radical intensity were studied in detail. 

Based on the above studies, it can be inferred that MWCNTs showed antioxidant characteristics. Unlike the conventional antioxidant Vitamin E, MWCNTs do not hinder the process of crosslinking but rather promote it. However, no studies so far have been reported, regarding to the combined influence of irradiation and MWCNTs reinforcement on the wear behaviour of UHMWPE. The exploration of MWCNTs in biomedical application is underway. Since a large part of the human body consists of carbon. 

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Crystallization enthalpies of the irradiated blends were lower than predicted by the additivity rule. The crosslinking of the amorphous domains lowered the melting enthalpies. Irradiated blends showed separated crystallizations, at their characteristic temperatures of crystallization, independent of composition. On irradiation, the crystallization temperature of PE did not change, whereas that of UHMWPE slightly decreased. Separate irradiation of UHMWPE and PE favored molecular scission over the crosslinking, whereas irradiation of the blends favored crosslinking. The melting temperature, T, increased linearly with... 

The effect of irradiation on the crystallinity of UHMWPE has already been described. Irradiation of UHMWPE in the melt leads to a high yield of crosslinks with effectively no chain scission occurring, and with increasing crosslink density a change from lamellar to micellar-like crystallization was found [83]. More recent thorough studies of UHMWPE often at sterilizing doses of around 25-50 kGy, show both scission and crosslinking as well as a transition zone within and below the polymer mass [84-87]. 

In the medical praxis, UHMWPE have been used successfully as one half of the bearing couple (against metallic alloys or ceramics) in total hip and total knee joint replacements crosslinked under irradiation [82]. Even y-irradiated UHMWPE at doses higher than 100 kGy presents the incipient ffacmre development [83], the alkyl macroradicals are involved in crosslinking and in a smaller proportion they promote oxidation [84, 85]. The prosthesis manufactures by the irradiation of UHMWPE have long durability, because the application of radiation treatment induces an increased crystallinity and promotes sterilization in the whole volume of material. 

Figure 3.13a b Carbonyls (P=0) and hydroperoxides changes (POOH) during room temperature post irradiation of UHMWPE 60 kGy irradiated under air pure UMWPE ( ), UHMWPE + 0.0011 moll vitamin E ( ), UHMWPE+ 0.0011 moll" vitaminE (A), UHMWPE + 0.011 moll vitamin E (X) [57]. (NB 0.0011 moll" corresponds to a weight ratio of 0.05%). 

Highly crosslinked UHMWPE can be produced by irradiation of a blank UHMWPE with ionizing radiation, in particular by X-rays, y-rays or electron beams, in order to produce radicals. The subsequent treatment of the irradiated material consists in exciting free radicals, which have not recombined, by means of microwave radiation or ultrasound. The process is claimed to ensure a substantially complete recombination of the free radicals. In addition, the crosslinking of the UHMWPE is also further optimized (30). 

O K. MURATOGLU, D.O.O CONNOR, C.R. BRAGDON, J. DELANEY, M. JASTY, W.H. HARIS, E. MERRILL, P. VENUGOPALAN, Gradient crosslinking of UHMWPE using irradiation in molten state for total joint arthroplasty , Biomaterials, 23 (2002)717-724... 

Table 15.4 Properties of UHMWPE and irradiated version (from Pruitt, 2005)...

In the late 1990s, highly crosslinked UHMWPE was found to have an effectively zero wear rate. Muratoglu et al. (2001) described how 40 mm thick discs of UHMWPE were irradiated in air using a lOMeV electron... 

Wang (2001) found that irradiation crosslinked UHMWPE had a significantly lower wear rate than un-crosslinked material. The radiation dose must be high to obtain the optimum effect (Fig. 15.19). Rieker et al. (2003) showed that the wear surfaces of highly crosslinked UHMWPE implants after 18 months in vivo, consisted of folds (Fig. 15.20). Such folds are also found in conventional UHMWPE, but fatigue leads to their detachment from the surface. The folds on the surface of the crosslinked polymer appear to stay in place. Crosslinking leads to a reduction in crystallinity, hence a... 

