Literature Review Ultra high molecular weight polyethylene UHMWPE due to its various properties such as biocompatibility high wear resistance low friction coefficient suitable stiffness and fatigue resistance has been emerged as a proven bearing material for implants in various arthroplasties such as acetabular cups and tibial inserts 13 18 20 Beside the extensive use of UHMWPE in total joint replacements for many years the primary problem is the constant generation of wear debris resulting from articulation against the metallic parts These wear particles induced osteolysis and aseptic loosening and has been considered as critical failure which limits the service life of prosthesis 21 In order to consider the former concern i e wear resistance the process of crosslinking or irradiation of UHMWPE are used and highly cross linked PE 22 However main interest continues to refer the long term oxidation of highly cross linked UHMWPE following the formation of free radicals during the process of irradiation These free radicals may responsible for continuing long term reactions hence leading to oxidative degradation in the presence of molecular oxygen 23 24 and transforming UHMWPE into brittle polymer with disappointing performance 25 Many orthopedics manufacturers have utilized the process of high energy radiation accompanied by the thermal stabilization processes to stabilize the cross linked polyethylene against oxidation Both of these processes appear to optimize the wear resistance of highly cross linked UHMWPES which usually take place below annealing or above re melting the melting transition temperature of 1 2 UHMWPE Anyhow the actual crystallinity of the polymer is decreased by re melting consequently influencing the polymer s mechanical properties such as yield and ultimate stress but annealing maintains these properties 26 Moreover as during annealing process crystals do not melt hence complete oxidative stability is not guaranteed by annealing methods because of the fact that fewer mobile free radicals entrapped within the crystalline phase of polymer do not have the ability of recombination Additionally the manipulation of α tocopherol vitamin E has been considered as the substitute of the thermal stabilization process 27
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Several approaches based on composite materials worth mentioning to improve the mechanical performance of UHMWPE include self reinforcement 28 31 reinforcement with carbon fibers and nano particles 32 among others carbon fiber reinforced CFR UHMWPE hybrids were regarded for orthopedic prosthesis and were commercially launched too Poly II Zimmer Inc Warsaw Indiana USA in 1970s 33 But the disastrous transient clinical failures ultimately led to the desertion of poly II In recent days with the intensive focus on UHMWPE development activated by radiation and thermal processing methods there has been a renewed curiosity about UHMWPE for biomedical applications Nano composites based on carbon nanotubes CNT have acquired enormous concern due to their high chemical stability as well as improved properties principally mechanical behavior thermal and optical properties and electrical conductivities 34 Owing to exceptional characteristics CNT reinforced polymeric composites have remained center of interest by researchers 35 The effect of gamma irradiation on CNT based nanocomposites with polypropylene PP matrix has been focused in a current study described by our group 36 There are reports of limited successful outcomes for UHMWPE reinforced with MWCNTs 1 3 However there is still a disagreement related to the biocompatibility of CNT at the present time because some researchers investigated their cytotoxicity 37 38 while other authors reported that CNTs are considered as perfect substrates for cellular development 39
42 Carbon nanotubes CNTs have induced an intense interest for a variety of applications such as biomaterials sensors catalyst supports and energy storage devices mainly because of its physical and mechanical characteristics It has been described in literature that significant features for the characterization of multi walled carbon nanotubes MWCNTs are surface properties imperfection sites and graphitization 43 45 Numerous techniques are employed in order to achieve MWCNTs with optimize properties These techniques include chemical etching ultrasonic treatment mechanical treatment and high energy irradiation 46 47 It has been revealed in modern researches that irradiation of CNTs with energetic particles like electrons and ions can be applied to produce molecular connections between the CNTs and composite materials And the repositioning of carbon atoms in the shells without structural disordering is brought out by the electron bombardment 48 The process of ion irradiation can be used for functionalization of CNTs and subsequently provides a new direction to improve the mechanical and electrical aspects of CNTs 49 γ ray irradiation has drawn full attraction for the reason that it is considered as optimized method to improve the physical and chemical properties of MWCNTs Furthermore gamma ray irradiation is exploited to acquire short MWCNTs by cutting them in the presence of dilute sulfuric acid improving the surface functioning of the carbon fiber CF 50 51 and for increasing cross linking in the matrix because of the intensified interaction between the nanotubes and polymers It has been demonstrated by Skakalova et al that
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