The demand of orthopedic implants increases considerably in recent years due to large numbers of aging population, and patients suffering from bone disease and trauma. Nowadays, metallic implants such as titanium alloys are widely used for load-bearing prosthesis in orthopedics. However, the stiffness of metallic implants exceeds largely from that of human bones, causing stress-shielding effect that reduces bone density and eventually detaches the implant from bones.
Polymer nanocomposites are promising materials for orthopedic applications because their mechanical properties can be tailored by various combinations of polymers and fillers. However, polymer surfaces do not anchor osteoblasts (bone cells) since they are bioinert. In contrast, ceramic and carbonaceous materials enhance the adhesion and growth of osteoblasts. Hydroxyapatite (HA) (Ca10(PO4)6(OH)2) is a mineral component of natural bones with good bioactivity. The HA microparticles with a loading level of 40 vol% has been added to high-density polyethylene to form biocomposites. The addition of high filler content to polymers generally leads to low mechanical strength and poor processability. In this regard, HA nanorods (nHA) with low loading levels of 6.7 vol.%, and 4.4 - 21.5 vol% are added to polypropylene (PP) and polyetheretherketone (PEEK) respectively to form biocomposites using melt compounding and injection molding techniques. The morphological, mechanical and thermal properties as well as biocompatibility of PP and PEEK composites were studied using scanning electron microscopy, tensile tests, thermogravimetric analysis, cell cultivation and cell proliferation assay measurements.
Polypropylene generally exhibits low elastic modulus (1.41 GPa) and tensile strength. The addition of 6.7 vol% nHA to PP only increases its stiffness to 2.22 GPa. This stiffness value is still far smaller than that of human cortical bones having a value of 7 GPa. In this respect, filler hybridization is a sound approach to enhance the stiffness of PP/nHA nanocomposites. Hexagonal boron nitride (hBN) with a layered structure exhibits good mechanical properties and biocompatibility. hBN platelets (30 - 150 nm) of 2.2 - 9.7 vol.% are incorporated into PP for enhancing its elastic modulus, i.e. from 1.41 GPa to 1.54 - 1.76 GPa. Moreover, hybridization 7 vol% hBN with 6.7 vol% nHA further increases its modulus to 2.38 GPa. As a result, such PP/hBN-nHA hybrid composite cannot be used for load-bearing implant applications. It can only be used for maxillofacial surgery purposes. From the cell cultivation and proliferation measurements, both the PP/ 6.7vol% nHA and PP/7vol% hBN-6.7vol% nHA exhibit good biocompatibility.
Polyetheretherketone with higher elastic modulus (3.8 GPa) than PP is also selected as the matrix material in this study. The aims are to achieve higher stiffness of the resulting composites. Binary and ternary PEEK nanocomposites are fabricated by adding 4.4 - 21.5 vol% nHA, and 1.6 - 1.9 vol.% carbon nanofiber (CNF)-nHA fillers. Carbon nanofibers with an elastic modulus of 180 GPa and good biocompatibility are effective hybrid fillers for PEEK. Tensile test results show that elastic modulus of PEEK/nHA nanocomposites increases with increasing nHA content. The PEEK/9.3vol% nHA nanocomposite exhibit higher tensile strength than conventional HAPEX microcomposite. Thermogravimetric measurements indicate that the nHA addition improves thermal stability of PEEK.
Elastic moduli of PEEK/nHA nanocomposites can reach 6.2 and 7.85 GPa by adding 15 and 21.5 vol% nHA. The modulus of PEEK/21.5vol% nHA exceeds that of cortical bones. However, its tensile elongation is 0.69%, being slightly smaller than that of cortical bones (1 - 3%). Thus this composite is excluded for making load-bearing implants due to its brittle nature. Therefore, 1.9 vol% CNF is added to 15 vol% PEEK/nHA nanocomposite, producing PEEK/15vol% nHA - 1.9vol% CNF nanocomposite with elastic modulus of 6.54 GPa and tensile elongation of 2.83%.
Cell culture and proliferation tests reveal that human osteoblasts adhere firmly and spread exclusively on the composite surfaces. 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and (2-(4-Iodophenyl)-3-(4-nitrophenyl)-5-(2,4-disulfophenyl)-2H-tetrazolium (WST) results demonstrate that nHA enhances proliferation of osteoblasts on the surfaces of PEEK/nHA composites considerably.
Alkaline phosphatase activity (ALP) assay shows good activity of osteoblast cells on the composite specimens with higher nHA contents. Moreover, CNF addition further increases ALP activity of PEEK/9.3vol% nHA and PEEK/15vol% nHA nanocomposites.
The PEEK/9.3vol% nHA nanocomposite with good mechanical, thermal and biological performances is an attractive biomaterial for use in maxillofacial surgery. The PEEK/15vol% nHA-1.9vol% CNF nanocomposite with enhanced tensile strength and excellent biocompatibility shows large potential for load-bearing implant applications.