Figure 11.1 shows a collection of FTIR spectra through the thickness of a shelf-aged, EtO-sterilized cup where bulk oxidation was evident. Gamma irradiation of the UHMWPE was ruled out by examination of the transvinylene region of the ETIR spectra. The absorption at 1718 cm i is attributed to ketones. In this case, the oxidation was associated with poor consolidation of the UHMWPE, rather than the result of the sterilization process itself. These results suggest that prosthetic UHMWPE needs stabilization, as the totality of commercial polyolefines. Biocompatible stabilizers, such as vitamin E (an a-tocopherol), are easily available on the market and already employed in a number of different applications (Costa et al. 1998b, Costa et al. 2000). 

Shen EW., HA. McKeUop, and R. Salovey. 1996. Irradiation of chemically crosslinked UHMWPE. 7 Poly Sci Poly Phys 34 1063-1077. 

FTIR trace of UHMWPE irradiated to 15 Mrads and gamma sterilized in air, with key peak locations indicated. 

Wear testing by Essner and colleagues in a multidirectional hip simulator for up to 10 million cycles demonstrated that Duration exhibited 32% less volumetric wear than air-irradiated control UHMWPE (1997). However, in a recent radiostereometric study by Nivbrant and colleagues, the clinical wear rate of Duration was not significantly different than an air-irradiated control UHMWPE after 2 years of follow-up (2003). 

In the Longevity process, the UHMWPE bars are warmed, placed in a carrier on a conveyor, and are exposed to electron beam radiation, with a total dose of 100 kGy. The UHMWPE does not heat above the melt transition during the crosslinking. After irradiation, the UHMWPE is heated above the melt temperature (>135°C) for stabilization of free radicals. Components are then machined from the Longevity material, enclosed in gas-permeable packaging, and sterilized by gas plasma. 

Abt N.A., and W. Schneider. 2003. Influence of irradiation on the properties of UHMWPE. In Highly crosslinked and thermally treated ultra-high molecular weight polyethylene for joint replacements. S.M. Kurtz, R. Gsell, and J. Martell, Eds. West Conshohoken, PA American Society for Testing and Materials. 

The functional properties of polymers can be ameliorated by the irradiation of their nanocomposites [86, 87]. The addition of MWCNT to low density polyethylene increases the radiation resistance in comparison to the pure LDPE, which was dependent on the MWCNT content [88]. WCNT nanocomposites were gamma irradiated at 90 kGy to improve the interaction between MWCNTs and the polymer matrix [89]. 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. 

UHMWPE and AI2O3 samples wem provided by Mathys Ltd. Bettlach (Switzeriand) and were the same materials as those used in the production of artificial h -joints. The UHMWPE samples had been y-irradiated with a dose of 25-30 kGy in a nitrogen atmosphere. Medical quality AI2O3 specimens were cleaned in an ultrasonic bath with, consecutivdy. isopropanol, ethanol and pure water for 10 min each followed by drying with a N2 gas jet The roughness (Ra) of UHMWPE samples was 0.1-02 (im and that of AkOs was 4-8 nm. 

After sliding UHMWPE against CoCrMo in all solutions, fluorescence was detected on the surface of the metal. The fluorescent spots showed similar sizes and shapes when they were generated in PBS, and visualized by postincubation with labeled albumin, as when they were generated in BSA and BSF containing labeled albumin. It could, therefore, be concluded that these spots were due to transfer of UHMWPE to the CoCrMo surface. This postulation was supported by AFM phase imaging, which showed a contrast between the CoCrMo surface, and the measurement of force-distance curves that were consistent with the effective modulus of UHMWPE. The measured effective modulus of 1 0.2 GPa lay within the range of bulk UHMWPE (0.85) and the surface of 7-irradiated UHMWPE (up to 2.6 GPa). ... 

Ratner et al. [132] reported the creatirai of UHMWPE composites formed via a similar hot compaction route, and also describe the affect of a chemical crosslinking pretreatment on commercial UHMWPE fibres. This crossUnking treatment was shown to increase the mechanical properties of the final composite constructs and may also enhance interfibrillar bonding. The use of crossUnking to enhance the mechanical properties of hot compacted PE fibres was also reported by Ward and Hine, although in this case, crosslinking was achieved by gamma irradiation in an atmosphere of acetylene [33]. 

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