Advances in Enhancing Mechanical Performance of Ultrahigh

Chapter 14

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Advances in Enhancing Mechanical Performance of Ultrahigh Molecular Weight Polyethylene Used for Total Joint Replacement Yan-Fei Huang, Jia-Zhuang Xu,* and Zhong-Ming Li* College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, 610065, Chengdu, People’s Republic of China *E-mail: [email protected] (J. Z. Xu); [email protected] (Z. M. Li)

Wear, mechanical properties and oxidative stability are the critical factors for the application and longevity of UHMWPE implants used in total joint replacements. In spite of the success in wear and oxidation stability, the application of UHMWPE implant in younger, more physically active patients has evoked an urgent need to enhance the mechanical properties. In this chapter, we summarize the recent advances in mechanical reinforcement of UHMWPE. A variety of routes to attain this goal are reviewed in terms of reinforcement manner, which is grouped into additive-based reinforcement and self-reinforcement. For the additive-based reinforcement, the influence of filler types, size, shape and functionalization on the mechanical performance is discussed. For the self-reinforcement, structural manipulation is the key and several paradigms are present. The prospects for future development in this field are also discussed.

Introduction Because of its excellent wear and fatigue resistance as well as good biocompatibility, ultrahigh molecular weight polyethylene (UHMWPE) has been successfully applied in the load-bearing articular pair for total hip and knee replacements since the 1960s (1). From the wealthy of clinical retrospective studies, wear, oxidative stability, and mechanical properties are three pivotal © 2017 American Chemical Society Ito et al.; Advances in Bioinspired and Biomedical Materials Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


facets in determining the performance and longevity of the total joint replacement (TJR) (2). Peri-prosthetic osteolysis trigged by UHMWPE wear debris is a primary cause for the failure of TJR along with the implant loosening (3, 4). Crosslinking by ionizing radiation (e.g. electron beam or γ-rays) has been broadly recognized as the most promising approach to ameliorate wear resistance of implants (5–7), but undesirably leaves behind residual free radicals in the crystalline phase of irradiated polymer bearings (8, 9). These free radicals migrate and react with diffused oxygen during the long-term service, resulting in chain scission and oxidative embrittlement through a cascade of oxidation reaction (10–13). To solve the above hurdle, thermal treatment is implemented to prevent the oxidation (14–17). Although being effective to eliminate the residual free radicals, the post-irradiation re-melting results in reduced crystallinity and ductility of UHMWPE (18, 19), and hence deteriorates the mechanical performance and fatigue resistance of the components, which are responsible for the increased incidence of rim fracture after impingement (20, 21). Post-irradiation annealing retains the crystalline structure and thus the mechanical performance of UHMWPE implants, but the radicals cannot be quenched thoroughly. An alternative strategy to lower the residual amount of radicals is introducing radical scavengers and/or antioxidants, among which vitamin E (VE) is the most widely used and has already been commercialized. VE stabilizes the irradiated UHMWPE by donating a proton of phenolic OH to macroradicals and itself becomes a radical with resonance structure. The oxidation cycle of polyethylene is interrupted without the post-thermal treatment. Thus, the mechanical properties of UHMWPE can be maintained. However, the presence of VE could impede the crosslinking in turn due to consumption of the radicals induced by irradiation (22–24). Although mechanical performance is gained little attention compared to wear and oxidation resistance, it is truly a vital factor to determine the longevity of the UHMWPE implants, especially for high stress applications, such as total knee implants. As reported, the maximum principal stress suffered by UHMWPE bearings can reach as high as 45 MPa for knee prosthesis during normal walking (25). This value is beyond the yield strength (ca. 20 ~ 25 MPa) of UHMWPE. More seriously, the inherent low yield stress would be further impaired after irradiation (26). Moreover, the number of younger and more active patients are increasing continually (27). It is projected that the demand for TJR by patients less than 65 years old would be 52% of primary total hip replacements and 55% ~ 62% of primary or revision total knee replacement (27). For 45 to 54 years age category, the future demand was anticipated to grow by 17 times and 5.9 times for total knee replacement and total hip replacement, respectively, from 2006 to 2030. Therefore, there is a pressing need to develop the UHMWPE materials that possess optimized mechanical performance without sacrificing the wear and oxidation resistance so as to prolong the longevity of the joint implants. In this chapter, we summarize the recent advances in mechanical enhancement of UHMWPE materials. From the perspective of polymer processing, there are two approaches to enhance the mechanical properties of the host matrix, i.e., additive-based reinforcement and self-reinforcement. The former can be achieved by incorporation of second reinforcing phase, such as carbon fiber 274 Ito et al.; Advances in Bioinspired and Biomedical Materials Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

(CF) (28), carbon nanotubes (CNTs) (29), graphene nanoplatelets (GNPs) (30), and hydroxyapatite (HA) (31), etc. The latter can be realized by manipulating the crystalline superstructure in drawing, flow or pressure fields (32). Progress in additive-based reinforcement and self-reinforcement of UHMWPE will be included in this chapter and the prospects for the future development of the high-performance UHMWPE TJR materials are also discussed.


