Yan-Fei Huang

Mechanical properties and biocompatibility of melt processed, self-reinforced ultrahigh molecular weight polyethylene

The low efficiency of fabrication of ultrahigh molecular weight polyethylene (UHMWPE)-based artificial knee joint implants is a bottleneck problem because of its extremely high melt viscosity. We prepared melt processable UHMWPE (MP-UHMWPE) by addition of 9.8 wt% ultralow molecular weight polyethylene (ULMWPE) as a flow accelerator. More importantly, an intense shear flow was applied during injection molding of MP-UHMWPE, which on one hand, promoted the self-diffusion of UHMWPE chains, thus effectively reducing the structural defects; on the other hand, increased the overall crystallinity and induced the formation of self-reinforcing superstructure, i.e., interlocked shish-kebabs and oriented lamellae. Aside from the good biocompatibility, and the superior fatigue and wear resistance to the compression-molded UHMWPE, the injection-molded MP-UHMWPE exhibits a noteworthy enhancement in tensile properties and impact strength, where the yield strength increases to 46.3 ± 4.4 MPa with an increment of 128.0%, the ultimate tensile strength and Youngs modulus rise remarkably up to 65.5 ± 5.0 MPa and 1248.7 ± 45.3 MPa, respectively, and the impact strength reaches 90.6 kJ/m(2). These results suggested such melt processed and self-reinforced UHMWPE parts hold a great application promise for use of knee joint implants, particularly for younger and more active patients. Our work sets up a new method to fabricate high-performance UHMWPE implants by tailoring the superstructure during thermoplastic processing.

Improved performance balance of polyethylene by simultaneously forming oriented crystals and blending ultrahigh-molecular-weight polyethylene

Polyethylene as a versatile polymer is being increasingly used for parts whose surfaces are in contact with moving metallic components or solid particles. This needs polyethylene to be greatly improved in mechanical properties as well as wear resistance. To this end, in the current work, various contents of ultrahigh-molecular-weight polyethylene (UHMWPE) were added into high-density polyethylene (HDPE) for enhancement of wear resistance, while the oriented crystals, i.e., shish-kebabs, were induced by shear flow for mechanical reinforcement. With 30 wt% UHMWPE was added, highly improved performance balance was achieved. The tensile strength rose from 26.4 MPa for normal HDPE samples to 68.5 MPa for the modified HDPE blends. The same trend was observed for impact toughness, where the impact strength increased from 6.3 to 34.1 kJ m−2. Moreover, addition of UHMWPE could reduce the wear rate from 22.1 to 7.6 mg MC−1. A very interesting phenomenon was observed, in which the overall properties of the modified HDPE blends were constantly enhanced with the increase of UHMWPE content though UHMWPE itself does not have much better mechanical properties than the oriented HDPE. This was ascribed to the amplified shear effect as a result of UHMWPE addition. The exceptionally high melt viscosity of UHMWPE assumes a gel state even at high temperature, making it just deform and hardly flow under the shear field, which amplifies the flow velocity difference between UHMWPE phase and HDPE melt. The amplified shear effect resulted in more pronounced molecular orientation and thus formation of a higher content of shish-kebab microstructure. Our work indicated that the melt processing-structure control strategy can desirably manipulate polyethylene products with desired properties.

Self-reinforced polyethylene blend for artificial joint application

By means of purposeful material design and melt manipulation, we present a highly feasible approach to simultaneously improve the mechanical properties, fatigue and wear resistance of an ultrahigh molecular weight polyethylene (UHMWPE)-based self-reinforced polyethylene (PE) blend for artificial joint replacement. The fluidity of the PE blend was achieved by blending low molecular weight polyethylene (LMWPE) with radiation cross-linked UHMWPE. The use of the cross-linked UHMWPE restrained the molecular diffusion between the LMWPE and UHMWPE phases, and hence increased the content of UHMWPE up to 50 wt% under the premise of desirable fluidity for injection molding. The combination of the shear flow field and pre-additive precursors successfully induced numerous interlocking shish-kebab structures in the LMWPE phase. Mechanical reinforcement was thus attained, where the ultimate tensile strength was significantly improved from 27.6 MPa for the compression-molded UHMWPE to 81.2 MPa for the PE blend, and the impact strength was increased from 29.6 to 35.2 kJ m-2. The fatigue and wear resistance were far superior to those of the compression-molded UHMWPE. Compared to the results reported in our previous study (40 wt% UHMWPE), the increased UHMWPE content caused the LMWPE phase melt to flow faster, thus amplifying the shear rate in the interfacial region between the two phases and depressing the relaxation of oriented molecular chains. The crystalline orientation was preserved, especially in the inner layer, leading to further enhancement of the mechanical properties. These results suggest that such a self-reinforced PE blend is of benefit to lowering the risk of failure and prolonging the life span of the implant under adverse conditions.

Injection-molded hydroxyapatite/polyethylene bone-analogue biocomposites via structure manipulation

Due to insufficient mechanical performance, such as low tensile strength, the application of hydroxyapatite (HA)/high-density polyethylene (HDPE) biocomposites has been limited to use as minor load-bearing bone substitutes. In the current work, we propose to impose an intense shear flow during injection molding to tune the microstructure of the HA/HDPE biocomposites, by which an anisotropic biomimetic structure and superior mechanical properties were gained. Morphological observations manifested that the imposed intense shear induced a large amount of oriented self-reinforced superstructure, i.e., interlocked shish-kebabs, which brought not only structure similarity with the natural bone but also considerable mechanical reinforcement. For the 20 wt% HA/HDPE biocomposite, the tensile strength and bending strength of the structured sample rose from 22.4 and 20.2 MPa for the normal sample to 60.4 and 44.0 MPa, increasing by 169% and 118%, respectively, which already reaches the bounds of human cortical bone. The Youngs modulus increased to 1462.0 MPa, with an augment of 37%. The impact toughness of the structured biocomposite (64.6 kJ m-2) showed as over 5 times larger than the normal biocomposite (10.1 kJ m-2). Besides, the dispersion of the HA in the biocomposites especially at the high filler content was enhanced, playing a positive role in sustaining the bioactivity. All these results indicate that the structured HA/HDPE biocomposites hold great promise for use in high load-bearing orthopedic applications.