Ultra-high molecular weight polyethylene bioactive composites with carbonated hydroxyapatite

To develop an orthopedic material for bone substitution, the substitute material must mimic living tissue from an anatomical and physiological point of view. The high wear and impact resistance besides the low friction coefficient, make ultra-high molecular weight polyethylene (UHMWPE) a suitable material to be used in orthopedic applications. However, UHMWPE is a bioinert material, not providing a proper interaction with the bone tissue surrounding to the implant. One way to mitigate this issue is improving UHMWPE bioactivity. This can be done by adding bioactive fillers in the polymeric matrix. In this work, UHMWPE composites were prepared by twin-screw extrusion. The fillers used were carbonated hydroxyapatite (CHA) and hybrids formed by precipitating CHA in collagens (hydrolyzed and type II). The results show that the fillers used caused a slight reduction in UHMWPE crystallinity degree, while both crystallization and melting temperatures remained almost unchanged. Dynamic-mechanical thermal analysis indicated a weak adhesion between filler and polymeric matrix, which is good from the biological point of view since the bioactive filler surface will be available to apatite deposition. The obtained materials exhibited good mechanical properties and in vitro bioactivity assay showed that all of the prepared materials are bioactive.

Thermal and mechanical behavior of ultra-high molecular weight polyethylene/collagen blends

Bone defects or diseases significantly affect quality of life, thus the development of materials with improved performance that can be used as bone substitutes is increasingly studied. As an alternative, ultra-high molecular weight polyethylene (UHMWPE) has been employed for orthopedic applications since it combines high wear resistance, high impact resistance and low friction coefficient. However, it is a bioinert material and difficult to process. In the present work, the addition of collagen (hydrolyzed or type II), one of the constituents of natural bone, to UHMWPE was studied aiming to improve its processability and possibly its biocompatibility. The blends were prepared by compression and twin-screw extrusion. The results show that addition of higher amounts of both collagens to UHMWPE reduced the degree of crystallinity. However, crystallization and melting temperatures were not affected. The thermogravimetric analysis exhibited two thermal events correlated to the degradation of collagens (Tmax~300 °C) and of UHMWPE (Tmax~480 °C), corroborating the FTIR analysis that presented bands corresponding to these materials. The extrusion process promoted a better dispersion of the collagens, especially the hydrolyzed one. In addition, the obtained materials presented better mechanical properties when extruded. Torque reduction during extrusion showed that hydrolyzed collagen aid processing, even more than collagen due to its smaller molecular weight.

Wear behaviour of UHMWPE reinforced by carbon nanofiller and paraffin oil for joint replacement

The majority of artificial joints incorporate biomedical grade Ultra High Molecular Weight Poly Ethylene (UHMWPE), whose wear is considered most important in controlling service time of the whole joint. The aim of this work was to improve wear resistance of UHMWPE through the addition of 0.5-2.0wt% of Carbon Nano Filler (CNF) and 2% wt of Paraffin Oil (PO) using ball milling (BM) and extrusion techniques (EX). The wear tests on these nanocomposites were conducted by a pin on disc in dry (air) and wet media (simulated synovial fluid or artificial lubricant, and bovine synovial fluid or natural lubricant). Mechanical tests (tensile and hardness), physical analysis (calorimetric, density, wet ability, roughness) and morphological observations were also performed. The experimental results showed that natural lubricant provides the greatest reduction in wear rate while the largest one occurred in air. Furthermore, the BM mixed nanocomposites with a filler load of 1.0% exhibited the best wear resistance among all the samples with an improvement of 42%, 64% and 83% in air, artificial and natural lubricant, respectively. This is due to its higher ductility and thermal features, and lower wet ability in the two lubricants.