Additive-Based Reinforcement UHMWPE composites reinforced with suitable second phase fillers have been developed for aerospace, industrial, and biomedical applications, with the general aim of improving the mechanical performance of nascent UHMWPE. In the fields of orthopedics, these UHMWPE-based composites are being researched as a potential alternative to the current highly cross-linked UHMWPE (33). Here, we will give an introduction about some representative fillers, such as CF, HA, graphene and so on. Considering the application of UMMWPE as biomaterials, some concerns regarding the biocompatibility of the fillers are also be discussed in the following text. CF Reinforced UHMWPE Composites CF with outstanding tensile strength and elastic modulus has been first used as reinforced fillers for the UHMWPE-based composites. In the 1970s, CF/UHMWPE composites, commercially known as Poly IITM (Zimmer, Inc., Warsaw, IN), were first applied as the orthopedic implants. The mechanical properties of UHMWPE showed notable increase after inclusion of short chopped CFs. Unfortunately, the resistance to the pitting and delamination of Poly IITM was reduced in the clinical setting. As a consequence, clinical failures, such as osteolysis and complete mechanical damage of the tibial bearing inserts, occurred within a short time after implantation (34, 35). These disastrous deteriorations were a result of the poor crack propagation resistance of the Poly IITM because the weak interfacial adhesion between CF and UHMWPE matrix, which inversely served as stress concentrators and crack nucleation sites (36, 37). In comparison to conventional UHMWPE, eight times faster fatigue-crack-propagation rate was found in Poly IITM. After premier clinical application, Poly IITM with unsatisfactory performance was eventually withdrawn (1). HA Reinforced UHMWPE Composites Hydroxyapatite (HA) with excellent intrinsic mechanical and bioactive properties is an attractive filler to enhance the mechanical performance of UHMWPE. Yang et al. found a rising tendency of the compression strength with increasing HA content from 5 wt% to 20 wt% (Figure 1a) (38). Additionally, an increased ball indentation hardness with increasing addition of HA was also reported (Figure 1b). They concluded the reason for such an improvement as follow: First, the inherently high strength of HA made the composites 275 Ito et al.; Advances in Bioinspired and Biomedical Materials Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


strong. Second, the reinforcement particles could impede the movement of UHMWPE molecular chains, thus improving compression strength. Size of the HA particles played an important role in improving mechanical properties of the HA/UHMWPE composites. It was reported that compression strength and ball indentation hardness of nano-HA/UHMWPE composites were higher than those of micro-HA/UHMWPE composites. Considering these facts, researchers tend to recruit nano-sized HA to reinforce UHMWPE. For instance, Mirsalehi et al found that Young’s modulus and hardness were 362.5% and 200% higher in the nanocomposite with 50 wt% nano-HA than pure UHMWPE, respectively.

Figure 1. (a) Compression strength as a function of filling content for micro and nano-HA/UHMWPE samples. (b) Ball indentation hardness as a function of filling content for micro- and nano-HA/UHMWPE samples. Figures reproduced with permission from ref. (38). Copyright 2016, John Wiley & Sons, Inc.

Besides mechanical properties, HA particles affect the tribological behaviors. Decreased stable friction coefficient and wear volume of UHMWPE were found with content of micro-HA lower than 15 wt%, or nano-HA lower than 10 wt% (38). In Mirsalehi’s work (39), UHMWPE nanocomposite with 50 wt% nano-HA exhibited a friction coefficient of 38.86% lower than that of pure UHMWPE. Dispersion of HA particles in UHMWPE is a serious challenge due to the extremely high viscosity of UHMWPE (40). Various processing aids were applied to reduce the viscosity of UHMWPE composites, including but not limited to, polyethylene glycol, paraffin oil, etc. Fang et al. developed a method to well disperse HA into UHMWPE by combining wet ball milling and swelling (41–43). UHMWPE and nano-sized HA particles were mechanically mixed by the ball mill and compression molding. Then, a pharmaceutical grade paraffin oil was employed to swell the slabs so as to enhance the UHMWPE chain mobility and HA-UHMWPE interface adhesion before the final hot press. A homogeneous dispersion of HA-rich phase and a UHMWPE-rich phase were achieved in the resultant composite. Compared to unfilled UHMWPE, Young’s modulus and yield strength of HA/UHMWPE composite were increased by 90% and 50%, respectively. 276 Ito et al.; Advances in Bioinspired and Biomedical Materials Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


In order to enhance the utilization efficiency of HA, a gradient structure in the HA/UHMWPE composites was designed by Bin et al. (44). Four slabs with HA contents ranging from 0 vol% to 31.5 vol% were prepared by a sol-gel process. Then, these slabs were hot-molded at 150 and 180 °C to form the gradient distribution of HA particles (Figure 2). Due to the different composition, different layer showed varied mechanical properties. At the surface layer (ΙV) with 31.5 vol% HA content, flexural stress was the highest, but flexural strain reached the lowest (Figure 3). At the inner layers (І ~ ΙΙΙ), the flexural strength is low, while the flexural strain is high. Taking advantage of the gradient structure, a simultaneously high strength and ductility were achieved (Figure 3). The balanced strength and strain showed great promise since it avoided the cracking under bending stress. Besides, high HA content led to a positive effect on tribological properties. Friction coefficient at the surface layer of the gradient HA/UHMPWE composites reduced to ca. 0.10 from ca. 0.16 for neat UHMWPE. The gradient composites with integration of strength, strain and wear resistance provide great merits as an acceptable cup of bearing material in an artificial joint.

Figure 2. (a) Hip prosthesis in orthopedics used for operation, (b) acetabular cup as a bearing material in the femoral, (c) a model of gradient composite of the acetabular cup, and (d) gradient composite used in the present experiment. Figures reproduced with permission from ref. (44). Copyright 2013, American Chemical Society.

Figure 3. Bending stress and strain for UHMWPE films prepared by melting (m) and gelation (g), UHMWPE/HA dry gel composite with 31.5 vol % HA content was molded at 150 °C (a) and 180 °C (b). Figures reproduced with permission from ref. (44). Copyright 2013, American Chemical Society. 277 Ito et al.; Advances in Bioinspired and Biomedical Materials Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Graphene-Reinforced UHMWPE Composites Graphene has recently been introduced as a robust reinforced filler to UHMWPE, due to its excellent mechanical properties, viz., 0.5 ~ 1 TPa elastic modulus and 130 GPa intrinsic strength (45–51). Till now, the reported graphene and its derivatives include graphene oxide (GO), graphene nanoplatelet (GNP), graphite nanoplatelets (GNS), fluorinated grapheme, etc. Despite of different surface functional groups and thickness, most graphene-base materials present an enhancement effect on the mechanical properties of UHMWPE. Lahiri et al. prepared GNP-reinforced UHMWPE films by electrostatic spaying and found that elastic modulus of UHMWPE showed an enhancement of 27% and 124% with addition of 0.1 wt% and 1 wt% GNP, respectively (Figure 4a) (30). Yield strength followed an increase trend with 59% and 90% enhancement at 0.1 wt% and 1 wt% GNP, respectively (Figure 4a). Similar enhancing effect was obtained by using GNS as the filler (52). At 0.25 wt% GNS loading, tensile strength (Figure 4b) and modulus (Figure 4c) of the composites were increased by 38.0% and 73.6%, respectively. An augment in yield strength of UHMWPE was observed with the increase of GO content (Figure 4d) (53). But the reinforcement effect of GO was not as notable as GNP and GNS. On one hand, the Young’s modulus was almost invariant with the increasing GO concentrations (Figure 4d). On the other hand, both ultimate tensile strength and elongation at break decreased even though GO was less than 0.5 wt%. Usually, there is an optimum concentration of graphene with respect to mechanical reinforcement for graphene/UHMWPE composites. Ultimate tensile strength and toughness of UHMPWE reached to upmost values at 0.1 wt% GNP (Figure 4a) (30). A deterioration in these two properties occurred by further increasing GNP content. Similar phenomena was conformed in Chen’s work: 0.5 wt% GO endowed the UHMWPE with the maximum values of ultimate tensile strength and tensile elongation, slightly higher than those of the pure UHMWPE (53). As the concentration of GO further increased to 1.0 wt%, these values decreased instead of increased forward. The turning of the mechanical properties after a specific concentration is mainly due to a trade-off between the intrinsic properties of fillers and the negative effects caused by the filler agglomeration at high loading. Poor dispersion of the filler prevented its wrapping by the polymer, and eventually leads to weak bonding with the matrix. On this occasion, microstructural flaw-like microcracks were generated and caused the loss of toughness and ductility. Graphene also enhanced other mechanical properties, such as hardness. Different from the tensile properties which reached the peak values at an optimal filler concentration, hardness of the UHMWPE/graphene composites progressively increased with filler content. Addition of 0.1, 0.3, 0.5, and 1.0 wt% GO gradually improved the micro-hardness of GO/UHMWPE composites from 5.18 Hv for neat UHMWPE to 5.78, 5.80, 5.84 and 5.97 Hv, respectively (Figure 5a). Similar increasing tendency in the micro-hardness was also observed in another GO/UHMMWPE composite (Figure 5b) (54), and FG/UHMWPE composite (Figure 5c) (55). The enhanced micro-hardness was contributed by the load transfer from the matrix to the graphene. 278 Ito et al.; Advances in Bioinspired and Biomedical Materials Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Another merit is expected that the presence of grapheme may improve the wear resistance of UHMWPE, which is of great importance for the load-bearing materials. But there are some controversies with regard to the friction coefficient. The stable friction coefficient of FG/UHMWPE composite decreased with the increase of FG (Figure 6a) (55), while it increased in GO/UHMWPE composites (Figure 6b) (54). As for the wear rate, a consistent wear reduction was observed between these two works. The volume wear rates of both FG/UHMWPE composites (Figure 6c) and GO/UHMWPE composites (Figure 6d) decreased obviously with filler content. The wear rate was reduced by 50% and 40% at 5 wt% FG and 3 wt% GO, respectively. These results indicated that the nano-fillers had efficient abilities to improve the wear resistance of the UHMWPE matrix.

Figure 4. (a) Stress strain plots of three films obtained from tensile tests. (b) and (c) Influence of filler contents on mechanical properties of composites. (d) Typical stress-strain curves of the GO/UHMWPE composites. Inset is the magnified curves of the marked area. Figure (a) reproduced with permission from ref. (30). Copyright 2012, American Chemical Society. Figure (b) and (c) reproduced with permission from ref. (52). Copyright 2012, John Wiley & Sons, Inc. Figure (d) reproduced with permission from ref. (53). Copyright 2012, Elsevier.

279 Ito et al.; Advances in Bioinspired and Biomedical Materials Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Figure 5. (a) The microhardness of the GO/UHMWPE composites with different amounts of GO. (b) The microhardness of GO/UHMWPE composites with the different GO contents. (c) Micro-hardness of FG/UHMWPE composites with different FG contents. Figure (a) reproduced with permission from ref. (53). Copyright 2012, Elsevier. Figure (b) reproduced with permission from ref. (54). Copyright 2012, Springer International Publishing AG. (c) reproduced with permission from ref. (55). Copyright 2016, Elsevier.

Before graphene/UHMWPE composites can fulfill their potential in TJR applications, the biocompatibility remains a significant concern. It was reported that after 5 days incubation, the osteoblast survivability of 1 wt% GNP/UHMWPE composites was reduced to 86.7% in comparison to neat UHMWPE (30). Such reduction was largely related to the exposed GNP clusters since their sharp edges might be detrimental to osteoblast viability. On the contrary, Chen et al. reported a relatively positive effect of GO on the proliferation of the MC3T3-E1 cells (53). At the same incubation time, there was no obvious change in the cell adsorption between UHMWPE/GO and neat UHMWPE. The good biocompatibility of the UHMWPE/GO composites can be attributed to good intrinsic biocompatibility and hydrophilic nature of GO. 280 Ito et al.; Advances in Bioinspired and Biomedical Materials Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Figure 6. (a) Variations friction coefficient and average friction coefficient of FG/UHMWPE composites. (b) Variations of friction coefficient of the GO/UHMWPE composites. The inset is the statistic friction coefficient as a function of GO content during the wear tests. (c) The wear rate of FG/UHMWPE composites with different FG contents after the friction tests. (d) Variation of wear rate of the GO/UHMWPE composites as a function of GO content after the wear tests. Figure (b) and (d) reproduced with permission from ref. (54). Copyright 2012, Springer International Publishing AG. Figure (a) and (c) reproduced with permission from ref. (55). Copyright 2016, Elsevier.

Self-Reinforcement Self-reinforcement is an advanced approach to enhancing the mechanical performance of semi-crystalline polymers without the need of adding any robust phases. It is realized through manipulation of crystalline morphology/structure by dint of external fields, such as flow, drawing or pressure fields (32, 56–58). Compared to other methods, self-reinforcement approaches allow the material to obtain the optimized properties without sacrifice of the purity (56). This homogeneity is of paramount importance for biomaterials because any heterogeneous additives may potentially cause chronic inflammation, foreign body reactions and immunological responses. 281 Ito et al.; Advances in Bioinspired and Biomedical Materials Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Self-Reinforced UHMWPE through High-Pressure Crystallization UHMWPE presents a hexagonal phase at elevated temperature and high pressure (59–61), where the mobility of crystal stems were promoted. This phase transformation induces the formation of extended chain crystals and increased overall crystallinity (59, 62, 63), showing great promise in improving mechanical properties. It was reported that the degree of crystallinity was boosted from 50.2% for ambient pressure-crystallized UHMWPE to 70.9% for high pressure-crystallized UHMWPE (HP-UHMWPE) (64). The increased crystallinity was accompanied by lamellar thickening. The tensile modulus and yield stress of HP-UHMWPE were improved by approximately 36% and 10%, respectively. In addition, the value of fatigue threshold of HP-UHMWPE were increased, signifying an improved fatigue fracture resistance. High-pressure crystallization coupled with crosslinking contributed to a polymer with both wear and fatigue resistance. Under the same irradiation dose, the crystallinity of irradiated HP-UHMWPE was higher than the normal irradiated UHMWPEs and the remelted counterparts (Figure 7a) (65). Yield strength of the HP-UHMWPEs irradiated to 25 and 65 kGy were improved (Figure 7b). However, 100-kGy irradiated UHMWPE did not show improved strength. It was explained by the fact that the increased crosslinking density caused by high irradiation dose decreased the chain mobility that hindered crystallization and thus the strength improvement. The bidirectional pin-on-disk wear rate of the irradiated HP-UHMWPE were slightly less than the normal irradiated UHMWPEs. This was probably due to the decrease in the plasticity of the UHMWPE after high pressure crystallization. Introducing a compatible plasticizing agent into UHMWPE raised the chain mobility during high pressure crystallization, leading to higher crystallinity. It was reported that adding 0.2 wt% VE increased the crystallinity of HP-UHMWPE from 58.3% for virgin UHMWPE to 73.8% (66). The ultimate tensile strength and yield strength were improved by 18.2% and 32.7%, respectively. Concomitantly, fatigue strength was increased (Figure 8a) and wear rate was decreased (Figure 8b). This is promising because fatigue damage and fatigue behavior under adverse loading conditions remain major concerns for the use of alternate bearing surfaces made of UHMWPE in total hip and especially total knee arthroplasty.

Self-Reinforced UHMWPE through a Shear Flow Field Structural manipulation of a semi-crystalline polymer by control over the shear flow field permits the construction of a smart and satisfactory internal morphology and structure, which has a profound influence on the mechanical performance of a polymer product. Considering this fact, some new processing techniques such as dynamic packing injection molding (67), shear-controlled orientation injection molding (68), and push-pull processing (69), have been developed. Recently, in our group, a modified injection molding machine called oscillation shear injection molding (OSIM) was employed to engender the 282 Ito et al.; Advances in Bioinspired and Biomedical Materials Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


injection molded parts with rich self-reinforced superstructure (70, 71). The OSIM apparatus, which is equipped with two hydraulically actuated pistons moved reversibly at the same frequency during the packing stage, can provide intensive shear field with shear rate ranging from several s-1 up to hundreds of s-1. By utilizing OSIM technology, a big stride towards structural manipulation was made in our recent explorations.

Figure 7. (a) Crystallinity of irradiated, melted, HPC UHMWPEs as a function of radiation dose. The lines designate data groups between which there are significant differences (p < 0.05). (b) Yield strength of UHMWPE as a function of radiation dose after irradiation, after melting, and after HPC. The lines designate data groups between which there are significant differences (p < 0.05). (c) Wear rate of irradiated, melted, HPC UHMWPE as a function of radiation dose. The lines designate data groups between which there are significant differences (p < 0.05). Figures reproduced with permission from ref. (65). Copyright 2009, John Wiley & Sons, Inc.



Figure 8. (a) Fatigue crack propagation resistance of HPC UHMWPE as a function of vitamin E concentration. (b) Wear rate of HPC UHMWPE as a function of vitamin E concentration. Figures reproduced with permission from ref. (66). Copyright 2009, Elsevier.

The top priority behind the structural manipulation of the internal morphology is to make UHMWPE melt processable. To this end, low molecular weight polyethylene (LMWPE, Mw = 1.2 × 105 g/mol) was blended with UHMWPE to obtain affordable fluidity. The results showed that the upper limit of UHMWPE content was 40 wt%; further increasing the UHMWPE resulted in a failure of processing due to the extremely high melt viscosity (32). To make the maximum use of the superior mechanical properties, wear resistance and fatigue strength of UHMWPE, the content of UHMWPE in its blend should be increased as much as possible. In order to achieve this goal, we crosslinked the UHMWPE by electron beam before processing and then blended it with LMWPE. On this occasion, molecular diffusion between the LMWPE and UHMWPE phases were restrained by the crosslinked junctions, and hence the content of UHMWPE could be increased up to 50 wt% under the premise of affordable fluidity (32). The reduced melt viscosity of 50 wt% crosslinked UHMMWPE/LMWPE (xUHMWPE blend) compared to 50 wt% UHMWPE/LMWPE (UHMWPE blend) is evidenced by the rheological results (Figure 9). At the same UHMWPE loading (50 wt%), the dynamic viscosity (η′) of xUHMWPE blend was dramatically lower than that of the UHMWPE blend at the observed frequency. Another method to further increase the UHMWPE concentration is to recruit an ultralow molecular weight polyethylene (ULMWPE, Mw = 2.8 × 104 g/mol) as the flow accelerator. Interestingly, we found that the UHMWPE blends could still be injection molded even when the UHMWPE loading was as high as 90 wt % (72). After achieving desirable melt viscosity, the UHMWPE-based materials were injection molded and structural manipulated by OSIM technology. In OSIM xUHMWPE, shish-kebabs andcopy disordered lamellae were detected in LMWPE phase (Figure 10a), and UHMWPE phase (Figure 10b), respectively (32). The formation of shish-kebabs in LMWPE phase not only offset the 284 Ito et al.; Advances in Bioinspired and Biomedical Materials Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


deterioration of mechanical properties of UHMWPE due to the presence of LMWPE itself, but also contributed to a remarkably enhanced mechanical performance compared to compression molded UHMWPE (CM UHMWPE). Similar with OSIM xUHMWPE, large amounts of shish-kebabs were also observed in ULMWPE-rich region (Figure 10c) in 90 wt% UHMWPE/ULMWPE blend (MP-UHMWPE) (72). However, oriented lamellae rather than disordered lamellae were formed in UHMWPE-rich region (Figure 10d). The reason for such divergence between these two blends could be ascribed to the improved mobility of UHMWPE chains owing to the existence of ULMWPE. In OSIM xUHMWPE, the crosslinking between UHMWPE chains suppressed molecular diffusion of LMWPE into xUHMWPE. It, thus, was relatively difficult for xUHMWPE to be structurally manipulated and randomly distributed crystalline lamellae were formed. For MP-UHMWPE, a proportion of ULMWPE was liable to penetrate into UHMWPE granules to intensify the chain mobility. The orientation-induced crystallization induced by shear flow gave rise to large amounts of oriented lamellae in UHMWPE phase. It was worth noting that the oriented crystals both in OSIM xUHMWPE and MP-UHMWPE sample were closely packed and the individual kebabs or lamellae penetrated into each other, exhibiting a unique interlocked state (marked by the yellow rectangle in Figure 10a, 10b and 10c). This interlocked state created a stronger enhanced effect on the mechanical performance since the interlocking between adjacent kababs or lamellae could prevent the slippage of molecular chains and born strong stress when an external force is exerted.

Figure 9. Dynamic viscosities of the UHMWPE blend (50 wt% UHMWPE/LMWPE) and xUHMWPE blend (50 wt% xUHMWPE/LMWPE). Frequency sweeps were measured at 200 °C from 100 to 0.01 Hz with a strain of 1%. Figures reproduced with permission from ref. (32). Copyright 2014, Royal Society of Chemistry.



Figure 10. SEM images of the etched surfaces: (a), (b) LMWPE phase in OSIM xUHMWPE. (c) ULMWPE-rich phase and (d) UHMWPE-rich phase in OSIM 90wt% UHMWPE/ULMWPE. Figure (a) and (b) reproduced with permission from ref. (32). Copyright 2014, Royal Society of Chemistry. Figure (c) and (d) reproduced with permission from ref. (72). Copyright 2014, Elsevier.

The arc-like diffractions of (110) plane across the thickness of MP-UHMWPE demonstrated the existence of oriented crystals (Figure 11a). Degree of orientation of MP-UHMWPE was approximately 0.85 (Figure 11c). While for CM-UHMWPE, only isotropic diffraction circles appeared with degree of orientation approaching to zero (Figure 11b and 11c). The lamellae orientation in inner layer of MP-UHMWPE is quite interesting and is seldom reported before. The corresponding reason was ascribed to the so-called shear amplification effect, which was caused by the big difference in melt viscosity between the two types of PE. Apart from inducing self-reinforced structure, the intensive shear flow also promoted self-diffusion of chain entanglements across the granule boundaries, reducing the structural defects with a concomitant decrease of stress concentration in MP-UHMWPE. The generation of self-reinforced structures, i.e., interlocked shish-kebabs and oriented lamellae, combined with the reduced structural defects rendered MP-UHMWPE with improved mechanical properties (Figure 12). Tensile properties and impact strength exhibited a significant improvement in comparison to CM-UHMWPE. Particularly, yield strength and ultimate tensile strength were increased by 128.0% and 57.5%, respectively. In addition to mechanical 286 Ito et al.; Advances in Bioinspired and Biomedical Materials Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


properties, the biocompatibility of MP-UHMWPE was not changed compared to the control sample. All fibroblast cells were attached, spread, and proliferated well on the MP-UHMWPE and CM-UHMWPE sheets without considerably reduced in numbers after 2 days incubating.

Figure 11. (a) WAXD patterns of (a) MP-UHMWPE and (b) CM-UHMWPE along the transverse direction, the shear flow direction is vertical; (c) degree of orientation of (A) MP-UHMWPE and (B) CM-UHMWPE along the transverse direction. Figures reproduced with permission from ref. (72). Copyright 2014, Elsevier.



Figure 12. The yield strength, ultimate tensile strength, Young’s modulus, Izod impact strength, fatigue strength and elongation at break (from left to right) of MP-UHMWPE and CM-UHMWPE. Figures reproduced with permission from ref. (72). Copyright 2014, Elsevier.

Self-Reinforced UHMWPE with Simultaneously Improved Mechanical Performance and Wear Resistance Crosslinking by electron beam and structural manipulation by shear flow fields permitted UHMWPE with simultaneously high wear resistance and mechanical properties. The friction coefficient of the melt-processed 95 wt% UHMWPE/ULMWPE (MP-xUHMWPE) was lower than that of CM-UHMWPE (Figure 13a1), and the wear rate substantially decreased by 87.6% (Figure 13a2). A relatively smooth surface with no obvious wear scratch was observed on MP-xUHMWPE (Figure 13b1 and 13b2), whereas typical damage modes, such as wear scratches, flake-like features, and areas with gross material removal, appeared in CM-UHMWPE (Figure 13c1 and 13c2). Such wear reduction is related to the decreased plasticity led by radiation crosslinking. Generally speaking, adhesive/abrasive wear occurs in UHMWPE when the material is plastically deformed in the direction of the applied stress to such an extent that its strength in the orthogonal direction weakens. Under this circumstance, wear particles are likely to break up from the surface, causing macroscopic wear. The introducing of crosslinked junctions in our work could restrict the mutual slippage of the molecular chains and thus decreased the plasticity. Therefore, MP-xUHMWPE showed improved wear resistance. Similar with MP-UHMWPE, the structural manipulation also induced copious amount of interlocked oriented lamellae throughout the whole sample of MP-xUHMWPE (73). The rigid structure of the oriented lamellae increased the hardness and thus contributed to the wear reduction. This was deduced from the fact that the wear rate is inversely proportional to the hardness according to the Archard wear equation (74). 288 Ito et al.; Advances in Bioinspired and Biomedical Materials Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Figure 13. (a1) Friction coefficient and (a2) wear rates of MP-xUHMWPE and CM-UHMWPE; worn surface of (b1) MP-xUHMWPE and (c1) CM-UHMWPE; (b2) magnified image of the square in b1; (c2) magnified image of the square in c1. Figures reproduced with permission from ref. (73). Copyright 2016, Elsevier. Due to the formation of the oriented lamellae, a significant enhancement in tensile properties were obtained (73), which compensated the loss in mechanical properties caused by radiation crosslinking (75, 76). For instance, the ultimate tensile strength and Young’s modulus rose dramatically from 41.6 and 668.5 MPa for CM-UHMWPE up to 62.8 and 1175.0 MPa for MP-xUHMWPE, respectively. Precipitously, MP-xUHMWPE presented unspoiled oxidation stability. The oxidation index of MP-xUHMWPE is 0.7, much lower than that (4.7) of xUHMWPE particles after accelerated aging in air at 80 °C for 8 weeks (Figure 14a). The severe oxidation of xUHMWPE was reasonable because the residual free radicals entrapped in the crystalline phase could easily trigger a cascade of oxidation during the long-term service (77). While for MP-xUHMWPE, the elevated temperature during melt processing allowed the free radicals to recombine together and moved out of the crystalline phase towards the crystalline-amorphous interface. This procedure was highly comparable to the “post-irradiation melting” strategy whose stabilization mechanism is also based on the recombination of free radicals encouraged by high temperature during melting 289 Ito et al.; Advances in Bioinspired and Biomedical Materials Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


(14). One the other hand, the intensive shear force facilitated the chain mobility and therefore speeded the mutual recombination of the free radicals. These two factors contributed to the improved oxidative stability of MP-xUHMWPE. TGA measurement conducted under air atmosphere further proved the better oxidation stability of MP-xUHMWPE than xUHMWPE particles (Figure 14b). The slight weight increase before the primary weight loss in TGA curve was identified as the occurrence of oxidization (78, 79). MP-xUHMWPE showed a 37.8 °C higher oxidation initiation temperature and 7.4 °C lower oxidation peak temperature, indicating again the much better oxidative stability of MP-xUHMWPE.

Figure 14. (a) Oxidation index and (b) typical decomposition curves of xUHMWPE and MP-xUHMWPE. The inset of (b) was a magnified view revealing a small but noticeable weight gain in the thermos-oxidation process. Figures reproduced with permission from ref. (73). Copyright 2016, Elsevier.

Summary This chapter reviewed two approaches to reinforce UHMWPE, i. e. the additive-based reinforcement and self-reinforcement. Incorporation of suitable second-phase fillers strongly influenced the performance of UHMWPE. Regardless of the type of the fillers, good dispersion is a prerequisite for the desirable mechanical properties. In general, the strengthening of UHMWPE strongly depended on the filler concentration. Fillers improved the mechanical properties of UHMWPE at the low contents; but after the preliminary stage, the material presented a reduction in properties with further increasing filler concentration. These outcomes are related to the agglomeration of fillers at high content. The biocompatibility of UHMWPE after introducing reinforced fillers is a big concern, but no consistent results were achieved until now. Self-reinforcement via structural manipulation shows enormous advantages in biomedical application without the interference of chemical purity of the UHMWPE. By means of introducing external fields, such as pressure and shear flow field, the crystallinity was increased and the self-reinforced structure was generated. This strategy is desired to improve the mechanical performance of UHMWPE. When combined with such structural manipulation and irradiation crosslinking, simultaneously enhanced mechanical, tribological properties and oxygen resistance of UHMWPE were achieved. 290 Ito et al.; Advances in Bioinspired and Biomedical Materials Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

In the further study, more systematic biocompatibility test, fatigue resistance, and tribological tests under some realistic conditions that mimic the in vivo kinematics should be done for filler reinforced UHMWPE. And for self-reinforced UHMWPE, the crystalline morphology and structure need to be further manipulated for the optimized comprehensive performance. Besides, more biological evaluation should be conducted to notarize the biocompatibility and reliability of the self-reinforced UHMWPE as potential candidates to TJR materials.


Acknowledgments The authors gratefully acknowledge the financial support from the Program of National Natural Science of China (51533004, 51421061, 51403139), Sichuan Youth Science and Technology Innovation Research Team (2014TD0002), Outstanding Young Scholars Research Fund of Sichuan University (2016SCU04A17), and State Key Laboratory of Polymer Materials Engineering (sklpme2016-3-08). We would like to express sincere thanks to the BL16B of the Shanghai Synchrotron Radiation Facility (SSRF, Shanghai, China) for the kind help on WAXD and SAXS measurements.

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